Aluminium(iii) porphyrin based dimers and trimers: synthesis, spectroscopy and photochemistry

The well-known oxophilicity of aluminum(III) ion and the well-defined redox and photochemical properties of aluminum(III) porphyrins have been exploited to synthesize the functionally active, 'axial-bonding’-type free base–aluminum(III) porphyrin dimer (H2–Al) and free base–[aluminum(III) porphyrin]2 trimer (H2–Al2), as well as the corresponding copper(II) (Cu–Al, Cu–Al2) and zinc(II) (Zn–Al, Zn–Al2) derivatives. These dimeric and trimeric species have been fully characterized by mass (FAB), UV/VIS, proton nuclear magnetic resonance (1D and 1H-1H COSY) and electron spin resonance spectroscopies and also by the differential pulse voltammetric method. Comparison of their spectroscopic and electrochemical data with those of the corresponding monomeric porphyrins reveals that there is no apparent ring-to-ring interactions in these 'vertically’ linked dimers and trimers. The fluorescence quantum yields and singlet state life times were found to be lower for H2–Al and H2–Al2 in comparison with those of the monomeric chromophores. Electronic energy transfer from the aluminum(III) porphyrin to the free base subunit is detected in both H2–Al and H2–Al2. Finally, a comparison is made between the presently reported aluminum(III) porphyrin based arrays and the previously reported analogous arrays based on tin(IV), germanium(IV) and phosphorous(V) porphyrins with regard to their architectural features, spectroscopic properties and photochemical activities.