Synthesis and excited-state photodynamics of a chlorin-bacteriochlorin dyad - Through-space versus through-bond energy transfer in tetrapyrrole arrays

Article abstract of DOI:10.1111/j.1751-1097.2007.00258.x

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromoba

Full text of DOI:10.1111/j.1751-1097.2007.00258.x

Photochemistry and Photobiology, 2008, 84: 786–801
Synthesis and Excited-state Photodynamics of a
Chlorin–Bacteriochlorin Dyad—Through-space Versus
Through-bond Energy Transfer in Tetrapyrrole Arrays
Chinnasamy Muthiah1, Hooi Ling Kee2, James R. Diers3, Dazhong Fan1,
Marcin Ptaszek1, David F. Bocian*3, Dewey Holten*2 and Jonathan S. Lindsey*1
1Department of Chemistry, North Carolina State University, Raleigh, NC
2Department of Chemistry, Washington University, St. Louis, MO
3Department of Chemistry, University of California Riverside, Riverside, CA
Received 10 October 2007, accepted 1 November 2007, DOI: 10.1111 ⁄ j.1751-1097.2007.00258.x
(3–7). Chlorophylls and bacteriochlorophylls are important
ABSTRACT
naturally occurring pigments that exhibit strong absorption
features in the red and NIR regions of the spectrum,
respectively (Chart 1). Over the past several years, we have
been working to develop de novo synthetic methods for
preparing chlorins (dihydroporphyrins) and bacteriochlorins
(tetrahydroporphyrins) that retain the fundamental spectral
and photophysical characteristics of the naturally occurring
macrocycles, yet are synthetically malleable, as required for
basic scientific studies or diverse applications (8–18). The one
or two reduced pyrrole rings in the synthetic chlorins or
bacteriochlorins, respectively, each incorporate a geminal
dimethyl group to block adventitious dehydrogenation, ren-
dering the synthetic hydroporphyrins far more stable than
naturally occurring pigments.
We have recently exploited the new synthetic methods to
gain access to chlorins bearing substituents that afford
significantly enhanced red absorption spectral features. Such
auxochromes include acetyl (13), triisopropylsilylethynyl (13),
formyl (19), phenyl (20) and vinyl (13) groups located at the
3,13-positions. The substituents were introduced at the
3,13-positions because the electronic transition that gives rise
to the Qy band is polarized along this axis. The chlorin
acetylation methodology has also been extended to gain facile
access to 131-oxophorbines, the core molecular framework of
chlorophylls (14). Studies of the photophysical, vibrational
and redox properties of a large family of these synthetic
chlorins along with molecular-orbital and four-orbital analysis
have given insights into the factors that control the effects of
peripheral substituents on the electronic characteristics of
these hydroporphyrin complexes (20,21).
Understanding energy transfer among hydroporphyrins is of
fundamental interest and essential for a wide variety of
photochemical applications. Toward this goal, a synthetic free
base ethynylphenylchlorin has been coupled with a synthetic
free base bromobacteriochlorin to give a phenylethyne-linked
chlorin–bacteriochlorin dyad (FbC-pe-FbB). The chlorin and
bacteriochlorin are each stable toward adventitious oxidation
because of the presence of a geminal dimethyl group in each
reduced pyrrole ring. A combination of static and transient
optical spectroscopic studies indicate that excitation into the Qy
band of the chlorin constituent (675 nm) of FbC-pe-FbB in
toluene results in rapid energy transfer to the bacteriochlorin
constituent with a rate of ꢀ(5 ps))1 and efficiency of >99%.
The excited bacteriochlorin resulting from the energy-transfer
process in FbC-pe-FbB has essentially the same fluorescence
characteristics as an isolated monomeric reference compound,
namely a narrow (12 nm fwhm) fluorescence emission band at
760 nm and a long-lived (5.4 ns) Qy excited state that exhibits a
¨
significant fluorescence quantum yield (Ff = 0.19). Forster
calculations are consistent with energy transfer in FbC-pe-FbB
occurring predominantly by a through-space mechanism. The
energy-transfer characteristics of FbC-pe-FbB are compared
with those previously obtained for analogous phenylethyne-
linked dyads consisting of two porphyrins or two oxochlorins.
The comparisons among the sets of dyads are facilitated
by density functional theory calculations that elucidate the
molecular-orbital characteristics of the energy donor and
acceptor constituents. The electron-density distributions in the
frontier molecular orbitals provide insights into the through-bond
electronic interactions that can also contribute to the energy-
transfer process in the different types of dyads.
A very large number of dyads and larger constructs that
contain porphyrins have been prepared for studies in artificial
photosynthesis (22). A sizable number of covalently linked
architectures that contain chlorins also have been prepared
(for a review of the early literature, see Ref. 23; for represen-
tative chlorin dyads and multads prepared more recently, see
Refs. 24–45). Extensive work has also been devoted to the
preparation and study of self-assembling chlorins (46). How-
ever, relatively few bacteriochlorin-containing dyads have been
prepared (47–54). Moreover, to our knowledge, no chlorin–
bacteriochlorin dyads have been prepared that incorporate
INTRODUCTION
Molecules with intense absorption in the red and near-infrared
(NIR) regions are attractive for applications encompassing
solar cells (1), medical imaging (2) and photodynamic therapy
*Corresponding author emails: holten@wustl.edu (Dewey Holten), David.
Bocian@ucr.edu (David F. Bocian), jlindsey@ncsu.edu (Jonathan S. Lindsey)
ꢀ 2008 The Authors. JournalCompilation. The AmericanSocietyofPhotobiology 0031-8655/08
786

Photochemistry and Photobiology, 2008, 84 787
a
relatively rigid intervening linker. The approach of
self-assembly ultimately may prove to be the simplest and
most versatile strategy for constructing multi-pigment archi-
tectures for light-harvesting applications. At present, however,
studies of energy-transfer dynamics are greatly facilitated by
the use of a covalent strategy wherein the constituents of
interest are joined at a defined distance of separation in a well-
defined architecture.
In the present study, we have taken advantage of the new
synthetic routes to hydroporphyrins to prepare a dyad
consisting of a free base (Fb) chlorin (C) and bacteriochlorin
(B) joined by
a phenylethyne (pe) linker (FbC-pe-FbB,
Chart 2). The properties of each chlorin or bacteriochlorin
constituent are established by comparison with two bench-
mark monomers (Chart 2). One benchmark lacks the linker
moiety (FbC1, FbB1) and the other benchmark contains the
linker moiety (FbC2, FbB2). We then investigate the energy-
transfer characteristics of the FbC-pe-FbB dyad using a
combination of static and transient optical spectroscopy.
A key objective of the present study was to understand how
the mechanism of energy transfer in covalently linked hydro-
porphyrin arrays compares with that in analogous porphyrin
arrays (55), given the profound differences in spectral charac-
teristics of the respective arrays. To address this issue, the
Chart 1.
Chart 2.

