Tuning photophysical properties with ancillary ligands in Ru(II) mono-diimine complexes

Abstract

The series of complexes [XRu(CO)(L-L)(L')₂][PF₆] (X = H, TFA, Cl; L-L = 2,2'-bipyridyl, 1,10-phenanthroline, 5-amino-1,10-phenanthroline and 4,4'-dicarboxylic-2,2'-bipyridyl; L'₂ = 2PPh₃, Ph₂ PC₂H₄PPh_2, Ph₂PCH═CHPPh₂) have been synthesized from the starting complex K[Ru(CO)₃(TFA)₃] (TFA = CF₃CO₂) by first reacting with the phosphine ligand, followed by reaction with the L-L and anion exchange with NaPF₆. In the case of L-L = phenanthroline and L'₂ = 2PPh₃, the neutral complex Ru(Ph₃P)(CO)(1,10-phenanthroline)( TFA)₂ is also obtained and its solid state structure is reported. Solid state structures are also reported for the cationic complexes where L-L = phenanthroline, L₂ = 2PPh₃ and X = Cl and for L-L = 2,2'-bipyridyl, L₂ = 2PPh₃ and X = H. All the complexes were characterized in solution by a combination of ¹H and ³¹P NMR, IR, mass spectrometry and elemental analyses. The purpose of the project was to synthesize a series of complexes that exhibit a range of excited-state lifetimes and that have large Stokes shifts, high quantum yields and high intrinsic polarizations associated with their metal-to-ligand charge-transfer (MLCT) emissions. To a large degree these goals have been realized in that excited-state lifetimes in the range of 100 ns to over 1 μs are observed. The lifetimes are sensitive to both solvent and the presence of oxygen. The measured quantum yields and intrinsic anisotropies are higher than for previously reported Ru(II) complexes. Interestingly, the neutral complex with one phosphine ligand shows no MLCT emission. Under the conditions of synthesis some of the initially formed complexes with X = TFA are converted to the corresponding hydrides or in the presence of chlorinated solvents to the corresponding chlorides, testifying to the lability of the TFA Ligand. The compounds show multiple reduction potentials which are chemically and electrochemically reversible in a few cases as examined by cyclic voltammetry. The relationships between the observed photophysical properties of the complexes and the nature of the ligands on the Ru(II) is discussed.

Keywords: Electrochemistry; Metal-to-ligand charge-transfer; Phosphine ligands; Photophysical properties; Ruthenium complexes.

Fig. 1

Solid state structure of (a) C₄₉H₃₈C_lF₆N₃O₄P₃Ru₁ (5), (b) C₃₅H₂₃F₆N₂O₅P₁Ru₁ (6) and (c) C₄₇H₃₉F₆N₂O₁P₃Ru₁ (10) showing the 90% probability thermal elipsoids and the calculated positions of the hydride and hydrogen atoms.

Fig. 2

CV of a 1.0 mM solution of 7 in CH₂Cl₂ containing 0.10 M [NBu₄][PF₆], at a GC working electrode; v = 0.2 V s⁻¹ (● starting potential).

Fig. 3

CV of a 1.0 mM solution of 5 in CH₂Cl₂ containing 0.10 M [NBu₄][PF₆], at a GC working electrode; v = 0.2 V s⁻¹ (● starting potential).

Fig. 4

(a) Absorption and (b) emission spectra of 9 in ethanol.

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