Thermo-/mechano-chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-sorting

Although strong chiral self-sorting (e.g., sergeants and soldiers principle, majority rules) often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers, oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here we report a benzo[h]quinoline-based iridi-um(III) complex (Ir) with helical chirality at its metal centre, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer-dimer transformation. While a five-coordinate monomer is formed in ra-cemic or enantiopure Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Ir solution. Estimation of dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature UV-Vis and 1H NMR spectra reveal a strong preference for homochiral dimerization (largest known value for coordination complex, Khomo/Khetero > 50) as a result of steric repulsions of peripheral alkyl chains in Ir heterochiral dimers. Notably, crystals of homochiral dimer are metastable, under-going a distinct colour change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behaviour, leading to emergence of dynamic functionality based on the rational design of discrete chiral assemblies.