An All-Atom Theory of Electron Transfer at Nanocrystal/Molecule Interfaces: A Hybrid LCAO/DFT Approach

Interfacial electron transfer (IET) probabilities and rates between CdSe and CdS nanocrystals of diameters 2, 4, and 6 nm, and an anthraquinone dimer (AQ₂H⁺), have been studied. To account for electronic configuration mixing, the previously reported all-atom hybrid BA-LCAO/DFT approach to QD/acceptor systems has been extended to include: (a) electron/hole interaction, and (b) exciton–exciton coupling. To carry out these calculations, at first, we generated a one-particle orbital basis of the QDs at the all-atom BA-LCAO level of theory, and orbitals of AQ₂H⁺ at a conventional DFT level. We then applied an exciton interaction theory for treating initial and final states of the quantum dot (“QD” = QD*/AQ₂H⁺) and charge separated (“CS” = QD⁺/AQ₂H) systems, and for evaluating the state-to-state coupling elements. These calculations revealed that (i) singlet–triplet splitting is significant in the QD systems, while practically negligible in the CS systems; (ii) quantum confinement effects are responsible for weak exciton correlation in the small QD and CS systems and much stronger exciton correlation in the larger ones; (iii) IET in all charge separation processes occurs via excitons close to the band edges, and (iv) charge recombination (“CR” = QD/AQ₂H⁺) in the smaller quantum dots proceeds via low energy holes, while a strong involvement of deep VB orbitals (hot holes) was observed in the 6 nm quantum dots. Furthermore, the quantum coupling strength in both CS and CR processes exhibits an exponential decrease with the size of the nanocrystal, as dictated by the corresponding decay of charge density on the surface of these QDs. At the present level of modeling, we find that quantum coupling is stronger in the QD(CdS)/AQ₂H⁺ than in the QD(CdSe)/AQ₂H⁺ dyads.