Influence of viscosity and dielectric relaxation on the intramolecular adiabatic proton transfer in the electronically excited free base of quinacrine

Abstract

Picosecond time-resolved fluorescence spectroscopy has been used to probe the excited-state reaction of the free base of quinacrine (QA) in polar and apolar solvents. All solutions reveal bi-exponential time behaviour of the fluorescence over the whole spectral region from 450–610 nm. Based among other things on the observed temperature-independent deuterium kinetic isotope effect we conclude that the observed behaviour arises from an adiabatic excited-state intramolecular proton-transfer (ESIPT) reaction. 2D-NOE NMR spectra indicate an initial conformation of the alkyl chain in which the diethylamino group is above the aromatic ring and the lack of an intramolecular hydrogen bond. There is a large effect of viscosity, η, on the reaction rate. In alkanes this seems to arise mainly from a direct effect of hydrodynamic solvent friction on the dynamics of the alkyl chain motion, which has to bring proton donor and proton acceptor site together within a hydrogen-bonding distance. The dependence of the rate constant k_r for ESIPT on the longitudinal dielectric relaxation time τ_l and on viscosity has been studied in detail for solutions of QA in linear alkanols. A linear dependence of 1/k_r on τ_l with non-zero intercept is found. This does not arise from a causal relation between k_r and dielectric relaxation. The viscosity dependence can be expressed accurately as k_r=k_r0 exp(–γη) with k_r0= 1.08 × 10¹⁰ s^–1 and γ= 0.06 cP^–1. This yields a linear dependence of 1/k_r on τ_l, because η is linearly correlated with τ_l. Relations of the type observed between k_r on the one hand and τ_l or η on the other hand offer the possibility to determine the values of the last two quantities for alkanols via an optical detection method.