Excited-State Coordination Chemistry: A New Quenching Mechanism Thomas Dougherty, † Charles Hicks, † Anthony Maletta, † Jianwei Fan, ‡ Irina Rutenberg, § and Harry D. Gafney* ,† Department of Chemistry & Biochemistry City UniVersity of New York, Queens College Flushing, New York 11367 Department of Chemistry & Biochemistry Manhattan College, RiVerdale, New York 10471 Department of Chemistry, City UniVersity of New York Queensborough Community College Bayside, New York 11364 ReceiVed July 21, 1997 The quenching of Ru(II) diimines has provided unique insights into the excited-state chemistry of transition-metal complexes. Quenching by inorganic and organic reagents, molecular oxygen, and hydrogen ion has elucidated an excited-state redox chemistry, inter- and intramolecular energy transfer processes, and striking excited-state acid-base properties. 1-11 Here, we present evidence for another type of quenching mechanism which results in an excited-state coordination chemistry and leads to an unusual photochemical reaction that increases the metal content of the complex. In this case, optical excitation of Ru(bpy) 2 (dpp) 2+ (bpy ) bipyridyl; dpp ) 2,3-dipyridylpyrazine) produces an metal- to-ligand charge transfer (MLCT) state localized on the dpp ligand. 12 The increased electron density in the excited state increases the basicity of the dpp ligand’s peripheral nitrogens, 11 and this increase in basicity leads to coordination to a second metal ion. Ru(bpy) 2 (dpp) 2+ has been widely used to assemble polymetallic complexes. 12-16 Refluxing the complex in the presence of a variety of transition metal salts and complexes results in coordina- tion at the peripheral nitrogens and, depending on the number of dpp ligands attached to Ru(II), formation of polymetallic com- plexes. Refluxing Ru(bpy) 2 (dpp) 2+ and PtCl 6 2- in ethanol, for example, changes the solution color from reddish-orange to purple, and chromatographic separation of the reaction mixture confirms [Ru(bpy) 2 (dpp)PtCl 4 ] 2+ formation. The color change is due to coordination of Pt(IV) at the dpp peripheral nitrogens which shifts the MLCT transition to dpp from 470 nm in Ru(bpy) 2 (dpp) 2+ to 525 nm in [Ru(bpy) 2 (dpp)PtCl 4 ] 2+ . In water at 30 °C and μ ) 1.0 M (NaCl), the rate law for the thermal reaction is R ) k th [Ru(bpy) 2 (dpp) 2+ ][PtCl 6 2- ] where k th ) 1.8 ( 0.4 × 10 -4 M -1 s -1 and, in the 30 to 50 °C range, E a ) 15 ( 2 kcal/mol. Spectra recorded periodically during a 457-nm photolysis of an aqueous solution 10 -4 M in Ru(bpy) 2 (dpp) 2+ and 10 -2 M in PtCl 6 2- (Figure 1) show a decline in absorbance at 470 nm and a corresponding increase at 525 nm. The spectral changes, which occur in a matter of minutes, are identical to those recorded during the thermal reaction, and thin-layer chromatography of the photolyte and isolation of the photoproduct confirm [Ru(bpy) 2 - (dpp)PtCl 4 ] 2+ formation. Isosbestic points at 398 and 483 nm (Figure 1) indicate a quantitative conversion through g60-70% reaction. Similar spectral changes indicative of dimer formation occur with OsCl 6 2- , RhCl 6 3- , and PdCl 6 2- , although with the latter complex, the extent of the quantitative conversion is limited by the hydrolysis of the hexachloride. At μ ) 3.0 M (NaCl), the Stern-Volmer constants from intensity and lifetime quenching of the 675-nm emission of Ru- (bpy) 2 (dpp) 2+ by PtCl 6 2- , 403 ( 35 M and 397 ( 43 M, respectively, are within experimental error (Figure 2) and in excellent agreement with the Stern-Volmer constant, 413 ( 62 M, obtained from the ratio of the slope to intercept of plots of the reciprocal of the quantum efficiency of [Ru(bpy) 2 (dpp)PtCl 4 ] 2+ formation, Φ bi , versus the reciprocal of the concentration of PtCl 6 2- (Figure 3). The equivalence of the values obtained from the different techniques establishes that, at high ionic strength, formation of [Ru(bpy) 2 (dpp)PtCl 4 ] 2+ occurs via a diffusional encounter between the excited Ru(bpy) 2 (dpp) 2+ and PtCl 6 2- . The intercept in Figure 3 yields a limiting value of Φ bi ) 0.18 ( 0.02, and taking 135 ( 14 ns as the lifetime of the MLCT state of Ru(bpy) 2 (dpp) 2+ , 12 the relation K sv ) k b τ yields 2.84 ( 0.56 × 10 9 M -1 s -1 for the bimolecular rate constant. Bimetallic formation occurs with higher efficiency at low ionic strength, but the reaction occurs via optical excitation of [Ru(bpy) 2 (dpp) 2+ , PtCl 6 2- ] ion pairs. Bimetallic formation is an unusual photochemical reaction that does not occur with metals known to quench by electron- or energy-transfer processes. Fe 3+ and Cr 3+ quench by electron and energy transfer, respectively, and form stable diimine complexes. Cr 3+ is also substitution inert, yet neither ion reacts with * Ru- (bpy) 2 (dpp) 2+ to form a bimetallic, Φ bi e10 -3 . Bimetallic formation is also energetically inconsistent with either photoin- duced electron or energy transfer. The emission from Ru(bpy) 2 - (dpp) 2+ places the MLCT state localized on dpp 1.48 eV above the ground state, while the absorption spectrum of PtCl 6 2- indicates that the 1 T 1 state lies 3.5 eV above the ground state. Lower energy, weaker absorptions of PtCl 6 2- place the spin forbidden 3 T 2 state at 2.8 eV, and from the Tanabe-Sugano diagram, the 3 T 1 state is calculated to lie at 2 eV above the ground state. 17 Clearly, uncertainties exist in the latter, yet neither state is expected to be <1.48 eV, making energy transfer endergonic. The reaction is also not due to a trivial photolysis of the hexachlorides followed by coordination to Ru(bpy) 2 (dpp) 2+ . 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Soc. 1998, 120, 4226-4227 S0002-7863(97)02439-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/15/1998