7,7,8,8-Tetraaryl-o-quinodimethane Stabilized by Dibenzo Annulation: A Helical π-Electron System That Exhibits Electrochromic and Unique Chiroptical Properties

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.

Thermodynamically Stable o-Quinodimethane: Synthesis, Structure, and Reactivity

Thermal isomerization of cyclobutaphenanthrene to o-quinodimethane was investigated. Sterically congested substituents or electron-donating substituents on the four-membered ring promoted the ring-opening, affording o-quinodimethane in a relatively stable form. Isolation of the newly prepared o-quinodimethane allowed its structural elucidation and investigation of its potential reactivities. Dual [4+2] cycloaddition of an aryne and o-quinodimethane afforded tetrabenzopentacene, demonstrating the synthetic application of the isolated compound.

Tetrathiafulvalene-Inserted Diphenoquinone: Synthesis, Structure, and Dynamic Redox Property

A new tetrathiafulvalene (TTF) derivative is synthesized, which is substituted with two phenoxy radicals on one 1,3-dithiole ring, and may have either open-shell diradical or closed-shell extended-quinoidal ground states. X-ray single crystal analysis and NMR measurements prove that this molecule has a closed-shell extended quinoidal structure both in the solid state and in solution. DFT calculations show the donor-acceptor electronic properties of this molecule with a well-separated HOMO-LUMO distribution and a small HOMO-LUMO energy gap. Because of this donor-acceptor character, this molecule gives both the dication and the dianion species by electrochemical oxidation and reduction. Furthermore, during the redox process between the neutral and dication states, this molecule exhibits unique changes in the cyclic voltammogram upon repeating the cycles or varying the scan rate. The observed electrochemical behavior is explained by the conformational changes in the electrochemically generated species, thus indicating that this molecule is classified as a dynamic redox system.

Redox-Switchable Bis-fused Tetrathiafulvalene Analogue: Observation and Control of Two Different Reduction Processes from Dication to Neutral State

Derivatives of a new bis-fused donor composed of TTF and extended TTF with an anthraquinoid spacer (TTFAQ) (2) were successfully synthesized. X-ray structure analysis of the tetrakis(methylthio) derivative 2Aa and its I₃^- salt revealed that the TTFAQ moieties of both 2Aa and 2Aa^•+ adopt the so-called saddle conformation similar to most neutral TTFAQs. The results obtained from the X-ray structure analysis and cyclic voltammetry suggest that a positive charge in 2Aa^•+ is unevenly distributed on the TTF moiety, while both positive charges of 2²⁺ are mainly located on the TTFAQ moiety. In the first two-electron redox processes, an extra cathodic wave attributed to the coexistence of a different reduction process from the oxidation process was observed for most of the derivatives.

Zirconacyclopentadiene-Annulated Polycyclic Aromatic Hydrocarbons

Syntheses of large polycyclic aromatic hydrocarbons (PAHs) and graphene nanostructures demand methods that are capable of selectively and efficiently fusing large numbers of aromatic rings, yet such methods remain scarce. Herein, we report a new approach that is based on the quantitative intramolecular reductive cyclization of an oligo(diyne) with a low-valent zirconocene reagent, which gives a PAH with one or more annulated zirconacyclopentadienes (ZrPAHs). The efficiency of this process is demonstrated by a high-yielding fivefold intramolecular coupling to form a helical ZrPAH with 16 fused rings (from a precursor with no fused rings). Several other PAH topologies are also reported. Protodemetalation of the ZrPAHs allowed full characterization (including by X-ray crystallography) of PAHs containing one or more appended dienes with the ortho-quinodimethane (o-QDM) structure, which are usually too reactive for isolation and are potentially valuable for the fusion of additional rings by Diels-Alder reactions.

11,11,12,12-Tetracyano-4,5-pyrenoquinodimethanes: air-stable push-pull o-quinodimethanes with S2 fluorescence

The synthesis and properties of 11,11,12,12-tetracyano-4,5-pyrenoquinodimethanes (4,5-TCNPs), a new family of isolable and air-stable o-quinodimethanes, are reported. The ortho disposition of the dicyanomethane substituents strongly polarizes the pyrene framework to promote broad and intense intramolecular charge-transfer transitions. In addition, spectroscopic studies reveal that 4,5-TCNPs violate Kasha's rule and emit from the S2 level.

Electrochromic and unique chiroptical properties of helically deformed tetraarylquinodimethanes generated from less-hindered dicationic precursors upon reduction

Electron-donating 1,1,4,4-tetraarylbutadiene is a representative electrochromic dye, and the same chromophore can be found in 9,10-bis(diarylmethylene)-9,10-dihydrophenanthrene (dibenzo-oQD) albeit in a fixed s-cis geometry. Unlike thermodynamically unstable 7,7,8,8-tetraaryl-o-quinodimethane, spontaneous electrocyclization is prohibited by dibenzo-annulation. Several derivatives of dibenzo-oQD were successfully generated despite highly strained geometry caused by steric hindrance between the bulky diarylmethylene units. Their precursors are phenanthrene-9,10-diyl bis(diarylmethylium) dyes (PDM2+) stabilized by four electron-donating alkoxy groups. The redox pairs of dibenzo-oQD/PDM2+ exhibit vivid change in color upon redox interconversion (electrochromism). Both dibenzo-oQD and PDM2+ adopt a helical conformation, whose configuration is unstable. When a chiral alkoxy substituent is attached on each of the aryl groups, the point chirality is successfully transmitted to helicity of PDM2+. Resulting diastereomeric biasing is the key for dibenzo-oQD/PDM2+ to exhibit the two-way-output response (e.g., UV–vis and CD). In addition, much more strained quinodimethane derivatives, 1,2-bis(diarylmethylene)acenaphthene and 1,16-diaryldibenzo[b,n]perylene, were also generated from the dicationic precursors, demonstrating that the reductive transformation can serve as a useful protocol to generate severely deformed π-conjugated systems.