Kinetics of the Decay of Excited Singlet State into a Pair of T-Excitons in Rubrene Films: Mechanism and Manifestation of Exciton Migration

The kinetics of the decay (splitting) of the excited singlet S_1^* -state of rubrene molecules into a pair of triplet-excitons (T-excitons) in rubrene films, usually represented in terms of the kinetics p s ( t ) of the decay of fluorescence (KDF) from the S_1^* -state, is analyzed in detail. The KDF is known to be significantly controlled by the process of diffusive migration and annihilation of the generated T-excitons. In the analysis, two migration models are considered: the two-state model (TSM), treating the migration effect as a result of transitions between the [TT] state of coupled T-excitons (at small TT-distances r ) and the [T+T]-state of freely migrating T-excitons (at large distances r ), as well as the free migration model (FMM), neglecting the effect of the [TT] state. Within the TSM and FMM, the expressions for p s ( t ) are derived, which are applied to describe the KDF p_s^exp( t ) , measured in amorphous rubrene films. Within the experimentally investigated range of times, 0.4–200 ns, the TSM is shown to reproduce the behavior of the experimental KDF p_s^exp( t ) much more accurately than the FMM. At longer times t ≳ 1pt 10 3 ns, a substantial difference ( ≳ 1pt 25% ) between p_s^exp( t ) and the FMM-predicted KDF p s ( t ) is found, which is far beyond the experimental error ( ≲ 3