Synthesis and photophysical characterization of porphyrin, chlorin and bacteriochlorin molecules bearing tethers for surface attachment

Article abstract of DOI:10.1111/j.1751-1097.2007.00195.x

The ability to tailor synthetic porphyrin, chlorin and bacteriochlorin molecules holds promise for diverse studies in artificial photosynthesis. Toward this goal, the synthesis and photophysical characterization of five tetrapyrrole compounds is described

Full text of DOI:10.1111/j.1751-1097.2007.00195.x

Photochemistry and Photobiology, 2007, 83: 1513–1528
Synthesis and Photophysical Characterization of Porphyrin, Chlorin and
Bacteriochlorin Molecules Bearing Tethers for Surface Attachment
Chinnasamy Muthiah1, Masahiko Taniguchi1, Han-Je Kim1, Izabela Schmidt1, Hooi Ling Kee2,
Dewey Holten*2, David F. Bocian*3 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 25 June 2007; accepted 12 July 2007; DOI: 10.1111/j.1751-1097.2007.00195.x
the bacteriochlorin is a tetrahydroporphyrin wherein a b,b¢-
bond in each of two rings positioned trans to each other is
saturated. The progressive saturation of the macrocycle
(porphyrin fi chlorin fi bacteriochlorin) leads to an in-
creased wavelength and intensity of the lowest-energy absorp-
tion band. In particular, porphyrins absorb relatively weakly
in the red region; chlorins absorb strongly in the red region;
bacteriochlorins absorb very strongly in the near-infrared
region (1). A comparison of the absorption spectra (2) of these
three types of macrocycles is shown in Fig. 1. Thus, the use of
porphyrins as surrogates for chlorins and bacteriochlorins
achieves synthetic expediency yet sacrifices solar coverage.
The use of porphyrins in lieu of chlorins or bacteriochlorins
also leaves unexamined key attributes of the latter molecules
that differ from porphyrins. Such features include (i) less
positive oxidation potential (i.e. greater ease of oxidation),
(ii) more negative reduction potential (i.e. more difficult
reduction), (iii) lower energy first singlet excited state and
(iv) lower symmetry (C2v or D2h versus D4h for metallopor-
phyrins), and hence an excited state that exhibits features of a
linear oscillator rather than a planar oscillator. Thus, there are
multiple rationales, in addition to the biomimetic imperative
and the overt reason of increased spectral coverage, to pursue
the examination of chlorins and bacteriochlorins in artificial
photosynthetic systems (3).
ABSTRACT
The ability to tailor synthetic porphyrin, chlorin and bacterio-
chlorin molecules holds promise for diverse studies in artificial
photosynthesis. Toward this goal, the synthesis and photophysical
characterization of five tetrapyrrole compounds is described.
Each compound bears a surface attachment group. One set
contains three meso-substituted porphyrins that differ only in the
nature of a surface-binding tether—isophthalic acid, ethynyl-
isophthalic acid or cyanoacrylic acid. The other set includes a
porphyrin, chlorin and bacteriochlorin each of which bears an
ethynylisophthalic acid tether. The ester derivative of each
compound was prepared for solution photophysical characteriza-
tion studies. The photophysical studies include determination (in
toluene or acetonitrile) of the electronic absorption and fluores-
cence spectra, fluorescence yield and lifetime of the lowest excited
singlet state. The excited-state lifetimes range from 1 to 5.6 ns
for the five compounds. The radiative rate constant for the
excited-state decay was estimated from the photophysical data
(fluorescence yield and excited-state lifetime) and from Strickler–
Berg analysis of the absorption and fluorescence spectra. The
synthesis and characterization of the tetrapyrrole compounds
underpin their use as sensitizers in molecular-based solar cells.
INTRODUCTION
Examination of the photoreactivity of chlorins and bacterio-
chlorins in artificial solar-conversion constructs requires the
ability to organize the molecules in well-defined architectures. In
this regard, the syntheses of these classes of non-saturated
macrocycles have presented obstacles. Three approaches gener-
ally have been employed for the synthesis of model (bacte-
rio)chlorins. (i) Reduction or derivatization of porphyrins: This
approach is the simplest yet least versatile, typically affording
mixtures of isomers with porphyrins that bear distinct patterns
of substituents (4,5). (ii) Semisynthesis: The chemical modifica-
tion of naturally occurring (bacterio)chlorophylls takes advan-
tage of ample quantities of naturally available starting materials.
Although inherently limited by the presence of nearly a full
complement of substituents arrayed about the perimeter of the
macrocycle, numerous studies have exploited this approach to
probe the role of diverse substituents about the chlorin macro-
cycle (6–15). A major challenge toward semisynthesis of
bacteriochlorophylls resides in the intrinsic lability of the
A chief objective of artificial photosynthesis is to learn how to
design and prepare synthetic constructs that duplicate the
various steps of natural photosynthetic systems. Toward that
goal, a vast amount of work has been carried out with
synthetic porphyrins as surrogates for the chlorophylls and
bacteriochlorophylls of photosynthetic systems. The rationale
for use of porphyrins rather than chlorins or bacteriochlorins
stems from the close structural resemblance of the macrocycles
and the more facile synthesis of porphyrins versus chlorins or
bacteriochlorins. Porphyrins, chlorins and bacteriochlorins
share a common macrocyclic skeleton containing 20 carbons
and four inner nitrogens yet differ in the degree of saturation
of the p system. The porphyrin is fully unsaturated, the chlorin
is a dihydroporphyrin wherein one b,b¢-bond is saturated and
*Corresponding author emails: holten@wustl.edu (Dewey Holten); dbocian@
ucr.edu (David Bocian); jlindsey@ncsu.edu (Jonathan S. Lindsey)
ꢀ 2007 The Authors. JournalCompilation. The AmericanSocietyofPhotobiology 0031-8655/07
1513

1514 Chinnasamy Muthiah et al.
100,000
80,000
60,000
40,000
20,000
0
300
400
500
600
700
800
Wavelength (nm)
Figure 1. Absorption spectra for a porphyrin (solid line), a chlorin
(dashed line) and a bacteriochlorin (dotted line). All three compounds
are magnesium chelates: magnesium octaethylporphyrin, chlorophyll a
and bacteriochlorophyll a, respectively. The peak violet-blue (Soret)
region molar absorption coefficients are 408 mM)1 cm)1 (408 nm),
112 mM)1 cm)1 (428 nm) and71 mM)1 cm)1 (361 nm), respectively (2).
molecules (16–22). (iii) De novo synthesis: This approach, which
we have pursued, entails the total synthesis of chlorins and
bacteriochlorins from simple, commercially available precur-
sors. In this approach, each target chlorin or bacteriochlorin is
designed to contain a geminal dimethyl group in each reduced
ring to prevent adventitious dehydrogenation (which ultimately
would cause reversion to the porphyrin). The use of the geminal
dimethyl group and the typical objective of a limited number of
substituents make this approach far more accessible than the
total synthesis of the naturally occurring (bacterio)chlorophylls.
The core structures of the stable chlorin and bacteriochlorin are
shown in Chart 1. The synthetic route to chlorins is quite
versatile and has enabled introduction of substituents at each
site at the perimeter of the macrocycle except position 7 (23–34).
The synthetic route to bacteriochlorins has been developed quite
recently. To date, substituents can be introduced at the 2, 12 and
15-positions (35,36). The denovo syntheses afford stable chlorins
and bacteriochlorins, and enable the macrocycles to be tailored
in a controlled manner without elaborate synthesis.
A long-term objective is to employ the stable synthetic
chlorins and bacteriochlorins in a variety of fundamental studies
in artificial photosynthesis. One attractive approach relies on the
attachment of porphyrinic molecules to surfaces for studies of
interfacial photoinduced electron-transfer reactions. In this
regard, porphyrins bearing a wide variety of tethers have been
synthesized for attachment to metal-oxide surfaces. The tethers
include carboxylic acid (37), benzoic acid (38–41), vinylbenzoic
acid (41,42), phenylphosphonic acid (43), vinylphenylphos-
phonic acid (41), benzylphosphonic acid (44), a-cyanoacrylic
acid (45,46), rhodanine (47), isophthalic acid (48), vinyliso-
phthalic acid (42), ethynylbenzoic acid (49) and thiophenecarb-
oxylic acid (50). Tethers that have been employed for attaching
dyes (other than porphyrins) to metal-oxide surfaces include
bidentate moieties such as isophthalic acid (42), cyanoacrylic
acid (42,45,46) and alkenyl or alkynyl homologs thereof (49–57).
With synthetic access to chlorins and bacteriochlorins in
hand, one objective was to prepare a set of tetrapyrrole
compounds (porphyrin, chlorin and bacteriochlorin) bearing
identical tethers for surface attachment. To this end, we have
prepared a porphyrin (ZnP-EI), a chlorin (ZnC-EI) and a
Chart 1
bacteriochlorin (FbB-EI) each bearing an ethynylisophthalic
acid tether (Chart 2). The three compounds with identical
tethers enables examination of the effects of increasing red-
region and decreasing blue-region absorption, thermodynamic
features and fundamental photophysical attributes as a func-
tion of increasing macrocycle saturation (porphyrin fi chlo-
rin fi bacteriochlorin). We have also prepared two other
meso-functionalized porphyrins bearing different tethers. The

