Femtosecond Fluorescence and Absorption Dynamics of an Azobenzene with a Strong Push−Pull Substitution

The ultrafast photoisomerization of a push−pull substituted azobenzene (4-nitro-4‘-(dimethylamino)azobenzene, NA) is studied by means of femtosecond fluorescence and absorption spectroscopy. The fluorescence dynamics is biphasic. The initial fluorescence with a narrow and intense spectrum decays in ∼100 fs. This decay is accompanied by the rise of broad red-shifted and much weaker emission. The same time constants recur in the transient absorption spectra which hold additional information on the ground-state dynamics. The ground state recovers in 0.8 ps, demonstrating that only the longer time constant is associated with an internal conversion process. Small spectral changes occurring thereafter (∼5 ps) point to vibrational cooling in the ground state. The results are analyzed in comparison with the behavior of the parent compound azobenzene. Though the push−pull substitution of azobenzene strongly alters the character of its excited states, the photodynamics are surprisingly robust with respect to that substitution.