Femtosecond fluorescence dynamics of rotation-restricted azobenzenophanes:: new evidence on the mechanism of trans → cis photoisomerization of azobenzene

The ultrafast relaxation dynamics of two rotation-restricted (azobenzeno-2S-phane and azobenzeno-4S-phane) and one rotation-free (4,4'-dimethylazobenzene) azobenzene derivatives were investigated using femtosecond fluorescence up-conversion on both S-1(n,pi*) and S-2(pi,pi*) excitations. On S-2 excitation, pulse-limited kinetics with a decay coefficient of similar to 100 fs corresponding to ultrafast S-2 -> S-1 relaxation is found to be common for all molecules under investigation regardless of the molecular structure. This indicates that a direct rotational. relaxation on the S-2 surface is unfavorable. On S-1 excitation, we observed biphasic fluorescence decay with a femtosecond component attributed to the decay of the Franck-Condon state prepared by excitation and a picosecond component attributed to the deactivation of the relaxed molecule on the S, surface. This picosecond component is slowed by at least a factor of 2 for the rotation-restricted 2S-bridged molecule compared to that of the rotation-free molecule; for the even stronger rotation-restricted azobenzeno-4S-phane, the decrease is by a factor of 10. These differences in deactivation suggest that the relaxed states and probably the trajectories for rotation-free and rotation-restricted molecules are different on the S, surface, which should be important for the quantum yield of photoisomerization.