Photochemistry and Photobiology, 2007, 83 1515
Chart 2
Noncommercial compounds. Compounds 1 (59), 2 (60), 3 (59), 4 (61),
5 (61), Zn-5 (61), 11 (27) and 12 (36) were prepared by following
literature procedures.
porphyrins are zinc chelates bearing a cyanoacrylic acid tether
(ZnP-A) or an isophthalic acid tether (ZnP-I). The chlorin is
also a zinc chelate whereas the bacteriochlorin lacks a metal
(free base). A free base bacteriochlorin was examined rather
than a zinc bacteriochlorin because effective synthetic methods
for metalating bacteriochlorins are not yet available.
The synthesis of the five tetrapyrrole compounds shown in
Chart 2 is described here, along with the photophysical
characterization of the ester derivatives (ZnP-A¢, ZnP-I¢,
ZnP-EI¢, ZnC-EI¢, FbB-EI¢). The performance of the acid
derivatives of the five compounds upon attachment to TiO2
surfaces (in Gratzel-type solar cells) along with detailed
analysis of the electronic (optical ⁄ redox) properties of the
sensitizers in terms of the underlying molecular-orbital char-
acteristics will be reported elsewhere (3).
Synthesis procedures. Dibutyl[5,10-dihydro-1,9-bis(4-methylbenzoyl)-
5-(4-methylphenyl)dipyrrinato]tin(IV) (2): Following a reported pro-
cedure (60), a solution of EtMgBr (187 mL, 187 mmol, 1.0 M solution
in tetrahydrofuran [THF]) was added slowly to a tap-water-cooled
flask containing a solution of 1 (8.80 g, 37.5 mmol) in toluene
(375 mL) under argon. The resulting mixture was stirred at room
temperature for 30 min. A solution of p-toluoyl chloride (12.5 mL,
94.0 mmol) in toluene (94 mL) was added for 10 min, and the resulting
solution was stirred for 30 min. The reaction mixture was poured into
saturated aqueous NH4Cl (750 mL) and ethyl acetate (750 mL). The
organic layer was washed (water, brine), dried (Na2SO4) and filtered.
The filtrate was concentrated to dryness. The residue was treated with
triethylamine (TEA, 15.7 mL, 112 mmol) and Bu2SnCl2 (11.4 g,
37.5 mmol) in CH2Cl2 (150 mL) at room temperature for 30 min.
The mixture was filtered over a silica pad (CH2Cl2). The eluate was
concentrated to dryness. The residue was dissolved in a minimum
amount of diethyl ether. Methanol was added, yielding a precipitate,
which upon filtration afforded a pale yellow solid (14.1 g, 54%).
Characterization data (1H NMR, 13C NMR and FAB-MS spectra)
were consistent with those obtained from samples prepared previously
(62).
5,10,15-Tris(4-methylphenyl)porphyrin (4): Following a reported
procedure (60), a solution of 2 (5.30 g, 7.53 mmol) in dry THF ⁄ MeOH
(288 mL, 10:1) was treated with NaBH4 (5.70 g, 150 mmol, 20 equiv)
in small portions with rapid stirring at room temperature. After 2 h,
analysis by thin layer chromatography (TLC) indicated incomplete
conversion of 2. Therefore, an additional amount of NaBH4 (5.70 g)
was added, and the reaction mixture was stirred for 2 h. The reaction
mixture was quenched by slow addition of saturated aqueous NH4Cl.
The reaction mixture was extracted with CH2Cl2. The organic layer
MATERIALS AND METHODS
General methods. 1H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were collected at room temperature in CDCl3 unless noted
otherwise. Absorption spectra were obtained at room temperature.
The tetrapyrrole compounds were analyzed using laser desorption
mass spectrometry (LD-MS) in the absence of a matrix (58). Fast atom
bombardment mass spectrometry (FAB-MS) data are reported for the
molecule ion or protonated molecule ion. Metalation of free base
compounds was monitored using fluorescence spectroscopy. Melting
points are uncorrected. All commercially available reagents were used
as received. All palladium-coupling reactions were carried out using
standard Schlenk-line techniques.

1516 Chinnasamy Muthiah et al.
was separated, dried (K2CO3) and concentrated to afford 2-diol as a
yellow foam-like solid. The freshly prepared 2-diol was condensed with
dipyrromethane 3 (1.09 g, 7.53 mmol) in CH2Cl2 (2.99 L) under
catalysis with Yb(OTf)3 (5.93 g, 11.3 mmol) at room temperature for
30 min (63,64). A sample of 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ, 5.09 g) was added, and the reaction mixture was stirred
for 1 h at room temperature. The reaction mixture was then
neutralized with TEA (7.5 mL) and filtered through a pad of silica
(eluted with CH2Cl2). The first fraction was collected and concen-
trated. The resulting solid was subjected to column chromatography
(silica, hexanes ⁄ CH2Cl2 [1:1]) to afford a purple solid (1.60 g, 37%).
Characterization data (1H NMR, LD-MS, FAB-MS and UV–Vis
spectra) were consistent with those obtained from samples prepared via
earlier routes (61,62).
5-Bromo-10,15,20-tris(4-methyphenyl)porphyrin (5): Following a
reported procedure (61), a solution of 4 (1.00 g, 1.72 mmol) in CHCl3
(430 mL) was treated with N-bromosuccinimide (NBS, 306 mg,
1.72 mmol) and pyridine (1.70 mL) at 0ꢁC for 30 min. Acetone
(2 mL) was added. The reaction mixture was concentrated to dryness.
The resulting solid was purified by column chromatography (silica,
hexanes ⁄ CH2Cl2 [1:1]) to give a purple solid (1.12 g, 99%). Charac-
terization data (1H NMR, LD-MS, FAB-MS, and UV–Vis spectra)
were consistent with those obtained from samples prepared previously
(61).
Zn(II)-5-Bromo-10,15,20-tris(4-methyphenyl)porphyrin (Zn-5): A
solution of Zn(OAc)2Æ2H2O (3.33 g, 15.2 mmol) in methanol (35 mL)
was added to a solution of 5 (1.00 g, 1.52 mmol) in CHCl3 (140 mL)
with stirring at room temperature. After 16 h, the reaction mixture was
concentrated, whereupon CH2Cl2 (50 mL) was added. The organic
layer was washed (saturated aqueous NaHCO3, H2O), dried (Na2SO4)
and concentrated. The resulting solid was purified by column
chromatography (silica, hexanes ⁄ CH2Cl2 [1:1]) to afford a purple
solid (0.93 g, 85%). Characterization data (1H NMR, LD-MS, FAB-
MS and UV–Vis spectra) were consistent with those obtained from
samples prepared previously (61).
5-[Bis(ethoxycarbonyl)methyl]-10,15,20-tris(4-methylphenyl)por-
phyrin (6): Diethyl malonate (97.4 mg, 0.608 mmol) was added
dropwise to NaH (24.3 mg, 0.608 mmol, 60% dispersion in mineral
oil) in dimethylsulfoxide (DMSO, 10.0 mL) at 100ꢁC. The mixture was
stirred until all solids had dissolved (ꢀ30 min), and then allowed to
cool to room temperature. Bromoporphyrin 5 (100 mg, 0.152 mmol)
was added all at once, and the mixture was heated at 100ꢁC for 20 h
under argon. The reaction mixture was allowed to cool to room
temperature and then diluted with ethyl acetate (50 mL). The organic
layer was washed with H2O, dried (Na2SO4) and concentrated. The
resulting residue was purified by column chromatography (silica,
hexanes then hexanes ⁄ CH2Cl2 [1:1.5]) to give a purple solid (90.1 mg,
80%): 1H NMR d )2.72 (br, 2H), 1.15 (t, J = 7.2 Hz, 6H), 2.69 (s,
3H), 2.70 (s, 6H), 4.28–4.32 (m, 4H), 7.33 (s, 1H), 7.54–7.57 (m, 6H),
8.06–8.09 (m, 6H), 8.80 (d, J = 4.8 Hz, 2H), 8.83 (d, J = 4.8 Hz, 2H),
8.95 (d, J = 4.8 Hz, 2H), 9.51 (d, J = 4.8 Hz, 2H); 13C NMR d 14.2,
21.7 (two overlapped resonances), 58.3, 62.4, 108.6, 120.5, 121.5, 127.5,
127.7, 134.6, 134.7, 137.6, 139.0, 139.5, 143.3, 170.3, resonances from
the a- and b-carbons of the porphyrin were not observed because of
NH tautomerism; LD-MS obsd 738.1; FAB-MS obsd 739.3318, calcd
739.3284 (C48H42N4O4); kabs (CH2Cl2) 419, 516, 551, 591 nm.
5-Formyl-10,15,20-tris(4-methylphenyl)porphyrin (7): A mixture
containing 6 (50.0 mg, 0.0676 mmol) in 6.0 mL of 20% aqueous
HCl acid was refluxed for 24 h. The reaction mixture was allowed to
cool to room temperature, and then diluted with ethyl acetate (80 mL).
The organic layer was washed (saturated aqueous NaHCO3, H2O),
dried (Na2SO4) and concentrated. The resulting residue was purified
(silica, hexanes then hexanes ⁄ CH2Cl2 [1:2]) to afford a trace amount
(ꢀ5% of the total) of a meso-methyl porphyrin followed by the title
compound as a purple solid (32.1 mg, 78%): 1H NMR d )2.00 (br,
2H), 2.71 (s, 3H), 2.72 (s, 6H), 7.54–7.58 (m, 6H), 8.03–8.06 (m, 6H),
8,70 (d, J = 4.8 Hz, 2H), 8.77 (d, J = 4.8 Hz, 2H), 8.77
(d, J = 4.8 Hz, 2H), 9.98 (d, J = 4.8 Hz, 2H), 12.45 (s, 1H); 13C
NMR (75 MHz) d 21.7 (two overlapped resonances), 107.6, 123.1,
126.1, 127.7, 127.8, 134.4, 134.5, 138.0, 138.1, 138.4, 138.9, 195.0,
resonances from the a- and b-carbons of the porphyrin were not
observed because of NH tautomerism; LD-MS obsd 608.3; FAB-MS
obsd 609.2675, calcd 609.2654 (C42H32N4O); kabs (CH2Cl2) 426, 530,
571, 601 nm.
Zn(II)-5-Formyl-10,15,20-tris(4-methylphenyl)porphyrin (Zn-7): A
solution of Zn(OAc)2Æ2H2O (173 mg, 0.787 mmol) in methanol (2 mL)
was added to a solution of 7 (32.0 mg, 0.0525 mmol) in CHCl3 (8 mL)
with stirring at room temperature. After 5 h, TLC analysis indicated
that the reaction was complete with the appearance of a new, polar
band (green). The reaction mixture was concentrated, whereupon ethyl
acetate (50 mL) was added. The organic layer was washed (saturated
aqueous NaHCO3, H2O), dried (Na2SO4) and concentrated. The
resulting solid was purified by column chromatography (hexanes then
CH2Cl2 ⁄ methanol [95:5]) to afford a green solid (33.5 mg, 95%): 1H
NMR (THF-d8) d 2.67 (s, 3H), 2.70 (s, 6H), 7.56–7.59 (m, 6H), 8.01–
8.05 (m, 6H), 8.68 (d, J = 4.8 Hz, 2H), 8.75 (d, J = 4.8 Hz, 2H), 8.95
(d, J = 4.8 Hz, 2H), 10.15 (d, J = 4.8 Hz, 2H), 12.57 (s, 1H); 13C
NMR (75 MHz, THF-d8) d 21.7 (two overlapped resonances), 109.6,
124.1, 127.2, 128.1, 128.2, 130.3, 131.8, 133.1, 134.9, 135.1, 135.2,
138.2, 138.3, 141.0, 141.2, 150.2, 150.3, 152.7, 154.5, 195.8; LD-MS
obsd 670.2; FAB-MS obsd 670.1710, calcd 670.1711 (C42H30N4OZn);
kabs (CH2Cl2) 430, 564, 604 nm.
Zn(II)-5-(2-Cyano-2-methoxycarbonylvinyl)-10,15,20-tris(4-methyl-
A
phenyl)porphyrin (ZnP-A¢):
mixture of Zn-7 (7.30 mg,
0.0108 mmol), methyl cyanoacetate (4.80 lL, 0.0540 mmol) and
piperidine (16.1 lL, 0.108 mmol) in anhydrous methanol (2.0 mL)
was refluxed under argon. After 6 h, the reaction mixture was
concentrated under reduced pressure. The resulting solid was chro-
matographed (silica, hexanes then CH2Cl2) to afford a green solid
(6.20 mg, 76%): 1H NMR (300 MHz) d 2.70 (s, 3H), 2.72 (s, 6H), 4.17
(s, 3H), 7.53–7.60 (m, 6H), 8.04–8.08 (m, 6H), 8.86 (d, J = 4.8 Hz,
2H), 8.90 (d, J = 4.8 Hz, 2H), 9.04 (d, J = 4.8 Hz, 2H), 9.38
(d, J = 4.8 Hz, 2H), 10.92 (s, 1H); 13C NMR (75 MHz) d 21.7 (two
overlapped resonances), 50.9, 111.6, 111.7, 113.3, 117.3, 123.5, 125.0,
127.3, 127.5, 129.2, 129.7, 131.1, 132.3, 132.7, 133.0, 134.5, 134.6,
137.7, 139.7, 150.5, 150.7, 151.2, 157.5, 169.3; LD-MS obsd 751.7;
FAB-MS obsd 751.1946, calcd 751.1926 (C46H33N5O2Zn); kabs (tolu-
ene) 449, 565, 621 nm.
Zn(II)-5-(2-Cyano-2-carboxyvinyl)-10,15,20-tris(4-methylphenyl)-
porphyrin (ZnP-A): Following a reported procedure (45), a mixture of
Zn-7 (23.0 mg, 0.0342 mmol), cyanoacetic acid (145 mg, 0.171 mmol)
and piperidine (0.102 mL, 1.03 mmol) in anhydrous methanol
(1.5 mL) was refluxed for 16 h under argon. While the solution was
allowed to cool to room temperature, CH2Cl2 (25 mL) and H2O
(50 mL) were added. The resulting solution was shaken vigorously,
whereupon the aqueous layer was adjusted to pH ꢀ2 with 2 M
aqueous H3PO4. The organic layer was then separated and concen-
trated. The resulting solid was purified by column chromatography
(silica, hexanes then CH2Cl2 ⁄ methanol [85:15]) to give a green solid
(20.0 mg, 79%): 1H NMR (DMSO-d6) d 2.66 (s, 3H), 2.67 (s, 6H),
7.59–7.61 (m, 6H), 8.03–8.05 (m, 6H), 8.72 (d, J = 4.8 Hz, 2H), 8.75
(d, J = 4.8 Hz, 2H), 8.84 (d, J = 4.8 Hz, 2H), 9.37 (d, J = 4.8 Hz,
2H), 10.54 (s, 1H), 11.71 (br, 1H); LD-MS obsd 738.2, 693.3 (CO2
cleavage); ESI-MS obsd 737.0; FAB-MS obsd 737.1764, calcd
737.1769 (C45H31N5O2Zn); kabs (THF) 430, 565, 620 nm.
Dimethyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolyl)isophthalate (8):
A solution of dimethyl 5-iodoisophthalate (320 mg, 1.00 mmol), TEA
(0.420 mL, 3.00 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(0.220 mL, 1.50 mmol) was treated with [1,1¢-bis(diphenylphosphi-
no)ferrocene]dichloropalladium(II)
[PdCl2(dppf),
44.0 mg,
0.0600 mmol] in acetonitrile (4 mL). After being stirred for 5 h at
85ꢁC under argon (on the Schlenk line), the mixture was allowed to cool
to room temperature and then concentrated to dryness. Et2O was added,
and the mixture was filtered (to remove triethylammonium iodide). The
filtrate was concentrated and treated with hexanes (30 mL). The
resulting mixture was filtered. The filtrate was concentrated and
chromatographed (silica, CHCl3 containing 1% TEA) to afford a white
solid (160 mg, 50%): mp 134–135ꢁC; 1H NMR d 1.36 (s, 12H), 3.94 (s,
6H), 8.63 (d, J = 1.6 Hz, 2H), 8.76 (t, J = 1.6 Hz, 1H); 13C NMR d
166.5, 140.1, 133.5, 130.3, 84.6, 52.5, 25.1; Anal. calcd for C16H21BO6: C,
60.03; H, 6.661; Found: C, 60.24; H, 6.56.
5-[3,5-Bis(methoxycarbonyl)phenyl]-10,15,20-tris(4-methylphenyl)por-
phyrin (FbP-I¢): Following a reported procedure (65), samples of 5
(50.0 mg, 0.0760 mmol),
8 (25.5 mg, 0.0760 mmol), anhydrous
K2CO3 (157 mg, 1.14 mmol) and Pd(PPh3)4 (17.6 mg, 0.0152 mmol,
20 mol%) were weighed in a 25 mL Schlenk flask. The flask was
pump-purged with argon three times. Toluene ⁄ N,N-dimethylforma-
mide (DMF) (9.0 mL, 2:1) was added, and the mixture was heated to

