Spin-flip excitation of molecules mediated by photoexcited silicon nanocrystals

Spin-flip excitation of molecules by photons is forbidden in the dipole approximation. Usually it requires the presence of a light-absorbing donor which subsequently transfers energy of electronic excitation to an acceptor molecule. Here we report on a specific type of spin-flip excitation process based on assemblies of silicon nanocrystals. This system has a highly developed surface being accessible for organic molecules, tunable energy of excitons due to quantum confinement effects, and a very long radiative lifetime of indirect excitons. These factors are found to be critical for achieving highly efficient energy transfer from excitons confined in Si nanocrystals to a variety of organic molecules. We show that energy transfer is governed by electron exchange mechanism.