Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting

Abstract

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and 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 butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, 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 a racemic or an enantiopure Bu-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 Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet−visible (UV−vis) and 1 H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, K homo /K hetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies.

Macrocycle‐Based Charge Transfer Cocrystals with Dynamically Reversible Chiral Self‐Sorting Display Chain Length‐Selective Vapochromism to Alkyl Ketones

“…Würthner and co‐workers reported that the core‐twisted perylene bisimide self‐assembled into homochiral one‐dimensional (1D) fibers and heterochiral two‐dimensional lamellar liquid crystals under different cooling rates of the monomer solution, which exhibited completely distinctive absorption properties [8] . Recently, Takimoto et al [9] . demonstrated a thermo‐/mechano‐chromism originating from the monomer and homochiral dimer transformation of a helical chiral iridium(III) complex, thus realizing switchable chiral supramolecular polymerization with naked‐eye visualization.…”

Macrocycle‐Based Charge Transfer Cocrystals with Dynamically Reversible Chiral Self‐Sorting Display Chain Length‐Selective Vapochromism to Alkyl Ketones

“…Würthner and coworkers reported that the core-twisted perylene bisimide self-assembled into homochiral one-dimensional (1D) fibers and heterochiral two-dimensional lamellar liquid crystals under different cooling rates of the monomer solution, which exhibited completely distinctive absorption properties. [8] Recently, Takimoto et al [9] demonstrated a thermo-/mechano-chromism originating from the monomer and homochiral dimer transformation of a helical chiral iridium(III) complex, thus realizing switchable chiral supramolecular polymerization with naked-eye visualization.…”