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

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 x 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 x 10(10) s(-1) for PyrQ-D in CH3OD was 1.68 times slower than that (2.27 x 10(10) 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.

Automated summary

Solvent-catalyzed proton transfer via the relay of hydrogen bonds was studied using a new class of 1H-pyrrolo[3,2-g]quinolines and their derivatives. The fluorescence dynamics showed a relationship between precursor and successor molecules and an increasing overall excited-state proton transfer rate with increasing basicity. Molecular dynamics simulation revealed cyclic H-bonded PyrQs incorporating methanol molecules, and different proton tunneling rates were observed for PyrQ and PyrQ-D.