Room temperature phosphorescence in longer-wavelength red light region found in benzothiadiazole-based dyes

Abstract

Room temperature phosphorescence in the longer-wavelength red-light region has attracted considerable interest owing to its development in biological imaging and sensing. However, red phosphorescence is highly limited because the phosphorescence color emitted from organic dyes has mostly been restricted to shorter-wavelengths in the blue, green, and yellow regions. Red phosphorescence is observed in an electron-accepting benzothiadiazole dye bearing a methoxy group and bromine atom in the crystal solid state. Although the benzothiadiazole dye has an inherent characteristic of phosphorescence in the monomeric state, the phosphorescence behavior in the crystalline solid state is assisted secondarily by aggregation through π⋯π interactions among benzothiadiazole moieties, C–H⋯O interactions among methoxy groups, and Br⋯Br interactions among bromine atoms. After the intersystem crossing accelerated by the heavy-atom effect of the introduced bromine atom, the given excited triplet state can be stabilized by these multiple intermolecular interactions, resulting in the restriction of the nonradiative channel and acceleration of the phosphorescence radiation. This red phosphorescence can be developed into other dyes bearing key electron-acceptor moieties, bromine atoms, and methoxy groups.