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 approximately 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(1) 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(1) surface, which should be important for the quantum yield of photoisomerization.