Ground-state proton transport along a blended-alcohol chain: Accelerated by accumulated proton-donating ability

The ground-state reverse proton transfer (GSRPT) of 7-hydroxyquinoline (7HQ) along a hydrogen (H)-bonded mixed-alcohol chain made of different two alcohol molecules having dissimilar proton-donating abilities, designed as a biomimetic system of a proton wire composed of various amino acids, has been investigated in nonpolar aprotic media of n-alkanes using time-resolved transient-absorption spectroscopy with variation of alcohol combinations and medium viscosities. Solvent-inventory experiments have been carried out by varying the composition of alcohols systematically in the heterogeneous H-bonded alcohol chain to understand the molecular dynamics and the elementary mechanisms of GSRPT. Similarly to excited-state proton transfer, GSRPT takes place concertedly without accumulating any reaction intermediate but asymmetrically via a rate-determining tunneling process, and GSRPT is accelerated by the accumulated proton-donating abilities of two alcohol molecules participating in the H-bond chain by push-ahead effect. However, in the ground state, the reorganization of the H-bond bridge in a cyclic 7HQ·(alcohol)₂ complex to form an optimal precursor configuration for efficient proton tunneling takes place prior to intrinsic proton transfer, and the rate constant of GSRPT is governed mainly by configurational optimization. Consequently, the large contribution of the configurational optimization to GSRPT leads to the weaker push-ahead effect and the less-asymmetric character of GSRPT than the respective ones of excited-state proton transfer whose rate constant is determined mostly by tunneling.