diphenylethylene chromophore. At low temperature in rigid glass
solvents the chromophores are trapped in this arrangement, but
the large angle between the planes of the two ring systems (ca.
50’) precludes any significant interchromophore interaction. We
identify this species with that observed in the low-temperature
photochemical experiments. At higher temperatures, the chro-
mophores can rotate away from each other around the C(9)-C-
(1 5’) bond to give the open compound observed near room tem-
Comparison with Dianthracene Photodissociation.
of their structural similarity, it is interesting to compare the
photodissociation of lepidopterenes with that of the anthracene
photodimers. Although the dissociation proceeds by the same
general pathway in both cases, the efficiency of the adiabatic path
shows a dramatic difference: for lepidopterene it approaches 90%
at room temperature, whereas in the anthracene photodimers it
amounts to only 0.05% of the total product yield.’* This difference
can be explained in terms of the qualitative theoretcial ideas
developed by Michl,19 which are based on the Woodward-
Hoffmann rules for the conservation of orbital symmetry.
In the case of photochemically allowed [4+4; 2+2] eliminations
(e.g., dianthracene photodissociation), the first excited singlet (S)
state of reactants correlates with that of the products, the ground
(G) state of reactants correlates with a doubly excited (D) state
of products, and vice versa. Avoided crossing of the D and G states
leads to a “pericyclic minimum” on the excited electronic surface
close to a local maximum on the ground-state surface, an ideal
geometry for nonradiative transitions.
Trapping of electronically
excited reactants at the pericyclic minimum leads predominantly
to partitioning between ground-state reactants and products
(18) S. Yamamoto and K.-H. Grellmann,
Chem. Phys. Lett.,
141 (1977), and references
(20) J. B. Birks, “Photophysics of Aromatic Molecules”, Wiley-Intersci-
ence, London, 1970, p 121.
(diabatic path), although escape from the pericyclic minimum
can lead to products via an adiabatic path in small yield.
In contrast, concerted
(e.g., lepidopterene photodissociation) are symmetry forbidden
(ground-state allowed). Here, the first excited singlet state of
reactants correlates with a higher excited state of the products,
leading to a barrier on the excited-state surface.19 Since there
is no pericyclic minimum, diabatic leakage to the ground state
is unimportant. The adiabatic process
become more efficient,
depending only on the size of the excited-state barrier and the
rates of competing nonradiative processes.
Wavelength Dependence of Photodissociation.