Acidity constants triplet state

Sodium Acetate-Sodium Chloride Mixtures. Ramasamy and Hurtubise (12) obtained RTF and RTF quantum yields, triplet formation efficiency, and phosphorescence lifetime values for the anion of p-aminobenzoic acid adsorbed on sodium acetate and on several sodium acetate-sodium chloride mixtures. Rate constants were calculated for phosphorescence and for radiationless transition from the triplet state. The results showed that several factors were important for maximum RTF from the anion of p-aminobenzoic acid. One of the most important of these was how efficiently the matrix was packed with sodium acetate molecules. A similar conclusion was found for RTF however, the RTF quantum yield increased more dramatically than the RTF quantum yield. 

Triplet state acidities, pKj, have been obtained by Porter and his school from phosphorescence studies, using flash photolysis techniques. The singlet, triplet and ground state acidity constants of some organic molecules are given in Table 4.2. 

Singlet, triplet and ground state acidity constants of some organic molecules... 

The pK values of phenols in singlet and triplet states are valuable guide to substituent effect in the excited states, specially for the aromatic hydrocarbons. In general, the conjugation between substituents and -electron clouds is very significantly enhanced by electronic excitation without change in the direction of conjugative substituent effect. The excited state acidities frequently follow the Hammett equation fairly well if exalted substituent constants a are used. 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

BH+(S ) and B(Si), the protonated and unprotonated base species in the Sx state, will be represented by x and y respectively, and the rate constant suffixes denote transitions to the ground state by solvent and other quenching (q), and by fluorescence (f), or transitions by intersystem crossing to the triplet state (T). Primed constants refer to the base species B(Si) and unprimed to the conjugate acid BH+(S1). In the protonation equilibrium the rate... 

Kg, Kg, K denote, respectively equilibrium dissociation constants in the ground state, the first excited singlet state, and the first triplet state. As usual, the strength of bases in their ground and excited states is expressed by the pK and pK values of their conjugate acids. 

Irradiation of o-nitrobenzaldehyde in benzene gives o-nitrosobenzoic acid ((/) 0.5) in a process that occurs via a triplet state having a lifetime of 0.6 ns, and the transient enol (49). A mechanism is proposed and this is outlined in Scheme 9. The kinetics of the photo-oxidation of benzaldehyde have been studied in the liquid phase and the rate constants extracted found to be in close agreement with those obtained from the photodecomposition of PhC020H. ... 

A laser study of the prototropic equilibrium of triplet benzophenone (BP) has been reported [111]. Acid-base properties in the triplet state of aromatic ketones in HjO-CHjCN (4 1) mixtures have been studied by means of nanosecond laser flash photolysis. The acidity constants p /T) in the triplet state are determined by means of the T T, absorbance titration curve, the Ware plot, and the Rayner-Wyatt plot, whose values agree well among them, showing that the acid-base equilibrium in the... 

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