Vibrational energy transfer in azodendrimer and azobenzene Original Research Article

Dynamics of intramolecular vibrational energy relaxation in a third-generation aryl ether azodendrimer and an azobenzene after the excitation of stretching modes of aromatic rings is analyzed using the dynamic reaction coordinate method in which the electronic state is described semiempirically on the basis of the PM3 scheme. It is found that the behavior of the vibrational relaxation depends sensitively on the magnitude of the excitation energy and on the species of the excited mode so that the relaxation time ranges over several orders of magnitude from picoseconds to nanoseconds. The vibrational energy transfer begins to take place when a threshold for the excitation energy or the elapsed time is exceeded, and this behavior of relaxation is not temporally homogeneous but is triggered by the resonant energy transfer to some specific modes satisfying the frequency matching condition. With the aid of a phenomenological, nonlinear coupled oscillator model, we also discuss the relation of the present analysis to a unique cis-to-trans isomerization reaction of azodendrimers induced by weak infrared irradiation.