The suppression of intramolecular charge transfer emission by tautomerism in 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine: Intramolecular proton transfer versus intermolecular proton transfer

Abstract

2-(4′-Aminophenyl)-1H-imidazo[4,5-c]pyridine (APIP-c) emits twisted intramolecular charge transfer (TICT) emission in methanol. However, when hydroxyl group was introduced in APIP-c, the hydroxy compound 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (AHPIP-c) does not emit TICT emission in methanol. But it undergoes excited state intramolecular proton transfer (ESIPT) to emit tautomer emission. The cause for the suppression of TICT by ESIPT in AHPIP-c was not clear. Therefore, DFT calculations were performed using polar continuum model with explicit methanol molecule to understand the cause. The vibrational frequency analyses were performed to know the strength of the bond in the ground and excited states. The potential energy curves for both intra- and intermolecular proton transfer processes were constructed in the ground and excited states. Our theoretical studies reveal that the relative strength of the intermolecular hydrogen bond and intramolecular hydrogen bond plays a crucial role in determining the dominating excited state process.