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Electronic Tuning of Mixed Quinoidal‐Aromatic Conjugated Polyelectrolytes: Direct Ionic Substitution on Polymer Main‐Chains

: We introduce the direct ionic substitution strategy of generating strongly electron-accepting ionic motifs and showcase its utility by synthesizing and characterizing a series of ionic azaquinodimethane (iAQM) compounds. The facile introduction of cationic substituents onto the quinoidal para-azaquinodimethane (AQM) core gives rise to a strongly electron-accepting building block, which can be readily employed in the synthesis of ionic small molecules and conjugated polyelectrolytes (CPEs). A succinct synthesis utilizing low-cost reagents affords a wide variety of functionality in these mixed aromatic-quinoidal systems that are otherwise difficult to access. Electrochemical measurements alongside theoretical calculations indicate notably low-lying LUMO values for the iAQM species. The optical band gaps measured for these compounds are highly tunable based on structure—ranging from 2.30 eV in small molecules down to 1.22 eV in polymers. The iAQM small molecules and CPEs showcase the effects of combining the donor-acceptor strategy and the bond-length alternation reduction strategy of band gap reduction. To make use of their notable optical properties, the application of the iAQM CPEs as active agents in photothermal therapy was also demonstrated. rings. iAQM polymers P1 and P2 show broad absorbances spanning parts of the visible and NIR portions of the electromagnetic spectrum, with an absorption edge at λ = 1010 nm and 940 nm, respectively. The corresponding optical band gaps conjugated ring system being used as a building block in low band gap CPEs. The synthesis of the iAQMs presented herein features high overall yields, functional group versatility, as well as the use of low-cost and widely-available reagents. Optical, electrochemical, and theoretical studies revealed that the ionic substituents endow the iAQM molecules with low-lying LUMO energy levels as well as small and highly tunable band gaps. The efficacy shown above in tuning the energetics of a quinoidal conjugated polyelectrolyte system using direct ionic substitution reveals future opportunities in using this strategy to generate more functional ionic semiconductors with desirable optoelectronic properties. Furthermore, the versatile reactivity of the AQM ditriflates towards nucleophilic displacement and metal-catalyzed cross-coupling is currently being pursued. Conjugated Polyelectrolytes are a class of conjugated polymers with ionic substituents. We demonstrate the efficacy of a strategy for forming low band-gap conjugated polyelectrolytes via direct ionic substitution of a mixed quinoidal-aromatic polymer. We also show that these materials display high activity in photothermal therapy applications.