Understanding the role of charge transfer states in singlet fission in Perylenediimide Crystals

Singlet fission (SF) has been extensively studied in scientific research as a promising approach to surpass the Shockley–Queisser theoretical efficiency limit. As a promising SF material, Perylenediimide (PDI)-based chromophore exhibit excellent photostability, large extinction coefficient, and high triplet yield. However, the mechanism of SF is rarely understood. Experimental results show different optical behavior in absorption and emission spectra between two phases of PDI crystals due to structure disorder . Theoretical investigations have revealed that charge transfer (CT) states play a vital role in SF process as an intermediate state. Employing the Frenkel-Holstein theoretical framework, a vibronic-exciton model is developed to describe the exciton transfer between singlet excitation states and triplet pair state in aggregated PDI crystals. We find that the symmetry of the CT states quasi-particle wavefunction plays the vital role in the optical signature of SF state. In-phase intermolecular coupling integral leads to J-aggregates, allowing the electronic wavefunction of charge transfer (CT) states to couple with the triplet pair state, while out-of-phase coupling results in H-aggregates, preventing this interaction. Vibronic progression of the absorption lineshape in H-aggregates breaks the symmetry of the wavefunction, thereby activating the contribution from the SF in the absorption spectrum, generating phonon-assisted SF. We finally compare the simulated absorption spectra with experimental measurements, capturing insights into the nature of singlet fission in EP-PDI chromophores.