Synthesis of porphyrin dyads with potential use in solar energy conversion

Article abstract of DOI:10.1039/a907428g

A convenient procedure for the synthesis of porphyrin derivative dyads is described. The dyads consist of a free base porphyrin covalently linked to a zinc porphyrin or ferrocene by an amide bond. 5-(4-Substituted phenyl)- 10,15,20-tris(4-methylphenyl) po

Full text of DOI:10.1039/a907428g

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J O U R N A L O F
Synthesis of porphyrin dyads with potential use in solar
energy conversion
Fernando Fungo, Luis A. Otero, Leonides Sereno, Juana J. Silber and Edgardo N. Durantini*
Departamento de Qu Âõ mica y F Âõ sica, Universidad Nacional de R Âõ o Cuarto, Agencia Postal Nro 3,
R Âõ o Cuarto 5800, Argentina. E-mail: edurantini@exa.unrc.edu.ar
C H E M I S T R Y
Received 14th September 1999, Accepted 2nd December 1999
A convenient procedure for the synthesis of porphyrin derivative dyads is described. The dyads consist of a free
base porphyrin covalently linked to a zinc porphyrin or ferrocene by an amide bond. 5-(4-Substituted phenyl)-
10,15,20-tris(4-methylphenyl) porphyrins were synthesized from meso-(4-methylphenyl) dipyrromethanes 1,
which was obtained with appreciable yield (83%). The reaction of dipyrromethane 1 with a mixture of two
appropriate substituted benzaldehydes affords the desired meso-substituted porphyrins, which can be easily
separated by ¯ash chromatography. These porphyrins bearing either one 4-acetamidophenyl group 2 or 4-
carboxymethylphenyl group 3, and three 4-methylphenyl peripheral functional groups, were prepared with
notable yields (15±17%) in a two-step one-¯ask reaction. Basic hydrolysis of the porphyrins 2 and 3 yielded
amino 4 and acid porphyrin 5, respectively. Treatment of 5 with zinc acetate afforded the corresponding metal
complex Zn-acid porphyrin 6. The dyads 7 and 8 were obtained by the coupling reaction between the acid
chloride derivatives of either Zn-acid porphyrin 6 or ferroceneacetic acid and amino porphyrins 4, respectively.
The present strategy may be easily used for preparation of other similar dyad derivatives. These compounds
could have interesting applications in electronic materials. Preliminary studies of light energy conversion by
SnO electrodes coated with porphyrin dyads 7 and 8 were performed. The results show that dyads 7 and 8
2
may be suitable for solar energy conversion devices.
1
2,13
aldehyde condensation.
tical in nature and usually multiple porphyrin products are
However, this approach is statis-
Introduction
1
2
The porphyrins lie at the focus of several different ®elds of
1
research. Like the photosynthetic reaction center, many
obtained. The isolation requires slow and long chromato-
graphic separation. Even so, obtaining the pure porphyrin is
1
4
arti®cial assemblies incorporate porphyrin and metallopor-
phyrin derivatives as light receptors in energy conversion and in
not always possible and the yield can be very poor. On the
other hand, porphyrins bearing four different meso-substitu-
tents (ABCD-porphyrin) can be synthesized via a stepwise
synthetic approach. Rational syntheses of ABCD-porphyrins
2
,3
the design of molecular-scale electronic devices.
Also,
porphyrin derivatives have been used in solar energy conver-
sion, which takes place in the spectral sensitization of wide
bearing four different aryl rings were performed from
15±17
4
±6
band gap semiconductors. The energy difference between the
conduction band edge of an n-type semiconductor and the
oxidation potential of the excited adsorbed dye, provides a
MacDonald's routes.
The ability to place four different
groups around the periphery of the porphyrins should enable
synthesis of sophisticated building block porphyrins. The yield
in the porphyrin condensation is not high and this condensa-
tion is mainly useful for synthesis of regioisomerically pure
3
ABCD-porphyrins. However, for AB -porphyrins the stepwise
method described above requires many steps (i.e. about 8
7
,8
driving force for photo-induced charge injection.
