text: Excited state intramolecular proton transfer (ESIPT) is a process in which photoexcited molecules relax their energy through tautomerization by transfer of protons. Some kinds of molecules could have different minimum-energy tautomers in different electronic states, and if the molecular structure of minimum-energy tautomer in the excited state is proton-transferred geometry between neighboring atoms, proton transfer in excited state can occur. The tautomerization often takes the form of keto-enol tautomerism.