Ruthenium-containing ionic liquids were reversibly converted to amorphous coordination polymers or oligomeric liquids by the alternate application of ultraviolet light or heat, thus enabling control of their ionic conductivity and viscoelasticity.

The control of physical properties of soft matters via external stimuli is useful for various applications. Poly(ionic liquid)s are versatile liquid polymers exhibiting ionic conductivity, though the development of photoresponsive poly(ionic liquid)s is still limited.Here we report the synthesis and properties of a photoreactive poly(ionic liquid) 1a consisting of a Ru sandwich complex [Ru(C5H5){C6H3(OC6H12CN)3}] + and a polymeric anion -[-CH2-CH(SO2N − SO2CF3)-]n-. Upon UV photoirradiation, the liquid transformed to a rubbery elastomer 2a through the photochemical reaction of the cations. The resultant elastomer is a unique hybrid coordination polymer comprising of a cationic coordination network and anionic covalent chains. The elastomer returned to 1a upon heating. The application of light and heat resulted in a coordination structure transformation that caused reversible changes in viscoelasticity and ionic conductivity. The liquid 1a was synthesized by polymerization of ionic liquid 1b containing an H2C=CHSO2NSO2CF3 − anion. UV photoirradiation transformed 1b into a viscous elastomer 2b consisting of a coordination network, though the elastomer changed into 1a upon heating via polymerization of the anion.

Metal complexes with thiocyanate (SCN–) ligands typically exhibit S- or N-coordinated linkage isomers. In this study, to explore ionic liquids that exhibit stimuli-responsiveness based on linkage isomerization, we synthesized an ionic liquid containing a cationic half-sandwich thiocyanate complex, [Ru­(C6H6)­(NCS)­L]­Tf2N (L = N-hexyl-2-pyridinemethanimine, Tf2N = bis­(trifluoromethanesulfonyl)­amide anion). The as-synthesized ionic liquid was a 0.7:0.3 mixture of N- and S-coordinated isomers, presenting as an extremely viscous liquid exhibiting a glass transition at 0 °C. Isomerization from the N- to the S-coordinated isomer occurred upon UV photoirradiation or heating, although thermal isomerization was accompanied by significant decomposition. The N- and S-coordinated isomers were separated into brown and orange liquids, respectively, using gel permeation chromatography. Each isomer exhibited a small solvatochromic absorption shift in organic solvents, with different solvent dependences observed for the two isomers.

Nitro-nitrito isomerization is a well-known photoreaction exhibited by metal complexes in the solid state. We previously reported that the isomerization ratio in the salts of a cationic platinum(II) nitrito complex [Pt(L)(NO2)] + (L = N 1 -(2-(dimethylamino)ethyl)-N 2 ,N 2 -dimethyl-N 1pentyl-1,2-ethanediamine) is correlated to the reaction cavity surrounding the nitrito ligand. In this study, to further elucidate the effect of the packing structure on the reaction, we investigated the photoisomerization of the same cation in different packing environments. The salt of the cationic complex with the PF6 anion gave three polymorphs (-, -, and -forms) and a pseudopolymorph (form, acetonitrile solvate), of which the -form is formed from the -form upon desorption of acetonitrile. These polymorphs exhibited 36-100% conversions at 180 K depending on the reaction cavity. In addition, the -form exhibited a space group change upon photoirradiation. The salt with the (CF3SO2)2N − anion was an ionic liquid, taking both the crystalline and glassy states at low temperature.The crystalline state exhibited approximately 30% conversion at 100 K, whereas photoisomerization was not observed in its glassy state as investigated by IR spectroscopy. In these salts, less densely packed crystals deteriorated upon photoirradiation.Molecular structures, IR spectra, bond lengths and angles, and crystallographic parameters (PDF).