Effect of pH in the photophysics of 2-hydroxy 1-naphthaldehyde in micellar systems

The effect of variation of pH of different (non-ionic, anionic and cationic) micellar media on the photophysics of 2-hydroxy 1-naphthaldehyde (HNL) as monitored through its anionic band variation has been deliberated. Cationic micelle facilitated anion emission of HNL gets enhanced with increase of pH but the neutral emission remains the same. Addition of acid in cationic micelle causes a decrease in anion band intensity along with a blue shift while no such effect was observed in anionic or non-ionic micelles. Guest HNL is found deeper in non-ionic micelle and also the barrier for formation of anion conformer is increased. A possible drifting of guests from bulk water to micellar interior in anionic and non-ionic micelle is indicated through decreased anion band intensity. Addition of salt in the medium has the same effect on emission as with addition of base. In anionic micelle addition of salt causes to decrease the neutral band vastly and the anion band decreases due to combined effect of increased non-radiative decay rate and greater accessibility of HNL to hydroxyl ions in the interfacial region. Increase in temperature lowers the anion emission band intensity due to decrease in aggregation number, which is substantiated by change of entropy in the temperature range. With addition of base the lifetime of anion band is decreased in anionic and non-ionic micelles due to lower polarity in micelle while a marginal increase is observed in cationic micelle as the micelle itself promotes anion formation.