EXCITED-STATE PROTONATION OF PHENYLPYRIDINES

A study of the excited state behavior of 2-, 3- and 4-phenylpyridines indicates that their fluorescence in water is due to excited state protonation, with a relative efficiency of 0.17:0.62:1.0 respectively, which decreases with the pK(a) value of the phenylpyridinium ion. No fluorescence is observed for the neutral molecule in organic solvents nor in basic medium. 2-Phenylpyridine, which has the smallest torsional energy and ground state pK(a) value, undergoes a sixfold increase in fluorescence as the phenylpyridinium ion in acidic medium relative to that in water. Results are consistent with pK(a)*=9.0+/-0.5 for the phenylpyridinium ion of the three molecules. Fluorescence and phosphorescence data support the view that these molecules become increasingly planar in the triplet state: their fluorescence behavior is similar to that of nitrogen heterocyclic (substituted pyridine), but their phosphorescence behavior is similar to that of substituted benzene, biphenyl. CNDO/S semi-empirical calculations for 3-phenylpyridine satisfactorily predict an inversion of n,pi and pi,pi states upon protonation.