Axial-ligand control of the photophysical behavior of ruthenium(II) tetraphenyl- and octaethylporphyrin: contrasting properties of metalloporphyrin (.pi.,.pi.) and (d,.pi.) excited states

The photophysical behavior of the ruthenium(II) porphyrins depends dramatically on the axial ligands coordinated to the central metal ion. The authors 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)/sub 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 (Me/sub 2/SO), or ethanol (EtOH). They assign the lowest excited state of all the RuP(CO)(L) complexes, including those with L absent, as the lowest excited triplet state, /sup 3/(..pi..,..pi..*), of the porphyrin ring. /sup 3/(..pi..,..pi..*) appears to form in high yield from the ring excited singlet, /sup 1/(..pi..,..pi..*), in less than or equal to 30 ps. On the other hand, they assign the lowest excited state of the RuP(L)/sub 2/ complexes, except for RuTPP(Me/sub 2/SO)/sub 2/, as a metal-to-ring (d,..pi..*), charge-transfer (CT) state. Although /sup 3/(..pi..,..pi..*) appears to be the lowest excited state in all the RuP(CO)(L) complexes investigated, they propose that the deactivation of this state nonetheless proceeds, in part, via a shorter lived (d,..pi..*) state at higher energy. The photodissociation quantum yield is measured to be approx. 10/sup -4/ for two of the complexes.