788 Chinnasamy Muthiah et al.
112.3, 121.1, 123.3, 129.5, 133.0, 134.8, 135.1, 141.6, 142.4, 178.8.
Anal. Calcd. for C27H32Br2N2OSi: C, 55.11; H, 5.48; N, 4.76. Found:
C, 55.92; H, 5.71; 4.52; FAB-MS obsd 586.0676, calcd 586.0650
(C27H32Br2N2OSi).
Notes: (1) All of the workup operations should be performed
without heating, and preferably under chilled conditions. (2) The
chromatography solvent mixture (hexanes ⁄ CH2Cl2 ⁄ ethyl acetate) was
stored over anhydrous K2CO3 prior to use.
energy-transfer characteristics of the chlorin–bacteriochlorin
dyad FbC-pe-FbB are compared with those of a porphyrin–
porphyrin dyad and an oxochlorin–oxochlorin dyad, each of
which contains a phenylethyne linker (39). The chlorin–
bacteriochlorin, oxochlorin–oxochlorin and porphyrin–por-
phyrin dyads exhibit strong red ⁄ NIR, modest red and weak
red absorption, respectively. Insights into the mechanism of
energy transfer are obtained by Forster energy-transfer calcu-
lations and by density functional theory (DFT) calculations.
The former concern the through-space (TS) interactions
between the donor and acceptor constituents, while the latter
elucidate the molecular-orbital (MO) characteristics of the
energy donor and acceptor constituents. The electron-density
distributions in the frontier MOs provide insights into
through-bond (TB) electronic interactions that can also
contribute to the energy-transfer process. Collectively, the
results and comparisons of the energy-transfer characteristics
of this set of dyads provide added insight into the design
principles that produce ultrafast and essentially quantitative
excited-state energy flow among tetrapyrrole constituents in
covalently linked arrays.
5-(2-Phenylethynyl)-10,15,20-triphenylporphyrin.
A solution of
Zn(II)-5-(phenylethynyl)-10,15,20-triphenylporphyrin (4.2 mg, 0.0060
mmol) in CH2Cl2 (1.0 mL) was treated dropwise with trifluoroacetic
acid (TFA, 25 lL, 0.18 mmol) for 1 min. The solution was stirred at
room temperature for 3 h. CH2Cl2 was added, and the reaction
mixture was washed (saturated aqueous NaHCO3, water, brine) and
dried (Na2SO4). The organic layer was concentrated and chromato-
graphed (silica, hexanes then hexanes ⁄ CH2Cl2 [1:2]) to give a purple
solid (2.8 mg, 73%): 1H NMR d –2.32 (brs, 2H), 7.49–7.50 (m, 1H),
7.56–7.60 (m, 2H), 7.72–7.81 (m, 9H), 8.04 (d, J = 7.2 Hz, 2H), 8.17–
8.22 (m, 6H), 8.76 (s, 4H), 8.92 (d, J = 4.4 Hz, 2H), 9.75 (d,
J = 4.4 Hz, 2H); LD-MS obsd 639.6; ESI-MS obsd 639.2526, calcd
639.2543 [(M + H)+, M = C46H30N4]; kabs (toluene) 434, 533, 574,
609, 668 nm; kem (toluene) 671 nm; Ff (Ar-purged toluene) 0.11; s
(Ar-purged toluene) 11.5 ns.
Zn(II)-3,13-Bis(2-phenylethynyl)-17,18-dihydro-18,18-dimethyl-10-
mesitylporphyrin (ZnC1). Following a standard procedure (13,57,58),
samples of ZnC3 (19.3 mg, 0.0284 mmol) and phenylacetylene
(18.7 lL, 0.175 mmol) were coupled using Pd2(dba)3 (5.20 mg,
0.00568 mmol) and P(o-tol)3 (10.4 mg, 0.0341 mmol) in toluene ⁄ TEA
(5:1, 12 mL) at 60ꢁC under argon. After 6 h, phenylacetylene (18.7 lL,
0.175 mmol), Pd2(dba)3 (5.20 mg, 0.00568 mmol), and P(o-tol)3
(10.4 mg, 0.0341 mmol) were added to the reaction mixture. After
12 h, the reaction mixture was concentrated under reduced pressure.
The resulting residue was chromatographed (silica, hexanes then
hexanes ⁄ CH2Cl2 [1:1]) to afford a green solid (14.8 mg, 72%): 1H
NMR (300 MHz) d 1.88 (s, 6H), 2.04 (s, 6H), 2.60 (s, 3H), 4.55 (s, 2H),
7.23 (s, 2H), 7.41–7.54 (m, 6H), 7.78–7.92 (m, 4H), 8.38
(d, J = 4.5 Hz, 1H), 8.53 (s, 1H), 8.66 (s, 1H), 8.84 (s, 1H), 8.86
(d, J = 4.5 Hz, 1H), 8.99 (s, 1H), 9.82 (s, 1H), LD-MS obsd 721.3;
FAB-MS obsd 720.2261, calcd 720.2231 (C47H36N4Zn); kabs (toluene)
426, 652 nm.
MATERIALS AND METHODS
Synthesis. General. 1H NMR (400 MHz) and 13C NMR (75 MHz)
spectra were collected at room temperature in CDCl3 unless noted
otherwise. Absorption spectra were obtained at room temperature in
toluene or CH2Cl2. Chlorins and the chlorin–bacteriochlorin dyad
were analyzed by laser desorption mass spectrometry (LD-MS) in the
absence of a matrix (56). Fast-atom bombardment mass spectrometry
(FAB-MS) or electrospray ionization mass spectrometry (ESI-MS)
data are reported for the molecule ion or protonated molecule ion.
Melting points are uncorrected. Preparative size-exclusion chromato-
graphy (SEC) was performed using Bio-Rad Bio-Beads SX-1 (200–400
mesh). Analytical SEC was performed on a HPLC using two columns
˚
3,13-Bis(2-phenylethynyl)-17,18-dihydro-18,18-dimethyl-10-mesityl-
porphyrin (FbC1). A solution of ZnC1 (9.0 mg, 0.012 mmol) in
CH2Cl2 (0.4 mL) was treated dropwise with TFA (29 lL, 0.37 mmol)
for 2 min. The solution was stirred at room temperature for 3 h.
CH2Cl2 was added, and the reaction mixture was washed (saturated
aqueous NaHCO3, water, brine) and then dried (Na2SO4). The organic
layer was concentrated and chromatographed (silica, hexanes then
hexanes ⁄ CH2Cl2 [6:4]) to afford a green solid (7.1 mg, 87%): 1H NMR
(300 MHz) d –1.74 (brs, 1H), –1.45 (brs, 1H), 1.88 (s, 6H), 2.06 (s, 6H),
2.61 (s, 3H), 4.65 (s, 2H), 7.25 (s, 2H), 7.44–7.58 (m, 6H), 7.85–7.99
(m, 4H), 8.43 (d, J = 4.5 Hz, 1H), 8.72 (s, 1H), 8.79 (s, 1H), 8.91 (d,
J = 4.5 Hz, 1H), 9.00 (s, 1H), 9.25 (s, 1H), 9.98 (s, 1H), LD-MS obsd
(500, 500 A in series, 30 cm each) with absorption spectral detection
(752 nm) and elution with tetrahydrofuran (THF, 0.8 mL min)1). All
commercially available materials were used as received. All palladium-
coupling reactions were carried out using standard Schlenk-line
techniques. The Sonogashira coupling reactions were carried out using
tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] and the ligand
tri-o-tolylphosphine [P(o-tol)3] in the absence of copper reagents
(57,58). Palladium insertion and transmetalation of hydroporphyrins
have not been observed under these conditions. Toluene and tri-
ethylamine (TEA) were degassed with argon for 5 min prior to use.
Noncommercial compounds. The following compounds were
prepared via literature procedures: Zn(II)-5-(2-phenylethynyl)-10,15,
20-triphenylporphyrin (59), 1-formyl-5-(4-(2-(triisopropylsilyl)ethynyl)
phenyl)dipyrromethane (1) (60), 8-bromo-2,3,4,5-tetrahydro-1,3,3-
trimethyldipyrrin (3) (13), Zn(II)-3,13-dibromo-17,18-dihydro-10-
mesityl-18,18-dimethylporphyrin (ZnC3) (13), FbB1 (12), FbB2 (17)
and 15-bromo-5-methoxy-8,8,18,18-tetramethyl-2,12-di-p-tolylbacter-
iochlorin (FbB3) (17).
660.3; FAB-MS obsd 659.3189, calcd 659.3175 [(M
C47H38N4]; kabs (toluene) 429, 676 nm.
+
H)+, M =
3,13-Bis(2-phenylethynyl)-17,18-dihydro-18,18-dimethyl-10-(4-ethyn-
ylphenyl)porphyrin (FbC2). A solution of 3,13-bis(2-phenylethynyl)-17,
18-dihydro-18,18-dimethyl-10-(4-(2-(triisopropylsilyl)ethynyl)phenyl)-
porphyrin (FbC5) (22.1 mg, 0.0227 mmol) in 3.0 mL of THF was
treated with tetrabutylammonium fluoride (TBAF, 0.138 mL, 5 equiv,
1 M in THF) and stirred for 1 h at room temperature. The mixture was
poured into CH2Cl2. The organic layer was extracted (5% NaHCO3,
H2O), dried (Na2SO4) and concentrated. The resulting solid was
chromatographed (silica, hexanes then hexanes ⁄ CH2Cl2 [4:1]) to give a
green solid (7.2 mg, 41%): 1H NMR d –1.91 (brs, 1H), –1.50 (brs, 1H),
2.06 (s, 6H), 3.31 (s, 1H), 4.67 (s, 2H), 7.44–7.60 (m, 6H), 7.85–7.90 (m,
4H), 7.96–8.00 (m, 2H), 8.10–8.12 (m, 2H), 8.58 (d, J = 4.5 Hz, 1H),
8.82 (s, 1H), 8.88 (s, 1H), 8.98 (d, J = 4.5 Hz, 1H), 9.02 (s, 1H), 9.31 (s,
1H), 10.03 (s, 1H); LD-MS obsd 640.7; FAB-MS obsd 640.2607, calcd
640.2627 (C46H32N4); kabs (toluene) 429, 676 nm.
8,9-Dibromo-1-formyl-5-[4-(2-(triisopropylsilyl)ethynyl)phenyl]di-
pyrromethane (2). Following a procedure for 8,9-dibromination of
1-formyldipyrromethanes (13), a solution of 1 (410 mg, 0.952 mmol) in
dry THF (9.5 mL) at )78ꢁC under argon was treated portionwise with
N-bromosuccinimide (NBS, 339 mg, 1.90 mmol). The reaction mixture
was stirred for 1 h at )78ꢁC. The mixture was allowed to warm to
)20ꢁC, whereupon a mixture of hexanes was added. The reaction
mixture was then allowed to warm to 0ꢁC. The organic layer was
washed with ice-cold water, dried (K2CO3) and concentrated at
ambient temperature. The resulting brown solid was purified by
column chromatography (silica, hexanes ⁄ CH2Cl2 ⁄ ethyl acetate [7:2:1])
to afford a yellow solid (0.406 g, 73%): mp 78–80ꢁC (dec); 1H NMR
(300 MHz, THF-d8) d 1.02–1.14 (m, 21H), 5.42 (s, 1H), 5.68–5.71 (m,
1H), 5.88–5.92 (m, 1H), 6.80–6.83 (m, 1H), 7.17 (d, J = 8.1 Hz, 2H),
7.41 (d, J = 8.1 Hz, 2H), 9.41 (s, 1H), 10.84 (brs, 1H), 11.24 (brs, 1H);
13C NMR (THF-d8) d 12.4, 19.2, 45.1, 90.9, 98.4, 100.3, 108.4, 111.2,
Alternative streamlined procedure for FbC2. A solution of Zn(II)-3,
13-bis(2-phenylethynyl)-17,18-dihydro-18,18-dimethyl-10-(4-(2-(triiso-
propylsilyl)ethynyl)phenyl)-porphyrin (ZnC5) (20.8 mg, 0.0242 mmol)
in 2.6 mL of THF was treated with TBAF (72.5 lL, 3 equiv, 1 M in
THF) and stirred for 0.5 h at room temperature. The mixture was

