SENSING PH-DEPENDENT FLUORESCENCE IN A HYDROCARBON SOLVENT WITH 2,6-DIPHENYLPYRIDINE

Fluorescence measurements of 2,6-diphenylpyridine in cyclohexane indic ate the formation of a 1:1 hydrogen bonded complex upon addition of tr ifluoroacetic acid, with a formation constant of 1.23 +/- 0.07 x 10(3) M-1, which increases the molecular fluorescence quantum yield from 0. 014 to 0.61 for the complex. This behavior is potentially useful for s ensing pH-dependent processes and acid generation in a hydrocarbon sol vent, and occurs via an intermolecular hydrogen bond transfer in the e xcited state since the dielectric constant of the solvent is too low t o support the excited-state proton transfer. MOPAC calculations correc tly predict the formation of the 1:1 complex with an intermolecular N- H bond distance of 2.51 Angstrom, and a stabilization energy of 3.6 kc al/mol. (C) 1998 Elsevier Science S.A.