Metal-to-ligand Charge Transfer Absorption Research Articles

Photoinduced Electron and Energy Transfer and pH-Induced Modulation of the Photophysical Properties in Homo- and Heterobimetallic Complexes of Ruthenium(II) and Rhodium(III) Based on a Heteroditopic Phenanthroline–Terpyridine Bridge

Homo- and heterobimetallic complexes of compositions [(bpy)2Ru(II)(phen-Hbzim-tpy)Ru(II)(tpy/tpy-PhCH3/H2pbbzim)](4+) and [(bpy)2Ru(II)(phen-Hbzim-tpy)Rh(III)(tpy-PhCH3/H2pbbzim)](5+), where phen-Hbzim-tpy = 2-[4-(2,6-dipyridin-2-ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-f][1,10]phenanthroline, bpy = 2,2'-bipyridine, tpy = 2,2':6',2"-terpyridine, tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2"-terpyridine, and H2pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine, have been synthesized and characterized by elemental analyses, electrospray ionization mass spectrometry, and (1)H NMR spectroscopy. The absorption spectra, redox behavior, and luminescence properties of these bimetallic complexes have been thoroughly investigated and compared with those of monometallic [(bpy)2Ru(II)(phen-Hbzim-tpy)](2+) and [(tpy-PhCH3)Rh(III)(tpy-Hbzim-phen)](3+) model compounds. The electrochemistry of the complexes shows a reversible Ru(II/III) oxidation in the anodic region and an irreversible Rh(III/I) reduction and several ligand-based reductions in the cathodic region. Steady-state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer from the metal-to-ligand charge-transfer (MLCT) excited state of the [(bpy)2Ru(II)(phen-Hbzim-tpy)] chromophore to the MLCT state of the tpy-containing chromophore [(phen-Hbzim-tpy)Ru(II)(tpy/tpy-PhCH3/H2pbbzim)] occurs in all three unsymmetrical homobimetallic complexes. On the other hand, for both heterometallic dyads, an efficient intramolecular photoinduced electron transfer from the excited ruthenium moiety to the rhodium-based unit takes place. The rate constants for the energy- and electron-transfer processes have been determined by time-resolved emission spectroscopy. The influence of the pH on the absorption, steady-state, and time-resolved emission properties of complexes has been thoroughly investigated. The absorption titration data were used to determine the ground-state pK values, whereas the luminescence data were utilized for determination of the excited-state acid dissociation constants. In effect, deprotonation of the azole NH moieties of the complexes leads to a substantial lowering of the MLCT absorption and emission band energies.

Characterization of Low Energy Charge Transfer Transitions in (terpyridine)(bipyridine)Ruthenium(II) Complexes and their Cyanide-Bridged Bi- and Tri-Metallic Analogues

The lowest energy metal-to-ligand charge transfer (MLCT) absorption bands found in ambient solutions of a series of [Ru(tpy)(bpy)X](m+) complexes (tpy = 2,2':3',2''-terpyridine; bpy = 2,2'-bipyridine; and X = a monodentate ancillary ligand) feature one or two partly resolved weak absorptions (bands I and/or II) on the low energy side of their absorption envelopes. Similar features are found for the related cyanide-bridged bi- and trimetallic complexes. However, the weak absorption band I of [(bpy)(2)Ru{CNRu(tpy)(bpy)}(2)](4+) is missing in its [(bpy)(2)Ru{NCRu(tpy)(bpy)}(2)](4+) linkage isomer demonstrating that this feature arises from a Ru(II)/tpy MLCT absorption. The energies of the MLCT band I components of the [Ru(tpy)(bpy)X](m+) complexes are proportional to the differences between the potentials for the first oxidation and the first reduction waves of the complexes. Time-dependent density functional theory (TD-DFT) computational modeling indicates that these band I components correspond to the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition, with the HOMO being largely ruthenium-centered and the LUMO largely tpy-centered. The most intense contribution to a lowest energy MLCT absorption envelope (band III) of these complexes corresponds to the convolution of several orbitally different components, and its absorption maximum has an energy that is about 5000 cm(-1) higher than that of band I. The multimetallic complexes that contain Ru(II) centers linked by cyanide have mixed valence excited states in which more than 10% of electronic density is delocalized between the nearest neighbor ruthenium centers, and the corresponding stabilization energy contributions in the excited states are indistinguishable from those of the corresponding ground states. Single crystal X-ray structures and computational modeling indicate that the Ru-(C≡N)-Ru linkage is quite flexible and that there is not an appreciable variation in electronic structure or energy among the conformational isomers.

