Tripletspectra of 4‐Nitrostilbenes in Polar and Non‐Polar Solvents at Different Temperatures and the Configuration of the Triplet State

The absorption spectra of triplet states of various substituted nitrostilbenes (e.g. 4‐nitro‐, 4,4′‐dinitro‐,4‐nitro‐4′‐methoxy‐, and 4‐nitro‐4′‐dimethylaminostilbene) were measured in solvents of different polarity at room temperature and in glycerol triacetate at temperatures ranging from +25 to ‐80°C, corresponding to a viscosity range of 10−2 to 1011 poise. The spectra derived from 4‐nitro‐and 4,4′‐dinitrostilbene do not vary with the polarity of the solvent while those of 4‐nitro‐4′‐methoxy‐, and 4‐nitro‐4′‐dimethylaminostilbene are shifted to longer wavelengths on increasing solvent polarity. The triplet spectra of all nitrostilbenes investigated are very similar in the liquid and the glassy state of the solvent. For trans‐4‐nitrostilbenes the yield of triplet intermediate is temperature and viscosity independent. From these effects and the observed strong decrease of the direct trans → cis quantum yield in rigid media it is concluded that the triplet state of the nitrostilbenes has predominantly the planar trans configuration.