Ultrafast electron injection in TiO₂ surface-anchored aluminum(III) porphyrin stacks

Two surface-anchored aluminum(III) porphyrin (AlPorF[Formula: see text] stacks, AlPorF 3 (Py)-COO/TiO 2 and AlPorF 3 (Ph)-COO/TiO 2 , have been constructed to investigate the interfacial electron injection from the AlPorF 3 into the conduction band of the TiO 2 nanoparticles as a function of stacking topology. The Lewis acid properties of AlPorF 3 were combined with the electronic and surface properties of TiO 2 to obtain the investigated porphyrin stacks. The axial Lewis base, pyridyl (Py) unit, in AlPorF 3 (Py)-COO/TiO 2 directs the porphyrins to stack on the TiO 2 surface in a layered fashion. The absence of a Lewis base in AlPorF 3 (Ph)-COO/TiO 2 is unable to form such defined stacks. The AlPorF 3 (Py)-COO/TiO 2 and AlPorF 3 (Ph)-COO/TiO 2 were characterized by steady-state and transient spectroscopic techniques. Transient absorption spectral studies show that surface-stacked hybrids exhibit electron injection from AlPorF 3 to the conduction band of TiO 2 . However, the injection efficiencies and kinetics are not very different in the investigated stacks AlPorF 3 (Py)-COO/TiO 2 and AlPorF 3 (Ph)-COO/TiO 2 indicating that the axial self-assembly does not alter the electronic communication within the AlPorF 3 layer to a significant level to perturb the photodynamics.