Singlet energy transfer in bis(phenylethynyl)phenylene-bridged zinc-free base hybrid diporphyrins

Article abstract of DOI:10.1039/a606700j

A set of bis(phenylethynyl)phenylene-bridged diporphyrins 1-3 has been prepared by the acid-catalysed double condensation reaction of bis(4-formylphenylethynyl)benzene 8 and 9, and 3,5-di-ferf-butylbenzaldehyde 11 with bis(3-hexyl-4-methylpyrrol-2-yl)methane 12 or the Pd⁰-catalysed coupling reaction of acetylene-substituted porphyrin 14 with 1,4-diiodobenzene. Intramolecular singlet excitation energy transfer in Zn-free base hybrid diporphyrins 1-3(ZH) has been studied by picosecond time-resolved fluorescence spectroscopy. The determined k_EN values are in the order of 1(ZH) > 2(ZH) ≈ 3(ZH). The energy transfer mechanism is discussed in terms of varying contribution of through-space and through-bond interactions.

Full text of DOI:10.1039/a606700j

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Singlet energy transfer in bis(phenylethynyl)phenylene-bridged zinc–
free base hybrid diporphyrins
Shigeki Kawabata,*,a Iwao Yamazaki,b Yoshinobu Nishimura b and
Atsuhiro Osuka *,c
a
Department of Liberal Arts and Sciences, Faculty of Engineering,
Toyama Prefectural University, Kosugi, Toyama 939-03, Japan
b Department of Chemical Process Engineering, Faculty of Engineering, Hokkaido University,
Sapporo 060, Japan
c Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-01, Japan
A set of bis(phenylethynyl)phenylene-bridged diporphyrins 1–3 has been prepared by the acid-catalysed
double condensation reaction of bis(4-formylphenylethynyl)benzene 8 and 9, and 3,5-di-tert-butyl-
benzaldehyde 11 with bis(3-hexyl-4-methylpyrrol-2-yl)methane 12 or the Pd0-catalysed coupling reaction of
acetylene-substituted porphyrin 14 with 1,4-diiodobenzene. Intramolecular singlet excitation energy
transfer in Zn–free base hybrid diporphyrins 1–3(ZH) has been studied by picosecond time-resolved
fluorescence spectroscopy. The determined kEN values are in the order of 1(ZH) > 2(ZH) ≈ 3(ZH). The
energy transfer mechanism is discussed in terms of varying contribution of through-space and through-
bond interactions.
The absorption of solar energy by antenna pigments and the
subsequent efficient transfer of excitation energy to photo-
synthetic reaction centres constitute key entries into the
generation of energetic charge-separated products in photo-
synthesis.1 For this purpose, photosynthetic organisms use
light-harvesting complexes as an antenna that has an elegant
alignment and fine-tuned photophysical and photochemical
properties, suitable for achieving remarkably high quantum
yield energy transfer over long distances through hundreds of
photosynthetic pigments. It seems that control of the energy
transfer may be achieved by holding each pigment in specific
spatial arrangements.2 This is also valid in artificial model
systems where rates of intramolecular energy transfer have been
shown to follow the trends predicted by Förster theory.3
On the other hand, recent reports have demonstrated that π-
conjugated systems can significantly enhance through-bond
electronic couplings between covalently linked energy donor
and acceptor pairs. For example, π-conjugated bridges includ-
ing ethyne,4–9 diyne,4,7–10 polyene4,11 and oligothiophene link-
ages11 have been demonstrated to mediate efficient electronic
interactions between the donor and the acceptor, thereby enabl-
ing efficient energy transfer over long distances, over which the
energy transfer is usually impossible without these π-conju-
gated bridges within the lifetime of the donor excited state. In
these covalently linked π-conjugated systems, electronic coup-
ling is not primarily determined by spatial arrangement but by
through-bond electronic interactions. Therefore, it should be
possible to construct a new energy-transfer network by combin-
ing both the spatial arrangements and the through-bond inter-
actions of π-conjugated systems.
distance, and thus decrease in the order 1 > 2 > 3, while the
through-bond interactions would be 1 ≈ 3 > 2, since the two
ethyne units are electronically conjugated in 1 and 3 but not
conjugated in 2. It is well-established that singlet–singlet energy
transfer can occur by Förster and/or Dexter mechanisms. Dis-
crimination of the two mechanisms may often be aided by
examining the geometry dependence of the energy transfer. The
Dexter mechanism may be predominant in the case of strong
electron exchange interactions due to the close contact of the
donor and the acceptor or the presence of π-conjugated bridge
between the donor and the acceptor, while the Förster mech-
anism may be predominant in the case of large spectral
overlap of the emission of the donor and the absorption of
the acceptor. In this paper, we report the synthesis and
singlet energy transfer of bis(phenylethynyl)phenylene-bridged
diporphyrins.
