Intramolecular excited proton transfer of 1-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol – A combined experimental and quantum chemical studies

•Absorption and emission spectral studies have been carried out.•HOMO–LUMO – probable charge transfer taking place.•ESIPT process explained in detail.•Highly efficient NLO chromophore.

Synthesis of the 1-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol [PPIN] is reported, spectral and fluorescent properties of the title compound are investigated. The feasibility of excited state intramolecular proton transfer (ESIPT) has been argued from the changeover of relative stability of the enol and the keto forms on photoexcitation from the S0-PES to the S1-PES. Critical evaluation on the modulations of geometrical parameters other than the proton transfer reaction coordinate has also been undertaken. The intramolecular hydrogen bonding (IMHB) interaction in PPIN has been explored by calculation of the hyperconjugative charge transfer interaction from the lone electron pair of ring nitrogen atom to the σ∗ orbital of O–H bond, under the provision of Natural Bond Orbital (NBO) analysis. However, DFT calculations together with the experimental results reveal that the excited species with the intramolecular N⋯HO hydrogen bond type undergoes rapid radiationless deactivation. This leads to a conclusion that the low-intensity dual-band fluorescence of the title compound in solution originates from the traces of the conformation with the –OH bond to azomethine nitrogen atom (ESIPT).

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