Therefore, the synthesis of well-de®ned asymmetric por-
phyrin derivatives is of great interest for the development of
new molecular structures. In the present work we are interested
in the synthesis of porphyrin dyads, in which the porphyrin
moiety is attached to either Zn-porphyrin 7 or ferrocene 8 by an
amide bond. Porphyrin dimers containing structural moieties
with both different singlet state energies and redox properties
can undergo singlet±singlet energy transfer or photoinduced
starting from pyrrole and aldehydes). More direct approaches
to trans-substituted porphyrins (ABAB-porphyrins) are pro-
vided by condensation of dipyrromethane with aldehyde. These
porphyrins require access to meso-substituted dipyrromethane,
which can be synthesized from the reaction of aldehyde with
9
18
electron transfer. Porphyrin±ferrocene dyad 8 has a good
chance of showing intramolecular photoinduced electron
excess of pyrrole catalyzed by acid.
This paper reports a convenient procedure for the synthesis
1
0
transfer. Thermodynamically, ¯uorescence quenching of the
porphyrin moiety could occur by photoinduced electron
transfer from ferrocene to the cyclic tetrapyrrole ®rst excited
singlet state to yield a charge-separated species. It has been
demonstrated that several covalently linked systems containing
synthetic porphyrin as electron acceptor can undergo photo-
of
5-(4-substituted
phenyl)-10,15,20-tris(4-methylphenyl)
porphyrins from meso-(4-methylphenyl) dipyrromethanes 1.
Therefore, dipyrromethanes 1 were synthesized from 4-
methylbenzaldehyde and excess of pyrrole. The reaction of
dipyrromethane
1 with a mixture of two appropriate
substituted benzaldehydes affords a mixture of three meso-
substituted porphyrins, which can be easily separated by ¯ash
chromatography. Thus, the desired meso-substituted por-
phyrins 2 and 3, bearing one 4-aminophenyl group or 4-
acetoxyphenyl group and three identical peripheral functional
groups have been prepared with appreciable yields (15±17%) in
a two-step one-¯ask reaction. Preliminary studies of solar
1
1
initiated electron transfer in high yield.
The proposed dyads require the synthesis of peripherally
asymmetric porphyrins, such as meso-substituted porphyrins
bearing a phenyl group substituted with the functional group to
make the link and three identical substituted phenyl groups
(AB -porphyrin). Such porphyrins, bearing two different types
3
of meso-substituents, can be prepared by a binary mixed
2
energy conversion by SnO electrodes coated with porphyrin
DOI: 10.1039/a907428g
J. Mater. Chem., 2000, 10, 645±650
This journal is # The Royal Society of Chemistry 2000
645

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dyads 7 and 8 were performed. The results showed that these
structures could be used as materials for light energy
1
9
conversion.
Results and discussion
Synthesis of porphyrins
The condensation of 4-methylbenzaldehyde with a large excess
of pyrrole (1 : 48 aldehyde : pyrrole molar ratio), catalyzed by
tri¯uoroacetic acid at room temperature, affords meso-(4-
methylphenyl) dipyrromethane 1. Flash chromatography on
silica gel, using n-hexane±ethyl acetate±triethylamine
(
80 : 20 : 1) as eluent, yields 83% of the dipyrromethane 1.
The use of a mildly basic medium (triethylamine $1%)
prevents the decomposition of the dipyrromethane on the silica
column, which is slightly acidic. Thus, dipyrromethane 1 is
stable in the puri®ed form upon storage at 0 ³C in a nitrogen
atmosphere and the absence of light. High purity is essential for
its application in the synthesis of asymmetric meso-substituted
porphyrins.
The 5-(4-substituted phenyl)-10,15,20-tris(4-methylphenyl)
porphyrins 2 and 3 were synthesized by the acid-catalyzed
condensation of dipyrromethane 1 (Scheme 1) and the corres-
ponding 4-substituted benzaldehydes in chloroform at room
temperature (Scheme 2).
Mixed-benzaldehyde±dipyrromethane condensations were
performedusingabout2.2 : 1.4 : 1molarratioofdipyrromethane
1, 4-methylbenzaldehyde and 4-substituted benzaldehyde,
respectively. This ratio of reactants gives the best yield of the
selected porphyrin. The ®rst reaction step was performed using
3 2
catalytic amounts of BF ?O(Et) , and chloroform as solvent, at
Scheme 2 Synthesis of porphyrins 4 and 5.
room temperature. In the second step, the reaction mixture was
subjected to oxidation with 2,3-dichloro-5,6-dicyano-1,4-ben-
zoquinone (DDQ). Thus, this mixed condensation affords a
mixture of three porphyrins, the 5,10,15,20-tetra(4-methylphe-
nyl) porphyrin, the 5-(4-substituted phenyl)-10,15,20-tris(4-
substituted phenyl) porphyrin 2 or 3 and the 5,15-bis(4-
substituted phenyl)-10,20-bis(4-substituted phenyl) porphyrin.
