EXCITED-STATE PROPERTIES OF 10-PHENYL-BENZO[B][1,6]NAPHTHYRIDINES IN SOLUTION

The relaxation pathways of excited 10-phenyl-benzo[b][1,6]naphthyridin e (PN), an analogue of acridine, and a series of 12 derivatives (R-6: CF3, Br, Cl, F, CH3, OCH3; R-8: SO2NH2, Br, Cl, CH3, OCH3, OC2H5) were studied in several solvents at 297 and 77 K. Fluorescence, intersyste m crossing and photodecomposition processes were examined by stationar y and transient techniques. At 297 K, the quantum yield of fluorescenc e (Phi(f)) increases significantly with solvent polarity and with elec tron-donating substitution, intersystem crossing partly compensating t his effect. The triplet is characterized by T-T absorption at 297 and 77 K and by phosphorescence at 77 K. Generally, intersystem crossing i s less sensitive to environment and substitution than Phi(f). Photodec omposition is negligible for PNs in acetonitrile at room temperature, but effective for PN and most of its R-8 derivatives in alcohols, e.g. 2-propanol, ethanol or methanol. This is ascribed to photoreduction, involving a spectroscopically hidden, probably (n, pi) singlet state.