TIME-RESOLVED FLUORESCENCE OF INTRAMOLECULAR CHARGE-TRANSFER SYSTEMS - EXPERIMENTAL RESULTS AND THEORETICAL PREDICTIONS

The time-resolved emission spectrum of dual fluorescent pyrrolidino-be nzonitrile in glycerol acetate is interpreted by employing a stochasti c model. The model system is made up of an internal coordinate describ ing the intramolecular adiabatic charge-transfer reaction and a polari zation coordinate describing the dynamic behaviour of the polar solven t. The time evolution of the system is governed by a coupled Smoluchow ski equation plus source and sink terms to account for the excitation from the ground state and subsequent decay. The model reproduces the d ynamic Stokes shift of the frequency maximum of the emission band rela ted to the charge-transfer (CT) state towards the red region, and it p rovides interpretation of the kinetic behaviour manifested by the syst em after pulse excitation at the short-wavelength absorption band, cor responding to the locally excited (LE) state. The observation of a dyn amic Stokes shift accompanying the time evolution of the CT band impli es a kinetic pathway which deviates from steepest descent to the CT st ate, the rate-determining step being the solvent relaxation.