Stimuli-Responsive Homochiral Coordination Polymers Undergoing Single-Crystal-to-Single-Crystal [2+2] Photodimerization.

The development of homochiral coordination polymers (HCPs) that combine structural robustness with stimuli-responsive functionality remains a significant challenge. Herein, we report two Zn-based HCPs, Zn(R) and Zn(S), constructed from alanine-derived chiral ligands and the photosensitive coligand 1,2-di(4-pyridyl)ethylene (dpe). Both compounds crystallize in the polar noncentrosymmetric (NCS) space group P21 and exhibit second-harmonic generation (SHG) activity arising from their chirality-induced NCS structures. Temperature-dependent in situ powder x-ray diffraction analyses reveal reversible dehydration-rehydration phase transitions, demonstrating the remarkable adaptability of the framework. Notably, the frameworks enforce a slip-stacked arrangement of dpe ligands that aligns neighboring olefinic bonds in accordance with Schmidt's criterion, thereby enabling UV-induced single-crystal-to-single-crystal [2+2] cycloaddition. Site-selective photodimerization to rctt-tetra(4-pyridyl)cyclobutane (rctt-tpcb) is achieved within 8 h of UV irradiation. Kinetic analyses yield Avrami exponents close to 2, indicating a predominantly two-dimensional solid-state transformation mechanism. Importantly, the mirror-image circular dichroism signals are retained throughout the photoreaction, confirming preservation of homochirality, while the SHG response persists after photoconversion. Quantitative analyses reveal a maximum rctt-tpcb conversion of 97%, accompanied by approximately 10% thermal cycloreversion in polycrystalline samples. These results establish a multifunctional HCP platform that combines reversible phase transformation, solid-state photoreactivity, and NCS optical functionality within a single framework.