Engineering Paramagnetic Spin Centers in Chemically Doped Organic Semiconductor.

Tuning the density of paramagnetic spin centers (PSCs) in π-conjugated systems enables coherent quantum dynamics, controllable magnetism, and molecular spin qubits, thereby opening new frontiers in molecular-scale quantum technologies. We investigate the fundamental mechanisms governing spin pairing and PSC concentration in sp²-carbon-conjugated polymeric systems using a Hubbard-style Hamiltonian, modified to incorporate electrostatic interactions and static defects, in combination with combinatorial analysis and Monte Carlo simulations. By analyzing linear polymers, 2D Lieb-type monolayers, and π-stacked Lieb structures, we elucidate how the interplay of various quantum-mechanical interactions such as spin–spin repulsion, spin–anion attraction, and anion–anion repulsion along with structural features such as 2D π-connectivity, building-block design, and pore size, collectively influence the propensity for spin-paired bipolaron formation and, consequently, the concentration of PSCs. Our results reveal that π-stacked 2D organic frameworks intrinsically suppress spin pairing, thereby facilitating enhanced PSC densities compared to monolayers and linear polymers consistent with experimental observations. Building upon good agreement with experiments, we identify key design principles that provide experimentally actionable guidelines for engineering high-spin-density organic materials and lay the foundation for the rational design of metal-free magnets and spin-active semiconductors for next-generation organic spintronics and quantum information technologies.