Contrasting photoelectrochemical behaviour of two isomeric supramolecular dyes based on meso-tetra(pyridyl)porphyrin incorporating four (μ3-oxo)- triruthenium(III) clusters

A saddle shaped tetracluster porphyrin species containing four [Ru3O(OAc)(6)(py)(2)](+) clusters coordinated to the N-pyridyl atoms of 5,10,15,20-tetra(3-pyridyl)porphyrin, H-2(3-TCPyP), has been investigated in comparison with the planar tetra(4-pyridyl) porphyrin analogue H-2(4-TCPyP). The steric effects from the bulky peripheral complexes play a critical role in the H-2(3-TCPyP) species, determining a non-planar configuration around the porphyrin centre and precluding any significant pi-electronic coupling, in contrast with the less hindered H-2(4-TCPyP) species. Both systems exhibit a photoelectrochemical response in the presence of nanocrystalline TiO2 films, involving the porphyrin excitation around 450 nm. However, only in the H-2(4-TCPyP) case do the cluster moieties also contribute to the photoinduced electron injection process at 670 nm, reflecting the relevance of the electronic coupling between the porphyrin centre and the peripheral complexes.