THEFLUORESCENCE OF ~-ANTHROIC ACIDAND ITSESTERS
2005
The Fluorescence of 9-Anthroic Acid and Its Esters. Environmental Effects on Excited-State Behavior1. by T. C. Wernerlb and David M. Hercules Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received November R5. 1968)
Previous workers have postulated excimer formation to account for the structureless fluorescence of 9-anthroic acid in acidic media and in aprotic solvents. The present work shows this interpretation to be incorrect. The solvent and pH dependence of the 9-anthroic acid fluorescence can be explained on the basis of an acid-base equilibrium. For the molecular form of 9-anthroic acid structureless fluorescence, similar to that of the esters of 9-anthroic acid, is observed. The large Stokes shift of the emission is a consequence of an excited-state rotation of the carboxyl group into the plane of the anthracene ring. This rotation, which can result in excitedstate six-membered ring formation through intramolecular hydrogen bonding, is shown to be dependent on temperature, solvent matrix, and size of the ester group. For the ionic form of 9-anthroic acid in protonic solvents rotation is inhibited owing to strong ground-state solvation and a structured anthracene-likefluorescence is observed. Fluorescence quantum yields and lifetimes measured in benzonitrile and ethanol are consistent with the similarity of the excited-state geometry of the acid and esters. Introduction The fluorescence spectrum of 9-anthroic acid (9-COOH) in ethanol is strongly concentration de~ e n d e n t . ~ The J emission a t -lou6 M has anthracenelike structure, while a t -lo+ M the emission changes to a broad, structureless band showing an unusually M large Stokes shift. Acidification of an solution shifts the emission to the structureless form and M solution shifts the emission alkalization of an to the structured form. In nonpolar solvents only the structureless spectrum is observed. Cherkasov and Bazilevskaya have postulated that the structured emission originates from both the ionic and molecular forms of the monomeric acid, while the broad emission originates from an excimera2J For diffusion-controlled excimer formation a t lO+M a rate constant of -lOl3 M-l sec-’ would be required, a value lo3 larger than normally observed rate constants for the solvents studied. To account for this discrepancy the formation of a ground-state dimer was proposed. Thus excimer formation would require only rearrangement of the ground-state dimer and would no longer be limited by diffusion. The fluorescence of an M solution of 0-COOH in ethanol a t 77°K approaches but does not coincide with the structured emission observed a t room temperature. This emission was interpreted as arising from the linear bound dimer which could not rearrange to form an excimer at 77°K.2p3 Some serious discrepancies are evident in the above interpretations. First, an anomalously large acid dimerization constant would be necessary for significant ground-state dimer formation a t concentrations