Photochemistry and Photobiology, 2008, 84 789
poured into CH2Cl2. The organic layer was extracted (5% NaHCO3,
H2O), dried (Na2SO4) and concentrated. The resulting crude product
was dissolved in CH2Cl2 (1.0 mL) and then treated dropwise with
TFA (101 lL, 1.30 mmol) for 2 min. The solution was stirred at room
temperature for 4 h. CH2Cl2 was added, and the reaction mixture was
washed (saturated aqueous NaHCO3, water, brine) and then dried
(Na2SO4). The organic layer was concentrated and chromatographed
(silica, hexanes then hexanes ⁄ CH2Cl2 [7:3]) to afford a green solid
(13.2 mg, 85%): 1H NMR d –1.91 (brs, 1H), –1.50 (brs, 1H), 2.06 (s,
6H), 3.31 (s, 1H), 4.67 (s, 2H), 7.44–7.60 (m, 6H), 7.85–7.90 (m, 4H),
7.96–8.00 (m, 2H), 8.10–8.12 (m, 2H), 8.58 (d, J = 4.5 Hz, 1H), 8.82
(s, 1H), 8.88 (s, 1H), 8.98 (d, J = 4.5 Hz, 1H), 9.02 (s, 1H), 9.31 (s,
1H), 10.03 (s, 1H); LD-MS obsd 640.7; FAB-MS obsd 640.2607, calcd
640.2627 (C46H32N4); kabs (toluene) 429, 676 nm.
concentrated and then further purified by preparative SEC (THF) and
column chromatography (silica, hexanes ⁄ CH2Cl2 [4:6]). (The final
silica column chromatography procedure removes any residual impur-
ities including polystyrene-derived fragments from the SEC column.)
The desired fraction was concentrated to give a purple solid (9.3 mg,
44%): 1H NMR (300 MHz) d –1.98 (brs, 1H), –1.62 (brs, 1H), –1.45
(brs, 1H), –1.17 (brs, 1H), 1.94, 2.01 (2s, 12H), 2.04 (s, 6H), 2.62 (s,
6H), 4.38 (s, 2H), 4.50–4.54 (m, 3H), 4.67 (s, 2H), 4.79 (s, 2H), 7.20–
7.64 (m, 12H), 7.85–8.30 (m, 10H), 8.6 (s, 1H), 8.67–8.77 (m, 3H), 8.83
(brs, 1H), 8.92–8.94(m, 2H), 9.04 (brs, 1H), 9.32 (brs, 1H), 9.87 (s, 1H),
9.88 (s, 1H); LD-MS obsd 1218.2; FAB-MS obsd 1219.5783,
calcd 1219.5751 [(M + H)+, M = C85H70N8O]; kabs (toluene) 387,
430, 554, 675, 756 nm. Note: The column chromatography (silica) of
FbC-pe-FbB should be performed in an expeditious manner, as the
compound decomposed during lengthy (ꢀ3 h) purification on
silica. Following isolation, the dyad was quite stable upon routine
handling in toluene and upon prolonged storage (>1 year at 0ꢁC).
Photophysical measurements. Static absorption (Varian Cary 100)
and fluorescence (Spex Fluorolog Tau 2) measurements were per-
formed as described previously for other compounds, typically for very
dilute (lM) solutions of the compounds in toluene (61,62). Fluore-
scence lifetimes were obtained using a phase-modulation technique
(61). Argon-purged solutions with an absorbance of £0.10 at kexc were
used for the fluorescence spectral and lifetime measurements. For
fluorescence spectra, the excitation or detection monochromator
typically had a band pass of 1.8 or 3.7 nm, respectively, and spectra
were obtained using 0.2 nm data intervals. The emission spectra were
corrected for detection-system spectral response. Fluorescence quan-
tum yields were determined for argon-purged solutions of the
compound relative to chlorophyll a in benzene (Ff = 0.325 [63]) and
were corrected for solvent refractive index. Transient absorption
measurements utilized 5–10 lM solutions excited with 130 fs, 8 lJ,
431 nm flashes and probed with white-light pulses of comparable
durations (64). All measurements were carried out at room temper-
ature.
Zn(II)-3,13-Dibromo-17,18-dihydro-18,18-dimethyl-10-[4-(2-(tri-
isopropylsilyl)ethynyl)-phenyl]porphyrin (ZnC4). Following a stream-
lined procedure (15), a solution of 2 (301 mg, 0.511 mmol) and 3
(138 mg, 0.511 mmol) in anhydrous CH2Cl2 (16 mL) was treated with
a
solution of p-toluenesulfonic acid (p-TsOHÆH2O, 486 mg,
2.55 mmol) in anhydrous methanol (4 mL) under argon. The red
reaction mixture was stirred at room temperature for 50 min. A sample
of 2,2,6,6-tetramethylpiperidine (0.866 mL, 5.00 mmol) was added.
The reaction mixture was concentrated. The resulting solid was
dissolved in CH3CN (51 mL) and subsequently treated with 2,2,6,6-
tetramethylpiperidine (2.17 mL, 12.7 mmol), Zn(OAc)2 (1.41 g,
7.66 mmol) and silver triflate (AgOTf, 394 mg, 1.53 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 ⁄ CH2Cl2
[1:2]) to afford a green solid (112.8 mg, 27%): 1H NMR (THF-d8) d
1.28 (brs, 21H), 2.04 (s, 6H), 4.60 (s, 2H), 7.82 (d, J = 7.8 Hz, 2H),
8.02 (d, J = 7.8 Hz, 2H), 8.43 (d, J = 4.0 Hz, 1H), 8.58–8.61 (m, 2H),
8.84 (s, 2H), 8.90 (d, J = 4.0 Hz, 1H), 9.68 (s, 1H); LD-MS obsd
814.3; FAB-MS obsd 814.0671, calcd 8.0680 (C39H40Br2N4SiZn); kabs
(toluene) 412, 622 nm.
Zn(II)-3,13-Bis(2-phenylethynyl)-17,18-dihydro-18,18-dimethyl-10-
[4-(2-(triisopropylsilyl)ethynyl)phenyl]porphyrin (ZnC5). Following
The fits of the transient-absorption time profiles for FbC-pe-FbB in
Fig. 3 utilized a function consisting of the instrument profile plus two
exponentials plus a constant. The shorter and longer components
reflect the lifetimes of the excited chlorin and bacteriochlorin compo-
nents, respectively. The same time constant for the shorter component
is derived within the reported error limits for any of the following
conditions: (1) the time constant for the longer component is a free
fitting parameter, (2) the time constant for the longer component is
fixed to have the 5.4 ns lifetime of the excited bacteriochlorin measured
by fluorescence detection, or (3) the time profiles are truncated at
ꢀ100 ps and fit to a function containing a single exponential decay.
Local and global data fitting were carried out using routines written
with IgorPro software (Wavemetrics, Inc.).
a
standard procedure (13,57,58), samples of ZnC4 (54.6 mg,
0.0667 mmol) and phenylacetylene (59.5 lL, 0.533 mmol) were
coupled using Pd2(dba)3 (12.2 mg, 0.0133 mmol) and P(o-tol)3
(24.4 mg, 0.0800 mmol) in toluene ⁄ TEA (5:1, 30 mL) at 60ꢁC under
argon. After 12 h, the reaction mixture was concentrated under
reduced pressure. The resulting residue was chromatographed (silica,
hexanes then hexanes ⁄ CH2Cl2 [1:1]) to afford a green solid (41.6 mg,
72%): 1H NMR (300 MHz) d 1.26–1.28 (brs, 21H), 2.03 (s, 6H), 4.54
(s, 2H), 7.44–7.54 (m, 6H), 7.78–7.85 (m, 4H), 7.90–7.93 (m, 2H), 8.00–
8.03 (m, 2H), 8.45 (d, J = 4.5 Hz, 1H), 8.50 (s, 1H), 8.76 (s, 1H), 8.80
(s, 1H), 8.81 (d, J = 4.5 Hz, 1H), 9.00 (s, 1H), 9.70 (s, 1H); LD-MS
obsd 855.3; FAB-MS obsd 858.3119, calcd 858.3096 (C55H50N4SiZn);
kabs (toluene) 427, 652 nm.
Calculations. Forster calculations. Calculations of the rate of
¨
excited-state energy transfer assuming a Forster TS mechanism were
carried out using PhotochemCad (65). The calculations were per-
formed for three phenylethyne-linked dyads in toluene: chlorin–
bacteriochlorin FbC-pe-FbB, oxochlorin–oxochlorin ZnO-pe-FbO
and porphyrin–porphyrin ZnP-pe-FbP. The following parameters
were utilized in the calculations for all three dyads: dielectric constant,
1.496; orientation factor j2 = 1.125 on the basis of similar calcula-
tions on other tetrapyrrole dyads (38,66); center-to-center distance,
3,13-Bis(2-phenylethynyl)-17,18-dihydro-18,18-dimethyl-10-[4-(2-
(triisopropylsilyl)ethynyl)phenyl]porphyrin (FbC5).
A solution of
ZnC5 (37.4 mg, 0.0434 mmol) in CH2Cl2 (1.4 mL) was treated
dropwise with TFA (101 lL, 1.30 mmol) for 2 min. The solution
was stirred at room temperature for 3 h. CH2Cl2 was added, and the
reaction mixture was washed (saturated aqueous NaHCO3, water,
brine) and then dried (Na2SO4). The organic layer was concentrated
and chromatographed (silica, hexanes then hexanes ⁄ CH2Cl2 [8:2]) to
afford a green solid (26.8 mg, 77%): 1H NMR d –1.92 (brs, 1H), –1.50
(brs, 1H), 1.25–1.28 (brs, 21H), 2.07 (s, 6H), 4.66 (s, 2H), 7.44–7.60 (m,
6H), 7.86–7.90 (m, 4H), 7.97–8.00 (m, 2H), 8.10–8.12 (m, 2H), 8.60 (d,
J = 4.5 Hz, 1H), 8.82 (s, 1H), 8.90 (s, 1H), 8.98 (d, J = 4.5 Hz, 1H),
9.02 (s, 1H), 9.31 (s, 1H), 10.02 (s, 1H); LD-MS obsd 796.9; FAB-MS
obsd 797.4073, calcd 797.4040 [(M + H)+, M = C55H52N4Si]; kabs
(toluene) 429, 676 nm.
Chlorin–bacteriochlorin dyad FbC-pe-FbB. Following a reported
procedure (17,57–59), samples of FbC2 (11.2 mg, 0.0175 mmol) and
FbB3 (11.5 mg, 0.0175 mmol) were coupled using Pd2(dba)3 (3.20 mg,
0.00350 mmol) and P(o-tol)3 (6.92 mg, 0.0227 mmol) in toluene ⁄ TEA
(5:1, 8.0 mL) at 60ꢁC under argon. After 6 h, Pd2(dba)3 (3.20 mg,
0.00350 mmol) and P(o-tol)3 (6.92 mg, 0.0227 mmol) were added to
the reaction mixture. After 13 h, the reaction mixture was concen-
trated under reduced pressure. The resulting residue was filtered
through a pad of silica (hexanes ⁄ CH2Cl2, [1:2]). The eluate was
˚
R = 15.6 A. For all three dyads, these calculations assumed that (1)
the dipole–dipole approximation is valid, (2) the same center-to-center
separation of the macrocycles is the relevant distance even though the
three dyads may differ in the extent to which electron density is
delocalized onto the linker and other peripheral groups, (3) the same
orientation factor applies even though there may be differences in the
alignment of the relevant transition dipoles. These assumptions are
reasonable considering the uncertainties involved. Moreover, the range
of uncertainties is not sufficient to account for the significant
differences in the Forster rates that are calculated among the different
classes of dyads (vide infra).
The Forster calculations on the three dyads also employed the
following dyad-specific parameters. Dyad FbC-pe-FbB: FbC fluore-
scence yield Ff = 0.42 and excited-state lifetime s = 8.0 ns (for
reference compound FbC2); FbB molar absorption coefficient
ꢀ = 120 000 M)1cm)1 at the Qy(0,0) maximum of 755 nm on the
basis of a literature value for the parent compound FbB1 (12); also see

