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Photochemistry of Flavothione and Hydroxyflavothiones: Mechanisms and Kinetics ¶
Author(s) -
Maçanita António L.,
Elisei Fausto,
Aloisi Gian Gaetano,
Ortica Fausto,
Bonifácio Vasco,
Dias António,
Leitão Emília,
Caldeira Maria João,
Maycock Christopher D.,
Becker Ralph S.
Publication year - 2003
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2003)0770022pofahm2.0.co2
Subject(s) - chemistry , photochemistry , kinetics , reaction rate constant , excited state , hydrogen atom abstraction , chemical kinetics , reaction mechanism , oxygen , dimer , radical , catalysis , organic chemistry , physics , quantum mechanics , nuclear physics
In this work we present a detailed study of the mechanism of photochemistry and thermal reactions, as well as of the kinetics of flavothione (FLT) in ethanol. Furthermore, we analyzed how the hydroxysubstitution pattern of FLT influenced both the kinetics and the mechanism relative to the parent FLT. We show that the primary photochemical reaction of FLT in the absence of oxygen is hydrogen (H)‐atom abstraction from the solvent by way of the excited triplet state of FLT. Several products result from thermal reactions of the resulting semireduced FLTH · radical, including more than one dimer. A full mechanism is proposed, and the relevant rate constants are evaluated. On the other hand, in the presence of oxygen and a low concentration of FLT, we found that the principal photoproduct is the parent flavone (FL). The reaction leading to photoxidation is not via 1 O 2 attacking a thione, but instead, it is via a reaction of the FLTH · radical with ground state oxygen. The kinetic data also demonstrate that the relative values of concentrations of reactants and the rate constants of the reactions can control the dominance of one mechanism over others. We also have examined the photochemical mechanisms and kinetics for several hydroxyflavothiones ( n ‐OHFLT) and compared them with FLT itself. We have found that the photochemical mechanism radically changes depending on the positions of substitution. These differences are directly related to the ordering of the excited states of the n ‐OHFLT. Specifically, FLT with lowest 3 n,π* states (FLT, 6‐hydroxyflavothione, 7‐hydroxyflavothione and 7,8‐dihydroxyflavothione) efficiently abstract H atoms to give the semireduced radical of the thione. The radical can (1) dimerize to form two different dimers; (2) react with oxygen to produce the parent FL; and (3) recombine with the solvent radical to yield the original FLT. In contrast, FLT with lowest 3 π,π* states (3‐hydroxyflavothione, 3,6‐dihydroxyflavothione and 3,7‐dihydroxyflavothione) behave as photosensitizers of oxygen to form singlet oxygen, which then reacts with the ground state of the substituted FLT. Finally, when T 2 (π,π*) is above S 1 (n,π*), as for 5‐hydroxyflavothione and 5,7‐dihydroxyflavothione, both the S 1 (n,π*) → T 1 (n,π*) intersystem crossing and photodegradation are inefficient.