The three porphyrins were easily separated by ¯ash
chromatography with high purity using dichloromethane±
methanol or acetone gradient. In both cases, the ®rst purple
band corresponds to meso-tetra-substituted porphyrin, the
second is the desired mono-amide (or ester) porphyrin 2 or 3
and the third more polar band belongs to bis-amido (or ester)
porphyrin. Thus, the desired porphyrins 2 and 3, bearing one
selected porphyrin, would require many steps. This present
strategy may be used for the preparation of other similar
porphyrin derivatives bearing only one different peripheral
phenyl substituent.
Amide and ester porphyrins 2 and 3 were hydrolyzed to
amine 4 and acid 5 porphyrin by heating in tetrahydrofuran±
methanol±KOH medium. Acid porphyrin 5 was treated with
zinc acetate to afford Zn-acid porphyrin 6 in 100% conversion.
Synthesis of dyads
Scheme 3 shows the synthesis of dyads 7 and 8 by coupling an
amide bond. To synthesize the desired porphyrin dyads, the
carboxylic acid group of the Zn-acid porphyrin or ferrocene-
acetic acid was transformed to an acid chloride using thionyl
chloride in dry toluene and pyridine. The formation of the acid
chloride was evidenced by TLC (dichloromethane±acetone 5%)
after mixing a small portion of the reaction mixture with
methanol. The acid chloride is not isolated but immediately
used for coupling with amino porphyrin 4. The reaction
mixture was allowed to react for 5 h, to yield after work up the
desired porphyrin dyads 7 and 8 with 76% and 79% yields,
respectively.
4
-acetamidophenyl or 4-carboxymethylphenyl group, respec-
tively, and three identical peripheral functional groups, were
obtained with appreciable yields of 15±17%.
Also, amide porphyrin 2 was previously synthesized by a
condensation of a binary mixture of 4-acetamidobenzaldehyde,
4
-methylbenzaldehyde and pyrrole in propionic acid. The
reaction work up is not simple because of the dif®culty of
removing propionic acid and the tar present, yielding after a
1
4
long procedure of puri®cation about 3.6%. Hence, the
dipyrromethane method presents two main advantages for
3
the synthesis of AB -porphyrins over the modi®ed Alder
1
3
method: the easier work up and the higher yields obtained.
The present method involves only a two-step one-¯ask
Absorption and ¯uorescence spectra
1
5±17
reaction, while the stepwise method,
which gives a pure
The absorption spectra of porphyrin monomers and dyads in
dichloromethane are shown in Fig. 1. The spectrum of the free
base amino porphyrin 4 exhibits a Soret absorption at 420 nm
and Q-band maxima at 518, 554, 594 and 650 nm, while Zn-
acid porphyrin 6 has maxima at 420 nm (Soret) and two Q-
bands at 549 and 588 nm. Fig. 1a shows the spectra in the 490±
7
00 nm region of the monomers 4 and 6, and the dyad 7. The
spectrum of the dyad 7 has a Soret band at 420 nm and Q-
bands at 518, 552, 592 and 650 nm. This spectrum is essentially
a linear combination of the spectra of the monomers, with only
minor differences in wavelength maxima and band shapes
Scheme 1 Synthesis of meso-(4-methylphenyl) dipyrromethane 1.
46 J. Mater. Chem., 2000, 10, 645±650
6

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Fig. 1 Normalized absorption spectra in dichloromethane of: (a)
amino porphyrin 4 (dashed curve), Zn-acid porphyrin 6 (dotted
curve), dyad 7 (solid curve) and of a linear combination of the spectra
of porphyrins 4 and 6 (open circles); (b) amino porphyrin 4 (dashed
curve) and dyad 8 (solid curve).
