Different competition mechanism between ESPT and TICT process regulated by protic and aprotic solvent in DHP

Abstract

In this study, we adopt density functional theory and time-dependent density functional theory (DFT/TDDFT) methods to compare and analyze the disparate competing mechanisms of excited state proton transfer (ESPT) and twisted intramolecular charge transfer (TICT) process of 2-(4′-diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b] pyridine (DHP) in protic and aprotic solvent. The results show that the DHP molecule undergoes excited state intramolecular proton transfer (ESIPT) in aprotic solvent ether. Differently, ESIPT is effectively inhibited in protic solvent methanol, whereas the coupling process of excited state double proton transfer (ESDPT) and TICT occurs. Combined with the analysis of potential energy surfaces, we clarify that DHP molecule in ether is more inclined to the channel of TICT → ESIPT → reverse TICT process to implement the ESIPT process. In DHP-MeOH complex, we confirm that the significant redshift phenomenon of fluorescence spectra observed in the experiment is attributed to the ESDPT induced TICT process. Our work offers a new avenue for switching ESPT and TICT competing mechanisms through protic and aprotic solvent, and considerably help the synthesis and development of highly efficient molecular photoswitch.