Nanoscale exponential distance dependence and electron-transfer model for intermolecular singlet exciton fission observed in rubrene-doped organic films

Abstract Singlet exciton fission, i.e., the splitting of a singlet exciton into two triplet excitons in molecular materials, is interesting due to its potential applications in organic photovoltaic devices. Traditionally, singlet fission can be explained by using an electron-transfer model in which twice concerted electron-transfer processes lead to the state conversion. In order to test this model, a highly efficient fission material, i.e., rubrene was mixed into other host materials which could be considered as spacers to separate doped rubrene molecules. Energy differences between LUMO levels of rubrene and host molecules constituted tunneling barriers for intermolecular electron-transfer between rubrene. Transient fluorescence spectroscopy was used to investigate the barrier effects in five series – two with varying barrier widths and three with varying barrier heights. The experimental results demonstrated that the transition rate of singlet fission exponentially decreased with the increasing distance between rubrene and the dynamics of fission process was apparently barrier-dependent. These findings could be regarded as clear confirmations of electron-transfer model for intermolecular singlet fission, and are of great importance for clarifying the true mechanism of singlet fission process.