Effects of substituents on synthetic analogs of chlorophylls. Part 3: The distinctive impact of auxochromes at the 7- versus 3-positions

Article abstract of DOI:10.1111/j.1751-1097.2012.01083.x

Assessing the effects of substituents on the spectra of chlorophylls is essential for gaining a deep understanding of photosynthetic processes. Chlorophyll a and b differ solely in the nature of the 7-substituent (methyl versus formyl), whereas chlorophyll a and d differ solely in the 3-substituent (vinyl versus formyl), yet have distinct long-wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited-state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 13¹-oxophorbines that contain ethynyl, acetyl, formyl and other groups at the 3-, 7- and/or 13-positions. Substituent effects on the absorption spectra are well accounted for using Gouterman's four-orbital model. Key findings are that () the dramatic difference in auxochromic effects of a given substituent at the 7- versus3- or 13-positions primarily derives from relative effects on the LUMO+1 and LUMO; (2) formyl at the 7- or 8-position effectively porphyrinizes the chlorin and (3) the substituent effect increases in the order of vinyl < ethynyl < acetyl < formyl. Thus, the spectral properties are governed by an intricate interplay of electronic effects of substituents at particular sites on the four frontier MOs of the chlorin macrocycle. Chlorophyll a and b differ solely in the nature of the 7-substituent, whereas chlorophyll a and d differ solely in the 3-substituent, yet have distinct long-wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited-state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 13 ¹-oxophorbines that contain a variety of substituents at the 3-, 7- and/or 13-positions. An intricate interplay of site-specific electronic effects of substituents on the four frontier MOs of the chlorin macrocycle is found to govern the spectral properties.

Full text of DOI:10.1111/j.1751-1097.2012.01083.x

Photochemistry and Photobiology, 2012, 88: 651–674
Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 3:
The Distinctive Impact of Auxochromes at the 7- versus 3-Positions
Joseph W. Springer1, Kaitlyn M. Faries1, James R. Diers2, Chinnasamy Muthiah3, Olga Mass3,
Hooi Ling Kee1, Christine Kirmaier1, Jonathan S. Lindsey*3, David F. Bocian*2 and Dewey Holten*1
1Department of Chemistry, Washington University, St. Louis, MO
2Department of Chemistry, University of California Riverside, Riverside, CA
3Department of Chemistry, North Carolina State University, Raleigh, NC
Received 18 November 2011, accepted 5 January 2012, DOI: 10.1111/j.1751-1097.2012.01083.x
of the 7-formyl versus 7-methyl group causes a bathochromic
shift of the B band, a hypsochromic shift of the Qy band, and
ABSTRACT
Assessing the effects of substituents on the spectra of chloro-
phylls is essential for gaining a deep understanding of photosyn-
thetic processes. Chlorophyll a and b differ solely in the nature of
the 7-substituent (methyl versus formyl), whereas chlorophyll a
and d differ solely in the 3-substituent (vinyl versus formyl), yet
have distinct long-wavelength absorption maxima: 665 (a) 646
(b) and 692 nm (d). Herein, the spectra, singlet excited-state
decay characteristics, and results from DFT calculations are
examined for synthetic chlorins and 131-oxophorbines that
contain ethynyl, acetyl, formyl and other groups at the 3-, 7-
and ⁄ or 13-positions. Substituent effects on the absorption
spectra are well accounted for using Gouterman’s four-orbital
model. Key findings are that (1) the dramatic difference in
auxochromic effects of a given substituent at the 7- versus 3- or
13-positions primarily derives from relative effects on the
LUMO+1 and LUMO; (2) formyl at the 7- or 8-position
effectively ‘‘porphyrinizes’’ the chlorin and (3) the substituent
effect increases in the order of vinyl < ethynyl < acetyl < for-
myl. Thus, the spectral properties are governed by an intricate
interplay of electronic effects of substituents at particular sites
on the four frontier MOs of the chlorin macrocycle.
also alters the ratio of the B and Qy band intensities. Quite
different behavior is observed for Chl d (and Pheo d), wherein
3-formyl replaces the 3-vinyl of Chl a. This change engenders a
bathochromic shift (665–697 nm) and hyperchromic effect on
the red-region absorption band, precisely the opposite of Chl b
versus Chl a (Fig. 1). Chl d is a primary pigment of the
cyanobacterium Acarychloris marina, which grows on didem-
nid ascidians below layers of the symbiont Prochlorin didemni
(also a cyanobacterium) whose rich Chl a content filters out
most of the light at wavelengths shorter than about 690 nm
(6). The spectral distinctions between the three native photo-
synthetic chromophore classes thus derive in large measure
from whether a formyl group is present at the 7-position
(Chl b), the 3-position (Chl d) or not at all (Chl a).
The hypsochromic effect of the formyl group at the
7-position stands in stark contrast to a widespread maxim in
organic photochemistry that the presence of a conjugative
group causes a bathochromic effect on the absorption spec-
trum. The term auxochrome to describe conjugative groups
originated in late 19th-century efforts to understand the
relation between color and constitution of synthetic dyes,
where only bathochromic effects were typically observed (7–
10). Similarly, mid-20th century tables of empirical rules for
calculating absorption spectra of substituted dienes and
dienones typically only show bathochromic increments for
incorporation of diverse functional groups (11). Thus, the
INTRODUCTION
Chlorophyll a (Chl a) and b (Chl b) are the two green pigments
that underlie the photosynthetic process in plants and many
cyanobacteria. Chl a and Chl b each bear a 3-vinyl group, an
isocyclic ring spanning the 13–15 positions, and a 7-methyl
versus 7-formyl group, respectively, attached to the chlorin
macrocycle (1). Chl a in toluene exhibits an intense B (Soret)
band at 432 nm and a Qy band at 665 nm of nearly
comparable intensity (Fig. 1); similar spectra are found in a
variety of solvents (2–5). Chl b exhibits a B band at 458 nm
and a Qy band at 646 nm; the former is more intense than that
of Chl a, whereas the latter is less intense (2–5). The respective
metal-free analogs pheophytin a (Pheo a) and b (Pheo b)
exhibit similar spectral differences (Fig. 1). Thus, the presence
classic book by Jaffe and Orchin (12)—consistent with Gillam
´
and Stern (13) a decade earlier—defined an auxochrome as ‘‘an
atom or group of atoms which, alone, does not give rise to
absorption in the ultraviolet but which, when conjugated to a
chromophore, causes a bathochromic shift and hyperchromic
effect.’’ On the other hand, Calvert and Pitts (11) offered a
more guarded definition of auxochromes as groups that
‘‘appreciably intensify and shift the absorption’’ and stated
that the shift can be bathochromic or hypsochromic. Mason
(14) also mentioned that substitution of arenes with a
conjugating substituent is ‘‘usually, but not invariably, accom-
panied by a redshift of the absorption band.’’ Attempts to
explain the origin of the effect of auxochromes of dyes became
interwoven with early concepts of resonance delocalization
(15). Indeed, Lewis and Calvin (16) stated that the effect of an
*Corresponding authors email: holten@wustl.edu (Dewey Holten),
dbocian@ucr.edu (David F. Bocian), jlindsey@ncsu.edu (Jonathan S. Lindsey)
ꢀ 2012 Wiley Periodicals, Inc.
Photochemistry and Photobiology ꢀ 2012 The American Society of Photobiology 0031-8655/12
651

652 Joseph W. Springer et al.
Chart 1. Nomenclature of chlorins (left) and 131-oxophorbines (right).
ents to differing extent with the four orbitals. Such interactions
depend on the nature and position of the substituent.
In our earlier studies (parts 1 and 2) of synthetic analogs of
the chlorophylls, we analyzed the spectral and photophysical
properties of large sets of zinc and free base chlorins bearing a
variety of substituents (e.g. acetyl, ethynyl and alkyl) at the
3- and ⁄ or 13-positions in terms of the four-orbital model
(20,21). The studies also encompassed a few chlorins that
contain the fifth ‘‘isocyclic’’ ring (i.e. 131-oxophorbines). The
nomenclature for chlorins and 131-oxophorbines, of which the
chlorophylls are members, is shown in Chart 1. Several analogs
with a subset of these groups (i.e. ethynyl and acetyl) at the
3,12- versus 3,13- position were also investigated (22). Sparsely
substituted parent macrocycles (chlorins, oxochlorins, phor-
bines and oxophorbines [23–26]) were probed to elucidate the
minimal structural and electronic characteristics necessary to
answer the question ‘‘what structural features make chloro-
phyll a green?’’ (26). However, none of these studies utilized a
formyl substituent, which is relevant to Chl b and Chl d. In
addition, although 7-substituted chlorins and oxophorbines
(relevant to Chl b) have been prepared (27,28), the electronic
and excited-state characteristics have not been analyzed and
compared with the same substituents (e.g. formyl, acetyl and
ethynyl) at other macrocycle sites.
Herein, we report the results of studies of the spectral and
photophysical properties of chlorins bearing formyl and other
common substituents at the 7- versus 3,13-positions, as well as
of the corresponding 7-substituted 131-oxophorbines (Charts 2
and 3). We also studied Chl a, b and d as well as Pheo a, b and
d in the same (noncoordinating) solvent toluene to facilitate
comparisons with the properties of synthetic chromophores
examined under the same conditions. Collectively, the goals of
these studies are to (1) gain insights into the architectural and
electronic characteristics that underpin the distinctive spectral
properties of Chl a, b and Chl d and (2) strengthen the
foundation for the utilization of tunable synthetic hydro-
porphyrin analogs in bioinspired light-harvesting and charge-
transfer systems for solar-energy conversion.
Figure 1. Chemical structure and absorption spectra of (A) chloro-
phyll a (red), chlorophyll b (blue) and chlorophyll d (green) and (B) the
pheophytin analogues in toluene at room temperature. The spectra
utilize the molar absorption coefficients reported in Ref 5 (the ordinate
has been multiplied by 10)4; the small absorption near 500 nm for
Chl d and Pheo d is due to carotenoid).
auxochrome ‘‘may be described as providing, through reso-
nance, entirely new electronic paths.’’ Such concepts were
consistent with one-electron particle-in-a-box treatments of
absorption in p-electron systems (wherein the lowest excited
state derives solely from electron promotion from the highest
occupied molecular orbital [HOMO] to the lowest unoccupied
molecular orbital [LUMO]). While the maxim still persists that
a conjugative group causes a bathochromic effect, the origin of
the absorption spectrum of cyclic polyenes including tetrapyr-
role macrocycles is now known to be comprehensible only by
consideration of the interplay of four frontier molecular orbitals
(in which the lowest excited state derives from an admixture of
two one-electron configurations, such as HOMO fi LUMO
and HOMO-1 fi LUMO+1; 17–19). Thus, the effects of
substituents on spectra stem from interactions of the substitu-
MATERIALS AND METHODS
Compounds. The previously synthesized chlorins include zinc chlorins
ZnC-E3E12 (29,30), ZnC-E3A12 (29,30), ZnC-E3M10 13 (29), ZnC-M10
E
(29), ZnC-M10A13 (29), ZnC-M10E13 (29), ZnC-V3M10 (29), ZnC (23),
ZnC-P10 (23), ZnC-T5 (23), ZnC-P15 (20), ZnC-A3M10A13 (29),
ZnC-M10F13 (31), ZnC-F3M10F13 (31), ZnC-T10 (27), ZnC-F7T10

Photochemistry and Photobiology, 2012, 88 653
Preparation
of
Zn(II)-17,18-dihydro-10-mesityl-13-methoxy-
carbonyl-18,18-dimethylporphyrin (ZnC-M10Es13). Application of a
literature procedure for metalation (26) to the free base chlorin FbC-
1
M
10Es13 afforded the title compound: H NMR d (300 MHz, CDCl3)
1.84 (s, 6H), 2.02 (s, 6H), 2.60 (s, 3H), 4.15 (s, 3H), 4.52 (s, 2H), 7.25 (s,
2H), 8.35 (d, J = 4.4 Hz, 1H), 8.53 (s, 1H), 8.72–8.74 (m, 2H), 9.01–
9.03 (m, 2H), 9.45 (s, 1H), 9.73 (s, 1H); LD-MS obsd 577.7; calcd
578.1660 (C33H30N4O2Zn); kabs (toluene) 415, 627 nm.
Preparation of Zn(II)-3,13-dibromo-17,18-dihydro-18,18-dimethyl-
10-mesitylporphyrin (ZnC-Br3M10Br13; 29). Following a streamlined
procedure (23,27), a solution of 8,9-dibromo-1-formyldipyrromethane
(29; 301 mg, 0.668 mmol) and 2,4,5,6-tetrahydro-1,3,3-trimethyldipyr-
rin (33; 150 mg, 0.557 mmol) in anhydrous CH2Cl2 (16 mL) was
treated with a solution of p-TsOHÆH2O (530 mg, 2.78 mmol) in
anhydrous methanol (4.0 mL) under argon. The red reaction mixture
was stirred at room temperature for 40 min. A sample of 2,2,6,6-
tetramethylpiperidine (1.04 mL, 6.11 mmol) was added. The reaction
mixture was concentrated. The crude solid was dissolved in CH3CN
(55.7 mL) and subsequently treated with 2,2,6,6-tetramethylpiperidine
(2.36 mL, 13.9 mmol), Zn(OAc)2 (1.53 g, 8.35 mmol) and AgOTf
(429 mg, 1.67 mmol). The resulting suspension was refluxed for 20 h
exposed to air. The crude mixture was filtered through a pad of silica
(CH2Cl2) followed by column chromatography of the resulting solid
[silica, hexanes, then hexanes ⁄ CH2Cl2 (1:1)] to afford a green solid
(136.2 mg, 36%). The data obtained by 1H NMR spectroscopy
(400 MHz), laser-desorption mass spectrometry (LD-MS), fast-atom
bombardment mass spectrometry (FAB-MS) and absorption spectros-
copy were consistent with the reported data for the title compound
obtained previously at lower scale (29).
Preparation
of
3,13-diacetyl-17,18-dihydro-18,18-dimethyl-10-
mesitylporphyrin (FbC-A3M10
A
13; 20,34). A solution of ZnC-Br3M10
Br13 (78.0 mg, 0.115 mmol) in CH2Cl2 (3.0 mL) was treated dropwise
with trifluoroacetic acid (266 lL, 22.2 mmol) over a 2 min period. The
solution was stirred at room temperature for 4 h. CH2Cl2 was added,
and the organic layer was washed (saturated aqueous NaHCO3 and
water) and then dried (Na2SO4). The organic layer was concentrated to
afford a purple solid. Following a procedure for Stille coupling on
chlorins (29), a mixture of the crude free base chlorin, tributyl(1-
ethoxyvinyl)tin (311 lL, 0.92 mmol) and (PPh3)2PdCl2 (16.1 mg,
0.0230 mmol) was refluxed in THF (5.5 mL) for 20 h in a Schlenk
line. The reaction mixture was treated with 10% aqueous HCl (30 mL)
at room temperature for 2 h. CH2Cl2 was added. The organic layer was
separated, washed (saturated aqueous NaHCO3, water and brine),
dried (Na2SO4) and concentrated. The resulting residue was chroma-
tographed [silica, hexanes ⁄ CH2Cl2 (1:1), then CH2Cl2] to afford a
purple solid (46 mg, 74%): 1H NMR (400 MHz, CDCl3) d )1.26 (brs,
2H), 1.85 (s, 6H), 2.03 (s, 6H), 2.63 (s, 3H), 3.06 (s, 3H), 3.27 (s, 3H),
4.60 (s, 2H), 7.25 (s, 2H), 8.35 (d, J = 4.4 Hz, 1H), 8.82 (s, 1H), 8.87 (d,
J = 4.4 Hz, 1H), 8.93 (s, 1H), 9.30 (s, 1H), 10.10 (s, 1H), 10.60 (s, 1H);
LD-MS obsd 542.6; FAB-MS obsd 542.2691, calcd 542.2682
(C35H34N4O2); kabs (toluene) 431, 687 nm.
Preparation of Pheophytin a. Following a reported procedure (35),
a sample of Chl a (0.5 mg, 0.0006 mmol) was dissolved in a drop of
cold ()18ꢁC) THF, and the solution was immediately diluted with
hexanes (11 mL). The solution was treated with 11 mL of aqueous
trichloroacetic acid (prepared by dissolving 25 g of trichloroacetic
acid in 100 mL of H2O). The solution was vigorously stirred for
15 min. The organic phase was washed with water (2 · 40 mL), 5%
aqueous NaHCO3 (1 · 40 mL), again with water (2 · 40 mL) and
Chart 2. Chlorins and 131-oxophorbines characterized here.
(27), ZnC-A7T10 (27), ZnC-E7T10 (27), ZnC-E3E13 (22), ZnC-E3A13
(22), ZnC-A12 (22) and ZnC-A13 (26); free base chlorins FbC (24), FbC-
P10 (24), FbC-T5 (24), FbC-P15 (24), FbC-M10 (24), FbC-M10E13 (20)
and FbC-M10Es13 (32); free base oxophorbines FbOP-M10 (28), FbOP-
A3M10 (28), FbOP-F7M10 (28), FbOP-A7M10 (28), FbOP-E7M10 (28)
and FbOP (26) and zinc oxophorbine ZnOP (26). Chl a and b were
obtained from Aldrich. Chl d was isolated from Chlorobium vibro-
forme.
concentrated to
a solid under reduced pressure at 30ꢁC. Ace-
tone (5 mL) was added, and the mixture was concentrated so as to
remove a trace of water. TLC analysis revealed a single green-brown
spot (Rf = 0.67, ethyl acetate ⁄ hexanes [1:2]) with a small amount of
a by-product. Chl a (a bright emerald green streaking spot, Rf = 0.5,
ethyl acetate ⁄ hexanes [1:2]) was not observed using TLC analy-
sis. The title compound was used as obtained: LD-MS obsd 870.6,
calcd 870.5659 (C55H74N4O5); kabs (ethanol) 410, 507, 537, 609,
667 nm.
Preparation of Pheophytin b. Following a reported procedure (35),
a sample of Chl b (2.5 mg, 0.0028 mmol) was dissolved in a drop of
cold ()18ꢁC) THF, and the solution was immediately diluted with
hexanes (50 mL). The solution was treated with 50 mL of aqueous
trichloroacetic acid (prepared by dissolving 25 g of trichloroacetic acid
in 100 mL of H2O). The solution was vigorously stirred for 15 min.
Synthesis procedures provided access to one new chlorin (ZnC-
)
M
10Es13
and increased amounts of valuable precursors (ZnC-
Br3M10Br13, FbC-A3M10 13) to other chlorins. The prior synthesis of
A
FbC-A3M10A13 was carried out by treatment of ZnC-Br3M10Br13 to
demetalation, introduction of the acetyl groups via Stille coupling, and
metalation to form ZnC-A3M10A13 followed by demetalation (20,29).
Herein, the synthesis of FbC-A3M10A13 was achieved by treatment of
ZnC-Br3M10Br13 to demetalation and Stille coupling on the crude free
base product to introduce the acetyl groups. Due to greater simplicity
and larger scale, a 7.5-fold larger quantity (46 mg) of FbC-A3M10A13
was obtained versus the prior report (20).

