Structure dependent activation of a Co molecular catalyst through photoinduced electron transfer from CdTe quantum dots

Abstract

Complexes of quantum dots with molecular catalysts are promising building blocks for photo-catalytic applications. Herein, we report the formation of stable complexes between colloidal CdTe quantum dots (CQDs) and two synthesized structurally different cobalt porphyrin derivatives (CoPp and CoPm, with phenyl and mesityl groups attached at the meso positions, respectively) through a sulfur bridge. Using both spectroscopy and computational methods, we found that the porphyrin adopts a “flat” binding mode on the CQD surface. We observed the coordination of the Co center on the CQD surface. This coordination is stronger for CoPp than for CoPm, resulting in a larger red shift in the absorption band. In addition, we measured a four fold increase in the electron transfer (ET) rate from the CQD to CoPp compared to that with CoPm by a transient absorption study and the charge recombination extended to tens of nanoseconds or longer depending on the structure of the porphyrin periphery. A spectrum measured after the ET points to a loss of coordination between the Co and CQD in a CoP/CQD complex. The experimental results are in agreement with density functional theory calculation results on the CoP complexes on CdTe surfaces, pointing to the porphyrin preferring to align along the CQD surface in the ground state. The change of porphyrin alignment from flat alignment before the excitation to upright alignment after the ET is a likely cause for the extended lifetime of the charge-separated (CS) state, due to an increase in the CS distance. Furthermore, the spectrum of the CS state can be assigned to catalytically active Co^IP, proposing the applicability of the complexes in CO₂ reduction.