Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S1 state.

Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the photoinduced isomerization reactions of a rotation-restricted trans-azobenzene (trans-AB) derivative capped by a crown ether (1), a chemically similar open derivative (2), and unsubstituted trans-AB (3) after excitation to the S1 (npi*) state at lamda=475 nm in dioxane solution. The observed biexponential temporal fluorescence profiles for 1 and 2 were almost indistinguishable within experimental error. The fitted fast fluorescence decay times (+/-2sigma) for the two compounds were tau1 (1) = (0.79 +/- 0.20) and tau1 (2) = (1.05 +/- 0.20) ps, compared to tau1 (3) = (0.37 +/- 0.06) ps. The second decay components could be described with tau2 (1) = (20.3 +/- 9.5) resp. tau2 (2) = (19.0 +/- 6.0) ps, vs. tau2 (3) = (3.26 +/- 0.85) ps. The very similar lifetimes strongly suggest that trans-cis isomerization of 1 and 2 after S1 excitation is governed by the same mechanism. Since 1 cannot isomerize by a simple large-amplitude rotation of one of the phenyl rings about the central NN bond, the isomerization dynamics of both ABs should be better described as "inversion" at the N atom(s) rather than large-amplitude "rotation".