654 Joseph W. Springer et al.
Chart 3. Previously studied chlorins and 131-oxophorbines used here for comparisons.
Water (50 mL) was added. The organic phase was washed with water
(4 · 50 mL) and concentrated to a solid under reduced pressure at
30ꢁC. Acetone was added, and the mixture was concentrated so as to
remove a trace of water. TLC analysis revealed a single spot (Rf = 0.7,
ethyl acetate ⁄ hexanes [1:1] or Rf = 0.3, ethyl acetate ⁄ hexanes [1:2])
with faint traces of two by-products. The title compound was used as
obtained: LD-MS obsd 883.9, calcd 884.5452 (C55H72N4O6); kabs
(ethanol) 437, 654 nm.
Preparation of Pheophytin d. Due to limited amount of material, a
solution of the compound was prepared by addition of a trace amount
of trifluoroacetic acid to a solution of Chl d in toluene. The resulting
spectrum matched that reported for Pheo d (5), and the solution was
used directly for photophysical studies.
Photophysical measurements. Static absorption (Varian Cary 100
or Shimadzu UV-1800) and fluorescence (Spex Fluorolog Tau 2 or
PTI Quantamaster 40) measurements were performed at room
temperature, as were all other studies. Measurement of the fluores-
cence quantum yield (Ff) and singlet excited-state lifetimes (sS)
utilized dilute (lM) Ar-purged toluene solutions. Samples for Ff
measurements had an absorbance £0.1 at the excitation wavelength.
Static emission measurements employed 2–4 nm excitation- and
detection-monochromator bandwidths and 0.2 nm data intervals.
Emission spectra were corrected for detection-system spectral
response. Fluorescence quantum yields were determined relative to
several different standards depending on the spectral characteristics.
These standards are (1) chlorophyll
a in deoxygenated toluene
(Ff = 0.325) (26), which is the value measured in benzene (36) and
(2) free base meso-tetraphenylporphyrin (FbTPP) in nondegassed
toluene, for which Ff = 0.070 was established with respect to the
zinc chelate ZnTPP in nondegassed toluene (Ff = 0.030) (37),
consistent with previous results on FbTPP (38). Fluorescence
lifetimes were obtained by two methods (and typically averaged) as
follows: (1) a phase-modulation technique in which samples were
excited at various wavelengths in the Soret region and detected
through appropriate colored glass filters (Spex Fluorolog Tau 2).
Modulation frequencies from 10 to 200 MHz were typically utilized
and both the fluorescence phase shift and modulation amplitude were
analyzed. (2) Decay measurements using time-correlated single
photon counting detection on an apparatus with an approximately
Gaussian instrument response function with a full width at half

Photochemistry and Photobiology, 2012, 88 655
maximum of ca 1 ns (Photon Technology International LaserStrobe
TM-3). Samples were excited in the near-UV region using excitation
pulses at 337 nm from a nitrogen laser or in the blue to green
spectral regions from a dye laser pumped by the nitrogen laser. Note
that some of the Ff and sS values from Ref 21 reported herein
(Tables 1–3) were updated (changes typically <15%) to reflect
averaging with additional repeat measurements. The additional
studies included using the second technique for the sS determinations
to complement the results from the first technique, which was used
exclusively in the earlier study.
Density functional theory calculations. DFT calculations were
performed in parallel mode (39) with either SPARTAN ’08 for
windows version 1.2.0 or SPARTAN ’10 for windows version 1.1.0 on
a PC equipped with an intel i7-975 cpu, 24 GB ram and three 300 GB
10 K rpm hard drives. The hybrid B3LYP functional and the 6-31G*
basis set were employed. The equilibrium geometries were fully
optimized using the default parameters of the Spartan program.
TDDFT calculations were performed in parallel mode (40) with
GAUSSIAN ’09 version B.01 64-bit for Linux using OPENSUSE
version 11.4. One of two PC systems was used for Gaussian runs. The
hardware for first PC system used for Gaussian runs is the same as that
used for the Spartan calculations and the second is a PC equipped with
an Intel i7-980 CPU, 24 GB RAM and two 600 GB 10 K rpm hard
drives. Geometries used for the TDDFT calculations were from
optimizations at the B3LYP ⁄ 6-31 + G(d,p) level. TDDFT single
point calculations were performed at the wB97xD ⁄ 6-311 + G(2d,2p)
level. The models used for TDDFT calculations and geometry
optimizations were selected based on a recent study published by Tian
et al. (41). Except for the occasional use of keyword scf = qc, default
parameter settings were used for Gaussian calculations.
Bx(0,0) and By(0,0) features may differ among compounds
(and the bands may not be of strictly pure x- and y-
polarization). The spectra in Fig. 2 show that the Bx(0,0) and
By(0,0) are coalesced into one feature (and similarly for the
weaker Bx(1,0) and By(1,0) when resolved) for the sparsely
substituted zinc chlorins (e.g. ZnC, ZnC-F7T10, ZnC-M10F13).
Reflecting a progressive decrease in symmetry, the splitting of
the Bx and By components increases as the number of
substituents is increased for the zinc chlorins (e.g. ZnC-
F3M10F13), for the free base chlorins, and for the oxophor-
bines. Figure 3 shows representative fluorescence spectra. The
emission spectra are dominated by the Qy(0,0) band, with
weaker vibronic features to longer wavelengths. Peak absorp-
tion and fluorescence positions are listed in Tables 1 and 2.
The wavelength of the Soret maximum (Bmax) for each
compound may reflect the Bx(0,0), By(0,0) or a combined
feature.
Tables 1 and 3 list the measured fluorescence yields (Ff) and
singlet excited-state lifetimes (sS). These observables are
)1
connected via the relationships sS = (kf + kic + kisc
)
=
(kf + knr))1 and Ff = kfÆsS, which contain the rate constants
for the decay pathways of the S1 excited state (the Qy state).
The rate constants are for S1 fi S0 fluorescence (kf),
S1 fi S0 internal conversion (kic), and S1 fi T1 intersystem
crossing (kisc). The sum of two nonradiative rate constants (kic
and kisc,) is denoted as knr. The inverses of values of kf and knr
(in units of ns) derived from the observed sS and Ff values are
also listed as the last two columns in Tables 1 and 3. The
relationships of the Einstein coefficients (42) indicate that a
greater radiative rate constant for spontaneous fluorescence
(kf) implies an increased oscillator strength for the corre-
sponding S1 fi S0 (i.e. Qy) absorption transition. In turn,
trends in the Qy(0,0) absorption intensity with substituent or
macrocycle type are generally reflected in the B ⁄ Qy(0,0) peak
intensity ratio, and similarly in the integrated intensities of the
B and Q manifolds (origin plus vibronic features). These
absorption intensity ratios are listed in Tables 1 and 2.
In the following subsections, comparisons are made con-
cerning the effects of different substituents at various positions
of the chlorin and oxophorbine macrocycles. Note that the
spectral data presented in Table 1 are also given in Table 2,
which includes comparisons with the most suitable benchmark
for each set of compounds. Such data aid assessment of the
impact of peripheral substituents on the spectral characteristics
(kmax values and intensity ratios) of a given compound relative
to the benchmark. Table 3 is similarly designed to aid in
quantitation of the substituent effects on the photophysical
properties (Ff, sS, kf, knr).
RESULTS AND DISCUSSION
Overview
The following sections first address the effects of substituents
on the spectral and photophysical properties of the new
chlorins and oxophorbines, along with comparisons to the
properties of selected analogs studied previously as well as the
native photosynthetic pigments. The results are then related to
the effects on the electron densities and energies of the frontier
molecular orbitals (MOs). This comparison is followed by a
closer examination of the absorption profiles in terms of the
four-orbital model, wherein the positions and intensities of the
spectral features are related to linear combinations of simple
one-electron promotions between the MOs. The four-orbital
analysis provides simple guidelines as to how substituent
patterns generally affect spectral properties; such insights give
a general perspective of the spectral properties of the photo-
synthetic pigments and are useful from a molecular-design
perspective.
Spectral and photophysical properties
General observations. Electronic ground-state absorption spec-
tra of the new chlorins and oxophorbines (Chart 2), relevant
analogs studied previously (Chart 3), and native chromo-
phores (Fig. 1) are shown in Fig. 2. Each spectrum contains
four main features with maxima that range approximately as
follows: Qy(0,0) (598–670 nm), Qx(0,0) (520–560 nm) and
overlapping Bx(0,0) and By(0,0) (the Soret bands; 390–
440 nm). These bands reflect optical transitions from the
ground state S0 to excited states S1, S2, S3 and S4 respectively.
Weaker (1,0) features are often seen roughly 1300 cm)1 to
shorter wavelength than the origin bands. Note that the Soret
region also appears to contain contributions from additional
transitions, and that the relative wavelength ordering of the
Effects of meso-aryl groups. For synthetic utility, construction
of chlorins and oxophorbines studied here (Chart 2) and close
analogs studied previously (21) generally contained a 10-aryl
group [p-tolyl (T), mesityl (M), phenyl (P)] that may differ for the
various sets of compounds (e.g. sets ZnC-X7T10 and FbOP-
X7M10 with variable 7-substituent X; Chart 3). Aryl groups have
been similarly placed at the other (e.g. 5, 15) meso-positions
(21,22,34). A key result is that essentially the same properties
are found for a given macrocycle and pattern of b-pyrrole
substituents independent of the nature and location of a single
meso-aryl group (Tables 2 and 3; entries 2–6 and 26–29). For
example, ZnC-T10 reported here and analogs studied previously

656 Joseph W. Springer et al.
Table 1. Spectral and photophysical properties of synthetic and native chromophores.*
)1
Bmax abs
(nm)
Qy(0,0) abs
(nm)
Qy(0,0)
emis (nm)
sf
(ns)
kf)1
(ns)
knr
(ns)
Compound
Source†
IB ⁄ IQy
‡
SB ⁄ SQy
§
Ff
7-chlorins
ZnC-F7T10
ZnC-A7T10
ZnC-E7T10
ZnC-T10
1
1
1
1
429
426
421
405
598
598
605
605
601
601
608
608
7.0
6.5
5.7
4.2
10.5
10.1
6.3
0.029
0.030
0.039
0.057
1.1
1.0
1.3
1.6
38
33
33
28
1.1
1.0
1.4
1.7
3.9
7-oxophorbines
FbOP-F7M10
FbOP-A7M10
FbOP-E7M10
3,12,13-chlorins
ZnC-F3M10F13
ZnC-M10F13
ZnC-M10Es13
FbC-M10Es13
ZnC-A13
1
1
1
441
439
434
653
654
660
655
656
662
4.6
4.1
2.7
6.5
6.1
6.0
0.17
0.17
0.28
9.0
9.4
13.2
53
55
47
11
11
18
1
1
1
1
1
1
439
418
415
405
411
418
667
633
627
653
627
636
672
637
630
656
631
641
1.4
2.0
2.4
1.9
1.9
1.8
2.2
2.9
3.4
5.2
2.5
2.4
0.31
0.25
0.24
0.29
0.23
0.19
6.3
5.3
4.8
9.8
4.3
4.4
20
21
20
34
19
23
9.1
7.1
6.3
14
5.6
5.4
ZnC-A12
Prior chlorins
ZnC
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
2
2
2
2
2
398
404
405
405
403
412
412
418
423
436
389
416
420
420
428
396
403
402
394
418
602
605
606
605
607
620
625
632
646
662
633
643
645
648
655
637
636
636
638
654
604
608
608
608
609
624
628
635
649
668
635
646
648
652
659
639
638
638
640
656
3.4
4.1
4.3
4.2
3.6
3.4
2.2
2.2
1.6
1.5
2.4
1.3
1.5
1.3
1.2
2.7
2.8
2.9
2.7
1.6
4.1
4.7
4.4
4.2
4.1
3.6
3.0
3.1
2.5
2.2
5.2
2.2
2.2
2.2
2.0
7.2
7.8
8.1
6.8
5.0
0.057
0.060
0.060
0.060
0.069
0.075
0.18
0.24
0.23
0.32
0.20
0.23
0.18
0.24
0.22
0.22
0.20
0.22
0.17
0.30
1.5
1.5
1.5
1.6
1.6
1.8
3.0
5.1
4.1
6.0
9.4
3.7
3.1
4.6
4.1
10.0
9.7
10.0
9.4
9.8
26
25
25
27
23
24
17
21
18
19
47
16
17
19
19
45
49
45
55
33
1.6
1.6
1.6
1.7
1.7
1.9
3.7
6.7
5.3
8.8
12
4.8
3.8
6.1
5.3
13
ZnC-P10
ZnC-M10
ZnC-T5
ZnC-P15
ZnC-V3M10
ZnC-M10E13
ZnC-M10A13
ZnC-E3M10E13
ZnC-A3M10A13
FbC
ZnC-E3E13
ZnC-E3E12
ZnC-E3A13
ZnC-E3A12
FbC-M10
FbC-P10
12
13
11
14
FbC-T5
FbC-P15
FbC-M10E13
Prior oxophorbines
ZnOP
4
4
5
5
419
408
413
446–
637
654
656
683
639
656
659
686
1.6
1.5
1.7
1.3
2.4
4.1
4.4
3.5
0.23
0.30
0.32
0.26
5.1
11.5
13.0
10.9
22
38
41
42
6.6
16
19
15
FbOP
FbOP-M10
FbOP-A3M10
Native chromophores
Chl a
4
1
1
1
1
1
432
458
449
415
439
428
665
646
692
671
657
697
671
651
697
676
662
702
1.3
2.7
1.0
2.0
4.6
1.4
1.6
3.1
2.9
3.8
6.1
4.0
0.33
0.19
0.24
0.24
0.15
0.16
6.2
3.4
6.7
7.6
7.5
6.5
19
18
28
32
50
41
9.2
4.2
8.8
10
8.8
7.7
Chl b
Chl d
Pheo a
Pheo b
Pheo d
*All data at room temperature in toluene unless noted otherwise. †Source of the original data. (1) This study; (2) Ref 21; (3) Ref 22; (4) Ref 26; (5)
Ref 34. Some of the Ff and sS values from Ref 21 were changed (by <15%) to reflect averaging with additional repeat measurements (including
using a second technique for the sS measurements as noted in the Materials and Methods section). ‡Ratio of the peak intensities of the Qy(0,0) band
to the Soret (B) maximum, which could be either the By(0,0) or Bx(0,0) band. §Ratio of the integrated intensities of the Qy manifold [Qy(0,0),
Qy(1,0)] to the Soret manifold [By(0,0), By(1,0), Bx(0,0), Bx(1,0)], for spectra plotted in cm)1. –Two peaks with comparable intensity are observed at
427 and 446 nm.
bearing a 10-mesityl, 5-p-tolyl or 5-phenyl group have the same
Qy(0,0) position within 2 nm (e.g. ZnC-T10 and ZnC-M10 in
Figs. 2A,D) and Ff and ss values that differ by <10%. Thus,
the nature of the 10-aryl group is irrelevant in comparisons
given below concerning the effects of substituents at different
b-pyrrole positions, or comparisons of chlorins or oxophor-
bines, either zinc chelates or free base forms.
perturbations to the photophysical characteristics relative to
the parent chlorins (ZnC or FbC) or oxophorbines (ZnOP or
FbOP; Tables 2 and 3; entries 1–6, 25–29, 32, and 33). The
parent compounds lack any peripheral substituents other than
the geminal dimethyl group. The spectral manifestations for
the zinc chlorins include a small (typically 3–4 nm) batho-
chromic shift of Qy(0,0), a modest (ca 25%) decrease in Qy(0,0)
intensity relative to the Soret (e.g. ZnC-T10 versus ZnC;
Fig. 2A) and <10% effects on Ff and ss (Tables 2 and 3).
Furthermore, on an absolute basis, the addition of one
meso-aryl ring (for synthetic utility) causes only minor

