Understanding the photophysics of 4-nitro-1-hydroxy-2-naphthoic acid: A controlled excited state proton transfer

This paper reports on the excited state intramolecular proton transfer (ESIPT) in 4-nitro-1-hydroxy-2-naphthoic acid (NHNA) as compared to the excited state photophysics of 1-hydroxy-2-naphthoic acid (HNA). Normally, HNA does not show an ESIPT, although the other regioisomer of the molecule, (3, 2HNA), does show an ESIPT. The calculated excited state double-well potential, charge density, bond length and dipole moment evince that, in contrast to the parent compound HNA, NHNA is conducive to ESIPT. Emission bands were attributed to the neutral form (410 nm), the intermolecular proton transfer form (490 nm) and the intramolecular proton transfer (zwitterionic) form (560 nm). NHNA showed phosphorescence arising from the open conformer, whereas HNA showed no phosphorescence. Intramolecular hydrogen bonding in NHNA seems to hinder the formation of the open conformer in polar solvent. A complete picture of the photophysical processes in NHNA after photoexcitation was obtained from the time-resolved emission along with kinetic analysis.