methylnaphthalene - J . Phys. Chem. 1984, 8 8, 5197-5204...

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J. Phys. Chem. 1984, 88, 5197-5204 5197 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- perature. Comparison with Dianthracene Photodissociation. Because 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. l9 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., 85, 73 (19) Michl, Photochem. Photobiol, 25, 141 (1977), and references (20) J. B. Birks, “Photophysics of Aromatic Molecules”, Wiley-Intersci- ( 1982). therein. 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 [4+2] photocycloelimination reactions (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 can become more efficient, depending only on the size of the excited-state barrier and the rates of competing nonradiative processes. Wavelength Dependence of Photodissociation.
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methylnaphthalene - J . Phys. Chem. 1984, 8 8, 5197-5204...

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