Comparative studies of excited state intramolecular proton transfer (ESIPT) and azo-hydrazone tautomerism in naphthalene-based fluorescent acid azo dyes by computational study

We found that benzothiazole is responsible for enhancing fastness properties of dyes. On comparing the total electronic energies of naphthol tautomers (-1425.16 eV) and its analogs (-2146.59 eV), we found that benzothiazolyl series is more stable than the naphthol series. Frontier Molecular Orbitals (FMOs) also show flow of charge transfer from the donor to the acceptor in benzothiazole-containing compounds, whereas it is absent in plane naphthol series. Among the benzothiazolyl isomers, the hydrazone form is found to be more stable and responsible for fluorescence possession. Highest Occupied Molecular Orbital (HOMO)-Lowest Occupied Molecular Orbital (LUMO) energy band gap also indicates the same. Electrophilicity index and hyper-hardness values of both the series were found to be positive which directly ratifies photostability and reactivity. Benzothiazolyl series was found to be more stable, hence light fastness, enhanced.