Grinding-Triggered Single Crystal-to-Single Crystal Transformation of a Zinc(II) Complex: Mechanochromic Luminescence and Aggregation-Induced Emission Properties.

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 ( fw) in THF/H2O (v/v) reached 70%, and fluorescence emission was enhanced. This phenomenon continued until fw was 90%, indicating aggregation-induced emission (AIE) property. The mechanism of AIE of ZnL2 in THF/H2O is the restriction of intramolecular rotation (RIR).