790 Chinnasamy Muthiah et al.
Ref. 67 for estimates for the 15-arylethynyl analog thereof. Dyad ZnO-
pe-FbO: ZnO fluorescence yield Ff = 0.04 and excited-state lifetime
s = 0.7 ns; FbO molar absorption coefficient ꢀ = 54 000 M)1cm)1 at
the Qy(0,0) maximum of 658 nm on the basis of values for reference
compounds (39). Dyad ZnP-pe-FbP: ZnP fluorescence yield
Ff = 0.035 and excited-state lifetime s = 2.4 ns on the basis of
values for related reference compounds (38,39); FbP molar absorption
coefficient ꢀ = 45 000 M)1cm)1 at the Qy (0,0) maximum of 574 nm,
which was obtained using the spectrum for 5-(2-phenylethynyl)-
10,15,20-triphenylporphyrin obtained herein and assuming that the
latter compound has the same e value at the Qy(1,0) maximum as that
for an analog in which the phenyl rather than the ethynyl moiety of the
phenylethynyl group is attached to the macrocycle, namely Zn(II)-5-(4-
ethynylphenyl)-10,15,20-trimesitylporphyrin (38). Finally, an addi-
tional calculation was performed on FbC-pe-FbB using Ff ꢀ 0.3 and
ꢀ ꢀ 80 000 M)1cm)1 to examine how reasonable extremes in these
parameters (based on data from related chlorins and bacteriochlorins)
would affect the calculated Forster rate.
Density functional theory calculations. Density functional theory
calculations were performed with Spartan ¢06 for Windows (Wave-
function, Inc.) (67) on a PC (Dell Optiplex GX270) equipped with a
3.2 GHz CPU and 3 GB RAM. The hybrid B3LYP functional and a
6–31G* basis set were employed. For the calculations on each
complex, the complete structure was used in the geometry optimiza-
tion. The equilibrium geometries were fully optimized using the default
parameters of the Spartan ¢06 program.
RESULTS
Synthesis
General. The synthetic approach to the free base chlorin–
bacteriochlorin dyad relies on the following pathways: (1) a
rational synthesis of chlorin building blocks (13), which
affords a chlorin containing 3,13-bis(phenylethynyl) and 10-
ethynylphenyl groups, and (2) a copper-free Pd-catalyzed
Sonogashira coupling (57–59) of the ethynylphenylchlorin and
a bromobacteriochlorin (17) to give the phenylethyne-linked
chlorin–bacteriochlorin construct.
Scheme 1.
2.0 molar equiv of NBS at )78ꢁC gave the 8,9-dibromo-1-
formyldipyrromethane 2 in 72% yield (Scheme 2). The bro-
mination occurs chemoselectively at the pyrrole unit in the
presence of the ethyne and the formylpyrrole moieties, and
regioselectively at the vicinal a-and b- (i.e. 8- and 9-) positions
of the pyrrole. The stability of dibromo compound 2 is similar
to that of analogous dibromo compounds (13). The 8,9-
dibromo-substitution pattern in 2 was established by NMR
spectroscopy (HH-COSY and NOESY experiments). Com-
pound 2 serves as the Eastern half for the chlorin-forming
reaction. The bromo-substituted Western half 3 was prepared
following literature procedures (13).
Benchmark compounds. Four compounds were employed for
comparison in the photophysical studies (Chart 2). Free base
bacteriochlorins FbB1 (12) and FbB2 (17) were prepared using
literature procedures, whereas the chlorins FbC1 and FbC2
were prepared herein. The synthesis of free base chlorin FbC1,
which bears 3,13-phenylethynyl groups and a 10-mesityl
substituent, is shown in Scheme 1. The conversion of 3,13-
dibromochlorin ZnC3 to the corresponding 3,13-bis(phenyl-
ethynyl)chlorin was performed by the copper-free Pd-mediated
Sonogashira coupling using phenylacetylene. This method
avoids possible transmetalation during the reaction with the
zinc chelate. Thus, reaction of 3,13-dibromochlorin ZnC3 (13)
with phenylacetylene in the presence of Pd2(dba)3 and P(o-tol)3
gave 3,13-bis(phenylethynyl)chlorin ZnC1 in 72% yield. De-
metalation of zinc chlorin ZnC1 with TFA in CH2Cl2 at room
temperature gave the free base 3,13-bis(phenylethynyl)chlorin
FbC1 in 87% yield. Compound FbC1 was characterized by
absorption spectroscopy, 1H NMR spectroscopy, LD-MS and
FAB-MS analyses. The synthesis of free base chlorin FbC2, a
benchmark and precursor to the dyad, is described below.
Chlorin formation. The synthesis of the 3,13-dibromochlorin
ZnC4 is shown in Scheme 2. The condensation of Eastern
half 2 and Western half 3 in anhydrous CH2Cl2 was
performed using a solution of p-TsOHÆH2O in anhydrous
methanol under argon, affording a clear reddish-brown
solution for 40–50 min. Neutralization with 2,2,6,6-tetra-
methylpiperidine followed by concentration of the reaction
mixture afforded a yellow solid. This crude tetrahydrobila-
diene-ab was subjected to metal-mediated oxidative cycliza-
tion (2,2,6,6-tetramethylpiperidine, Zn(OAc)2 and AgOTf) in
refluxing acetonitrile in the presence of air for 20 h,
affording the zinc chelate of the 3,13-dibromochlorin
(ZnC4) in 27% yield starting from 2. Compound ZnC4
Chlorin precursors. The synthesis of chlorins relies on the
condensation of an Eastern half and a Western half. The
synthesis of the Eastern half employed herein begins with
known 1-formyldipyrromethane 1 (60). Treatment of 1 with

