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.