Novel finding of the effect of triple bond on the photochemical cis-trans isomerization of C=C double bond

Article abstract of DOI:10.1246/cl.2001.1050

Both cis- and trans-1,2-bis(phenylethynyl)ethenes (1) gave fluorescence emission with considerably high quantum yield and intersystem crossing to the triplet state which underwent cis-trans isomerization. On the basis of the results, a novel potential energy surface of isomerization of 1 was proposed.

Full text of DOI:10.1246/cl.2001.1050

1
050
Chemistry Letters 2001
Novel Finding of the Effect of Triple Bond on the Photochemical
cis–trans Isomerization of C=C Double Bond
†
†
†
Hirokazu Sakakibara, Masashi Ikegami, Kakuzo Isagawa, Sachiko Tojo, Tetsuro Majima, and Tatsuo Arai*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571
†
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
(Received July 13, 2001; CL-010655)
Both cis- and trans-1,2-bis(phenylethynyl)ethenes (1) gave
emission was determined to be 0.31 and 0.42 for cis- and trans-
1, respectively. On direct irradiation with 328 nm light, 1
underwent mutual isomerization between cis- and trans-isomers
giving the trans-to-cis isomer ratio at the photostationary state
of ([t]/[c])pss = 44/56 in benzene. The quantum yields of
cis–trans and trans–cis isomerization were determined to be
fluorescence emission with considerably high quantum yield
and intersystem crossing to the triplet state which underwent
cis–trans isomerization. On the basis of the results, a novel
potential energy surface of isomerization of 1 was proposed.
0
.27 and 0.17, respectively. Contrary to the results reported by
7
(
Phenylethynyl)ethenes have already been prepared and
Turro for 1,2-bis(phenylethynyl)benzene, photochemical
Bergman cyclization was not observed in 1.
donor acceptor complexes between (phenylethynyl)ethenes and
electron acceptors have extensively been studied to construct a
On 308 nm laser excitation, cis- and trans-1 gave the simi-
lar transient absorption spectrum with the absorption maximum
at 450 nm and the lifetime of 370 ns (Figure 2). The observed
transient was quenched by oxygen with the rate constant of 5.6
1–6
new materials with useful optical and electronic properties.
In
addition, enediynes can produce a highly reactive 1,4-biradical
intermediate either thermally or photochemically which express-
7
9
–1 –1
es the biological activity. However, the photochemical cis–trans
× 10 M
s
indicating that the observed transient is assigned
isomerization of enediynes has scarcely been studied. In this
respect, we have prepared 1,2-bis(phenylethynyl)ethenes (1). We
wish to report here the novel effect of triple bond on the photo-
chemical as well as photophysical properties of ethenes. In brief,
on direct irradiation both cis- and trans-1 gave fluorescence
emission with considerably high quantum efficiency in addition
to the efficient intersystem crossing to the triplet state undergoing
cis–trans isomerization. Furthermore, not only the planar trans
to the triplet state. Furthermore, the transient was quenched by
the triplet quenchers such as azulene with the rate constant of
9
–1 –1
7.0 × 10 M s . These results suggest that 1 undergoes inter-
3
*
3
*
3 *
system crossing to the triplet state where t , p , and c could
be equilibrated. Since both cis- and trans-isomers of 1 seem to
have no considerable steric effect, they seem to be nearly the
same in energy and in population in the triplet state as well as in
the ground state. If this is the case, we can depict a novel
3
*
3
*
3 * 3 *
triplet state ( t ) and perpendicular triplet state ( p ), but also the
potential energy surface of cis–trans isomerization with c , t
3
*
3 *
cis triplet state ( c ) is equilibrated in the triplet state and one can
induce the deactivation from all of the three conformations by
choosing appropriate experimental conditions.
and p equilibrated with each other. The equilibration in the
excited triplet state is usually accomplished much faster than
9
the decay to the ground state.
Both cis- and trans-1 gave the fluorescence emission with
maxima at 364 and 387 nm and at 362 and 382 nm, respectively
in benzene (Figure 1). The quantum yield of fluorescence
In order to shed light on the above possibility, we have
studied the effect of azulene (Az) concentration on the photo-
8
,9
stationary state isomer ratios on triplet sensitization.
The
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1051
(
[t]/[c]) value observed on camphorquinone (E = 51.5 kcal
p
s
s
T
–
1
mol ) sensitization at 435 nm was 53/47 in the absence of Az,
but increased with increasing Az concentration as shown in
Figure 2. At first sight the Az effect seems to indicate the equili-
bration between t and p . If this is the case, the ([t]/[c])
value must linearly increase with increasing Az concentration.