Photochemistry and Photobiology, 2012, 88 657
Table 2. Comparisons of spectral properties of new compounds, analogues studied previously and native chromophores.*
Bmax
Qy(0,0)
k (nm) Dk (nm) Dm (cm)1
602
IB ⁄ IQy
SB ⁄ SQ
Entry Compound
Source† D reference‡ k (nm) Dk (nm) Dm (cm)1
)
)
Value D % Value D %
1
2
3
4
5
6
ZnC
ZnC-T10
2
1
2
2
2
2
1
1
1
2
1
2
2
1
1
2
2
3
3
3
3
1
1
4
2
2
2
2
2
2
1
4
5
1
1
1
5
1
1
1
1
1
1
398
405
405
404
405
403
429
426
421
412
418
418
412
415
439
436
423
416
420
420
428
411
418
419
389
396
403
402
394
418
405
408
413
441
439
434
446
432
458
449
415
439
428
3.4
4.2
4.3
4.1
4.2
3.6
7.0
6.5
5.7
3.4
2.0
2.2
2.2
2.4
1.4
1.5
1.6
1.3
1.5
1.3
1.2
1.9
1.8
1.6
2.4
2.7
2.8
2.9
2.7
1.6
1.9
1.5
1.7
4.6
4.1
2.7
1.3
1.3
2.7
1.0
2.0
4.6
1.4
4.1
3.9
4.4
4.7
4.2
4.1
ZnC
ZnC
ZnC
ZnC
7
7
6
7
5
24
21
16
7
13
13
7
10
21
18
11
)434
)434
)373
)434
)312
)1381
)1217
)938
)420
)768
)768
)420
)595
)1144
)988
)631
605
606
605
605
607
598
598
605
620
633
632
625
627
667
662
646
643
645
648
655
627
636
637
633
637
636
636
638
654
653
654
656
653
654
660
683
665
646
692
671
657
697
3
4
3
3
5
)7
)7
0
14
27
26
19
21
34
30
21
)82
)110
)82
24
26
21
24
6
67
55
36
)5
7
15
2
ZnC-M10
ZnC-P10
ZnC-T5
)82
ZnC-P15
ZnC
)137
193
193
0
7
8
9
ZnC-F7T10
ZnC-A7T10
ZnC-E7T10
ZnC-V3M10
ZnC-M10F13
ZnC-M10A13
ZnC-M10E13
ZnC-M10Es13
ZnC-F3M10F13
ZnC-A3M10A13
ZnC-E3M10E13
ZnC-E3E13
ZnC-E3E12
ZnC-E3A13
ZnC-E3A12
ZnC-A13
ZnC-T10
ZnC-T10
ZnC-T10
ZnC-M10
ZnC-M10
ZnC-M10
ZnC-M10
ZnC-M10
ZnC-M10F13
ZnC-M10A13
ZnC-M10E13
11
10
6.3
169
159
62
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
)373
)704
)679
)502
)553
)805
)717
)520
)21
)53
)49
)49
)44
)30
)32
)27
3.6
2.9
3.1
3.0
3.4
2.2
2.2
2.5
2.2
2.2
2.2
2.0
2.5
2.4
2.4
5.2
7.2
7.8
8.1
6.8
5.0
5.2
4.1
4.4
6.5
6.1
6.0
3.5
1.6
3.1
2.9
3.8
6.1
4.0
)18
)34
)30
)32
)23
)24
)29
)17
ZnC-E3E13
4
)229
2
)48
15
0
ZnC-E3A13
ZnC
8
13
7
)445
)795
)407
7
25
9
)165
)662
)226
)913
)8
)44
)5
)9
)39
)4
ZnC-A12
ZnC-A13
ZnC
ZnOP
FbC
21
)1259
35
)53
)41
FbC-M10
FbC-P10
FbC
FbC
FbC
7
14
13
5
22
9
)454
)893
)831
)326
)1329
)561
)1197
)297
)1537
)1434
)1172
)1792
)718
4
3
3
)99
)75
)75
)124
)408
)385
)507
)47
70
47
)92
)603
)661
442
)587
)387
318
13
17
21
13
38
50
56
31
FbC-T5
FbC-P15
FbC
5
FbC-M10E13
FbC-M10Es13
FbOP
FbC-M10
FbC-M10
FbC
17
16
21
2
)3
)2
4
27
28
)19
27
17
)14
26
)41
)30
)38
13
171
141
59
)24
)19
108
)23
33
130
)30
)31
)28
)21
7
19
5
FbOP-M10
FbOP-F7M10
FbOP-A7M10
FbOP-E7M10
FbOP-A3M10
Chl a
FbOP
FbOP-M10
FbOP-M10
FbOP-M10
FbOP-M10
ZnOP
28
26
21
33
13
26
17
7
48
39
36
)20
)33
94
Chl b
Chl d
Pheo a
Pheo b
Pheo d
Chl a
Chl a
FbOP
Pheo a
Pheo a
)1314
)876
)413
)1317
)732
81
)7
61
24
13
)556
5
*All data at room temperature in toluene unless noted otherwise. †Source of the original data. (1) This study; (2) Ref 21; (3) Ref 22; (4) Ref 26; (5)
Ref 34. ‡The reference compound used for calculation of the D values for each of the spectral and photophysical parameters.
Collectively, these results show that the impacts of a single
meso-aryl group are sufficiently small that they can be
effectively ignored in making comparisons of synthetic
Table 2 entry 33). Subsequent addition of the 7-formyl group to
produce FbOP-F7M10 gives an opposing (hypsochromic) shift in
Qy(0,0) (656 to 653 nm; )3 nm; 70 cm)1) and a substantial
(171%) increase in the B ⁄ Qy peak intensity ratio, namely a
relative decrease in the Qy(0,0) intensity (Table 2 entry 34).
Turning to other photophysical properties, the Ff and sS values
for FbOP-M10 (0.32, 13.0 ns) are slightly larger than those for
FbOP (0.30, 11.5 ns) and then decrease substantially upon
addition of the 7-formyl group to give FbOP-F7M10 (0.17,
9.0 ns; Table 3 entry 34). The latter diminution in Ff and sS can
be traced in part to the diminution in kf, which in turn reflects the
hypochromic effect on the Qy(0,0) absorption due to incorpo-
ration of the 7-formyl group (Table 2).
chlorophyll analogs (e.g. bearing
a 10-substituent) with
the native photosynthetic pigments, which generally lack
meso-substituents.
Impact of a 7-formyl group in free base oxophorbine. The
absorption spectra in Figs. 2B,C and the data listed in Table 2
show that incorporation of a 7-formyl group in a synthetic
oxophorbine has significant spectral manifestations. Quantita-
tion of the effects with the synthetic compounds entails a two-
step process. First, in keeping with the above-noted points, the
addition of a 10-mesityl group to the parent FbOP to form
FbOP-M10 has relatively small effects, which include a 2 nm
()47 cm)1) bathochromic shift (654 to 656 nm) of Qy(0,0) and a
13% increase of the B ⁄ Qy peak intensity ratio (Fig. 2B and
Figure 2C also shows that the spectrum of synthetic
oxophorbine FbOP-F7M10 is remarkably close to that of the
native chromophore Pheo b, except that the latter has slightly
longer wavelength Soret and Qy(0,0) absorption maxima. The

658 Joseph W. Springer et al.
Table 3. Comparisons of photophysical properties of new compounds, analogues studied previously and native chromophores.*
)1
Ff
sf
kf)1
Value (ns)
knr
Entry
Compound
Source†
D reference‡
Value
D %
Value (ns)
D %
D %
Value (ns)
D %
1
2
3
4
5
6
7
8
ZnC
ZnC-T10
2
1
2
2
2
2
1
1
1
2
1
2
2
1
1
2
2
3
3
3
3
1
3
4
2
2
2
2
2
2
1
4
5
1
1
1
5
1
1
1
1
1
1
0.057
0.057
0.060
0.060
0.060
0.069
0.029
0.030
0.039
0.075
0.25
0.24
0.18
0.24
0.31
0.32
0.23
0.23
0.18
0.24
0.22
0.23
0.19
0.23
0.20
0.22
0.20
0.22
0.17
0.30
0.29
0.30
0.32
0.17
0.17
0.28
0.26
0.33
0.19
0.24
0.24
0.15
0.16
1.5
1.6
1.5
1.5
1.6
1.6
1.1
1.0
1.3
1.8
5.3
5.1
3.0
4.8
6.3
6.0
4.1
3.7
3.1
4.6
4.1
4.3
4.4
5.1
9.4
10
26
28
25
25
27
23
38
33
33
24
21
21
17
20
20
19
18
16
17
19
19
19
23
22
47
45
49
45
55
33
34
38
41
53
55
47
42
19
18
28
32
50
41
1.6
1.7
1.6
1.6
1.7
1.7
1.1
1.0
1.4
1.9
7.1
6.7
3.7
6.3
9.1
8.8
5.3
4.8
3.8
6.1
5.3
5.6
5.4
6.6
12
13
12
13
11
14
14
16
19
11
11
18
15
9.2
4.2
8.8
10
8.8
7.7
ZnC
ZnC
ZnC
ZnC
0
5
5
5
21
)49
)47
)32
25
317
300
200
300
24
7
0
0
7
7
)31
)38
)19
20
253
240
100
220
19
8
)4
6
0
0
6
6
)35
)41
)18
19
344
319
131
294
28
ZnC-M10
ZnC-P10
)4
ZnC-T5
4
)12
36
18
18
)4
)16
)16
)32
)20
)5
ZnC-P15
ZnC
ZnC-F7T10
ZnC-A7T10
ZnC-E7T10
ZnC-V3M10
ZnC-M10F13
ZnC-M10A13
ZnC-M10E13
ZnC-M10Es13
ZnC-F3M10F13
ZnC-A3M10A13
ZnC-E3M10E13
ZnC-E3E13
ZnC-E3E12
ZnC-E3A13
ZnC-E3A12
ZnC-A13
ZnC-T10
ZnC-T10
ZnC-T10
ZnC-M10
ZnC-M10
ZnC-M10
ZnC-M10
ZnC-M10
ZnC-M10F13
ZnC-M10A13
ZnC-M10E13
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
33
28
18
37
)10
6
31
43
ZnC-E3E13
)22
)16
6
)21
ZnC-E3A13
ZnC
)8
304
)17
304
)11
187
2
0
)29
24
)13
251
)4
ZnC-A12
ZnC-A13
ZnC
ZnOP
FbC
240
)15
313
FbC-M10
FbC-P10
FbC
FbC
FbC
10
0
10
)15
36
32
50
6
3
6
)4
4
)4
17
)27
)24
)19
8
29
34
15
2
)14
)6
46
)17
58
28
8
0
8
)8
8
8
33
9.7
10
9.4
9.8
9.8
12
FbC-T5
FbC-P15
FbC
0
FbC-M10E13
FbC-M10Es13
FbOP
FbC-M10
FbC-M10
FbC
)2
)2
22
FbOP-M10
FbOP-F7M10
FbOP-A7M10
FbOP-E7M10
FbOP-A3M10
Chl a
FbOP
7
13
13
19
FbOP-M10
FbOP-M10
FbOP-M10
FbOP-M10
ZnOP
)47
)47
)13
)19
41
)42
)26
)20
)85
)33
9.0
9.4
13
)31
)28
2
)16
22
)45
8
)34
)1
)14
)42
)42
)5
)21
39
)54
)4
)39
)12
)23
11
6.2
3.4
6.7
7.6
7.5
6.5
Chl b
Chl d
Pheo a
Pheo b
Pheo d
Chl a
Chl a
FbOP
Pheo a
Pheo a
*All data at room temperature in toluene unless noted otherwise. †Source of the original data. (1) This study; (2) Ref 21; (3) Ref 22; (4) Ref 26; (5)
Ref 34. Some of the Ff and sS values from Ref 21 were changed (by <15%) to reflect averaging with additional repeat measurements (including
using a second technique for the sS measurements as noted in the Materials and Methods section). ‡The reference compound used for calculation of
the D values for each of the spectral and photophysical parameters.
photophysical properties of FbOP-F7M10 (Ff = 0.17 and
sS = 9.0 ns) are also reasonably close to those of Pheo b
(Ff = 0.15 and sS = 7.5 ns). The 10-mesityl group of FbOP-
F7M10 has little impact on these comparisons (Fig. 2B and
Tables 2 and 3). The results indicate that starting with the parent
chlorin (FbC), incorporation of the keto-containing 5-mem-
bered ring (ring E; Chart 2) followed by the 7-formyl group gives
a close spectral representation of the native chromophore (26).
On the basis of prior results (21) for ZnC-V3M10 versus ZnC-M10
(Fig. 2F), the addition of a 3-vinyl group to FbOP-F7M10 would
be expected to give an almost perfect spectral match with Pheo b.
Similarly, the data in Fig. 2F (and Table 2) suggest (26) that the
combination of the keto-bearing 5-membered ring and a 3-vinyl
group alone in the corresponding metal chelate should give a
close spectral match with Chl a, which lacks the 7-formyl group.
Nature of the 7-substituent. The spectral and photophysical
properties engendered by different 7-substituents were exam-
ined for a set of free base oxophorbines FbOP-X7M10 and a set
of zinc chlorins ZnC-X7T10, where the substituent X is formyl
(F), acetyl (A) or ethynyl (E). Consider the spectral effects for
the two sets (each referenced to the relevant 7-unsubstituted
benchmark) given by entries 7–9 and 34–36, respectively, in
Table 2. The bathochromic shift in the Soret band (Bmax
)
increases in the order ethynyl < acetyl < formyl, with acetyl
closer to formyl than ethynyl for both sets. The shifts are 16, 21
and 24 nm ()938, )1217 and )1381 cm)1) for the ZnC-X7T10
series and 21, 26 and 28 nm ()1172, )1434 and )1537 cm)1
)
for the FbOP-X7M10 series. Turning to the Qy(0,0) band, for
ZnC-X7T10 ethynyl gives no shift, whereas acetyl and formyl
both give
a ) hypsochromic shift; for
7 nm (193 cm)1

