Synthesis of dithiaporphyrin-based singlet-singlet energy transfer systems

Article abstract of DOI:10.1039/b102349g

The synthesis of the first alkynyl core-modified porphyrin building block, 5,10,15,20-tetrakis(4-ethynylphenyl)-21,23-dithiaporphyrin (N₂S₂ core), is reported. The building block was used to construct donor-appended dithiaporphyrin systems under mild palladium-coupling conditions. The porphyrin building block appended to four boron dipyrrin units, (BDPY)₄S₂TPP, did not show any energy transfer from the BDPY unit to the central dithiaporphyrin core. However, the pentaporphyrin containing a central dithiaporphyrin sub-unit and four peripheral normal porphyrin sub-units (N₄ core) showed efficient energy transfer from the peripheral N₄ porphyrins to the central N₂S₂ porphyrin.

Full text of DOI:10.1039/b102349g

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Synthesis of dithiaporphyrin-based singlet–singlet energy transfer
systems
Duraisamy Kumaresan, Neeraj Agarwal and Mangalampalli Ravikanth*
1
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai, 400 076, India
Received (in Cambridge, UK) 13th March 2001, Accepted 18th May 2001
First published as an Advance Article on the web 18th June 2001
The synthesis of the first alkynyl core-modified porphyrin building block, 5,10,15,20-tetrakis(4-ethynylphenyl)-21,23-
dithiaporphyrin (N2S2 core), is reported. The building block was used to construct donor-appended dithiaporphyrin
systems under mild palladium-coupling conditions. The porphyrin building block appended to four boron–dipyrrin
units, (BDPY)4S2TPP, did not show any energy transfer from the BDPY unit to the central dithiaporphyrin core.
However, the pentaporphyrin containing a central dithiaporphyrin sub-unit and four peripheral normal porphyrin
sub-units (N4 core) showed efficient energy transfer from the peripheral N4 porphyrins to the central N2S2 porphyrin.
Porphyrin arrays are ideal models for the study of photo-
synthetic processes. Several symmetrical porphyrin arrays have
been synthesized as model compounds for natural photo-
synthetic processes to study photo-induced processes.1 Since
the porphyrin units in symmetrical arrays are identical, it is
difficult to achieve selective excitation of the porphyrin unit
upon irradiation to study electron transfer or energy transfer.
Selective excitation of the porphyrin unit can be achieved easily
if the porphyrin units in the array are not identical (unsym-
metrical). Unsymmetrical porphyrin arrays that consist of two
different porphyrin sub-units are potential models for studying
photosynthetic processes. In the recent past attention has been
directed towards synthesizing covalently linked unsymmetrical
arrays such as porphyrin–corrole or porphyrin–chlorin in order
to obtain fast initial charge transfer and a slow back reaction,
so giving a long lived charge transfer state.2 However, the study
of unsymmetrical porphyrin arrays is in its infancy and more
studies will be required to probe photo-induced processes.
Interestingly, there has been only one report3 on unsymmetrical
arrays containing core-modified porphyrins. One of the most
interesting modifications of the porphyrin molecule would be a
change of the immediate environment around the central metal
atom through replacement of the nitrogens by other hetero-
atoms such as sulfur, oxygen, selenium, tellurium, etc.4 This
constitutes an attractive core modification and the insertion
of such atoms into the porphyrin core is expected to change the
electronic environment of the porphyrin π-system. The sub-
stitution of nitrogen by hetero-atoms reduces the porphyrin
ring core size. Thus, a series of hetero-atom substituted por-
phyrins form a group of core-modified porphyrins that exhibit
interesting properties in terms of both aromatic character and
their ability to stabilize metals in unusual oxidation states.5–9
Herein we report the first synthesis of a dithiaporphyrin build-
ing block (N2S2 core) bearing ethyne groups and its application
in the synthesis of dithiaporphyrin systems appended to energy
donors.10 We have chosen N,N Ј-difluoroboryl-1,9-dimethyl-5-
phenyldipyrrin (BDPY) and normal porphyrin (N4) to act as
energy donors. Thus, we report the synthesis of dithiaporphyrin
appended to four BDPY units and of a pentaporphyrin con-
taining one central N2S2 porphyrin sub-unit and four peripheral
N4 porphyrin sub-units. Steady state fluorescence spectra
confirmed the energy transfer from the N4 sub-units to the
N2S2 sub-unit in the pentaporphyrin. However, we did not
observe any energy transfer from BDPY to the N2S2 unit in
BDPY-appended dithia array, which is in contrast to the
boron–dipyrrin-appended N4 porphyrin array in which an
efficient energy transfer occurred from BDPY to the N4
porphyrin unit.
