Modulated photophysics of an ESIPT probe 1-hydroxy-2-naphthaldehyde within motionally restricted environments of liposome membranes having varying surface charges.

The present work demonstrates the modulation of excited state intramolecular proton transfer (ESIPT) emission of 1-hydroxy-2-naphthaldehyde (HN12) upon its interaction with the liposomal vesicles/bilayer of dimyristoyl-l-α-phosphatidylcholine (DMPC) and dimyristoyl-l-α-phosphatidylglycerol (DMPG) and its subsequent implementation as an efficient molecular reporter for probing of microheterogeneous environments of lipid-bilayer system. Modifications on the emission profile of HN12 in terms of remarkable emission intensity enhancement coupled with a hypsochromic shift induced by the presence of DMPC and DMPG membranes have been interpreted to be vivid manifestations of the interactions between the two partners. Steady-state anisotropy, red-edge excitation shift (REES), and time-resolved fluorescence measurements have been fruitfully exploited to complement other experimental findings. Probable binding site of HN12 in the lipid-bilayers has been assessed on the basis of intertwining the results of fluorescence quenching with other experimental results and is further substantiated from docking studies.