The rate constants of the reversible intramolecular excited-state charge-transfer (CT) reaction of 4-(dimethylamino)benzonitrile (DMABN; I) in toluene were determined as a function of temperature, from double-exponential fluorescence decays measured by time-correlated single-photon counting. The same decay times were found for the locally excited (LE) and the CT state, as shown by global analysis. For the CT state, the stabilization enthalpy DELTA-H (-6 kJ mol-1) and the change in entropy DELTA-S (-24 J K-1 M-1) were determined. A correlation of the charge-transfer time of DMABN with the longitudinal dielectric relaxation time was not observed. The presence of dimers of solute/solvent complexes in the ground state was excluded by H-1 NMR experiments. A reversible excited-state reaction was shown to occur in toluene for the planar molecules 1-methyl-5-cyanoindoline (NMCI; II) and 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMCQ; III) as well as for 3,5-dimethyl-4-(dimethylamino)benzonitrile (DMADBN; IV), based on the wavelength dependence of the double-exponential fluorescence decays. Such a dependence was not found for N-methyl-4-aminobenzonitrile (MABN; IH), 4-aminobenzonitrile (ABN; IHH), and 3,5-dimethyl-4-aminobenzonitrile (ADBN; IVHH), which is attributed to an increase in energy of the CT state relative to the LE state in these secondary and primary aromatic amines. The implications of these results with respect to the importance of bond rotation in twisted internal charge transfer (TICT) are discussed.