Photochemistry and Photobiology, 2008, 84 791
Scheme 2.
was characterized by 1H NMR spectroscopy, LD-MS and
FAB-MS analyses.
Scheme 3.
The synthesis of the chlorin benchmark and building block
FbC2 is shown in Scheme 3. The Sonogashira coupling of
dibromochlorin ZnC4 and phenylacetylene was carried out in
the presence of Pd2(dba)3 (20 mol%) and P(o-tol)3 in tolu-
ene ⁄ TEA (5:1) at 60ꢁC. After 18 h, workup including column
chromatography gave 3,13-bis(phenylethynyl)chlorin ZnC5 in
72% yield. Demetalation of ZnC5 with TFA afforded the free
base 3,13-bis(phenylethynyl)chlorin FbC5 in 77% yield. The
free base chlorin FbC5 serves as the benchmark compound for
the chlorin–bacteriochlorin dyad in photophysical studies. The
cleavage of the triisopropylsilyl group in free base chlorin FbC5
using TBAF afforded the desired chlorin FbC2 in only 40%
yield (along with the formation of several unknown chlorins),
whereas use of the zinc chelate ZnC5 gave quantitative
cleavage. Demetalation of the crude product with TFA
afforded the free base chlorin FbC2 in 85% yield starting from
ZnC5. The chlorins were characterized by absorption spectros-
copy, 1H NMR spectroscopy, LD-MS and FAB-MS analyses.
dyads (59). Thus, treatment of ethynylphenylchlorin FbC2 and
bromobacteriochlorin FbB3 under the standard reaction con-
ditions (20 mol% Pd2(dba)3 and P(o-tol)3 in toluene ⁄ TEA [5:1]
at 60ꢁC with 1 equiv of each hydroporphyrin) afforded
FbC-pe-FbB in 62% yield as judged by analytical SEC.
The purification of the FbC-pe-FbB proceeds as follows:
(1) removal of palladium reagents by silica-pad filtration,
(2) separation of the target dyad by gravity-flow SEC and (3)
final silica column chromatography to remove residual impur-
ities. This three-column sequence has been used extensively to
good effect in the synthesis of other dyads and larger arrays
(57,58). In this manner, FbC-pe-FbB was isolated in 44% yield
(Scheme 4). FbC-pe-FbB was characterized by absorption and
fluorescence spectroscopy, 1H NMR spectroscopy, LD-MS
and FAB-MS analyses.
Photophysical characterization
Phenylethyne-linked chlorin–bacteriochlorin dyad. The synthe-
sis of FbC-pe-FbB was achieved through use of the copper-free
Sonogashira coupling reaction (57,58), which has been applied
for the synthesis of analogous phenylethyne-linked porphyrin
Absorption spectra. The absorption spectra for the FbC-
pe-FbB dyad and the benchmark monomers chlorin FbC1
and bacteriochlorin FbB2 in toluene are shown in Fig. 1 (solid

792 Chinnasamy Muthiah et al.
Several points regarding the absorption spectra are
noteworthy. (1) The prominent low-energy feature (755 nm)
in the spectrum of FbC-pe-FbB is exclusively the bacterio-
chlorin Qy(0,0) band, which is expected to have a molar
absorption coefficient of ꢀ120 000 M)1 cm)1 by analogy
with the results for a related bacteriochlorin (12,68). (2) The
spectra of chlorin reference compounds FbC1, FbC2 and
FbC5 (Chart 2, Scheme 3) are basically indistinguishable.
This result indicates the minimal effect of attachment of the
ethyne group on the meso-aryl ring of the chlorin, similar to
previous findings on porphyrin analogs (66). (3) The main
absorption bands for the ethynylbacteriochlorin in FbC-pe-
FbB and benchmark compound FbB2 (387, 548, 754 nm) are
significantly redshifted compared to an analog FbB1, which
lacks the ethynyl substituent (373, 511, 731 nm). Similarly,
the bacteriochlorin Qy(0,0) ⁄ B(0,0) peak-intensity ratios for
FbC-pe-FbB and FbB2 are larger than in FbB1. These
spectral characteristics parallel those observed when an
ethynyl group is directly appended to synthetic chlorins
(20,21).
Fluorescence spectra, quantum yields and lifetimes. Fluorescence
spectra of FbC-pe-FbB, chlorin FbC1 and bacteriochlorin
FbB2 in toluene are shown in Fig. 1 (dashed spectra). The
wavelengths of the prominent fluorescence features are listed
in Table 1. Several points are particularly noteworthy
regarding these spectra. The chlorin fluorescence (681 nm)
observed for FbC-pe-FbB using several different excitation
wavelengths (387, 430, 515, 550 nm) is reduced in intensity at
least 50-fold compared to the level in monomers FbC1, FbC2
and FbC5. In contrast, the intensity of the bacteriochlorin
fluorescence (760 nm) in FbC-pe-FbB is similar to that found
for reference compound FbB2 regardless of whether the
chlorin or bacteriochlorin component of the dyad is primarily
excited; the corresponding fluorescence yield is Ff = 0.19
(Table 1). Similarly, the lifetime of the lowest-energy (Qy)
singlet excited state of the bacteriochlorin constituent of FbC-
Scheme 4.
pe-FbB in toluene (s = 5.4
experimental error as that measured for reference monomer
FbB2 (5.7 0.6 ns) or for an analogous 5-methoxy-15-
(arylethynyl)bacteriochlorin (Ff = 0.16, s = 5.3 ns) exam-
ined previously (68). Thus, upon excitation of the chlorin,
energy flows in very high yield to the bacteriochlorin. The
resulting excited bacteriochlorin then behaves as though it
were an isolated ethynylbacteriochlorin, with no (charge-
transfer) quenching due to the presence of the chlorin
constituent.
0.5 ns) is the same within
spectra). The wavelengths of the main absorption bands for
these compounds and additional reference monomers are listed
in Table 1. The spectrum of FbC-pe-FbB is essentially the sum
of the spectra of the benchmark monomers. This result is a
manifestation of relatively weak (but not insignificant) elec-
tronic coupling between the two constituents of the dyad. The
phenylethyne-linked arrays are thus quite distinct from the
arrays wherein an ethynyl moiety directly joins two tetrapyr-
role macrocycles; the latter exhibit profound spectral changes
because of strong electronic coupling between the macrocycles.
Table 1. Summary of photophysical data.*
Chlorin
Bacteriochlorin
s (ns)
Compound
kB (nm)
kQ (nm)
kem (nm)
Ff
s (ns)
Ff
y
FbC-pe-FbB
FbC5
FbC2
FbC1
FbB2
FbB1
387, 430
429
430
429
387
755
676
675
676
754
731
760
677
677
678
759
736
<0.01
0.39
0.42
0.0048
0.0002
0.5
0.5
0.19
5.4
0.5
8.0
8.0
8.1
0.37
0.5
0.19
0.18
5.7
4.8
0.6
0.5
373
*For compounds in toluene at room temperature.

