Weak magnetic fields (< 30 mT) can have a profound influence on radical pair reactions. This is, at first sight, puzzling since the magnetic interactions are typically much smaller than the thermal energy at room temperature. The radical pair is created in a highly non-equilibrium state, however, displaying significant mixing between its singlet and triplet spin states. The mixing rate is altered if a magnetic field is present via the Zeeman interaction. If the spin states have different chemical fates and/or different recombination rates any changes in the singlet-triplet mixing will create a variation in downstream products and/or reaction kinetics. This concept is commonly referred to as the radical pair mechanism (RPM). The RPM is the most likely explanation for migratory bird’s ability to use the Earth magnetic field for orientation and navigation. This hypothesis is remarkable for two reasons. First, the Earth’s magnetic field is extremely weak (» 50 mT). Second, to be a useful chemical compass, the magnetic field effect has to be dependent not only on the strength but also on the direction of an external magnetic field.
We have recently shown proof-of-principle for a chemical compass based avian magnetoreception through transient absorption experiments. A carotenoid-porphyrin-fullerene triad is the first molecule to ever display a directional response based on the RPM to magnetic fields comparable to Earth’s. Demonstrating the same response in cryptochromes, the protein believed to be responsible for the avian magnetic compass, remains challenging for technical reasons related to sensitivity and sample stability. Attempts to overcome these challenges has inspired the development of novel spectroscopic tools for investigating oriented materials, such as evanescent-wave broadband cavity-enhanced absorption spectroscopy, with a broad applicability to solid-liquid interface studies.