Proton-coupled charge-transfer reactions and photoacidity of N,N-dimethyl-3-arylpropan-1-ammonium chloride salts

Abstract

Photoinduced intermolecular proton-transfer processes from N,N-dimethyl-3-arylpropan-1-ammonium chloride salts (ArCl, with aryl as 1-pyrenyl, 9-anthryl, and 2-naphtyl) to a solvent molecule have been investigated by steady-state and dynamic spectroscopic methods. The intermolecular proton-transfers are coupled either to the formation of an exciplex or to a solvent-separated ion pair in what we have termed a ‘proton-coupled charge-transfer reaction’. A range of solvents has been observed to mediate both the ground-state conformations of the ArCl and the extent of electron transfer. Unlike typical photoacids, in which through-bond interactions control photoacidity, through-space charge-transfer interactions are responsible in the excited singlet states of the ArCl. Transient absorption experiments reveal a range of electronic comportments in the excited-states of the ArCl. Excited-state pK_a values of −3.4, 1.3, and −3.3 in THF were calculated using a Förster-like approach for the 1-pyrenyl, 9-anthryl, and 2-naphthyl salts, respectively. The observed rate of proton-transfer was found to be independent of the thermodynamic driving force and the short-term reversibility of these reactions has been approximated. The data suggest how other systems may be designed to facilitate this novel process.