Results and discussion
Synthetic schemes to bis(phenylethynyl)phenylene linkers and
diporphyrins are shown in Schemes 1–3. Palladium(0) mediated
coupling reactions between 4-(4,4-dimethyl-2,6-dioxan-1-yl)-
phenylacetylene (4) and 1,2-, 1,3- and 1,4-diiodobenzenes in
triethylamine12 gave protected bridges 5–7, which were hydro-
lysed under acidic conditions to give dialdehydes 8–10, respect-
ively (Scheme 1). 1,2- and 1,3-Bis(phenylethynyl)phenylene-
bridged diporphyrins 1(HH) and 2(HH) were prepared in
moderate yields by the trichloroacetic acid catalysed double
condensation 4,13 of dialdehydes 8 and 9 with 3,5-di-tert-
butylbenzaldehyde (11) and bis(3-hexyl-4-methylpyrrol-2-yl)-
methane (12) in acetonitrile–dichloromethane followed by p-
chloranil oxidation, respectively (Scheme 2). However, the very
poor solubility of 10 hampered the application of this double
condensation method. We therefore sought an alternative route
based on the palladium catalysed coupling reaction (Scheme
3).6,9 The acid catalysed condensation of 4-ethynylbenzaldehyde
(13) and 11 with 12 in a mixture of acetonitrile–dichloro-
methane followed by oxidation with a p-chloranil afforded
ethynylporphyrin 14 in 20% yield. In these mild reaction condi-
tions, a terminal ethyne does not need to be protected. In the
presence of PdCl2 and triphenylphosphine in triethylamine, the
Recently, we prepared polyyne- and polyene-bridged dipor-
phyrins and demonstrated that linear π-conjugated polyyne and
polyene bridges can enhance through-bond electronic coupling
between the donor and the acceptor, thus enabling efficient
energy and electron transfer over long distances.4 As part of
our continuing programme of developing artificial photo-
synthetic systems, we have now prepared a set of bis(phenyl-
ethynyl)phenylene-bridged diporphyrins 1–3 and examined
intramolecular singlet energy transfer from zinc–porphyrin
donor to free-base porphyrin acceptor. It is expected that the
through-space interactions would depend primarily on the
J. Chem. Soc., Perkin Trans. 2, 1997
479

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Scheme 2 Synthetic procedure of bis(phenylethynyl)phenylene dipor-
phyrins 1 and 2
Scheme 1 Synthetic procedure of bis(phenylethynyl)phenylene linker
coupling reaction of 14 with 1,4-diiodobenzene proceeded
smoothly to provide 1,4-bridged diporphyrin 3(HH) in 35%
yield.9 Partial zinc metallation was carried out by treatment of
1–3(HH) with a small amount of Zn(OAc)2–MeOH while the
extent of the metallation was monitored by TLC. After a suit-
able time, the metallation was stopped by addition of water and
the products were separated by silica gel column chromato-
graphy, giving singly zinc-metallated diporphyrins 1–3(ZH).
Scheme 3 Synthetic procedure of bis(phenylethynyl)phenylene dipor-
phyrin 3
The absorption spectra of dialdehydes 8–10 in tetrahydro-
furan (THF) are shown in Fig. 1. The absorption maximum of
10 is observed at a longer wavelength than those of 8 and 9,
480
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 1 Data for 1–3
Compound
kEN /sϪ1 a
τ/psb
τ/psc
kEN /sϪ1 d
r/10Ϫ10 m J/cm6 mmolϪ1
κ2
kEN /sϪ1 e
1(ZH)
2(ZH)
3(ZH)
4.8 × 109
9.4 × 108
9.3 × 108
195
793
796
217
692
723
4.4 × 109
5.7 × 108
5.7 × 108
13.1
22.7
26.2
4.7 × 10Ϫ14
3.7 × 10Ϫ14
3.5 × 10Ϫ14
0.69
0.97
1.1
3.7 × 1010
1.5 × 109
7.1 × 108
a
Rates of singlet energy transfer calculated on the basis of the steady-state fluorescence intensity at 585 nm according to the eqn. (1). b The decay
lifetime of the fluorescence of zinc porphyrin at 585 nm. c The increase time of the fluorescence of free base porphyrin at 700 nm. d Rates of singlet
energy transfer calculated on the basis of the fluorescence lifetime according to eqn. (2). e Predicted Förster energy transfer rate constant.
Fig. 1 UV–VIS absorption spectra of 8–10 in THF
suggesting larger electronic interactions in a 1,4-substituted
bridge than those in 1,2- and 1,3-substituted ones.