Complete Family of Mono-, Bi-, and Trinuclear ReI(CO)3Cl Complexes of the Bridging Polypyridyl Ligand 2,3,8,9,14,15-Hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene: Syn/Anti Isomer Separation, Characterization, and Photophysics

The syn and anti isomers of the bi- and trinuclear Re(CO)(3)Cl complexes of 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me(6)) are reported. The isomers are characterized by (1)H NMR spectroscopy and X-ray crystallography. The formation of the binuclear complex from the reaction of HATN-Me(6) with 2 equiv of Re(CO)(5)Cl in chloroform results in a 1:1 ratio of the syn and anti isomers. However, synthesis of the trinuclear complex from the reaction of HATN-Me(6) with 3 equiv of Re(CO)(5)Cl in chloroform produces only the anti isomer. syn-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} can be synthesized by reacting 1 equiv of Re(CO)(5)Cl with syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} in refluxing toluene. The product is isolated by subsequent chromatography. The X-ray crystal structures of syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} and anti-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} are presented both showing severe distortions of the HATN ligand unit and intermolecular π stacking. The complexes show intense absorptions in the visible region, comprising strong π → π* and metal-to-ligand charge-transfer (MLCT) transitions, which are modeled using time-dependent density functional theory (TD-DFT). The energy of the MLCT absorption decreases from mono- to bi- to trinuclear complexes. The first reduction potentials of the complexes become more positive upon binding of subsequent Re(CO)(3)Cl fragments, consistent with changes in the energy of the MLCT bands and lowering of the energy of relevant lowest unoccupied molecular orbitals, and this is supported by TD-DFT. The nature of the excited states of all of the complexes is also studied using both resonance Raman and picosecond time-resolved IR spectroscopy, where it is shown that MLCT excitation results in the oxidation of one rhenium center. The patterns of the shifts in the carbonyl bands upon excitation reveal that the MLCT state is localized on one rhenium center on the IR time scale.

Effects of Electronic Mixing in Ruthenium(II) Complexes with Two Equivalent Acceptor Ligands. Spectroscopic, Electrochemical, and Computational Studies

The lowest energy metal to ligand charge transfer (MLCT) absorption bands found in ambient solutions of [Ru(NH(3))(4)(Y-py)(2)](2+) and [Ru(L)(2)(bpy)(2)](+) complexes (Y-py a pyridine ligand and (L)(n) a substituted acetonylacetonate, halide, am(m)ine, etc.) consist of two partly resolved absorption envelopes, MLCT(lo) and MLCT(hi). The lower energy absorption envelope, MLCT(lo), in these spectra has the larger amplitude for the bis-(Y-py) complexes, but the smaller amplitude for the bis-bpy the complexes. Time-dependent density functional theory (TD-DFT) approaches have been used to model 14 bis-bpy, three bis-(Y-py), and three mono-bpy complexes. The modeling indicates that the lowest unoccupied molecular orbital (LUMO) of each bis-(Y-py) complex corresponds to the antisymmetric combination of individual Y-py acceptor orbitals and that the transition involving the highest occupied molecular orbital (HOMO) and LUMO (HOMO-->LUMO) is the dominant contribution to MLCT(lo) in this class of complexes. The LUMO of each bis-bpy complex that contains a C(2) symmetry axis also corresponds largely to the antisymmetric combination of individual ligand acceptor orbitals, while the LUMOs are more complex when there is no C(2) axis; furthermore, the energy difference between the HOMO-->LUMO and HOMO-->LUMO+1 transitions is too small (<1000 cm(-1)) to resolve in the spectra of the bis-bpy complexes in ambient solutions. Relatively weak MLCT(lo) absorption contributions are found for all of the [Ru(L)(2)(bpy)(2)](m+) complexes examined, but they are experimentally best defined in the spectra of the (L)(2) = X-acac complexes. TD-DFT modeling of the HOMO-->LUMO transition of [Ru(L)(4)bpy](m+) complexes indicates that it is too weak to be detected and occurs at significantly lower energy (about 3000-5000 cm(-1)) than the observed MLCT absorptions. Since the chemical properties of MLCT excited states are generally correlated with the HOMO and/or LUMO properties of the complexes, such very weak HOMO-->LUMO transitions can complicate the use of spectroscopic information in their assessment. As an example, it is observed that the correlation lines between the absorption energy maxima and the differences in ground state oxidation and reduction potentials (DeltaE(1/2)) have much smaller slopes for the bis-bpy than the mono-bpy complexes. However, the observed MLCT(lo) and the calculated HOMO-->LUMO transitions of bis-bpy complexes correlate very similarly with DeltaE(1/2) and this indicates that it is the low energy and small amplitude component of the lowest energy MLCT absorption band that is most appropriately correlated with excited state chemistry, not the absorption maximum as is often assumed.

