Influence of surfactants on the excited state photophysics of 4-nitro-1-hydroxy-2-naphthoic acid

This paper reports the changed dynamics of excited state proton transfer (ESPT) photophysics of 4-nitro-1-hydroxy-2-naphthoic acid (NHNA) in ionic and nonionic micelles, as revealed from time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra (TRANES). Both the neutral and anionic bands of NHNA are enhanced and blue-shifted inside the micelles. Quenching of emission by metal Cu2+ reveals that NHNA does not penetrate into the core of the micelles; rather it binds to the micelle/water interfacial region. The fast decay component is due to decay of normal species and the slow component is assigned to decay of anionic species. In all the micelles the decay time becomes longer by three times compared to that in water. The efficiency of formation of anionic state from the locally excited state gets enhanced in micellar media due to enhanced sojourn in the first excited state in increased viscous medium around NHNA. TRANES data also indicate that of the normal and anionic species in the aqueous phase the anionic species increases in the micelles as the time evolved.