The absorption spectra of diporphyrins 1–3(HH) and 1–
3(ZH) in THF, however, do not display significant difference
between the isomers (Fig. 2). The absorption bands due to the
Fig. 2 UV–VIS absorption spectra of 1–3(HH) and 1–3(ZH) in THF.
Each spectrum is normalized to the Soret absorption.
bridge moieties are slightly broadened and hidden by the strong
absorption of porphyrin. The absorption spectra of 1–3(ZH)
are essentially a superposition of those of free base and zinc
sity of 1:1 mixture of corresponding 1–3(ZZ) and 1–3(HH) at
porphyrin components. The Soret absorption peak in 3(HH)
585 nm, respectively (Table 1).
The energy transfer reaction in these compounds has also
and 3(ZH) are somewhat red-shifted relative to those in other
diporphyrins 1(HH) and 1(ZH), and 2(HH) and 2(ZH), but the
difference is not so large. Therefore we conclude that the elec-
tronic interaction between the porphyrins is not strong in the
ground state in this series.
The fluorescence spectra of 1–3(ZH) are different from the
superposition of those of the individual chromophores (Fig. 3).
been investigated by picosecond time-resolved fluorescence
spectroscopy.3 Time-resolved fluorescence spectra of 1(ZH)
taken by excitation at 532 nm in THF are shown in Fig. 4. The
fluorescence spectrum at 36 ps was assigned as primarily due to
the emission from the zinc porphyrin. Decay of the zinc por-
phyrin fluorescence was followed by an increase in the free base
The fluorescence intensity of the zinc porphyrin is reduced (λem
/
porphyrin fluorescence. Essentially the same time-resolved
fluorescence spectra were observed for 2(ZH) and 3(ZH). The
fluorescence decay times of the zinc porphyrin were roughly the
same as the fluorescence increase times of the free base porphy-
rins (Table 1). Since the fluorescence decay times are more reli-
able than the fluorescence increase times, we used the former
nm = 585 and 643) whereas that of the free base porphyrin is
enhanced (λem/nm = 632 and 695). This observation shows the
efficient singlet excitation energy transfer from the zinc porphy-
rin to the free base porphyrin. Under our dilute conditions (ca.
10Ϫ7 ), intermolecular singlet energy transfer reactions from
zinc porphyrin to free base porphyrin can be excluded. Rate
constants (kEN ) of the intramolecular energy transfer have been
calculated using eqn. (1), where τ0 is the fluorescence lifetime of
values in calculating the rate of the energy transfer (kEN
)
according to eqn. (2).
kEN = 1/τ Ϫ 1/τ0
(2)
kEN = (1/τ0)(I0 Ϫ I)/I
(1)
The two calculated kEN values based either on eqn. (1) or on
eqn. (2) display a similar trend; 1(ZH) > 2(ZH) ≈ 3(ZH). The
calculated kEN values based on eqn. (2) should be more reliable
the reference zinc porphyrin (1.46 ns), I is the fluorescence
intensity of 1–3(ZH) at 585 nm, and I0 is the fluorescence inten-
J. Chem. Soc., Perkin Trans. 2, 1997
481

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sition dipole moment and the spectral overlap integral (J)
[eqn. (3)].
kEN = 8.37 × 10Ϫ27 κ2rϪ6Jτ0Ϫ1 (solvent: THF, φZnP = 0.037) (3)
The calculated values of r, J, κ2 and energy transfer rate
constant kEN are summarized in Table 1. The centre-to-centre
distances (r) between the zinc porphyrin and the free base por-
phyrin are estimated by CPK model, assuming neither fluctu-
ation nor bend. Orientation factors are calculated by assuming
free rotation about the ethyne unit and by dynamic averaging in
the case that one porphyrin is fixed at 0Њ, 90Њ, 180Њ and 270Њ
about the ethyne unit and the other porphyrin rotates freely.
Comparison of the rates of the energy transfer in 1–3(ZH)
gave some insight into the mechanism of the energy transfer.
