Deprotonation-induced enhancement in fluorescence of 2-((2-hydroxybenzylidene)amino)phenol, a Schiff base

Excited state dynamics of neutral and anionic forms of 2-((2- hydroxybenzylidene)amino)phenol (HBAP), a Schiff base, has been elucidated using time resolved fluorescence spectroscopy. The neutral molecule exhibits keto-enol tautomerism. So, the fluorescent cis-keto excited state is formed by direct excitation as well as ultrafast excited state proton transfer (ESIPT) from the enolic excited state. It exhibits very feeble fluorescence, associated with a lifetime of tens of picosecond. The anion, however, exhibits significantly stronger fluorescence with lifetime of almost a nanosecond. The fluorescent excited state for the anion is formed by solvent relaxation around it and conformational relaxation of the molecule itself. Ion-dipole interaction in the ground state is manifested in the fluorescence properties. Comparison with very recent publications on other Schiff bases, it is established that enhancement of fluorescence, induced by deprotonation, is a general phenomenon for this class of molecules. [Display omitted]