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

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 (S 0 , S 1 , S 2 , T 1 , and T 2 ) of three aromatic thioketones and their isomers. The difference in the H-bond nature between S 2 and S 1 gives rise to ultrafast S 2 → S 1 internal conversion via the two-state conical intersection. Strong S 2 fluorescence observed usually in thiocarbonyl compounds is absent in aromatic thioketones with intramolecular H-bonds. Meanwhile, the relatively weak H-bond interactions in S 1 and T 1 states make the S 1 , T 2 , and T 1 states degenerate or quasi-degenerate. As a result, the T 2 state acts as a relay and enables both forward S 1 → T 1 and reverse T 1 → S 1 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.