Concurrent ground and excited state proton transfer of (E)-2-((napthalen-2-ylimino)-methyl)phenol: Modulation in micellar media

Abstract

The phenomena of ground and excited state proton transfer (GSPT and ESPT) of the synthesized Schiff base, (E)-2-((napthalen-2-ylimino)-methyl)phenol (NMP) have been unraveled in aqueous medium using various spectroscopic techniques. Changes in the absorption and emission spectra of an aqueous solution of the probe upon addition of an external base are ascribed to the proton transfer in both the ground and the excited states. Existence of equilibria between the neutral and the deprotonated species in the two electronic states is established from the observation of the isosbestic and isoemissive points in the absorption and emission spectra. Acid dissociation constants in the ground state (pKa) and photoexcited state (pKa*) of NMP in aqueous medium have been derived from the steady-state experiments using Weller’s method. The time resolved experiments clearly indicate the reversibility of the proton transfer reaction in the photo-excited state. The pKa* value determined from the kinetic measurements corroborates that obtained from the steady state methods. The higher value of pKa* than pKa is rationalized from the higher electron density on oxygen in the photoexcited state relative to the ground state, as obtained from the Mulliken charge densities through quantum chemical calculations. Extending our studies to the micellar environments, it is established that the deprotonation process is favored in cationic micelles while the same is disfavored in anionic or non-ionic micelles. This has been rationalized in terms of the surface charge characteristics of the individual micelles.