Upon irradiation with visible light (lambda > 420 nm) in C6D6 under Ar, dialkylsilicon porphyrins Si(TPP)R-2 (TPP = 5,10,15,20-tetraphenylporphinato) (R =CH2CH2CH3 (1) and CH2TMS (2)) reacted with nitroxy compounds, such as 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), 4-oxo-,2,6,6-tetramethylpiperidinyl-1-oxy, and 1,1,3,3-tetramethylisoindolinyl-2-oxy, to give dinitroxysilicon porphyrins 6. In the absence of nitroxy compounds, photolysis of I resulted in the formation of a long-lived (>50 days at 25 degrees C) EPR-active silicon diradical with a g-value of 2.0026, which did not react with nitroxy compounds in the dark but was smoothly trapped by TEMPO to give 6a upon excitation with visible light. Irradiation of 6 with visible Light in the presence of free nitroxy compounds resulted in reversible axial ligand exchange, which proceeded, in response to the switching on and off of the light, to reach an equilibrium.