Single-Crystal-to-Single-Crystal Transformation of Hydrogen-Bonded Triple-Stranded Ladder Coordination Polymer via Photodimerization Reaction.

A one-dimensional hydrogen-bonded triple-stranded ladder coordination polymer [Cd(bpe)(NO)(HO)] () (where bpe = -1,2-bis(4-pyridyl)ethylene) containing three parallel C═C double bonds was synthesized. This compound undergoes photochemical [2 + 2] cycloaddition and produces -tetrakis(4-pyridyl)cyclobutane (-tpcb) in up to 67% yield via Single-Crystal-to-Single-Crystal (SCSC) transformation. Triple-stranded ladder-like structures have never before displayed such a kind of SCSC transformation. Furthermore, photoirradiation of ground produces -tpcb in up to 100% yield in the solid state. On the basis of the alignment of three C═C olefinic bonds of bpe ligands in parallel, only two out of the three aligned bpe are expected to undergo [2 + 2] photodimerization. However, the quantitative yield from the solid-state photochemical [2 + 2] cycloaddition reaction has been achieved via grinding of crystals of to a powder. The effects of grinding on photoreactivity of were thoroughly studied using H NMR spectroscopy, thermogravimetric analysis (TGA), and Raman spectroscopy. These studies indicate that the molecular movements of the hydrogen-bonded ladders are reinforced due to the loss of coordinated water molecules and the further crystal repacking via bond-breaking/forming of the hydrogen-bonded assemblies during mechanical grinding. The 100% photodimerization of ground shows that the grinding accelerates internal molecular motions of ladder structures within the crystals lattice. The solid-state photoluminescence of , before and after UV irradiation, was investigated at room temperature, both indicative of interesting luminescent properties.