Photochemistry and Photobiology, 2007, 83 1517
95ꢁC. TLC of the reaction mixture indicated incomplete consumption
of the starting porphyrin after 6 h. Therefore, an additional amount
of Pd(PPh3)4 (17.6 mg) was added, and the reaction mixture was
stirred for 12 h at 95ꢁC. The solvent was removed. The residue was
Zn(II)-5-[2-(3,5-Dicarboxyphenyl)ethynyl]-10,15,20-tris(4-methy-
lphenyl)porphyrin (ZnP-EI): Following a reported procedure (45), a
solution of NaOH (96.7 mg, 20 equiv per CO2Me, 2.42 mmol) in H2O
(650 lL) and methanol (1.6 mL) was added to a refluxing solution of
porphyrin ZnP-EI¢ (52.0 mg, 0.0604 mmol) in THF (3.2 mL) and
methanol (1.6 mL) under argon. After 2 h, TLC analysis indicated that
all of ZnP-EI¢ had been consumed. The solution was allowed to cool to
room temperature, whereupon CH2Cl2 (30 mL), H2O (60 mL) and
2 M aqueous H3PO4 were added. The solution was shaken vigorously
in a separatory funnel (pH ꢀ2.0). The organic layer, which contained
the porphyrin, was isolated. Acetone (30 mL) was added to the organic
layer, and the solution was concentrated to a volume of ꢀ5 mL. The
product was precipitated by the addition of H2O. The resulting
mixture was centrifuged and then filtered to give a greenish purple
powder (46.0 mg, 91%): 1H NMR (DMSO-d6) d 2.66 (s, 3H), 2.68
(s, 6H), 7.58–7.64 (m, 6H), 8.01–8.07 (m, 6H), 8.57–8.58 (m, 1H), 8.70
(d, J = 4.8 Hz, 2H), 8.72 (d, J = 4.8 Hz, 2H), 8.79–8.80 (m, 2H),
8.89 (d, J = 4.8 Hz, 2H), 9.81 (d, J = 4.8 Hz, 2H), 13.70 (br, 2H);
LD-MS obsd 829.7; FAB-MS obsd 830.1846, calcd 830.1872
(C51H34N4O4Zn); kabs (THF) 440, 532, 573, 622 nm.
filtered through
a pad of silica (CH2Cl2) followed by column
chromatography of the resulting solid (silica, hexanes then hex-
1
anes ⁄ CH2Cl2 [1:2]) to afford a purple solid (46.2 mg, 78%): H NMR
(THF-d8) d )2.70 (br, 2H), 2.67 (s, 3H), 2.68 (s, 6H), 3.97 (s, 6H),
7.56–7.60 (m, 6H), 8.06–8.10 (m, 6H), 8.72 (d, J = 4.8 Hz, 2H),
8.83–8.85 (m, 4H), 8.86 (d, J = 4.8 Hz, 2H), 9.03–9.05 (m, 2H),
9.07–9.09 (m, 1H); 13C NMR (75 MHz, THF-d8) d 20.5, 21.7, 52.8,
117.0, 120.7, 121.0, 127.6 (two overlapped resonances), 129.4, 130.3,
131.3, 134.7 (two overlapped resonances), 137.7, 138.7, 139.2, 139.3,
143.4, 166.7, resonances from the a- and b-carbons of the porphyrin
were not observed because of NH tautomerism; LD-MS obsd 772.6;
FAB-MS obsd 772.3017, calcd 772.3050 (C51H40N4O4Zn); kabs
(CH2Cl2) 419, 515, 549, 593 nm.
Zn(II)-5-[3,5-Bis(methoxycarbonyl)phenyl]-10,15,20-tris(4-methyl-
phenyl)porphyrin (ZnP-I¢): A solution of Zn(OAc)2Æ2H2O (196 mg,
0.892 mmol) in methanol (2.0 mL) was added to a solution of FbP-I¢
(46.0 mg, 0.0595 mmol) in CHCl3 (8.0 mL) with stirring at room
temperature. After 16 h, the reaction mixture was concentrated and
filtered through a pad of silica (hexanes then CH2Cl2) to afford a
purple solid (45.8 mg, 92%): 1H NMR (THF-d8) d 2.67 (s, 3H + s,
6H), 3.96 (s, 6H), 7.54–7.58 (m, 6H), 8.04–8.09 (m, 6H), 8.72 (d,
J = 4.8 Hz, 2H), 8.84–8.86 (m, 4H), 8.88 (d, J = 4.8 Hz, 2H), 9.02–
9.04 (m, 2H), 9.06–9.08 (m, 1H); 13C NMR (75 MHz, THF-d8) d 21.6
(two overlapped resonances), 52.6, 118.1, 121.7, 122.0, 127.5 (two
overlapped resonances), 129.4, 130.1, 131.3, 132.3, 132.4, 132.7, 134.6
(two overlapped resonances), 137.4, 138.5, 140.0, 140.1, 144.1, 144.2,
149.9, 150.7, 150.8, 150.9, 166.7; LD-MS obsd 836.4; FAB-MS obsd
834.2192, calcd 834.2185 (C51H38N4O4Zn); kabs (THF) 426, 519, 557,
597 nm.
Zn(II)-5-(3,5-Dicarboxyphenyl)-10,15,20-tris(4-methylphenyl)por-
phyrin (ZnP-I): Following a reported procedure (45), a solution of
NaOH (53.6 mg, 1.34 mmol, 20 equiv per CO2Me) in H2O (0.40 mL)
and methanol (1.0 mL) was added to a refluxing solution of porphyrin
ZnP-I¢ (28.0 mg, 0.0335 mmol) in THF (2.0 mL) and methanol
(1.0 mL) under argon. After 2 h, TLC analysis indicated that all of
ZnP-I¢ had been consumed. After the mixture cooled to room
temperature, the mixture was concentrated to remove the THF and
methanol. The resulting mixture was diluted with H2O (5.0 mL), and
the mixture was adjusted to pH ꢀ2 using 2 M aqueous H3PO4. The
resulting precipitate was isolated by centrifugation and filtration to
give a purple powder (26.4 mg, 98%): 1H NMR (THF-d8) d 2.68 (s, 3H
+ s, 6H), 7.53–7.57 (m, 6H), 8.06–8.09 (m, 6H), 8.75 (d, J = 4.8 Hz,
2H), 8.85–8.87 (m, 4H), 8.87 (d, J = 4.8 Hz, 2H), 9.02–9.04 (m, 2H),
9.08–9.10 (m, 1H), resonances from the COOH of the porphyrin were
not observed; 13C NMR (THF-d8) d 21.7 (two overlapped resonances),
119.0, 121.4, 121.8, 128.0, 128.4, 130.8, 131.1, 132.3, 132.4, 132.8,
135.3, 135.4, 137.8, 138.5, 139.4, 140.3, 141.6, 143.9, 145.2, 150.7, 151.3
(two overlapped resonances), 151.4, 167.7; LD-MS obsd 806.1;
FAB-MS obsd 806.1806, calcd 806.1872 (C51H34N4O4Zn); kabs
(THF) 425, 520, 557, 597 nm.
5-Ethynylisophthalic acid (9):
25.7 mmol, 20 equiv per CO2Me) in H2O (2.0 mL) and methanol
(5.0 mL) was added to refluxing solution of dimethyl 5-eth-
A solution of NaOH (1.03 g,
a
ynylisophthalate (140 mg, 0.642 mmol) in THF (10 mL) and methanol
(5.0 mL) under argon. After 2 h, TLC analysis indicated that all of the
dimethyl 5-ethynylisophthalate had been consumed. After the mixture
cooled to room temperature, the mixture was concentrated in vacuum
to remove the THF and methanol. The resulting mixture was diluted
with H2O (5.0 mL), and the mixture was adjusted to pH ꢀ1 using
aqueous 10% HCl. The resulting precipitate was isolated by centri-
fugation and filtration. The filtered material was washed with H2O to
give a white powder (122 mg, 100%): mp 104–106ꢁC [lit. mp 106.2ꢁC
(68)]; 1H NMR (DMSO-d6) d 4.46 (s, 1H), 8.13–8.15 (m, 2H), 8.43–
8.44 (m, 1H), 13.56 (br, 2H); 13C NMR (DMSO-d6) d 81.5, 82.8, 123,
130, 132, 136, 166; Anal. calcd for C10H6O4: C, 63.16; H, 3.18. Found:
C, 63.34; H, 3.42.
1,3-Bis[2-(trimethylsilyl)ethoxycarbonyl]-5-ethynylbenzene (10):
a general procedure (69), samples of 9 (177 mg,
Following
0.931 mmol) and 2-(trimethylsilyl)ethanol (226 mg, 1.91 mmol) were
dissolved in 5.0 mL of DMF. A sample of N,N-dicyclohexylcarbodii-
mide (DCC, 394 mg, 1.91 mmol) was added followed by 4-dimethyla-
minopyridine (23.0 mg, 0.186 mmol). A voluminous white precipitate
formed immediately. After 14 h, ethyl acetate was added, and the
mixture was filtered. The filtrate was concentrated and then treated
with ethyl acetate. The resulting solution was washed with 5%
NaHCO3 and dried over Na2SO4. The solvent was removed under
reduced pressure. The resulting residue was purified by column
chromatography (silica, hexanes ⁄ ethyl acetate [95:5]) to give a white
solid (125 mg, 34%): mp 51–52ꢁC; 1H NMR d 0.09 (s, 18H), 1.13–1.18
(m, 4H), 3.16 (s, 1H), 4.42–4.47 (m, 4H), 8.28–8.30 (m, 2H), 8.50–8.63
(m, 1H); 13C NMR d )1.2, 17.7, 64.2, 79.1, 111.8, 123.2, 130.7, 131.7,
137.1, 150.0; Anal. calcd for C20H30O4Si2: C, 61.50; H, 7.74. Found: C,
61.64; H, 7.85.
17,18-Dihydro-10-mesityl-15-[2-(3,5-bis(2-(trimethylsilyl)ethoxy-
carbonyl)phenyl)ethynyl]-18,18-dimethyl-5-(4-methylphenyl)porphy-
Zn(II)-5-[2-(3,5-Bis(methoxycarbonyl)phenyl)ethynyl]-10,15,20-tris-
(4-methylphenyl)porphyrin (ZnP-EI¢): Following a reported procedure
for Sonogashira coupling with arylporphyrins (61,66,67), samples of
Zn-5 (70.0 mg, 0.0968 mmol) and dimethyl 5-ethynylisophthalate
(25.3 mg, 0.116 mmol) were coupled using tris(dibenzylideneace-
tone)dipalladium(0) [Pd2(dba)3, 53.2 mg, 0.0580 mmol] and tri-o-
tolylphosphine [P(o-tol)3, 133 mg, 0.435 mmol] in toluene ⁄ TEA (5:1,
36 mL) at 50ꢁC under argon. After 16 h, the reaction mixture was
concentrated under reduced pressure. The resulting residue was
chromatographed (silica, hexanes then CH2Cl2 ⁄ methanol [95:5]) to
afford a greenish purple solid (74.0 mg, 89%): 1H NMR (THF-d8) d
2.67 (s, 3H), 2.70 (s, 6H), 4.02 (s, 6H), 7.54–7.59 (m, 6H), 8.03–8.09 (m,
6H), 8.69–8.71 (m, 1H), 8.77–8.78 (m, 4H), 8.82–8.85 (m, 2H), 8.95 (d,
J = 4.8 Hz, 2H), 9.80 (d, J = 4.8 Hz, 2H); 13C NMR (75 MHz,
THF-d8) d 21.7 (two overlapped resonances), 52.8, 93.6, 95.7, 122.1,
122.8, 123.4, 125.5, 127.3, 127.4, 130.4, 130.7, 131.3, 131.7, 132.2,
133.1, 134.5, 134.6, 134.9, 136.4, 137.2, 140.0, 140.2, 150.0, 150.9 (two
overlapped resonances), 152.4, 166.0; LD-MS obsd 860.6; FAB-MS
obsd 858.2218, calcd 858.2185 (C53H38N4O4Zn); kabs (THF) 441, 531,
572, 623 nm.
rin (FbC-EI¢): Following
a general procedure for Sonogashira
coupling (61,66,67), 15-bromochlorin 11 (75.3 mg, 120 lmol) and 10
(58.6 mg, 150 lmol) were coupled using Pd2(dba)3 (16.5 mg,
18.0 lmol) and P(o-tol)3 (43.7 mg, 144 lmol) in toluene ⁄ TEA (5:1,
48 mL) in a Schlenk flask at 60ꢁC under argon. After 5 h, Pd2(dba)3
(16.5 mg, 18.0 lmol) and P(o-tol)3 (43.7 mg, 144 lmol) were added to
the reaction mixture. After 12 h, the mixture was concentrated under
reduced pressure. The residue was chromatographed (silica, hexanes
then hexanes ⁄ CH2Cl2 [1:1]) to afford a reddish purple solid (44.0 mg,
39%): 1H NMR d )1.05 (br, 1H), )0.91 (br, 1H), 0.16 (s, 18H), 1.25 (t,
J = 8.5 Hz, 4H), 1.86 (s, 6H), 2.07 (s, 6H), 2.60 (s, 3H), 2.66 (s, 3H),
4.55 (t, J = 8.5 Hz, 4H), 4.77 (s, 2H), 7.23 (s, 2H), 7.48–7.51 (m, 2H),
7.97–8.00 (m, 2H), 8.22–8.24 (m, 1H), 8.35–8.37 (m, 1H), 8.55–8.57 (m,
1H), 8.68–8.73 (m, 5H), 8.75 (s, 1H), 9.16–9.18 (m, 1H); LD-MS obsd
936.4; FAB-MS obsd 936.4459, calcd 936.4466 (C58H64N4O4Si2); kabs
(toluene) 425, 565, 660 nm.
Zn-(II)-17,18-Dihydro-10-mesityl-15-[2-(3,5-bis(2-(trimethylsilyl)-
ethoxycarbonyl)phenyl)ethynyl]-18,18-dimethyl-5-(4-methylphenyl)-
porphyrin (ZnC-EI¢): A solution of FbC-EI¢ (37.5 mg, 40.0 lmol) in

