Irreversible photoisomerization behavior of 2-stilbazole ? covalently bound to porphyrin

Article abstract of DOI:10.1039/b109377k

Irreversible cis-to-trans photoisomerization behavior was observed for a stilbazole-porphyrin (1), and its zinc complex (2). trans-1 and trans-2 did not isomerize to the corresponding cis-isomer under irradiation with UV light, although stilbazoles readily undergo the trans-to-cis isomerization upon UV irradiation. In contrast, cis-to-trans photoisomerization was observed for both cis-1 and cis-2; the isomerization of cis-stilbazole readily proceeds by visible light irradiation via complexation with metalloporphyrins.

Full text of DOI:10.1039/b109377k

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Irreversible photoisomerization behavior of 2-stilbazole† covalently
bound to porphyrin
Hiroshi Sugimoto,* Keigo Kuramoto and Shohei Inoue
1
Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science,
Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. E-mail: hrssgmt@ci.kagu.sut.ac.jp;
Fax: ϩ(81) 3-5261-4631; Tel: ϩ(81) 3-3260-4272
Received (in Cambridge, UK) 15th October 2001, Accepted 10th June 2002
First published as an Advance Article on the web 5th July 2002
Irreversible cis-to-trans photoisomerization behavior was observed for a stilbazole–porphyrin (1), and its zinc
complex (2). trans-1 and trans-2 did not isomerize to the corresponding cis-isomer under irradiation with UV light,
although stilbazoles readily undergo the trans-to-cis isomerization upon UV irradiation. In contrast, cis-to-trans
photoisomerization was observed for both cis-1 and cis-2; the isomerization of cis-stilbazole readily proceeds by
visible light irradiation via complexation with metalloporphyrins.
Introduction
Photoresponsive molecules are of much interest and import-
ance in relation to photoresponsive systems in nature and as
building blocks to construct photoresponsive supramolecular
systems.1 We have recently found that photoisomerizable
stilbazole coordinates to metal complexes of tetraphenyl-
porphyrin to different extents depending on the structure of the
geometric isomers of stilbazole due to steric repulsion between
the metalloporphyrin and stilbazole.2 Thus, the complexation
of zinc tetraphenylporphyrin with 2-stilbazole is reversibly
photoswitchable under the action of UV and visible light
through the trans–cis isomerization of the stilbazole (Scheme
1).3 A related compound, stilbene, differs much from stilbazole,
since stilbene undergoes both trans-to-cis and cis-to-trans
photoisomerization under photoirradiation, whereas the cis-to-
trans photoisomerization of stilbazole proceeds less efficiently
than the trans-to-cis process when metalloporphyrins or
appropriate sensitizers promote the cis-to-trans isomerization
of stilbazole.4,5
More recently, we have demonstrated an interesting novel
example of a “photoresponsive molecular switch”: a light-
driven on–off switching system for chemical reactions,
composed of aluminium porphyrin and stilbazole. This system
can regulate the reaction of carbon dioxide and propylene
oxide to give propylene carbonate, catalyzed by aluminium
tetraphenylporphyrin in the presence of 2-stilbazole under UV
or visible light through the photoisomerization of stilbazole.6
In the course of our related studies on porphyrin–stilbazole
chemistry, we synthesized a strategically designed porphyrin
having a covalently linked 2-stilbazole moiety in the periphery,
such as 5-[2Ј-(2Љ-pyridylethenyl)phenyl]-10,15,20-triphenyl-
porphyrin (stilbazole–porphyrin; 1), in order to produce more
efficient photoresponsive systems. Contrary to expectation,
we found interesting and unusual phenomena, such as the
irreversible photoisomerizations of stilbazole–porphyrin (1)
and its zinc complex (2) in which neither trans-1 nor trans-2
isomerized into the corresponding cis-isomer, but, in sharp
contrast, cis-1 and cis-2 did undergo photoisomerization. Thus,
we wish to report the details of these results in the present
paper.‡
Scheme 1
Results and discussion
5-[2Ј-(2Љ-Pyridylethenyl)phenyl]-10,15,20-triphenylporphyrin
(stilbazole–porphyrin; 1) was synthesized by the reaction be-
tween 5-(2Ј-formylphenyl)-10,15,20-triphenylporphyrin (4) and
(2-pyridylmethyl)triphenylphosphonium bromide via 5-[2Ј-
(1Љ,3Љ-dithiacyclohexan-2Љ-yl)phenyl]-10,15,20-triphenylpor-
phyrin, which was prepared from protected o-phthalaldehyde,
benzaldehyde, and pyrrole (Scheme 2). The product was a
mixture of trans-1, with a trans 2-(2Ј-pyridyl)ethenyl unit in the
periphery, and cis-1, carrying a peripheral cis-stilbazolyl unit.
