Laser Photolysis Studies of Rhodium(III) Porphyrins. Photodissociation of Axial Phosphine Ligand in the Temperature Range 300−200 K

Abstract

Photodissociation of the axial ligand from triphenylphosphine complexes of rhodium(III) octaethyl- and tetraphenyporphyrin halides, (X-)(PPh3)RhIIIOEP and (X-)(PPh3)RhIIITPP (X = Cl, Br, and I), in toluene solutions are investigated by the 355-nm laser flash photolysis. The photodissociation yield of PPh3, Φ, from (X-)(PPh3)RhIIIOEP is markedly dependent on the nature of X: Φ = 0.79 for X = I, Φ = 0.48 for X = Br, and Φ = 0 for X = Cl. Similar trends are observed for (X-)(PPh3)RhIIITPP: Φ = 0.13 for X = I and Φ = 0 for X = Br and Cl. The excited triplet state is not detected for (I-)(PPh3)RhIIIOEP and (Br-)(PPh3)RhIIIOEP but for (Cl-)(PPh3)RhIIIOEP and (X-)(PPh3)RhIIITPP. On the basis of the oxygen quenching study and the temperature dependence of the dissociation quantum yields, the excited state of (I-)(PPh3)RhIIIOEP and (Br-)(PPh3)RhIIIOEP from which the dissociation occurs is ascribed to the excited singlet state. For (I-)(PPh3)RhIIITPP, either the excited singlet and triplet state is found to be responsible for the dissociation of the axial PPh3. From the quantum yield measurements for the photodissociation of the axial PPh3, Φ, and the triplet formation, ΦST, in the temperature range 200−300 K, the ligand dissociation and the intersystem crossing processes of (I-)(PPh3)RhIIITPP are demonstrated to be competitive at the excited singlet state. The sum of the quantum yields, Φ plus ΦST, obtained at 300 K is smaller than unity. This result is discussed by assuming that the energy dissipation process at the excited singlet state is due to the formation of the transient species that have insufficiently dissociated bonds between X and Rh and/or PPh3 and Rh: the species partly return to the parent molecule without dissociation of the axial PPh3.