Long-range hydrogen-bond relay catalyses the excited-state proton transfer reaction

Abstract

Solvent (e.g., water)-catalyzed proton transfer (SCPT) via the relay of hydrogen (H)-bonds plays a key role in proton migration. In this study, a new class of 1H-pyrrolo[3,2-g]quinolines (PyrQs) and their derivatives were synthesized, with sufficient separation of the pyrrolic proton donating and pyridinic proton accepting sites to probe excited-state SCPT. There was dual fluorescence for all PyrQs in methanol, i.e., normal (PyrQ) and tautomer 8H-pyrrolo[3,2-g]quinoline (8H-PyrQ) emissions. The fluorescence dynamics unveiled a precursor (PyrQ) and successor (8H-PyrQ) relationship and the correlation of an increasing overall excited-state SCPT rate (k_SCPT) upon increasing the N(8)-site basicity. k_SCPT can be expressed by the coupling reaction k_SCPT = K_eq × k_PT, where k_PT denotes the intrinsic proton tunneling rate in the relay and K_eq denotes the pre-equilibrium between randomly and cyclically H-bonded solvated PyrQs. Molecular dynamics (MD) simulation defined the cyclic PyrQs and analyzed the H-bond and molecular arrangement over time, which showed the cyclic PyrQs incorporating ≧3 methanol molecules. These cyclic H-bonded PyrQs are endowed with a relay-like proton transfer rate, k_PT. MD simulation estimated an upper-limited K_eq value of 0.02-0.03 for all studied PyrQs. When there was little change in K_eq, the distinct k_SCPT values for PyrQs were at different k_PT values, which increased as the N(8) basicity increased, which was induced by the C(3)-substituent. k_SCPT was subject to a deuterium isotope effect, where the k_SCPT of 1.35 × 10¹⁰ s^-1 for PyrQ-D in CH₃OD was 1.68 times slower than that (2.27 × 10¹⁰ s^-1) of PyrQ in CH₃OH. MD simulation provided a similar K_eq for PyrQ and PyrQ-D, leading to different proton tunneling rates (k_PT) between PyrQ and PyrQ-D.