Fast Redox Perturbation of Aqueous Solution by Photoexcitation of Pyranine

Abstract

Intense illumination (60-120 MW/cm2) of an oxygen-free aqueous solution of pyranine (8-hydroxypyrene-l,3,64risulfonate) by the third harmonic frequency of an Nd-Yag laser (355 nm) drives a two successive-photon oxidative process of the dye. The first photon excites the dye to its first electronic singlet state. The second photon interacts with the excited molecule, ejects an electron to the solution and deactivates the molecule to a ground state of the oxidized dye (a+.). The oxidized product, a+., is an intensely colored compound ( A , , , = 445 nm, E = 43000 2 1000 M-I ern-') that reacts with a variety of electron donors like quinols, ascorbate and ferrous compounds. In the absence of added reductant, a+. is stable, having a lifetime of -10 min. In acidic solutions the solvated electrons generated by the photochemical reaction react preferentially with H+. In alkaline solution the favored electron acceptor is the ground-state pyranine anion and a radical, @-., of the reduced dye is formed. The reduced product is well distinguished from the oxidized one, having its maximal absorption at 510 nm with E = 25000 2 2000 M-l cm-l. The oxidized radical can be reduced either by @-. or by other electron donors. The apparent second-order rate constants of these reactions, which vary from lo6 u p to lo9 M-I s-l, are slower than the rates of diffusion-controlled reactions. Thus the redox reactions are limited by an energy barrier for electron transfer within the encounter complex between the reactants.

Criteria for selecting fluorescent dye tracers for soil hydrological applications using Uranine as an example

“…Dye solutions for which the exponent 2.303c × m × d from Eq. (1) is less than 0.05 can be linearly approximated by (Aeby et al, 2001;Vanderborght et al, 2002): (Kotlyar et al, 1996) 4.8·10 -4 (0.25) ÷ 3.8·10 -3 (2) c = 3.4·10 -5 (0.017 g l -1 ) *1-U: Uranine excited at 491 nm wavelength; 1-U350: Uranine excited at 350 nm wavelength; 2-BS: Brilliant Suflaflavine; 2-SB: Sulforhodamine B; 3-RWT: Rhodamine WT; 4-U: Uranine; 4-P: Pyranine.…”

Redox Photochemistry of Thiouredopyrenetrisulfonate¶

“…These absorption changes have been followed kinetically and characterized. The resulting oxidized radical is a strong oxidant (TUPS + /TUPS:+0.88 V), which eventually regains its electron from the redox partner (16,21).…”

Aplicação de modelos simples em fenômenos envolvendo monocamadas e micelas

“…Com um pKa de aproximadamente 7 no estado fundamental e pKa* < pH local médio < pKa, é possível tratar a cinética experimental como um processo de pseudo-primeira ordem: É importante notar que o transiente de absorbância do ânion PO -não retorna ao nível pré-pulso. Esse fenômeno tem a ver com a foto-oxidação da piranina e ocorre tanto em micelas e vesículas como em água (Kotlyar et al, 1996). Considerando que a reprotonação está numa escala de tempo muito mais rápida, o ajuste foi realizado usando-se uma monoexponencial mais um A escolha da piranina para estudo, além das suas propriedades fotoácidas, está na provável localização do seu ânion (PO -) no centro do pool da micela reversa de AOT (Valeur & Bardez, 1989;Politi et al, 1985;Kondo et al, 1982), facilitando em muito a modelagem teórica, já que as equações de DebyeSmoluchowski e de Poisson-Boltzmann apresentadas em 3.2 foram trabalhadas para o caso de simetria radial esférica.…”