Laser Flash Photolysis of Carbazole in Solution: Cation Radical as a Source of Carbazolyl Radical

In the course of 266 nm nanosecond laser flash photolysis of carbazole (CBL) in acetonitrile, we discovered a new transient absorption band centered at 360 nm that has been heretofore unreported despite numerous reports on similar topics. To put some limits on possible transients responsible for this absorption band and thus to solve the mechanism of CBL photolysis, we employed the strategy of selectively blocking the CBL active sites by various modifications in the structure. This strategy was supported by the use of the solvent effect and triplet quenching by molecular oxygen. As a result, the mechanism of carbazole photolysis has been elucidated, part of which was our new discovery that the carbazolyl radical can be formed by the deprotonation of the cation radical. The proposed mechanism has been supported by the reaction with TEMPO, theoretical calculations, and also LC-MS/UV analysis of the stable photoproducts. Given the high impact of CBL-based compounds as one of the key compounds in material science (e.g., OLEDs, TADF, and other light-emitting materials), the understanding of the observed radical-driven processes that occur in the photolysis of carbazole seems to be of great interest.