Photophysics of doubly-charged quinine: Steady state and time-dependent fluorescence

Abstract

The quinine dication in aqueous solution (1 N H 2SO 4) gives two fluorescence lifetimes (τ 1 = 2.80 ns and τ 2 = 19.36 ns) at ambient temperature. τ 2 shows a small increase with an increase in acid concentration between 0.1 N and 15 N. Quenching by Cl − shows that τ 1 and τ 2 are differentially quenched. The Stern—Volmer quenching constant K SV for τ 1 is 10 M −1 and for τ 2 is 75 M −1. In addition, K SV is dependent on emission wavelength. In acidified solution, τ 2 increases with an increase in emission wavelength, whereas τ 1 exhibits a behaviour which resembles a two-state mechanism with a negative amplitude in the region of longer emission wavelength. However, the two-state theory does not give an entirely satisfactory mechanism for the time-dependent emission. Time-resolved emission spectroscopy (TRES) shows a spectral relaxation which partially explains the dependence of τ 2 on emission wavelength in accordance with Bakhshiev formulation. Transient and steady state fluorescence studies from 80 to 290 K show that at 160 K there is a rapid relaxation process resulting in an increase in τ 2 and a sudden spectral shift. We propose that the complex behaviour of quinine decay consists of two major relaxation processes: a charge-transfer process which occurs around 160 K and a solvent reorientation process which occurs in the fluid medium.