Deciphering the Optical Switching Mechanism of CdSe/CdS\nQDs Luminescence by Diarylethene Molecular Photoswitches: A Stochastic\nModel Analysis

Hybrid\nsystems (HSs) of functional organic ligands and colloidal\nquantum dots (QDs) exhibit unique physical properties that do not\nemerge in QDs themselves owing to a synergetic effect and are expected\nto be applied to luminescence switching and photoenergy conversion\nmaterials. However, HSs have two inherent issues. The high surface-to-volume\nratio of QDs makes them prone to generating trap states during the\nhybridization process, resulting in a performance decline of the HSs.\nThe second issue is that the number of attached ligands on a single\nQD is distributed. The observed properties represent the average ensemble\nof these compositions, making it difficult to extract elementary processes.\nIn this study, we employed Z-type ligand exchange and analyzed the\nenergy transfer process using a stochastic model to address these\nchallenges. The luminescence quantum yield of diarylethene derivative\n(DAE)-CdSe/CdS HSs, formed through Z-type ligand exchange, remained\nnearly constant compared to that of the as-prepared CdSe/CdS QDs without\nadditional formation of trap states. We elucidated the energy transfer\nprocesses from excited CdSe/CdS QDs to closed-form DAE (c-DAE) using\na stochastic model, enabling us to estimate the rate constant of energy\ntransfer from a single QD to a single c-DAE and the mean number of\nc-DAE as a function of UV irradiation time. The distance from the\nQD to c-DAE calculated from the energy transfer rate is in good agreement\nwith the estimated value from the QD diameter and adsorbed c-DAE length.\nIn addition, we found that the trap states on the surface of the QDs\nwere reversibly formed by the photochromic reaction, and the number\nof trap states linearly increased with UV irradiation time. This methodology\nhas the potential to be extended to other QDs and functional organic\nligand combinations, offering more accurate understanding of the structural\nanalysis and elementary exciton processes of HSs.