Photochemistry and Photobiology, 2012, 88 659
Figure 2. Absorption spectra of selected chlorins and oxophorbines.
Effect of a 13- versus 7-formyl substituent. A 13-formyl group
has dramatically different effects than a 7-formyl group. This
point can be seen from the data for the addition of a 13-formyl
group to ZnC-M10 to give ZnC-M10F13 versus those described
elsewhere in the text for the addition of a 7-formyl group to
ZnC-T10 to give ZnC-F7T10 (entries 11 versus 7 in Tables 2 and
3). Spectral distinctions are shown in Fig. 2D versus Fig. 2A.
The 13- versus 7-formyl group gives a smaller bathochromic
shift in Bmax (13 nm [)768 cm)1] versus 24 nm [)1381 cm)1]),
a bathochromic shift in Qy(0,0) (27 nm [)704 cm)1]) versus a
hypsochromic shift ()7 nm [193 cm)1]), a decrease (53%)
versus an increase (67%) in the B ⁄ Qy peak intensity ratio, and
a corresponding decrease (34%) versus an increase (169%) in
the B ⁄ Qy integrated intensity ratio. Stated differently, a
13-formyl group gives an increase in the relative Qy(0,0)
intensity along with a shift of the band to longer wavelength,
whereas the 7-formyl group causes just the opposite effects on
both Qy(0,0) intensity and wavelength.
Figure 3. Fluorescence spectra of selected chlorins and oxophorbines.
The differences extend to the photophysical characteristics.
While a 13-formyl group increases Ff from 0.060 to 0.25 and sS
from 1.5 to 5.3 ns, the opposite derives from a 7-formyl group,
namely decreases in Ff from 0.057 to 0.029 and sS from 1.6 to
1.1 ns (entries 11 and 7 of Table 3). The differential effects of
13- versus 7-formyl groups result in part from a 16% increase
versus a 36% decrease in kf, consistent (via the relationships of
the Einstein coefficients [42]) with the above-noted effects on
the relative Qy(0,0) absorption intensities. A 13- versus 7-formyl
group gives rise to a decrease versus an increase in the overall
nonradiative decay rate constant knr = (kic + kisc) of the Qy
(i.e. S1) excited state (Table 3). The energy gap law for
nonradiative decay (42) would suggest that the S1 fi S0
FbOP-X7M10 ethynyl gives a 4 nm ()92 cm)1) bathochromic
shift, whereas acetyl and formyl give hypsochromic shifts of
2 nm (47 cm)1) and 3 nm (70 cm)1). For both sets, the peak
intensity ratio of the Soret and Qy(0,0) bands (IB ⁄ IQ in
y
Table 2) increases along the series ethynyl < acetyl < formyl.
The percentage increases from the 7-unsubstituted benchmark
are 36%, 55% and 67% for the ZnC-X7T10 series and 59%,
141% and 171% for the FbOP-X7M10 series. Turning to the
photophysical properties, with a few exceptions the Ff and sS
are reduced 20–50% from the values for the benchmark
lacking a 7-substituent.

660 Joseph W. Springer et al.
internal conversion should show the opposite effect because of
the lower excited-state energy with 13- versus 7-formyl. Thus,
the differential effect on knr is likely associated mainly with
S1 fi T1 intersystem crossing, and in particular how a formyl
group at the two positions leads to a difference in the
electron-density distribution in the molecule. The latter point
is addressed below in the context of MO calculations.
Collectively, these observations underscore the broad, pro-
foundly different consequences of a formyl group at the 13-
versus 7-positions of the chlorin macrocycle. The same is
true, but with smaller impacts, for other auxochromes at the
two positions. The latter point is illustrated by comparisons
given in the next subsection.
impact than acetyl, which in turn has a more substantial
impact than ethynyl.
2. For each group, addition of the 3-substituent to give a 3,13-
substituted complex generally has a greater impact on
spectral properties than for the 13-substituent. For exam-
ple, comparing ZnC-F3M10F13 (referenced to ZnC-M10F13)
versus ZnC-M10F13 (referenced to ZnC-M10), the former
has
a
larger bathochromic shift in Bmax (21 nm
[)1144 cm)1] versus 13 nm [)768 cm)1]) and a larger
bathochromic shift in Qy(0,0) (34 nm [)805 cm)1] versus
27 nm [)704 cm)1]). Analogous results are found for the
acetyl- and ethynyl-substituted chlorins (Table 2).
3. Comparison of the above results with those obtained
previously (20,21) for ZnC-V3M10 versus ZnC-M10 (Table 2
entry 10) indicates that a 3-formyl (or 13-formyl) group
gives larger bathochromic shifts in the Soret and Qy bands
than a 3-vinyl group.
4. The spectral consequences of a 3-substituent parallel those
for a 13-substituent (bathochromic shifts in Soret and Qy
and decrease in the B ⁄ Qy intensity ratio) in being pro-
foundly different than for a 7-substituent (bathochromic
shift in Soret, hypsochromic shift in Qy and increase in the
B ⁄ Qy intensity ratio). The comparative properties of
synthetic chlorins (and oxophorbines) thus mimic the
similar functionally significant differences in spectral prop-
erties of Chl d versus a (formyl versus vinyl at the 3-position)
and Chl b versus a (formyl versus methyl at the 7-position),
as shown in Fig. 1.
Formyl versus other groups at the 13-position. With zinc
chlorin ZnC-M10F13 in hand, trends in properties with 13-
substituents analogous to those given above for the 7-position
can be made with the same groups (formyl, acetyl and
ethynyl). Such comparisons make use of compounds ZnC-
M10A13 and ZnC-M10E13 studied previously (20,21) and can be
extended to include the 13-ester analog ZnC-M10Es13 charac-
terized here and oxophorbine ZnOP studied previously (26),
which contains the 13-keto group integral to ring E (Chart 1).
The properties of these compounds (each with respect to the
appropriate reference) are given under entries 11–14 and 24 of
Tables 2 and 3. The tables also list data for several free base
analogs, including FbC-M10Es13 characterized here (entries
30–31). The main findings are as follows:
1. Acetyl or ethynyl at the 13-position results in substantial
differences from the same group at the 7-position, analo-
gous to the above-noted differences for formyl.
2. In analogy with the results for the 7-position, the impact of
a 13-substituent on spectral properties generally follows the
trend: ethynyl < acetyl < formyl, with the effect of acetyl
being closer to formyl than ethynyl.
5. It was noted in the text above concerning the spectra in
Fig. 2F (and the data in Table 2) that the addition of a
keto-bearing 5-membered ring to a zinc chlorin to form the
corresponding 131-oxophorbine (ZnOP) plus the addition
of a 3-vinyl group will be expected to give a close spectral
match with Chl a, which bears both of these structural
elements (in addition to many others; Fig. 1). Comparison
of Fig. 2D–F also reveals that the Qy(0,0) bands of ZnC-
F3M10F13 and ZnC-A3M10A13 lie at wavelengths 2–3 nm
longer and shorter, respectively, than that of Chl a, and the
Soret bands are only 4–8 nm longer. Thus, these two
synthetic chlorins also give a good spectral representation
of Chl a, and have comparable photophysical properties
(sS = 6 ns, Ff = 0.3) as well. The spectral match makes
sense considering the relative effects of the key substitutions
along the Qy axis (rings A and C; Chart 1), namely keto-
bearing 5-membered ring > formyl > acetyl > vinyl.
Thus, for applications in which a spectrum and photophys-
ical characteristics similar to those of Chl a are required, the
3,13-disubstituted zinc chlorins provide relatively simple
alternatives.
3. Formyl and acetyl groups at the 13-position have compa-
rable effects on Ff and sS.
4. A 13-ester group has somewhat smaller effects on spectral
and photophysical properties than a 13-acetyl group.
5. Starting with the parent chlorin, addition of the keto-
containing 5-membered ring to give the oxophorbine
engenders spectral changes that are slightly larger than
13-formyl, and changes in excited-state properties (Ff and
sS) that are slightly smaller and closer to the effects of 13-
acetyl (entries 11, 12 and 24 in Tables 2 and 3).
Formyl versus other substituents at the 3-position. Assessment
of the properties of ZnC-F3M10F13 versus ZnC-M10F13 (entries
15 and 11 of Tables 2 and 3) gives insight into the effect of a
3-formyl group, just as the above comparison of the latter
compound (ZnC-M10F13) versus ZnC-M10 reveals the effect of
a 13-formyl group. The spectral evolution for this trio of
chlorins is shown in Fig. 2D. The collective data allow
comparisons with analogous sets studied previously (21),
namely ZnC-A3M10A13 versus ZnC-M10A13 versus ZnC-M10
for acetyl and ZnC-E3M10E13 versus ZnC-M10E13 versus
ZnC-M10 for ethynyl (entries 16 ⁄ 12 and 17 ⁄ 13 in Tables 2
and 3). The main results for addition of a 3-substituent to give
a 3,13-disubstituted zinc chlorin are as follows:
Photophysical properties of the native chromophores. The
photophysical properties of the native chromophores have been
investigated previously in a variety of solvents, with Chl a being
the most widely studied. The ranges of values and average for sS
(in ns) at room temperature (unless noted otherwise) are as
follows: Chl a (4.6–7.8; average 5.9; 43–56); Chl b (3.2–6.3;
average 4.0; 43–45,48,49,54–59); Chl d (6.2; 55); Pheo a (5.3–8.1;
average 6.5; 48,50,56,58,60); Pheo b (7.3 at 77 K, 7.7 at 4.2 K;
56). The ranges of values and average for Ff at room temper-
ature are as follows: Chl
a
(0.07–0.50; average 0.30;
1. Paralleling the results described above for the 7- and 13-
positions, formyl at the 3-position has a somewhat greater
36,38,44,45,48,49,61–68); Chl
b
(0.06–0.17; average 0.11;

Photochemistry and Photobiology, 2012, 88 661
36,44,45,49,54,57,59,61,62,64,65,67,68); Pheo
a
(0.11–0.29;
give ZnC-F7T10 (Table 3). Again, one consideration here is the
effect of the formyl group on the electron-density distribution
in the molecule and thus on the intersystem crossing rate.
For Pheo b versus a versus d, the sS value is 7.5 ns versus
7.6 ns versus 6.5 ns and the Ff value is 0.15 versus 0.24 versus
0.16 (entries 41–43 of Table 3). These measured quantities
in turn reflect radiative rate constant kf of (50 ns))1 versus
(32 ns))1 versus (41 ns))1 and net nonradiative decay rate
constant knr of (8.8 ns))1 versus (10 ns))1 and (7.7 ns))1. Thus,
the excited-state lifetimes are comparable for all three native
free base chromophores and only modestly longer than the
Mg-containing chlorophyll analogs (except for Chl a). The
radiative rate constants are all diminished compared to the
chlorophyll analogs, consistent with the reduced Qy(0,0)
absorption intensity (relative to the Soret) of the pheophytins
compared to the chlorophylls (Fig. 1).
For a direct comparison between native and synthetic
analogs it was noted above that the absorption spectrum of
Pheo b is mimicked well by that of oxophorbine FbOP-F7M10
except that the former has slightly longer wavelength absorp-
tion maxima (Fig. 2C). The photophysical properties of Pheo b
(Ff = 0.15 and sS = 7.5 ns) are also mimicked reasonably
well by those of FbOP-F7M10 (0.17, 9.0 ns). The 10-mesityl
group in the synthetic compound has little impact on photo-
physical properties (Fig. 2B and Tables 2 and 3).
average 0.20; 36,61,64,65,67–69); Pheo b (0.07–0.14; average
0.10; 64,65,67,68). Herein, we have studied all six native
chromophores in the same solvent (toluene) that we have used
previously (20–22,26) with the synthetic analogs to facilitate
comparisons. The sS and Ff values that we have found here for
the chromophores in toluene fall within the range of values
observed previously and are in good agreement with the values
in the same or similar (e.g. benzene) media, where comparisons
can be made. Specific comparisons are as follows.
A very important parameter for the function of native
photosynthetic chromophores is the singlet excited-state life-
time (sS) because the inverse of this quantity is the total
intrinsic decay rate with which energy or charge transfer
competes in the native antenna or reaction-center complexes.
Related and of interest is the fluorescence yield (Ff), which is
given by the product of sS and the radiative decay rate
constant kf. The latter can be related to the strength of the Qy
absorption manifold.
For Chl b versus a versus d, the sS value is 3.4 ns versus
6.2 ns versus 6.7 ns and the Ff value is 0.19 versus 0.33 versus
0.24 (entries 38–40 of Table 3). These measured quantities
in turn reflect a radiative rate constant kf of (18 ns))1 versus
(19 ns))1 versus (28 ns))1 and net (internal conversion plus
intersystem crossing) nonradiative decay rate constant knr of
(4.2 ns))1 versus (9.2 ns))1 versus (8.8 ns))1. The most signif-
icant change is that Chl b has a roughly two-fold increased knr
versus Chl a, which results in the two-fold reduction in excited-
state lifetime. The enhanced nonradiative decay is likely
associated with enhanced intersystem crossing. This is so
because the higher energy Qy state of Chl b versus a would
be expected to decrease the contribution from internal
conversion. A similar effect was described elsewhere in the
text for the incorporation of a 7-formyl group in ZnC-T10 to
Molecular orbital characteristics
Figure 4 gives the electron-density distributions and energies of
the frontier MOs of representative chlorins and 131-oxophor-
bines. The MOs are the highest occupied (HOMO), the lowest
unoccupied (LUMO), the HOMO-1 and the LUMO+1. These
four orbitals are the most relevant to the optical and photo-
physical characteristics of the molecules. Table 4 lists the MO
Figure 4. Electron densities and energies (eV) of frontier MOs of representative chlorins and oxophorbines.

