Excimer Emission in Protonated Pyridine Systems. 2. Excimer Emission of Protonated Dipyridylalkanes in Solution at Room Temperature and 77 K

Article abstract of DOI:10.1021/j150666a006

Emission properties of (2-pyridyl)-(CH2)n-(2-pyridyl) (n = 2, 3, 5, and 7) and (4-pyridyl)-(CH2)n-(4-pyridyl) (n = 2, 3, and 5) were studied in solution in the presence of trifluoroacetic acid at room temperature and 77 K.At room temperature, only 1,3-di(2-pyridyl)propane exhibits a very weak fluorescence at approximately 6900 cm^-1 to the red of the normal fluorescence.This band is ascribed to an intramolecular excimer fluorescence between the protonated pyridine moieties.However, 4,4'-dipyridylalkanes do not fluoresce.At 77 K, being specific to a mixed solvent of tetrahydrofuran, methanol, and methyltetrahydrofuran (4:3:1 by volume), the protonated 2,2'-dipyridylalkanes exhibit a structureless band around around 325 nm besides the normal fluorescence band, but they exhibit no excimer fluorescence.On the other hand, the protonated 4,4'-dipyridylalkanes apparently exhibit only a structureless band around 325 nm.The 325-nm fluorescence band comes from a dimerlike excimer in which the protonated pyridine moieties interact intermolecularly in the excited state.The efficiency of the dimerlike excimer formation is independent of the length of methylene chain.From studies on the effects of solvent and temperature on the dimerlike excimer emission, it is found that the formation of the dimerlike excimer is strongly related to a solvent cage effect.