| dc.description.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 KSV for τ1 is 10 M−1 and for τ2 is 75 M−1. In addition, KSV 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. |
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