PHOTOINDUCED PROCESSES IN AN ALUMINUM-PORPHYRIN / NAPHTHALENEDIIMIDE AXIAL DYAD.

Porphyrins are widely studied cromphorores, well-known as constituents of both natural and artificial supramolecular systems which show interesting photophysical features such as photoinduced electron and/or energy transfer. Recently, attention has been paid to the possibility of exploiting Aluminium(III) porphyrins as supramolecular building blocks1,2, due to the stability of the axial coordination between the Al-porphyrin and ligands carrying oxygen-containing groups. Here we present an example of such a supramolecular system, a dyad based on an Al(III)-tetra-phenyl-porphyrin moiety linked by axial coordination to a naphtalene-diimide unit carrying a carboxylate group. The photophysical properties of the dyad system and of appropriate model systems have been studied by means of both stationary and time-resolved spectroscopic techniques, including single-photon counting, nanosecond laser flash photolysis and ultrafast transient absorption spectroscopy. The experimental data indicate the occurrence of photoinduced electron transfer in the dyad, with clear evidence given by solvent effects on excited-state kinetics. Density functional theory (DFT) calculations have been performed to gain a better insight on the electronic structure and provide a reliable geometry for the system in order to remedy the lack of any pentacoordinated Al-porphyrin x-ray structure in literature.