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

Pant, Debi ; Tripathi, U. C. ; Joshi, G. C. ; Tripathi, H. B. ; Pant , D. D. (1990) Photophysics of doubly-charged quinine: Steady state and time-dependent fluorescence Journal of Photochemistry and Photobiology A: Chemistry, 51 (3). pp. 313-325. ISSN 1010-6030

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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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