Photogenerated Spin states in Quantum Dot – Molecule Conjugates

The inherent spin polarization in photogenerated spin-correlated radical pairs makes them promising candidates for quantum computing and quantum sensing applications. Notably, these states are spin-polarized (non-Boltzmann populated) at moderate temperatures, and can therefore be initialized in well-defined quantum states. This feature allows them to be probed and manipulated using microwave pulses within standard electron paramagnetic resonance spectrometers equipped with pulsed lasers. Quantum dot – molecule conjugates offer a tunable platform for hosting these spin-correlated radical pairs. We have therefore designed a series of quantum dot – molecular systems that can produce photogenerated spin-polarized states. The molecules are chosen for their ability to undergo efficient charge separation, and the nanoparticle materials, ZnO quantum dots, are chosen for their promising spin properties. Transient and steady state optical spectroscopy performed on ZnO quantum dot–molecular conjugates shows that reversible photogenerated charge separation is occurring. Transient and pulsed electron paramagnetic resonance experiments are then performed on the photogenerated radical pair, which demonstrate that (1) the radical pair is polarized at moderate temperatures and well modeled by existing theories, (2) the spin states can be accessed and manipulated with microwave pulses, and (3) the spin system can be tuned with quantum dot size and molecular linker length. This work opens the door to a new class of promising qubit materials that can be photogenerated in polarized states and hosted by highly tailorable inorganic nanoparticles.