Excited state mechanisms in crystalline carbazole: the role of aggregation and isomeric defects

Abstract

The molecule of carbazole (Cz) is commonly used as a building block in organic materials for optoelectronic applications, acting as a light-absorbing, electron donor and emitting moiety. Crystals from Cz and its derivatives display ultralong phosphorescence at room temperature. However, different groups have reported inconsistent quantum efficiencies for the same compounds. In a recent experimental study by Liu et al. (Nat. Mater. 2021, 20, 175–180), the ultralong phosphorescence properties of Cz have been associated with the presence of small fractions of isomeric impurities from commercially available Cz. In this paper, we use state-of-the-art computational approaches to investigate light-induced processes in crystalline and doped Cz. We revisit the role of aggregation and isomeric impurities on the excited state pathways and analyse the mechanisms for exciton, Dexter energy transfer and electron transport based on Marcus and Marcus–Levich–Jortner theories. Our excited state mechanisms provide a plausible interpretation of the experimental results and support the formation of charge-separated states at the defect/Cz molecular interface. These results contribute to a better understanding of the factors that enhance the excited state lifetimes in organic materials and the role of doping with organic molecules.