BINDING-ENERGIES OF TRIPLET EXCIMERS IN POLY(N-VINYLCARBAZOLE) SOLID FILMS FROM LASER-BASED KINETIC SPECTROSCOPY BETWEEN 15-K AND 55-K

Time-resolved delayed luminescence spectra from solid films of poly(N-vinylcarbazole) have been recorded in the temperature interval between 15 and 55 K. Below 40 K the phosphorescence spectra are primarily nonexcimeric in character but become totally excimeric at 55 K. Rate constants for the phosphorescence decay in this same temperature interval yield linear Arrhenius plots but with a discontinuous change in slope at 40 K. The activation energy above 40 K is 2.0 kJ/mol and is associated with the trapping of nonexcimeric triplets. When combined with previous determinations of the activation energies for detrapping of triplet excimers, binding energies of 2.5 and 12.1 kJ/mol are found for the shallow and deep excimers relative to the energy of the limited free rotor, the energy of which corresponds to the top of the activation barrier. The molecular configuration corresponding to these triplet excimer states is dissociative on the ground-state potential energy surface. The dissociation energies are 30.2 kJ/mol for the shallow trap and 35.4 kJ/mol for the deep trap. These energies are larger than the corresponding ones for singlet excimers in agreement with earlier predictions. © 1990 American Chemical Society.