Excited-State Proton Transfer of Cyanonaphthols in Protic Ionic Liquids: Appearance of a New Fluorescent Species

Excited-state proton transfer (ESPT) of 5-cyano-2-naphthol (5CN2) and 5,8-dicyano-2-naphthol (DCN2) in three different protic ionic liquids (PILs), triethylammonium trifluoromethanesulfonate ([N₂₂₂H][CF₃SO₃]), triethylammonium methanesulfonate ([N₂₂₂H][CH₃SO₃]), and triethylammonium trifluoroacetate ([N₂₂₂H][CF₃COO]), was studied by time-resolved fluorescence. In [N₂₂₂H][CF₃SO₃], both 5CN2 and DCN2 showed fluorescence only from ROH* (normal form of substituted naphthol in the excited states), indicating that no ESPT occurred in [N₂₂₂H][CF₃SO₃]. For 5CN2 in [N₂₂₂H][CH₃SO₃], fluorescence bands from ROH* and RO^-* (anionic form of substituted naphthol in the excited states) were observed, indicating that 5CN2 could dissociate proton to surrounding solvents and form RO^-*. More interestingly, 5CN2 in [N₂₂₂H][CF₃COO] and DCN2 in [N₂₂₂H][CH₃SO₃] and [N₂₂₂H][CF₃COO] showed an anomalous fluorescence band around 470 nm (5CN2) or around 520 nm (DCN2) which has not been reported previously. The kinetics of each fluorescent component of 5CN2 and DCN2 was analyzed on the basis of the time profile of fluorescence intensity. Plausible ESPT schemes of 5CN2 and DCN2 were discussed on the basis of the kinetics and the basicity of anion in PILs.