Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole
Spectroscopic studies of the biologically active compound 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT), have been performed. Absorption studies in the UV-Vis region for FABT in polar solvents, like water or ethanol, exhibit the domination of the enol form over its keto counterpart, with a broad absorption band centered around 340 nm. In non-polar solvents such as n-heptane or heavier alkanes the 340 nm absorption band disappears and an increase of the band related to the keto form (approximately 270 nm) is observed. Fluorescence spectra (with 270 nm and 340 nm excitation energies used) show a similar dependence: for FABT in 2-propanol a peak at about 400 nm dominates over that at 330 nm while in n-heptane this relation is reversed. The solvent dependent equilibrium between the keto and enol forms is further confirmed by FTIR and Raman spectroscopies. As can be expected, this equilibrium also shows some temperature dependences. We note that the changes between the two tautomeric forms of FABT are not related to the permanent dipole moment of the solvent but rather to its dipole polarizability.
The keto/enol equilibrium as a function of different solvents and temperature has been the subject of many theoretical and experimental studies [1–4]. Molecules displaying intramolecular hydrogen bonding between proton donor groups such as −OH or −NH and proton acceptor groups such as =N- or C=O are of a growing interest [5, 6]. They are used as chelating agents, polymer UV stabilizers , laser dyes , biological and medical agents [9, 10], and potentially as molecular switches and molecular memories . The keto-enol equilibrium is a characteristic property of such heterocyclic systems  and is known to be sensitive to solvent effects and temperature [3, 4, 13, 14].
For these kinds of molecules with possible intramolecular hydrogen bonding it is accepted that the more polarized keto form dominates in polar solvents while the less polarized enol form dominates in non-polar or slightly polar solvents [3, 15]. In some Schiff bases this effect is different, with the more polarized keto form dominating in non-polar solvents [16, 17]. Such keto/enol equilibrium was observed by means of fluorescence spectroscopy also in 9-anthrole  and similar compounds.
In this study we present experimental work on the 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT), see Fig. 1 . This molecule is able to form an intramolecular hydrogen bond between the −OH and =N-N= groups , see Fig. 1. Electronic absorption and fluorescence, FTIR and Raman spectra were recorded in several solvents, both polar and non-polar, from water and alcohols up to heavier alkanes. The presence of solvents modifies the electronic spectra both in absorption and fluorescence. For non-polar solvents with high polarizability such as n-heptane, the electronic absorption spectra display a clear band at 273 nm, related to the keto form. The band centered at 340 nm, related to the enol form, is too weak to be observed in absorption spectra but is clearly visible in fluorescence spectra.