First, the fact that 2(ZH) and 3(ZH) display almost the same
kEN values is clear evidence for the through-bond electronic
interaction in 3(ZH), since the centre-to-centre distance is
shorter in 2(ZH) and thus a larger kEN value is expected
through dipole–dipole interaction (Förster mechanism). Sec-
ond, the through-bond electronic interaction is inferred to be
weaker than the direct dipole–dipole interaction in 1(ZH) since
the kEN value observed is ca. one order of magnitude larger
than that in 3(ZH). The dipole–dipole interaction can operate
even at a long distance and becomes ‘steeply’ important at a
shorter distance between the donor and the acceptor. Thus,
such through-space interaction is predominant over the
through-bond interaction in 1(ZH). The calculated kEN value
for 1(ZH) is one order of magnitude larger than the observed
value. This may show the limitation of the theory based on the
point-dipole approximation at a small distance as reported pre-
viously.3 Comparison of the rate of the energy transfer of the
linear dimer 3(ZH) (kEN = 5.7 × 108 sϪ1; r = 26.2 × 10Ϫ10 m) with
Fig. 3 Fluorescence spectra of 1(ZH) (lower solid line), 2(ZH) (centre
solid line), 3(ZH) (upper solid line) and 1:1 mixture of 1(ZZ) and
1(HH) (dotted line) in THF
that of the diphenylpolyyne-bridged hybrid dimer 15 (kEN
=
1.9 × 109 sϪ1; r = 27.0 × 10Ϫ10 m)4 indicates lower ability of a
1,4-bis(phenylethynyl)phenylene bridge in mediating the
through-bond electronic interactions than that of diphenyl-
polyyne bridges of analogous length.15
As an extension of this model study, the synthesis of the
related hybrid diporphyrins for study of intramolecular elec-
tron transfer is now in progress, since the electron exchange
interaction is more important in electron transfer reactions.
Bis(phenylethynyl)phenylene bridges are useful in keeping
donors and acceptors at well-defined geometry, and are capable
of mediating the through-bond electronic interactions.
Experimental
Fig. 4 Time-resolved fluorescence spectra of 1(ZH) in THF. The exci-
tation wavelength is 532 nm. Each spectrum is normalized to the max-
imum intensity.
UV–VIS spectra were recorded on a Hitachi 150-20 spectro-
meter and steady-state fluorescence spectra were taken on a
Shimadzu RF-5300PC spectrofluorometer. 1H NMR spectra
were recorded on a JEOL JNM EX-400 spectrometer, and
coupling constants (J) are given in Hz. Mass spectra were
recorded on JEOL HX-100 and AX-500 spectrometers. For
porphyrin compounds, the positive-FAB ionization method
was used, accelerating voltage 10 kV, Xe atom as the primary
ion source, and a mixture of 3-nitrobenzyl alcohol–CHCl3 as
the FAB matrix. IR spectra were recorded on a Shimadzu
FTIR-8100 spectrometer. Fluorescence lifetimes were measured
on 10Ϫ7  air-saturated solutions with a picosecond time-
than those based on eqn. (1), since the comparison of the
steady-state fluorescence intensity suffers from errors derived
from the presence of fluorescent impurities.
If the energy transfer occurs only by a simple Coulombic
interaction, the energy transfer rate constant can be pre-
dicted by Förster theory.14 In this theory the rates of singlet
energy transfer are expressed in terms of the orientation
factor (κ), the centre-to-centre distance (r) between the tran-
482
J. Chem. Soc., Perkin Trans. 2, 1997

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correlated single photon counting system.16 Solvents and
reagents were purified by standard methods before use.
1,4-Bis(4-formylphenylethynyl)benzene 10
The synthetic procedure was the same as for 8. This compound
was hardly soluble in common organic solvents. Yield 133 mg,
quant.; milky white powder; mp 270 ЊC (decomp.) (Found: C,
82.00; H, 4.22. C24H14O2 requires C, 86.21; H, 4.22%);
λmax(THF)/nm 342 (ε/dm3 molϪ1 cmϪ1 55 000); νmax/cmϪ1 1703s,
1601m, 1206m and 828m; δH (CDCl3) 10.04 (2 H, s, CHO),
7.89 (4 H, d, J 7.8, ArH), 7.69 (4 H, d, J 8.3, ArH) and 7.57
(4 H, s, centre-ArH); m/z 334.0995 (Calc. 334.0994 for
C24H14O2).