Photochemistry and Photobiology, 2008, 84 793
Figure 2. Time-resolved absorption difference spectra for FbC-pe-FbB
in toluene at room temperature at two time delays following excitation
with a 130 fs, 431 nm flash.
Figure 1. Normalized absorption spectra (solid) and fluorescence
spectra (dashed) for (A) FbC-pe-FbB, (B) benchmark bacteriochlorin
monomer FbB2 and (C) benchmark chlorin monomer FbC1 in toluene
at room temperature. The excitation wavelengths used to elicit the
fluorescence are 554 nm (FbC-pe-FbB), 548 nm (FbB2) and 550 nm
(FbC1).
Transient absorption studies. Figure 2 shows representative
spectra
time-resolved
absorption
difference
(DA =
Atransient state ) Aground state) for FbC-pe-FbB in toluene. Typical
kinetic profiles are shown in Fig. 3. The data were acquired
using a 130 fs excitation flash at 431 nm, which is absorbed
primarily (ꢀ85%) by the chlorin constituent of the dyad (see
Fig. 1). As a consequence, the chlorin Qy(0,0) ground-state
absorption band at 675 nm bleaches with the instrument
response. The feature at this wavelength in the spectrum at
0.5 ps (Fig. 2) also contains a contribution from chlorin
Qy(0,0) excited-state stimulated emission. This emission is
stimulated by the white-light probe pulse and is expected to
have a shape similar to the static fluorescence profile of the
chlorin, which is only slightly redshifted from the Qy(0,0)
ground-state absorption band (see Fig. 1C). The combined
Qy(0,0) bleaching and stimulated-emission feature of the
chlorin at 675 nm disappears by 30 ps (Fig. 2). This decay
Figure 3. Representative kinetic profiles and fits for (A) the decay of
the chlorin feature at 675 nm consisting of ground-state absorption
bleaching plus excited-state stimulated emission and (B) growth of the
analogous bacteriochlorin feature at 760 nm.
The decay of the chlorin feature at 675 nm is paralleled by
a growth in the analogous combined Qy(0,0) bleaching plus
stimulated emission due to the bacteriochlorin at 760 nm.
occurs with a time constant of 4.7
0.2 ps, determined from
This feature grows with a time constant of 4.9
0.2 ps
global analysis of the decay profiles at 2 nm wavelength
increments across this feature (Fig. 3A).
(Fig. 3B). As can be seen from the spectrum at 0.5 ps (Fig. 2),
a small fraction of the bacteriochlorin feature develops more

794 Chinnasamy Muthiah et al.
rapidly (with the instrument response). This latter contribu-
tion reflects the fact that the bacteriochlorin component of
the dyad absorbs ꢀ15% of the excitation photons. The
bacteriochlorin Qy(0,0) feature at 760 nm is fully developed
by ꢀ30 ps, in concert with the decay of the chlorin Qy(0,0)
feature. Subsequently, the difference spectrum does not
change shape over the 2 ns time course of the measurements;
however, the spectrum decays slightly in amplitude consistent
with the 5.4 ns lifetime of the Qy excited state of the
bacteriochlorin determined above using fluorescence detection
(Table 1).
In practice, calculation of the Forster TS process is
straightforward whereas that of the Dexter TB process is
beyond the scope of the present paper. Hence, the presence of
a TB process is typically inferred by the observation of an
energy-transfer rate that is substantially faster than the rate
calculated on the basis of TS (i.e. Forster) considerations.
Thus, the observed energy-transfer rate is the sum of the TS
and TB mechanisms, as shown in Eq. 2:
kENT ¼ kTS þ kTB
ð2Þ
Evidence for the TB mechanism can be obtained in
several ways: (1) by alteration of the energy levels of the
linker, (2) by structural constraints on the linker–
donor ⁄ acceptor attachment site such that conjugation
between the linker and the donor or acceptor is altered,
and (3) by attaching the linker to sites on the donor or
acceptor where the frontier MOs have significant electron
density versus nodes. None of these alterations is expected to
change the TS contribution to energy transfer as long as the
spectral features, distance of separation and transition-dipole
orientation factor remain unchanged. We previously inves-
tigated a wide variety of porphyrin–porphyrin dyads and
found that the dominant mechanism of energy transfer was
a TB process mediated by the linker, with only a minor
contribution via the Forster TS process (55). As part of this
earlier work, sets of porphyrin–porphyrin arrays were
designed to probe the TB process by each of the three
aforementioned structural ⁄ electronic alterations.
In the following sections, we first assess the TS versus TB
contributions to energy transfer in the chlorin–bacteriochlorin
dyad FbC-pe-FbB. In this regard, we expected that energy
transfer in this dyad would have a substantial, if not dominant,
contribution from a TS mechanism, given the strong oscillator
strength of the donor and acceptor transitions that give rise to
the lowest energy emission ⁄ absorption bands (i.e. in the
red ⁄ NIR). We then address the related question as to what
extent TB electronic interactions might be present, as evi-
denced by the electron-density distributions in the frontier
MOs of the energy donor and acceptor constituents of the
dyad. To provide additional perspective on the interplay of TS
and TB contributions to energy transfer in tetrapyrrole dyads
in general, we then compare the energy-transfer characteristics
of the chlorin–bacteriochlorin dyad FbC-pe-FbB with those
previously obtained for analogous phenylethyne-linked dyads
consisting of two porphyrins or two oxochlorins (39). These
comparisons are facilitated by Forster TS energy-transfer
calculations on the latter classes of dyads, as well as DFT
calculations on their energy-donor and acceptor constituents
that are aimed at assessing potential TB interactions (as
dictated by the electron-density distributions in the frontier
MOs).
Rate and yield of excited-state energy transfer. The character-
istics of the energy-transfer process are readily derived using
standard analysis embodied in Eq. (1)
kENT ¼ ðsdyadÞꢁ1 ꢁ ðsmConÞꢁ1
ð1aÞ
C
/
¼ kENT ꢂ sdCyad
ð1bÞ
ENT
Here, smCon is the lifetime of the excited chlorin benchmark
monomer, which is governed by the rates of fluorescence,
internal conversion and intersystem crossing. The lifetime of
the excited chlorin in dyad FbC-pe-FbB is taken to be the
average of the time constants for decay ⁄ growth of the
chlorin ⁄ bacteriochlorin features in the transient absorption
spectra described above, namely sdCyad = 4.8 ps. Thus, the
lifetime for the excited chlorin in FbC-pe-FbB is considerably
shorter than that for benchmark FbC1 (smCon = 8.1 ns)
because of energy transfer to the bacteriochlorin. Substitution
of the measured lifetimes into Eq. (1) gives a rate constant of
kENT = (4.8 ps))1 and a yield of FENT > 0.99 for the energy-
transfer process. Thus, excitation energy transfer from chlorin
to bacteriochlorin is rapid and essentially quantitative.
DISCUSSION
Understanding the mechanism(s) of energy transfer in mul-
ticomponent architectures is essential for the rational design
of systems wherein energy flow to a designated site occurs in
an efficient manner. In covalently linked architectures, energy
transfer can occur by two parallel mechanisms (55). One
mechanism is the well-known Forster process, which entails
the resonant coupling of the transition-dipole moments of the
excited donor and the ground-state acceptor. The expected
rate and efficiency of this process can be calculated for the
energy donor and acceptor interacting TS in the geometry
fixed by the covalent linker that joins the constituents in the
dyad, even though the presence of the linker is not a factor in
the calculation. The Forster process falls off as 1 ⁄ R6, where
R is typically taken as the center-to-center distance of the
donor and acceptor chromophores. The other mechanism is
the Dexter process, which entails the overlap of the wave-
functions of the excited donor and ground-state acceptor.
The Dexter process typically falls off exponentially with
distance. For dyads wherein the components are held more
than a few bond lengths apart, a significant TS component to
this electron-exchange interaction is precluded, and the
process is driven by TB coupling mediated by the covalent
linker that joins the donor and acceptor constituents of the
dyad.
Mechanism of energy transfer in chlorin–bacteriochlorin
dyad FbC-pe-FbB
The Forster calculations on the yield and rate of the TS
excitation energy-transfer from the chlorin to bacteriochlorin
in FbC-pe-FbB result in values of FTS = 0.99 and
kTS = (5.6 ps))1. This very fast TS rate is driven by the
strong transition dipole strengths of the hydroporphyrin

