Synthesis and Light-Induced Actuation of Photo-Labile 2-Pyridyl-1,2,3-Triazole Ru(bis-bipyridyl) Appended Ferrocene Rotors

Abstract

To realise useful control over molecular motion in the future an extensive toolbox of both actionable molecules and stimuli-responsive units must be developed. Previously, our laboratory has reported 1,1′-disubstituted ferrocene (Fc) rotor units which assume a contracted/π-stacked conformation until complexation of cationic metal ions causes rotation about the Ferrocene (Fc) molecular ‘ball-bearing’. Herein, we explore the potential of using the photochemical ejection of [Ru(2,2′-bipyridyl)₂]²⁺ units as a stimulus for the rotational contraction of new ferrocene rotor units. Fc rotors with both ‘regular’ and ‘inverse’ 2-pyridyl-1,2,3-triazole binding pockets and their corresponding [Ru(2,2′-bipyridyl)₂]²⁺ complexes were synthesised. The rotors and complexes were characterised using nuclear magnetic resonance (NMR) and ultraviolet (UV)-visible spectroscopies, Electro-Spray Ionisation Mass Spectrometry (ESI–MS), and electrochemistry. The 1,1′-disubstituted Fc ligands were shown to π-stack both in solution and solid state. Density Functional Theory (DFT) calculations (CAM-B3LYP/6-31G(d)) support the notion that complexation to [Ru(2,2′-bipyridyl)₂]²⁺ caused a rotation from the syn- to the anti-conformation. Upon photo-irradiation with UV light (254 nm), photo-ejection of the [Ru(2,2′-bipyridyl)₂(CH₃CN)₂]²⁺ units in acetonitrile was observed. The re-complexation of the [Ru(2,2′-bipyridyl)₂]²⁺ units could be achieved using acetone as the reaction solvent. However, the process was exceedingly slowly. Additionally, the Fc ligands slowly decomposed when exposed to UV irradiation meaning that only one extension and contraction cycle could be completed.