1518 Chinnasamy Muthiah et al.
CH2Cl2 (10 mL) was treated with
a
methanol solution (1 mL)
1H), 9.15 (s, 1H), resonances from the NH and COOH protons of the
bacteriochlorin were not observed because of D-H exchange; LD-MS
obsd 769.0; FAB-MS obsd 769.3382, calcd 769.3390 (C49H44N4O5);
kabs (methanol) 387, 554, 755 nm.
containing Zn(OAc)2Æ2H2O (176 mg, 0.800 mmol) at room tempera-
ture for 17 h. The reaction mixture was diluted with CH2Cl2 (50 mL),
washed with saturated aqueous NaHCO3, dried (Na2SO4) and filtered.
The filtrate was concentrated to dryness. The residue was chromato-
graphed (silica, hexanes then hexanes ⁄ CH2Cl2 [1:2]) to afford a
greenish purple solid (35.5 mg, 85%): 1H NMR d 0.16 (s, 18H), 1.24 (t,
J = 8.5 Hz, 4H), 1.87 (s, 6H), 2.03 (s, 6H), 2.58 (s, 3H), 2.64 (s, 3H),
4.54 (t, J = 8.5 Hz, 4H), 4.67 (s, 2H), 7.20 (s, 2H), 7.44–7.48 (m, 2H),
7.90–7.93 (m, 2H), 8.10 (d, J = 4.3 Hz, 1H), 8.24 (d, J = 4.3 Hz, 1H),
8.41–8.43 (m, 2H), 8.51 (d, J = 4.3 Hz, 1H), 8.56 (d, J = 4.3 Hz, 1H),
8.61–8.63 (m, 2H), 8.65–8.67 (m, 1H), 9.03 (d, J = 4.3 Hz, 1H); 13C
NMR d )1.1, 17.7, 21.3, 21.6, 21.7, 31.6, 44.6, 51.5, 64.2, 91.6, 92.8,
94.3, 96.1, 125.0, 125.8, 126.0, 126.4, 127.1, 127.7, 127.9, 129.5, 129.6,
131.8, 132.6, 133.6, 134.2, 136.1, 137.39, 137.51, 138.5, 138.9 (two
overlapped resonances), 139.2, 145.8, 146.6, 147.5, 148.2, 155.00,
155.05, 164.3, 165.9, 171.6; LD-MS obsd 998.1; FAB-MS obsd
998.3563, calcd 998.3601 (C58H62N4O4Si2Zn); kabs (toluene) 431,
624 nm.
Zn(II)-15-[2-(3,5-Bis(2-(3,5-dicarboxyphenyl)ethynyl]-17,18-dihy-
dro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)porphyrin (ZnC-EI):
A solution of ZnC-EI¢ (20.0 mg, 20.0 lmol) in dry DMF (2 mL) was
treated with KF (23.2 mg, 0.400 mmol) at 75ꢁC for 12 h. The reaction
mixture was concentrated to dryness. The resulting residue was
chromatographed (silica, CH2Cl2 ⁄ methanol [5:1]) to afford a fraction
containing the mono-ester product (8.2 mg). Further elution with
methanol gave a fraction containing the title compound, which was
dried, dissolved in THF, filtered to remove silica gel and concentrated
to dryness to afford the title compound (8.8 mg). A solution of the
fraction containing the mono-ester in dry DMF (1.0 mL) was treated
with KF (11.6 mg, 0.200 mmol), chromatographed and worked-up in
the same manner to afford additional title compound (3.7 mg).
Altogether, 12.5 mg (78% yield) of the title compound was obtained
as a dark green solid: 1H NMR (methanol-d4) d 1.88 (s, 6H), 2.05
(s, 6H), 2.56 (s, 3H), 2.63 (s, 3H), 4.70 (s, 2H), 7.21 (s, 2H), 7.47–7.49
(m, 2H), 7.87–7.89 (m, 2H), 7.96 (d, J = 4.3 Hz, 1H), 8.10 (d,
J = 4.3 Hz, 1H), 8.28 (d, J = 4.7 Hz, 1H), 8.41–8.43 (m, 2H),
8.47–8.50 (m, 3H), 8.62–8.63 (m, 1H), 9.04 (d, J = 4.7 Hz, 1H), a
resonance from the COOH moieties of the chlorin was not observed
because of D-H exchange; LD-MS obsd 798.2; FAB-MS obsd
798.2194, calcd 798.2185 (C48H38N4O4Zn); kabs (methanol) 429, 600,
623 nm.
5-Methoxy-8,8,18,18-tetramethyl-2,12-bis(4-methylphenyl)-15-[2-(3,5-
bis(2-(trimethylsilyl)ethoxycarbonyl)phenyl)ethynyl]bacteriochlorin
(FbB-EI¢): Following a standard procedure (36,61,66,67), a mixture of
12 (26.0 mg, 0.0394 mmol), 10 (18.5 mg, 0.0473 mmol), Pd2(dba)3
(5.41 mg, 0.00591 mmol) and P(o-tol)3 (15.6 mg, 0.0512 mmol) was
placed into a 50 mL Schlenk flask. Toluene ⁄ TEA (15.0 mL [5:1]) was
added, and the reaction mixture was stirred at 50ꢁC. After 6 h, Pd2(dba)3
(5.41 mg, 0.00591 mmol) and P(o-tol)3 (15.6 mg, 0.0512 mmol) were
added. After 18 h, the mixture was concentrated to dryness. The
resulting residue was purified by column chromatography (silica,
hexanes then hexanes ⁄ CH2Cl2 [3:7]) to afford a purple solid (24.0 mg,
63%): 1H NMR (THF-d8) d )1.54 (br, 1H), )1.25 (br, 1H), 0.16 (s, 18H),
1.25 (t, J = 8.5 Hz, 4H), 1.90 (s, 6H), 1.94 (s, 6H) 2.61 (s, 6H), 4.39 (s,
2H), 4.49 (s, 3H), 4.54 (t, J = 8.5 Hz, 4H), 4.69 (s, 2H), 7.57–7.60 (m,
4H), 8.10–8.15 (m, 4H), 8.66–8.70 (m, 3H), 8.83 (s, 1H), 8.86 (s, 1H), 8.96
(s, 1H), 9.20 (s, 1H); 13C NMR (THF-d8) d )1.1, 14.3, 17.7, 22.9, 31.0,
31.5, 45.5, 46.1, 47.5, 52.4, 64.2, 92.7, 93.5, 94.1, 96.4, 98.1, 116.9, 120.1,
125.6, 127.1, 129.0, 129.7, 130.00, 130.03, 130.5, 131.2, 131.3, 131.8,
133.6, 133.7, 133.9, 134.1, 134.9, 136.2, 137.1, 137.3, 137.7, 137.8, 154.0,
163.3, 165.8, 169.5, 170.6; LD-MS obsd 969.8; FAB-MS obsd 968.4772,
calcd 968.4728 (C59H68N4O5Si2); kabs (CH2Cl2) 387, 549, 758 nm.
5-Methoxy-8,8,18,18-tetramethyl-2,12-bis(4-methylphenyl)-15-[2-(3,5-
dicarboxyphenyl)ethynyl]bacteriochlorin (FbB-EI): A solution of FbB-
EI¢ (19.0 mg, 0.0195 mmol) in 2.0 mL of THF was treated with
tetrabutylammonium fluoride (TBAF, 0.195 mL, 10 equiv, 1 M THF)
and stirred for 2 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, THF fi ethanol) to give a purple solid (14.4 mg, 95%): 1H
NMR (methanol-d4) d 1.90 (s, 6H), 1.93 (s, 6H) 2.61 (s, 6H), 4.39 (s,
2H), 4.49 (s, 3H), 4.69 (s, 2H), 7.58–7.63 (m, 4H), 8.06–8.12 (m, 4H),
8.53–8.55 (m, 2H), 8.70–8.71 (m, 1H), 8.77 (s, 1H), 8.80 (s, 1H), 8.90 (s,
Optical spectroscopy. Static absorption spectra (Cary 100) and
fluorescence spectra and quantum yields (Spex Tau2; 5 nm bandpass)
employed argon-purged solutions (1–10 lM for absorption and 0.3–
1 lM for emission). Fluorescence yields were obtained using argon-
purged samples with ZnTPP in toluene (Ff = 0.030) (70) as a standard
for ZnP-I¢ and ZnP-EI¢, and chlorophyll a in benzene (Ff = 0.325) (71)
as the standard for ZnC-EI¢ and FbB-EI¢. Fluorescence lifetime studies
employed 0.3–3 lM argon-purged solutions using a phase modulation
technique (72). The absorption coefficients (ꢀ) for the chlorin Zn(II)-
17,18-dihydro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)porphyrin
(ZnC-T5M10
) and free base bacteriochlorin 5-methoxy-8,8,18,18-
tetramethyl-2,12-bis(4-methylphenyl)bacteriochlorin (MeO-BC) in
benzonitrile were determined from the known values in toluene (23,
35) as follows—the solvent was removed from three 3 mL aliquots of a
stock solution of the compound (having measured absorbance spectrum
and concentration) via use of a benchtop vacuum centrifuge. Then 3 mL
of benzonitrile was added to each dried sample. The absorbance spectra
were measured and the resulting three determinations of ꢀ were
averaged.
RESULTS AND DISCUSSION
Synthesis of tetrapyrrole compounds
Tethered porphyrins. The synthetic approaches for the target
porphyrins are as follows: (1) synthesis of the meso-acrylic acid
derivative (ZnP-A) via a Knoevenagel condensation of a meso-
formylporphyrin and cyanoacetic acid, (2) synthesis of the
meso-isophthalic acid derivative (ZnP-I) via a Suzuki coupling
reaction with a bromo-porphyrin followed by basic hydrolysis
and (3) synthesis of the meso-ethynylisophthalic acid derivative
(ZnP-EI) via a Sonogashira coupling reaction with a bromo-
porphyrin followed by basic hydrolysis. A key issue was to
identify the suitability of the tether for surface attachment.
The target bromoporphyrin (Zn-5) for conversion to the
three tethered porphyrins is a known compound (61). A
refined synthesis at larger scale with improved procedures is
described here and shown in Scheme 1. The diacylation of
5-(4-methylphenyl)dipyrromethane (1) (59) with p-toluoyl
chloride under the standard reaction conditions (60) followed
by the treatment with Bu2SnCl2 in CH2Cl2 containing TEA
afforded the corresponding tin complex 2 (60). The tin
complex 2 was readily isolated by filtration through a pad of
silica, whereupon the resulting solid was crystallized using
diethyl ether and methanol. A large-scale synthesis of known
porphyrin 4 (61,62) was carried out using the tin complex 2
and dipyrromethane 3 (59). Thus, the reduction of tin complex
2 (5.30 g) with NaBH4 gave the putative dicarbinol 2-diol.
Condensation (63) of 2-diol with dipyrromethane 3 (1.09 g) in
the presence of improved acid catalysis conditions [Yb(OTf)3
in CH2Cl2 (64)] and oxidation with DDQ gave porphyrin 4
(1.60 g) in 37% yield. Bromination of 4 using 1 equiv of NBS
afforded the bromoporphyrin 5 (61) in 99% yield. Treatment
of 5 with Zn(OAc)2Æ2H2O in CHCl3 ⁄ methanol (4:1) at room
temperature gave Zn-5 in 85% yield.
The porphyrin bearing a cyanoacrylic acid (ZnP-A) tether is
readily prepared by derivatization of a meso-formylporphyrin.
A number of routes to meso-formylporphyrins have been
devised (73). A new route for the synthesis of meso-formyl-
porphyrins, which was discovered serendipitously, is shown in
Scheme 2. Treatment of bromoporphyrin 5 with diethyl-

