Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

Abstract

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.

Sequence-sorted redox-switchable hetero[3]rotaxanes

“…For NDIC7 we obtained a crystal structure (ESI † section 8). In the crystal, the dihedral angle between the phenyl group and the NDI is 70.01(11)°, which is considerably flatter than for NDIC8 45 (84.18(8)°), in order to facilitate π-π interactions between the NDIs of neighbouring NDIC7 in the solid state. The ring sizes of NDIC8, NDIC7 and TTFC8 are wider than those of DBC8 and BC7, respectively, as the 4.766( 6) Å (NDIC7) / 4.730(4) Å (NDIC8) distance of the two phenolic O atoms in the NDI-substituted macrocycles as well as the S-S distance of 3.3136(13) Å in TTFC8 46 are substantially larger than the O-O distance of 2.5642(5) Å in DBC8.…”

“…15 In a study on TTFand NDI-decorated crown ethers with different cavity sizes, crown [8] ethers revealed a preference for dibenzylammonium and crown [7] wheels for benzyl alkyl ammonium stations, suggesting integrative self-sorting for more functional wheels. 45 Herein, we propose a blueprint to sequence-sorted heterocircuit [3]rotaxanes using one directional axle and a library of five macrocycles carrying three different types of functional units. This approach allows for unrestricted selection and combination of the functional units in rotaxane structure.…”

“…47 Recently, we have shown that the spectroelectrochemical properties of redox-active crown ethers are not significantly influenced by the crown ether size. 45 This allows us to use the crown ether sizes to program the position of the functional unit on the axle without changing the spectroelectrochemical properties of the functional units. Therefore, the hetero [3]rotaxane combinations carrying DBC8 / NDIC7 and NDIC8 / BC7 are considered as "sequence pseudoisomers".…”

Sequence-sorted redox-switchable hetero[3]rotaxanes

“…47 Recently, we have shown that the spectroelectrochemical properties of redox-active crown ethers are not significantly influenced by the crown ether size. 45 This allows us to use the crown ether sizes to program the position of the functional unit on the axle without changing the spectroelectrochemical properties of the functional units. Therefore, the hetero [3]rotaxane combinations carrying DBC8 / NDIC7 and NDIC8 / BC7 are considered as "sequence pseudoisomers".…”

“…15 In a study on TTF-and NDI-decorated crown ethers with different cavity sizes, crown [8] ethers revealed a preference for dibenzylammonium and crown [7] wheels for benzyl alkyl ammonium stations, suggesting integrative self-sorting for more functional wheels. 45 Fig. 1 Schematic representation of [2] and [3]rotaxanes containing a directional axle (dark blue) and one or two macrocycles (grey) carrying functional units (orange and green).…”