Biomimetic engineering spin relaxation in flavin radical pairs for singlet–triplet qubits

Abstract

Non-adiabatic spin relaxation in the flavin spin-correlated radical pairs (SCRPs), which amounts to a spin flip-flop essentially, is addressed for tailoring their coherence times towards biomimetic molecular spin qubits, the paramagnetic systems that exhibit long spin-coherence times. Their molecular electron-spin counterparts, the single-molecule magnet (SMM) and free radical qubits, operate with an active spin of an unpaired electron in paramagnetic molecules of coordinated block-metal complexes (CMCs) and stable free radicals, respectively. Parental biological cofactors, such as CMCs (Ln3+-based, heme b, cobalamin) and flavins (riboflavin, flavin mononucleotide, flavin adenine dinucleotide), use unpaired spins of central metal and flavin semiquinone free radicals for vital reactions in living nature. Moreover, stable free radicals of flavin semiquinones pave a new way for the design of optically initiated singlet–triplet qubits with superposed spin states 0/1. This is due to their coupling in radical pairs by spin correlations as a result of blue light-generated spin-selective electron transport. Artificially designed CMCs with both quantum spin tunneling and magnetic irreversibility below blocking paramagnetic temperature, the fingerprint of SMM, were studied over 3 decades, being already incorporated in metal-organic frameworks and scaffolds. Long-lived flavin SCRPs gained interest as metal-free qubit spin materials more recently, for the sake of bio-mimicking their remarkable ability in quantum photosensing and response to weak magnetic field direction. It is attributed to the spin-selective recombination reaction of RPs through non-adiabatic interconversion, as the electron spins couple to the geophysical and local magnetic fields, compatible with the radical pair mechanism. If so, the lifetime of flavin SCRP should exceed 700 ns at ambient temperatures to fit the Larmor frequency of Earth’s field. The spin dynamics behind photoinduced SCRPs formation, interconversion, and separation are broadly studied at the border of quantum biology, spin chemistry, and physics. The dominant role of the complex radiationless intersystem crossing processes is understood, but the underlying mechanisms are still highly debated. Spin relaxation, which limits the lifetime of the correlated singlet– triplet state and dephasing of flavin SCRP, is examined here, considering manifold interactions and surroundings, dramatically affecting reverse intersystem crossing in the presence of weak magnetic field interference. A set of predictive, spin relaxation-accountable design criteria and input parameters for engineering the biomimetic radical pair qubits is underpinned, and emergent flavin architectures are illustrated. A breakthrough release of fluorescent-protein qubit for quantum biosensing has gingered up this effort.