Photochemistry and Photobiology, 2007, 83 1519
Scheme 1
malonate in the presence of NaH ⁄ DMSO at 100ꢁC for 20 h
gave the diethylmalonate porphyrin 6 in 80% yield. H NMR
1
spectroscopy, LD-MS and FAB-MS analyses established the
structure of diethylmalonate porphyrin 6. The characteristic
methine proton in 6 shows a sharp singlet at d 7.33 p.p.m. in
the 1H NMR spectrum. Treatment of 6 with 20% aqueous
HCl under reflux gave meso-formylporphyrin 7 in 78% yield.
Although unexpected, this route of meso-bromination, substi-
tution with diethyl malonate and in situ transformation to give
the meso-formyl substituted free base porphyrin, provides a
convenient alternative to traditional Vilsmeier formylation,
which typically requires use of copper porphyrins. The
reaction of 7 with Zn(OAc)2Æ2H2O gave Zn-7 in 95% yield.
Scheme 2
The synthesis of cyanoacrylic acid derivative ZnP-A is also
shown in Scheme 2. The Knoevenagel condensation (45) of
Zn-7 with cyanoacetic acid in the presence of piperidine gave the
crude piperidine salt, which upon subsequent treatment with
phosphoric acid (pH ꢀ 2) gave the carboxylic acid ZnP-A in
good yield (79%) without demetalation. The Knoevenagel
condensation was also applied to prepare the cyanoacrylic ester
derivative ZnP-A¢ as a benchmark compound. Thus, treatment

