Sterically-Controlled Excited-State Intramolecular Proton Transfer Dynamics in Solution

Excited-state intramolecular proton transfer (ESIPT) is a fundamental ultrafast photochemical process. Although it has been intensively studied for the development of novel photonic devices such as organic light-emitting diodes, the relation between ESIPT reaction and intramolecular charge transfer (ICT) is still a subject of debate. Furthermore, the effects of the molecular geometry and of the substituent on ESIPT and ICT processes are still unclear. To address these issues, we synthesized a set of four compounds designed to control the electron density distribution of the proton-donating (PD) group and the steric hindrance between the PD and the adjacent phenyl groups: three 2-(1-phenyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol derivatives, PIPP-Xs (X = H, F, and OMe), and 2-(1-phenyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)naphthalen-2-ol (PIPN). We then investigated their ESIPT and ICT dynamics as well as the related structural changes using femtosecond transient absorption spectroscopy and t...