1,2-Bis[4-(4,4-dimethyl-2,6-dioxan-1-yl)phenylethynyl]benzene 5
4-(4,4-Dimethyl-2,6-dioxan-1-yl)phenylacetylene 4 (1.30 g, 6
mmol) and 1,2-diiodobenzene (0.99 g, 3 mmol) were dissolved
in dry triethylamine.12 Pd(OAc)2 (28 mg), triphenylphosphine
(66 mg), and a small amount of PdCl2(PPh3)2 were added, and
the mixture was refluxed under Ar for 2 h, then stirred over-
night at room temp. The mixture was poured into 2  HCl and
extracted with benzene, washed with aqueous NaHCO3, and
dried over anhydrous Na2SO4. The product was purified by sil-
ica gel column chromatography with benzene, recrystallized
from THF–hexane. Yield 340 mg, 22%; white powder, mp
194 ЊC (Found: C, 80.41; H, 6.71. C34H34O4 requires C, 80.60;
H, 6.76%); νmax/cmϪ1 2951m, 2857m, 1385m, 1096s, 1018m
and 831m; δH (CDCl3) 7.57 (4 H, d, J 8.3, ArH), 7.56 (2 H, dd,
J 5.4 and 3.9, centre-ArH), 7.49 (4 H, d, J 8.3, ArH), 7.30
(2 H, dd, J 5.6 and 3.2, centre-ArH), 5.40 (2 H, s, acetal), 3.78
(4 H, d, J 11.2, CH2), 3.66 (4 H, J 10.3, d, CH2), 1.30 (6 H, s,
Me) and 0.80 (6 H, s, Me); m/z 506.2445 (Calc. 506.2457 for
C34H34O4).
1,2-Bis{4-[5-(3,5-di-tert-butylphenyl)-2,8,12,18-tetrahexyl-
3,7,13,17-tetramethyl-15-porphyrinyl]phenylethynyl}benzene
1(HH)
Dialdehyde 8 (50 mg, 0.15 mmol), 11 (263 mg, 1.2 mmol) and
12 (513 mg, 1.5 mmol) were dissolved in dry CH3CN (8 ml) and
dry CH2Cl2 (5 ml).4,13 Trichloroacetic acid (74 mg, 0.45 mmol)
in dry CH3CN (3 ml) was added and stirred at room temp-
erature overnight under Ar in the dark. A solution of p-chloranil
(588 mg, 2.4 mmol) in dry THF (30 ml) was added, and the
mixture was stirred further for 1 d. After evaporation of the
solvent, the residue was dissolved in a small amount of CHCl3
and the solution was passed through a short activated alumina
column. The products were separated by silica gel column
chromatography with CH2Cl2, and recrystallized from CH2Cl2–
MeOH. Yield 36 mg, 12% based on the amount of 8 used;
violet crystals; λmax(THF)/nm 408 (relative absorption intensity
1000), 506 (95), 537 (25), 576 (35) and 628 (7); δH (CDCl3) 10.13
(4 H, s, meso-H), 8.16 (4 H, d, J 8.3, ArH), 8.12 (4 H, d, J 8.3,
ArH), 7.86 (4 H, s, ArH), 7.84–7.86 (2 H, dd, J 5.9 and 3.4,
centre-ArH), 7.77 (2 H, s, ArH), 7.50–7.52 (2 H, dd, J 5.9 and
3.4, centre-ArH), 3.90 (8 H, t, J 7.8, Hex-1), 3.85 (8 H, t, J 7.8,
Hex-1), 2.58 (12 H, s, Me), 2.40 (12 H, s, Me), 2.11 (8 H, p, J
7.6, Hex-2), 2.01 (8 H, p, J 7.1, Hex-2), 1.65 (8 H, m, Hex-3),
1.47 (44 H, s + m, But + Hex-3), 1.40 (8 H, m, Hex-4), 1.30 (8 H,
m, Hex-5), 1.15–1.20 (8 H, m, Hex-4), 1.08–1.15 (8 H, m,
Hex-5), 0.82 (12 H, t, J 7.3, Hex-6), 0.69 (12 H, t, J 7.3,
Hex-6) and Ϫ2.45 (4 H, br, NH); m/z 2058 (Calc. 2056 for
C146H190N8).
1,3-Bis[4-(4,4-dimethyl-2,6-dioxan-1-yl)phenylethynyl]benzene 6
The synthetic procedure was the same as for 5 except for using
1,3-diiodobenzene instead of 1,2-diiodobenzene. Yield 395 mg,
26%; pale-yellow powder; mp 231 ЊC (Found: C, 80.26; H, 6.81.
C34H34O4 requires C, 80.60; H, 6.76%); νmax/cmϪ1 2953m, 2853m,
1385m, 1100s, 1021m and 831m; δH (CDCl3) 7.72 (1 H, s,
centre-ArH), 7.54 (4 H, d, J 8.3, ArH), 7.50 (4 H, d, J 8.3,
ArH), 7.48 (2 H, d, J 6.4, centre-ArH), 7.33 (1 H, t, J 7.6,
centre-ArH), 5.40 (2 H, s, acetal), 3.78 (4 H, d, J 11.2, CH2),
3.66 (4 H, d, J 10.74, CH2), 1.30 (6 H, s, Me) and 0.81 (6 H, s,
Me); m/z 506.2446 (Calc. 506.2457 for C34H34O4).
