Polycyclic Aromatic Nitrogen Heterocycles. Part IV: Effect of Solvent Polarity, Solvent Acidity, Nitromethane and 1,2,4-Trimethoxybenzene on the Fluorescence Emission Behavior of Select Monoaza- and Diazaarenes

Fluorescence emission spectra are reported for naphth[2′l′8′7′: 4,10,5]anthra[l,9,8cdef]cinnoline, benzo[lmn][3,8]phenanthroline (also called 2,7-diazapyrene), benz[4,10]anthra[l,9,8cdef]cinnoline, naphtho[8,1,2hij]pyreno[9,10,ldef]phthalazine, acenaphtho[l,2b]pyridine, benzo[a]phenazine, indeno[l,2,3ij][2,7]naphthyridine, and indeno-[l,2,3ij]isoquinoline dissolved in organic nonelectrolyte solvents of varying polarity and acidity. Results of these measurements indicate that naphth[2′,1′,8′,7′:4,10,5]anthra[l,9,8cdef]cinnoline exhibits some signs of probe character as evidenced by changing emission intensity ratios; however, numerical values did not vary systematically with solvent polarity. The effect of nitromethane and 1,2,4-trimethoxybenzene as selective quenching agents on both the unprotonated and protonated PANHs was also examined. Nitromethane was found to quench fluorescence emission of roughly two-thirds of the alternant unprotonated PANHs studied to date. Emission intensities of the protonated PANHs remained essentially constant and were not affected by nitromethane. 1,2,4-Trimethoxybenzene, on the other hand, quenched the fluorescence emission of several unprotonated and all protonated PANHs examined.