Solid-State [2+2] Photoreaction of Isostructural Cd(II) Metal Complexes and Solid-State Fluorescence

A green method to synthesize cyclobutane derivatives has been developed over the past three decades in the form of solid-state [2+2] photochemical reactions. These solid-state reactions also play a major role in the structural transformation of hybrid materials. In this regard, crystal engineering has played a major role in designing photoreactive molecular systems. Here, we report three novel binuclear Cd(II) complexes with the molecular formula [Cd₂(4spy)₄L₄], where 4spy = 4-styryl pyridine and L = p-toluate (1); 4-fluorobenzoate (2); and 3-fluorobenzoate (3). Although three different benzoates are used, all three complexes are isostructural, as corroborated through SCXRD experiments. Structural analysis also helped in identifying two potential photoreactions. These are both intra- and intermolecular in nature and are driven by the head-to-head (HH) and head-to-tail (HT) alignment of 4spy linkers within these metal complexes. ¹H NMR spectroscopy studies showed evidence of a quantitative head-to-head photoreaction in all these three complexes, and SCXRD analysis of the recrystallization of the photoproducts also provided confirmation. TGA studies of these photoreactive complexes showed an increase in the thermal stability of the complexes due to the solid-state photoreaction. Photoluminescence studies of these complexes have been conducted, showing a blue shift in emission spectra across all three cases after the photoreaction.