Sensing pH-dependent fluorescence in a hydrocarbon solvent with 2,6-diphenylpyridine

Fluorescence measurements of 2,6-diphenylpyridine in cyclohexane indicate the formation of a 1:1 hydrogen bonded complex upon addition of trifluoroacetic 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 sensing pH-dependent processes and acid generation in a hydrocarbon solvent, and occurs via an intermolecular hydrogen bond transfer in the excited state since the dielectric constant of the solvent is too low to support the excited-state proton transfer. MOPAC calculations correctly 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 kcal/mol. (C) 1998 Elsevier Science S.A.