Results and discussion
The synthetic method followed here to prepare the desired
dithiaporphyrin building block, 2, is outlined in Scheme 1. The
unknown diol 1 was prepared by following the method of
Ulman and Manassen.11 One equivalent of 2,5-dilithiothio-
phene was condensed with two equivalents of 4-(2-trimethyl-
silylethynyl)benzaldehyde12 in ice-cold dry tetrahydrofuran,
which was followed by recrystallization from toluene to yield
69% of diol 1 as a white solid. One equivalent of 1 was con-
densed with one equivalent of pyrrole in CHCl3 in the presence
of a catalytic amount of BF3ؒOEt2. TLC analysis showed a single
yellow spot of the desired compound, 2. Column chrom-
atography with silica using CH2Cl2 gave the dithiaporphyrin
building block, 2 in 17% yield. The porphyrin building block
1
was characterized by H NMR, absorption and fluorescence
spectroscopies and by MALDI mass spectrometry. The tri-
methylsilyl group was removed by stirring 2 with K2CO3 in
THF–methanol at room temperature for 4 h to afford depro-
tected porphyrin, 3, in 80% yield. The deprotection reaction
1
was confirmed by H NMR, which clearly showed the dis-
appearance of the peak at 0.39 ppm due to the methyl groups
and the appearance of a peak at 3.9 ppm due to the alkyne
proton. The building block 3 was used initially to couple with
four equivalents of N,N Ј-difluoroboryl-1,9-dimethyl-5-(4-iodo-
phenyl)dipyrrin (BDPY-I) in toluene–triethylamine at 35 ЊC in
the presence of a catalytic amount of Pd2(dba)3 and AsPh3.13
After work-up and column chromatography, the dithia-
porphyrin appended to four BDPY units, S2TPP(BDPY)4, 4,
1
was obtained in 58% yield and it was characterized by H
NMR, absorption and fluorescence spectroscopies and by
MALDI mass spectrometry. The symmetric nature of 4 is
clearly evident from its 1H NMR spectrum [Fig. 1(a)]. Both the
thiophene and the pyrrole protons of the porphyrin unit
appeared as singlets at lower field and the pyrrole protons of
the BDPY units appeared as a doublet at higher field. MALDI
mass spectrometry showed a characteristic molecular ion peak
at 1921.3. The absorption spectrum of 4 along with those of 3
and BDPY, presented in Fig. 2, showed bands corresponding to
both BDPY and 3 sub-units. However, the Soret band of 4
experienced a slight red shift compared to 3, indicating a weak
interaction between the sub-units. For the purposes of com-
parison, we also synthesized a normal N4 porphyrin appended
1644
J. Chem. Soc., Perkin Trans. 1, 2001, 1644–1648
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102349g

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to four BDPY units, H2TPP(BDPY)4, (5), which exhibited
similar characteristic features in its absorption spectrum.14
The emission spectrum of 4, along with that of 5, recorded
at 485 nm is shown as an inset in Fig. 2. As is evident from
Fig. 2, in the case of 4, excitation at 485 nm (where BDPY is the
predominant absorber) resulted in emission from both the units
with a strong emission from the BDPY unit, indicating that
there is no energy transfer from the BDPY unit to the thia-
porphyrin unit. The quantum yield of the BDPY (φ = 0.16)
component in 4 is similar to that of the free BDPY unit
(φ = 0.18), which supports the lack of energy transfer from the
BDPY unit to the dithiaporphyrin unit. On the other hand,
in the case of 5, excitation at 485 nm resulted in a strong
quenching of the BDPY emission (φ = 0.006) and increased
emission from the N4 porphyrin unit suggesting that there is
energy transfer from the BDPY unit to the N4 porphyrin unit.
The energy transfer that occurred in 5 was independent of the
excitation wavelength.
BDPY unit to the dithiaporphyrin unit in 4, we synthesized
a dithiaporphyrin unit appended to N4 porphyrin units in
place of the BDPY units. In this system, we expected the N4
porphyrin unit to act as an energy donor and transfer energy
to the central N2S2 unit. The desired N4 porphyrin building
block, 5,10,15-tris(3,5-di-tert-butylphenyl)-20-(4-iodophenyl)-
porphyrin (6), was synthesized by following the building-block
approach.15 The reaction of one equivalent of 3 with 4.5 equiv-
alents of 6 was carried out in toluene–triethylamine (5 : 1) in
the presence of a catalytic amount of AsPh3 and Pd2(dba)3.