1520 Chinnasamy Muthiah et al.
of Zn-7 with methyl cyanoacetate in the presence of piperidine
afforded the cyanoacrylic ester derivative ZnP-A¢ in 76% yield.
The synthesis of the porphyrin bearing an isophthalic acid
(ZnP-I) tether was achieved through use of a Suzuki coupling
reaction. The reported procedure for the synthesis of phenyl-
linked porphyrin dyads and triads via the Suzuki coupling
reaction was applied (65). The first step entailed utilization of a
Suzuki coupling reagent with slight modifications of a litera-
ture method (74). Treatment of 4,4,5,5-tetramethyl-1,3,2-
dioxaborolane with dimethyl 5-iodoisophthalate in dry
CH3CN using PdCl2(dppf) and TEA at 85ꢁC afforded the
expected product 8 in 50% yield (Scheme 3). Equimolar
amounts of bromoporphyrin 5 and 8 were coupled in
toluene ⁄ DMF (2:1) using Pd(PPh3)4 (20 mol%) and K2CO3.
After 6 h at 95ꢁC, TLC analysis of the reaction mixture
indicated that some of the starting porphyrin remained
unreacted. Therefore, an additional amount of Pd(PPh3)4
(20 mol%) was added, and the reaction mixture was allowed
to stir for 12 h at 95ꢁC. The purification of the crude reaction
mixture by silica column chromatography afforded the por-
phyrin FbP-I¢ in 78% yield. Treatment of FbP-I¢ with
Zn(OAc)2Æ2H2O in CHCl3 ⁄ methanol (4:1) at room tempera-
ture gave ZnP-I¢ in 92% yield. The hydrolysis (45) of the ester
in ZnP-I¢ using NaOH followed by acidic workup gave the
isophthalic acid derivative ZnP-I in 98% yield.
The synthesis of a meso-ethynyl substituted porphyrin
(ZnP-EI) was readily achieved using a copper-free Sonoga-
shira coupling reaction (61). Thus, the reaction of Zn-5 with
dimethyl 5-ethynylisophthalate (75) in the presence of
Pd2(dba)3 and P(o-tol)3 gave ethynyl-porphyrin ZnP-EI¢ in
89% yield (Scheme 4). The hydrolysis (45) of the ester in ZnP-
EI¢ with sodium hydroxide (20 equiv per ester group) followed
by acidic workup gave the ethynylisophthalic acid derivative
ZnP-EI in good yield (91%).
A tethered chlorin. The route to a chlorin bearing a meso-
ethynylisophthalic acid (ZnC-EI) tether was slightly modified
from that employed for porphyrins. The chief difference was to
use a protecting group that could be removed under mild
conditions and thereby obtain the target containing free
carboxylic acids at a late stage in the synthesis without
affecting the macrocycle. To achieve this we chose the
2-(trimethylsilyl)ethyl moiety for the cleavable ester group.
The synthesis of the corresponding linker 10 is shown in
Scheme 5. Treatment of dimethyl 5-ethynylisophthalate with
NaOH in THF ⁄ MeOH ⁄ H2O (5:5:1) at reflux for 2 h followed
by acidic workup (10% aqueous HCl) gave the free ethynyl-
isophthalic acid 9 in quantitative fashion. The ethynyliso-
phthalic acid 9 is known; however, the prior method of
Scheme 4
Scheme 3

Photochemistry and Photobiology, 2007, 83 1521
Scheme 5
synthesis employed a different route (68). Following a litera-
ture procedure (69), linker 10 was obtained by reaction of 9
with 2-(trimethylsilyl)ethanol in the presence of DCC and
DMAP in 34% yield.
Synthesis of the chlorin bearing the meso-ethynylisophthalic
acid tether is shown in Scheme 6. The Sonogashira coupling of
meso-substituted bromochlorin 11 (27) with 10 in the presence
of Pd2(dba)3 and P(o-tol)3 in toluene ⁄ TEA (5:1) afforded FbC-
EI¢ in 39% yield. Treatment of FbC-EI¢ with Zn(OAc)2Æ2H2O
in CH2Cl2 ⁄ methanol (4:1) at room temperature gave ZnC-EI¢
in 85% yield. The deprotection of the 2-(trimethylsilyl)ethyl
groups in ZnC-EI¢ was performed using KF to afford the free
acid ZnC-EI in 78% yield.
Scheme 6
Characterization of tetrapyrrole compounds
A tethered bacteriochlorin. The route to a bacteriochlorin
bearing a meso-ethynylisophthalic acid tether followed that of
the chlorin synthesis. Stable synthetic bacteriochlorins have
become available only recently (35). The bacteriochlorin
bearing a 5-methoxy substituent (MeO-BC) was found to be
readily brominated at the 15-position upon treatment with 1
equiv of NBS in THF at room temperature, thereby affording
15-bromobacteriochlorin 12 in 73% yield (36). The Sonoga-
shira coupling of meso-bromobacteriochlorin 12 with 10 was
carried out in the same manner as for the corresponding
chlorin or porphyrin. Thus, the reaction of 12 and 10 in the
presence of Pd2(dba)3 and P(o-tol)3 in toluene ⁄ TEA (5:1) at
50ꢁC afforded the bacteriochlorin conjugate FbB-EI¢ in 63%
yield (Scheme 7). Deprotection (69) of the 2-(trimethylsi-
lyl)ethyl groups in FbB-EI¢ was readily achieved using TBAF
(5 equiv per ester group) to afford the corresponding free acid
FbB-EI.
Each of the tetrapyrrole compounds for solar-cell studies bears
a carboxylic acid for surface attachment (ZnP-A, ZnP-I, ZnP-
EI, ZnC-EI, FbB-EI). Corresponding tetrapyrrole compounds
bearing ester moieties were employed as benchmarks in
solution photophysical characterization studies (ZnP-A¢,
ZnP-I¢, ZnP-EI¢, ZnC-EI¢, FbB-EI¢).
Composition. All porphyrins, chlorins and bacteriochlorins
were characterized by 1H NMR spectroscopy, LD-MS and
FAB-MS analyses. In addition, 13C NMR spectroscopy was
performed for all compounds except those with insufficient
solubility. In general, the porphyrinic compounds with insuf-
ficient solubility were those bearing free carboxylic acids (with
the exception of ZnP-I). Additional characterization is
described in the following sections.
The cyanoacrylic acid derivative ZnP-A¢ (or ZnP-A) is a
trisubstituted alkene that in principle can exist in cis or trans

1522 Chinnasamy Muthiah et al.
(A)
ZnP-A'
x13
(B)
ZnP-I'
x20
(C)
ZnP-EI'
x18
(D)
ZnC-EI'
Scheme 7
x5
1
isomeric forms. H NMR spectroscopy and chromatography
of ZnP-A¢ (or ZnP-A) indicate the presence of only one
isomer. It is essential to establish the configuration, given that
the longest-wavelength absorption band of 2,2-disubstituted
ethenyl-porphyrins (or hydroporphyrins) shifts depending on
the cis or trans configuration. For example, Tamiaki and
Kouraba reported that trans-isomers of the synthetic ethenyl-
chlorins exhibited a longer-wavelength (up to 15 nm) transi-
tion than that of the corresponding cis-isomers (8). Only two
cyanoacrylic acid-substituted porphyrins (or hydroporphyrins)
have been reported to our knowledge, and each was reported
to exist as the trans-isomer (8,45).
(E)
FbB-EI'
400
500
600
700
800
900
The configuration of ZnP-A¢ was assigned upon compari-
son of the chemical shift of the 1H NMR spectrum for the
corresponding resonances of vinyl protons of the following
compounds (in CDCl3): (1) styrene, 6.72 p.p.m. (76); (2) (E)-
and (Z)-3-phenyl-2-propenenitrile; trans 7.30 p.p.m., cis
7.03 p.p.m. (77); (3) trans- and cis-cinnamic acid; trans
7.81 p.p.m., cis 7.07 p.p.m. (78); (4) (E)-2-cyano-3-phenyl-2-
propenoic acid, 8.32 p.p.m. (79); and (5) zinc-(II)-5,15-divinyl-
10,20-diphenyl porphyrin, 9.24 p.p.m. (80). On the basis of
these data, the expected chemical shift of the resonance for the
vinyl proton adjacent to the porphyrin meso carbon (51-H) of
ZnP-A¢ was calculated to be 10.64 p.p.m. for the trans-isomer
and 10.17 p.p.m. for the cis-isomer, respectively. The observed
resonance appeared at 10.92 p.p.m. Accordingly, the config-
Wavelength (nm)
Figure 2. Normalized absorption (solid) and fluorescence (dashed)
spectra in toluene at room temperature of (A) ZnP-A¢, (B) ZnP-I¢, (C)
ZnP-EI¢, (D) ZnC-EI¢ and (E) FbB-EI¢.
uration of ZnP-A¢ is provisionally assigned as the trans-isomer
(wherein the carboxy group is trans to the porphyrin) as shown
in Chart 2.
Absorption spectra. The absorption and fluorescence
spectra for compounds ZnP-I¢ (zinc porphyrin), ZnP-EI¢
(15-ethynyl-substituted zinc porphyrin), ZnP-A¢ (15-cyanoac-
rylate-substituted zinc porphyrin), ZnC-EI¢ (15-ethynyl-substi-