† The IUPAC name for 2-stilbazole is 2-styrylpyridine.
‡ A different type of zinc complex of stilbazole-linked porphyrin was
synthesized by Burrell et al. in which the structures of molecular
assembly were discussed.15
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This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b109377k

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These geometrical isomers were successfully separated by
column chromatography on silica gel. H NMR and UV–vis
group in cis-2 is considered to experience much difficulty in
1
coordinating to the intramolecular Zn, self-assembled aggre-
gations8 cannot be excluded, in which no spectral changes were
observed for cis-2 over a wide concentration range.
spectra for the fractions that eluted faster (F-1) and more slowly
(F-2) showed that they were isomeric stilbazole–porphyrins (1)
with different geometries of the peripheral stilbazolyl unit. F-1
exhibited 1H NMR signals in C6D6 assignable to olefinic
In order to investigate the isomerization behavior of the
stilbazolyl groups in the zinc complexes trans-2 and cis-2,
irradiation with UV light or visible light was performed for
C6D6 solutions of 2 in NMR sample tubes, and 1H NMR
spectra were recorded periodically.¶ When solution of cis-2 in
C6D6 (2.0 × 10Ϫ3 mol dmϪ3) was irradiated with visible light (λ >
380 nm), isomerization of cis-2 to trans-2 was observed to take
place (Scheme 3), as demonstrated by the decrease in intensity
of the signals due to cis-2 and the increase in those due to
trans-2.
3
protons at δ 7.5 and 7.9 ppm with JHH = 16.0 Hz and its
characteristic absorption maximum in the UV due to the
stilbazolyl unit occurred at λmax = 302 nm in CHCl3 (ε = 41400).
In contrast, the NMR signals assignable to the olefinic protons
of F-2 were observed at δ 6.2 and 6.7 ppm (3JHH = 12.5 Hz) and
an absorption maximum appeared at λmax = 290 nm (ε = 17600)
in the UV spectrum. Since geometrical isomers of stilbazole are
easily identified by their 1H NMR and UV–vis spectra in
1
which the olefinic protons of trans-isomer possess H NMR
The isomerization proceeded smoothly, and the trans content
of 2 increased to 38, 67, and 91% after 10, 30, and 60 min,
respectively (Fig. 1).
signals at relatively lower magnetic field with larger coupling
constants than those of the cis-isomer, and the absorption
maximum in the UV region for the trans-isomer appears at
longer wavelength with a larger ε value than that for the cis-
isomer,4,7 F-1 and F-2 were thus identified as trans-1 and cis-1,
respectively.
