Photoluminescence spectroscopy of dibenzopentacene single-crystals: multiple emissive states across temperature, time, and magnetic field in a pursuit of exothermic singlet fission

Abstract

Dibenzopentacene is a close analogue of pentacene, but it has attracted much less attention than its acene cousin, and its solid-state photophysics remains nearly unknown. In this work we present the first study of dibenzopentacene single crystals using photoluminescence spectroscopy, combining steady-state and time-resolved measurements at different temperatures and in different magnetic fields. A complex evolution of excitation was found between 5 and 300 K, with in total five co-existing and interconverting emissive states. Free excitons dominate the PL spectra below 20 K, but then convert to self-trapped excitons (STEs) at higher temperatures. The interconversion between free exciton and STE states has been found to be thermally activated, and the barriers were determined from the temperature dependence of emission lifetimes. A 20-fold decrease of the PL intensity between 120 K and room temperature is ascribed to the singlet fission, which is confirmed by magneto-PL measurements. However, the large driving force expected for exothermic singlet fission in dibenzopentacene appears to be not fully consistent with its overall photoluminescence behavior, especially at low temperatures. This work highlights that the relationship between photoluminescence and singlet fission for systems with a large energy difference between S₁ and T₁ + T₁, in which the reversibility of SF is hard to expect, is still not well understood.