Phosphine and phosphonite complexes of a Ru(II) porphyrin. 2. Photophysical and electrochemical studies

The photophysical and electrochemical properties of a series of mono- and bis-phosphine complexes of a 5,15-diphenyl-substituted ruthenium porphyrin, (MeOH)Ru(II)(CO)(DPP) 1, were investigated. The ligands used were diphenyl(phenylacetenyl)phosphine (DPAP), diethyl (phenylacetenyl)phosphonite PAP(OEt)(2)], tris(phenylacetenyl)phosphine (PA)(3)P], and bis(diphenylphosphino)acetylene (DPPA). All complexes display two reversible one-electron oxidations at: 0.61 and 1.0 V vs SCE (1), 0.42-0.51 and 0.97-1.05 V (PR(3))Ru(II)(CO)(DPP)], and 0.06-0.25 and 0.82-0.95 V (PR(3))(2)Ru(II)(DPP)]. As predicted by EHMO calculations, the first oxidation is porphyrin or phosphorus centered, whereas the second one is ruthenium centered. Bulk electrolysis at the first oxidation potential yields stable monocations. Simulation of the cyclic voltammogram of (DPAP)Ru(II)(CO)(DPP) in CH(2)Cl(2) demonstrates the kinetic lability of the complex, and the association constant found (K = 1.27 x 10(6) M(-1)) is in accordance with the value determined by UV-vis titration (K = 1.2 +/- 0.3 x 10(6) M(-1)). Coordination of one phosphine ligand to Ru(II)(CO)(DPP) leads to a red shift in both the absorption and luminescence spectra. Shifts are typically 10 nm for the B- and Q-band absorptions and are not affected by the nature of the phosphorus ligand. The intense luminescence of (PR(3))Ru(II)(CO)(DPP), red-shifted by 21-28 nm compared to 1, can be attributed to originate from a (3)(pi,pi) excited state, and it exhibits lifetimes from 150 to 240 micros. In the bis-phosphine complexes (PR(3))(2)Ru(II)(DPP), the Q-band absorption is broadened and does not show any distinct peak. Judged from EHMO calculation, this could arise from a low-energy charge-transfer state involving the phosphorus ligand. The luminescence is efficiently quenched due to radiationless decay from a charge-transfer excited state, involving either the metal center or the phosphorus ligand; an unambiguous assignment could not be made.