Mixing of aromatic (A) and quinonoid (Q) units has not only proved to be an effective synthetic strategy in designing polymers with delocalized electrons, it often yields a low band gap system as well.[1] While many conjugated polymers display A-ground state, only a few examples of Q-ground state are known. In our approach, in designing low band gap polymers, we have computationally studied more than two dozen heteroaromatic molecules with repeat units (up to n=5) of potential polymers with a Q-ground states which are synthetically attainable. Among various monomer repeat units explored, eight systems which energetically favors Q-ground states compared to their A isomeric structures were successfully obtained using density functional theory (DFT); one is illustrated below. Some of these systems are found to possess low band gaps. These Q-ground states systems were further characterized using inter-ring bond lengths parameters. The systems with 6-membered ring forming a π-conjugated backbone failed to energetically stabilize the Q-ground states while many examples of stable Q-ground states of 5-membered ring in the backbone were obtained.[2] Quinonoid systems often show unusual open-shell diradical characteristics exhibiting interesting electronic and optical properties.[3] From unrestricted open-shell DFT calculations on quinonoid repeat units of polymers under study, terminated with hydrogen atoms at both ends, we computed y<sub>0</sub>, a measure of diradical character. Only the Q-ground states systems and a few A-systems showed diradical character. Further, the diradical character increases with increasing size (typically converging for n &gt; 10). Mixing the Q-ground state systems with the A-ground state systems under study will be the next step with the expectation that some of these combinations will yield Q-ground state polymers with high diradical characters and very low band gaps.

Designing pi-conjugated polymers with quinonoid repeat units

ACS Spring Meeting 2021

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