Interrogating the Azo-Hydrazo Proton Transfer Process in the Excited State Using a Multipronged Spectroscopic Study: Effect of Annulation and Solvent

Excited state intramolecular proton transfer (ESIPT) in organic dyes is one of the most studied photophysical processes by experimental and theoretical chemists alike. Of the various subclasses of ESIPT, the azo-hydrazo tautomerism occurs in ortho azo-dyes derived from phenols, mainly β-naphthol. In the current study, we present a steady-state and time-resolved spectroscopic investigation on the azo-hydrazo proton transfer process in two azo dyes derived from β-naphthol, 1-phenylazo-2-naphthol (PDNO) and 1-naphthylazo-2-naphthol (NDNO), which differ by a phenyl ring in solvents of differing polarity and proticity. Steady-state investigations implicate the presence of the ground-state azo-hydrazo tautomerism. The emission lifetime measurements reveal that the azo and hydrazo tautomers have different radiative lifetimes, with the hydrazo tautomer decaying faster than the azo tautomer. The femtosecond-picosecond transient absorption measurements provide critical information on the nonradiative decay processes associated with the dyes and demarcate the temporal behavior of the nonradiative relaxation between the two dyes. NDNO, having an extra phenyl ring, reveals slower relaxation times compared to PDNO, irrespective of the solvent. The relaxation time is longest in chloroform and shortest for nonpolar hexane, irrespective of dye. Considering both the steady-state and time-resolved measurements, we propose how the spectrodynamics in azo dyes derived from β-naphthol could be manipulated by tuning the annulation as well as the surrounding solvent system, which aids the fundamental understanding of excited-state photoprocesses like the azo-hydrazo tautomerism.