Chemistry Letters 2000
1095
spectra peaked at 610 nm was assigned to the triplet state (T–T
absorption spectrum) since it was quenched by oxygen. The
T–T absorption spectrum is very similar to that of trans-1
observed on excitation with a 308 nm laser pulse (λmax = 610
nm and 410 nm, Figure 1(b)), which has been assigned to the
triplet tautomer of trans-1.5 Since they both have absorption
peaked at 610 nm and have lifetime of 1.2 µs, we can assign
that they are the same triplet state and both are the triplet tau-
tomer of trans-1. The quantum yield of intersystem crossing
We have performed semiempirical calculations (PM3 /
4CI)7,8 in order to study the potential energy surface of the pho-
toisomerization and hydrogen atom transfer in the ground, the
excited singlet, and the excited triplet states. The results agree
well to the experimental results. For example, there is no
3
potential minimum at (cis-1)* and the heat of formation (∆Hf)
of 3(cis-1)* is 18.7 kJ mol–1 higher than that of 3(cis-1')*; it indi-
cates the occurrence of adiabatical hydrogen atom transfer of
cis-1 in the excited triplet state.
from (trans-1)* to (trans-1')* on excitation of trans-1 at 308
nm was 0.09 in benzene. The quantum yield of intersystem
crossing in cis-1 was determined to be 0.06 and is very close to
the quantum yield of photoisomerization (0.05).
These results indicate that the photoisomerization proceeds in
the excited triplet state and photoirradiation of cis-1 forms the
triplet tautomer of trans-1.
We have reported various types of photochemical isomer-
ization of arylethenes.9 In these cases the mode of isomeriza-
tion as well as the potential energy surfaces of isomerization
was understood by the effect of aryl group on the triplet ener-
gies of planar trans and perpendicular geometries. The present
findings of the one-way isomerization open a novel mechanism
of highly specific isomerization induced by the remote hydro-
gen atom transfer where two adiabatic photochemical process-
es, hydrogen atom transfer and cis-trans isomerization takes
place by the absorption of only one photon. We are now
extending these findings to apply the intramolecular multi adia-
batic photochemical reactions.
1
3
From these experimental results, we have proposed a novel
mechanism for the photoisomerization of 1 as shown in Figure
2. The photoexcitation of cis-1 gives the triplet tautomer of cis-
1 either via intramolecular hydrogen atom transfer in the singlet
state and subsequent intersystem crossing, or via intersystem
crossing followed by intramolecular hydrogen atom transfer in
the triplet state. The triplet tautomer of cis-1 (3(cis-1')*) iso-
merizes adiabatically to give the triplet tautomer of trans-1
(3(trans-1')*), which has sufficient lifetime to be detected by the
laser flash photolysis. Such one-way characteristics from 3(cis-
1')* to 3(trans-1')* can be explained by the very low triplet ener-
gies of these tautomers. The tautomer has a conjugation of
tetraenone structure and therefore, the planar trans triplet
3(trans-1')* should be the most stable conformation in the excit-
ed triplet state. Actually, the triplet energy of trans-1' was esti-
mated to be 173 kJ mol–1, which is much lower than the triplet
energy of trans-1 (229 kJ mol–1). Thus, in the excited triplet
state of 1, the tautomer (3(cis-1')*) formed by intramolecular
hydrogen atom transfer in the excited state undergoes twisting
around the double bond to give the perpendicular triplet state
(3p'*), but the perpendicular geometry is no longer a funnel for
the deactivation to the ground state and the deactivation takes
place solely at 3(trans-1')*.
This work was supported by a Grant-in-Aid for Scientific
Research (No. 10440166) and a Grant-in-Aid for Scientific
Research on Priority Areas (A) (No. 10146103) from the
Ministry of Education, Science, Sports, and Culture, Japan and
by Research Foundation for Opto-Science and Technology.
References and Notes
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4
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7
The semiempirical calculations were performed using
CAChe MOPAC ver. 94.10 on Macintosh G3 with parame-
ters in Reference 8.
8
9
J. J. P. Stewart, J. Comp. Chem., 10, 209 (1989).
T. Arai and K. Tokumaru, Chem. Rev., 93, 23 (1993).