Ultrafast electron injection in TiO₂surface-anchored aluminum(III) porphyrin stacks

Two surface-anchored aluminum(III) porphyrin (AlPorF3) stacks, AlPorF3(Py)-COO/TiO2 and AlPorF3(Ph)-COO/TiO2, have been constructed to investigate the interfacial electron injection from the AlPorF3 into the conduction band of the TiO2 nanoparticles as a function of stacking topology. The Lewis acid properties of AlPorF3 were combined with the electronic and surface properties of TiO2 to obtain the investigated porphyrin stacks. The axial Lewis base, pyridyl (Py) unit, in AlPorF3(Py)-COO/TiO2 directs the porphyrins to stack on the TiO2 surface in a layered fashion. The absence of a Lewis base in AlPorF3(Ph)-COO/TiO2 is unable to form such defined stacks. The AlPorF3(Py)-COO/TiO2 and AlPorF3(Ph)-COO/TiO2 were characterized by steady-state and transient spectroscopic techniques. Transient absorption spectral studies show that surface-stacked hybrids exhibit electron injection from AlPorF3 to the conduction band of TiO2. However, the injection efficiencies and kinetics are not very different in the investigated stacks AlPorF3(Py)-COO/TiO2 and AlPorF3(Ph)-COO/TiO2 indicating that the axial self-assembly does not alter the electronic communication within the AlPorF3 layer to a significant level to perturb the photodynamics.