Exploration of Solid-State [2 + 2] Photodimerization in the Coordination Polymers of Rigid and Linear Diene: Single-Crystal-to-Single-Crystal Transformation of a 2D Coordination Polymer to a 3D Coordination Polymer

Coordination polymers serve as an important platform to promote highly selective solid-state reactions between organic struts. A new class of amide-containing cyclobutane molecules has been synthesized using a [2 + 2] photodimerization reaction in coordination polymers (CPs) of rigid and linear diene 3,3′-(1,4-phenylene)bis(N-(3-pyridyl)acrylamide), 4PMA, with rigid as well as flexible dicarboxylates as coligands. Four Co(II) CPs studied here produce 2D layers with rhomboidal grids (isophthalate (CP-1) and p-carboxy cinnamate (CP-2)), while p-phenylene acrylate (CP-3) yields a 3D framework with cds topology and p-phenylene diacetate forms 2D layers with a bimetallic secondary building unit (SBU). The [2 + 2] cycloaddition reaction upon exposure to 365 nm UV light on the methanol solvate of 4PMA remained elusive despite a favorable polymeric alignment in its crystal structure. Single crystals of CPs upon irradiation resulted in photodimerization; however, the crystal structure suggests polymerization (CP-1 and CP-2) or no reaction (CP-3). This unusual behavior is attributed to deviations in geometrical parameters from the ideal, serving as the driving force behind the photodimerization reaction. Nevertheless, the 2D CP of CP-4 exhibited resilience during dimerization, leading to the formation of a 3D coordination polymer of the fsc net of CP-4P through a single-crystal-to-single-crystal structural transformation. The separated dimer product displays quenched photoluminescence and blue-shifted spectra compared with the monomer.