The dynamics of excited state structural relaxation of 4-dimethylaminobenzonitrile (DMABN) and related compounds

The excited state structural relaxation of 4 -dimethylaminobenzenes with various para-acceptor substituents having double-band emission, local excited (LE) and charge transfer (CT), has been investigated. Fluorescence measurements at different temperatures and in different solvents have confirmed the existence of viscosity-dependent, temperature, and polarity-activated relaxation. The kinetics analysis has shown that the radiative deactivation rate constants of the individual LE and CT states differ by 7 – 112 -fold. The dipole moment changes at the excitation for CT states are significantly larger than those for LE states. The spectral-kinetics behavior of compounds studied agrees with the models A→ A → B or A→ A B , where A is the local excited planar and B is the relaxed twisted state of the molecule. The rate constants of the twisted state formation have been calculated in the temperature range 293 – 77 K. The activation energies of forward process for compounds studied have been estimated.