radicalpair

A radical pair refers to two molecules or molecular fragments that each contain an unpaired electron and are formed in a single chemical event, often via photoinduced electron transfer in donor–acceptor systems. The pair typically remains electronically coupled long enough for their unpaired electrons to influence each other, before recombination or separation occurs.

The pair is formed in a defined spin state, usually a singlet, and the electron spins experience hyperfine interactions with nearby nuclei as well as external magnetic fields. This leads to coherent interconversion between singlet and triplet spin states. The subsequent chemical fate of the pair depends on its spin state: singlet pairs often recombine to return to the ground state, while triplet pairs may yield different products or escape as separated radicals.

Magnetic field effects and the radical pair mechanism: The rate of singlet–triplet interconversion and thus reaction outcomes can be modulated by magnetic fields. This sensitivity to weak magnetic fields is a central feature of the radical pair mechanism and has been proposed as a basis for magnetoreception in some animals, as well as exploited to control reaction yields in chemistry.

Lifetimes, measurement, and theory: Radical pairs typically exist from nanoseconds to microseconds. They are studied with time-resolved spectroscopy and related techniques. Theoretical descriptions commonly use quantum density matrix formalisms that include hyperfine couplings, exchange interactions, and dipolar couplings, as well as decoherence and solvent effects that influence dynamics.

Importance and applications: The concept is fundamental in spin chemistry and photochemistry, with implications for chemical reaction control, quantum biology concepts such as magnetoreception, and potential applications in organic electronics and spintronics.