Laterally Bound Co Porphyrin on CdTe QD : A Long-Lived Charge-Separated Nanocomposite

Abstract

Cobalt porphyrin (CoP) derivatives are potential compounds for photocatalytic CO2 reduction which must be activated by photoinduced electron transfer from a suitable electron donor. Herein, we have prepared and studied the photophysics of CdTe quantum dots (CQD) coupled with CoP derivatives where CQDs act as the light antenna and the electron donor and CoP acts as the electron acceptor. To facilitate the nanocomposite formation of CoP with CQD, CoP has been equipped with a −COOH anchoring group which leads to strong complexation between CQD and CoP as observed in the absorption spectra by a gradual shift in the Soret absorption band. This is attributed to the lateral binding geometry of CoP through the −COOH anchoring group and Co-center coordination to CQD, which helps to bring CoP close to the CQD. Our DFT calculations have identified that this lateral geometry is more favorable than the upright orientation on the CdTe (110) surface. The redox levels have been determined from cyclic voltammetry which shows that the electron transfer (ET) from CQD to CoP is feasible. The strong luminescence quenching of CQD in the presence of CoP has also suggested quantitative CQD/CoP nanocomposite formation and pointed to the ET from QDs to CoP. The charge carrier dynamics have been monitored using femtosecond transient absorption (TA) spectroscopy. The TA spectral analysis has shown efficient ET in CQD/CoP which proves that our 4 nm CQD acts as an efficient electron donor for the CoP counterpart. The CQD excited state lifetime is shortened along with delayed Soret band bleaching of CoP in this nanocomposite. From the global fitting of TA data, the estimated average ET time constant from CQD to a CoP molecule is approximately 70 ps, and the charge recombination time is ≫5 ns. Also, differences in the TA spectra after ET have been observed which can be associated with the changes in the binding geometry of CoP on the CQD surface, which is lateral in the case of the ground-state complex to the upright orientation after the ET process. Hence, the studied CQD/CoP nanocomposites are promising materials to initiate CO2 reduction through photoexcitation of the CQD that activates the CoP molecular catalyst through the ET.