Spin-Polarized Anthracene Triplet States Sensitized by Si QDs Revealed by Time-Resolved EPR Spectroscopy.

Photoexcited quantum dot (QD)-chromophore hybrid systems hold great promise for photon upconversion, a process that could significantly boost the efficiency of solar energy technologies and be useful for spintronics and quantum information science. However, the magnetic interactions that govern the generation and transfer of spin polarization in these systems are not well understood. Here, we report the first time-resolved electron paramagnetic resonance (EPR) study of triplet energy transfer (TET) in colloidal Si QDs conjugated with two anthracene-based organic molecules: 9-ethylanthracene and 9-vinylanthracene. Triplet formation and spin dynamics monitored with time-resolved continuous-wave and pulsed EPR spectroscopy reveal Dexter-type TET from Si QDs to the anthracene moiety. Notably, the electron spin polarization of these conjugates displayed unusual fully emissive patterns, a surprising result that cannot be explained by the conventional spin-orbit or radical-pair mechanisms typically seen in purely organic donor-acceptor systems. The mechanism of this novel TET is discussed.