662 Joseph W. Springer et al.
energies for all new chlorins and oxophorbines studied here,
selected analogs studied previously, and native chromophores
to aid in the assessment of the effects of individual substituents.
In this section, the effects of the nature and position of
substituents on the individual frontier MOs are described.
Connections to the spectral properties are analyzed in the next
section. Key findings are as follows:
1. A single meso-aryl group has only small effects on the
frontier MOs for both the zinc chelates and free base forms
(entries 2–6 and 26–29 in Table 4). The effects are generally
destabilizing (energy shifts to less negative values relative to
ZnC and FbC, respectively). The energy shifts are £0.04 eV
for the LUMO+1, LUMO and HOMO. The shifts are
somewhat larger (up to 0.09 eV) for the HOMO-1, which places
considerable electron density at the meso-positions (Fig. 4).
2. Addition of a 7-substituent to a zinc chlorin (ZnC-X7T10
set; entries 7–9 in Table 4) or a free base oxophorbine
(FbOP-X7M10 set; entries 34–36) shifts all four MOs to
more negative energies, with the degree of stabilization in
the order formyl > acetyl > ethynyl.
3. For both sets of complexes noted in point (2), the energy
shift upon 7-substitution is significantly larger (by roughly
two- to three-fold) for the LUMO+1 than the other three
MOs. The magnitudes of energy shifts for all three
substituents (formyl, acetyl and ethynyl) for the ZnC-
X7T10 series (exemplified by values for ZnC-F7T10 versus
ZnC-T10) decrease in the order LUMO+1 ()0.50 eV) >
HOMO ()0.26 eV) > HOMO-1 ()0.19 eV) ꢀ LUMO
()0.18 eV). The magnitudes of the energy shifts for all
three substituents for the FbOP-X7M10 series (exemplified
by values for FbOP-F7M10 versus FbOP-M10) decrease in
the order LUMO+1 ()0.48 eV) > HOMO-1 ()0.21 eV) >
HOMO ()0.18 eV) > LUMO ()0.14 eV).
4. Addition of a 13-substituent to a zinc chlorin (ZnC-M10X13
set; entries 11–14 in Table 4) shifts all four frontier MOs to
more negative energies, with the degree of stabilization in
the order formyl > acetyl > ester > ethynyl.
5. The orbital stabilization for each 13-substituent in the
ZnC-M10X13 series noted in point (4) follows the
order LUMO > HOMO > HOMO-1 ꢀ LUMO+1. For
Table 4. Molecular orbital energies and energy gaps for chlorins and oxophorbines.*
Entry Compound
Code† HOMO-1 HOMO LUMO LUMO+1 LUMO – HOMO LUMO+1 – HOMO-1 DEavg DEdiff
Zinc chelates
ZnC
1
2
3
4
5
6
7
8
a
X
r
)5.16
)5.07
)5.11
)5.12
)5.08
)5.13
)5.26
)5.26
)5.15
)5.13
)5.30
)5.23
)5.19
)5.20
)5.48
)5.33
)5.26
)5.32
)5.31
)5.36
)5.36
)5.28
)5.29
)5.34
)4.79
)4.75
)4.78
)4.77
)4.75
)4.78
)5.01
)4.99
)4.83
)4.79
)5.02
)4.95
)4.85
)4.92
)5.23
)5.09
)4.91
)4.94
)4.94
)5.04
)5.02
)4.97
)4.95
)5.12
)2.12
)2.10
)2.12
)2.11
)2.10
)2.14
)2.28
)2.30
)2.18
)2.22
)2.49
)2.42
)2.27
)2.36
)2.85
)2.69
)2.42
)2.43
)2.43
)2.57
)2.58
)2.44
)2.45
)2.57
)1.55
)1.53
)1.56
)1.55
)1.53
)1.55
)2.03
)1.96
)1.74
)1.61
)1.75
)1.69
)1.64
)1.65
)2.04
)1.90
)1.75
)1.75
)1.79
)1.81
)1.88
)1.69
)1.74
)1.83
2.67
2.65
2.66
2.66
2.65
2.64
2.73
2.69
2.65
2.57
2.53
2.53
2.58
2.56
2.38
2.40
2.49
2.51
2.51
2.47
2.44
2.53
2.50
2.55
3.61
3.54
3.55
3.57
3.55
3.58
3.23
3.30
3.41
3.52
3.55
3.54
3.55
3.55
3.44
3.43
3.51
3.57
3.52
3.55
3.48
3.60
3.54
3.51
3.14
3.10
3.11
3.12
3.10
3.11
2.98
3.00
3.03
3.05
3.04
3.04
3.07
3.06
2.91
2.92
3.00
3.04
3.02
3.01
2.96
3.07
3.02
3.03
0.94
0.89
0.89
0.91
0.90
0.94
0.50
0.61
0.76
0.95
1.02
1.01
0.97
0.99
1.06
1.03
1.02
1.06
1.01
1.08
1.04
1.07
1.04
0.96
ZnC-T10
ZnC-M10
ZnC-P10
b
ZnC-T5
ZnC-P15
ZnC-F7T10
ZnC-A7T10
ZnC-E7T10
ZnC-V3M10
ZnC-M10F13
ZnC-M10A13
ZnC-M10E13
ZnC-M10Es13
ZnC-F3M10F13
ZnC-A3M10A13
ZnC-E3M10E13
ZnC-E3E13
ZnC-E3E12
ZnC-E3A13
ZnC-E3A12
ZnC-A13
F
A
E
c
d
e
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
f
g
h
i
j
p
o
n
m
k
l
ZnC-A12
ZnOP
q
Free bases
FbC
25
26
27
28
29
30
31
32
33
34
35
36
37
)5.07
)5.02
)5.01
)4.98
)5.04
)5.10
)5.11
)5.26
)5.19
)5.40
)5.32
)5.27
)5.30
)4.89
)4.87
)4.87
)4.85
)4.87
)4.93
)5.01
)5.20
)5.17
)5.35
)5.28
)5.22
)5.29
)2.20
)2.18
)2.19
)2.17
)2.20
)2.33
)2.42
)2.62
)2.58
)2.72
)2.66
)2.64
)2.84
)1.67
)1.68
)1.68
)1.65
)1.66
)1.75
)1.77
)1.93
)1.91
)2.39
)2.29
)2.10
)2.13
2.69
2.69
2.68
2.68
2.67
2.60
2.59
2.58
2.59
2.63
2.62
2.58
2.45
3.40
3.34
3.33
3.33
3.38
3.35
3.34
3.33
3.28
3.01
3.03
3.17
3.17
3.05
3.02
3.01
3.01
3.03
2.98
2.97
2.96
2.94
2.82
2.83
2.88
2.81
0.71
0.65
0.65
0.65
0.71
0.75
0.75
0.75
0.69
0.38
0.41
0.59
0.72
FbC-M10
FbC-P10
FbC-T5
FbC-P15
FbC-M10E13
FbC-M10Es13
FbOP
FbOP-M10
FbOP-F7M10
FbOP-A7M10
FbOP-E7M10
FbOP-A3M10
OX
OF
OA
OE
*Obtained from DFT calculations. All values are in eV. †Compound designation code for data points in Figs. 5–7.

Photochemistry and Photobiology, 2012, 88 663
example, the values (in eV) are )0.37, )0.24, )0.19 and
)0.19 for ZnC-M10F13 and )0.30, )0.17, )0.12 and )0.13
for ZnC-M10A13 (both versus ZnC-M10).
in energy (DEdif) of the [LUMO – HOMO] and [LUMO+1–
HOMO-1] energy gaps given in Eq. (1).
DEavg ¼ ½DE(LUMOþ1ꢁ HOMO-1Þ
ð1aÞ
6. Addition of a 3-substituent (formyl, acetyl and ethynyl) to a
13-substituted zinc chlorin (ZnC-X3M10X13 series; entries
15–17 in Table 4) shifts all four MOs to more negative
energies, with the degree of stabilization in the order
formyl > acetyl > ethynyl.
þ DE(LUMOꢁ HOMOÞꢂ=2
DEdif ¼ ½DE(LUMO+1ꢁ HOMO-1Þ
ð1bÞ
ꢁ DE(LUMO ꢁ HOMOÞꢂ
7. The orbital stabilization (energy shifts) for each 3-substituent
in the ZnC-X3M10X13 series noted in point (6) follows the
order LUMO > LUMO+1 > HOMO > HOMO-1. For
example, the values (in eV) are )0.36, )0.29, )0.21 and )0.18
for ZnC-F3M10F13 (versus ZnC-M10F13) and )0.27, )0.21,
)0.14 and )0.10 for ZnC-A3M10A13 (versus ZnC-M10A13).
8. The above-noted calculated orbital energy shifts for the
addition of a 3-formyl group to ZnC-M10F13 (to form ZnC-
F3M10F13) parallel the following values for addition of a
3-formyl group to ZnC-M10 (to form the fictive chlorin
ZnC-F3M10, which has MO energies of )5.31, )5.01, )2.54
and )1.38 eV): LUMO ()0.42), LUMO+1 ()0.27), HOMO
()0.23), HOMO-1 ()0.20). For each orbital, the shift is
0.18–0.32 eV greater than those for the analogous addition
of a 3-vinyl group to ZnC-M10 (to form the chlorin ZnC-
V3M10 studied previously [21]) of )0.10, )0.05, )0.01 and
)0.02 eV, respectively (Table 4). These comparisons
exemplify the more substantial stabilization of each fron-
tier MO by 3-formyl versus 3-vinyl. This point will be
explored further below in relation to the observed spectral
effects.
9. The trends in LUMO energies reflect trends in (but not
absolute values of) the reduction potentials, and similarly
for the HOMO energies and oxidation potentials, as we
have found for a large series of zinc chlorins (21). In
particular, the more negative LUMO and HOMO energies
(Table 4) for ZnC-F7T10 ()2.28 and )5.01 eV) versus
ZnC-T10 ()2.10 and )4.75 eV) indicate that the addition
of a 7-formyl group makes the chlorin easier to reduce and
harder to oxidize. A similar conclusion is reached for the
free base oxophorbines FbOP-F7M10 ()2.72 and )5.35 eV)
versus FbOP-M10 ()2.58 and )5.17 eV).
Parameter DEavg places the energy center of gravity (COG)
of the By(0,0) and Qy(0,0) bands. The Qy(0,0) band lies lower
and the By(0,0) band higher than the average energy by one-
half the magnitude of a splitting parameter (ry) that in turn
depends on DEdif and the configuration-interaction energy (d)
as given by Eqs. (2) and (3).
EB ¼ DEavg þ ry=2
ð2aÞ
ð2bÞ
ð2cÞ
y
EQ ¼ DEavg ꢁ ry=2
y
ry ¼ DEdif=½2sinð2gyÞꢂ
gy ¼ ½tanꢁ1ðDEdif=2dÞꢂ=2
ð3Þ
The configuration-interaction energy derives from the
electron-density distribution in the molecule (and thus elec-
tron-electron repulsion) and should be relatively constant for a
series of related molecules. In addition to the By–Qy splitting
(ry; Eq. [2c]), the relative intensities of the By and Qy bands is
driven by DEdif via Eq. (4), using gy in Eq. (3).
RQ =RB ¼ ½1 ꢁ cosð2gyÞꢂ=½1 þ cosð2gyÞꢂ
ð4Þ
y
y
The intensity relationship follows because the constructive
(By) versus destructive (Qy) interference of the (nominally
equal) transition-dipole moments associated with the
HOMO-1 fi LUMO+1 and HOMO fi LUMO configura-
tions dictates the relative oscillator strengths of the By and Qy
absorption transitions. In particular, as DEdif increases (e.g.
due to substituents), the transition-dipole cancellation for the
Qy state decreases and intensity is shifted from By to Qy, and
vice versa. Mathematically, as DEdif increases, gy in Eq. (2)
increases, cos(2gy) decreases and the numerator in Eq. (4)
grows at the expense of the denominator, and vice versa.
According to Eq. (2), this same increase in DEdif increases the
energy spacing between By and Qy and vice versa.
It is useful to note the contribution of the (HOMO fi
LUMO) and (HOMO-1 fi LUMO+1) configurations to the
states responsible for the main optical features. For porphy-
rins, the two configurations are effectively degenerate and
contribute equally to the By and Qy states. For chlorins, prior
DFT calculations (70–72), prior other theoretical work (73–75)
and our own DFT calculations using Spartan (39) and
Gaussian (40) give a (HOMO fi LUMO):(HOMO-1 fi
LUMO+1) composition of the Qy state in the range 65:35–
80:20 for chlorins (depending on the molecule and calculation).
Spectral analysis using the four-orbital model
The basic model. In this section, the substituent effects on the
energies of the frontier MOs (HOMO-1, HOMO, LUMO and
LUMO+1) of the synthetic chlorins and oxophorbines
are explored further using Gouterman’s four-orbital model
(17–19). Within this model, the By and Qy states are comprised
of linear combinations of the (y-polarized) electronic
configurations that result from HOMO fi LUMO and
HOMO-1 fi LUMO+1 one-electron promotions. (Similarly
Bx and Qx are derived from HOMO fi LUMO+1 and
HOMO-1 fi LUMO.) The By state reflects the symmetric
combination and the Qy state the antisymmetric combination
of the configurations. The respective constructive versus
destructive interference of the associated transition dipole
moments results in a strong By(0,0) band (in the Soret
manifold) and a weaker Qy(0,0) band, consistent with the
observed spectra (Figs. 1 and 2).
Spectral analysis for the synthetic chlorins and oxophorbines. In
this section, the four-orbital model is used to analyze the
spectral properties of the new 7-substituted chlorins and
oxophorbines, and 3,13-disubstituted chlorins (Chart 2). The
The key parameters that are most closely tied to the optical
properties are the average energy (DEavg) and difference

664 Joseph W. Springer et al.
scope is broadened by the inclusion of effects of selected
additional substituents utilized previously at the 3,13-positions
(Chart 3; 20–22,26). Figures 5 and 6 show the MO energies
(top panels), energy gaps (middle panels) and four-orbital
parameters (bottom panels) plotted against the energy ⁄ wave-
length (Table 2) of the Soret maximum (Bmax; left panels) and
Qy(0,0) band (right panels) for the various compounds. The
intensity ratio of the Qy and By manifolds simulated using Eq.
(4) and a configuration-interaction energy of 0.4 eV is plotted
against the observed integrated intensity ratio of the Qy
manifold and the entire Soret manifold in Fig. 7. Note that for
the synthetic chlorins, Bmax defines the position of the By(0,0)
band, which is intimately connected with the Qy(0,0) band
within the four-orbital model, because splitting with Bx(0,0) is
typically negligible. The splitting of two bands increases as the
symmetry is diminished and is quite apparent for the free base
oxophorbines (Figs. 1 and 2). However, the consistent use of
Bmax rather than relying on specific (and in some cases
uncertain) assignments of the By(0,0) and Bx(0,0) bands for the
latter compounds does not alter any conclusions drawn here
from the analysis. In the following, the effects of substitu-
ents at the 3- and 13-positions of the synthetic zinc chlorins
are analyzed first, followed by discussion of the 7-substituted
zinc chlorins and free base oxophorbines.
Figure 5 shows that the trends in Bmax and Qy(0,0)
absorption energy ⁄ wavelength with MO-related properties
for formyl (and ester) at the 13- or 3,13-positions are in
keeping with those for other substituents (acetyl, ethynyl and
vinyl; Tables 2 and 4) that were part of an extended set of
analogs studied previously (21). As noted above, formyl, acetyl
and other substituents employed at the 3- and 13-positions
stabilize the frontier MOs to varying degrees. The plots in
Fig. 5A,D show that stabilization of each orbital accompanies
the bathochromic shifts in the By(0,0) and Qy(0,0) bands. In
Figure 5. Calculated frontier MO energies (top panels), energy gaps (middle panels) and four-orbital parameters (bottom panels) versus the
measured Bmax (left panels) and Qy(0,0) (right panels) absorption energy (bottom axis) and wavelength (top axis) for 3,13-disubstituted zinc
chlorins characterized here (closed symbols), a selection of 3,13-disubstituted zinc chlorins studied previously (open symbols) and oxophorbine
ZnOP (half-filled symbols). Top: energies LUMO+1 (diamonds), LUMO (circles), HOMO (squares), HOMO-1 (stars). Middle: energy gaps
LUMO+1 – HOMO-1 (down triangles) and LUMO – HOMO (up triangles). Bottom: DEavg (squares) and DEdif (circles). The indicated values are
the slopes of the trend lines. The compound code for the data points is given in the third column of Table 4.

