DEUTERIUM-ISOTOPE EFFECTS ON THE FLUORESCENCE OF PHENYLPYRIDINES

A comparison of the fluorescence behavior of 2-, 3- and 4-phenylpyridi nes in H2O and D2O confirms the excited state protonation of these mol ecules in the excited singlet state. In the case of 2-phenylpyridine, the ratio of the fluorescence yield in water to the value measured in D2O is 2.73. Results indicate that two factors control the isotope eff ect on the fluorescence quantum yield: the faster mobility of the prot on relative to the deuterium ion leading to a higher fluorescence yiel d, and an inverse effect associated with the slowing down of the depro tonation step in D2O during intersystem crossing. The inverse effect o bserved with 4-phenylpyridine is attributed to a larger triplet yield and increased basicity relative to the 2- and 3-isomers. The absence o f the excited state proton transfer and isotope effect with 2,6-diphen ylpyridine is the result of inverted n,pi and pi,pi states and a lower pK(a) relative to the monosubstituted phenylpyridines. (C) 1997 Elsev ier Science S.A.