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.
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