Synthesis and Self-Assembly of Chiral Cylindrical Molecular Complexes: Functional Heterogeneous Liquid-Solid Materials Formed by Helicene Oligomers

Chiral cylindrical molecular complexes of homo- and hetero-double-helices derived from helicene oligomers self-assemble in solution, providing functional heterogeneous liquid-solid materials. Gels and liotropic liquid crystals are formed by fibril self-assembly in solution; molecular monolayers and fibril films are formed by self-assembly on solid surfaces; gels containing gold nanoparticles emit light; silica nanoparticles aggregate and adsorb double-helices. Notable dynamics appears during self-assembly, including multistep self-assembly, solid surface catalyzed double-helix formation, sigmoidal and stairwise kinetics, molecular recognition of nanoparticles, discontinuous self-assembly, materials clocking, chiral symmetry breaking and homogeneous-heterogeneous transitions. These phenomena are derived from strong intercomplex interactions of chiral cylindrical molecular complexes.

[Synthesis, Aggregation, Self-assembly, and Dynamic Properties of Helicene Oligomers]

 Biological systems exhibit dynamic phenomena at the macroscopic level as a result of the hierarchical integration of phenomena at the molecular level. For example, a number of amino acids compose actin proteins, which form three-dimensional structures determined by the sequence of amino acids. They form fibers by self-assembly, which then form ordered structures such as meshes, lyotropic liquid crystals (LCs), and bundles. The dynamic and reversible polymorphism between these nano- to centimeter-sized ordered structures is essential for biological functions such as cell division, contraction, and locomotion. To understand biological systems and create new functional materials, it is essential to develop a methodology to integrate phenomena at the molecular level into those at the macroscopic level using synthetic molecules. In this research, synthetic oligomers containing helicenes, which exhibit reversible structural transitions between cylindrical double helices and random coils in response to thermal stimuli, were employed as building blocks for the development of such a methodology. The properties of homo- and hetero-double helices at the molecular level were first controlled by taking advantage of the diversity of their molecular structures. Then, nano- to micrometer-sized structures were constructed by the self-assembly of hetero-double helices, which include fibers/gels, vesicles, and lyotropic LCs, and their dynamic properties were controlled by molecular design.

Molecular switching involving metastable states: molecular thermal hysteresis and sensing of environmental changes by chiral helicene oligomeric foldamers

Molecular switching is a phenomenon in which the molecular structure reversibly changes in response to external stimulation. It is crucial in biology and is used in various biological sensing applications and responses. In contrast to the well-studied molecular switching involving two or more thermodynamically stable states, switching involving metastable states exhibits notable non-equilibrium thermodynamic properties. Synthetic chiral helicene oligomeric foldamers that exhibit molecular thermal hysteresis in dilute solution are examples. Molecular switching can be used for sensing environmental changes, including temperature threshold, temperature decrease/increase, rate of temperature decrease, counting the numbers 1 and 2, and concentration increase.

"Inverse" thermoresponse: heat-induced double-helix formation of an ethynylhelicene oligomer with tri(ethylene glycol) termini

Ethynylhelicene oligomers with TEG terminal groups showed a unique thermoresponse in aqueous solvents: double-helix formation upon heating and disaggregation upon cooling.

An ethynylhelicene oligomer [(M)-d-4]-C₁₂-TEG with six tri(ethylene glycol) (TEG) groups at the termini was synthesized, and double-helix formation was studied using CD, UV-Vis, vapor pressure osmometry, dynamic light scattering, and ¹H NMR. [(M)-d-4]-C₁₂-TEG reversibly changed its structure between a double helix and a random coil in response to heating and cooling in aromatic solvents, non-aromatic polar organic solvents, and aqueous solvent mixtures of acetone/water/triethylamine. Notably, [(M)-d-4]-C₁₂-TEG in acetone/water/triethylamine (1/2/1) formed a double helix upon heating and disaggregated into random coils upon cooling. The double helix/random coil ratio sharply changed in response to temperature changes. This is an unprecedented “inverse” thermoresponse, which is opposite to the “ordinary” thermoresponse in molecular dimeric aggregate formation. This phenomenon was explained by the dehydration of the terminal TEG groups and the formation of condensed triethylamine domains upon heating.

Energy Aspects of Thermal Molecular Switching: Molecular Thermal Hysteresis of Helicene Oligomers

Molecular switching is a phenomenon by which a molecule reversibly changes its structure and state in response to external stimuli or energy. Herein, molecular switching is discussed from thermodynamic and kinetic aspects in terms of energy supply with an emphasis on the thermal switching exhibited by helicene oligomers. It includes the inversion of relative thermodynamic stability induced by temperature changes and molecular thermal hysteresis in a closed system. The thermal phenomenon associated with the oligomers involves population/concentration changes between metastable states under nonequilibrium thermodynamic control.

Double helix formation of oligoresorcinols in water: thermodynamic and kinetic aspects

We previously reported that the oligoresorcinols formed double-stranded helices in neutral water through interstrand aromatic interactions. In the present study, we synthesized a new series of oligomers from the 2mer to the 15mer to explore the thermodynamics, kinetics, and mechanism of the double helix formation of the oligoresorcinols in water. The double helix formation was dependent on the chain length of the oligomers and significantly affected by solvent, pH, salt, and temperature. The free energy change (-DeltaG) for the double helix formation linearly increased with the chain length from the 4mer to the 11mer (DeltaDeltaG = -0.94 kcal mol(-1) unit(-1)), whereas it did not change for the oligomers longer than the 11mer. The van't Hoff analysis of the 9mer revealed that the double helix formation was an enthalpically driven process (DeltaH = -27 +/- 1.5 kcal mol(-1) and DeltaS = -70 +/- 5 cal mol(-1) K(-1)), which was consistent with the upfield shifts in the (1)H NMR spectra and the hypochromicity of the absorption spectra as a result of the interstrand aromatic interactions in water. Furthermore, the kinetic analysis of the chain exchange reaction between the double helices of the optically active and optically inactive 11mers revealed a small DeltaS(double dagger), suggesting that the chain exchange proceeds not via the dissociation-association pathway, but via the direct exchange pathway.

Sequence- and chain-length-specific complementary double-helix formation

The artificial sequential strands consisting of two, three, or four m-terphenyl groups joined by diacetylene linkers with complementary binding sites, either the chiral amidine (A) or achiral carboxyl (C) group, were synthesized in a stepwise manner. Using circular dichroism and (1)H NMR spectroscopies along with liquid chromatography, we showed that, when three dimeric molecular strands (AA, CC, and AC) or six trimeric molecular strands (AAA, CCC, AAC, CCA, ACA, and CAC) were mixed in solution, the complementary strands were sequence-specifically hybridized to form one-handed double-helical dimers AA.CC and (AC) 2 or trimers AAA.CCC, AAC.CCA, and ACA.CAC, respectively, through complementary amidinium-carboxylate salt bridges. Upon the addition of CCA to a mixture of AAA, AAC, and ACA, the AAC.CCA double helix was selectively formed and then isolated from the mixture by chromatography. Moreover, the homo-oligomer mixtures of amidine or carboxylic acid from the monomers to tetramers (A, AA, AAAA, C, CC, and CCCC) assembled with a precise chain length specificity to form A.C, AA.CC, and AAAA.CCCC, which were separated by chromatography.

Double-stranded helical polymers consisting of complementary homopolymers

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.