1,4-Bis[4-(4,4-dimethyl-2,6-dioxan-1-yl)phenylethynyl]benzene 7
The synthetic procedure was the same as for 5 except for using
1,4-diiodobenzene instead of 1,2-diiodobenzene. Yield 477 mg,
31%; white powder; mp 305 ЊC (Found: C, 80.46; H, 6.82.
C34H34O4 requires C, 80.60; H, 6.76%); νmax/cmϪ1 2955m, 2853m,
1383m, 1102s, 1019m, 837m and 814m; δH (CDCl3) 7.54 (4
H, d, J 8.3, ArH), 7.50 (4 H, d, J 8.3, ArH), 7.50 (4 H, s, centre-
ArH), 5.40 (2 H, s, acetal), 3.78 (4 H, d, J 11.2, CH2), 3.66 (4 H,
d, J 11.2, CH2), 1.30 (6 H, s, Me) and 0.81 (0.81, s, Me); m/z
506.2501 (Calc. 506.2457 for C34H34O4).
Monozinc complex 1(ZH)
To a solution of 1(HH) (20 mg) in CH2Cl2 (20 ml) were added a
few drops of a saturated MeOH solution of Zn(OAc)2. After a
while, the solution was washed with water, dried with
anhydrous Na2SO4, and concentrated in vacuo. The products
were separated by silica gel column chromatography with
CH2Cl2, recrystallized from CH2Cl2–MeOH. Yield 5 mg, 25%;
violet crystals; λmax(THF)/nm 416 (relative absorption intensity
1000), 506 (44), 543 (50), 577 (28) and 628 (2); m/z 2120 (Calc.
2117 for C146H188N864Zn).
1,2-Bis(4-formylphenylethynyl)benzene 8
A solution of 5 (203 mg, 0.4 mmol) in CH2Cl2 (20 ml) was
stirred overnight with 50% trifluoroacetic acid (20 ml) at 0 ЊC.
The solution was washed with saturated NaHCO3 solution and
dried over Na2SO4. Product was recrystallized from THF–
hexane. Yield 103 mg, 77%; pale-yellow powder; mp 154 ЊC
(Found: C, 85.95; H, 4.18. C24H14O2 requires C, 86.21; H,
4.22%); λmax(THF)/nm 302 (ε/dm3 molϪ1 cmϪ1 71 000) and 333
(37 000); νmax/cmϪ1 2361m, 1698s, 1603m, 1210m and 828m;
δH (CDCl3) 10.04 (2 H, s, CHO), 7.88 (4 H, d, J 7.8, ArH), 7.71
(4 H, d, J 8.3, ArH), 7.62 (2 H, dd, J 5.6 and 3.2, centre-ArH)
and 7.40 (2 H, dd, J 5.9 and 3.4 centre-ArH); m/z 334.0996
(Calc. 334.0994 for C24H14O2).
Bis(phenylethynyl)-1,3-phenylene-bridged dimer 2(HH)
The synthetic procedure was the same as for 1(HH). Yield 34
mg, 11% based on the amount of 9 used; violet crystals;
λmax(THF)/nm 409 (relative absorption intensity 1000), 506
(84), 537 (20), 576 (30) and 628 (6); δH (CDCl3) 10.25 (4 H, s,
meso-H), 8.14 (4 H, d, J 7.8, ArH), 8.07 (1 H, s, centre-ArH),
7.99 (4 H, d, J 7.8, ArH), 7.92 (4 H, s, ArH), 7.80 (2 H, s, ArH),
7.76 (2 H, d, J 7.3, centre-ArH), 7.55 (1 H, t, J 7.6, centre-ArH),
4.00 (16 H, t + t, Hex-1), 2.58 (12 H, s, Me), 2.46 (12 H, s, Me),
2.20 (16 H, m, Hex-2), 1.74 (16 H, m, Hex-3), 1.51 (36 H, s,
But), 1.49 (16 H, m, Hex-4), 1.38 (16 H, m, Hex-5), 0.91 (24 H,
t, J 7.1, Hex-6) and Ϫ2.39 (4 H, br, NH); m/z 2058 (Calc. 2056
for C146H190N8).
1,3-Bis(4-formylphenylethynyl)benzene 9
The synthetic procedure was the same as for 8. Yield 113 mg,
85%; pale-yellow powder; mp 183 ЊC (Found: C, 85.98; H, 4.27.
C24H14O2 requires C, 86.21; H, 4.22%); λmax(THF)/nm 308 (ε/
dm3 molϪ1 cmϪ1 52 000) and 325 (49 000); νmax/cmϪ1 2361m,
1688s, 1603m, 1210m and 824m; δH (CDCl3) 10.04 (2 H, s,
CHO), 7.89 (4 H, d, J 8.3, ArH), 7.77 (1 H, s, centre-ArH), 7.69
(4 H, d, J 8.3, ArH), 7.56 (2 H, d, J 7.6, centre-ArH) and 7.40
(1 H, t, J 7.6, centre-ArH); m/z 334.0990 (Calc. 334.0994 for
C24H14O2).