Metallation of the two isomers of the stilbazole–porphyrin
(trans-1 and cis-1) by Zn(OAc)2 gave the zinc porphyrins trans-2
and cis-2 almost quantitatively without isomerization, as con-
firmed by TLC analysis of the reaction mixtures. The UV–vis
absorption spectra in CHCl3 for both trans-2 (Soret band: λmax
= 428 nm) and cis-2 (λmax = 430 nm) were similar to those of
the zinc tetraphenylporphyrin–pyridine complex (428 nm) and
zinc tetraphenylporphyrin–cis-2-stilbazole complex (430 nm),
irrespective of the concentration of 2 ([2] = 1.0 × 10Ϫ2–1.0 ×
10Ϫ6 mol dmϪ3). Upfield shifts of the signals due to the pyridyl
group were observed in the 1H NMR spectra in C6D6 for
trans-2 and cis-2, when compared respectively with the spectra
of trans-1 and cis-1. These observations indicate that central
zinc atoms of 2 are coordinated by the pyridine moieties.§ As
for the structure of trans-2, molecular modeling studies have
shown that the pyridyl group in trans-2 easily coordinates intra-
molecularly to the Zn atom. On the other hand, since the pyridyl
§ The Soret band of zinc tetraphenylporphyrin [Zn(TPP)] in CHCl3
shifts to 420 nm from that of the metal-free tetraphenylporphyrin at 418
nm. The absorption maximum of Zn(TPP) at 420 nm does not change,
regardless of the amount of coexisting trans-2-stilbazole (1–1000
equiv.), whereas a new absorption maximum appears at 430 nm in the
presence of cis-2-stilbazole, and the original absorption at 420 nm dis-
appears only when more than 800 equiv. of cis-stilbazole, with respect
to Zn(TPP), are added to Zn(TPP) ([Zn(TPP)] = 2.0 × 10Ϫ2 mol dmϪ3).
Fig. 1 Time-courses of cis-to-trans photoisomerization of 1 (᭺) and 2
(᭹) under irradiation with visible light (λ > 380 nm) in C6D6 ([1]0 or [2]0
= 2.0 × 10Ϫ3 mol dmϪ3) at 23 ЊC under nitrogen. Mole fractions of the
trans-isomers were determined by 1H NMR.
¶ The wavelengths were the same as those employed for photoiso-
merizations in bimolecular systems.2,3
Scheme 2
J. Chem. Soc., Perkin Trans. 1, 2002, 1826–1830
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Coordination to zinc is thought to bring the stilbazole group
closer to the porphyrin, which is more favorable for energy
transfer upon irradiation.
The photoresponses of 2-stilbazole covalently linked to
porphyrin, such as 5-[2Ј-(2Љ-pyridylethenyl)phenyl]-10,15,20-
triphenylporphyrin (stilbazole–porphyrin; 1) and its zinc
complex (2), observed in the present study were very different
from those of the bimolecular systems composed of zinc
porphyrin and stilbazole,4b,9 and this is the first example of a
visible-light driven, irreversible photoisomerization of olefins.
In summary, photoisomerization of cis-1 and -2 occurred
only on irradiation with visible light. trans-1 and -2 did not
isomerize following irradiation with either UV light or visible
light. Our results are essentially different from the related
observation reported by Wildes and Whitten,9b since trans-to-
cis photoisomerization was not observed at all in our
stilbazole–porphyrin systems (1 and 2). Wildes and Whitten
reported the efficient cis-to-trans photoisomerization of
1-(1Ј-naphthyl)-2-(4Ј-pyridyl)ethylene in the presence of the
magnesium complex of etioporphyrin-I, where trans-to-cis
ratios of 95 : 5–99 : 1 at the photostationary state were reported;
they also observed trans-to-cis isomerization in the same
system.
Our observations can be related to the unique isomerization
processes of some 1,2-disubstituted olefins having aromatic
polycyclic groups that Arai and Tokumaru reported in the
1980s, in which irradiation with UV light caused the olefins
to undergo only cis-to-trans isomerization, and the reverse
isomerization from trans-form to cis-form did not occur. They
discussed the mechanism of the irreversible isomerization from
the view point of triplet energy surfaces of aromatic olefins.10
The porphyrin moiety as a triplet sensitizer absorbs visible light
and probably undergoes rapid intersystem crossing to the triplet
followed by triplet–triplet energy transfer to the stilbazole unit,
and thus gives the trans-isomer after the possible adiabatic
cis-to-trans isomerization. The excited stilbazole is considered
to have an equilibrium between the twisted triplet and the
trans-triplet states. It is likely that the energy transfer from the
stilbazole in the trans-triplet state to the porphyrin moiety
affords the trans-isomer exclusively, while deactivation of the
twisted triplet gives a mixture of trans- and cis-isomers. Since
1 and 2 have both chromophores, such as stilbazole and
porphyrin, in the same molecule, the former process should
occur in preference.