Scheme 3 Synthesis of porphyrin dyads 7 and 8.
generated photocurrent when the electrodes are illuminated
with monochromatic light, by using eqn. (1),
(
Fig. 1a). The spectra of dyad 8 and the amino porphyrin 4 are
compared in Fig. 1b. The Soret band of dyad 8 has a maximum
at 420 nm and the Q-bands are at 518, 553, 593 and 650 nm.
The ferroceneacetic acids do not absorb in the region shown
and moreover the dyad 8 spectrum is not changed with respect
to the amino porphyrin 4. Thus, in both dyads, the absorption
spectra are consistent with only a weak interaction between the
moieties and the two chromophores retain their individual
identities.
The corrected emission spectra were taken in dichloro-
methane exciting the sample at 550 nm. The free base amino
porphyrin 4 features maxima at 652 and 715 nm, while Zn-acid
porphyrin 6 has maxima at 598 and 646 nm. The ¯uorescence
spectrum of dyad 7 presents two maxima at 653 and 716 nm.
These results show that singlet±singlet energy transfer ef®-
ciently occurs in dyad 7. Porphyrin±ferrocene dyad 8 has an
emission spectrum with maxima at 653 and 717 nm, which has
a spectral shape similar to porphyrin 4. However, emission is
quenched in the dyad 8 by about 20% relative to monomer 4.
IPCEꢀ%†~100ꢀisc1240†=ꢀIincl† (1)
where isc is the short circuit photocurrent (A cm ), Iinc is the
22
22
incident light intensity (W cm ), and l is the excitation
wavelength (nm). The photo-induced charge separation
ef®ciencies, with IPCE around 10% were found for both
dyads (Fig. 2).
There is a close correspondence between the photocurrent
action spectrum and the absorption spectrum of the electrodes
(
Fig. 2). This fact con®rms that light absorption by the
porphyrin dyads is the initial step in the generation of
photo-induced charge transfer mechanism, like in the natural
6
photosynthetic apparatus.
Conclusion
Two basic steps were used sequentially in the synthesis: (1)
meso-(4-methylphenyl) dipyrromethane 1 was formed from 4-
methylbenzaldehyde and excess pyrrole catalyzed by acid;
Photoelectrochemical results
(
2) the desired porphyrins 4 and 5, bearing one 4-acetamido-
Preliminary results on photoelectrochemical properties of
dyads 7 and 8 over SnO
showed good light harvesting. The incident-photon-to-photo-
current ef®ciency (IPCE) was obtained through measuring the
phenyl or 4-carboxymethylphenyl group and three identical
peripheral methyl groups, were obtained with appreciable
yields of 15±17% by condensation of dipyrromethane 1 with
appropriate benzaldehydes. The resultant amido or ester
2
nanostructured semiconductor
J. Mater. Chem., 2000, 10, 645±650
647

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(
250 microns) thin layer chromatography plates from Analtech
and silica gel (230±400 mesh) for column chromatography
from Aldrich were used. Fluorescence spectra were recorded on
a Spex FluoroMax ¯uorimeter.
Starting materials
All the chemicals from Aldrich were used without further
puri®cation, except hydroquinone, which was recrystallized
from toluene. Toluene, dichloromethane and chloroform (GR
Ê
grade) from Merck were distilled and stored over 4 A
molecular sieves.
Synthesis of meso-(4-methylphenyl)dipyrromethane (1)
A solution of 4-methylbenzaldehyde (1.2 g, 10 mmol) and
pyrrole (33.3 mL, 480 mmol) was degassed by bubbling with
argon for 15 min, then tri¯uoroacetic acid (231 mL, 3 mmol)
was added. The solution was stirred for 30 min at room
temperature, at which point no starting aldehyde was shown
by TLC analysis (cyclohexane±ethyl acetate±triethylamine;
8
(
0 : 20 : 1). The mixture was diluted with dichloromethane
150 mL), washed with aqueous 0.1 M NaOH and then washed
with water. The organic phase was dried with MgSO , ®ltered
4
and the solvent was removed under reduced pressure. The
unreacted pyrrole was removed by vacuum distillation at
4
0 ³C. The product was puri®ed by ¯ash chromatography
silica gel, cyclohexane±ethyl acetate±triethylamine; 80 : 20 : 1)
(
and yielded 1.96 g (83%) of the pure meso-(4-methylphenyl)di-
z
pyrromethane 1. MS m/z 236.1 (M ) (236.1315 calculated for
16 16 2
C H N ). Spectroscopic data of 1 agree with those previously
1
8
reported.
