Mapping Magnetic Fields Using a Radical Pair Reaction

Abstract

Magnetically sensitive chemical reactions are a dramatic example of quantum coherence under ambient conditions. The interaction energy of a 10 mT magnetic field with an electron spin is less than kBT/400 at room temperature, and thus one might expect magnetic fields to have negligible influence on chemical reactions. However, in certain photochemical processes, the outcome is determined largely by coherent spin dynamics rather than by thermodynamics. In chemical systems showing a magnetic field effect (MFE), photoinduced electron transfer generates a radical pair (RP) in which the electrons are initially in an entangled spin state, yet are distant enough to be only weakly interacting. The electron spins evolve, each under the influence of the local magnetic interactions. Hyperfine couplings to nearby magnetically active nuclei (mostly 1H, 14N, and 13C) induce differential precession of the electron spins and thus favor intersystem crossing (ISC). An external magnetic field induces an electronic Zeeman splitting of the T+ and T– states. Very small magnetic fields can enhance ISC through the “low field effect”,1 while larger magnetic fields decouple the T+ and T– states from the singlet and thereby suppress ISC. Even larger fields can again enhance ISC if the two electrons have different g values. Spin-correlated radical pairs are important in polymer chemistry, organic chemistry, material science, and photosynthesis. Magnetic field effects on spin-correlated radical pairs have been proposed as the primary transduction mechanism in the magnetic sense of birds and insects, although this hypothesis has not been confirmed by direct measurements on the putative sensor cryptochrome proteins.2 In this report, a fluorescent chemical indicator of magnetic field was used to visualize the magnetic field around ferromagnetic nanostructures. The indicator was a chain-linked electron donor–acceptor molecule, phenanthrene-(CH2)12-O-(CH2)2-dimethylaniline,3 that forms spin-correlated radical pairs upon photoexcitation. Its fluorescence intensity changes with external magnetic field applied. The magnetic field altered the coherent spin dynamics, yielding an 80% increase in exciplex fluorescence in a 0.1 T magnetic field. The magnetic field distributions were quantified to precision of 1.8 × 10-4 T by image analysis and agreed with finite-element nanomagnetic simulations.