Scheme 3
In contrast, unexpectedly, isomerization of trans-2 to cis-2
following irradiation with UV light (λ = 290–360 nm) did not
occur under similar conditions (Scheme 3), whereas trans-2-
stilbazole itself readily isomerizes to the cis-form following
irradiation with UV light4 under identical conditions.
Furthermore, it was confirmed that cis-2 did not isomerize
after irradiation with UV light and that trans-2 did not
isomerize after irradiation with visible light. An intramolecular
coordinative interaction between the central Zn atom and the
pyridyl group in trans-2 is thought to prevent the stilbazolyl
moiety from isomerizing.
With regard to the unexpected phenomena mentioned above,
isomerization of the metal-free porphyrins trans-1 and cis-1
was investigated next in order to further examine the effect of
the interaction between the pyridyl group and the Zn on the
photoisomerization of the stilbazolyl moiety in 2. It was
found that trans-1 did not isomerize into cis-1 on irradiation
with UV light. More unexpectedly, cis-to-trans photoisomeriz-
ation of cis-1 was observed under irradiation with visible light
(Scheme 3); the cis-form stilbazole is known to undergo efficient
isomerization upon irradiation with visible light via complex-
ation of the pyridine group with metalloporphyrins4b,9 or in the
presence of appropriate sensitizers.5 We also confirmed that
irradiation with UV light did not cause photoisomerization of
cis-1, as observed for cis-2, and that irradiation with visible
light did not bring about the photoisomerization of trans-1.
These results are quite unusual and very interesting because
there is, in appearance, no chance for the stilbazolyl moiety of 1
to interact with the porphyrin due to the lack of a central Zn
atom. Therefore, the isomerization behavior observed for
trans-1 and cis-1 indicates that interactions possibly exist
between the porphyrin macrocycle and the stilbazole unit,
although the chemical shift value of the signals due to the N–H
protons in trans-1 (Ϫ2.0 ppm in C6D6) and to those in cis-1
(Ϫ1.8 ppm) was not very different from the chemical shift
value for the N–H signal in tetraphenylporphyrin (Ϫ2.1 ppm).
The slower isomerization of cis-1 to trans-1 compared with
the isomerization of cis-2 to trans-2 (Fig. 1) indicates that the
interaction between pyridine and zinc in cis-2 facilitates the
isomerization4b,9 much more than the possible interaction of
the stilbazolyl moiety with the metal-free porphyrin in cis-1.
The participation of visible light in the photoisomerizations
of 1 and 2 suggests that the porphyrin moiety plays an
essential role. This is of much interest in connection with
photomorphogenesis in plants where a tetrapyrrole-containing
chromoprotein, such as phytochrome, is involved as photo-
transducer.
Experimental
Materials
Dichloromethane (CH2Cl2) was washed successively with conc.
H2SO4, water, and aq. NaHCO3, dried over CaCl2, and distilled
over CaH2 in a nitrogen atmosphere. Deuterated chloroform
(CDCl3) was fractionally distilled over CaH2 under nitrogen.
Benzene (C6H6) and deuterated benzene (C6D6) were distilled
over sodium benzophenone ketyl in a nitrogen atmosphere.