Synthesis of meso-substituted porphyrins
-(4-Aminophenyl)-10,15,20-tris(4-methylphenyl) porphyrin
5
4). A solution of 4-methylbenzaldehyde (174 mg, 1.45
(
mmol), 4-acetamidobenzaldehyde (307 mg, 1.88 mmol) and
meso-(4-methylphenyl)dipyrromethane 4 (718 mg, 3.04 mmol)
in 250 mL of chloroform was purged with argon for 15 min.
Fig. 2 Absorption spectrum (solid curve) and photocurrent action
spectrum (open circles) of electrode ITO/SnO /Dyad, for (a) dyad 7 and
b) dyad 8.
2
(
3 2
Then BF ?O(Et) (0.44 mmol, 176 mL of 2.5 M stock solution
in chloroform) was added. The solution was stirred for 80 min
at room temperature. Then, DDQ (340 mg, 1.50 mmol) was
added and the mixture was stirred for an additional 1 h at room
temperature. The solvent was removed under reduced pressure
and ¯ash column chromatography (silica gel, dichloro-
methane±methanol 1%, gradient from 0.01 to 1% methanol)
yielded 155 mg (15%) of the pure selected 5-(4-acetamidophe-
nyl)-10,15,20-tris(4-methylphenyl) porphyrin 2 as the second
porphyrin moving band. TLC analysis (dichloromethane±
methanol 3%) Rf 0.46. MS m/z 713.3 (M ) (713.3158
39 5 1
calculated for C49H N O ).
To a solution of acetamido porphyrin 2 (110 mg, 0.15 mmol)
in 50 mL of THF±methanol 2 : 1 was added 10 mL of KOH
porphyrins 4 and 5 were hydrolyzed to amine and acid
porphyrin, respectively, by heat in tetrahydrofuran±methanol±
KOH medium. Zn-acid porphyrin 6 was obtained by treatment
of 5 with zinc acetic acid. Dyads 7 and 8 were formed by
coupling reaction between the corresponding acid chloride and
amino porphyrin 4.
These dyads have two moieties separated by a relatively rigid
amide linkage, which provides electronic coupling between the
2
0
chromophores.
Particularly, dyad 7 contains structural
z
moieties with different singlet state energies, undergoing
singlet±singlet energy transfer ef®ciently to the free base after
2
1
irradiation with visible light. In the porphyrin±ferrocene dyad
, the redox properties of both moieties indicate a possible
photo-induced electron transfer quenching of porphyrin
8
5
0%. The reaction mixture was stirred under an argon
1
0
atmosphere for 22 h at 60 ³C. Then, the mixture was diluted
with dichloromethane and washed with aqueous sodium
carbonate. The organic solvent was evaporated under reduced
pressure. Flash column chromatography (silica gel, dichloro-
methane±acetone 2%) afforded 298 mg (76%) of the desired
emission. However, further evidence for electron transfer
from ferrocene to the cyclic tetrapyrrole is required. Further
studies concerning the mechanism of photocurrent generation
are presently in progress in our laboratory.
5
-(4-aminophenyl)-10,15,20-tris(4-methylphenyl) porphyrin 4.
z
Experimental
47 37 5
MS m/z 671.3 (M ) (671.3052 calculated for C H N ).
Spectroscopic data of amine porphyrin 4 coincide with those
14,22
previously reported.
General
Absorption spectra and FT-IR spectra were recorded on a
Shimadzu UV-2401PC and on
a
Nicolet Impact 400,
5-(4-Carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrin
(5). A solution of 4-methylbenzaldehyde (186 mg, 1.55 mmol),
4-carboxymethylbenzaldehyde (328 mg, 2.00 mmol) and meso-
(4-methylphenyl)dipyrromethane 1 (804 mg, 3.41 mmol) in
250 mL of chloroform was purged with argon for 15 min.