Photochemistry and Photobiology, 2007, 83 1523
Qy(0,0) transition (81,82). The band positions and assignments
are given in Table 1, which also presents the peak intensity
ratio (IB ⁄ IQ) of the B and Qy(0,0) bands. The table also gives
estimates for the molar absorption (extinction) coefficients (ꢀ)
on the basis of data for related compounds that we have
studied previously, including Zn(II)-tetraphenylporphyrin
(ZnTPP) (70,83) and Zn(II)-5-(4-ethynylphenyl)-10,15,20-tri-
phenylporphyrin (ZnU) (84); similarly, the values for the
three porphyrins in MeCN were estimated on the basis of data
for ZnU in benzonitrile (84). These estimates are used below to
obtain the radiative rate constant for decay of the S1 excited
state of each compound, which can be compared with values
derived from fluorescence yields and excited-state lifetimes.
Such comparisons provide cross-checks of the various param-
eters, including the molar absorption coefficients, for which
direct measurement errors of 30% or more are common.
The longest-wavelength Q absorption band for the com-
pounds in toluene shows a progressive redshift in position (k)
and increase in peak intensity with respect to the Soret (B)
(A)
ZnP-A'
x12
(B)
ZnP-I'
x29
band along the series ZnP-I¢ (k = 591 nm; IB ⁄ IQ
=
(C)
106) < ZnP-EI¢ (614 nm; 18) < ZnP-A¢ (621 nm; 13)
< ZnC-EI¢ (624 nm; 4.8) < FbB-EI¢ (757 nm; 0.93) (Fig. 2).
The largest combined redshift and intensification of the visible
absorption bands are observed upon (i) incorporation of the
cyanoacrylate tether in the porphyrin (ZnP-A¢ versus ZnP-I¢),
and (ii) progressing from chlorin to bacteriochlorin (ZnC-EI¢
to FbB-EI¢). Generally similar trends are observed for the
compounds in acetonitrile (Fig. 3).
ZnP-EI'
x17
Fluorescence spectra, quantum yields and lifetimes. The
Qy(0,0) and Qy(0,1) fluorescence bands are roughly mirror
symmetric to the Qy(0,0) and Qy(1,0) absorption features. The
shifts in the positions of the fluorescence features parallel those
in the absorption bands with a change in electronic structure
(tether and macrocycle reduction) in both toluene and aceto-
nitrile (Figs. 2 and 3 and Table 1). The exception to this trend
is ZnP-A¢, which shows a much broader spectrum than the
other compounds, with a loss of resolution of the vibronic
features. In addition, this compound shows an approximately
five-fold larger (Stokes) shift between the Q(0,0) absorption
and fluorescence features than the other two porphyrins, and
an even larger difference compared to the chlorin and
bacteriochlorin (Table 1).
(D)
ZnC-EI'
x7
(E)
FbB-EI'
Along the series of compounds in toluene, the fluorescence
quantum yield (Ff) increases in the following order: ZnP-I¢
(0.033) < ZnP-A¢
(0.063) < ZnP-EI¢
(0.12) ꢀ ZnC-EI¢
(0.12) < FbB-EI¢ (0.16). The lifetime of the lowest excited sin-
glet state increases in the order: ZnP-A¢ < (0.98 ns) < ZnP-I¢
(2.3 ns) < ZnP-EI¢ (2.5 ns) < ZnC-EI¢ (2.6 ns) < FbB-EI¢
(5.3 ns). These values for the porphyrins can be compared with
that of ZnTPP [Ff = 0.030 (70); s = 2.1 ns (85)]. Comparable
trends in fluorescence quantum yield and excited singlet-state
lifetime are observed in acetonitrile (Table 2). Close examina-
tion of these data indicates that the fluorescence quantum yield
(Ff) and lifetime (s) for ZnP-A¢ do not follow the same trend as
the other four compounds. For example, the fluorescence
quantum yield for ZnP-A¢ is greater than that for ZnP-I¢
(0.063 versus 0.033), but the reverse is true for the excited-state
lifetime (0.98 versus 2.3 ns).
400
500
600
700
800
900
Wavelength (nm)
Figure 3. Normalized absorption (solid) and fluorescence (dashed)
spectra in acetonitrile at room temperature of (A) ZnP-A¢, (B) ZnP-I¢,
(C) ZnP-EI¢, (D) ZnC-EI¢ and (E) FbB-EI¢.
tuted zinc chlorin) and FbB-EI¢ (15-ethynyl-substituted
free base bacteriochlorin) in toluene are shown in Fig. 2.
Analogous data for the compounds in acetonitrile are given in
Fig. 3. The absorption spectra for all the compounds contain a
near-UV Soret (B) band and a series of Q bands to longer
wavelengths. For the porphyrins, the long-wavelength absorp-
tion band represents the degenerate Qx,y(0,0) transitions,
whereas the band for the chlorin and bacteriochlorin is the
Excited-state decay properties. The fluorescence quantum
yield and excited-state lifetime often, but not always, parallel
one another among a set of tetrapyrrole compounds. A

1524 Chinnasamy Muthiah et al.
Table 1. Absorption and fluorescence spectral data.*
Bmax
Qx(1,0) absn
Qx(0,0) absn
Qy(0,0) absn
(IB ⁄ IQ)† absn Qy(0,0)‡ emsn
k (nm)
Dm§
k
e
k
e
k
e
k
e
Compound Solvent (nm) (mM)1 cm)1
)
(nm) (mM)1 cm)1
)
(nm) (mM)1 cm)1
)
(nm) (mM)1 cm)1
)
(cm)1
)
ZnP-A¢
ZnP-I¢
Toluene 449
MeCN 443
Toluene 426
MeCN 424
Toluene 441
MeCN 438
Toluene 431
MeCN 430
Toluene 387
MeCN 384
414
322
475
542
599
720
213
188
116
113
565
565
551
558
567
573
520
526
517
513
23
19
23
19
23
19
4.8
3.9
11
14
621
630
591
598
614
624
565
571
550
546
31
27
621
630
591
598
614
624
624
624
757
755
31
27
4.5
8.4
33
43
44
26
13
12
106
65
18
17
4.8
7.2
663
667
599
605
620
629
626
628
761
761
1020
880
230
190
180
130
50
100
70
90
4.5
8.4
33
43
9.4
8.2
44
47
ZnP-EI¢
ZnC-EI¢
FbB-EI¢
120
108
0.93
1.1
*All measurements were performed at room temperature in toluene or acetonitrile (MeCN). The molar absorption coefficient for each compound was
derived from the relative amplitudes of the bands (Figs. 2 and 3), using the value at a specific band maximum that was estimated as follows. The value
at the Qx,y(1,0) maximum of ZnP-A¢, ZnP-I¢ and ZnP-EI¢ in toluene is based on data for benchmark porphyrins ZnTPP (70,83) and ZnU (84);
similarly, the value for the three porphyrins in MeCN was estimated based on data for ZnU in benzonitrile (84). The value for ZnC-EI¢ in toluene at
the Qy(0,0) band is based on the data for Zn(II)-17,18-dihydro-10-mesityl-18,18-dimethyl-5-(4-methylphenyl)porphyrin (ZnC-T5 M10) in toluene
(23). The value estimated for FbB-EI¢ in toluene is based on the data for 5-methoxy-8,8,18,18-tetramethyl-2,12-bis(4-methylphenyl)bacteriochlorin
(MeO-BC) in toluene (35). The values for ZnC-EI¢ and FbB-EI¢ at the Qy(0,0) maximum in MeCN are based on those obtained for these compounds in
benzonitrile determined here (see Materials and Methods). †Ratio of the peak intensities of the Soret (B) and Qy(0,0) absorption bands. ‡Qy(0,0)
fluorescence maximum. §Difference in the energies of the Qy(0,0) absorption and fluorescence bands (i.e. the Stokes shift).
Table 2. Summary of photophysical data.
SB
[(k )
)1]– (ns)
nr
Cmpd
Solvent
Ff*
sf† (ns)
[(kf))1]‡ (ns)
[(kSf B
)
)1]§ (ns)
[(knr
)
)1] || (ns)
ZnTPP
ZnP-A¢
Toluene
Toluene
MeCN
Toluene
MeCN
Toluene
MeCN
Toluene
MeCN
Toluene
MeCN
0.030#
0.063
0.044
0.033
0.026
0.12
0.098
0.12
0.11
2.1**
0.98
0.96
2.3
2.1
2.5
2.4
2.6
2.9
5.3
70
16
22
70
79
21
24
21
25
32
41
57
24
33
56
68
28
37
24
35
16
19
2.2
1.0
1.0
2.4
2.2
2.8
2.7
3.0
3.3
6.4
6.5
2.2
1.0
1.0
2.4
2.2
2.7
2.6
2.9
3.2
7.9
7.9
ZnP-I¢
ZnP-EI¢
ZnC-EI¢
FbB-EI¢
0.16
0.14
5.6
*Fluorescence quantum yield ( 15%) determined using B-band excitation and chlorophyll a in benzene (Ff = 0.325) (71) as a standard for FbB-
EI¢ and ZnC-EI¢, and ZnTPP in toluene (Ff = 0.030) (70) as a standard for ZnP-EI¢, ZnP-I¢ and ZnP-I¢. †Excited singlet-state lifetime ( 10%)
determined using fluorescence modulation spectroscopy and B-band excitation. ‡The time constant (inverse of rate constant) for the radiative
(spontaneous fluorescence) decay pathway of the excited singlet obtained using the fluorescence quantum yield and lifetime via the equation
(kf))1 = s ⁄ Ff. §The time constant (inverse of rate constant) for the radiative decay pathway of the excited singlet obtained using the Strickler–Berg
relationship (Eq. 6) and the absorption and fluorescence spectra (Figs. 2 and 3 and Table 1). ||The overall time constant (inverse of the combined
rate constant) for the nonradiative decay pathways (internal conversion plus intersystem crossing) of the excited singlet obtained using Eq. 5. –The
overall time constant (inverse of the combined rate constant) for the nonradiative decay pathways (internal conversion plus intersystem crossing) of
the excited singlet obtained using Eq. 8. #From Seybold and Gouterman (70). **From Tomizaki et al. (85).
parallelism is generally expected because these two observables
are related by the rate constants for the radiative (spontaneous
fluorescence) (kf) and internal conversion (kic) decay pathways
of the excited singlet state to the ground state, and for
intersystem crossing (kisc) to the triplet excited state by the
following equations.
Combining Eqs. (1) and (2) gives the radiative rate constant
via the expression
Uf
kf ¼
:
ð3Þ
s
An estimate for kf can be obtained for each compound in a
given solvent using the measured values for Ff and s. The
inverse of kf (units of nanoseconds) is listed in the fifth column
of Table 2. It is also convenient to combine the rate constants
for the two nonradiative decay routes of the lowest excited
singlet state to obtain
1
s ¼
ð1Þ
ð2Þ
kfþkicþkisc
kf
Uf ¼
kfþkicþkisc