Synthesis of 5-[2Ј-(1Љ,3Љ-dithiacyclohexan-2Љ-yl)phenyl]-
10,15,20-triphenylporphyrin (3)11
For the preparation of 1, 5-[2Ј-(1Љ,3Љ-dithiacyclohexan-2-
yl)phenyl]-10,15,20-triphenylporphyrin was first prepared
according to Lindsey’s method. To a 2 L two-necked round-
bottom flask connected to a three-way stopcock and a reflux
condenser, 2-(1Ј,3Ј-dithiacyclohexan-2Ј-yl)benzaldehyde12 (1.1
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g, 5 mmol), CH2Cl2 (2 L), benzaldehyde (1.5 mL, 15 mmol), and
pyrrole (1.4 mL, 20 mmol) were added successively, and the
flask was wrapped in aluminium foil. After purging of the solu-
tion with dry nitrogen for 30 min, BF3–OEt2 (0.82 mL, 6.6
mmol) was added via a hypodermic syringe, and the mixture
was stirred for 1 h at room temperature in a nitrogen atmos-
phere. Then, 2,3,5,6-tetrachloro-1,4-benzoquinone (DDQ; 3.4
g, 15 mmol) was added to the reaction mixture, and the result-
ing solution was gently further stirred at room temperature for
1 h. Triethylamine (0.9 mL, 6.6 mmol) was added and the vola-
tile fractions were removed from the reaction mixture under
reduced pressure. The resulting dark purple residue was sub-
jected to column chromatography on silica gel (Wakogel C-300)
with CH2Cl2–n-hexane (2 : 1) as eluent. The second fraction,
purple in color, was collected and evaporated to dryness. The
purple residue was recrystallized from CHCl3–n-hexane to give
5-[2Ј-(1Љ,3Љ-dithiacyclohexan-2Љ-yl)phenyl]-10,15,20-triphenyl-
porphyrin (3; 550 mg, 15%) as a purple powder. CHN analysis:
found C, 77.81; H, 5.19; N, 7.76 (calcd for C48H36N4S2: C, 78.66;
H, 4.95; N, 7.64%). FAB-MS for C48H37N4S2 (MHϩ): m/z 733.
methyl)triphenylphosphonium bromide (1.73 g, 17.3%) as
white crystals. FAB-MS for C24H21BrNP: C24H21NPϩ; m/z 354,
(C24H21NPϩ)2BrϪ; m/z 787. Mp 260–261 ЊC (decomp.). 1H
NMR (CDCl3): δ 5.8 (d, 2H, CH2), 7.1 (d, 1H, 3-pyridyl-H),
7.6–7.9 (m, 16H, phenyl and 5-pyridyl-H), 8.0 (t, 1H, 4-pyridyl-
H), 8.3 (s, 1H, 6-pyridyl-H).
Synthesis of 5-[2Ј-(2Љ-pyridylethenyl)phenyl]-10,15,20-
triphenylporphyrin (stilbazole–porphyrin; 1)
5-(2Ј-Formylphenyl)-10,15,20-triphenylporphyrin (4; 193 mg,
0.3 mmol) and (2-pyridylmethyl)triphenylphosphonium
bromide (192 mg, 0.3 mmol) were dissolved in dry CH2Cl2
(10 mL) under nitrogen, and the mixture was stirred at room
temperature prior to the addition of diazabicyclo[5.4.0]undec-
7-ene (0.13 mL, 0.8 mmol). After 30 min, the reaction mixture
was directly subjected to column chromatography on silica
gel (Wakogel C-300) using CH2Cl2 as eluent. The second and
third of the three major fractions were collected separately
and recrystallized from CHCl3–MeOH (1 : 1) to give (E)-5-[2Ј-
(2Љ-pyridylethenyl)phenyl]-10,15,20-triphenylporphyrin (trans-
1; 101 mg, 47%) and cis-1 (64 mg, 30%), respectively. trans-1:
CHN analysis: found C, 84.25; H, 5.01; N, 9.60 (calcd for
C51H35N5: C, 85.33; H, 4.91; N, 9.76%). Mp 251–253 ЊC
1
Mp 320–322 ЊC (decomp.). H NMR (CDCl3): δ Ϫ2.7 (s, 2H,
NH), 1.6 (m, 2H, SCH2CH2CH2S), 1.9 and 2.3 (dt, 4H, SCH2),
4.8 (s, 1H, SCH(Ph)S), 7.5 (m, 12H, m-phenyl and p-phenyl),
8.2 (m, 7H, o-phenyl), 8.7–8.8 (d, 8H, pyrrole).