Photochemistry and Photobiology, 2008, 84 795
energy donor and acceptor constituents of the dyad. The
finding that the calculated Forster rate is only marginally
smaller than the measured value of kENT = (4.8 ps))1 implies
that the TS mechanism dominates energy transfer in the dyad.
However, before drawing a firm conclusion, it is useful to
explore how the uncertainties in the parameters used in the
Forster calculations (e.g. the fluorescence yield of the energy
donor and the molar absorption coefficient of the energy
acceptor) affect the calculated TS energy-transfer rate.
Accordingly, an additional calculation was performed in
which the above parameters were altered to encompass
reasonable lower limits based on data from related chlorins
and bacteriochlorins (see Materials and Methods). This
calculation resulted in a slightly slower Forster rate of
kTS = (12 ps))1. The latter value when compared with the
measured value of kENT = (4.8 ps))1 would imply that the TS
and TB mechanisms actually make comparable contributions
to the energy-transfer dynamics in FbC-pe-FbB (see Eq. 2).
Regardless, energy transfer is nearly quantitative because the
measured rate, as well as the Forster rate calculated with a
range of reasonable parameters, is at least 500-fold faster than
the rate of the competitive intrinsic (monomer-like) decay
processes in the excited chlorin.
Further insight into the extent to which TB electronic
interactions might complement the clearly substantial TS
contribution to energy transfer in FbC-pe-FbB can be gained
via examination of the electron-density distributions in the
frontier MO constituents of the dyad. The calculated electron-
density distributions are shown for the associated reference
compounds in Fig. 4. The characteristics of all four orbitals are
relevant because the Qy excited states of both the chlorin and
bacteriochlorin are composed of admixtures of promotions
from the highest-occupied molecular orbital (HOMO) to the
lowest-unoccupied molecular orbital (LUMO) and the second-
highest occupied molecular orbital (HOMO)1) to the second-
lowest unoccupied molecular orbital (LUMO+1) (69). In this
regard, recent DFT calculations indicate that the contributions
of the (HOMO fi LUMO) ⁄ (HOMO)1 fi LUMO+1) pro-
motions to the Qy excited state of chlorins and bacteriochlorins
are approximately 60 ⁄ 40 and 70 ⁄ 30, respectively (70). This ratio
of contributions is 50 ⁄ 50 for porphyrins.
Inspection of Fig. 4 reveals the following characteristics of
the frontier MOs for chlorin–bacteriochlorin dyad FbC-pe-
FbB.
(1) For the ethynylphenyl-substituted chlorin (Fig. 4A),
there is relatively little electron density delocalized onto the
ethynylphenyl group in any of the frontier MOs. (Note that
‘‘ethynylphenyl’’ implies the phenyl group is attached to the
chlorin macrocycle and the ethyne moiety is a substituent on
the phenyl ring. Conversely, ‘‘phenylethynyl’’ implies attach-
ment of the ethynyl group to the macrocycle. See Chart 2.)
(2) For the phenylethynyl-substituted bacteriochlorin
(Fig. 4B), there is little electron-density delocalized onto the
phenylethynyl group in either the HOMO or LUMO; however,
the extent of electron-density delocalization in the HOMO–1
and LUMO+1 is significant. In this regard, it should be noted
that the phenyl of the phenylethynyl group can rotate with
respect to the ethyne, and that the rotational barrier (governed
by steric interactions, etc.) is only a fraction of kT. Further-
more, the rotation angle impacts the extent to which electron
density delocalized from the macrocycle onto the ethyne
Figure 4. Electron-density distributions and energies of the frontier
molecular-orbitals of the reference monomeric components of FbC-pe-
FbB.
moiety is further delocalized onto the phenyl ring. For the
minimized structure shown in Fig. 4B, the phenyl group is
effectively coplanar with the bacteriochlorin macrocycle, so

796 Chinnasamy Muthiah et al.
that electron density in the HOMO)1 and LUMO+1 is
delocalized both onto the ethyne and the phenyl. For other
rotational angles, the electron density on the ethyne would
remain substantial but that on the phenyl will diminish.
(3) For both the chlorin and bacteriochlorin, the HOMO
has essentially no electron density at the meso position, which
is the site of linker attachment for both constituents of the
dyad. The latter result is expected because the HOMO of
hydroporphyrins is an a2(p) orbital, which is derived from the
porphyrin a1u(p) orbital that is the HOMO–1 for most meso-
substituted porphyrins (see below).
Collectively, the electron-density distributions predicted
for the constituents of FbC-pe-FbB imply relatively weak TB
electronic interactions. The relevant electron-density distri-
butions include the near-absence of electron density on the
linker in any of the frontier MOs of the energy-donor
chlorin, and the minimal electron-density on the linker of
the HOMO and LUMO of the energy-acceptor bacterio-
chlorin (which make the major contribution to the wave-
function of the Qy excited state [70]). Thus, the most
consistent picture of energy transfer in FbC-pe-FbB is one
wherein the TS contribution dominates. This is the result
predicted by the Forster calculations performed using
spectral parameters derived from benchmark monomers
versus the calculation performed using spectral parameters
that give a lower bound for TS interactions. Below, the
energy-transfer characteristics of the chlorin–bacteriochlorin
dyad are compared with those of different classes of
tetrapyrrole dyads.
Comparison with results on other tetrapyrrole dyads
The energy-transfer mechanism in the chlorin–bacteriochlo-
rin dyad can be compared with those that are operative in
dyads that we have previously examined. These latter dyads
are composed of either porphyrin (P) or oxochlorin (O)
constituents linked by the same phenylethyne linker present
in FbC-pe-FbB. In each case, the linker is attached to the
meso (as opposed to b-pyrrole) positions of the tetrapyrrole
constituents. The arrays studied earlier include the dyads
denoted ZnP-pe-FbP and ZnO-pe-FbO in Chart 3. In all
cases, the energy acceptor (i.e. bacteriochlorin in FbC-pe-
FbB, free base porphyrin in ZnP-pe-FbP and free base
oxochlorin in ZnO-pe-FbO) is the macrocycle that is directly
appended to the ethynyl group of the linker. This follows
because the singlet excited-state energy of the bacteriochlorin
is intrinsically lower than that of the chlorin, likewise for the
free base oxochlorin and porphyrin versus the metalated
analogs. The ethynyl group appended to the energy acceptor
serves to further lower the singlet excited-state energy, as
evidenced by the substantial (up to 30 nm) absorption band
shifts resulting from direct attachment of an ethynyl (or
phenylethynyl) group to tetrapyrroles (20,21,39,71–73). For
example, reference compound 5-(2-phenylethynyl)-10,15,20-
triphenylporphyrin (prepared here as a benchmark for the
energy acceptor in the Forster calculations for ZnP-pe-FbP)
has its Qx(0,0) absorption band at 668 nm (1.85 eV)
compared with 649 nm (1.91 eV) for an analog in which
the phenyl, rather than ethynyl, moiety of the phenylethynyl
group is attached to a meso position of the porphyrin
macrocycle.
Chart 3.
Porphyrin–porphyrin dyad. In the case of porphyrin–porphy-
rin dyad ZnP-pe-FbP, we found previously that energy
transfer from the zinc to free base subunit has a rate constant
of kENT > (1 ps))1 and an efficiency of FENT > 99%. In
contrast, the calculated Forster rate is kTS = (57 ps))1. The
fact that the TS rate is at least 50-fold smaller than the
measured value suggests that energy transfer in ZnP-pe-FbP is
dominated by an electron-exchange-mediated TB mechanism
(39). We previously found that the latter mechanism also
dominates in related porphyrin dyads employing a diphenyl-
ethyne linker or p-phenylene linker (55).
The significant TB contribution to energy transfer in the
porphyrin–porphyrin dyad can be rationalized in terms of the
characteristics of the frontier molecular orbitals (Fig. 5) as
follows.

