Time-resolved fluorescence of intramolecular charge-transfer systems: Experimental results and theoretical predictions

The time-resolved emission spectrum of dual fluorescent pyrrolidino-benzonitrile in glycerol acetate is interpreted by employing a stochastic model. The model system is made up of an internal coordinate describing the intramolecular adiabatic charge-transfer reaction and a polarization coordinate describing the dynamic behaviour of the polar solvent. The time evolution of the system is governed by a coupled Smoluchowski equation plus source and sink terms to account for the excitation from the ground state and subsequent decay. The model reproduces the dynamic Stokes shift of the frequency maximum of the emission band related to the charge-transfer (CT) state towards the red region, and it provides interpretation of the kinetic behaviour manifested by the system after pulse excitation at the short-wavelength absorption band, corresponding to the locally excited (LE) state. The observation of a dynamic Stokes shift accompanying the time evolution of the CT band implies a kinetic pathway which deviates from steepest descent to the CT state, the rate-determining step being the solvent relaxation.