Fluorescence Based on Excited-State Intramolecular Proton Transfer of 1-(2-Benzothiazolyl)-2-hydroxytryptanthrin

1-(2-Benzothiazolyl)-2-hydroxytryptanthrin (T2OH1BT) with benzothiazole as a proton acceptor at the 1-position of 2-hydroxytryptanthrin (T2OH) was synthesized and its photophysical properties were investigated. T2OH1BT was found to emit fluorescence at 630-670 nm in polar solvents due to proton dissociation of the hydroxy group, and 575-610 nm in low polar solvents due to excited-state intramolecular proton transfer. T2OH1BT emitted fluorescence in the solid state, thereby differentiating it from common tryptanthrin derivatives which cannot emit fluorescence in the solid state due to their planar structure.