Lactim–lactam tautomerism through four member hydrogen bonded network in isoindole fused imidazole system: A combined spectroscopic and theoretical approach to photophysical properties

• Photophysical study in heterocycles of isoindole fused imidazole system (ADII).
• Lactim–lactam tautomerism reaction across four member hydrogen bonded network.
• Effect of intermolecular hydrogen bonding interaction on ESIPT process.
• Computational study on conformational stability and PEC for ESIPT process.

This article presents the photophysical study of a pharmaceutically important isoindole fused imidazole derivative, namely 1-(2-hydroxy-5methyl-phenyl)-3,5-dioxo-1H-imidazo-[3,4-b] isoindole (ADII) using steady state absorption, emission and time resolved emission spectroscopy. The molecule possesses a four member intramolecular hydrogen bonded ring composed of OH and N atom suitable for intramolecular proton transfer process. Absorption study of ADII suggests the existence of its lactim and lactam tautomeric form in the ground state. The appearance of large Stokes shifted emission signifies lactim–lactam isomerization by excited state proton transfer reaction. Quantum chemical calculations at Density Functional Theory (DFT) (B3LYP/6-311+G**) and Hartree–Fock (6-311+G**) levels have also been performed in support of the experimental findings. Both the level of theory suggests the existence of two tautomeric forms in the ground electronic state and the calculated potential energy surfaces along the proton transfer coordinate suggest the preference of excited state proton transfer reaction than the ground state with respect to activation barrier. The pH variance experiment is also carried out for investigating different absorbing and emitting species of ADII in aqueous and ACN medium.

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