Switching the ESIPT and TICT process of DP-HPPI via intermolecular hydrogen bonding

Intermolecular hydrogen bonding prominently figures in numerous photophysical and photochemical processes. It is worth discussing whether it can act as a switch to effectively convert excited-state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) process. In this paper, the effect of intermolecular hydrogen bonding on the ESIPT and TICT process of DP-HPPI in DCM and ACN solvent has been explored by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The analytical results show that DP-HPPI experiences the ESIPT process via intramolecular hydrogen bonding in DCM. However, the ESIPT process is prohibited due to the involvement of intermolecular hydrogen bonding in ACN, triggering the TICT process. From the study results of photophysical properties in DP-HPPI, we reveal that DP-HPPI undergoes reversible ESIPT in DCM. Moreover, we elucidate the detailed proton transfer pathway and the advantage of reverse proton transfer over forward proton transfer. Notably, an unobstructed TICT process occurs in the S1 state after light irradiation of DP-HPPI in ACN. Our work successfully closes the ESIPT process and initiates the TICT process via intermolecular hydrogen bonding, and provides new insights into the design and synthesis of efficient optical devices.