A Genuine Intramolecular Proton Relay System Undergoing Excited-State Double Proton Transfer Reaction

7-Hydroxyquinoline-8-carboxylic acid (1), which possesses dual intramolecular hydrogen bonds, undergoes excited-state intramolecular double proton transfer, (ESIDPT) resulting in a quinolinone-like tautomer emission (λmax ∼ 470 nm). ESIDPT of 1 is cooperative, as evidenced by chemically blocking either proton donating site. While the overall rate of ESIDPT is higher than the system response limit, (220 fs)–1, the theoretical approach favors a concerted, asynchronous ESIDPT with a rather small or negligible barrier, demonstrating an intrinsic proton relay system that undergoes intramolecular double proton transfer in the electronic excited state.