Multi-input–Multi-output Molecular Response System Based on Dynamic Redox Behavior of Hexaphenylethane-type Electron Donors with the Tetrahydrophenanthrazepine Skeleton: Strong Chiroptical Signals through the Transmission of Point Chirality to Helicity

Redox-active tetraaryldibenzoquinodimethanes

Quinodimethanes (QDs) are a class of non-aromatic π-conjugated compounds that are well-known to be interconvertible scaffolds in many response systems. While parent ortho- and para-QDs (o-QD and p-QD) can be easily converted to benzocyclobutenes or oligomers/polymers by the formation of C-C bonds at α-positions, the attachment of four phenyl groups to these reactive sites makes o-Ph₄QD and p-Ph₄QD long-lived. We have demonstrated that further dibenzo-annulation of such tetraaryl QD units also drastically increases their stability, and many tetraarylated dibenzoquinodimethane derivatives have been developed. This Feature Article shows our milestones in creating functional redox systems, where drastic changes in structure occur upon electron transfer (dynamic redox "dyrex" behaviour).

ECD spectroelectrochemistry: A review

The electronic circular dichroism (ECD) spectroscopy is probably the most important chiraloptical method, and the role of chirality in contemporary chemistry, pharmacy, and material science constantly increases. On the other hand, the electrochemical methods are also very sensitive tools for studying multivarious redox processes. Nevertheless, the first ECD spectroelectrochemical (SEC) study was only published by Daub, Salbeck and Aurbach in 1988, and since then, the ECD SEC method has been mentioned in only thirty papers. By the summer of 2020, the ECD SEC studies were mainly focused around molecular systems for organic, and marginally, inorganic chiroptical switching studies of biochemical redox reactions. The review provides more details about the ECD SEC studies carried out so far. At the end, we suggest some future applications for the ECD spectroelectrochemistry.

Hash-Mark-Shaped Azaacene Tetramers with Axial Chirality

The tetramers of azapentacene derivatives with unique hash mark (#)-shaped structures were prepared in a quite facile manner. The #-shaped tetramers are optically active due to possessing extended biaryl skeletons, and the structure of the tetramer composed of four dihydrodiazapentacene (DHDAP) units (1) was investigated as the first example of this kind of molecule. The tetramer 1 showed characteristic chiroptical properties reflecting its orthogonally arranged quadruple DHDAP moieties, as well as redox activity. The solution of enantiopure 1 exhibited intense circularly polarized luminescence (CPL) with a dissymmetry factor of 2.5 × 10^-3. The absolute configuration of the enantiomers of 1 was experimentally determined by X-ray crystal analysis for the dication salt of the enantiomer of 1 with SbCl₆^- counterions. The solutions of enantiopure 1²⁺·2[SbCl₆^-] also showed NIR circular dichroism (CD) spectra over the entire range from visible to 1100 nm, enabling the modulation of the chiroptical properties by redox stimuli.

Two-way chromic interconversion of the 2,2′-biphenol-6,6′-diyl dication with 5H,10H-dioxapyrene or 9H,10H-4,5-dihydroxyphenanthrene

Two-proton or two-electron transfer of the title biphenolic dication proceeds nearly simultaneously to induce 2,6′/2′,6- or 6,6′-bond formation to give dioxapyrene or dihydrophenanthrene derivatives, respectively, with vivid changes in color.

Drastic change in racemization barrier upon redox reactions: novel chiral-memory units based on dynamic redox systems

The helical configuration of dication dyes 2(2+) with a dihydrodibenzoxepin unit remained unchanged even at high temperature, whereas the corresponding neutral electron donors 1 with a tetrahydrophenanthroxepin skeleton easily underwent racemization. Due to their electrochemical bistability, electron exchange between 1 and 2(2+) is prohibited. Thus, the above electrochromic pairs can serve as novel chiral-memory units where redox reactions trigger switching between an "erasable/writable"-state (1) and a "memorizing"-state (2(2+)).

Ultralong carbon-carbon bonds in dispirobis(10-methylacridan) derivatives with an acenaphthene, pyracene, or dihydropyracylene skeleton

Acenapthalene, pyracene, and dihydropyracylene attached to two units of spiroacridan are a novel class of hexaphenylethane (HPE) derivatives that have an ultralong Csp3-Csp3 bond (1.77-1.70 A). These sterically challenged molecules were cleanly prepared by C-C bond formation through two-electron reduction from the less-hindered dications. These ultralong bonds were realized based on several molecular-design concepts including enhanced "front strain" through "multiclamping" by means of fusing or bridging aryl groups in the HPE molecule. The lengths of these ultralong bonds and their relation to the conformation (torsional angle) were also validated by means of theoretical calculations. Bond-fission experiments revealed that the bonds are more easily cleaved than standard covalent bonds to produce the corresponding dication upon oxidation with an increase in the length of the C-C bond.

Ultralong C-C bonds in hexaphenylethane derivatives

The longer C-C bond than the standard (1.54 Å) is so weakened that it is cleaved easily, as found in the parent hexaphenylethane (HPE). However, the compounds with an ultralong C-C bond (1.75 Å) can be isolated as stable solids when the bond-dissociated species does not undergo any reactions other than bond reformation. This is the central point in designing the highly strained HPEs, which were obtained by two-electron reduction of the corresponding dications. Steric repulsion of "front strain" is the major factor to expand the central C-C bond of HPEs. During the detailed examination of the ultralong C-C bond, the authors discovered the intriguing phenomenon of "expandability": the C-C bond length can be altered over a wide range by applying only a small amount of energy (1 kcal mol-1) supplied by crystal packing force. This observation indicates that the much longer C-C bond than the shortest nonbonded contact (1.80 Å) will be realized under the rational molecular design concept.