The progress of the reaction was monitored by analytical size
exclusion chromatography (SEC) and TLC.16,17 The reaction
mixture remained homogeneous throughout the coupling pro-
cess. After 20 hours, the peak from the starting material had
decreased and the pentamer peak had reached a maximum
with a small amount of higher oligomers (Fig. 3). The reaction
was stopped and solvents were removed in vacuo. The crude
pentamer was passed through a silica gel column to remove
unreacted materials and was subjected to SEC chromatography.
To understand the failure of the energy transfer from the
Scheme 1 Synthetic scheme for the 21,23-dithiaporphyrin building block and its donor-appended systems.
J. Chem. Soc., Perkin Trans. 1, 2001, 1644–1648
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Fig. 3 Size exclusion chromatograms of the reaction solution of 3
with 6 under palladium coupling conditions to synthesize 7 and
chromatogram of 7 after purification.
Fig. 1 1H NMR spectra of (a) 4 and (b) 7 recorded in CDCl3. The
peak marked by an asterisk corresponds to an impurity present in the
solvent.
Fig. 4 Q-bands and Soret band (inset) absorption spectra of 7 along
with those of its monomers. The concentrations used were: Soret band
(inset), ∼2 × 10Ϫ6 M; and Q-bands, 5 × 10Ϫ5 M.
protons. The absorption spectra of 7, along with those of the
two monomers, in both the Q-band and the Soret region are
shown in Fig. 4. The absorption spectrum of 7 resembles, but
does not equal, the sum of the spectra of the corresponding
monomeric units. The absorption spectrum was dominated by
the intense Soret band (inset in Fig. 4) of the four N4 porphyrin
constituents. The weaker five Q-bands of the porphyrins lie
between 500 and 750 nm. Both the Q-bands and the Soret band
exhibited slight red shifts and broadening, indicating a weak
interaction between the normal and the dithiaporphyrin
sub-units. The emission spectrum of 7 along with those of its
corresponding monomers (recorded at 420 nm) are shown in
Fig. 5. Upon excitation of 7 at 420 nm (where the N4 porphyrin
unit is the dominant absorber), fluorescence was observed
exclusively from the N2S2 unit. The quantum yield of the
N4 unit in 7 (φf = 0.002) was greatly diminished compared to
that of the corresponding N4 monomer and the intensity of
the fluorescence bands of the N2S2 unit was enhanced, which
strongly supports the occurrence of energy transfer from the
N4 unit to the N2S2 unit. Similar emission spectra were observed
at different wavelengths at which the N4 porphyrin is a strong
absorber. These results indicate that very efficient energy trans-
fer occurs from the N4 porphyrins to the central N2S2 porphyrin
Fig. 2 Absorption spectra of 4, 3 and BDPY recorded in toluene. The
concentration used was ∼2 × 10Ϫ6 M. The inset shows the emission
spectra of 4, 5 and BDPY in toluene.
The pentamer obtained from the SEC column contained small
amounts of impurities of higher and lower porphyrin oligomers
and, hence, it was further subjected to preparative HPLC to
afford the pure pentaporphyrin (H2TPP)4S2TPP (7), comprising
one free base N2S2 sub-unit and four N4 sub-units, in 37%
1
yield. The pentaporphyrin, 7 was characterized by H NMR,
absorption and emission spectroscopies and by MALDI mass
1
spectrometry. The H NMR spectrum of 7 in the 7–10 ppm
region is shown in Fig. 1(b). The thiophene protons appeared as
a singlet, indicating the symmetric nature of the pentamer. The
pyrrole protons and aryl protons appeared as a multiplet due
to overlap of the N2S2 porphyrin unit and N4 porphyrin unit
1646
J. Chem. Soc., Perkin Trans. 1, 2001, 1644–1648

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added and it was then extracted with ether (3 × 50 ml). The
organic layers were combined and washed with brine and
dried over anhydrous Na2SO4. The crude product obtained on
evaporation of the solvent was recrystallized from toluene
1
and afforded the pure diol as a white solid (1.35 g, 69%). H
NMR (CDCl3, δ in ppm) 0.34 (s, 18H), 2.12 (br s, 2H), 6.03
(s, 2H), 6.76 (s, 2H), 7.44 (m, 4H), 7.53 (m, 4H). Calcd for
C27H32Si2O2S: C, 68.8; H, 6.59; found: C, 68.5; H, 6.61%. Mp
115 ЊC.
