The photochemistry of cis-ortho-, meta-, and para-aminostilbenes

The photochemical behavior of the three isomers of cis-aminostilbene is reported here and compared to that of cis-stilbene as well as the corresponding trans-aminostilbenes. The absorption spectra of cis-, ortho-, and meta-aminostilbene are characterized by multiple low oscillator strength bands as a result of symmetry-induced configuration interaction. Fluorescence is observed for cis-meta-aminostilbene but not for the other two isomers, even at 77 K. The fluorescence spectrum of cis-meta-aminostilbene at 77 K is broad and red shifted relative to that of the trans isomer with a large Stokes shift, and its singlet lifetime in methyltetrahydrofuran at 77 K is 17.4 ns. The photocyclization quantum yield for cis-meta-aminostilbene in cyclohexane solution is the largest measured for a monosubstituted cis-stilbene and the photoisomerization quantum yield is unusually low. In contrast, the photochemical behavior of the ortho and para isomers is similar to that of most substituted cis-stilbenes and is dominated by photoisomerization. The excited-state potential energy surface for cis-meta-aminostilbene is proposed to be perturbed, as in the case of trans-meta-aminostilbene, resulting in striking differences in the photochemical behavior of the three positional isomers. The photochemical behavior of the ortho and para isomers is dominated at low temperatures by a nonradiative channel that has been observed for other substituted cis-stilbenes.