Experimental and Computational Study on Photophysical Properties of Mesoionic Chalcogenones

N‐Heterocyclic carbene adducts with main group elements (NHC=E) have aroused great interest and have been widely investigated in coordination chemistry. Among them, N‐heterocyclic carbene adducts with chalcogens (NHC=Ch) have been known for a long time. Their investigations mostly focused on synthesis, coordination chemistry and electrochemistry. Their photophysical properties still remain unexplored. In this work, the photophysical properties of mesoionic carbene adducts with sulfur and selenium have been investigated both in solution and solid state. These compounds showed blue fluorescence in dichloromethane. While in solid state, orange to red room‐temperature phosphorescence can be observed, and dual emission was found in mesoionic thiones. Furthermore, time‐dependent density functional theory (TD‐DFT) calculations were used to obtain insights into the luminescent mechanism. Mesoionic chalcogenones have been developed to investigate their photophysical properties. Mesoionic thiones 1 a and 1 b emit both green fluorescence and red phosphorescence at room temperature, and mesoionic selones 2 a and 2 b show orange to red room‐temperature phosphorescence (RTP). Notably, 1 a emits white colour under illumination with a 365 nm UV lamp.