Synthetic light-driven rotary molecular motors show complicated structural dynamics during the rotation process. A combination of DFT calculations and various spectroscopic techniques is employed to study the effect of the bridging group in the lower half of the molecule on the conformational dynamics. It was found that the extent to which the bridging group can accommodate the increased folding in the transition state is the main factor in rationalizing the differences in barrier height and, as a consequence, the rotary speed. These findings will be essential in designing future rotary molecular motors.
We have developed effective reaction conditions for the Suzuki cross-coupling of chlorinated hexaphenylbenzene derivatives. A chloro group on a hexaphenylbenzene framework exhibits a low reactivity to Suzuki cross-coupling, and only nickel catalysts bearing alkyl-substituted phosphine ligands achieved the coupling. With this as a key step, we succeeded in the selective preparation of a C 2v -symmetric hexaphenylbenzene derivative containing two kinds of aryl group.
Abstract Separation of a homogeneous mixture of different components to reach an ordered out-of-equilibrium state in solution has attracted continuous attention. While this can be achieved using external chemical fuels or photo energy, an alternative energy source is heat. Here we realize a temperature-controlled cycle of transitions between ordered and disordered states based on a mixture of two kinds of building blocks that self-assemble into cubic structures (nanocubes). An almost statistical mixture of nanocubes (disordered state) is thermodynamically most stable at lower temperature (25 °C), while homoleptic assemblies composed of single components are preferentially produced at higher temperature (100 °C) followed by rapid cooling. The scrambling of the building blocks between the nanocubes takes place through the exchange of free building blocks dissociated from the nanocubes. Based on this mechanism, it is possible to accelerate, retard, and perfectly block the scrambling by the guest molecules encapsulated in the nanocubes.
Abstract True understanding of dispersion interaction in solution remains elusive because of difficulty in the precise evaluation of its interaction energy. Here, the effect of substituents with different polarizability on dispersion interactions in water is discussed based on the thermodynamic parameters determined by isothermal titration calorimetry for the formation of discrete aggregates from gear-shaped amphiphiles (GSAs). The substituents with higher polarizability enthalpically more stabilize the nanocube, which is due to stronger dispersion interactions and to the hydrophobic effect. The differences in the thermodynamic parameters for the nanocubes from the GSAs with CH 3 and CD 3 groups are also discussed to lead to the conclusion that the H/D isotope effect on dispersion interactions is negligibly small, which is due to almost perfect entropy-enthalpy compensation between the two isotopomers.
The self-assembly processes of two kinds of octahedron-shaped M6L4 cages consisting of cis-protected Pd(II) complexes and organic tritopic ligands were investigated. Whether kinetically trapped species larger than the cages are produced or the M6L4 cage is assembled without the formation of such kinetic traps is determined by a balance between the rates of oligomerization and intramolecular cyclization, which is affected by slight changes in the chemical structure of the tritopic ligand. A numerical analysis of the experimental data based on a reaction network model where 249 reactions between the possible 56 species were considered revealed the self-assembly pathways of one of the two M6L4 cages.
Abstract A kinetic trap is the metastable species that is transiently or constantly produced during the reaction by trapping in a deep energy well. In most cases, the reactivity of kinetically trapped species is relatively low under the reaction conditions. Herein, we report another type of kinetically trapped species that is an incomplete cage ( IC ) intermediate produced during the self‐assembly of a Pd 2 L 4 cage from ditopic ligand (L) and Pd II ions with a certain lifetime, although IC has a high enough reactivity to be converted into the cage with the reaction of free L, which was confirmed by the reaction of the isolated IC and L under the self‐assembly conditions. IC was kinetically trapped not because IC lies on the bottom of a deep energy well but because the conversion of the intermediates essential for the conversion of IC to the cage preferentially takes place; IC was kinetically trapped independently of the shape of the energy landscape of the self‐assembly.
Abstract Molecular dynamics (MD) simulations in aqueous solution were performed for hexameric nanocubes of methylated (16) and demethylated (26) gear-shaped amphiphiles. To elucidate the difference between these nanocubes, the principal component analysis for the MD simulation results in aqueous solution and the normal mode analysis in the gas phase were also applied to the nanocube, 16 and 26. The structural fluctuation of the lowest-frequency motion on 26 is larger than that on 16 around the triple π stacking of 3-pyridyl groups. For the structural stability and fluctuation of the nanocube, interactions among the constituent gear-shaped molecules play more important roles than the solvophobic effect due to solvent molecules.
Abstract Sensing systems of nonpolar gas molecules without functional groups such as natural gas and liquefied petroleum gas (LPG) remain difficult to develop because of lacking selective detection of such molecules over other gas molecules. Here we report a supramolecular fluorescence sensor for LPG using a 2-nm-sized cube-shaped molecular container i.e. a nanocube self-assembled from six molecules of gear-shaped amphiphiles (GSA) in water. The nanocube selectively encapsulates LPG, while it does not bind other gas molecules. Upon encapsulation of LPG in the nanocube, the fluorescence from the nanocube is enhanced by 3.9 times, which is caused by the restricted motion of the aromatic rings of GSA in the nanocube based on aggregation-induced emission. Besides the high selectivity, high sensitivity, quick response, high stability of the nanocube for LPG, and easy preparation of GSA satisfy the requirement for its practical use for an LPG sensor.
The self-assembly process of a Pd4L8 double-walled square (DWS) was investigated. As was seen in Pd2L4 cages, Pd(II)-linked coordination self-assembly processes are generally affected by the rigidity of multitopic ligands. However, the self-assembly of a Pd4L8 DWS from rigid ditopic ligands took place with the formation of two kinds of metastable species [submicrometer-sized species and a Pd3L6 double-walled triangle (DWT)]. This result suggests that the self-assembly process of the DWS is largely affected by the geometry of the final product and not by that of the ditopic ligand.
The self-assembly process of supramolecular coordination rings composed of dinuclear PtII complexes and organic ligands was investigated by an NMR-based quantitative approach (QASAP: quantitative analysis of the self-assembly process), which enabled the monitoring of the growth of oligomers during the self-assembly. Even though the oligomers could adopt various conformations, the macrocycles were assembled without the generation of the oligomers containing more components than the macrocycles. More information can be found in the Full Paper by S. Hiraoka et al. on page 838. The self-assembly process of supramolecular coordination rings composed of dinuclear PtII complexes and organic ligands was investigated by an NMR-based quantitative approach (QASAP: quantitative analysis of the self-assembly process), which enabled the monitoring of the growth of oligomers during the self-assembly. Even though the oligomers could adopt various conformations, the macrocycles were assembled without the generation of the oligomers containing more components than the macrocycles. More information can be found in the Full Paper by S. Hiraoka et al. on page 838.
