Excited state proton transfer based fluorescent molecular probes and their application in studying lipid bilayer membranes.

Molecules with ionizable protons, with different proton transfer efficiencies in the excited and ground states, show excited state prototropism (ESPT). In suitable proton donating/accepting environments, ESPT of a molecule can result in the observation of emissions from different prototropic species, each characterized by different emission spectra and different emission lifetimes. In condensed media, the immediate environment around the ESPT molecule can significantly influence the emission spectral parameters of different prototropic species. This forms the basis of ESPT based fluorescence sensing. The concept of ESPT has been widely used for probing dynamical and structural information of micro-heterogeneous media like micelles, polymers, lipid bilayer membranes, etc. ESPT molecules like naphthol and intra-molecular ESPT (ESIPT) molecules like hydroxyflavones etc., are said to be good multi-state fluorescent molecular probes if (i) the partitioning of these molecules to a micro-heterogeneous organized medium is more efficient, and (ii) the molecules possess distinct excitation and emission wavelengths corresponding to their different prototropic forms. The fluorescence of different prototropic forms shows a sensitive response towards the change in the local environment around the micro-heterogeneous organized medium concerning the physical properties, local structure, and dynamics. This review mainly comprises the work carried out on ESPT fluorescence molecular probing of biomimetic liposomes/lipid bilayer membranes from 1990 onwards.