Photoelectrochemical properties of Zn-naphthalocyanine (ZnNc) and its modified compound, Zn-2,3-tetraquinoxalinotetraazaporphyrin (ZnTQP) were investigated in thin films. The ZnNc electrode vacuum-deposited on an indium tin oxide (ITO) substrate exhibited almost an ohmic I-V curve in the dark and small cathodic photocurrents under illumination, which was typical for the p-type semiconduction of the ZnNc layer, in a photoelectrochemical cell. The drop-casted film electrode of ZnTQP, on the other hand, showed rectified I-V characteristics in the dark and high anodic photocurrents under illumination, which was attributed to the n-type semiconducting character of the ZnTQP layer. These different photoelectrochemical behaviors were characterized by photocurrent action spectra and ultraviolet photoelectron spectroscopy. The substitution with quinoxalino groups into the Nc macrocyclic system gave rise to lowering of the HOMO energy level for the TQP molecule. This electronic energy shift generated donor electron states in the band gap after contact with the substrate or electrolyte and enabled the photooxidation at the valence band edges corresponding to the Soret-and Q-band excitations. The pH dependence of the anodic photocurrents for the ZnTQP electrode indicated that the photooxidation occurred through a hole injection from the HOMO and underlying sub-HOMO states, separately, for the Soret- and Q-band excitations. For the ZnNc electrode, the Q-band illumination produced cathodic photocurrents, whereas the Soret band produced anodic photocurrents. It suggested that under the Soret-band excitation holes generated in the sub-HOMO state directly contributed to the photooxidation.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry