articles in 2019

The signs of circularly polarized luminescence and circular dichroism of binaphthyl-pyrene fluorophores with the same axial chirality can be controlled by altering the bonding positions of the two fluorescent pyrene units in the solution state. The relative geometrical arrangement of the two pyrene rings plays a substantial role in determining the signs of the observed chiroptical properties.

The intramolecular asymmetric photocycloaddition between 1,4-naphthoquinone and vinyl ether with a small (R)-point-chiral group on the tether was studied. Under photoirradiation, [2+2] cyclobutane products were exclusively obtained for this intramolecular system. The effect of solvent polarity on the stereoselectivity was significant, with predominant formation of the (2R,S3) over the (2R,R3) isomer in non-polar solvents being reversed in slightly polar dichloromethane. The detailed temperature-dependent study revealed the dominant diastereo-differentiating processes, which were switched between the ground-state equilibrium, the relative rate of bond formation in the triplet manifold, as well as deactivation processes between the pro-(S3) and pro-(R3) precursors, depending on the temperature domain examined. The enthalpic contribution (ΔΔH≠) was always compensated by the entropic factor (ΔΔS≠), implying the importance of solvation on the diastereo-differentiation steps. The mechanism of photocycloaddition, especially for the face-selective processes, is thoroughly discussed, which is supported by quantum chemical calculations on the ground-state circular dichroism (CD) spectral behavior as well as on the diastereomeric transition states in the triplet excited state.

The intramolecular asymmetric photocycloaddition between 1,4-naphthoquinone and vinyl ether with a small (R)-point-chiral group on the tether was studied. Under photoirradiation, [2+2] cyclobutane products were exclusively obtained for this intramolecular system. The effect of solvent polarity on the stereoselectivity was significant, with predominant formation of the (2R,S3) over the (2R,R3) isomer in non-polar solvents being reversed in slightly polar dichloromethane. The detailed temperature-dependent study revealed the dominant diastereo-differentiating processes, which were switched between the ground-state equilibrium, the relative rate of bond formation in the triplet manifold, as well as deactivation processes between the pro-(S3) and pro-(R3) precursors, depending on the temperature domain examined. The enthalpic contribution (ΔΔH≠) was always compensated by the entropic factor (ΔΔS≠), implying the importance of solvation on the diastereo-differentiation steps. The mechanism of photocycloaddition, especially for the face-selective processes, is thoroughly discussed, which is supported by quantum chemical calculations on the ground-state circular dichroism (CD) spectral behavior as well as on the diastereomeric transition states in the triplet excited state.

A unique and effective interaction between the peripheral aromatic blades makes hexaarylbenzenes (HABs) attractive in fundamental research as well as for various applications such as molecular wires, sensors, and supramolecular assemblies. The chiroptical responses of HABs are susceptible to environmental factors such as solvent and temperature owing to the dynamic conformational transitions between the conformers. In this study, pressure dependence on the propeller chiral HABs in two different solvents was studied in detail. The effective differential volumes for two different equilibria were determined by quantitative analyses of CD spectra, affording very large differential volumes from the propeller to toroidal conformer (ΔVT-C) of +43 and +42 cm3 mol−1, for H2 and H6, respectively, in methylcyclohexane. The value of H6 was further enhanced to +72 cm3 mol−1 in hexane, the largest value for the typical unimolecular conformational change. Such a response of propeller chirality in HABs is expedient in designing more advanced piezo-sensitive materials.

Oxidation of β-to-β directly linked and sulfur-bridged meso-amino NiII-porphyrin dimers with PbO2 gave helical tetrapyrrin (biliverdin analogue)-fused NiII-porphyrins. These ring cleaving reactions differ markedly from the previously reported oxidation of a β–β linked NiII-porphyrin dimer carrying one amino group, which gave an azepine-fused porphyrin dimer. The tetrapyrrin-fused NiII-porphyrins display intense NIR absorption bands at 1200–1400 nm and reversible redox processes because of the highly π-conjugated networks and rigid structures. These tetrapyrrin-fused NiII-porphyrins were separated to stable enantiomers, which showed clear Cotton effects in their CD spectra with Δϵ of 102 order.

Stereoisomeric β-cyclodextrin (CD) dimers linked with a sulfur atom or an arene spacer were designed to create a tethered dual CD capsule for precisely manipulating the regio- and enantioselectivities of the photocyclodimerization of 2-anthracenecarboxylate (AC) to four stereoisomeric classical 9,10:9′,10′-cyclodimers and two nonclassical 5,8:9′,10′-cyclodimers. Among the dimeric CD hosts prepared, exo-3-thia-β-CD dimer formed 1:1 and 1:2 host–guest complexes with AC in aqueous solutions, the former of which hindered but the latter facilitated the AC photocyclodimerization with regio- and enantioselectivities much higher than those obtained with native β-CD or the rest of the β-CD dimers. The stereochemical outcomes turned out to be highly sensitive to and hence critically manipulable by the linking position and configuration of the connected saccharide units and the linker length, as well as the external variants, such as temperature, pH, and added salt. Eventually, the photocyclodimerization of AC mediated by the dimeric β-CD host gave enantiopure syn-head-to-tail-9,10:9′,10′-cyclodimer in 97–98% yield in a pH 5.1 buffer solution at 0.5 °C and also in an aqueous CsCl solution at −20 °C.