Monozinc complex 2(ZH)
The synthetic procedure was the same as for 1(ZH). Yield 8 mg,
40%; violet crystals; λmax(THF)/nm 417 (relative absorption
intensity 1000), 506 (38), 543 (44), 577 (24) and 628 (2); m/z
2121 (Calc. 2117 for C146H188N864Zn).
J. Chem. Soc., Perkin Trans. 2, 1997
483

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5-(3,5-Di-tert-butylphenyl)-15-(4-ethynylphenyl)-2,8,12,18-tetra-
hexyl-3,7,13,17-tetramethylporphyrin 14
References
1 R. Van Grondelle, J. P. Dekker, T. Gillbro and V. Sundstrom,
Biochim. Biophys. Acta, 1994, 1187, 1.
2 G. Mcdermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-
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3 A. Osuka, K. Maruyama, I. Yamazaki and N. Tamai, Chem. Phys.
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4 S. Kawabata, N. Tanabe and A. Osuka, Chem. Lett., 1994, 1797;
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5 S. Prathapan, T. E. Johnson and J. S. Lindsey, J. Am. Chem. Soc.,
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6 J. S. Lindsey, S. Prathapan, T. E. Johnson and R. W. Wagner, Tetra-
hedron, 1994, 50, 8941; R. W. Wagner, T. E. Johnson, F. Li and
J. S. Lindsey, J. Org. Chem., 1995, 60, 5266.
3,5-Di-tert-butylbenzaldehyde 11 (656 mg, 3 mmol), bis(3-
hexyl-4-methylpyrrol-2-yl)methane 12 (2.06 g, 6 mmol) and 4-
ethynylbenzaldehyde 13 (390 mg, 3 mmol) were dissolved in dry
CH3CN (32 ml) and dry CH2Cl2 (10 ml).13 Trichloroacetic acid
(74 mg, 0.45 mmol) in dry CH3CN (3 ml) was added and stirred
at room temp. overnight under Ar in the dark. A solution of p-
chloranil (2.94 g, 12 mmol) in dry THF (100 ml) was added,
and the mixture was stirred further for 1 d. After evaporation of
the solvent, the residue was dissolved in a small amount of
CHCl3 and the solution was passed through a short activated
alumina column. The products were separated by silica gel col-
umn chromatography with CH2Cl2 (second fraction), recrystal-
lized from CH2Cl2–MeOH. Yield 605 mg, 20%; violet crystals;
δH (CDCl3) 10.23 (2 H, s, meso-H), 8.07 (2 H, d, J 8.3, ArH),
7.91 (2 H, s, ArH), 7.89 (2 H, d, J 8.3, ArH), 7.80 (1 H, s, ArH),
3.98 [8 H, m(t + t), Hex-1], 3.34 (1 H, s, CCH), 2.50 (6 H, s, Me),
2.45 (6 H, s, Me), 2.17 (8 H, m, Hex-2), 1.72 (8 H, m, Hex-3),
1.50 (26 H, s + m, But + Hex-4), 1.36 (8 H, m, Hex-5), 0.90 (12 H,
t, J 7.3, Hex-6) and Ϫ2.42 (2 H, br, NH); m/z 991 (Calc. 991 for
C70H94N4).
7 V. S.-Y. Lin, S. G. DiMagno and M. J. Therien, Science, 1994,
264, 1105; P. J. Angiolillo, V. S.-Y. Lin, J. M. Vanderkooi and
M. J. Therien, J. Am. Chem. Soc., 1995, 117, 12 514.
8 D. P. Arnold and L. J. Nitschinsk, Tetrahedron, 1992, 48, 8781;
D. P. Arnold and G. A. Heath, J. Am. Chem. Soc., 1993, 115, 12 197;
J. J. Gosper and M. Ali, J. Chem. Soc., Chem. Commun., 1994, 1707.
9 Pd-catalysed coupling reaction of meso-(ethynyl)NiOEP with 1,4-
diiodobenzene was reported to give a 1,4-bis(ethynyl)phenylene
bridged dimer in 60% yield, but the similar reaction with 1,2-
diiodobenzene gave the corresponding 1,2-bis(ethynyl)phenylene
bridged dimer in some amount which was not isolated due to
thermal unstability. D. P. Arnold and L. J. Nitschinsk, Tetrahedron
Lett., 1993, 34, 693; D. P. Arnold, D. A. James, C. H. L. Kennard
and G. Smith, J. Chem. Soc., Chem. Commun., 1994, 2131.
