N-(Di)icosyl-Substituted Benzo[a]phenoxazinium Chlorides: Synthesis and Evaluation as Near-Infrared Membrane Probes

Five benzo[a] phenoxazinium chlorides containing alkyl chains with twenty carbon atoms at the 5- or 9-positions of the tetracyclic ring were efficiently synthesised and characterised by UV/Vis and NIR spectroscopy. The absorption and emission maxima in ethanol occur in the range 627-641 nm and 645-676 nm, respectively, with quantum yields varying from 0.14 to 0.38. Preliminary photophysical studies with these fluorochromophores in zwitterionic [2,3-bis(palmitoyl-oxy) propyl-2-(trimethylammonio)ethyl phosphate, DPPC] and cationic (N,N-dimethyl-N-octadecyloctadecan-1-aminium bromide, DODAB) vesicles were carried out. The results showed that the new benzo[a] phenoxazinium derivatives are able to detect the gel to liquid-crystalline lipid phase transition through variations in either the extent of H-aggregation or in the acid-base equilibrium.