Synthesis, Spectroscopy, Electrochemistry, Spectroelectrochemistry, Langmuir-Blodgett Film Formation, and Molecular Orbital Calculations of Planar Binuclear Phthalocyanines

A planar binuclear phthalocyanine and its dizinc and dicobalt derivatives, in which two phthalocyanine units share a common benzene ring, have been studied by spectroscopy, electrochemistry, and spectroelectrochemistry. Their Langmuir-Blodgett film-forming properties have been examined, and the results of molecular orbital calculations on these and related systems are also presented. The properties are compared with those of the corresponding mononuclear control molecules. UV-visible-near IR absorption and magnetic circular dichroism spectroscopies and cyclic and differential pulse voltammetry indicate that, in these compounds, two relatively independent chromophore units interact and therefore that the two phthalocyanine planes are not completely planar in solution. The electrochemical and spectroelectrochemical measurements show the formations of various mixed-valence oxidation and reduction species in the binuclear compounds. The nonmetalated and dizinc complexes exhibit both S₁ and S₂ emission. The radiative lifetimes (τ) of the dizinc complex are smaller than those of the metal-free binuclear derivative for both S₁ and S₂emission, while the quantum yields (ϕF) are larger. Furthermore, for S₂ emission, the ϕF and τ values of the binuclear compounds are larger than those of the corresponding mononuclear control molecules. Molecular orbital calculations within the framework of the Pariser-Parr-Pople approximation reproduce the splitting of the Q absorption band and further show that the splitting becomes larger the smaller the size of the shared common aromatic unit. In monolayers spread on water, the metal-free binuclear complex appears to have a slipped-stack conformation, tilted from the air-water interface normal plane. In Langmuir-Blodgett films, it may form a slipped-stack molecular arrangement with the stacking axis parallel to the substrate and/or a flat-lying conformation on the substrate surface.