Spin-selective charge recombination in complexes of CdS quantum dots and organic hole acceptors

This paper describes the mechanisms of charge recombination on both the nanosecond and microsecond time scales in a donor-acceptor system comprising thiol-modified bis(diarylamino)4,4'-biphenyl (TPD) molecules attached to a CdS quantum dot (QD) via the thiolate linker. Transient absorption measurements, in conjunction with EPR and magnetic field effect studies, demonstrate that recombination on the nanosecond time scale is mediated by radical pair intersystem crossing (RP-ISC), as evidenced by the observation of a spin correlated radical ion pair, the formation of the localized (3)*TPD state upon charge recombination, and the sensitivity of the yield of (3)*TPD to an applied magnetic field. These experiments show that the radical spins of the donor-acceptor system have weak magnetic exchange coupling (|2J| < 10 mT) and that the electron donated to the QD is trapped in a surface state rather than delocalized within the QD lattice. The microsecond-time scale recombination is probably gated by diffusion of the trapped electron among QD surface states. This study demonstrates that magneto-optical studies are useful for characterizing the charge-separated states of molecule-QD hybrid systems, despite the heterogeneity in the donor-acceptor geometry and the chemical environment of the radical spins that is inherent to these systems.