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
