Luminescence and hydrogen bonding in quinoline and isoquinoline

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Abstract
A purification scheme has been developed which is capable of separating the two isomers quinoline and isoquinoline. The emission spectra of the purified compounds were measured at 77 K in a 3-methylpentane (3MP) glass and in a 3MP glass with small amounts of hydrogen bonder added. The hydrogen bonders used were hexafluoroisopropanol (HFIP), trifluoroethanol (TFE), and absolute ethanol (EtOH). An upper limit on the ratio of fluorescence to phosphorescence quantum yields has been set at 20 ppm for quinoline in 3MP, which is 1–2 orders of magnitude smaller than previously reported. Isoquinoline still fluoresces in 3MP, which we believe to be caused by a residual hydrogen bonder not removed by the purification scheme. The excitation and lifetime data obtained suggest that the phosphorescence in these compounds is originating in a nonhydrogen-bonded species, while the fluorescence is originating in a hydrogen-bonded species, regardless of solvent. A new emission has also been detected in the presence of small amounts of hydrogen bonder. This violet fluorescence is believed to be the result of an excited state protonation of the heterocycle. The ground state is believed to be nonprotonated, but strongly or ’’closely’’ hydrogen bonded. A summary of impurity induced absorption and luminescence data obtained by other authors is included to help clarify the spectral properties of quinoline and isoquinoline.