Nature of excited-state dependent hydrogen bonds and their critical role in determining the photophysical properties of aromatic thioketones

Abstract

In this work, how the excited-state dependent hydrogen bond (H-bond) interactions control photophysical processes have been uncovered by accurate electronic structure calculations for the five lowest-lying states (S₀, S₁, S₂, T₁, and T₂) of three aromatic thioketones and their isomers. The difference in the H-bond nature between S₂ and S₁ gives rise to ultrafast S₂ → S₁ internal conversion via the two-state conical intersection. Strong S₂ fluorescence observed usually in thiocarbonyl compounds is absent in aromatic thioketones with intramolecular H-bonds. Meanwhile, the relatively weak H-bond interactions in S₁ and T₁ states make the S₁, T₂, and T₁ states degenerate or quasi-degenerate. As a result, the T₂ state acts as a relay and enables both forward S₁ → T₁ and reverse T₁ → S₁ processes to occur efficiently, which provides new insights into the mechanism of thermally activated delayed fluorescence (TADF), and could be used to improve the design principle of purely organic TADF materials.