Photochemistry and Photobiology, 2012, 88 665
Figure 6. Calculated frontier MO energies (top panels) and energy gaps (middle panels) and four-orbital parameters (bottom panels) versus the
measured Bmax (left) and Qy(0,0) (right) absorption-band energy (bottom axis) and wavelength (top axis) for 7-substituted zinc chlorins (closed
symbols) and 7-substituted free base oxophorbines (open symbols; Chart 1). Top: energies LUMO+1 (diamonds), LUMO (circles), HOMO
(squares), HOMO-1 (stars). Middle: energy gaps LUMO+1 – HOMO-1 (down triangles) and LUMO – HOMO (up triangles). Bottom: DEavg
(squares) and DEdif (circles). The indicated values are the slopes of the trend lines for 7-unsubstituted, 7-acetyl and 7-formyl chlorins (solid lines)
and oxophorbines (dashed lines) and in the right panels for 7-unsubstituted and 7-ethynyl oxophorbines (dotted). The compound code for the data
points is given in the third column of Table 4.
particular, the plot for each orbital (and also each plot in
panels B, C, E and F) has the unsubstituted reference zinc
chlorin ZnC (labeled ‘‘a’’) at the top right (highest MO and
spectral energy), the diformyl complex ZnC-F3M10F13 (labeled
‘‘h’’) at the lower left (lowest MO and spectral energy) and the
mono-formyl analog ZnC-M10F13 (labeled ‘‘d’’) in between.
However, for both bands, the slope of the trend line for the
LUMO (m = 2.6 or 3.2) is much greater than that for the
HOMO (m = 1.6 or 1.8) and the slope for the LUMO+1
(m = 1.7 or 2.1) is modestly greater than that for the
HOMO-1 (m = 1.3 or 1.6). Figure 5B,E show that these
differences translate into a more steep decrease of the
[LUMO – HOMO] energy gap (m = 1.0 or 1.4) than
the [LUMO+1 – HOMO-1] energy gap (m = 0.4 or 0.6) with
the two spectral positions. In turn, the unequal slopes of the
two MO energy gaps translate into an increase in DEdif that
accompanies the decrease (bathochromic shift) in the COG
parameter DEavg (right to left in Fig. 5C,F). The increase in
DEdif with 3- and 13-substituents also correlates with the
increase in Qy(0,0) intensity relative to the Soret manifold
(Fig. 7, circles) as predicted by the four-orbital model.
Figure 6 shows the analogous plots of MO-related para-
meters versus spectral positions for the 7-substituted zinc
chlorins (solid lines and closed symbols) and free base
oxophorbines (open symbols and dashed lines). Figure 7
(squares and diamonds) shows the associated plots for the
relative spectral intensities. Although there are fewer
7-substituted compounds in each set (unsubstituted and
mono-formyl, -acetyl and -ethynyl), differences from the effects
of 3- and 13-substituents (Fig. 5) are quite apparent. It is
simplest in viewing each plot in Fig. 6 to compare the position
of the 7-formyl zinc chlorin (labeled ‘‘F’’) or 7-formyl free base
oxophorbine (labeled ‘‘OF’’) with that for the unsubstituted zinc
chlorin (labeled ‘‘X’’) or free base oxophorbine (labeled ‘‘OX’’).

666 Joseph W. Springer et al.
energy gaps that underlie the spectral properties. Tracing the
origins of spectral properties to the effects of the substituents
and sites on the energies of specific MOs has been aided by
analysis using the four-orbital model.
Walking a given substituent around the macrocycle
The pronounced difference in the spectral impact of a formyl
group at the 7- versus 3- or 13-positions prompted us to inquire
as to whether such differences also pertain to other macrocycle
positions, and in particular whether there are unique charac-
teristics of the 7-position (or 8-position), which lies in the
pyrrole ring trans to the reduced ring in the chlorins (Chart 1).
To this end, DFT calculations were performed in which either
a single formyl, acetyl, vinyl or ethynyl group was walked
around a zinc chlorin lacking any substituent other than the
geminal dimethyl group in the reduced ring. The results are
shown in Figs. 8–10.
In Fig. 8, the top panels (A,D,G and J) show the energies of
the individual MOs, the middle panels (B,E,H and K) the MO
energy gaps [LUMO+1 – HOMO-1] and [LUMO – HOMO],
and the bottom panels (C,F,I,L) the four-orbital parameters
DEavg and DEdif. Figures 9 and 10 show simulated positions of
the By(0,0) and Qy(0,0) bands and their COG based solely on
DEavg and DEdif and a constant value of d = 0.4 eV for the
configuration-interaction energy (rounded from the values of
0.42 eV found by Gouterman (17) for porphyrins and 0.49 eV
found by Kee et al. (21) for a set of zinc chlorins). A constant
energy correction of DEcor = 0.5 eV was used to shift the MO
DEavg to lie within the typical Bmax–Qy spectral COG range for
zinc chlorins. It is noteworthy that the purpose of such
simulations is not to predict absolute spectral characteristics,
but rather to elucidate the origins of trends and contrasting
properties with substituent, which are independent of the exact
values of d and DEcor employed.
The simulations in Figs. 9 and 10 generally correctly predict
the effects of a formyl, acetyl, vinyl or ethynyl group at the 7-
versus 3- or 13-positions of the synthetic zinc chlorins
(Tables 1 and 2 and Fig. 2). This agreement is expected based
on the four-orbital analysis given in the preceding section that
utilized the calculated MO energies for the exact compounds
studied (Figs. 5–7). However, the origins of primary effects
may be more readily apparent using fictive macrocycles with a
single auxochrome. For example, the simulations given in
Fig. 10B correctly predict that 3-formyl gives a bathochromic
shift in By(0,0) and a bathochromic shift and small hyper-
chromic effect on Qy(0,0); Fig. 10A shows that these effects
arise from a reduced energy (bathochromic shift) of the COG
(DEavg) coupled with an increase in DEdif, the latter driving
the increased band splitting (Eq. 2) and intensity ratio (Eq. 4).
The simulations given in Fig. 10B also correctly predict that
7-formyl gives a larger bathochromic shift in By(0,0) and
hypochromic effect on Qy(0,0); Fig. 10A shows that these
effects arise from a comparable (to 3-formyl) reduction in the
COG (DEavg) coupled with a reduction (rather than an increase
for 3-formyl) in DEdif. Inspection of Fig. 10A reveals how the
relative signs and magnitudes of substituent-induced changes
in DEavg and DEdif ultimately dictate whether the By(0,0) and
Qy(0,0) bands will undergo shifts in the same or opposite
directions (and the DEdif-driven change in relative intensities).
These basic insights also apply to the analogous effects of
Figure 7. Simulated (using Eq. [4] and d = 0.4 eV) versus observed
integrated intensities of the Qy and B absorption manifolds for 7-
substituted free base oxophorbine series (filled diamonds), 7-substi-
tuted zinc chlorin series (filled squares) and zinc chlorins studied here
(filled circles) and previously (open circles) substituted at the 3-, 13- or
3,13-positions (along with controls, such as containing a common
10-aryl group). The designations of the compounds (data points) are
indicated in the third column of Table 4.
In addition, for the Qy(0,0) bands, the 7-ethynyl group (labeled
‘‘E’’ or ‘‘OE’’) causes a shift that is either in the opposite
direction (free base oxophorbines) or minimal (zinc chlorins)
compared with formyl (or acetyl). In the following, we focus on
the effects of 7-formyl (labeled ‘‘F’’ or ‘‘OF’’) and the similar,
but typically modestly smaller effects of 7-acetyl (labeled ‘‘A’’ or
‘‘OA’’).
Stabilization of the MO by incorporation of a 7-formyl
group is connected with the observed bathochromic shift in
Bmax and a hypsochromic shift in Qy(0,0). The slopes of the
trend lines in Fig. 6A,D reveal these shifts are associated with
a substantially greater substituent effect on the LUMO+1 than
the other three MOs, with the effect of the LUMO being
comparable to the HOMO-1, which is modestly smaller than
that for the HOMO. These trends can be compared with those
described above for 3- and 13-substituents (Fig. 5), where the
substituent dependence of the LUMO has the greatest impact,
with the effects of the LUMO+1 being comparable to that of
the HOMO (as opposed to HOMO-1). The MO shifts for
7-formyl (and 7-acetyl) translate into opposing slopes of the
trendlines for the LUMO+1 – HOMO-1 versus LUMO –
HOMO for Bmax (Fig. 6B) and a reversal in both slopes for
Qy(0,0) relative to Bmax (Fig. 6E versus Fig. 6B). In turn, the
resulting effect of these two 7-substituents to bathochromically
shift Bmax and hypsochromically shift Qy(0,0) is connected
with a decrease in the average MO energy gap parameter DEavg
combined with a decrease in the MO energy gap difference
parameter DEdif. The decreased DEdif is connected with a
reduced By(0,0)–Qy(0,0) splitting and a decrease in the relative
Qy(0,0) intensity (via Eqs. [3] and [4]), the latter being shown in
Fig. 7 (squares and diamonds).
Collectively, these considerations show that the dramatic
difference in the spectral impact of the same substituent,
especially formyl, at the 7- versus 3- or 13-positions primarily
derives from the differential impacts on the LUMO+1 and
LUMO orbitals, and the resulting differences in the MO

Photochemistry and Photobiology, 2012, 88 667
Figure 8. MO energies (top panels), energy gaps (middle panels) and four-orbital parameters (bottom panels) for a zinc chlorin containing a single
formyl (A–C), acetyl (D–F), vinyl (G–I) or ethynyl (J–L) group at various macrocycle sites. The solid bar at )2.14 eV (under the points for the
7- and 8-positions) denotes the energy of the degenerate ‘‘LUMOs’’ of the unsubstituted zinc porphyrin and the solid bar at )5.21 eV denotes the
energy of the accidentally degenerate porphyrin ‘‘HOMOs.’’ The horizontal dashed lines denote the reference values for ZnC with no substituents.
3-formyl in Chl d versus 7-formyl in Chl b relative to no formyl
in Chl a (Fig. 1).
from walking a single substituent around the periphery of a
zinc chlorin are given in the following.
Although the simulations correctly predict an increase or
decrease in the Qy(0,0) ⁄ By(0,0) intensity ratio, making quan-
titative intensity comparisons with the observed spectra are
difficult for the following reasons:
1. A change in DEdif reflects a change in the ratio of integrated
intensities (oscillator strengths) of the Qy and By manifolds;
this ratio may be obscured when spectra are normalized to
the Soret peak intensity (because there are uncertainties in
extinction coefficients) if the bands have different band-
widths.
2. The Soret region of the observed spectra contains overlap-
ping contributions from the By(0,0), Bx(0,0), By(1,0) and
Bx(1,0) and likely from other nonfour-orbital transitions.
Nonetheless, these points do not diminish the fact that the
four-orbital simulations generally correctly predict both the
substituent effects on the Qy(0,0) and By(0,0) positions and
relative intensities. Furthermore, starting at the observed
spectra (Figs. 1 and 2) and simulated spectra (Figs. 9 and
10), it is a straightforward path via the two key four-orbital
parameters (DEavg and DEdif; Fig. 8 bottom panels) and the
two associated MO energy gaps (Fig. 8 middle panels) to the
MOs themselves (Fig. 8 top panels), thereby tracing the effects
of the nature and position of a substituent on the spectral
characteristics to their root origin. Specific insights gleaned
In the top panels of Fig. 8, the solid bar at )2.14 eV
(under the points for the 7- and 8-positions) denotes the
energy of the two degenerate ‘‘LUMOs’’ of the unsubstituted
zinc porphyrin, and the solid bar at )5.21 eV denotes
the energy of the two accidentally degenerate porphyrin
‘‘HOMOs.’’ Reduction of ring D (Chart 1) destabilizes one
porphyrin orbital of each pair, which become the chlorin
LUMO+1 and HOMO, whereas two other porphyrin orbitals
are far less affected, translating into the chlorin LUMO and
HOMO-1. These effects are seen in the top panels in Fig. 8
by comparing the MO energies for the unsubstituted chlorin
(left-most, half-filled points) with those for the porphyrin
analog (solid bars).
Relative to the unsubstituted chlorin, MO shifts generally
increase in the following order: vinyl (Fig. 8G) < ethynyl
(Fig. 8J) < acetyl (Fig. 8D) < formyl (Fig. 8A). The electro-
nic properties of the formyl group accentuate the contrasting
effects at the 7- and 8-positions versus other sites, which are
summarized as follows:
1. Formyl at all positions stabilizes the LUMO+1 compared
to the unsubstituted chlorin, with the effect at the 7- and
8-positions being so substantial as to return the energy
level essentially back down to that of the unsubstituted
porphyrin.

668 Joseph W. Springer et al.
Figure 10. (A) Simulations of the By(0,0) and Qy(0,0) band positions
and the energy center of gravity (COG) obtained from the four-orbital
parameters depicted in the bottom panels of Fig. 9 for a zinc chlorin
containing no substituents or a single formyl group at the 3-, 7-, 10-
and 13- positions. (B) The same data as in panel A for three cases, but
with the bands given 20-nm wide Gaussian skirts. The intensities of
By(0,0) bands are normalized to unity and the intensities of the Qy(0,0)
bands are given by the result in Eq. (4) times the measured ZnC
Qy(0,0) ⁄ By(0,0) peak intensity ratio of 0.29, which serves as
reference.
a
Figure 9. Simulations of the By(0,0) and Qy(0,0) band positions and the
energy center of gravity (COG) obtained from the four-orbital
parameters depicted in the bottom panels of Fig. 8 for a zinc chlorin
containing a single formyl (F), acetyl (A), vinyl (V) or ethynyl (E) group
at various macrocycle sites. The dashed lines denote the wavelength
(bottom axis) or energy (top axis) for no substituent (vertical axis
position ‘‘no’’). For all simulations, a value of the d = 0.4 eV was used
and an energy correction of DEcorr = 0.5 eV was applied to DEavg to
place the center of gravity in the typical spectral region.
4. Formyl at all positions stabilizes the HOMO-1, but the
effect is about half as large at the b-pyrrole- (including 7-
and 8-) positions versus the meso-positions and thus keeps
the level closest to that of the unsubstituted chlorin and
porphyrin.
5. Analogous but slightly smaller effects generally occur for
acetyl at the b-pyrrole positions. The more substantial
differences of acetyl versus formyl at the meso-positions
(open symbols) likely derive from the precise orientations
with respect to the macrocycle plane; the DFT-minimized
structures have the C=O basically in-plane for 5-, 10- or
15-formyl, slightly out-of-plane for 20-formyl, and sub-
stantially out-of-plane for 5-, 10-, 15- or 20-acetyl. Further
refinement on this point is beyond the level of the present
calculations.
2. Formyl at all positions stabilizes the LUMO, although the
minimum effect is for the 7- and 8-positions such that only a
modest stabilization is observed compared to the unsubsti-
tuted chlorin and porphyrin.
3. Formyl at all positions stabilizes the HOMO, albeit slightly
more so at the b-pyrrole (closed symbols) versus meso sites
(open symbols), and for the 7- and 8-positions the shift
brings the level ca 65% of the way back down toward that
of the unsubstituted porphyrin.

