The phosphorescent triplet states of aza-aromatics and their protonated cations in rigid solutions of ethanol and 1-butyl-3-methylimidazolium hexafluorophosphate

The lowest excited triplet (T1) states of 2,2′-bipyridine (BPY), 6,6′-dimethyl-2,2′-bipyridine (DMBPY), 2,2′-biquinoline (BQ), 2-phenylpyridine (2-PPY), 3-phenylpyridine (3-PPY), 4-phenylpyridine (4-PPY), quinoline (QN) and isoquinoline (iso-QN) have been studied through measurements of the phosphorescence and EPR spectra in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and in ethanol. BPY, DMBPY and BQ have the trans conformation in the ground (G) and T1 states in [BMIM][PF6] and in ethanol. Although only neutral forms of BPY, DMBPY, BQ and 4-PPY were observed in ethanol, their singly protonated cations were observed in [BMIM][PF6] in the G and T1 states. In the T1 states, the solvent effects of [BMIM][PF6] on the electron spin distributions of the aza-aromatics studied and their singly protonated cations are small.