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Publication costs assisted by the Max-Planck-lnstitut fur Kohlenforschung, Institut fur Strahlenchemie Laser flash photolysis of nitro-substituted stilbenes (4-nitrostilbene, 4,4'-dinitrostilbene, 4-nitro-4'methoxystilbene, and 4-nitro-4'-dimethylaminostilbene) permits the observation of transients. The lifetimes of the transients (around 100 nsec) and their spectra vary strongly with the polarity of the solvent for 4-nitro-4'-methoxystilbene and for 4-nitro-4'-dimethylaminostilbene, but less so for the other nitrostilbenes. No transients were observed from 4-cyano-4'-dimethoxystilbene, 4,4'-dimethoxystilbene, 4-aminostilbene, and 4,4/-diaminostilbene. It is assumed that the transients are the triplet states of the nitrostilbenes. This is supported by the fact that (1) the transients of the nitrostilbenes are quenched at a diffusion-controlled rate by such triplet quenchers as oxygen, azulene, and ferrocene, (2) the transient from 4-nitro-4'-dimethylaminostilbene is formed by interaction with the triplet state of triphenylene (rate constant for the energy transfer k = 8 X 109 M-1 sec-1, and (3) the lifetime of the transient from 4nitro-4,-methoxystilbene is identical with the lifetime of the expected triplet intermediate derived from the influence of added quenchers (azulene or ferrocene) on the position of the cis-trans photostationary state (cf. subsequent paper). Under our experimental conditions the transients produced from the cis and the trans form of 4-nitro-4'-methoxystilbene and two other nitrostilbenes have identical spectra and lifetimes. The Xmax values of the transients produced in nonpolar liquid solvents show a blue shift in comparison to the values measured in the glassy state in EPA at 77°K. The Xmax values of nitronaphthalene and other nitroaromatics do not show this shift. In agreement with Dainton, et al., this result is explained by assuming that the triplet intermediate may be in a somewhat more twisted form in the liquid than it is in the glass state.

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