Experimental study on the dynamic process and charge-transfer model of intermolecular singlet exciton fission

The charge-transfer model is an important theoretical mechanism to explain the intermolecular singlet exciton fission process. However, the rationality of this model is still on debate, and there is also a lack of research on the electron-transfer process involved in this model. In this work, highly efficient fission molecule, i.e. rubrene, was selected to be the research object. By using co-deposition technique, we fabricated four series of rubrene-doped films in which rubrene molecules were uniformly mixed with other inert organic molecules, forming amorphous composite solid. For each sample, steady-state photoluminescence spectra and their time-resolved fluorescence decay curves were measured at room temperature. Theoretically, based on a traditional three-state reaction model of "S-1+S-0 <->(1)(TT)(i)<-> T-1+T-1", all measured fluorescence decay curves could be well fitted by using a set of coupled rate equations. Then the important rate constants involved in singlet exciton fission process were obtained by curve-fitting. By using the molar volume and molar mass of all materials, the averaged intermolecular distances between doped Rubrene molecules were calculated. It was found that the electron-transfer process between adjacent Rubrene molecules showed the character of quantum tunneling. And the obtained rate constants for state conversion of "S-1+S-0 ->(1)(TT)(i)" exhibited a clear exponential attenuation with increasing intermolecular distance. These results were in line with the charge-transfer model and could be regarded as experimental confirmation. We believe that all the results present in this work are of significance for clarifying the microscopic picture and physical mechanism of exciton fission.