Photochemistry and Photobiology, 2012, 88 669
6. The effects of vinyl or ethynyl can be in the same or
opposite direction than formyl or acetyl depending on the
site of macrocycle substitution.
Walking a formyl substituent around the chlorin versus
131-oxophorbine macrocycle
The MO-based analysis in the previous section has elucidated
the molecular origins of the spectral consequences of placing a
single substituent (formyl, acetyl, vinyl and ethynyl) at various
sites around the perimeter of a zinc chlorin. The analysis
invites the question as to whether the similar behavior or
deviations can be expected for the free base chlorin analogues,
and for the related zinc and free base 131-oxophorbines. The
oxophorbines are more directly relevant to the native chloro-
phylls and pheophytins, which also contain the keto-bearing
five-membered ring. Figures 11 and 12 give the results of
walking a formyl group around all four molecules (ZnC, FbC,
ZnOP and FbOP); the results for ZnC given above (Figs. 8
and 9) are reproduced here for ease of comparison with the
other three cases.
Figure 11 gives the MO energies (top panels), MO energy
gaps (middle panels) and resulting four-orbital parameters
(bottom panels) on which the spectral characteristics depend.
The left-hand panels in Fig. 12 give the simulated wavelengths
(and energies) of the By(0,0) and Qy(0,0) bands and their COG.
The right-hand panels give schematic stick spectra showing the
Qy(0,0) position and the integrated intensity of the Qy
manifold normalized (in each case) to the integrated intensity
of the By manifold. In Fig. 11 panels A,D,G and J, the solid
In effect, of all macrocycle sites, the 7- and 8-positions are
unique in allowing maximal effects on the LUMO+1 and
minimal effects on the LUMO, as well as generally comparable
(all positions) and small to modest effects on the HOMO and
HOMO-1. In contrast, the maximal effect of 3- and 13-
substituents is typically on the LUMO. Furthermore, the
distinguishing characteristics of the 7- and 8- positions are
emphasized by the electronically hot yet structurally compact
formyl group compared with other auxochromes such as
acetyl, vinyl and ethynyl. A simplified view is that the
distinguishing characteristics of the 7- or 8-positions and the
formyl (or acetyl) group combine to effectively ‘‘porphyrinize’’
the chlorin. If not exactly true in terms of the precise effects on
all frontier MOs (top panels in Fig. 8), this perspective is
appropriate when considering the relevant MO energy gaps
(middle panels) and four-orbital parameters (bottom panels)
that underlie the spectral properties (Figs. 9 and 10). This
analysis thus traces in a relatively straightforward manner the
molecular origins of the distinguishing spectral characteristics
of incorporation of a formyl (or acetyl) group at the 7- versus
3- (or 13-) positions of synthetic chlorins and oxophorbines, as
well as the distinguishing spectral features of the native
photosynthetic pigments Chl a, b and d.
Figure 11. MO energies (top panels), energy gaps (middle panels) and four-orbital parameters (bottom panels) for four fictive compounds bearing a
single formyl group at various macrocycle positions. The compounds are zinc (A–C) and free base (D–F) chlorin and zinc (G–I) and free base (J–L)
131-oxophorbine unit. The solid bars in each panel denote the energies of the frontier MOs (LUMO+1, LUMO, HOMO and HOMO-1) of the
unsubstituted porphyrin analog; fewer than four orbitals are shown for ZnC and FbC because pairs of orbitals are degenerate or effectively so (see
text for details). The horizontal dashed lines denote the reference values for ZnC with no substituents.

670 Joseph W. Springer et al.
Figure 12. Simulations of the By(0,0) and Qy(0,0) band positions and the energy center of gravity (COG) obtained from the four-orbital
parameters depicted in the bottom panels of Fig. 11 for four fictive molecules bearing a single formyl group at various macrocycle positions. The
compounds are zinc (A) and free base (B) chlorin and zinc (C) and free base (D) 131-oxophorbine unit. The dashed line for each case denotes
the wavelength (bottom axis) or energy (top axis) for no substituent (vertical axis position ‘‘no’’). For all simulations, a value of the d = 0.4 eV
was used and an energy correction of DEcorr = 0.5 eV was applied to DEavg to place the center of gravity in the typical spectral region. Panels
E–H give analogous stick spectra showing the Qy(0,0) position and integrated intensity of the Qy manifold normalized to that of the By
manifold.
bars give the energies of the frontier MOs of the unsubstituted
porphyrin analog. The analogs for 131-oxophorbines ZnOP
and FbOP are the respective zinc and free base porphyrin with
an annulated keto-bearing five-membered ring, in other words
17,18-didehydro-131-oxophorbines given that a phorbine is a
chlorin (Chart 4). The ‘‘porphyrinic’’ MO values are as
follows: zinc porphyrin ()5.22 eV [HOMO-1], )5.21 eV
[HOMO], )2.14 eV [LUMO and LUMO+1]); free base
porphyrin ()5.30 eV [HOMO-1], )5.15 eV [HOMO],
)2.24 eV [LUMO], )2.22 eV [LUMO+1]); zinc 17,18-didehy-
1. All four molecules (ZnC, FbC, ZnOP and FbOP) show
the same dramatic difference in the impact of a formyl
group at the 7- and 8-positions compared with other
sites, namely a bathochromic shift in By(0,0) and a
hypsochromic shift (zinc chelates) or virtually no change
(free bases) in Qy(0,0) and the hypochromic effect on the
Qy(0,0) band (Fig. 12). Again, these effects arise from
substantial LUMO+1 stabilization by 7- or 8-formyl with
less change in the other orbitals, particularly the LUMO
(Fig. 11). In this regard, the ‘‘porphyrinization’’ of the
chlorin or oxophorbine by 7- or 8-formyl is even more
pronounced for FbC and FbOP than for ZnC and ZnOP,
respectively, as can be seen from the individual MOs
and the low DEdif value (which is at a minimum for
porphyrins).
dro-131-oxophorbine
[HOMO], )2.60 eV [LUMO], )2.40 eV [LUMO+1]); free base
17,18-didehydro-131-oxophorbine
()5.60 eV [HOMO-1],
()5.53 eV
[HOMO-1],
)5.40 eV
)5.33 eV [HOMO], )2.67 eV [LUMO], )2.48 eV [LUMO+1]).
The main points that emerge are as follows:

Photochemistry and Photobiology, 2012, 88 671
analogs ZnC-E3A12 versus ZnC-E3A13 and ZnC-E3E12 versus
ZnC-E3E13. The main results are as follows:
1. The simulations (Figs. 8 and 9) suggest that both 12- and
13-acetyl will give a bathochromic shift in both the By(0,0)
and Qy(0,0) bands, and an hyperchromic effect on the latter
with respect to the former, in agreement with observations
on ZnC-A12 and ZnC-A13 (Fig. 2 and Table 2).
2. The simulations suggest that the bathochromic shifts in
both the By(0,0) and Qy(0,0) bands will be larger (by ca
10 nm) for 12- versus 13-acetyl, in agreement with obser-
vation. The comparable shifts for the two bands for both
compounds arise because both 12- and 13-acetyl engender
comparable shifts in the COG (via DEavg) without a
significant difference in the splitting (via DEdif).
Chart 4. Porphyrin analogues of the chlorins and 131-oxophorbines.
3. The comparable DEdif in turn predicts that 12- and 13-acetyl
groups should give comparable Qy(0,0) intensities relative
to By(0,0), consistent with observations. Closer examination
of this point indicates that the simulated intensity (and
splitting) should be about 2% lower for 12- versus 13-acetyl,
which is reversed from the small observed difference in
relative intensities (Fig. 2E). However, this small difference
is understandable considering the above-noted points con-
cerning detailed comparisons of simulated and observed
intensity ratios.
4. The observed spectral differences for ZnC-E3E12 versus
ZnC-E3E13 are smaller than those for ZnC-E3A12 versus
ZnC-E3A13 and ZnC-A12 versus ZnC-A13 (Table 2, entries
18–23). This observed smaller 12- versus 13-position differ-
ence for ethynyl compared with acetyl is predicted by the
four-orbital simulations given in Fig. 9D compared with
Fig. 9B.
2. The presence of oxophorbine unit makes the above-noted
effects [decreased By(0,0)–Qy(0,0) splitting and decreased
Qy(0,0) ⁄ By(0,0) intensity ratio] slightly emphasized for
7- versus 8-formyl, and more so for ZnOP than FbOP
(Fig. 12C,D,G,H). Thus, Chl b has the formyl group placed
at the position of greatest impact for such major spectral
distinctions from Chl a.
3. In general, the By(0,0) band is bathochromically shifted,
the Qy(0,0) band is hypsochromically shifted, and the
Qy(0,0) ⁄ By(0,0) intensity ratio is reduced for the free base
chlorin or oxophorbine compared to the zinc chelate.
4. The presence of the oxophorbine unit in ZnOP versus
ZnC and FbOP versus FbC diminishes the magnitude of the
bathochromic shift caused by a 5-formyl group and
enhances that due to a 10-formyl group.
Inspection of Figs. 11 and 12 shows that these effects can be
traced to the effects of the formyl group at the various
positions in the various macrocycles on the individual MOs,
particularly the LUMO+1 relative to the LUMO (and the
HOMO and HOMO-1) for the 7- and 8-substituents. While
comparisons can be made, the main effects depicted by the
simulations are consistent with the observed spectra of the
synthetic chlorins (Fig. 2) and the native chromophores
(Fig. 1). Collectively, the results indicate that the differential
effects of a strong auxochrome at various macrocycle positions
are generally similar for the chlorins and oxochlorins, and for
the zinc chelates and free base forms; differences do exist (and
origins traced) that can be utilized by judicious choice of
substituent site, such as the 7-position (versus the 8-position,
etc.) of the formyl group in Chl b.
5. All three pairs of compounds show a ca 20% reduction in
Ff and variable impact on sS for the same substituent
(acetyl or ethynyl) at the 12- versus 13-position.
6. The above-noted differences ultimately arise from the in-
equivalence of the chlorin 12- and 13-positions (Chart 1).
Inspection of Fig. 8 indicates that the inequivalence does not
significantly affect the HOMO-1, and causes reversed effects
for 12- versus 13-substitution on the HOMO compared to
the LUMO and LUMO+1. As a consequence, differences in
the spectral and electronic properties of the lowest singlet
excited-state of the 12-versus 13-substituted chlorins origi-
nate more from the [LUMO – HOMO] configuration than
the [LUMO+1 – HOMO-1] contribution.
Collectively, these observations indicate that the four-
orbital simulations of the effects of acetyl (or ethynyl) at the
12- versus 13-positions reproduce the key spectral differences
and give insights into their origin. Further inspection of Figs. 8
and 9 suggest that analogous differences are to be expected for
a substituent (e.g. acetyl, formyl and vinyl) at the 2- versus
3-positions. The latter considerations are relevant to the
spectral properties of native chlorophylls and bacteriochloro-
phylls, which typically differ in the substituent pattern at these
two positions.
Effect of the same substituent at the 12- versus 13-positions
The spectral simulations shown in Fig. 9 for fictive zinc
chlorins bearing formyl or acetyl at various macrocycle
positions predict that such groups at the 12- versus 13-
positions should give small yet measurable differences in
spectral properties. It is of interest to compare such a
prediction with observation as a calibration on the ability of
the four-orbital approach to give insights into the origins of
such modest changes in spectral characteristics. To this end,
zinc chlorins ZnC-A12 and ZnC-A13 were prepared. Absorp-
tion spectra are shown in Fig. 2E and the spectral and
photophysical parameters are listed in Tables 2 and 3 (entries
22 and 23). For additional comparisons, the tables (entries 18–
21) also list data for two previously studied (22) pairs of
CONCLUSIONS AND OUTLOOK
A widespread view in organic photochemistry is that the
incorporation of a conjugative group in a chromophore causes
a bathochromic shift of the absorption spectrum. While
correct in many instances, the spectra of Chl a versus b

672 Joseph W. Springer et al.
3. Strain, H. H., M. R. Thomas and J. J. Katz (1963) Spectral
absorption properties of ordinary and fully deuteriated chloro-
phylls a and b. Biochim. Biophys. Acta 75, 306–311.
4. Rowan, K. S. (1989) Photosynthetic Pigments of Algae, pp. 65–
111. Cambridge University Press, Cambridge.
5. Kobayashi, M., M. Akiyama, H. Kano and H. Kise (2006)
Spectroscopy and structure determination. In Chlorophylls and
Bacteriochlorophylls: Biochemistry, Biophysics, Functions and
provide a prominent exception to this (simplistic) view. The
long-wavelength absorption band of the ‘‘less-conjugated’’
Chl a (7-methyl) appears at 665 nm, whereas that of the
‘‘more-conjugated’’ Chl b (7-formyl) absorbs at 646 nm. Thus,
the effects of substituents on chlorin absorption spectra cannot
be satisfactorily addressed by a simple ‘‘particle-in-a-box’’
method (in which the lowest excited state is composed solely of
the [LUMO – HOMO] configuration), but rather can be
understood with Gouterman’s four-orbital model (in which
the lowest excited state is composed of a linear combination of
the [LUMO – HOMO] and [LUMO+1 – HOMO-1] configu-
rations). The results gathered here show that the presence of a
potent auxochrome at the 7-position primarily stabilizes the
LUMO+1, but has little effect on the LUMO, and gives
modest effects on the HOMO and HOMO-1. The resulting
differences in the MO energy gaps are at the heart of the
observed spectral differences of Chl b versus Chl a. Thus, the
spectral change caused by the 7-formyl group stems not from a
change in the path or extent of p-conjugation, as was once
believed to be the origin of auxochromic effects
(7,8,12,13,15,16), but rather by stabilization preferentially of
one of the four frontier orbitals of the cyclic polyene.
Upon consideration of all possible sites for incorporation of
a formyl group, the 7- and 8-positions are most susceptible to
this pattern of effects on orbital energies. The overall impact of
the 7-formyl group is to ‘‘porphyrinize’’ the chlorin in terms of
the resulting spectra and the relative energies of the frontier
orbitals. Indeed, the major absorption bands of Chl b are
positioned ‘‘inside’’ (i.e. the B and Qy bands appear at longer
and shorter wavelengths, respectively) those of Chl a. On the
other hand, Chl d can be regarded as a somewhat more
extreme form of Chl a given the presence of a formyl group in
lieu of a vinyl group at the 3-position.
Applications (Edited by B. Grimm, R. J. Porra, W. Rudiger and H.
¨
Scheer), pp. 79–94. Springer, Dordrecht, The Netherlands.
6. Larkum, A. W. D. and M. Kuhl (2005) Chlorophyll d: the puzzle
¨
resolved. Trends Plant Sci. 10, 355–357.
7. Maccoll, A. (1947) Colour and constitution. Quart. Rev. 1, 16–58.
8. Ferguson, L. N. (1948) Relationships between absorption spectra
and chemical constitution of organic molecules. Chem. Rev. 43,
385–446.
9. Venkataraman, K. (1952) The Chemistry of Synthetic Dyes, Vol. I,
pp. 323–400. Academic Press, New York.
10. Dahne, S. (1978) Color and constitution: one hundred years of
¨
research. Science 199, 1163–1167.
11. Calvert, J. G. and J. N. Pitts Jr (1966) Photochemistry, pp. 263–
268. John Wiley and Sons, Inc., New York.
12. Jaffe, H. H. and M. Orchin (1962) Theory and Applications of
´
Ultraviolet Spectroscopy, pp. 175–177, 587. John Wiley and Sons,
Inc., New York.
13. Gillam, A. E. and E. S. Stern (1954) An Introduction to Electronic
Absorption Spectroscopy in Organic Chemistry, pp. 44–45. Edward
Arnold (Publishers) Ltd., London.
14. Mason, S. F. (1970) Color and the electronic states of organic
molecules. In The Chemistry of Synthetic Dyes, Vol. III (Edited by
K. Venkataraman), pp. 169–221. Academic Press, New York.
15. Bury, C. R. (1935) Auxochromes and resonance. J. Am. Chem.
Soc. 57, 2115–2117.
16. Lewis, G. N. and M. Calvin (1939) The color of organic sub-
stances. Chem. Rev. 25, 273–328.
17. Gouterman, M. (1959) Study of the effects of substitution on the
absorption spectra of porphin. J. Chem. Phys. 30, 1139–1161.
18. Gouterman, M. (1961) Spectra of porphyrins. J. Mol. Spectrosc. 6,
138–163.
19. Gouterman, M. (1978) Optical spectra and electronic structure of
porphyrins and related rings. In The Porphyrins Vol. III (Edited by
D. Dolphin), pp. 1–165. Academic Press, New York.
20. Kee, H. L., C. Kirmaier, Q. Tang, J. R. Diers, C. Muthiah, M.
Taniguchi, J. K. Laha, M. Ptaszek, J. S. Lindsey, D. F. Bocian
and D. Holten (2007) Effects of substituents on synthetic analogs
of chlorophylls. Part 1: synthesis, vibrational properties and ex-
cited-state decay characteristics. Photochem. Photobiol. 83, 1110–
1124.
21. Kee, H. L., C. Kirmaier, Q. Tang, J. R. Diers, C. Muthiah, M.
Taniguchi, J. K. Laha, M. Ptaszek, J. S. Lindsey, D. F. Bocian
and D. Holten (2007) Effects of substituents on synthetic analogs
of chlorophylls. Part 2: redox properties, optical spectra and
electronic structure. Photochem. Photobiol. 83, 1125–1143.
22. Mass, O., M. Ptaszek, M. Taniguchi, J. R. Diers, H. L. Kee, D. F.
Bocian, D. Holten and J. S. Lindsey (2009) Synthesis and pho-
tochemical properties of 12-substituted versus 13-substituted
chlorins. J. Org. Chem. 74, 5276–5289.
23. Ptaszek, M., B. E. McDowell, M. Taniguchi, H.-J. Kim and J. S.
Lindsey (2007) Sparsely substituted chlorins as core constructs in
chlorophyll analogue chemistry. Part 1: synthesis. Tetrahedron 63,
3826–3839.
24. Taniguchi, M., M. Ptaszek, B. E. McDowell and J. S. Lindsey
(2007) Sparsely substituted chlorins as core constructs in chloro-
phyll analogue chemistry. Part 2: derivatization. Tetrahedron 63,
3840–3849.
The availability of a set of >30 synthetic chlorins, which
were prepared to probe the effects of substituents, in
conjunction with DFT calculations on these and other
fictive chlorins, has enabled the insights delineated above.
Understanding the interplay of the four frontier molecular
orbitals with substituents at diverse sites should facilitate the
rational design of chlorins for specific photochemical pur-
poses and deepen understanding of photosynthetic processes
in general.
Acknowledgements—This work was supported by grants from the
Division of Chemical Sciences, Geosciences, and Biosciences, Office of
Basic Energy Sciences of the U.S. Department of Energy to D.F.B.
(DE-FG02-05ER15660), D.H. (DE-FG02-05ER15661) and J.S.L.
(DE-FG02-96ER14632). We would like to thank Dr. Michael Krayer
for preparation of chlorin ZnC-M10Es13, Drs. Jayeeta Bhaumik,
Marcin Ptaszek and Masahiko Taniguchi for gifts of synthetic chlorins
and Drs. Dariusz Niedzwiedzki and Robert Blankenship for a gift of
Chl d.
25. Taniguchi, M., M. Ptaszek, B. E. McDowell, P. D. Boyle and J. S.
Lindsey (2007) Sparsely substituted chlorins as core constructs in
chlorophyll analogue chemistry. Part 3: spectral and structural
properties. Tetrahedron 63, 3850–3863.
26. Mass, O., M. Taniguchi, M. Ptaszek, J. W. Springer, K. M. Faries,
J. R. Diers, D. F. Bocian, D. Holten and J. S. Lindsey (2011)
Structural characteristics that make chlorophylls green: interplay
of hydrocarbon skeleton and substituents. New J. Chem. 35,
76–88.
REFERENCES
1. Scheer, H. (1991) Structure and occurrence of chlorophylls. In
Chlorophylls (Edited by H. Scheer), pp. 3–30. CRC Press, Boca
Raton, FL.
2. Smith, J. H. C. and A. Benitez (1955) Chlorophylls: analysis in
plant materials. In Modern Methods of Plant Analysis, Vol. IV
(Edited by K. Paech and M. V. Tracey), pp. 142–196. Springer-
Verlag, Berlin.

