Manipulating vapochromic or solvatochromic properties of platinum(ii) pincer complexes through ligand modifications

Colour-changing chemical sensors have found important applications in the detection of low concentrations of volatile organic compounds (VOCs). Among the most promising molecular materials are platinum pincer complexes that display rapid colour changes when exposed to a variety of VOCs. In the solid state, these rapid responses have been linked to changes in non-covalent interactions between the platinum pincer molecules and the guest VOCs. To gain a better understanding of the interactions involved, we have studied the manipulation of vapochromic or solvatochromic properties in a series of square planar platinum(ii) pincer complexes through structural modification of the monodentate ligand occupying the fourth coordination site. The platinum(ii) complexes are based on the 1,3-di(pyridine)benzene tridentate linker (N^C^N) skeleton with the fourth site occupied by a monodentate, anionic ligand L. The formulae of the complexes synthesised are [Pt(N^C(C(O)OMe)^N)(L)] (L = (NCO) (6), (NCS) (7), (OC(O)Me) (8), (OC(O)CF₃) (9), (OS(O)₂CF₃) (10), and (OS(O)₂(C₆H₄Me)) (11)), and the crystalline solids and solutions of these materials have been tested for solvatochromic or vapochromic changes with VOCs including dichloromethane, acetonitrile, diethyl ether, methanol and water. The complexes 6 and 7 crystallised as yellow solids, with no solvent voids in the crystal lattice. Single-crystal X-ray analyses showed that the intermolecular Pt⋯Pt separations were too long for direct Pt⋯Pt interactions. Neither material displayed solvatochromic or vapochromic properties. However, when the fourth ligand was an acetate group, complex 8, the solid displayed vapochromism, changing colour from an orange water-containing crystalline form to an anhydrous yellow form, and a blue form when treated with methanol vapour. A crystal structure analysis showed that in the orange form adjacent pincer molecules were linked together through a hydrogen bonding network involving lattice water molecules, supported by π⋯π stacking interactions. Complexes 9 and 10 both showed solvatochromism, forming a bright yellow solid when crystallised from dichloromethane but forming an orange solid when recrystallised from acetonitrile. Complex 11 displayed both vapochromic and solvatochromic properties. This complex was isolated as a purple solid, turning yellow upon treatment with methanol droplets or vapour, but this colour change could be reversed upon addition of acetonitrile.