The fluorescence spectrum of benzo(f)quinoline

Spectral changes in the fluorescence of benzo(f)quinoline due to hydrogen bonding with different proton donors have been studied in cyclohexane at room temperature. The fluorescence is noticeably enhanced in intensity with a shift to longer wavelengths by hydrogen bonding with ethanol and benzyl alcohol, while it is strongly quenched by hydrogen bonding with phenol. Moreover, hydrogen-bonded benzo(f)quinoline with acetic acid or trichloroacetic acid is non-fluorescent, but protonated compounds with these acids emit fluorescence in a region longer in wavelength than that of the fluorescence of free benzo(f)quinoline. The equilibrium constants of hydrogen bond formation calculated from the changes in fluorescence intensity with addition of different proton donors are somewhat larger than those obtained from the changes in the absorption spectrum. The basicity of benzo(f)quinoline in the excited electronic state has been discussed with respect to the hydrogen bonding ability in the state.