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

Abstract

The quinine dication in aqueous solution (1 N H₂SO₄) gives two fluorescence lifetimes (Τ₁ = 2.80 ns and Τ₂ = 19.36 ns) at ambient temperature. Τ₂ shows a small increase with an increase in acid concentration between 0.1 N and 15 N. Quenching by Cl^- shows that Τ₁ and Τ₂ are differentially quenched. The Stern-Volmer quenching constant K_SV for Τ₁ is 10 M^-1 and for Τ₂ is 75 M^-1. In addition, K_SV is dependent on emission wavelength. In acidified solution, Τ₂ increases with an increase in emission wavelength, whereas Τ₁ 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 Τ₂ 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 Τ₂ 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.