Spin–lattice relaxation mechanism of magnetic field effects on singlet fission in amorphous molecular semiconductors

Singlet fission (SF) in molecular semiconductors is a photophysical process of spontaneous splitting of the excited singlet state into a pair of triplet excitons (TT-pair). This process is usually strongly influenced by spin-selective back geminate TT-annihilation (TTA). Spin selectivity manifests itself in magnetic field effects (MFEs) on both TTA and SF kinetics, the study of which allows us to reveal some specific features of this kinetics. In our work, we analyze the mechanism of MFE generation in TTA and SF processes in amorphous molecular semiconductors. In this mechanism, the MFEs are assumed to be determined by magnetic field dependent spin-lattice relaxation (SLR) in TT-pairs, generated by the zero-field splitting interaction (in T-excitons), fluctuating due to T-exciton hopping over arbitrarily oriented molecules in amorphous semiconductors. The SLR-transitions are described with a semiempirical model, which makes it possible to obtain the SF-kinetic functions in analytical form. The mechanism of SLR-assisted MFEs is found to be very efficient in TTA and SF processes. The obtained results are analyzed in detail and applied to interpret experimentally observed SF-kinetic dependences in various magnetic fields. In particular, it is shown that the proposed model of SLR-generated MFEs enables one to describe the effect of crossing of SF-kinetic functions, corresponding to different magnetic fields.