5,10,15,20-Tetrakis[4-(2-trimethylsilylethynyl)phenyl]-21,23-
dithiaporphyrin, 2
A stream of argon was passed through chloroform (25 ml)
in a 100 ml one-necked, round-bottomed flask for 10 min.
Compound 1 (100 mg, 0.205 mmol) and pyrrole (0.040 ml,
0.575 mmol) were added and argon purging was continued for
an additional 10 min. BF3ؒOEt2 (0.040 ml of a 2.5 M stock
solution) was added and the reaction mixture was stirred for
1 h. During this period the colour of the reaction mixture was
slowly turned from colourless to dark brown. DDQ (100 mg,
0.441 mmol) was then added and the reaction mixture was
stirred in open air for an additional 1 h. The solvent was
removed in vacuo and the crude compound was purified by
silica column using CH2Cl2. The desired compound 2 was
Fig. 5 Emission spectra of 7 along with those of its monomers. The
inset shows the emission spectra of 8 recorded at different wavelengths.
unit. To further confirm the energy transfer in pentaporphyrin
7, we introduced Zn2ϩ into the four peripheral N4 units by the
standard Zn(OAc)2 method to give (ZnTPP)4S2TPP (8) and
recorded the steady state fluorescence spectra at different wave-
lengths. Excitation at 550 nm, where zinc porphyrin absorbs
four times as much as the free base porphyrin, resulted in
emission exclusively from the central N2S2 unit, indicating
an efficient energy transfer from zinc porphyrin to the N2S2
porphyrin unit. This kind of efficient energy transfer is not
possible in pentaporphyrins comprising only N4 free base sub-
units where all the porphyrin units are the same and absorb
in the same region. Thus, the pentaporphyrin 7 reported here
is unique in the sense that it can be excited selectively and
it shows an efficient energy transfer from the central N2S2 unit.
The synthesis and detailed photodynamics of several such
pentaporphyrins containing N2O2, N3O, N3S and N3SO as
central cores are presently under investigation in our laboratory.
1
obtained as a purple solid (17 mg, 17%). H NMR (CDCl3,
δ in ppm) 0.39 (s, 36H, CH3), 7.92 (AAЈBBЈ, 8H, Ar), 8.17
(AAЈBBЈ, 8H, Ar), 8.65 (s, 4H, β-pyrrole), 9.65 (s, 4H,
β-thiophene). LD-MS C64H60N2S2Si4 calcd. av. mass 1033.7,
obsd. m/z 1033.1. Anal. calcd: C, 74.4; H, 5.76; N, 2.71; found:
C, 75.2; H, 5.82; N, 2.75%. UV–Vis λmax/nm (ε/molϪ1 dm3 cmϪ1)
439 (159000), 516 (10200), 551 (5600), 632 (1500), 698 (1900).
5,10,15,20-Tetrakis(4-ethynylphenyl)-21,23-dithiaporphyrin, 3
To a solution of 2 (80 mg, 0.077 mmol) in 16 ml of THF–
methanol (3 : 1) K2CO3 (42 mg, 0.304 mmol) was added and
the reaction mixture was stirred at room temperature for 4 h.
The progress of the reaction mixture was monitored by TLC.
The reaction mixture was rotary-evaporated to dryness and
the greenish purple powder thus obtained was dissolved in
50 ml of CH2Cl2. The organic layer was washed with 10%
NaHCO3 (2 × 50 ml), dried (Na2SO4), filtered and rotary-
evaporated to dryness. Column chromatography (silica, CH2-
Cl2) afforded 45.9 mg (0.0616 mmol, 80%) of porphyrin 3.
1H NMR (CDCl3, δ in ppm) 3.33 (s, 4H, CCH), 7.95 (AAЈBBЈ,
8H, Ar), 8.20 (AAЈBBЈ, 8H, Ar), 8.67 (s, 4H, β-pyrrole),
9.67 (s, 4H, β-thiophene). LD-MS C52H28N2S2 calcd. av. mass
744.9, obsd. m/z 746.4. Anal. calcd: C, 82.1; H, 3.79; N, 3.76;
found: C, 82.1; H, 3.71; N, 3.79%. UV–Vis λmax/nm (ε/molϪ1
dm3 cmϪ1) 439 (168000), 516 (12200), 552 (4600), 633 (1000),
698 (1900).