3
*
3 *
3 *
give p , where t and p are equilibrated. The unimolecular
3
*
3
*
3 *
pss
,9
deactivation takes place at p , since the decay rate constant from
p is much faster than the decay rate constant from the planar t
8
3
*
3 *
However, the ([t]/[c])pss value became almost constant to give the
as already described. On the other hand, introdcution of anthryl
–3
value of 61/39 at Az concentration of 4.0 × 10 M. Since Az
quenched the triplet state exclusively by energy transfer mecha-
nism with kaz as the diffusion controlled rate constant, the above
group on the ethylenic carbon resulted in one-way cis–trans iso-
3
*
3
*
3 *
merization, where t is much more stable than c and p and
3
*
therefore deactivation takes place exclusively from t .
3
*
results indicate that Az interacts not only with t , but also with
The present findings open a new type of potential energy
3
*
3 *
c to result in energy transfer to give triplet Az and trans- and
cis-1. In this case, most of the deactivation takes place by energy
surafces of cis–trans isomerization, where three conformers, t ,
3
*
3 *
p , and c are equilibrated and the deactivation may take place
from these conformations by addition of appropriate quenchers.
3
3 *
transfer from both t* and c to Az to give trans- and cis-1
3
*
3 *
1
1
resulting in the isomer ratio of ([t]/[c])pss = 61/39 in the presence
Actually, Az quenched c and t giving exclusively c and t,
respectively, while unimolecular deactivation took place solely
–3
of Az with the concentration higher than 2 × 10 M, while the
3
*
3
*
1
1
deactivation takes place exclusively from p to give trans- and
cis-1 with the isomer ratio of ([t]/[c])pss 53/47 in the absence of
from p giving c and t with nearly the same ratio.
In summary, the phenylethynyl group instead of phenyl
group changed the behavior of stilbene in the excited singlet
state to undergo efficient fluorescence emission and intersystem
crossing in addition to the cis–trans isomerization.
Furthermore, three energy minima exist in the triplet state
3
*
Az. Thus, we can estimate the equilibrium constant between t
3
*
and c from the above value to be 61/39.
The triplet lifetime of 1 (370 ns) is much longer than that of
stilbene (60 ns), which indicates the existence of equilibration
3
*
3 *
3
*
3 *
between the planar triplet states ( c and t ) and p . Since the
potential surface of 1, where c has a considerable population
3
*
3 *
3 *
deactivation rate constant from t to trans-1 is reasonably esti-
and equilibrated with t and p .
3
*
mated to be the same with that from c to cis-1, the equilibrium
constant between the planar triplet state ( t and c ) and the per-
pendicular triplet state ( p ) can be estimated by observed triplet
3
*
3 *
This work was partly supported by a Grant-in-Aid for
Scientific Research (Nos. 09450319, 10440166, and 13305058)
from the Ministry of Education, Science, Sports and Culture,
Japan.
3
*
lifetime. Thus, the triplet lifetime (τ ) can be expressed by eq 1,
T
where k and k ' are the rate constants for the decativation from
d
d
3
*
3
*
3 *
the planar triplet state ( t and c ) and p , respectively, and K is
3
*
the equilibrium constant between the planar triplet state ( t and
References
3
*
3
*
1
2
3
F. Diederich, D. Philp, and P. Seiler, J. Chem. Soc., Chem. Commun.,
994, 205
D. Philp, V. Gramlich, P. Seiler, and F. Diederich, J. Chem. Soc.,
Perkin Trans. 2, 1995, 875.
A. Hilger, J.-P. Gisselbrecht, R. R. Tywinski, C. Boudon, M.
Schreiber, R. E. Martin, H. P. Luthi, M. Gross, and F. Diederich, J.
Am. Chem. Soc., 119, 2069 (1997).
c ) and p . The values k and k ' were previously estimated to
d
–1
d
1
4
–1
7
9
be 2 × 10 s and 2 × 10 s , respectively.
3
*
3 *
3 *
Thus, K = 0.12 ={[ p ]/([ t ]+[ c ])} can be obtained. By
using this value, one can roughly estimate the proportion of the
4
5
K. P. C. Vollhardt and L. S. Winn, Tetrahedron Lett., 26, 709 (1985).
H. Taniguchi, K. Hayashi, K. Nishioka, Y. Hori, M. Shiro, and T.
Kitamura, Chem. Lett., 1994, 1921.
3
*
3
*
3 *
three conformations to be t : p : c = 54 : 11 : 35.
On the basis of these arguments, the potential energy sur-
face of cis–trans isomerization of 1 in the triplet state was
depicted as shown in Figure 4.
6
K. Sonogashira, Y. Tohda, and N. Hagihara, Tetrahedron Lett., 1975,
4467.
7
8
A. Evenzahav and N. J. Turro, J. Am. Chem. Soc., 120, 1835 (1998).
J. Saltiel and M. L. Charlton “Rearrangement in Ground and Excited
State,” ed. by P. de Mayo, Academic Press New York (1980), Vol. 3,
p 25.
The potential energy surfaces of cis–trans isomerization
8,9
depended on the substituents on the ethylenic double bond. On
triplet sensitization stilbene underwent two-way isomerization.
3
*
9
T. Arai and K. Tokumaru, Chem. Rev., 93, 23 (1993).
In the triplet state c is unstable and undergoes isomerization to