Evidence for triplet inter-chain polaron pairs in poly(phenylene vinylene) and their transformations

Photoluminescence of films of poly(phenylene vinylene) (PPV), and changes of its intensity under conditions of electron spin-resonance as a function of temperature and light intensity were studied. Three types of resonant signals were found in magnetic resonance spectrum:(i) a narrow (17mT), (ii) a broad (130mT) enhancement signals at g=2, and (iii) the signal at g=4. The results permitted to conclude that Coulomb bound polaron pairs are produced in PPV under photoexcitation. The narrow signal was shown to appear due to a microwave induced resonant transition in triplet polaron pairs. The resonant transitions changed the rate of geminate recombination of the pairs that was revealed to lead to formation of triplet intra-chain excitons. Those excitons annihilated in the second order reactions showing themselves as delayed fluorescence. The annihilation rate was found to be influenced by resonant transitions in triplet exciton pairs as well. The lifetime of triplet intra-chain excitons was estimated from microwave modulation frequency dependence. The results showed that energy level of the lowest polaron pair state situated below that of singlet intra-chain exciton can act as a sink of the excitation energy influencing the quantum yields of the photoluminescence, electroluminescence and photoconductivity.