The Fluorescence Properties of Benzoic Acid in Aqueous Acidic Medium

The room temperature fluorescence of the benzoic acid monomer is reported for the first time in aqueous and ethanolic solutions. The appearance of fluorescence, which is dependent upon the lowest 1(π，π*) ，lying energetically below the 3(π，π*)，is found to be affected by PH, buffer, solute concentration, and solvents. For the non fluorescence monomer, in low polar organic solvents and for the benzoate anion in aqueous solvents, the 1(π，π*) configuration is though to lie above the 3(π，π*)，the fluorescence is assigned to the undissociated monomer of benzoic acid, and this represents the first reported fluorescence from the monomeric species of benzoic acid. In addition, the two bands in the benzoic acid fluorescence in aqueous acidic solutions and that from benzoic acid in ethanol are shown to be related。
Until recently, aromatic carbonyl compounds, such as benzoic acid, have been found not to fluoresce. The nonfluorescent nature of these compounds has been ascribed to efficient intersystem crossing, even though in an often overlooked study, in 1910, Ley and Engelhardt did observe fluorescence for benzoic acid in ethanol. In 1961, Elman, Burkhalter, and Ladou found benzoic acid to fluoresce under the extreme condition of a 70% sulfuric acid solution, in 1970, observed fluorescence for a wide range of aromatic ketones, including benzophenone, in carbon tetrachloride solutions at 23℃. In 1971, Rusakowicz, Byers and Leermakers obtained fluorescence for benzoic acid in 85%phosphoric acid. Tournon and El-Bayoumi studied the room temperature emission of several phenyl carboxylic acids in ethanol solutions, where only the monomer form of the acid is present, and found several of these acids to fluoresce, while benzoic acid did not. This lack of emission further sustained the concept of a nonfluorescent monomer in benzoic acid. They also noted that that the benzoic acid dimer fluoresces at room temperature. This present study shows that fluorescence can be obtained from the monomerform of benzoic acid in hydroxylic solvents.