Slow proton transfer dynamics of a four member intramolecular hydrogen bonded isoindole fused imidazole system: A spectroscopic approach to photophysical properties

•Photophysics of pharmaceutically important heterocyclic of isoindole fused imidazole system.•Excited state lactim–lactam tautomerism reaction studied spectroscopically.•Proton transfer in four member hydrogen bonded network favors over six member ring.•Irreversible and slow (kPT ∼ 109 s−1) proton transfer dynamics.

Spectral studies of 1-(2-hydroxy-5-chloro-phenyl)-3,5-dioxo-1H-imidazo-[3,4-b] isoindol (ADCL) in various solvents reveal the occurrence of excited state intramolecular proton transfer (ESIPT) process. Excitation at the shorter wavelengths preferentially leads to dual emission from an equilibrium mixture of excited state normal (N*, lactim form) and tautomeric (T*, lactam form) form due to the operation of proton transfer process. The proton transfer dynamics is found to be very slow in various organic solvents and it occurs in the nano-second time scale. This may be due to high energy barrier along the proton transfer coordinate through a strained four member hydrogen bonded network compared to the usual six member hydrogen bonded ring.

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