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Spin chemistry of photogenerated spin-correlated radical pairs (SCRPs) offers a practical approach to control chemical reactions and molecular emissions using weak magnetic fields. This capability to harness magnetic field effects (MFEs) paves the way for developing SCRPs-based molecular qubits. Here, we introduce a new series of donor-chiral bridge-acceptor (D-χ-A) molecules that demonstrate significant MFEs on fluorescence intensity and lifetime in solution at room temperature – critical for quantum sensing. By precisely tuning the donor site through torsional locking, distance extension, and planarization, we achieved remarkable control over key quantum properties, including field-response range and linewidth. In the most responsive systems, emission lifetimes increased by over 200%, and total emission intensity was modulated by up to 30%. This level of tunability, and rational design principle of optically addressable molecular qubits, represents a major leap toward functional synthetic molecular qubits, advancing the field of molecular quantum technologies. 

The degree of torsional hindrance can significantly contribute to anti-Arrhenius behavior of charge recombination, wherein recombination rates decrease as temperature increases.