Changing Mechanical Strength in Cr(III)- Metallosupramolecular Polymers with Ligand Groups and Light Irradiation

We have demonstrated the ability to control the mechanical properties of metallosupramolecular materials via choice of ligand binding group, as well as with external light irradiation. These photoresponsive Cr­(III)-based materials were prepared from a series of modified hydrogenated poly­(ethylene-co-butylene) polymers linked through metal–ligand interactions between a Cr­(III) metal center and pyridyl ligand termini of the polymers. The introduction of these Cr­(III)-pyridine bonds gave rise to new mechanical and optical properties of the polymer materials. Depending on the type of pyridyl ligand, density functional theory calculations revealed changes in coordination to the Cr­(III), which ultimately led to materials with significantly different mechanical properties. Electronic excitation of the Cr­(III) materials with 450 and 655 nm CW lasers (800 mW/cm²) resulted in generation of excited state photophysical processes which led to temporary softening of the materials up to 143 kPa (41.5%) in storage modulus (G′) magnitude. The initial mechanical strength of the materials was recovered when the light stimulus was removed, and no change in mechanical properties was observed with light irradiation where there was no absorbance by the Cr­(III) moiety. These materials demonstrate that introduction of metal–ligand bonding interactions into polymers enables the design and synthesis of photoresponsive materials with tunable optical-mechanical properties not seen in traditional polymeric materials.