Excimer emission in protonated pyridine systems. 1. Fluorescence spectroscopy of protonated pyridine and its methyl derivatives in rigid glass solution at 77 K

Abstract

The emission properties of pyridine and mono- and dialkylpyridines have been studied in solution in the presence of trifluoroacetic acid at room temperature and 77 K. At room temperature, mono- and dialkylpyridines exhibit a weak and broad fluorescence band with a peak at about 300 nm except for pyridine and 4-n-alkyl- and 2-methylpyridines. This fluorescence originates from a (..pi../sup -/*) state of protonated mono- and dialkylpyridines. However, they exhibit no excimer fluorescence even in a highly concentrated system. At 77 K, in the mixed solvent of tetrahydrofuran, methanol, and methyltetrahydrofuran (4:3:1 by volume) in the presence of trifluoroacetic acid, mono- and dialkylpyridines exhibit a broad and structureless fluorescence band at about 325 nm, in addition to the normal fluorescence band. 4-n-Alkyl- and 2-methylpyridines apparently exhibit only a very weak fluorescence band at about 325 nm, but pyridine is nonfluorescent even at 77 K. It is concluded from the observations of absorption and fluorescence excitation spectra and the fluorescence characteristics that this broad and structureless band is ascribed to a particular excimer (termed dimerlike excimer fluorescence for convenience) which originates from the interaction between protonated monoalkylpyridines (or dialkylpyridines). The analysis of temperature and solvent dependence of fluorescence spectra and the phasemore » transition of the mixed solvent show that the cage of the mixed solvent plays an important role in the dimerlike excimer formation. Further, on the basis of a four-electron ASMO approximation, the dimerlike excimer fluorescence is assigned to result from the in-plane twisted and plane parallel configuration of a compact pair of protonated pyridines.« less