Self-trapping limited exciton diffusion in a monomeric perylene crystal as revealed by femtosecond transient absorption microscopy

Abstract

Self-trapping and singlet–singlet annihilation of the free excitons in a monomeric (β) perylene crystal were studied by using femtosecond transient absorption microscopy. The free exciton generated by the photo-excitation of the β-perylene crystal relaxed to the self-trapped exciton with a rate constant of 7 × 10¹⁰ s⁻¹. The singlet–singlet annihilation of the free exciton observed under the high excitation density conditions was competed with the self-trapping of the free exciton; we estimated the annihilation rate constant for the free exciton to be 1 × 10⁻⁸ cm³ s⁻¹ from the excitation density dependence of the free exciton decay. After self-trapping of the free exciton, no annihilation was observed in the 100 ps time range, suggesting that the diffusion coefficient was reduced drastically by self-trapping. The results show that the major factor limiting the exciton diffusion in the β-perylene crystal is a relaxation of the free exciton to the self-trapped exciton, and not the lifetime of the exciton. Though the singlet–singlet annihilation rate constants and fluorescence lifetime of the β-perylene crystal are similar to those of the anthracene crystal, the estimated exciton diffusion length (2 nm) in the β-perylene crystal is much smaller than that (100 nm) in the anthracene crystal as a result of the exciton self-trapping.