1
(decomp.). FAB-MS for C51H35N5 (MHϩ): m/z 718. H NMR
Synthesis of 5-(2Ј-formylphenyl)-10,15,20-triphenylporphyrin
(C6D6): δ Ϫ2.0 (s, 2 H, NH), 6.1 (t, 1H, 5-pyridyl-H), 6.3
(4)13
(d, 1H, 3-pyridyl), 6.4 (t, 1H, 4-pyridyl), 7.5 and 7.9 (2 d, both
3
1H, JHH = 16.0 Hz, CH᎐CH), 7.6–7.8 (m, 12H, m-phenyl and
᎐
Deprotection of the obtained 5-[2Ј-(1Љ,3Љ-dithiacyclohexan-
2Љ-yl)phenyl]-10,15,20-triphenylporphyrin was achieved by
reaction with BF3–OEt2 and DDQ. To a CH2Cl2 solution
(300 mL) of 5-[2Ј-(1Љ,3Љ-dithiacyclohexan-2Љ-yl)phenyl]-
10,15,20-triphenylporphyrin (292 mg, 0.4 mmol) were added
BF3–OEt2 (5.0 mL, 41 mmol) and DDQ (5.0 g, 22 mmol), and
the mixture was stirred at room temperature overnight in the
air. The reaction mixture was washed three times with
aqueous Na2CO3 (10%) and dried over MgSO4. Evaporation to
dryness gave a purple powder that was recrystallized from
CHCl3–MeOH (1 : 1) to give 5-(2Ј-formylphenyl)-10,15,20-
triphenylporphyrin (4; 230 mg, 90%) as purple crystals. CHN
analysis: found C, 84.02; H, 4.92; N, 8.63 (calcd for C45H30N4O:
p-phenyl), 7.8 (d, 1H, 6-pyridyl), 8.1–8.4 (m, 7H, o-phenyl),
9.1 (m, 8H, pyrrole). UV–vis (CHCl3): λmax/nm (10Ϫ3 εmax) 646
(4.2), 591 (6.9), 551 (8.5), 516 (21.9), 421 (278), 302 (41.4).
cis-1: CHN analysis: found C, 84.83; H, 5.11; N, 9.74 (calcd
for C51H35N5: C, 85.33; H, 4.91; N, 9.76%). Mp 249–251 ЊC.
FAB-MS for C51H35N5 (MHϩ): m/z 718. 1H NMR (C6D6):
3
δ Ϫ1.8 (s, 2 H, NH), 6.2 and 6.7 (2 d, both 1H, JHH
=
12.5 Hz, CH᎐CH), 6.8 (t, 1H, 5-pyridyl-H), 7.2 (t, 1H, 4-
᎐
pyridyl), 7.5–7.7 (m, 12H, m-phenyl and p-phenyl), 8.0 (d, 1H,
3-pyridyl), 8.7 (d, 1H, 6-pyridyl), 8.2–8.4 (m, 7H, o-phenyl),
9.1 and 9.4 (2 d, 2H and 6H, pyrrole). UV–vis (CHCl3): λmax/nm
(10Ϫ3 εmax) 647 (2.8), 591 (4.3), 550 (5.3), 515 (13.4), 420 (272),
290 (17.6).
1
C, 84.09; H, 4.70; N, 8.72%). Mp 356–359 ЊC (decomp.). H
NMR (CDCl3): δ Ϫ2.7 (s, 2H, NH), 7.7 (m, 12H, m-phenyl and
p-phenyl), 8.4 (m, 7H, o-phenyl), 8.6 (d, 2H, pyrrole), 8.9 (s, 6H,
pyrrole), 9.5 (s, 1H, CHO).
Preparation of zinc porphyrin (2)
The zinc complexes of trans-1 and cis-1 (trans-2 and cis-2) were
prepared by the reaction of trans-1 and cis-1 with Zn(OAc)2. To
a round-bottom flask (200 mL) containing a CHCl3 solution
(100 mL) of trans-1 (143 mg, 0.2 mmol), a saturated MeOH
solution of Zn(OAc)2 (7 mL) was added, and the mixture was
stirred at room temperature for 2 h. Then, the reaction mixture
was washed with brine (100 mL × 3) and dried with Na2SO4.
