Excited state properties of 10-phenyl-benzo[b][1,6]naphthyridines in solution

Abstract

The relaxation pathways of excited 10-phenyl-benzo[b][1,6]naphthyridine (PN), an analogue of acridine, and a series of 12 derivatives (R-6: CF 3, Br, Cl, F, CH 3, OCH 3; R-8: SO 2NH 2, Br, Cl, CH 3, OCH 3, OC 2H 5) were studied in several solvents at 297 and 77 K. Fluorescence, intersystem crossing and photodecomposition processes were examined by stationary and transient techniques. At 297 K, the quantum yield of fluorescence ( φ f) increases significantly with solvent polarity and with electron-donating substitution, intersystem crossing partly compensating this effect. The triplet is characterized by TT absorption at 297 and 77 K and by phosphorescence at 77 K. Generally, intersystem crossing is less sensitive to environment and substitution than φ f. Photodecomposition 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, π ∗ ) singlet state.