Triplet exciton migration in rigid solution of poly(N-vinylcarbazole) and N-vinylcarbazole–fumaronitrile alternating copolymer

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Abstract
The migration of triplet excitation energy in poly(N‐vinylcarbazole) (PVCz) has been investigated in rigid solution at 77°K. The behavior of triplet energy migration in PVCz is compared with that in N‐vinylcarbazole–fumaronitrile (VCz–FN) 1:1 alternating copolymer, in which carbazole chromophores are regularly separated by a main chain segment of five carbon atoms, and in 0.5 mol/1 N‐ethylcarbazole, a model compound of PVCz. In every case, the delayed emission spectra are composed of phosphorescence and delayed fluorescence, the latter resulting from the triplet–triplet annihilation between two long‐lived mobile triplets. When doped with naphthalene, remarkable difference of the behaviors of triplet energy migration was observed between PVCz and VCz–FN copolymer, i.e., in the emission decay time and in the manner of phosphorescence quenching with naphthalene. In PVCz the phosphorescence lifetime remains 7.6 sec despite the effective intensity quenching of both phosphorescence and delayed fluorescence; thus, the expected relation that the decay time of the delayed fluorescence should be half the phosphorescence time, as is often found in aromatic hydrocarbon crystals, was not found. However, in the case of the VCz–FN alternating copolymer, both intensity and lifetime of phosphorescence are quenched by naphthalene. Such differences in the behaviors of phosphorescence emissions are discussed in terms of the difference of the rate of migration along the polymer chain and the rate of phosphorescence between these polymers. From the quenching experiments with naphthalene, the migration coefficients of triplet energy have been estimated to be 3.2×10−12 for PVCz and 4.9×10−13 cm2 sec−1 for VCz–FN copolymer, respectively. Also, in solid film the triplet energy migration has been demonstrated by the observation of delayed fluorescence. Triplet energy migrates more effectively in solid film than in rigid solution.