Photochemistry and Photobiology, 2007, 83 1525
same in the S0 and S1 states (breaking the mirror symmetry
relationship).
knr ¼ kicþkisc
Combining Eqs. (4) and (1) gives
:
ð4Þ
ð5Þ
Comparisons among the values obtained for the com-
pounds in toluene are useful in showing reasonable consistency
of the various measurements and parameters. For the por-
phyrin bearing the aryl ester tether (ZnP-I¢), the radiative rates
kf = (70 ns))1 and kSf B = (56 ns))1 agree well with each other.
These values are also in concert with those for zinc tetra-
phenylporphyrin (ZnTPP), namely kSf B ꢀ (60 ns))1 obtained
by Seybold and Gouterman (70) and kSf B = (57 ns))1 and
kf = (70 ns))1 obtained here (Table 2). For the porphyrin
bearing the ethynylaryl ester tether (ZnP-EI¢), the radiative
rates kf = (21 ns))1 and kSf B = (28 ns))1 agree well, as do
those for the porphyrin bearing the cyanoacrylate tether (ZnP-
A¢), for which kf = (16 ns))1 and kfSB = (24 ns))1. Thus,
amongst the three zinc porphyrins, both kf and kSf B increase in
the order ZnP-I¢ < ZnP-EI¢ < ZnP-A¢. This trend reflects an
increasing radiative probability, which is in keeping with the
increased intensity of the Qx,y(0,0) band that occurs (in parallel
with a redshift) along this series (Figs. 2 and 3 and Table 1).
The rates kf = (21 ns))1 and kSf B = (24 ns))1 for zinc
chlorin ZnC-EI¢ are both comparable to those for the
porphyrin analog ZnP-EI¢. The deviation between the two
estimates for the radiative rate is largest for the free base
bacteriochlorin FbB-EI¢ and the reason is unclear. In partic-
ular, the value of kf = (32 ns))1 is comparable to the average
value kf = (27 ns))1 obtained from the fluorescence yield and
excited-state lifetime for a series of synthetic free base and zinc
bacteriochlorins (H. L. Kee and D. Holten, unpublished). On
the other hand, the shorter value, kSf B = (16 ns))1, obtained
for FbB-EI¢ from the Strickler–Berg analysis is consistent with
the greater oscillator strength in the Qy absorption manifold
compared to that for zinc chlorin ZnC-EI¢ (Table 1).
knr ¼ sꢁ1 ꢁ kf:
An estimate for knr can be obtained for each compound in a
given solvent using the measured value of s and the value of kf
obtained using Eq. (3) (seventh column of Table 2).
Another estimate for the radiative rate constant can be
obtained by integrating the absorption profile for the S0 fi S1
vibronic transitions. For this purpose, it is common to utilize
the Strickler–Berg relationship (86), which takes into account
the fact that the S0MS1 (induced) absorption and (spontaneous
fluorescence) emission bands are not superimposed but obey
an (approximate) mirror symmetry relationship. The Strickler–
Berg formula is as follows:
Z
ꢀ
ꢁꢁ1
gl
e
~m
ꢁ3
kSf B ¼ 2:88 ꢂ 10ꢁ9n2 mf
ð6Þ
~
~
dm
Av gu
Here, n is the refractive index of the solvent and gl ⁄ gu is the
ratio of the degeneracy of the lower (S0) state to that of the
upper (S1) state. The integration is over the S0 fi S1 absorp-
tion profile (i.e. encompassing the Qy(0,0) and Qy(1,0) bands)
expressed as the molar absorption coefficient (ꢀ) versus the
~
wavenumber (m) position. The quantity in Eq. 6 in brackets is
derived from the fluorescence profile as follows:
R
ꢀ
ꢁꢁ1
~
~
IðmÞdm
ꢁ3
R
~
mf
¼
ð7Þ
Av
~mꢁ3Ið~mÞd~m
The calculation of kSf B for each of the five tetrapyrrole esters
utilized the absorption and fluorescence spectra in Figs. 2 and
3 along with the estimated molar absorption coefficients in
Table 1. The calculations were performed both using Origin
(Microcal) software and PhotochemCad (2). Due to the
overlap of the x- and y-polarized Q-band absorption transi-
tions of porphyrins ZnP-A¢, ZnP-EI¢ and ZnP-I¢, a degeneracy
ratio of 1/2 was used for each of these compounds (and the
reference compound ZnTPP). The degeneracy ratio is 1 for
ZnC-EI¢ and for FbB-EI¢. The values for the inverse of the kfSB
(units of nanoseconds) calculated using the Strickler–Berg
analysis are listed in the sixth column of Table 2. In analogy
with the logic underpinning Eq. (4), the value of kSf B can be
used to obtain a second measure of the effective nonradiative
excited-state decay rate as follows:
The S1 fi S0 radiative decay route is in competition with the
S1 fi S0 internal conversion and S1 fi T1 intersystem-crossing
nonradiative decay pathways. The net rate for the latter two
routes [Eq. (5)] is virtually the same for porphyrin ZnP-I¢
[knr = (2.2 ns))1
] and the reference compound ZnTPP
(Table 1). The effective nonradiative decay for arylethynyl
porphyrin ZnP-EI¢ [knr = (2.8 ns))1] and arylethynyl chlorin
ZnC-EI¢ [knr = (3.0 ns))1] is only modestly slower, while that
for free base bacteriochlorin FbB-EI¢ [knr = (6.4 ns))1] is
considerably reduced. The largest effective nonradiative decay
rate is found for porphyrin ZnP-A¢ [knr ꢀ (1 ns))1]. This
difference is most likely due to enhanced internal conversion to
the ground state. This possibility is consistent with the much
larger (Stokes) shift between the Q(0,0) absorption and
fluorescence bands (Table 1) and the much broader bands in
both spectra for this compound compared to the other
tetrapyrroles studied here. These combined data are analogous
to those observed for porphyrins that are rendered nonplanar
by steric interactions involving the substituents, leading to
increased macrocycle conformational excursions, which may
be enhanced upon photoexcitation (87–89). Regardless of the
ultimate origin, the presence of the cyanoacrylate group in the
tether for ZnP-A¢ (and presumably the cyanoacrylic acid group
in ZnP-A) results in enhanced nonradiative decay that
shortens the excited-state lifetime. However, this lifetime is
still sufficiently long (ꢀ1 ns) that effective electron transfer
from the photoexcited sensitizer to a semiconductor surface
should be possible, as is the case for the other tetrapyrrole
SB
nr
k
¼ sꢁ1 ꢁ kSf B
:
ð8Þ
The results are listed in the last column of Table 2.
Inspection of Table 2 indicates that there is generally good
agreement of the values for the radiative rate obtained from
the measured fluorescence lifetime and yield (kf) and that
obtained from the Strickler–Berg analysis of the absorption
and fluorescence profiles (kfSB). The largest deviation is found
for the free base bacteriochlorin FbB-EI¢. The agreement
between the values is good considering the errors in the various
measurements and in the assumptions underlying the Stric-
kler–Berg analysis. The latter can be an issue for some
porphyrins because the overtone vibronic transitions derive
most of their intensity from Herzberg-Teller coupling rather
than from Franck-Condon overlap, which may not be the

1526 Chinnasamy Muthiah et al.
bide-a’s from methyl E ⁄ Z-pyropehophorbide-a 131-ketoximes.
J. Heterocycl. Chem. 42, 835–840.
sensitizers studied here, which have even longer excited-state
lifetimes. The effect of electronic coupling through the linker
and thermodynamic considerations owing to the different
macrocycles is the subject of a companion article in which the
contributions of the light-harvesting and redox characteristics
to solar-cell efficiency among the five sensitizers are
analyzed (3).
15. Kelley, R. F., M. J. Tauber and M. R. Wasielewski (2006)
Intramolecular electron transfer through the 20-position of a
chlorophyll a derivative: An unexpectedly efficient conduit for
charge transport. J. Am. Chem. Soc. 128, 4779–4791.
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linked dimeric derivatives of chlorophyll a, pyrochlorophyll a,
chlorophyll b, and bacteriochlorophyll a. J. Org. Chem. 45, 1969–
1974.
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.F.B. (DE-FG02-
05ER15660), D.H. (DE-FG02-05ER15661) 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 NCSU Facility was obtained from the North
Carolina Biotechnology Center and the NSF.
17. Struck, A., E. Cmiel, I. Katheder, W. Schafer and H. Scheer
(1992) Bacteriochlorophylls modified at position C-3: Long-range
intramolecular interaction with position C-132. Biochim. Biophys.
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18. Mironov, A. F., M. A. Grin, A. G. Tsiprovskii, A. V. Segenevich,
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