Photophysics of reduced silicon tetraphenylporphyrin

The contrasting photophysical properties of two silicon (IV) tetraphenylporphyrins, Si(TPP)(py)₂ and Si(TPP)Cl₂, have been investigated using static absorption and fluorescence spectroscopy and ultrafast transient absorption measurements. The parent Si(TPP)Cl₂, in which the porphyrin macrocycle has its normal 2-oxidation state, has a fluorescence yield of 0.027, and a lifetime of 1.8 ns for the lowest excited singlet state. In marked contrast, the reduced, anti-aromatic complex Si(TPP)(py)2, with the macrocycle in the 4- oxidation state, has an extremely low fluorescence yield (≤0.0004) and a 750-fold shorter excited-state lifetime (2.4 ps) in the same solvent (pyridine). The rapid deactivation of photoexcited Si(TPP)(py)₂ to the ground state is likely associated with its ruffled structure and the presence of low-energy excited states in its electronic manifold.