Switchable NIR emission enabled by phase-driven barrierless ESIPT in 1-NH2

In this Letter, we report phase-dependent excited-state relaxation pathways and fluorescence mechanisms of the 1-NH₂ molecule using quantum chemical simulations. In the liquid phase, the synergistic coupling between excited-state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) facilitates nonradiative decay through a conical intersection (CI) channel, leading to the quenching of Keto* fluorescence. Conversely, in the solid phase, restricted molecular rotation blocks the CI channel and promotes barrierless ESIPT, yielding strong Keto* emission in the near-infrared (NIR) region. Reorganization energy analysis further elucidates the origin of redshifted emission. This work reveals a novel, to the best of our knowledge, phase-dependent fluorescence switching mechanism in 1-NH₂, offering valuable theoretical guidance for developing smart luminescent materials.