Photoisomerization of ttt-DPH
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3185
proposed by Hohlneicher and Dick,65 and the small torsional
barrier in the 1Bu state is the pathway favored by Troe and
Weitzel for stilbene photoisomerization.66 Other possibilities can
be envisioned.
(12) Saltiel, J.; Wang, S.; Watkins, L. P.; Ko, D.-H. J. Phys. Chem. A
2000, 104, 11443-11450.
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Terminal bond twisting is sensitive to d-substitution, k1H/k1D
) 1.37 for both olefinic positions, consistent with analogous
observations on trans-stilbene,37-43 and with theoretical expecta-
tions.36,44,47 Whether the central bond is similarly sensitive to
d-substitution remains to be established. The relatively low
Arrhenius frequency factor Atpt associated with crossing the
torsional barrier for the central bond suggests that the process
may be diabatic as in a variant of the OS mechanism for stilbene
(e.g., a partially avoided crossing between the 1Bu state and a
higher nAg state, n > 2).9 The polar solvent AN enhances the
Arrhenius frequency factor Aptt for the terminal bonds to the
point where the possibility that this process has been rendered
adiabatic must be entertained.
In view of the tendency in vogue to explain the radiationless
deactivation of polyenes by invoking ultrafast crossing of CIs,
perhaps the most unexpected conclusion is that decay from 1ttt-
DPH* is dominated by eq 4, an essentially barrierless radia-
tionless pathway, which is insensitive to d-substitution at one
or both terminal bonds of the triene moiety. Nuclear motions
of CC bonds along the triene backbone may be involved in this
process as is proposed for polyenes generally53,54 and as has
been found recently for what may be the analogous transition
in all-trans-â-carotene.63,64 However, branching between pho-
toisomers and the original ground state is supposed to occur in
the ground state after internal conversion through a CI.53,54 Such
a model clearly does not apply to DPH. The major radiationless
decay process is completely divorced from trans f cis photoi-
somerization, is barrierless within experimental uncertainty,
shows no DIE, and occurs in the ns time scale. If ultrafast decay
characterizes crossing a CI, then a CI is not involved here. For
a CI via partial 1,3-bond formation, or CC bonding on the path
to a bicyclobutane to play a role in eq 4, it would have to be
neither ultrafast nor along cis-trans photoisomerization reaction
coordinates. More work is required to elucidate the mechanism
of this highly effective radiationless decay channel.
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Acknowledgment. The National Science Foundation, most
recently by Grant CHE 9985895, supported this research.
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