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

Abstract

The kinetics of the decay (splitting) of the excited singlet ${\text{S}}_{1}^{*}$-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 ${\text{S}}_{1}^{*}$-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 $p_{s}^{{{\text{exp}}}}\left( t \right)$, 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 $p_{s}^{{{\text{exp}}}}\left( t \right)$ much more accurately than the FMM. At longer times t $\gtrsim {\kern 1pt}$ 10³ ns, a substantial difference ($\gtrsim {\kern 1pt} 25\%$) between $p_{s}^{{{\text{exp}}}}\left( t \right)$ and the FMM-predicted KDF p_s(t) is found, which is far beyond the experimental error ($\lesssim$3%).