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) × 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.