Ultrafast Hole Transfer From CdSe Quantum Dots to Nitroxide Free Radicals*

Organic molecules coupled to inorganic semiconductor quantum dots (QDs) have been proposed to be interesting candidates for photocatalytic applications.$^{\mathrm{\thinspace }}$Among these molecules, nitroxide free radicals are proven to be efficient photoluminescence (PL) quenchers when coupled with II-IV QDs but the mechanism of this PL quenching has not been well-understood. Here, we assessed the mechanism of the PL quenching of photoinduced colloidal CdSe QDs by 4-Amino, 2,2,6,6-tetramethylpiperidine-1-oxyl radical (4-Amino-TEMPO). Analysis of the time-resolved photoluminescence and transient absorption spectroscopies show a hole transfer from the valence band of CdSe QDs to 4-Amino-TEMPO. This reductive quenching happens in sub-picosecond timescale. Such ultrafast hole extraction of colloidal CdSe QDs by 4-Amino-TEMPO implies that these radicals are efficient hole acceptors when coupled directly to CdSe QDs, therefore suggesting CdSe QDs/ Nitroxide free radicals hybrid systems can play a significant role in the future of optoelectronic applications.