Spin-Polarized Radicals with Extremely Long Spin-Lattice Relaxation Time at Room Temperature in a Metal-Organic Framework

Abstract

The generation of spin polarization is key in quantum information science and dynamical nuclear polarization. Polarized electron spins with long spin-lattice relaxation times (T1) at room temperature are important for these applications, but have been difficult to achieve. We report the realization of spin-polarized radicals with extremely long T1 at room temperature in a metal-organic framework (MOF) in which azaacene chromophores are densely integrated. Persistent radicals are generated in the MOF by charge separation after photoexcitation. Spin polarization of triplet generated by photoexcitation are successfully transferred to the persistent radicals. Pulse ESR measurements reveal that the T1 of the polarized radical in the MOF is as long as 274 s at room temperature. The achievement of extremely long spin polarization in MOFs with nanopores accessible to guest molecules will be an important cornerstone for future highly sensitive quantum sensing and efficient dynamic nuclear polarization.

Radical qubits photo-generated in acene-based metal–organic frameworks

“…The nanoporous structures and the interaction between the host electron spins and the guest molecules offer a promising platform for quantum sensing in the fields of chemistry and biology. [1][2][3][4][5][6] Several kinds of porous systems have been proposed, such as metal-organic frameworks (MOFs) containing a paramagnetic metal centre, 1,[7][8][9][10] MOFs and covalent-organic frameworks (COFs) with organic radicals, 6,[11][12][13][14][15] and chromophore triplets in MOFs. 16 A key challenge in applying these materials to quantum science is to extend the coherence time or spin-spin relaxation time (T 2 ), which is the lifetime of the superposition quantum states.…”

“…We have recently developed a dense chromophore-integrated MOF, which exhibits photoinduced radicals with significantly long spin-lattice relaxation time (T 1 ) and T 2 values even at room temperature. 6 Our previous MOF system (DAT-MOF-1) consists of a ligand containing the 5,12-diazatetracene (DAT) chromophore 20 and the 4,4′-biphenyldicarboxylic acid (BPDC) coligand bound to Zn-paddlewheel units. The co-ligand and Zn paddlewheels are considered to be redox-inert species.…”

“…In our previous report, the typical aromatic radical g value of 2.0023 was observed in DAT-MOF-1. 6,30 The observed radicals were assumed as oxidized/reduced DAT molecules formed by photoinduced charge separation. We observed similar values for DAT-MOF-2 (g = 2.0018) and DAT-MOF-3 (g = 2.0023) (Fig.…”

“…In addition to the ligand-derived absorption band below 600 nm, a new broad absorption band between 600-900 nm appeared immediately after laser irradiation, and its intensity decreased after 12 h in the dark. 6,31 Since the new broad absorption band also appeared in the chemically oxidized DAT-MOF-1, the generation of the DAT cation radical by photoirradiation was indicated. Fig.…”