UC Irvine

This thesis describes the first demonstration of a quantum dot photoacid. Photoacids are a class of molecules with a protic bond that is weakened upon photo-excitation. CdSe quantum dots were synthesized and were made to have mixed-ligand surfaces with a majority of water-solubilizing sulfonate functional groups and fewer acidic electron-donor functional groups, notably 4-mercaptophenol (MPh). Sulfonate groups are very acidic and therefore remain charged over a wide range of pH values. Through judicious choice of synthesis and ligand-exchange conditions, water-solubilizing 2-mercaptoethanesulfonate (MES) was introduced in addition to an equal concentration of protic and dipolar MPh groups. The electronic states of surface-bound MPh groups are such that they can serve as electron-transfer donors to excited-state CdSe quantum dots. The challenge to characterize the photoacidity of this class of chromophores is that the charge-separated CdSe– and MPh+ state does not efficiently emit photons. Therefore, a recently developed electrochemical impedance spectroscopy technique was studied in great detail to understand the cause of observed changes in impedance and then utilized to definitively demonstrate that this new class of quantum dot dyes were photoacidic. This light-driven energy conversion process is uniquely suited to convert light into ionic power, which can be used for direct desalination of saltwater.