respectively. NMR spectra were taken on a Varian Gemini
spectrometer at 300 MHz. Mass spectra were taken with a
Varian Matt 312 operating in EI mode at 70 eV and with a
Vestec Laser Tec Research Instrument by laser desorption time
of ¯ight mass spectroscopy. TLC Uniplate Silica gel GHLF
3 2
Then, BF ?O(Et) (0.46 mmol, 184 mL of 2.5 M stock solution
648
J. Mater. Chem., 2000, 10, 645±650

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in chloroform) was added and the solution was stirred for
2H, J 8.4 Hz, 5 Ar 2,6-H), 8.85±8.96 (m, 8H, pyrrole). FT-IR
2
1
8
1
0 min at room temperature. After that, DDQ (352 mg,
.55 mmol) was added and the mixture was stirred for 1 h at
nmax/cm (KBr) 3311, 3023, 2921, 2855, 1698, 1470, 1284,
z
1188, 971, 809. MS m/z 897.2 (M ) (897.2254 calculated for
Fe). Anal. Calcd. C 78.92, H 5.28, N 7.80; found C
78.79, H 5.37, N 7.71%.
room temperature. The solvent was removed under reduced
pressure and ¯ash chromatography (silica gel, dichloro-
methane±methanol 3%, gradient) afforded 188 mg (17%) of
59 47 5 1
C H N O
5-(4-carboxymethylphenyl)-10,15,20-tris(4-methylphenyl) por-
phyrin 3. MS m/z 714.3 (M ) (714.2997 calculated for
z
2
Preparation of SnO nanocrystalline ®lms
C H N O ).
4
9
38
4
2
This ester porphyrin 3 (100 mg, 0.140 mmol) was hydrolyzed
in 50 mL of THF±CH OH (2 : 1) and 6 mL of KOH 40%. The
reaction was stirred under argon atmosphere for 16 h at
0 ³C. Then, the mixture was neutralized with hydrochloric
acid solution and extracted with dichloromethane. The organic
solvent was evaporated under reduced pressure and ¯ash
column chromatography (silica gel, dichloromethane±metha-
Optically transparent electrodes were cut from indium tin oxide
(ITO) coated glass plates (1.3 mm thickness, 100 V/square)
3
obtained from Delta Technologies. A SnO
2
particle diameter of
20±30 A from Alfa Chemicals was used without further
puri®cation. A 1.5% SnO suspension was prepared by dilution
of the commercial suspension with water containing a
surfactant (0.01% Triton X-100, Aldrich). ITO/SnO electrodes
Ê
6
2
2
2
5
nol 8%) afforded 77 mg (79%) of the desired acid porphyrin 5.
z
MS m/z 700.3 (M ) (700.2841 calculated for C48H N O ).
36 4 2
were prepared by coating using Kamat's procedure. An
aliquot of 0.1 mL of the diluted suspension was spread onto the
2
clean ITO surface with a 3.5 cm area, followed by drying of
Spectroscopic data of acid porphyrin 5 agree with those
2
previously reported.
2±24
2
the electrodes over a warm plate. Finally, the SnO ®lms were
annealed at 450 ³C for 1 h. The resulting ®lms, which are
transparent in the visible region, have strong absorption in the
UV with an onset at around 355 nm. (This onset absorption
corresponds to a bulk band gap of 3.5 eV.)
Zinc 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) por-
phyrin (6). To a solution of acid porphyrin 5 (50 mg,
0
.071 mmol) in 20 ml of dichloromethane was added 5 mL of
Dyad adsorption onto the semiconductor ®lm was accom-
plished by soaking the annealed ®lm in a saturated n-hexane or
petroleum ether solution of the dyad for 2 h. This produces
strong coloration of the semiconductor ®lm. A wire was
connected to the ITO surface with an indium solder.
a saturated solution of zinc(II) acetate in methanol. The mixture
was stirred for 30 min at room temperature. Solvents were
evaporated under reduced pressure and ¯ash chromatography
(silica gel, dichloromethane±methanol 4%) gave 52 mg (95%)
z
of pure Zn-acid porphyrin 6. MS m/z 763.6 (M ) (763.64
calculated for C H N O Zn).