Photochemistry and Photobiology, 2012, 88 673
27. Muthiah, C., M. Ptaszek, T. M. Nguyen, K. M. Flack and J. S.
Lindsey (2007) Two complementary routes to 7-substituted chlo-
rins. Partial mimics of chlorophyll b. J. Org. Chem. 72, 7736–7749.
28. Muthiah, C., D. Lahaye, M. Taniguchi, M. Ptaszek and J. S.
Lindsey (2009) Regioselective bromination tactics in the de novo
synthesis of chlorophyll b analogues. J. Org. Chem. 74, 3237–3247.
29. Laha, J. K., C. Muthiah, M. Taniguchi, B. E. McDowell, M.
Ptaszek and J. S. Lindsey (2006) Synthetic chlorins bearing
auxochromes at the 3- and 13-positions. J. Org. Chem. 71, 4092–
4102.
30. Laha, J. K., C. Muthiah, M. Taniguchi, B. E. McDowell, M.
Ptaszek and J. S. Lindsey (2009) Additions and corrections: syn-
thetic chlorins bearing auxochromes at the 3- and 13-positions.
J. Org. Chem. 74, 5122.
31. Muthiah, C., J. Bhaumik and J. S. Lindsey (2007) Rational routes
to formyl-substituted chlorins. J. Org. Chem. 72, 5839–5842.
32. Ptaszek, M., D. Lahaye, M. Krayer, C. Muthiah and J. S. Lindsey
(2010) De novo synthesis of long-wavelength absorbing chlorin-
13,15-dicarboximides. J. Org. Chem. 75, 1659–1673.
J. Cioslowski and D. J. Fox (2009) The Calculation Methods used
in Gaussian ‘09, Revision B.01. Gaussian, Inc., Wallingford, CT.
41. Tian, B., E. S. E. Eriksson and L. A. Eriksson (2010) Can range-
separated and hybrid DFT functionals predict low-lying excita-
tions? A Tookad case study. J. Chem. Theory Comput. 6, 2086–
2094.
42. Birks, J. B. (1970) Photophysics of Aromatic Molecules, pp. 142–
192. Wiley-Interscience, London.
43. Brody, S. S. (1957) Instrument to measure fluorescence lifetimes in
the millimicrosecond region. Rev. Sci. Instrum. 28, 1021–1026.
44. Brody, S. S. and E. Rabinowitch (1957) Excitation lifetime of
photosynthetic pigments in vitro and in vivo. Science 125, 555.
45. Brody, S. S. (1960) Delay in intermolecular and intramolecu-
lar energy transfer and lifetimes of photosynthetic pigments.
Z. Elektrochem. 64, 187–194.
46. Butler, W. L. and K. H. Norris (1963) Lifetime of the long-
wavelength chlorophyll fluorescence. Biochimica Biophys. Acta 66,
72–77.
47. Muller, A., R. Lumry and H. Kokubun (1965) High performance
¨
33. Krayer, M., T. Balasubramanian, C. Ruzie
´
, M. Ptaszek, D. L.
phase fluorometer constructed from commercial subunits. Rev.
Sci. Instrum. 36, 1214–1226.
Cramer, M. Taniguchi and J. S. Lindsey (2009) Refined syntheses
of hydrodipyrrin precursors to chlorin and bacteriochlorin build-
ing blocks. J. Porphyrins Phthalocyanines 13, 1098–1110.
34. Mass, O., D. R. Pandithavidana, M. Ptaszek, K. Santiago, J. W.
Springer, J. Jiao, Q. Tang, C. Kirmaier, D. F. Bocian, D. Holten
and J. S. Lindsey (2011) De novo synthesis and properties of
analogues of the self-assembling chlorosomal bacteriochloro-
phylls. New J. Chem. 35, 2671–2690.
48. Dzhagarov, B. M. (1972) Effect of structural factors on intra-
molecular transitions in tetrapyrrole molecules. Izv. Akad. Nauk
SSSR, Ser. Fiz. 36, 1093–1096 (English translation: Bull. Acad.
Sci. USSR 36, 984–986).
49. Hindman, J. C., R. Kugel, M. R. Wasielewski and J. J. Katz
(1978) Coherent stimulated light emission (lasing) in covalently
linked chlorophyll dimers. Proc. Natl Acad. Sci. USA 75, 2076–
2079.
35. Lotjonen, S. and P. H. Hynninen (1983) An improved method
¨
¨
for the preparation of (10R)- and (10S)-pheophytins a and b.
Synthesis 708–710.
36. Weber, G. and F. W. J. Teale (1957) Determination of the abso-
lute quantum yield of fluorescent solutions. Trans. Faraday Soc.
53, 646–655.
50. Yuen, M. J., L. L. Shipman, J. J. Katz and J. C. Hindman (1980)
Concentration quenching of fluorescence from chlorophyll-a,
pheophytin-a, pyropheophytin-a and their covalently-linked pairs.
Photochem. Photobiol. 32, 281–296.
51. Kaplanova, M. and K. Cermak (1981) Effect of reabsorption on
the concentration dependence of fluorescence lifetimes of chloro-
phyll a. J. Photochem. 15, 313–319.
52. Connolly, J. S., A. F. Janzen and E. B. Samuel (1982) Fluores-
cence lifetimes of chlorophyll a: solvent, concentration and oxygen
dependence. Photochem. Photobiol. 36, 559–563.
53. Natarajan, L. V., J. E. Ricker, R. E. Blankenship and R. Chang
(1984) Solvent influences on the singlet quenching of chlorophyll a
by 2,5-dimethyl-p-benzoquinone. Photochem. Photobiol. 39, 301–
306.
54. Jabben, M., N. A. Garcia, S. E. Braslavsky and K. Schaffner
(1986) Photophysical parameters of chlorophylls a and b—fluo-
rescence and laser-induced optoacoustic measurements. Photo-
chem. Photobiol. 43, 127–131.
55. Niedzwiedzki, D. and R. Blankenship (2010) Singlet and triplet
excited state properties of natural chlorophylls and bacteriochlo-
rophylls. Photosyn. Res. 106, 227–238.
56. Avarmaa, R. A. and R. P. Tamkivi (1978) Dependence of decay
times of chlorophyll fluorescence on luminescence and excitation
wavelengths. Optik. Spektrosk. 45, 247–254.
57. Kelly, A. R. and L. K. Patterson (1971) Model systems for
photosynthesis II. Concentration quenching of chlorophyll b
fluorescence in solid solutions. Proc. R. Soc. Lond. A 324, 117–
126.
58. Avarmaa, R., T. Soovik, R. Tamkivi and V. Tonissoo (1977)
Fluorescence life-times of chlorophyll a and some related com-
pounds at low temperatures. Studia Biophysica 65, 213–218.
59. Pfarrherr, A., K. Teuchner and D. Leupold (1991) Chlorophyll
b in solution: fluorescence lifetimes, absorption and emission
spectra as criteria of purity. J. Photochem. Photobiol. B: Biol. 9,
35–41.
60. Dmitrievsky, O. D., V. L. Ermolaev and A. N. Terenin (1957)
Direct life-time measurements of excited molecules of chlorophyll
and analogous pigments in different media. Dokl. Akad. Nauk
SSSR 114, 751.
61. Forster, L. S. and R. Livingston (1952) The absolute quantum
yields of the fluorescence of chlorophyll solutions. J. Chem. Phys.
20, 1315–1320.
62. Latimer, P., T. T. Bannister and E. Rabinowitch (1956) Quan-
tum yields of fluorescence of plant pigments. Science 124, 585–
586.
37. Seybold, P. G. and M. Gouterman (1969) Porphyrins XIII: fluo-
rescence spectra and quantum yields. J. Mol. Spectrosc. 31, 1–13.
38. Gradyushko, A. T., A. N. Sevchenko, K. N. Solovyov and M. P.
Tsvirko (1970) Energetics of photophysical processes in chloro-
phyll-like molecules. Photochem. Photobiol. 11, 387–400.
39. Except for molecular mechanics and semi-empirical models, the
calculation methods used in Spartan ’08 or ’10 have been docu-
mented in: Shao, Y., L. F. Molnar, Y. Jung, J. Kussmann,
C. Ochsenfeld, S. T. Brown, A. T. B. Gilbert, L. V. Slipchenko,
S. V. Levchenko, D. P. O’Neill, R. A. DiStasio Jr, R. C. Lochan,
T. Wang, G. J. O. Beran, N. A. Besley, J. M. Herbert, C. Y. Lin,
T. Van Voorhis, S. H. Chien, A. Sodt, R. P. Steele, V. A. Rasso-
lov, P. E. Maslen, P. P. Korambath, R. D. Adamson, B. Austin,
J. Baker, E. F. C. Byrd, H. Dachsel, R. J. Doerksen, A. Dreuw,
B. D. Dunietz, A. D. Dutoi, T. R. Furlani, S. R. Gwaltney,
A. Heyden, S. Hirata, C.-P. Hsu, G. Kedziora, R. Z. Khalliulin,
P. Klunzinger, A. M. Lee, M. S. Lee, W. Liang, I. Lotan, N. Nair,
B. Peters, E. I. Proynov, P. A. Pieniazek, Y. M. Rhee, J. Ritchie,
E. Rosta, C. D. Sherrill, A. C. Simmonett, J. E. Subotnik,
H. L. Woodcock III, W. Zhang, A. T. Bell, A. K. Chakraborty,
D. M. Chipman, F. J. Keil, A. Warshel, W. J. Hehre, H. F. Schaefer
III, J. Kong, A. I. Krylov, P. M. W. Gill and M. Head-Gordon
(2006) Advances in methods and algorithms in a modern quantum
chemistry program package. Phys. Chem. Chem. Phys. 8, 3172–
3191.
40. Frisch, M. J., G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hrat-
chian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S.
Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.
E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dap-
¨
prich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,

674 Joseph W. Springer et al.
63. Broyde, S. B. and S. S. Brody (1967) Emission spectra of chlo-
rophyll-a in polar and nonpolar solvents. J. Chem. Phys. 46, 3334–
3340.
64. Dvornikov, S. S., V. N. Knyukshto, K. N. Solovev and M. P.
Tsvirko (1979) Phosphorescence of chlorophyllis a and b and their
pheophytins. Opt. Spectrosc. (USSR) 46, 385–388.
65. Kelly, A. R. and G. Porter (1970) Model systems for photosyn-
thesis I. Energy transfer and light harvesting mechanisms. Proc. R.
Soc. Lond. A 315, 149–161.
66. Leupold, D., A. Struck, H. Stiel, K. Teuchner, S. Oberlander
¨
and H. Scheer (1990) Excited-state properties of 20-chloro-
chlorophyll a. Chem. Phys. Lett. 170, 478–484.
67. Kawai, N. and K. Morishige (1994) Absorption and fluorescence
properties of chlorophyll a and b and pheophytin a and b in aqueous
solutions of nonionic surfactants. Bunseki Kagaku 43, 791–797.
68. Kawai, N. and K. Morishige (1994) Visible absorption and fluo-
rescence properties of chlorophyll a and b and pheophytin a and b
in the nonionic surfactant-nonaqueous solvent system. Bunseki
Kagaku 43, 1155–1162.
70. Linnanto, J. and J. Korppi-Tommola (2006) Quantum chemical
simulation of excited states of chlorophylls, bacteriochlorophylls
and their complexes. Phys. Chem. Chem. Phys. 8, 663–687.
71. Petit, L., A. Quartarolo, C. Adamo and N. Russo (2006) Spec-
troscopic properties of porphyrin-like photosensitizers: insights
from theory. J. Phys. Chem. B 110, 2398–2404.
¨
72. Balaban, T. S., P. Braun, C. Hattig, A. Hellweg, J. Kern, W.
Saenger and A. Zouni (2009) Preferential pathways for light-
trapping involving b-ligated chlorophylls. Biochim. Biophysica
Acta 1787, 1254–1265.
73. Weiss Jr, C. (1972) The pi electron structure and absorption spectra
of chlorophylls in solution. J. Mol. Spectrosc. 44, 37–80.
74. Petke, J. D., G. M. Maggiora, L. L. Shipman and R. E. Christ-
offersen (1978) Stereoelectronic properties of photosynthetic and
related systems. Ab initio configuration interaction calculations on
the ground and lower excited singlet and triplet states of magne-
sium chlorin and chlorin. J. Mol. Spectrosc. 73, 311–331.
75. Petke, J. D., G. M. Maggiora, L. Shipman and R. E. Christof-
fersen (1979) Stereoelectronic properties of photosynthetic and
related systems–V. Ab initio configuration interaction calculations
on the ground and lower excited singlet and triplet states of ethyl
chlorophyllide a and ethyl pheophorbide a. Photochem. Photobiol.
30, 203–223.
69. Gurinovich, G. P., A. N. Sevchenko and K. N. Solov’ev (1971)
Spectroscopy of Chlorophyll and Related Compounds, pp. 134–364.
National Technical Information Service, US Department of
Commerce, Springfield, VA.