Electronic excitation energy transfer in poly-N-epoxypropyl-3,6-dibromocarbazole

The mechanisms of electronic excitation energy transfer in films of poly-N-epoxypropyl-3,6-dibromocarbazole doped with pyrene, rubrene, and bis[2-(2'-benzothienyl)-pyridinato-N,C$^{3'}$](acetylacetonate)iridium(III) (Btp$_2$Ir(acac)) have been studied in the range $T$ = 5–295 K. It has been established that the mechanisms of transfer depend on the relative position of the $S_1$ and $T_1$ levels of the base and dopant: if the $S_1$ level of the dopant lies between the $S_1$ and $T_1$ levels of the base and its $T_1$ level is below than that of the base (pyrene molecules), only the triplet-triplet transfer takes place, whereas if the $S_1$ level of the dopant lies below the $T_1$ level of the base (pyrene and Btp$_2$Ir(acac) molecules), the triplet-singlet transfer is observed in addition to the triplet-triplet transfer. It has been assumed that the spin-forbidden triplet-singlet energy transfer is allowed because of the strong spinorbit interaction induced by two heavy bromine atoms in chromophores of the polymer. This transfer proceeds through the long-range dipole-dipole interaction at low temperatures and the electron exchange interaction in the vicinity of room temperature.