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 theoretical calculations. Although the four compounds commonly exhibit a dual emission originating from the excited enol (E*) and keto (K*) tautomers, their emission properties, such as emission maxima and lifetimes, are systematically modulated by substitution at the para-position of the PD group. The experimental and time-dependent density functional theory calculation results showed that the substitution of an electron-withdrawing group at the para-position of the PD group and the planarity between the PD and proton-accepting (PA) groups play important roles in inducing an efficient ESIPT characterized by increased emission of the K* tautomer. On the other hand, the photoexcitation for PIPP-Xs induces the formation of cis-K*, which is the most stable structure, whereas in PIPN the E* tautomer generated by the photoexcitation is rapidly converted to two species, cis-K* and per-K* with time constants of <0.2 and 0.5 ps, respectively. Furthermore, the per-K* state of PIPN has a charge transfer characteristic, suggesting intramolecular charge migration induced by the formation of per-K* state. This distinctive dynamics of PIPN is due to its pretwisted structure between PD and PA groups. The results provided in this study demonstrate that the molecular geometry plays an important role in the ESIPT and ICT processes.