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 (np*) state at l ¼ 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 (AE2s) for the two compounds were t 1 (1) ¼ (0.79 AE 0.20) and t 1 (2) ¼ (1.05 AE 0.20) ps, compared to t 1 (3) ¼ (0.37 AE 0.06) ps. The second decay components could be described with t 2 (1) ¼ (20.3 AE 9.5) resp. t 2 (2) ¼ (19.0 AE 6.0) ps, vs. t 2 (3) ¼ (3.26 AE 0.85) ps. The very similar lifetimes strongly suggest that trans-cis isomerization of 1 and 2 after S 1 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''.