After the volatile fractions had been removed under reduced
pressure, the purple powder obtained was recrystallized from
CHCl3–MeOH to give trans-2 as purple crystals (134 mg, 86%).
trans-2: FAB-MS for C51H33N5Zn (MHϩ): m/z 780. Mp >237 ЊC
(decomp.). 1H NMR (C6D6): δ 5.9 (t, 1H, 5-pyridyl-H), 6.3
Synthesis of (2-pyridylmethyl)triphenylphosphonium bromide
2-Methylpyridine (18.6 mL, 0.2 mol) and benzoyl peroxide
(0.4 g) were added successively to a suspension of N-bromo-
succinimide (71.2 g, 0.2 mol) in CCl4 (500 mL) with vigorous
stirring, and the mixture was kept at 60 ЊC for 18 h. The
reaction mixture was then cooled in an ice bath and filtered.
The filtrate was concentrated to ca. 100 mL, washed with water,
and then dried over MgSO4. CCl4 and unreacted 2-methyl-
pyridine were removed under reduced pressure (10 mmHg, 30
ЊC) to give 2-bromomethylpyridine as a brown oil. This product
was used directly in the next step without further purification,
since it decomposed easily on standing. 1H NMR (CDCl3): δ 4.7
(s, 2H, CH2Br), 7.2 (m, 1H, 3-pyridyl-H), 7.5 (d, 1H, 5-pyridyl-
H), 7.8 (m, 1H, 4-pyridyl-H), 8.6 (d, 1H, 6-pyridyl-H).14
(d, 1H, 3-pyridyl), 6.4 (t, 1H, 4-pyridyl), 6.6 and 8.2 (2 d, both
3
1H, JHH = 16.0 Hz, CH᎐CH), 6.8 (d, 1H, 6-pyridyl), 7.6–7.9
᎐
(m, 12H, m-phenyl and p-phenyl), 8.3–8.6 (m, 7H, o-phenyl),
9.2–9.3 (m, 8H, pyrrole). UV–vis (CHCl3): λmax/nm (10Ϫ3 εmax
)
A benzene solution (30 mL) of 2-bromomethylpyridine (4.3
g, 25 mmol) was added to a benzene solution (30 mL) of
triphenylphosphine (6.6 g, 25 mmol), and the mixture was
stirred at reflux overnight. The precipitates were collected by
filtration, and washed with benzene to afford the crude product
as a yellow powder. Charcoal was added to a CHCl3 solution of
the crude product, and the black suspension was stirred for
several minutes before the charcoal was filtered off. The filtrate
was evaporated to dryness to leave a white powder. The residual
powder was recrystallized from n-hexane to give (2-pyridyl-
598 (6.6), 558 (22.8), 428 (510.3), 313 (44.7). cis-2 was prepared
similarly to trans-2 in 77% yield. cis-2: FAB-MS for
C51H33N5Zn (MHϩ): m/z 780. Mp >252 ЊC (decomp.). 1H NMR
(C6D6): δ 3.5 (br, 1H, 6-pyridyl-H), 5.3 (t, 1H, 5-pyridyl-H), 6.0
3
(t, 1H, 4-pyridyl), 6.2 and 6.9 (2 d, both 1H, JHH = 12.3 Hz,
CH᎐CH), 6.3 (m, 2H, 3-pyridyl), 7.2 (t, 1H, o-phenyl), 7.3
᎐
(t, 1H, m-phenyl), 7.6–7.9 (m, 11H, m-phenyl and p-phenyl),
8.3–8.8 (m, 7H, o-phenyl), 8.9, 9.2, and 9.4 (3 d, 8H, pyrrole).
UV–vis (CHCl3): λmax/nm (10Ϫ3 εmax) 601 (6.2), 560 (19.0), 430
(430.1).
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Measurements
Elemental analyses were conducted on a Yanaco MT-6 CHN
1
Corder. H NMR measurements were performed using CDCl3
or C6D6 as solvent at 22 ЊC on a Bruker type DPX-400
spectrometer, where the chemical shifts were determined with
respect to TMS (δ 0.00 ppm) as internal standard. Mass spectra
were recorded on a JEOL JMS-SX102A spectrometer. Melting
points were uncorrected.
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