Photodissociation rate, protolytic

The emission spectra of 10-CPT in water-methanol mixtures exhibits dual fluorescence (Fig. 1 left). The appearance of the low energy emission band at 570 nm for 10-CPT in a neat methanol solution indicates an efficient PTTS process. The large fluorescence quantum yield and similarity of the emission at neutral and basic solutions is evidence of the excited anion (RO ) formation, in contrast to 6HQ, for which double PTTS leads to the tautomer [2], With the increase of water content in the mixtures, we observed a substantial decrease in the fluorescence intensity of the nondissociated form of 10-CPT at 430 nm and a concomitant increase of RO " intensity at 570 nm. This is a well-known effect in hydroxyaromatic compounds [4] and is attributed to the increase of the protolytic photodissociation rate with increasing water concentration. 

A decrease in the protolytic photodissociation rate of DCN2 with increasing pressure is also observed in supercritical C02/methanol mixtures with constant methanol molarity and molality [45]. This effect is currently under investigation. 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

For protolytic photodissociation of hydroxyaromatic compounds we may expect that in anionic micelles the excited anions of hydroxyaromatic compounds having the same sign of charge will leave the micelles, and in the cationic micelles - hydrogen ions will leave micelles. Exit rate constants of excited anions of hydroxyaromatic compound from the micelles were determined using the non-solubilized fluorescence quenchers. Simultaneously, we have proved that it is in the micellar phase that protolytic dissociation does proceed and not as a result of the preliminary exit of excited molecules of hydroxyaromatic compounds from the micellar phase to an aqueous one. 

A comparison of the rise time of naphtholate anion fluorescence in water and in vesicles shows that the rate constants of protolytic photodissociation in the membrane and in water have the same order of magnitude. 

Now we shall analyze the regularities in rate constants of protolytic photodissociation of hydroxyaromatic compounds in the micellar phase and in the membranes of vesicles ( Tables 1 and 5 ). For one and the same compounds in cationic micelles the acidity constants are usually greater than in the aqueous solution ( pA decreases ), and dissociation rate... 

The dependence of rate constants of protolytic photodissociation and backward protonation reaction of excited anions on the pK value in aqueous solutions and micelles... 

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