Photochemistry of Biologically Important Transition Metal Complexes. II. Carbonylpiperidinetetraphenylporphine Complexes of Iron(II) and Ruthenium(II)

Carbonylpiperidinetetraphenylporphineiron(II) and dipiperidinetetraphenylporphineiron(II) do not emit the characteristic porphyrin phosphorescence. It is suggested that the porphyrin singlets initially excited undergo an efficient radiationless transition to an excited state which lies below the lowest porphyrin triplet. It is concluded that this lower excited state initiates the release of coordinated CO which has been observed for a variety of biologically important carbonylporphyrins of iron(II). It is assumed that the reactive state is either an excited CT or, more likely, a LF state. If iron(II) is replaced by the homologous ruthenium(II) the lowest LF excited state lies apparently at higher energies than the lowest porphyrin triplet due to the larger LF splitting of the heavier ruthenium. Hence carbonylpiperidinetetra‐phenylporphineruthenium(II) dissolved in piperidine shows the typical porphyrin phosphorescence, but undergoes also a simultaneous photosubstitution of CO by piperidine with low efficiency. Quantum yields of CO release and relative emission intensities were determined at two irradiating wavelengths (412 and 530 nm) and at two temperatures (25 °C and 80 °C). The results led to the suggestion that the reactive LF state can be populated from the lower porphyrin triplet by thermal activation. In addition to this thermally activated photolysis a temperature independent contribution seems to be important at higher excitation energies. It is assumed that the deactivation cascade starting from higher excited singlets of the porphyrin ligand includes the population of the reactive LF state before the lowest porphyrin triplet is reached. It is also possible that it is not a LF state but a low‐lying CT state which is responsible for the photoreactivity.