4
8
34
4
2
Synthesis of dyads
Photoelectrochemical measurements
Dyad 7. Zn-acid porphyrin 6 (40 mg, 0.052 mmol) was
dissolved in 20 mL of dry toluene and 8 mL of pyridine. This
solution was stirred under argon for 10 min. Then, thionyl
chloride (10 mL, 0.14 mmol) was added. The reaction mixture
was stirred for 30 min at room temperature. The solvents were
removed by distillation at reduced pressure. Excess of pyridine
and thionyl chloride was removed under vacuum. The acid
chloride was redissolved in 16 mL of toluene and 6 mL of
pyridine. A solution of amide porphyrin 4 (40 mg, 0.060 mmol)
in 4 mL of toluene and 2 mL of pyridine was added. The
reaction was stirred under argon atmosphere for 6 h at room
temperature. Then, the mixture was partitioned between
dichloromethane and water. The organic layer was extracted
with dichloromethane and the solvents were evaporated under
reduced pressure. Flash column chromatography (silica gel,
Photoelectrochemical experiments were conducted in an
aqueous solution (0.01 M) of hydroquinone, with phosphate
buffer (pH~5.2) prepared from 0.05 M NaH PO4 and
NaOH. This solution was thoroughly degassed by bubbling
with Ar and an Ar atmosphere was maintained in the top of the
cell by passing a continuous stream.
2
The measurements were carried out in a 10 mm quartz
photoelectrochemical cell equipped with Ag/AgCl as reference,
and Pt foil as the auxiliary electrodes. A battery operated low
noise potentiostat, constructed in our laboratory, was used in
all photoelectrochemical measurements, which were recorded
on a Radiomiter-Copenaghe X-t recorder. Action spectra for
dye-coated SnO2 electrodes were obtained by sending the
output of a 150 W high-pressure Xe lamp (Photon Technology
Instrument, PTI) through a PTI high intensity grating
monochromator and recording the resulting steady-state
photocurrent. The electrodes were located at the focus of the
dichloromethane±acetone 5%) afforded 54 mg (74%) of pure
1
dyad 7. H NMR (300 MHz, CDCl
3
, TMS) d/ppm 22.78 (br s,
), 7.55 (d, 12H, J
.0 Hz, 10,15,20,10',15',20' Ar 3,5-H), 7.89 (d, 2H, 5 Ar 3,5-H),
.07 (d, 12H, J 8.0 Hz, 10,15,20,10',15',20' Ar 2,6-H), 8.15 (s, br
2
2
8
8
H, pyrrole N-H), 2.71 (s, 18H, Ar-CH
3
monochromator ouput (illuminated area: 1 cm ). All the
photoelectrochemical measurements were done in front face
con®guration. The incident light intensities at different
wavelengths were measured with a Coherent Laser-Mate Q
radiometer.
s, 1H, NHCO), 8.22 (d, 2H, J 8.3 Hz, 5 Ar 2,6-H), 8.26 (d, 2H,
J 8.2 Hz, 5' Ar 2,6-H), 8.34 (d, 2H, J 8.2 Hz, 5' Ar 3,5-H),
2
1
8
2
.80±9.00 (m, 16 H, pyrrole). FT-IR nmax/cm (KBr) 3310,
928, 2855, 1689, 1419, 1342, 1208, 1177, 998, 801. MS
z
Acknowledgements
69 9 1
m/z 1417.0 (M ) (1416.93 calculated for C95H N O Zn).
Anal. Calcd. C 80.47, H 4.90, N 8.89; found C 80.59, H
4
Authors are grateful to Fundaci o n Antorchas, Consejo
Nacional de Investigaciones Cient Âõ ®cas y T e cnicas of Argen-
tina, Consejo de Investigaciones Cient Âõ ®cas y Tecnol o gicas de
la Provincia de C o rdoba, Agencia Nacional de Promoci o n
Cient Âõ ®ca y T e cnica and SECYT de la Universidad Nacional de
R Âõ o Cuarto, for ®nancial support.
.98, N 8.77%.
Dyad 8. The reaction was performed as described above for
dyad 7, using 13 mg (0.052 mmol) of ferroceneacetic acid.
Flash column chromatography (silica gel, dichloromethane±
1
methanol 1.5%) yielded 32 mg (69%) of pure dyad 8. H NMR
(
H), 2.72 (s, 9H, 10,15,20 Ar-CH ), 3.11 (s, 2H), 4.17 (m, 2H),
300 MHz, CDCl
3
, TMS) d/ppm 22.77 (br s, 2H, pyrrole N-
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