Experimental
1H NMR spectra were recorded on a Varian 300 MHz using
tetramethylsilane as internal standard. Absorption and
fluorescence spectra were obtained with Shimadzu-160 and
Spex Fluoromax respectively. Mass spectra were obtained for
samples in neat form by laser desorption mass spectrometry
using a Bruker Proflex II. Toluene, THF and triethylamine
were obtained from S.D. Fine chemicals, India, and dried
by standard procedures before use. All general chemicals
were obtained from Qualigens, India. Aldehydes and pyrrole
were obtained from Lancaster. Column chromatography was
performed using 60–120 mesh silica obtained from Sisco
Research Laboratories, India. Size exclusion chromatography
was performed using Bio-Beads SX-1 in toluene obtained from
Biorad, USA.
(BDPY)4S2TPP, 4
Samples of 3 (10 mg, 0.0139 mmol) and BDPY-I (35 mg,
0.0839 mmol) were dissolved in 12 ml of toluene–triethylamine
(5 : 1). AsPh3 (40 mg, 0.131 mmol) was then added and the
solution was deaerated with argon for 15 min. The coupling
reaction was initiated by the addition of a catalytic amount of
Pd2(dba)3 (16 mg, 0.017 mmol). The reaction mixture was
stirred under argon at 35 ЊC. After 3 h, the solvent was removed
in vacuo; the crude material was dissolved in dichloromethane
and passed through a short silica column to remove the
palladium species, triphenylarsine and other side products. The
desired compound was then eluted with CH2Cl2–5% ether
2,5-Bis[4-(2-trimethylsilylethynyl)phenyl(hydroxy)methyl]-
thiophene, 1
Dried and distilled n-hexane (13 ml) was added to a 250 ml
three-necked, round-bottomed flask that was equipped with a
gas inlet tube, a reflux condenser and a rubber septum.
N,N,N Ј,N Ј-Tetramethylethylenediamine (0.924 g, 7.95 mmol,
1.2 ml) and n-butyllithium (5 ml of a 1.6 M solution in hexane)
were injected into the stirred solution. Thiophene (0.263 g,
4.12 mmol, 0.250 ml) was injected and the solution was
refluxed gently for 1 h. The reaction mixture was then allowed
to attain room temperature. 4-(2-Trimethylsilylethynyl)benz-
aldehyde (1.6 g, 7.91 mmol) in dry tetrahydrofuran (16 ml) was
added dropwise to the ice-cooled reaction flask. After addition
was complete, the reaction mixture was allowed to attain
room temperature, saturated ammonium chloride solution was
1
in 58% yield (20 mg). H NMR (CDCl3, δ in ppm) 1.54 (s,
24H, CH3), 6.13 (d, J = 4.5 Hz, 8H, BDPY pyrrole), 6.79 (d,
J = 4.5 Hz, 8H, BDPY pyrrole), 7.59 (AAЈBBЈ, 8H, Ar), 7.80
(AAЈBBЈ, 8H, Ar), 8.05 (AAЈBBЈ, 8H, Ar), 8.29 (AAЈBBЈ, 8H,
Ar), 8.75 (s, 4H, β-pyrrole), 9.75 (s, 4H, β-thiophene). LD-MS
J. Chem. Soc., Perkin Trans. 1, 2001, 1644–1648
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C120H80N10S2B4F8 calcd. av. mass 1921.4, obsd. m/z 1921.3.
UV–Vis λmax/nm (ε/molϪ1 dm3 cmϪ1) 442 (375000), 514
(154700), 552 (13000), 698 (2900).
1H NMR (CDCl3, δ in ppm) 1.55 (m, 216H, -CH3), 8.10 (m,
44H, Ar), 8.21 (d, J = 8 Hz, 8H, Ar), 8.34 (d, J = 8 Hz, 8H, Ar),
8.41 (d, J = 8 Hz, 8H, Ar), 8.88–8.97 (m, 36H, β-pyrrole), 9.88
(s, 4H, β-thiophene). LD-MS C324H324N18S2Zn4 calcd. av. mass
4795.9, obsd. m/z 4796.2. UV–Vis λmax/nm (ε/molϪ1 dm3 cmϪ1)
423 (623723), 440 (195495), 518 (13213), 551 (25683), 589
(7913), 633 (743), 699 (1762).
