Polycyclic Aromatic Nitrogen Heterocycles. Part III: Effect of Solvent Polarity and Solvent Acidity on the Fluorescence Emission Behavior of Select Azapyrenes and Phenanthroisoquinolines

Fluorescence emission spectra are reported for 1-azapyrene, 2-azapyrene, 4-azapyrene, 12-azabenzo[a|pyrene, phenanthro[9,10g]isoquinoline, phenanthro[2,3h|isoquinoline, and phenanthro[3,2h]isoquinoline dissolved in organic nonelectrolyte solvents of varying polarity and acidity. Results of these measurements indicate that 12-azabenzo[a]pyrene, phenanthro[2,3h]isoquinoline, and phenanthro[3,2h]isoquinoline exhibit modest probe character as evidenced by decreased I/II or I/III band emission intensity ratios with increasing solvent polarity. Alcoholic solvents such as trifluoroethanol protonate the nitrogen hetero-atom, resulting in loss of emission fine structure accompanied by a sizeable red-shift in emission wavelength(s). For 1-azapyrene and 2-azapyrene the observed fluorescence emission spectra showed both neutral and protonated forms of the solute in trifluoroethanol.