PHOTOINDUCED PROTON TRANSFERS IN 3,5-DI-TERT-BUTYLSALICYLIC ACID

The origin of the long wavelength fluorescence emission (peak-to-peak Stokes shifts similar to 9050-9450 cm(-1)) of 3,5-di-tert-butylsalicyl ic acid (t-BSA) has been investigated in a variety of organic solvents . Two types of emissions, I and II, can be identified. The type I emis sion is observed in hydrocarbon and nonpolar solvents. It is a relativ ely weak fluorescence (phi(f) less than or equal to 0.01), and the lam bda(F) lies at similar to 466 nm. The type II emission is relatively s trong, and phi(f) values ranging from 0.2 to 0.6 are obtained. It is o bserved in alcohols, acetate solvents, and acetonitrile. The lambda(F) is at similar to 430 nm. Mixed solvent experiments suggest that solve nt molecules H-bond with t-BSA to form cluster complexes in these solv ents and that these cluster complexes emit to give rise to the type II emission. While the fluorescence intensities of both types of emissio ns are quenched by acetic acid, the introduction of a trace amount of triethylamine is shown to enhance the type I emission (in cyclohexane) but not the type II emission (in ethanol). This observation, along wi th the similar fluorescence spectra between t-BSA and LitBSA (lithium 3,5-di-tert-butylsalicylate), suggests that the long wavelength emissi on of t-B SA originates either from the excited 3,5-di-tert-butylsalic ylate anion or a species derived from it. The lifetime for t-BSA is si gnificantly shorter than that of LitBSA in cyclohexane (0.7 versus 3-7 ns), and the short lifetime is attributable to the fluorescence quenc hing of the excited anion by reprotonation. Studies of the fluorescenc e of salicylic acid, methyl salicylate, 2-methoxybenzoic acid, and met hyl 2-methoxybenzoate indicate that (1) the two tert-butyl groups in t -BSA have no effect on the fluorescence and (2) salicylic acid and met hyl salicylate also exhibits similar long wavelength emissions. It is concluded from the structural effect study that an intramolecular prot on transfer, from the OH group to the carboxyl function, is the prereq uisite for the long wavelength emission. Since the long wavelength emi ssion of methyl salicylate is shown to be from an excited enol tautome r, formed by a keto --> enol tautomerization in the excited state, we suggest that photoinduced tautomerism occurs in t-BSA too. t-BA underg oes a very rapid deprotonation reaction upon photoexcitation. The exci ted 3,5-di-tert-butylsalicylate anion tautomerizes from the keto form to the enol form and then fluoresces to give the long wavelength emiss ion. The similarity in photophysical behavior between t-BSA and 2-hydr oxy-3-naphthoic acid is also discussed based on available literature d ata.