(BDPY)4H2TPP, 5
Compound 5 was prepared by following the method described
for 4. The coupling of 5,10,15,20-tetrakis(4-ethynylphenyl)-
porphyrin (10 mg, 0.0139 mmol) and BDPY-I (35 mg,
0.0839 mmol) in toluene–triethylamine (5 : 1) under palladium-
coupling conditions, as described above, resulted in 5 (36%,
9.5 mg) after column chromatography. 1H NMR (CDCl3,
δ in ppm) Ϫ2.73 (s, 2H, NH), 1.54 (s, 24H, CH3), 6.33 (d, J =
4.5 Hz, 8H, BDPY pyrrole), 6.79 (d, J = 4.5 Hz, 8H, BDPY
pyrrole), 7.59 (AAЈBBЈ, 8H, Ar), 7.79 (AAЈBBЈ, 8H, Ar), 7.99
(AAЈBBЈ, 8H, Ar), 8.26 (AAЈBBЈ, 8H, Ar), 8.92 (s, 8H, β-
pyrrole). LD-MS C120H82N12B4F8 calcd. av. mass 1887.4, obsd.
m/z 1887.1. UV–Vis λmax/nm (ε/molϪ1 dm3 cmϪ1) 425 (874316),
514 (214200), 554 (21730), 592 (11051), 648 (6705).
Acknowledgements
This work was supported by a grant from the Department of
Science & Technology and Council of Scientific & Industrial
Research, Govt. of India.
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5,10,15-Tris(3,5-di-tert-butylphenyl)-20-(4-iodophenyl)-
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Compound 6 was prepared by following the procedure reported
earlier.15
(H2TPP)4S2TPP, 7
5,10,15,20-Tetrakis(4-ethynylphenyl)-21,23-dithiaporphyrin (11
mg, 0.0148 mmol) and 5,10,15-tris(3,5-di-tert-butylphenyl)-20-
(4-iodophenyl)porphyrin (80 mg, 0.074 mmol) were dissolved in
toluene–triethylamine (5 : 1, 12 ml) in a 25 ml round-bottomed
flask. The flask was fitted with a reflux condenser and a glass
pipette was inserted through the top of the condenser into the
solution for argon purging. The reaction vessel was placed in an
oil bath preheated to 35 ЊC. The argon was purged for 15 min.
AsPh3 (34 mg, 0.111 mmol) and Pd2(dba)3 (18 mg, 0.019 mmol)
were then added and the reaction was stirred at 35 ЊC until
HPLC analyses showed the absence of starting materials (20 h).
The solvents were removed in vacuo and the crude compound
was passed through a small silica column using CH2Cl2 to
remove the excess of AsPh3 and the palladium impurities. The
porphyrin mixture containing unreacted monomeric materials,
the desired pentamer and higher molecular weight material was
dissolved in the minimum amount of toluene and loaded on to
the top of a preparative size exclusion chromatography column
(Bio-Beads SX-1 poured into toluene) and eluted with toluene.
On slow elution, the higher oligomers, desired pentamer and
lower molecular weight materials were clearly separated. The
required pentamer was collected and then subjected to another
SEC column to furnish the pure (H2TPP)4S2TPP in 37%
yield. 1H NMR (CDCl3, δ in ppm) Ϫ2.63 (s, 8H, NH), 1.54 (m,
216H, -CH3), 7.83 (m, 12H, Ar), 8.05 (m, 24H, Ar), 8.13 (m,
8H, Ar), 8.22 (d, J = 8 Hz, 8H, Ar), 8.34 (d, J = 8 Hz, 8H, Ar),
8.41 (d, J = 8 Hz, 8H, Ar), 8.88–8.97 (m, 36H, β-pyrrole), 9.89
(s, 4H, β-thiophene). LD-MS C324H332N18S2 calcd. av. mass
4542.5, obsd. m/z 4543.9. UV–Vis λmax/nm (ε/molϪ1 dm3 cmϪ1)
421 (770000), 441 (240000), 518 (28600), 555 (16900), 592
(6200), 648 (5600), 698 (2300).
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A solution of 7 (10 mg, 0.002 mmol) in 10 ml of CH2Cl2 was
treated with methanolic Zn(OAc)2 (25 mg, 0.114) and the
reaction mixture was stirred for 2 h. The solvent was removed
under vacuum and the resulting solid was purified on a silica
gel column using CH2Cl2 to give a purple solid in 90% yield.
1648
J. Chem. Soc., Perkin Trans. 1, 2001, 1644–1648