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

• Published in: Chemical science (Cambridge) 2023-07, Vol.14 (26), p.7237-7247
• Main Authors: Chang, Kai-Hsin, Peng, Yu-Chiang, Su, Kuan-Hsuan, Lin, Yi-Hsien, Liu, Jiun-Chi, Liu, Ying-Hsuan, Hsu, Chao-Hsien, Yang, Hsiao-Ching, Chou, Pi-Tai
• Format: Article
• Language: English
• Subjects: Basicity; Chemical bonds; Chemical reactions; Chemistry; Coupling (molecular); Deuterium; Excitation; Fluorescence; Hydrogen bonds; Isotope effect; Methanol; Molecular dynamics; Organic compounds; Protons; Quinoline; Relay; Simulation
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d3sc01441j

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 1 H -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 8 H -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 × 10 10 s −1 for PyrQ-D in CH 3 OD was 1.68 times slower than that (2.27 × 10 10 s −1 ) of PyrQ in CH 3 OH. 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 . The excited-state solvent-catalysed proton transfer of PyrQs requires a relay of 3 methanol molecules, where the N(8) proton-accepting site is the rate-determining step for the intrinsic proton tunnelling k pt .