Excited state proton transfer of some substituted naphthols in liposomes

Fluorescence probes based on excited state proton transfer (ESPT) have been recently developed for microheterogeneous media. The ESPT behaviour of a s eries of substituted naphthols (4-chloro-1-naphthol, 5-amino-1-naphthol, 4- methoxy-1-naphthol, 7-methoxy-2-naphthol, 3-amino-2-naphthol, 6-bromo-2-nap hthol and 1-bromo-2-naphthol) is investigated in liposomes, using steady st ate and time resolved fluorescence studies. Lt-Chloro-l-naphthol emerges as a successful ESPT probe even though 7-methoxy-2-naphthol and 3-methoxy-1-n aphthol are also useful. The possible reason for this success is accounted on the basis of the difference in proton dissociation constants in excited state (pK(a)*) and ground state (pK(a)). The failure of aminonaphthols as E SPT probe is due to the existence of multiple equilibrium between different species and the low quantum yield of these molecules in non-polar media. T he increased intersystem crossing in the case of bromonaphthol excludes its possibility as an ESPT probe. Among the seven-monosubstituted naphthols th at have been screened, 4-chloro-1-naphthol emerges as a better choice for a n ESPT probe. The neutral form fluorescence of this molecule correctly refl ects the phase transition behaviour of DMPC liposome membranes. This probe also successfully senses the cholesterol-induced phase changes in lipid bil ayer membranes.