Photochemistry and Photobiology, 2008, 84 797
(1) The HOMO of the porphyrins is a2u(p), which exhibits
substantial electron density at the meso position of the
macrocycle and is the site of linker attachment.
(2) Significant electron density is delocalized onto the ethyne
moiety of the phenylethynyl group for the HOMO and LUMO
of the phenylethynylporphyrin (Fig. 5B). However, there is
little delocalization further onto the phenyl for the minimized,
static structure shown. Nonetheless, even small rotations that
move the phenyl and macrocycle away from perpendicularity
will shift electron density from the intervening ethyne onto the
phenyl ring, as is seen in the minimized structure for the
analogous bacteriochlorin (Fig. 4B).
Collectively, the results and analysis support the dominance
of the TB mechanism for excited-state energy transfer between
meso-phenylethyne-linked porphyrins such as ZnP-pe-FbP. It
should be noted, however, that the dominance of the TB
mechanism for ZnP-pe-FbP does not imply that this mechanism
completely dominates energy transfer in all classes of porphyrin
dyads. In particular, we have shown that for porphyrin arrays
wherein the HOMOs of the constituents are switched from
a2u(p) to a1u(p), the overall energy-transfer rate is decreased by
an order of magnitude and that the TS process becomes
important (55). The decreased energy-transfer rate is a result
of diminished TB electronic coupling that arises because the
a1u(p) orbital has a node at the meso-site of linker attachment.
Oxochlorin–oxochlorin dyad. In the case of oxochlorin–oxo-
chlorin dyad ZnO-pe-FbO, we previously found that energy
transfer from the zinc to free base subunit has a measured
rate constant kENT = (10 ps))1 and an efficiency FENT
>
99%. The calculated Forster rate is kTS = (9.5 ps))1. The
finding that the measured and calculated Forster rates are
comparable to one another suggests that the TS mechanism
makes a substantial, perhaps the dominant, contribution to
energy transfer in the oxochlorin–oxochlorin dyad. Nonethe-
less, the perturbations to the optical spectra observed
previously upon the successive addition to the energy-
acceptor free-base oxochlorin of the ethyne, phenyl and then
the energy-donor zinc oxochlorin to construct the dyad
indicate that the inter-subunit TB interactions are significant,
albeit weaker than in porphyrin–porphyrin dyad ZnP-pe-FbP
(39). The potential TB contribution to excitation energy
transfer between the two meso-linked oxochlorins in ZnO-pe-
FbO versus the two meso-linked porphyrins in ZnP-pe-FbP
can be further assessed by extending the arguments given
above for ZnP-pe-FbP and FbC-pe-FbB.
(1) The HOMO of the phenylethynyl-substituted free base
oxochlorin remains the a2u(p)-derived orbital (Fig. 6B) that is
also the HOMO in the free base porphyrin (Fig. 5B). This
result is unexpected, given that (to our knowledge) all other
chlorin-type molecules examined to date exhibit a1u(p)-derived
HOMOs, including the ethynylphenyl-substituted zinc oxo-
chlorin constituent of ZnO-pe-FbO (Fig. 6A). The retention of
the a2u(p)-derived HOMO for the free base oxochlorin derives
from an interplay of factors that include a net electron-
withdrawing effect of the keto group of the oxochlorin to
stabilize all orbitals to some extent, a preferential destabilizing
Figure 5. Electron-density distributions and energies of the frontier
molecular-orbitals of the reference monomeric components of ZnP-pe-
FbP.
effect of macrocycle saturation in the oxochlorin on the a1u
-
of these factors is that the spacing between the HOMO and
HOMO–1 of the free base oxochlorin (Fig. 6B) is the smallest
among the complexes. Thus, for an equality of all other factors,
the contribution of the HOMO fi LUMO configuration of
versus the a2u-derived filled orbitals, and a destabilizing effect
of meso-ethyne substitution on the a2u(p)- versus a1u-derived
orbitals. In addition to the orbital ordering, another net effect

798 Chinnasamy Muthiah et al.
(2) On the other hand, the HOMO of the ethynylphenyl-
substituted zinc oxochlorin is an a1u(p)-derived orbital that has
much less electron density at the site of linker attachment when
compared with the a2u(p) HOMO of the zinc porphyrin
(Fig. 6A versus 5B). The resulting ordering of HOMO
and HOMO–1 is more typical of oxochlorins, chlorins and
bacteriochlorins (Figs. 4 and 6A), and emphasizes the impact
of the directly bonded ethyne group in the free base oxochlorin
described above (Fig. 6B). Thus, for an equality of all other
factors, the contribution of the HOMO fi LUMO configura-
tion of the zinc-chelate energy-donor chromophore to the
inter-subunit TB coupling will carry less weight for ZnO-pe-
FbO compared to ZnP-pe-FbP.
Collectively, the above characteristics indicate that the TB
contribution to energy transfer between the oxochlorin con-
stituents of ZnO-pe-FbO, while substantial, should be less than
half that between the porphyrin constituents of ZnP-pe-FbP.
On the other hand, the TS interaction in the oxochlorin dyad is
larger than that for the porphyrin dyad (due to the larger
transition dipole strengths of the hydroporphyrin energy
donor and acceptor subunits in the former dyad). This in turn
increases the TS contribution to the energy-transfer rate for
the oxochlorin dyad. Accordingly, the most consistent picture
of the energy-transfer process in the oxochlorin dyad is one
wherein both TS and TB processes contribute.
CONCLUSIONS
The results obtained here on a chlorin–bacteriochlorin dyad
and comparison with the findings from previous work on dyads
containing other tetrapyrrole macrocycles indicate that
through proper design, both TS and TB mechanisms can be
utilized to achieve ultrafast and essentially quantitative excited-
state energy transfer in tetrapyrrole-based arrays. In the case of
meso-phenylethyne-linked dyads, the TS rate decreases along
the series FbC-pe-FbB, ꢀ(5 ps))1 > ZnO-pe-FbO, ꢀ(10 ps))1
> ZnP-pe-FbP, ꢀ(60 ps))1. The magnitude of the TS contri-
bution to the overall energy-transfer rate (chlorin–bacterio-
chlorin > oxochlorin–oxochlorin > porphyrin–porphyrin)
parallels the intensification (and redshift) in the associated
fluorescence and absorption transitions in the constituent
macrocycles. In contrast, the TB contribution follows the
reverse trend for the same dyads (porphyrin–porphyrin >
oxochlorin–oxochlorin > chlorin–bacteriochlorin). This trend
parallels the extent of electron-density delocalization onto the
linker in the MOs that are the key contributors to the
wavefunction of the lowest energy singlet excited state.
Collectively, our results show that a favorable balance of
TS and TB contributions can be achieved to give ultrafast
and essentially quantitative excited-state energy transfer with
different combinations of constituents. One question that
arises from the present study is whether TB contributions to
energy transfer in arrays of hydroporphyrins could be
appreciably increased via tuning of the characteristics of the
frontier MOs (via substituent effects) and the use of linker
attachment sites where these orbitals have high electron
density. (Indeed, the oxochlorin–oxochlorin dyad discussed
here appears to exhibit some of these attributes.) If so, TB
interactions could act in concert with the already substantial
TS interaction to enhance the efficiency of excited-state
energy transfer in arrays having diverse architectures. These
Figure 6. Electron-density distributions and energies of the frontier
molecular-orbitals of the reference monomeric components of ZnO-pe-
FbO.
the free base energy-acceptor chromophore to the inter-sub-
unit TB coupling will carry roughly the same weight for ZnO-
pe-FbO and ZnP-pe-FbP.

Photochemistry and Photobiology, 2008, 84 799
16. 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.
17. Fan, D., M. Taniguchi and J. S. Lindsey (2007) Regioselective
15-bromination and functionalization of a stable synthetic bacte-
riochlorin. J. Org. Chem. 72, 5350–5357.
18. Muthiah, C., M. Ptaszek, T. M. Nguyen, K. M. Flack and
J. S. Lindsey (2007) Two complementary routes to 7-substituted
chlorins. Partial mimics of chlorophyll b. J. Org. Chem. 72, 7736–
7749.
concepts could be implemented in hydroporphyrin-based
synthetic light-harvesting systems for solar-energy conversion.
Finally, we note that the chlorin–bacteriochlorin dyad motif,
with suitable tailoring, might also find use in biomedical
applications, such as flow cytometry and optical molecular
imaging, owing to its narrow bandwidths and substantial
effective (>80 nm) Stokes shift between the red absorption
and NIR fluorescence transitions.
19. Muthiah, C., J. Bhaumik and J. S. Lindsey (2007) Rational routes
to formyl-substituted chlorins. J. Org. Chem. 72, 5839–5842.
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
excited-state decay characteristics. Photochem. Photobiol. 83,
1110–1124.
Acknowledgements—This research was supported by grants from the
Division of Chemical Sciences, Office of Basic Energy Sciences, Office
of Energy Research, U.S. Department of Energy to D.H. (DE-FG02-
05ER15661), D.F.B. (DE-FG02-05ER15660) and J.S.L. (DE-FG02-
96ER14632). Mass spectra were obtained at the Mass Spectrometry
Laboratory for Biotechnology at North Carolina State University.
Partial funding for the Facility was obtained from the North Carolina
Biotechnology Center and the National Science Foundation.
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. Harvey, P. D. (2003) Recent advances in free and metalated
multiporphyrin assemblies and arrays; a photophysical behavior
and energy transfer perspective. In The Porphyrin Handbook, Vol.
18 (Edited by K. M. Kadish, K. M. Smith and R. Guilard), pp.
63–250. Academic Press, San Diego.
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