Proton-switchable vapochromic behaviour of a platinum(ii)–carboxy-terpyridine complex

Abstract

We synthesized a carboxy-substituted Pt(II)–terpyridine complex, i.e. [PtCl(Hctpy)]Cl ([1H]Cl; Hctpy = 4′-carboxy-2,2′:6′,2′′-terpyridine), that shows interesting switchable vapochromic behaviour upon protonation/deprotonation of the carboxy group. The as-synthesized dark-blue amorphous-like solid [1H]Cl·3H₂O was converted to a yellow crystalline solid, [1H]Cl·H₂O, upon exposure to various polar organic solvent vapours (e.g., acetonitrile, ethanol, 1-propanol, and dichloromethane), which promote the removal of water molecules. The reaction of [1H]Cl·3H₂O with aqueous ammonia afforded a deprotonated bright-yellow crystalline complex, i.e. [PtCl(ctpy)]·H₂O (1·H₂O), which exhibits red luminescence with an emission maximum at 622 nm. Although the colour of 1·H₂O was not affected by exposure to various polar organic solvent vapours, interesting vapochromic luminescence with a remarkable red-shift of the emission maximum from 622 to 652 nm was observed upon exposure to saturated water vapour to form orange crystalline 1·3.5H₂O. X-ray structural analysis revealed that the planar and neutral complex molecule 1 forms a one-dimensional columnar structure with an intermolecular Pt⋯Pt distance of 3.518(2) Å in the orange crystalline 1·3.5H₂O, while the cationic molecule [1H]⁺ in the protonated form, [1H]Cl·H₂O, generates a dimeric structure with an intermolecular Pt⋯Pt distance of 3.439(2) Å. This difference suggests that the vapochromic behaviours of the protonated and deprotonated forms could be caused by structural changes induced by water-vapour adsorption/desorption, which affect the intermolecular Pt⋯Pt distance, thereby changing the energy of the metal–metal-to-ligand charge-transfer (MMLCT) transition. These contrasting results for the protonated and deprotonated complexes clearly indicate that the hydrophilicity of complex 1 is significantly affected by protonation/deprotonation of the carboxy group. In addition, quasi-reversible conversion between [1H]Cl·3H₂O and 1·H₂O was achieved by exposure of the protonated and deprotonated forms to triethylamine and humid hydrochloric acid vapours, respectively.