Influence of quantum dots on the aromaticity of thiosalicylic acid

When ligands are coordinated to quantum dots (QDs), the ring current of the ligand strongly influences the applications of the QDs, for example in solar cell technology. The Raman spectrum of the ligand can be used to probe and identify ions or measure ion concentrations. Here, we investigated, using a theoretical method, the aromaticities and Raman spectra of CdTe, CdSe, and CdS QDs coordinated with thiosalicylic acid ligands. We found that the aromaticity of the benzene ring in free thiosalicylic acid increased when it was used as a QD ligand. The ring currents of the benzene rings in the CdTe-ligand, CdSe-ligand, and CdS-ligand systems were stronger than the ring current of the benzene ring in free thiosalicylic acid; in other words, the QDs influence the ring current--they enhance the electron transfer rate of the benzene ring. We also discovered that the CdTe-ligand and CdSe-ligand systems have stronger ring currents than the CdS-ligand system. The high electronegativity and vacant d orbital of the sulfur atom influence the ring current of the ligand in the CdS-ligand system. Further, the Raman spectrum of free thiosalicylic acid was different from the spectra of the ligands in the QD-ligand systems: the Raman spectra of COO(-) in each QD-ligand system was enhanced compared with that of the COO(-) in free thiosalicylic acid.