Impact of exciton fine structure on the energy transfer in magic-sized (CdSe)₁₃ clusters

Magic-sized (CdSe)₁₃ clusters (MSCs) represent a material class at the boundary between molecules and quantum dots that exhibit a pronounced and well separated excitonic fine structure. The characteristic photoluminescence is composed of exciton bandgap emission and a spectrally broad mid-gap emission related to surface defects. Here, we report on a thermally activated energy transfer from fine-structure split exciton states to surface states by using temperature dependent photoluminescence excitation spectroscopy. We demonstrate that the broad mid-gap emission can be suppressed by a targeted Mn-doping of the MSC leading to the characteristic orange luminescence of the ⁴T₁ → ⁶A₁ Mn²⁺ transition. The energy transfer to the Mn²⁺ states is found to be significantly different than the transfer to the surface defect states, as the activation of the dopant emission requires a spin-conserving charge carrier transfer that only dark excitons can provide.