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.
Department of Chemistry and Biochemistry, National Chung Cheng University , Chia-Yi 62102, Taiwan R.O.C.
Chia-Yu
CY
Peng
Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University , Taipei 10617, Taiwan R.O.C.
Jiun-Yi
JY
Shen
Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University , Taipei 10617, Taiwan R.O.C.
Yi-Ting
YT
Chen
Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University , Taipei 10617, Taiwan R.O.C.
Pei-Jhen
PJ
Wu
Institute of Optoelectronic Sciences, National Taiwan Ocean University , Keelung 20224, Taiwan R.O.C.
Wen-Yi
WY
Hung
Department of Chemistry and Biochemistry, National Chung Cheng University , Chia-Yi 62102, Taiwan R.O.C.
Wei-Ping
WP
Hu
Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University , Taipei 10617, Taiwan R.O.C.
Pi-Tai
PT
Chou