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 series of substituted naphthols (4-chl oro-1-naphthol, 5-amino-1-naphthol , 4-methoxy1-naphthol, 7-methoxy-2-naphthol , 3-amino-2-naphthol, 6-bromo-2naphthol and 1-bromo-2-naphthol) is investigated in liposomes, using steady state and time resolved fluorescence studies. 4-Chloro-1-naphthol emerges as a successful ESPT probe even though 7-methoxy-2naphthol and 4-methoxy1-naphthol are also useful. The poss ible reason for thi s success is accounted on the basis of the difference in proton dissociati on constants in exc ited state (pK"*) and ground state (p.'(,). The fai lure of aminonaphthols as ESPT probe is due to the ex istence of multiple equilibrium between d ifferent spec ies and the low quantum yie ld of these molecules in non-polar med ia. The increased intersystem crossing in the case of bromonaphthol excludes its poss ibility as an ESPT probe. Among the sevenmonosubsti tuted naphthols that have been screened, 4-chl oro-1 -naphthol emerges as a better choice for an ESPT probe. The neutral form fluorescence of thi s molecule correct ly reflects the phase transition behaviour o f DMPC liposome membranes. This probe also successful ly senses the cholesterolinduced phase changes in lipid bilayer membranes.