  The yield of chemical reactions involving intermediate radical pairs is
influenced by magnetic fields well beyond the levels expected from energy
considerations. This dependence can be traced back to the microscopic dynamics
of electron spins and constitutes the basis of the chemical compass. Here we
propose a new experimental approach based on molecular photoswitches to achieve
additional control on the chemical reaction and to allow short-time resolution
of the spin dynamics. Our proposal enables experiments to test some of the
standard assumptions of the radical pair model and improves the sensitivity of
chemical magnetometers by two orders of magnitude.
