Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde

1,8-Dihydroxy-2-naphthaldehyde
(DHNA), having doubly
intramolecular hydrogen bonds, was strategically designed and synthesized
in an aim to probe a long-standing fundamental issue regarding synchronous
versus asynchronous double-proton transfer in the excited state. In
cyclohexane, DHNA shows the lowest lying S0 →S1 (π–π*) absorption at ∼400
nm. Upon excitation, two large Stokes shifted emission bands maximized
at 520 and 650 nm are resolved, which are ascribed to the tautomer
emission resulting from the first and second proton-transfer products,
denoted by TA* and TB*, respectively. The first proton transfer (DHNA* → TA*) is ultrafast (< system response of
150 fs), whereas the second proton transfer is reversible, for which
the rates of forward (TA* → TB*) and backward (TA* ←
TB*) proton transfer were determined to be (1.7 ps)−1 and (3.6 ps)−1, respectively. The fast equilibrium
leads to identical population lifetimes of ∼54 ps for both
TA* and TB* tautomers. Similar excited-state double-proton transfer
takes place for DHNA in a single crystal, resulting in
TA* (560 nm) and TB* (650 nm) dual-tautomer emission. A comprehensive
2D plot of reaction potential energy surface further proves that the
sequential two-step proton motion is along the minimum energetic pathway
firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay
type of intramolecular double-proton transfer reaction in the excited
state, which should gain fundamental understanding of the multiple
proton transfer reactions.