Excited-state hole transfer in quantum dot-ferrocene hybrids: influence of ligand-exchange chemistry on surface loading and charge-transfer dynamics

We investigated how the post-synthesis treatment of tetradecylphosphonic acid-capped CdTe (TDPA-CdTe) QDs with CdCl₂ and oleylamine (OAm) affected subsequent ligand-exchange reactions with ferrocenylhexanethiol (FcC₆SH) and the dynamics and yield of photoinduced hole transfer from CdTe to adsorbed ferrocenylhexanethiolate (FcC₆S^-). CdCl₂/OAm treatment promoted an 8-fold increase of the average number of adsorbed FcC₆S^- ligands (NFcC6S−) per CdTe QD; NFcC6S− increased with the concentration of FcC₆SH and reached a maximum of 198 ± 5. Higher per-QD loadings of FcC₆S^- increased the overall ensemble rate constant of CdTe-to-FcC₆S^- hole transfer (<i>k_Nht</i>), which reached (3.0 ± 0.4) × 10⁹ s⁻¹, and the efficiency of hole transfer (<i>η_ht</i>), which reached (98.2 ± 0.3)%. Values of <i>k_Nht</i> increased monotonically with NFcC6S−; at relatively low loadings of FcC₆S⁻, <i>k_Nht</i> varied linearly with NFcC6S−, indicating that hole-transfer pathways were additive. The intrinsic rate constant of hole transfer per adsorbed FcC₆S⁻ (<i>k_ht</i>), (2.75 ± 0.19) × 10⁷ s⁻¹, was 10-fold higher for CdCl₂/OAm-treated CdTe QDs than for TDPA-CdTe QDs. Surface-anchoring thiolates did not trap photogenerated holes; instead, holes were transferred to the Fe(II) center of FcC₆S⁻. Treatment of CdTe QDs with CdCl₂ and OAm thus promoted ligand-exchange with FcC₆S⁻, accelerated excited-state hole transfer, and afforded tunability of charge-transfer dynamics.