S. V. Litke

Photophysics of mixed ligand complexes of ruthenium(II) with 2,2′-bipyridyl and phosphines

Luminescence of mixed ligand complexes of ruthenium(II) of the types cis-Ru(bpy)2X2(I), cis-[Ru(bpy)2(PPh3)X](BF4)(II), and cis-Ru(bpy)(PP)X2(III) (X = CN, NO2, PPh3 is triphenyl phosphine; PP is 1,2-bis(diphenylphospino)ethane (dppe) and cis-,2-bis(diphenylphosphino)ethylene (dppene)) is studied in alcohol matrices (EtOH/MeOH, 4:1) frozen at 77 K. A sequence of complexes I–III exhibits an additive (in the number of phosphorous atoms) blue shift of the absorption and luminescence bands and an increase in the quantum yield of luminescence and in the excited-state lifetime. The rate constant of nonradiative deactivation of the excited state decreases more than by an order of magnitude in the sequence I–III of cyano complexes and only by three times in a sequence of nitro complexes. This is assumed to be caused by a specific (in the hydrogen bond type) interaction of nitro groups of complexes with a proton solvent.

Spectroscopy and photophysics of chloro-bis-bipyridyl complexes of ruthenium(II) with pyridine ligands

The absorption, luminescence, and luminescence excitation spectra of ruthenium(II) complexes cis-[Ru(bpy)2(L)Cl]+[bpy=2,2′-bipyridyl; L=NH3, pyrazine, pyridine, 4-aminopyridine, 4-picoline, isonicotinamide, 4-cyanopyridine, 4,4′-bipyridyl, or trans-1,2-bis(4-pyridyl)ethylene] in alcoholic (4: 1 EtOH-MeOH) solutions are studied. At 77 K, the quantum yields and decay times of the luminescence of the complexes are measured and the deactivation rate constants of the lowest electronically excited metal-to-ligand charge transfer state (3MLCT) are determined. The linear correlation between the energy of the lowest state 3MLCT dπ(Ru)>π*(bpy) of the cis-[Ru(bpy)2(L)Cl]+ complexes and the parameter pKa of the free 4-substituted pyridines and pyrazine used as ligands is established.

Photophysics of bis-bipyridyl complexes of ruthenium(II) with triphenylphosphine

We studied the spectral-luminescent characteristics of the luminescence of mixed-ligand polypyridine-phosphine complexes of ruthenium(II) cis-[Ru(bpy)2(PPh3)X](BF4)n with ligands 2,2′-bipyridyl (bpy) and triphenylphosphine (PPh3) and X = Cl−, Br−, CN−, NO2−, NH3, MeCN, pyridine (py), 4-aminopyridine (pyNH2), and 4,4′-bipyridyl (4,4′-bpy) in a 4: 1 EtOH-MeOH alcoholic mixture at 77 K. The radiative and nonradiative deactivation rate constants of the lowest electronically excited state of the complexes are determined. We find that triphenylphosphine has a greater effect on the photophysical characteristics of ruthenium(II) complexes compared to π-acceptor strong-field ligands, such as MeCN, CN−, and NO2−. At the same time, the characteristics of complexes cis-[Ru(bpy)2(PPh3)X]n+ considerably depend on the nature of the second monodentate ligand X, which is coordinated to ruthenium(II), and correlate with its position in the spectrochemical series of ligands.

Photophysics of the adsorbed bipyridyl complexes of ruthenium(II) with phosphines

Luminescence of the ruthenium(II) complexes cis-Ru(bpy)2(CN)2 (I), cis-[Ru(bpy)2(PPh3)CN](BF4) (II), and cis-Ru(bpy)(dppe)(CN)2 (III)[bpy=2.2′-bipyridyl, PPh3=triphenylphosphine, dppe=1,2-bis(diphenylphosphino)ethane], adsorbed on silicon oxide (Aerosil) were studied at a temperature of 77 K. The luminescence spectra, decay times, and quantum yields were measured, and the intermolecular rate constants of radiative transitions and nonradiative decay of the excited electronic state with the metal-to-ligand charge transfer (MLCT) were determined. It is found that the adsorption of the complex is accompanied by a decrease in the energy of the radiative MLCT state and by a considerable acceleration of its nonradiative decay. It is concluded that the interaction of the complexes with the surface adsorption centers occurs via formation of a strong hydrogen bond with a hydroxyl-hydrate cover, the interaction of complexes in the 3MLCT state being stronger than in the ground state. The additive (in the number of phosphorus atoms coordinated to the central ruthenium ion), a shift of the absorption and luminescence bands to shorter wavelengths in the sequence of complexes I–III, is retained when the complexes transform from solutions to the absorbed state.

