Ultrafast Proton-Coupled Electron-Transfer Dynamics in Amino-Type Indole-Triazolopyrimidine Derivatives

Excited-state intramolecular proton transfer (ESIPT) can be accompanied by electron transfer within a molecular skeleton, leading to a proton-coupled electron transfer (PCET) process. The kinetics of ESIPT are influenced by the concurrent electron transfer, which in turn affects the overall photophysical properties of the system. In this study, we elucidate the photoinduced PCET mechanism in an ESIPT system based on 7-(indol-2-yl)-triazolopyrimidine (In-TAP) derivatives, which feature an N–H···N-type intramolecular hydrogen bond between adjacent heteroaromatic rings. Chemical modifications introduce short-range and long-range charge-transfer characteristics, facilitating an ultrafast ESIPT reaction on a time scale of ∼150 fs, prior to the system reaching its equilibrium polarization of the solvent field. Strongly solvatochromic T* emission is attributed to the solvation-associated ESIET process following the ultrafast ESIPT reaction. This work proposes a novel and relatively rare N–H···N-type molecular framework that approximately follows the case-B PCET mechanism (PT occurs prior to ET), extending insights from the well-established systems such as 2-((2-(2-hydroxyphenyl)benzo-[d]oxazol-6-yl)methylene)-malononitrile (diCN-HBO), 2-((2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl)-methylene)-cyanoacetic acid (HBODC), and related analogs.