Photochemical reactions of rhodizonate and croconate dianions in aqueous solution.
Creators: name: Zhao, Binjun.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1993
Subjects: subject: Chemistry, Inorganic.

description: The thermal and photochemical reactions of the dianion of rhodizonic acid (1,2-dihydroxycyclohexenetetrone) and the kinetics of the photochemical reactions of the dianion of croconic acid (1,2-dihydroxycyclopentenetrione) have been studied in aqueous solution in the presence and absence of electron acceptors. For rhodizonate dianion, both thermal and photochemical reactions are initiated by electron transfer to an acceptor which is a sufficiently strong oxidizing agent. With hydrogen peroxide and ferricyanide, a square root dependence of the rate of disappearance of rhodizonate dianion on the concentration of additive was observed, whereas with tetracyanoethylene the rate was first order with respect to the concentration of additive. This difference in behavior is explained on the basis of the rate of separation of ions from the initial charge transfer complex. For croconate dianion, the quantum yield for disappearance of croconate dianion was small (10$\sp{-3}$) in neutral solution, but was substantially increased in basic solution and in the presence of electron acceptors. At pH 12 in the presence of 4-nitrobenzylbromide or biacetyl, a quantum yield of 1 was observed. The kinetics of the rate of disappearance of croconate dianion as a function of pH and concentration of acceptor showed that the excited dianion is oxidized by acceptors and reacts with hydroxyl ion. A mechanism is proposed that is shown to be consistent with the results. The main reaction product for rhodizonate dianion is croconate dianion, while for croconate dianion, the main reaction product is squarate dianion. Photopolymerization of acrylamide was initiated by the photochemical reactions of rhodizonate dianion and croconate dianion in the presence of various acceptors. Photoinitiation was most efficient with croconate dianion where a maximum quantum yield for polymerization of acrylamide of $2\times10\sp3$ was observed in neutral solution. Kinetic studies using croconate dianion as initiator showed that the rate of polymerization was a linear function of the concentration of monomer and proportional to the square root of the light absorbed by the croconate dianion. The rate of initiation of radicals calculated from the rate of polymerization was compared with the rate of production of radicals calculated from the steady state photolysis study of croconate dianion. Laser-induced flash photolysis study of rhodizonate and croconate dianions has been performed both in the presence and absence of electron acceptors. The rate constant for the recovery of the ground state from the excited state (k$\sb{\rm D})$ is $2.4\times10\sp5\ {\rm s}\sp{-1}$ for rhodizonate dianion and $2.8\times10\sp5\ {\rm s}\sp{-1}$ for croconate dianion in neutral solution. Absorption spectra of croconate monoanion radical with a maximum at 500 nm were observed in the presence of the additives 4-nitrobenzylbromide, biacetyl and methyl viologen. Rate constants for the diffusion-controlled electron transfer reactions from the excited croconate dianion to the acceptors (k$\sb{\rm T})$ were measured. The values of k$\sb{\rm T}$ were $\rm 2.0\times10\sp8\ M\sp{-1}s\sp{-1}$ for biacetyl, $\rm 2.8\times10\sp9\ M\sp{-1}s\sp{-1}$ for 4-nitrobenzylbromide and $\rm 3.7\times10\sp \ M\sp{-1}s\sp{-1}$ for methyl viologen. The rate constant for quenching of the excited state of croconate dianion by oxygen was $\rm 4.5\times10\sp9\ M\sp{-1}s\sp{-1}.$