Unraveling the phase-dependent ESPT pathways in multi-channel PiQ molecule: insights from TDDFT calculations.

The excited-state proton transfer (ESPT) behavior of the PiQ molecule was systematically investigated in both liquid and solid phases. The asymmetric molecular structure and intramolecular hydrogen bonding interactions were analyzed through geometry optimization, potential energy surface calculations, and frontier molecular orbitals analysis. It was demonstrated that in the liquid phase, PiQ undergoes a concerted excited-state intramolecular double proton transfer (ESDPT), resulting in green fluorescence at 510 nm. In contrast, in the solid phase, only the initial excited-state intramolecular single proton transfer (ESISPT). Crystalline packing in the solid state restricts molecular rearrangement, destabilizes the TD* (ESDPT) form, and thus inhibits the second step of proton transfer, resulting in the formation of TB* (ESISPT) and blue fluorescence emission at 487 nm. RMSD analysis supports this inhibition by revealing minimal geometric deviation between the ground and excited states in the solid phase. These findings demonstrate that phase-dependent ESPT pathways govern the emission behavior of PiQ, enabling visually perceptible phase recognition and offering a viable strategy for the development of multi-responsive luminescent materials.