Guest-responsive covalent frameworks by the cross-linking of liquid-crystalline salts: tuning of lattice flexibility by the design of polymerizable units

Cross-linked polymers prepared by the in-situ polymerization of liquid-crystalline salts were found to work as solid-state hosts with a flexible framework. As a component of such hosts, four kinds of polymerizable amphiphilic carboxylic acids bearing alkyl chains with acryloyloxy (A), dienyl (D), and/or nonreactive (N) chain ends (monomeric carboxylic acids; M(AAA), M(ANA), M(DDD), and M(DND)) were used. The carboxylic acids were mixed with an equimolar amount of a template unit, (1R,2S)-norephedrine (guest amine; G(RS)), to form the corresponding salts. Every salt exhibited a rectangular columnar LC phase at room temperature, which was successfully polymerized by (60)Co γ-ray-induced polymerization without serious structural disordering to afford the salt of cross-linked carboxylic acid (polymeric carboxylic acid; P(AAA), P(ANA), P(DDD), and P(DND)) with G(RS) . Owing to the noncovalency of the interactions between the polymer framework P and the template G(RS), the cross-linked polymers could reversibly release and capture a meaningful amount of G(RS). In response to the desorption and adsorption of G(RS), the cross-linked polymers dramatically switched their nanoscale structural order. A systematic comparison of the polymers revealed that the choice of polymerizable groups has a significant influence on the properties of the resultant polymer frameworks as solid-state hosts. Among these polymers, P(DDD) was found to be an excellent solid-state host, in terms of guest-releasing/capturing ability, guest-recognition ability, durability to repetitive usage, and unique structural switching mode.     

Convergent assembly of the spiroacetal subunit of didemnaketal B

A highly convergent synthesis of the C9-C28 spiroacetal subunit of didemnaketal B has been accomplished. Assembly of the C9-C15 alkylborate and C16-C21 enol phosphate by means of Suzuki-Miyaura coupling and acid-catalyzed cyclization of the derived dihydroxy enol ether enabled a rapid and efficient construction of the spiroacetal subunit. The C22-C28 side chain was incorporated via Nozaki-Hiyama-Kishi coupling to complete the synthesis.     

2,2′-Tethered Binaphthyl-Embedded One-Handed Helical Ladder Polymers: Impact of the Tether Length on Helical Geometry and Chiroptical Property

Synthetic breakthroughs diversify the molecules and polymers available to chemists. We now report the first successful synthesis of a series of optically-pure 2,2′-tethered binaphthyl-embedded helical ladder polymers based on quantitative and chemoselective ladderization by the modified alkyne benzannulations using the 4-alkoxy-2,6-dimethylphenylethynyl group as the alkyne source, inaccessible by the conventional approach lacking the 2,6-dimethyl substituents. Due to the defect-free helix formation, the circular dichroism signal increased by more than 6 times the previously reported value. The resulting helical secondary structure can be fine-tuned by controlling the binaphthyl dihedral angle in the repeating unit with variations in the 2,2′-alkylenedioxy tethering groups by one carbon atom at a time. The optimization of the helical ladder structures led to a strong circularly polarized luminescence with a high fluorescence quantum yield (28 %) and luminescence dissymmetry factor (2.6×10⁻³).     

Unique guest-inclusion properties of a breathing ionic crystal of K3[Cr3O(OOCH)6(H2O)3][alpha-SiW12O40].16 H2O

A microstructured ionic crystal, K(3)[Cr(3)O(OOCH)(6)(H(2)O)(3)][alpha-SiW(12)O(40)].16 H(2)O (1) was synthesized by the complexation of the Keggin-type polyoxometalate of [alpha-SiW(12)O(40)](4-) with a macrocation of [Cr(3)O(OOCH)(6)(H(2)O)(3)](+). Compound 1 possessed a straight channel, with an opening of approximately 0.5x0.8 nm, which contained the water of crystallization. The use of the macrocation with large size (0.7 nm) and small charge (+1) reduced the anion-cation interaction and was essential for the channel formation. The molecular structures of the polyoxometalate and the macrocation in 1 were retained under vacuum at 473 K. Analogues of 1 were synthesized with [alpha-PVW(11)O(40)](4-) or [Fe(3)O(OOCH)(6)(H(2)O)(3)](+). The water of crystallization in 1 was removed under vacuum at room temperature to form the closely packed guest-free phase 2. Compound 2 reversibly and repeatedly included water and polar organic molecules with two carbon atoms or less. Guest inclusion was highly selective and a difference of even one methylene group in the organic guest molecule was discriminated by the host. Polar organic molecules with longer methylene chains and nonpolar molecules such as dinitrogen and methane were completely excluded. The guest-inclusion properties could be explained by the ion-dipole interaction between the host and the guest, which is proportional to the dipole moment of the guest molecule and inversely proportional to the ion-dipole (host-guest) distance. Thus, small polar molecules were selectively absorbed. These distinctive guest-inclusion properties were successfully applied to the oxidation of methanol from a mixture of C(1) and C(2) alcohols. These results show unique guest inclusion and catalysis by rationally designed ionic crystals.     

Effect of core-shell micelle formation on the redox properties of phenothiazine-labeled poly(ethyl glycidy ether)-block-poly(ethylene oxide)

Redox properties of phenothiazine-labeled poly(ethyl glycidy ether)-block-poly(ethylene oxide) (PT-EGE_n-b-EO_m) are reversibly changed by core-shell micelle formation. In the temperature range higher than the critical micellization temperature (cmt), the anodic potential of PT group positively shifts and concomitantly its anodic current decrease, or levels off compared to those of the reference polymer PT-EO_m without the thermo-responsive EGE_n segment. The former alteration is caused by incorporation of hydrophobic PT groups into a core of the micelle and the latter by the decrease in the diffusion coefficient of PT groups due to formation of the core-shell micelles. The cmt value and the temperature-dependent alteration in the redox properties strongly depend on the polymer structure, especially the length of thermo-responsive EGE_n segment. The electrochemically determined hydrodynamic radii of the polymer aggregates seem to be overestimated, compared to the values reported for the aggregates of other thermo-responsive polymers with similar molecular weights, implying the presence of electrochemically inactive PT groups in the copolymers having longer thermo-responsive segments.     

Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance

We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole (HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density in the colored state. The thermal bleaching rate (tau1/2 = 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.