Sublevel-Selective Electron Spin Hyerpolarization of Triplet Radical Pairs in Metal–Organic Frameworks

Molecular spin systems are promising for spin-based technologies due to their tunable properties and compatibility with bottom-up assembly. Photogenerated radical pairs (RPs) exhibit spin hyperpolarization even under ambient conditions, making them valuable for quantum information science (QIS) and dynamic nuclear polarization (DNP). However, most studies have focused on isolated molecules in dense matrices, limiting their applicability. Here, we propose a new approach for integrating RPs into a metal-organic framework (MOF) as higher-order assemblies, enabling regulated generation of spin polarization at specific spin sublevels. An electron acceptor (A), N,N'-Di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxdiimide (DPNDI), was incorporated into MOF-525 (Zn) composed of an electron donor (D), Zn-tetrakis(4-carboxyphenyl)porphyrin (ZnTCPP), yielding MOF-525 (Zn)-DPNDI. The dense packing of D-A pairs enables efficient quenching of the singlet excited state of ZnTCPP, leading to the regulated photophysical pathway to form strongly-coupled triplet radical pairs (TRPs) with selective T0 state spin polarization. TRPs exhibit narrower EPR spectra than triplet states generated via intersystem crossing (ISC), which offers significant advantages for enhancing spectral addressability in QIS and improving efficiency of polarization transfer in DNP. The rigid intercalating structure of D-A in MOF prevents the relaxation of T0 state spin polarization even in the presence of various guest solvents and its spin-lattice relaxation time is modulated by adsorbed guest molecules. This work demonstrates a molecular framework capable of generating long-lived, sublevel-specific spin polarization, offering a scalable platform for quantum sensing and guest-targeted DNP.