PHOTOPHYSICS AND PHOTOCHEMISTRY OF AROMATIC NITROGEN-HETEROCYCLES - FLUORESCENCE FROM 2,2'-BIPYRIDINE AND 1,10-PHENANTHROLINE

The absorption and emission spectra of 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) have been measured in fluid solution at room temperature; the absorption spectra of the free base, monoprotonated, and diprotonated forms can be easily resolved in aqueous and nonaqueous solution. In neutral aqueous solution, bpy exhibits an additional absorption band (λmax 308 nm) which is removed by base and is attributed to a covalent hydrate species which is capable of being deprotonated. The free base form of bpy is nonfluorescent in nonaqueous solution; emission (λmax 328 nm) in neutral aqueous solution is attributed to fluorescence from the covalent hydrate species inasmuch as the emission is excited at 308 nm and is removed by base. Emission from protonated covalent hydrate species (λmax 425 nm) and monoprotonated bpy (λmax 335 nm) can be resolved; no fluorescence is detected from diprotonated bpy, apparently due to rapid excited state deprotonation. Emission is observed from free base (λmax 360 nm) and monoprotonated (λmax 410 nm) phen; no fluorescence is detected from diprotonated phen, apparently also due to rapid excited state deprotonation. Evidence from absorption and emission intensity measurements as a function of pH suggests the existence of covalent hydrates of free base and monoprotonated phen. © 1979 American Chemical Society.