Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S-1 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 S-1 (n pi*) state at lambda = 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 (+/- 2 sigma) for the two compounds were tau(1) (1) = (0.79 +/- 0.20) and tau(1) (2) = (1.05 +/- 0.20) ps, compared to tau(1) (3) = (0.37 +/- 0.06) ps. The second decay components could be described with tau(2) (1) (20.3 +/- 9.5) resp. tau(2) (2) = (19.0 +/- 6.0) ps, vs. tau(2) (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''.