Photoinduced Homolytic Bond Cleavage of the Central Si-C Bond in Porphyrin Macrocycles Is a Charge Polarization Driven Process.

Photoinduced cleavage of the bond between the central Si atom in porphyrin macrocycles and the neighboring carbon atom of an axial alkyl ligand is investigated by both experimental and computational tools. Photolysis and electron paramagnetic resonance measurements indicate that the Si-C bond cleavage of Si-phthalocyanine occurs through a homolytic process. The homolytic process follows a low-lying electronic excitation of about 1.8 eV that destabilizes the carbide bond of similar bond dissociation energy. Using electronic structure calculations, we provide insight into the nature of the excited state and the resulting photocleavage mechanism. We explain this process by finding that the electronic excited state is of a charge transfer character from the axial ligand toward the macrocycle in the reverse direction of the ground state polarization. We find that the homolytic process yielding the radical intermediate is energetically the most stable mechanistic route. Furthermore, we demonstrate using our computational approach that changing the phthalocyanine to smaller ring system enhances the homolytic photocleavage of the Si-C bond by reducing the energetic barrier in the relevant excited states.