Spectroscopic and quantum-chemical investigations of chloro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

The luminescence spectra of cis-[Ru(bpy)2(L)Cl]+ (bpy is 2,2′-bipyridyl; L is pyrazine, pyridine, 4-amino-pyridine, 4-picolin, isonicotinamide, 4-cyanopyridine, or 4,4′bipyridyl) complexes are studied in alcoholic (4: 1 EtOH-MeOH) solutions at 77 K. A linear correlation is found between the energy of the lowest electronically excited metal-to-ligand charge transfer (3MLCT) state dπ(Ru) → π* (bpy) and the parameter pKa of the free 4-substituted pyridines and pyrazine used as ligands L. The [B3LYP/6-31G + LanL2DZ(Ru)] hybrid method of the density functional theory is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of the ruthenium ion. It is shown that there exists a linear unambiguous correlation between the negative charge on the nitrogen atom (qNL) of ligands L coordinated in the complex and the parameters pKa of free ligands. The calculated energies of 3MLCT excited states almost linearly (correlation coefficient 0.958) depend on the charge qNL, which completely agrees with experimental data.

Spectroscopy of highly luminescent 1,3,5-triazapentadiene complexes of platinum(II) in solution and in the adsorbed and solid states and quantum-chemical interpretation

We have measured the absorption, luminescence, and luminescence excitation spectra, as well as the excited-state lifetimes and luminescence quantum yields, of 1,3,5-triazapentadiene complexes of platinum(II) in a solution, in the solid state, in an adsorbed state on a SiO2 surface, and in a polystyrene matrix at room temperature and at 77 K. We also have performed quantum-chemical calculations of the equilibrium geometry of the ground and excited states of the complexes and of the nature and structure of molecular orbitals.

Spectroscopy and quantum-chemical calculations of nitro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

The luminescence, absorption, and luminescence excitation spectra of complexes cis-[Ru(bpy)2(L)(NO2)]+ [bpy = 2,2′-bipyridyl, L = pyridine, 4-aminopyridine, 4-dimethylaminopyridine, 4-picoline, isonicotinamide, or 4,4′-bipyridyl] in alcoholic (4 : 1 EtOH–MeOH) solutions are studied at 77 K. A linear correlation is established between the energy of the lowest electronically excited metal-toligand charge transfer state dπ(Ru) → π*(bpy) of the complexes and the pKa parameter of the free 4-substituted pyridines used as ligands L. The B3LYP/[6-31G(d)+LanL2DZ(Ru)] hybrid density functional method is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of ruthenium(II) ions. It is shown that there exists a mutually unambiguous correspondence between the charge on the nitrogen atom of ligands L coordinated in the complex and the pKa parameter of ligands. The calculated energies of the electronically excited metal-to-ligand charge transfer states of complexes linearly (correlation coefficient 0.99) depend on the charge on the nitrogen atom of ligands L, which completely agrees with the experimental data.

Spectroscopy and photophysics of binuclear ruthenium(II) complexes with nitrogen-containing heterocyclic bridging ligands

The absorption spectra at room temperature and the spectra, the quantum yields, and the decay times of the luminescence at 77 K of binuclear complexes [X(bpy)2Ru(BL)Ru(bpy)2Cl]2+ (bpy = 2,2′-bipyridyl; X = Cl, BL = pyrazine, 4,4′-bipyridyl, trans-1,2-bis(4-pyridyl)ethylene, and trans-1,2-bis(4-pyridyl)ethane and X = NO2, BL = 4,4′-bipyridyl) in alcoholic (4: 1 EtOH-MeOH) solutions are studied. It is shown that the interaction between the metal centers (MCs) of the complexes affects the characteristics of the electronically excited states (EESs) of each of them and facilitates increasing the transition dipole moment Ru(dπ)→BL(π*). The deactivation rate constants of the lowest electronically excited metal-to-ligand charge transfer (3MLCT) state of the complexes are determined. In an asymmetric binuclear complex, the energy transfer from MC(NO2) to MC(Cl) is revealed, with the rate constant of this transfer being not smaller than 3.2 × 1010 s−1.