AN INVESTIGATION OF BINDING-ENERGIES OF TRIPLET EXCIMERS IN CARBAZOLYL-CONTAINING POLYMERS

The temperature-induced interchange between nonexcimeric and excimeric phosphorescence of poly[N-[(vinyloxy)carbonyl]carbazole] (PFCZ) and poly[N-[(allyloxy)carbonyl]carbazole] (PACZ) solid films has been investigated from 20 to 240 K. In addition, polystyrene doped with either N-ethylcarbazole (NEC) or N-(carboethoxy)carbazole has been studied. Activation energies for trapping of triplet excitons at excimer-forming sites in the matrix are all small, between 1.1 and 2.6 kJ/mol, and nearly the same for all of these carbazole species. On the other hand, activation energies for detrapping are much larger for the doped polymer films and for PACZ than for PFCZ or for poly(N-vinylcarbazole) which had been investigated earlier. The rate-determining step in triplet excimer formation apparently involves atom and electron rearrangement associated with bond formation as opposed to the translation of the mobile triplet exciton or internal rotations associated with the polymer chain. It is concluded that steric freedom of the reacting chromophore pair leads to excimers in a relaxed configuration whereas more restricted pairs produce excimers having a higher degree of steric strain.