Peter J. Kovi

Biprotonic versus intramolecular phototautomerism of salicylic acid and some of its methylated derivatives in the lowest excited singlet state

Abstract Salicylic acid demonstrates intramolecular proton transfer (phototautomerism) in its uncharged and singly charged anionic species in both chloroform and aqueous media. Methyl salicylate on the other hand, exhibits intramolecular phototautomerism in chloroform but in aqueous media phototautomerism appears to be biprotonic, involving diffusion limited protonation of the carboxyl group and dissociation of the hydroxyl group by the solvent. The biprotonic process in methyl salicylate is complete, during the lifetime of the excited state in moderately concentrated acid solutions but is incomplete in dilute acid media.

Photoluminescence of 6- and 7-aminoquinolines

The fluorescences of 6- and 7-aminoquinoline and 6-dimethylaminoquinaldine were studied in media of different acidities, polarities, hydrogen-bonding capabilities and temperatures. The excited state acidities and solvent dependences of the fluorescences of these compounds are typical of aminoquinolines substituted in the homocyclic ring. The acidities of the doubly protonated cations are so great in the lowest excited singlet state, however, that prototropic dissociation in the excited state occurs even in rigid media at 195°K. The lowest triplet state of the singly charged cation of 6-aminoquinoline is found to be nearly degenerate with the lowest excited singlet state of the same molecule. This is attributed to the great charge-transfer component of the S1←S0 transition which is not present in the T1←S0 transition, and the interaction of the S1 state with the polar solvent.

Electronic absorption and fluorescence study of ionization and intramolecular hydrogen-bonding in the α, β-o-hydroxynaphthoic acids

The electronic absorption and fluorescence spectra of 1-hydroxy-2-naphthoic acid and 2-hydroxy-1-naphthoic acid were studied throughout the pH range in water and the Hammett acidity range in sulfurie acid. As in salicylic acid and 3-hydroxy-2-naphthoic acid an intramolecular hydrogen bond between the hydroxyl proton and the carboxyl group is evident from the small pH dependent shifts of the spectra of the neutral molecules, upon dissociation, and the anomalous pKa values. The neutral molecules undergo biprotonic phototautomerizations to the zwitterions, in their lowest excited singlet states, in moderately concentrated sulfuric acid. The anomalous shifts of the fluorescences of the neutral molecules upon dissociation, are attributed to the effects of dissociation and intramolecular hydrogen bonding, upon the 1La and 1Lb states. Mention is made of the possible utility of fluorimetric titrimetry for qualitative analysis at trace levels.

An electronic spectral study of the influence of thermal and electronic processes on the determination of the excited singlet-state dissociation constants of 1- and 2-naphthoic acid

Abstract The electronic absorption and fluorescence spectra of 1-naphthoic acid and 2-naphthoic acid were studied as functions of prototropic form, solvent and temperature. The dissociation constants for the prototropic equilibria between the cations and neutral molecules were determined titrimetrically for ground and excited singlet states with respect to the revised Hammett acidity scale of Jorgenson and Hartter. Calculation of excited-state dissociation constants from the Forster cycle is critically evaluated with respect to thermal relaxation processes in electronically excited and ground states and their effects on the ordering of electronic states. The interpretation of excited-state prototropism in the 2-isomer is straightforward. However, in the 1-isomer there is strong indication of inversion of states which introduces complications into the interpretation of pK*a values obtained from fluorimetric titrations as well as calculations from shifts of electronic spectra.

Electronic Spectra and Electronic Structures of Prototropic Species Derived from the 9-Anthrol-Anthrone System

Abstract Electronic absorption and fluorescence spectroscopies are employed to show that anthrone and its cation are closer to carbonyl derivatives of benzene in electronic structure than to derivatives of anthracene. The anion derived from anthrone is, however, a phenolic anthracene anion by electronic structure and by chemical reactivity. The phenolic 9-anthranol tautomer observed in some nonaqueous media is not measurably present in water at room temperatures in the ground electronic state. However, although neither anthrone or 9-anthrol fluoresce in water, there is evidence from the pH dependence of the fluorescence of the anion, to indicate that at least some of the 9-anthrol tautomer is present in the lowest excited singlet state. Although there has been extensive study of the electronic spectra and prototropic behavior of hydroxyaromatics derived from benzene and naphthalene, relatively little information is available about the hydroxyanthracenes. Baba and Suzuki (1,2) have examined the electronic ...

Acidity Dependences of the Photoluminescences of Benzaldehyde and Acetophenone

Abstract Aldehydes and ketones derived from benzene are generally thought to phosphoresce but not to fluoresce (1). Although fluorescence has been observed from aldehydes and ketones derived from medium and large polycyclic aromatic hydrocarbons in hydroxylic solvents (2), benzaldehyde and aceto-phenone do not fluoresce even in water. However, in concentrated acid media we have observed intense fluorescence from the conjugate acids of benzaldehyde and acetophenone, an observation which has potential analytical significance. Moreover, in concentrated sulfuric acid media, we have observed phosphorescence from the cations derived from benzaldehyde and acetophenone enabling calculation of the dissociation constants of these cations in the lowest triplet state. These observations prompted the examination of the acidity dependences of the electronic spectra of benzaldehyde and acetophenone.

Ionization equilibria and electronic spectroscopy of the chelating quinolinecarboxylic acids

Abstract pH and Hammett acidity dependences of the electronic absorption and fluorescence spectra of quinaldic acid. quinoline-8-carboxylic acid and their methyl esters have been studied. Conversions of the quinoline-like absorption spectra of the anions to quinolinium-like absorption spectra of the uncharged species show that the uncharged ground state species derived from the free acids consist overwhelmingly of the zwitterions. The excited singly charged cation of quinaldic acid appears to undergo an acidity-dependent phototautomerism. However, the 8-isomer does not exhibit this phenomenon. The long emission wavelengths of the esters relative to the zwitterions indicate that of the two electron-withdrawing groups, the carboxyl group is stronger than the heterocyclic nitrogen in the excited state. The fluorescence of quinoline-8-carboxylic acid in sulfuric acid could be very useful for quantitative analysis of transition metal chelates of this ligand.

Intermolecular Hydrogen Bonding and Proton Transfer in the Ground and Lowest Excited Singlet States of the Ethyl Esters of 1- and 2-naphthoic Acid

Abstract The absorption spectra of ethyl 1-naphthoate in hexane and in water demonstrate normal dependences upon solvent hydrogen bonding and polarity properties while the absorption spectra of 1-naphthoic acid do not. This is attributed to inhibition of conjugation by steric interference between the carbethoxy group of the ester or the hydrated carboxyl group of the acid and the peri-hydrogen atom of the naphthalene ring in the 5-position, in the ground electronic state. The anomalous fluorimetric titration characteristics of the 1- and 2- naphthoic esters and of 1-naphthoic acid are attributed to hydrogen bonded exciplex formation in the lowest excited singlet state. In a recent study of the solvent and acidity dependences of the electronic absorption and fluorescence spectra of 1- and 2-naphthoic acid (1) several phenomena were observed which were rather unusual and whose explanations were beyond the scope of the previous study. Among these were 1) the red shift of the absorption spectra upon going fro...