Fluorenone sensitization in solution interconverts all-trans-1, 3, with its trans,cis,trans, cis,trans,trans, and cis,cis,trans (trace) isomers, tct-, ctt-, and cct-DPH, respectively. Photoisomerization quantum yields are reported for the three major isomers in degassed and air-saturated benzene. In degassed solutions the quantum yields are strongly concentration dependent due to quantum chain processes. The presence of air eliminates the quantum chain processes, as all DPH triplets are deactivated by oxygen. Triplet-triplet absorption spectra observed in the microsecond time scale starting from these three DPH isomers are identical. The concentration dependence of the DPH triplet lifetime is consistent with the concentration dependence of the isomerization quantum yields. The results indicate that in benzene DPH triplets exist as an equilibrium mixture of ttt, tct, and ctt isomers whose composition at 20 °C, 94% ttt, 5% tct, and 1% ctt is revealed by isomerization quantum yields in the presence of air. Photoisomerization quantum yields in the presence of air show that the isomeric triplets are fully equilibrated within less than 100 ns. † J.S. dedicates this paper to his teacher, George S. Hammond, whose illuminating and inspiring ways made this issue possible.
Quantum yields for the interconversion of the all- cis,trans,cis,3, in methylcyclohexane (MCH) or acetonitrile (AN) following 366 nm excitation show these processes to be relatively inefficient. Their dependence on the concentration of the DPH reveals significant participation of triplet states in the overall process. Despite very low intersystem crossing quantum yields (0.029 and 0.010 in MCH and AN, respectively) singlet and triplet contributions in the photoisomerization of all-trans-1,6-diphenyl-1,3,5-hexatriene are roughly equal in MCH, and, for the trans,cis,trans isomer, in AN. However, in AN the cis,trans,trans isomer forms nearly exclusively by a singlet pathway from the other two isomers. The cis,cis,trans isomer, a very minor component in photostationary states, appears to form primarily from the cis,trans,trans isomer whose excited singlet state also gives another isomer, tentatively identified as ctc-DPH. The major radiationless channel of the excited singlet state of each DPH isomer is direct decay to the original ground state. Barriers to torsional relaxation of the planar lowest DPH excited singlet states (2 1 A g and 1 1 B u ) must be significantly higher than previously supposed. Photoisomerization quantum yields of all-trans-DPH in the presence of fumaronitrile (FN) are also separated into singlet and triplet contributions. Fumaronitrile quenches DPH fluorescence and singlet contributions to the photoisomerization equally, but enhances DPH triplet formation and the triplet contribution to the photoisomerization. Radical cations of DPH form in AN but do not participate in isomer interconversion.
Irradiation of all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) in acetonitrile (AN) gives ctt-and tct-DPH by relatively inefficient pathways but mainly via the singlet excited state. Assuming that twisted singlet excited state intermediates partition equally to cis and trans ground state double bonds leads to the conclusion that the major nonradiative decay process of the singlet excited state of ttt-DPH ( 1 ttt-DPH*) is direct decay to the ttt-DPH ground state, φ nr ) 0.54. If CH stretching vibrations serve as accepting modes in this decay process, deuterium substitution should profoundly attenuate it. We report a comparative study of perhydro-ttt-DPH with di-and tetradeuterated ttt-DPH (ttt-DPH-d n , n ) 0, 2, and 4) involving deuteration of one and both terminal triene double bonds. NMR and HPLC analyses give k H /k D ) 1.36 ( 0.1 for terminal bond isomerization at 20.0 °C. The very small changes in τ f and φ f are consistent with such a kinetic deuterium isotope effect on the rate constant for terminal bond isomerization. The temperature dependencies of τ f , φ ctt , and φ tct for 1 ttt-DPH-d 0 * give energy barriers for torsional relaxations that are well above the 2A g -1B u energy gap. The major radiationless decay process is not related to trans f cis photoisomerization, is barrierless within experimental uncertainty, is completely insensitive to deuterium substitution, and occurs in the ns time scale. The mechanistic implications of these results are discussed. † Part of the special issue "George S. Hammond & Michael Kasha Festschrift". J.S. dedicates this paper to his inspiring teacher George S. Hammond with gratitude.
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