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 (kNht), which reached (3.0 ± 0.4) × 109 s−1, and the efficiency of hole transfer (ηht), which reached (98.2 ± 0.3)%. Values of kNht increased monotonically with NFcC6S−; at relatively low loadings of FcC6S−, kNht varied linearly with NFcC6S−, indicating that hole-transfer pathways were additive. The intrinsic rate constant of hole transfer per adsorbed FcC6S− (kht), (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.