10 H. L. Anderson, S. J. Martin and D. D. C. Bradley, Angew. Chem.,
Int. Ed. Engl., 1994, 33, 655; H. L. Anderson, Inorg. Chem., 1994,
33, 972.
11 F. Effenberger, H. Schlosser, P. Bauerle, S. Maier, H. Port and
H. C. Wolf, Angew. Chem., Int. Ed. Engl., 1988, 27, 281; N. Holl,
H. Port, H. C. Wolf, H. Stobel and F. Effenberger, Chem. Phys.,
1993, 176, 215; F. Wurthner, M. S. Vollmer, F. Effenberger,
P. Emele, D. U. Meyer, H. Port and H. C. Wolf, J. Am. Chem. Soc.,
1995, 117, 8090.
Bis(phenylethynyl)-1,4-phenylene-bridged dimer 3(HH)
Ethynylporphyrin 14 (100 mg, 0.1 mmol) and 1,4-diiodo-
benzene (10.9 mg, 0.033 mmol) were dissolved in triethylamine
(7 ml).9 PdCl2 (10 mg) and PPh3 (20 mg) were added, and the
mixture was refluxed under Ar for 4 h, then stirred overnight at
room temp. The solvent was removed in vacuo, the residue was
dissolved in CH2Cl2 and washed with 1  HCl, then water, dried
over Na2SO4. The products were purified by silica gel column
chromatography (Wakogel C-200 + Merck Kieselgel 7736, vol.
2:1) with CHCl3 containing stabilizer (EtOH 0.4–0.9%) (sec-
ond fraction), and recrystallized from CH2Cl2–MeOH. Yield
23.4 mg, 35%; violet crystals; λmax(THF)/nm 411 (relative
absorption intensity 1000), 506 (86), 537 (20), 576 (30) and 628
(5); δH (CDCl3) 10.26 (4 H, s, meso-H), 8.14 (4 H, d, J 7.6, ArH),
7.99 (4 H, d, J 7.8, ArH), 7.92 (4 H, s, ArH), 7.81 (2 H, s, ArH),
7.80 (4 H, s, centre-ArH), 4.0 (16 H, m, Hex-1), 2.58 (12 H, s,
Me), 2.47 (12 H, s, Me), 2.21 (16 H, m, Hex-2), 1.75 (16 H, m,
Hex-3), 1.51 (36 H, s, But), 1.50 (16 H, m, Hex-4), 1.38 (16 H,
m, Hex-5), 0.91 (24 H, t, J 7.3, Hex-6) and Ϫ2.40 (4 H, br, NH);
m/z 2058 (Calc. 2056 for C146H190N8).
12 J. R. Weir, B. A. Patel and R. F. Heck, J. Org. Chem., 1980, 45, 4926.
13 A. Osuka, B.-L. Liu and K. Maruyama, J. Org. Chem., 1993, 58,
3582; A. Osuka, B.-L. Liu and K. Maruyama, Chem. Lett., 1993,
949; A. Osuka, N. Tanabe, S. Nakajima and K. Maruyama, J. Chem.
Soc., Perkin Trans. 2, 1996, 199.
14 T. Förster, Discuss. Faraday Soc., 1959, 27, 7.
Monozinc complex 3(ZH)
15 Albinsson et al. have studied similar energy transfer reaction in
diporphyrins close related to 3(ZH). They found the rate of energy
transfer in 3(ZH) is ca. one-third of that in a model where the
central 1,4-phenylene unit was replaced by a 9,10-anthracenyl
unit. K. K. Jensen and B. Albinsson, XVI IUPAC Symposium on
Photochemistry at Helsinki, 1996, abstr. p. 313.
The synthetic procedure was the same as for 1(ZH). Yield 3 mg,
15%; violet crystals; λmax(THF)/nm 418 (relative absorption
intensity 1000), 506 (38), 544 (46), 578 (24) and 628 (2); m/z
2120 (Calc. 2117 for C146H188N864Zn).
16 I. Yamazaki, N. Tamai, H. Kume, H. Tuchiya and K. Oba, Rev. Sci.
Instrum., 1985, 56, 1187.
Acknowledgements
Work at Kyoto was partly supported by Grants-in-Aid for
Scientific Research (No. 07228232 and 07454249) from the
Ministry of Education, Science, Sports and Culture of Japan
and by Iwatani Naoji Foundation.
Paper 6/06700J
Received 30th September 1996
Accepted 29th October 1996
484
J. Chem. Soc., Perkin Trans. 2, 1997