Donor–acceptor (DA) conjugated polymers (CPs) with narrow bandgaps and open-shell character represent an emerging class of materials whose rich categories of behavior emanate from the collective properties of π-electrons and diminished intramolecular electron pairing. However, the structural and electronic heterogeneities that define these materials complicate bandgap control at low energies, and connections linking topology, exchange interactions, and (opto)electronic functionality remain nascent.We synthesized open-shell donor–acceptor copolymers comprised of a solubilizing thiadiazoloquinoxaline (TQ) acceptor and alternating cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) or dithieno[3,2-b:2',3'-d]thiophene (DTT) donors, which exhibit highly rigid backbones. Electron paramagnetic resonance and magnetic susceptibility measurements, complemented by quantum chemical calculations, demonstrate dramatic differences in the structural, electronic, topological, and magnetic structure as a result of atom-specific substitution. The CPDT–TQ copolymer demonstrates extensive π-conjugation with a quinoidal bonding pattern, a very narrow optical bandgap of less than 0.1 eV, and a high-spin ground state singlet-triplet splitting (ΔEST) of 7.17 × 10−3 kcal mol–1. Substitution of electron-rich sulfur at the donor bridgehead position in DTT–TQ results in diminished π-conjugation, an aromatic bonding pattern, and a low-spin ground state with ΔEST of –5.24 kcal mol–1. This work provides a framework to rationally understand the evolution of the electronic structure in open-shell DA CPs, control the ground state electronic structure, tune distance-dependent spin–spin coupling, and articulate the emergence of the novel properties of these materials.

Topology and Ground State Control in Open-Shell Donor–Acceptor Conjugated Polymers

ACS Spring Meeting 2021

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