Grinding-Triggered Single Crystal-to-Single Crystal Transformation of a Zinc(II) Complex: Mechanochromic Luminescence...

We first report single crystal X-ray analysis of ground crystals of mechanochromic luminescence (MCL) that shows single crystal-to-single crystal transformation (SCSCT). Single crystals of [ZnL₂] (1-SG, HL = 2-[[[4-(2-benzoxazolyl)phenyl]imino]-methyl]-5-(diethylamino)-phenol) were obtained upon slight grinding of single crystals of [ZnL₂]·0.5CH₃OH (1), both of which were characterized by single crystal X-ray diffraction. Crystals of 1 showed emission centered at 647 nm (red color), while crystals of 1-SG showed emission band at 624 nm (orange-red color) under UV light, indicating MCL property of the Zn(II) complex. Reversible MCL property with emission color change between red and yellow for 1 was observed upon high grinding and recrystallization. Single crystal X-ray analysis suggested that it is due to the alteration of molecular conformation of ligands in ZnL₂ instead of weak intermolecular interaction that 1 exhibits MCL. Investigation of the control Zn(II) complexes (2-4) indicated that flexible substituents and rotated aromatic rings are desirable to generate the MCL-active complexes. In addition, 1 was highly fluorescent in THF solution, but its fluorescence quenched upon addition of water. DFT calculations suggested that this is due to the formation of the excited hydrated ZnL₂ species via Zn-O coordination bond, which results in electron-driven proton transfer (EDPT). Aggregates formed as water fraction ( f_w) in THF/H₂O (v/v) reached 70%, and fluorescence emission was enhanced. This phenomenon continued until f_w was 90%, indicating aggregation-induced emission (AIE) property. The mechanism of AIE of ZnL₂ in THF/H₂O is the restriction of intramolecular rotation (RIR).