p-Azaquinodimethane: A Versatile Quinoidal Moiety for Functional Materials Discovery

Conspectus The past50 years of discoveryin organic electronics have beendriven in large part by the donor-acceptor design principle,wherein electron-rich and electron-poor units are assembled in conjugationwith each other to produce small band gap materials. While the utilityof this design strategy is undoubtable, it has been largely exhaustedas a frontier of new avenues to produce and tune novel functionalmaterials to meet the needs of the ever-increasing world of organicelectronics applications. Its sister strategy of joining quinoidaland aromatic groups in conjugation has, by comparison, received muchless attention, to a great extent due to the categorically poor stabilityof quinoidal conjugated motifs. In 2017 though, the p-azaquinodimethane (AQM)motif was first unveiled, which showed a remarkable level of stabilitydespite being a close structural analogue to p-quinodimethane,a notably reactive compound. In contrast, dialkoxy AQM small moleculesand polymers are stable even under harsh conditions and could thusbe incorporated into conjugated polymers. When polymerized with aromaticsubunits, these AQM-based polymers show notably reduced band gapsthat follow reversed structure-property trends to some of theirdonor-acceptor polymer counterparts and yield organic field-effecttransistor (OFET) hole mobilities above 5 cm(2) V-1 s(-1). Additionally, in an ongoing study, theseAQM-based compounds are also showing promise as singlet fission (SF)active materials due to their mild diradicaloid character. Anexpanded world of AQMs was accessed through their ditriflatederivatives, which were first used to produce ionic AQMs (iAQMs) sportingtwo directly attached cationic groups that significantly affect theAQM motif's electronics, producing strongly electron-withdrawingquinoidal building blocks. Conjugated polyelectrolytes (CPEs) createdwith these iAQM building blocks exhibit optical band gaps stretchinginto the near-infrared I (NIR-I) region and showed exemplary behavioras photothermal therapy agents. In contrast to these stableAQM examples, the synthetic explorationof AQMs also produced examples of more typical diradicaloid reactivitybut in forms that were controllable and produced intriguing and high-valueproducts. With certain substitution patterns, AQMs were found to dimerizeto form highly substituted [2.2]paracyclophanes in distinctly moreappreciable yields than typical cyclophane formation reactions. CertainAQM ditriflates, when crystallized, undergo light-induced topochemicalpolymerization to form ultrahigh molecular weight (>10(6) Da) polymers that showed excellent performances as dielectric energystorage materials. These same AQM ditriflates could be used to producethe strongly electron-donating redox-active pentacyclic structure:pyrazino[2,3-b:5,6-b ']diindolizine(PDIz). The PDIz motif allowed for the synthesis of exceedingly smallband gap (0.7 eV) polymers with absorbances reaching all the way intothe NIR-II region that were also found to produce strong photothermaleffects. Both as stable quinoidal building blocks and through theircontrollable diradicaloid reactivity, AQMs have already proven tobe versatile and effective as functional organic electronics materials.

Automated summary

In the past 50 years, the donor-acceptor design principle has played a significant role in the discovery of organic electronics. However, the alternative strategy of joining quinoidal and aromatic groups in conjugation has received less attention due to poor stability. The introduction of p-azaquinodimethane (AQM) in 2017 has shown remarkable stability and potential for different applications in organic electronics.