Axial-Ligand Control of the Photophysical Behavior of Ruthenium(II) Tetraphenyl- and Octaethylporphyrin. Contrasting Properties of Metalloporphyrin (?,?*) and (d,?*) Excited States

Article abstract of DOI:10.1021/j100314a026

The photophysical behavior of the ruthenium(II) porphyrins depends dramatically on the axial ligands coordinated to the central metal ion.We have measured the picosecond and slower time scale transient absorption spectra and kinetics, emission data, and ground-state absorption spectra for two classes of complexes: RuP(CO)(L) and RuP(L)2.Results are compared for complexes in which the porphyrin macrocycle (P) is tetraphenylporphyrin (TPP) or octaethylporphyrin (OEP) and the axial ligand L is piperidine (pip), pyridine (py), dimethyl sulfoxide (Me2SO), or ethanol (EtOH).We assign the lowest excited state of all the RuP(CO)(L) complexes, including those with L absent, as the lowest excited triplet state ³(?,?^*), of the porphyrin ring ³(?,?^*) appears to form in high yield from the ring excited singlet, ¹(?,?^*), in a metal-to-ring (d,?^*) charge-transfer (CT) state.We attribute this general switch of the lowest excited state from ³(?,?^*) in RuP(CO)(L) to (d,?^* in RuP(L)2 to the loss of ?-backbonding between the filled Ru(d?) orbitals and the empty CO(?^*) orbitals.The loss of axial ?-backbonding is expected to destabilize the d? orbitals, making them closer in energy to the empty e_g(?^*) orbitals of the porphyrin ring.This lowers the energy of (d,?^*) relative to ³(?,?^*) in RuP(L)2 compared to RuP(CO)(L).Although ³(?,?^*) appears to be the lowest excited state in all the RuP(CO)(L) complexes investigated, we propose that the deactivation of this state nonetheless proceeds, in part, via a shorter lived (d,?^*) state at higher energy.We speculate that the faster decay of (d,?^*) compared to ³(?,?^*) may be due to a better Franck-Condon factor for radiationless decay to the ground state.The recay route of ³(?.?^*) via a thermally accessible (d,?^*) in the RuP(CO)(L) complexes also may be the pathway for photodissociation of CO from these molecules, which in the presence of L results in the formation of RuP(L)2.The photodissociation quantum yield is measured to be ca. 1E-4 for two of the complexes.