Photodriven Spin Change of Fe(II) Benzimidazole Compounds Anchored to Nanocrystalline TiO2 Thin Films

Ferrous tris-chelate compounds based on 2-(2'-pyridyl)benzimidazole (pybzim) have been prepared and characterized for studies of spin equilibria in fluid solution and when anchored to the surface of mesoporous nanocrystalline (anatase) TiO(2) and colloidal ZrO(2) thin films. The solid state structure of Fe(pybzim)(3)(ClO(4))(2).CH(3)CN.H(2)O was determined by single-crystal X-ray diffraction at 110 K to be triclinic, P-1, a = 11.6873(18), b = 12.2318(12), c = 14.723(4) A, alpha = 89.864(13) degrees , beta = 71.430(17) degrees , gamma = 73.788(11) degrees , V = 1907.1(6) A(3), Z = 2, and R = 0.0491. The iron compound has a meridional FeN(6) distorted octahedral geometry with bond lengths expected for a low-spin iron center at 110 K. The visible absorption spectra of Fe(pybzim)(3)(2+) and Fe(pymbA)(3)(2+), where pymbA is 4-(2-pyridin-2-yl-benzimidazol-1-ylmethyl)-benzoic acid, in methanol solution were dominated by metal-to-ligand charge-transfer (MLCT) bands. Variable-temperature UV-visible absorption spectroscopy revealed dramatic changes in the extinction coefficient consistent with a high-spin ((1)A) left harpoon over right harpoon low-spin ((5)T) equilibrium. Thermodynamic parameters for the temperature-dependent spin equilibrium of Fe(pymbA)(3)(2+) in methanol were determined to be DeltaH(HL) = 3270 +/- 210 cm(-1) and DeltaS(HL) = 13.3 +/- 0.8 cm(-1) K(-1). The corresponding values for Fe(pybzimEE)(3)(2+), where pybzimEE is (2-pyridin-2-yl-benzimidazol-1-yl)-acetic acid ethyl ester, in acetonitrile solution were determined to be 3072 +/- 34 cm(-1)and 10.5 +/- 0.1 cm(-1) K(-1). The temperature-dependent effective magnetic moments of Fe(pybzimEE)(3)(2+) in acetonitrile solution were also quantified by the Evans method. Pulsed 532 nm light excitation of Fe(pybzim)(3)(2+) or Fe(pymbA)(3)(2+) in solution resulted in an immediate bleach of the MLCT absorption bands. Relaxation back to the equilibrium state followed a first-order reaction mechanism. Arrhenius analysis of the (5)T --> (1)A rate constant yielded an activation energy, E(a), of 1090 +/- 20 cm(-1) and 710 +/- 10 cm(-1) for Fe(pybzim)(3)(2+) and Fe(pymbA)(3)(2+) in methanol, respectively. The compound Fe(pymbA)(3)(2+) was found to bind to colloidal TiO(2) and ZrO(2) thin films. The absorption spectra of the surface-attached compounds were quantified from 295 to 193 K. Pulsed light excitation of Fe(pymbA)(3)/TiO(2) and Fe(pymbA)(3)/ZrO(2) resulted in the immediate bleach of the MLCT absorption bands. Relaxation was nonexponential but was well described by kinetic models based on a Gaussian distribution of activation energies or a Levy distribution of lifetimes. An Arrhenius analysis of the Gaussian data yielded average activation energies of 660 +/- 80 cm(-1) and 730 +/- 40 cm(-1) for Fe(pymbA)(3)(ClO(4))(2) on TiO(2) and ZrO(2) surfaces, respectively. The Levy distribution analysis did not adequately fit the Arrhenius model.

Sensitized near-infrared emission of Yb III from an Ir III–Yb III bimetallic complex

A new cyclometalated ligand 1,3-dimethyl-5-phenyl-1H-[1,2,4]-triazole (pdt) was designed and synthesized. And the corresponding Ir III complex Ir(pdt) 2(phen5f) (phen5f stands for 4,4,5,5,5-pentafluoro-1-(1′,10′-phenanthrolin-2′-yl)-pentane-1,3-dionate) was obtained. According to the measurement of the lowest triplet state energy level of Ir(pdt) 2(phen5f), it is suitable for sensitizing NIR (near-infrared) lanthanide ions instead of Eu III. The bimetallic complex [(pdt) 2Ir(μ-phen5f)YbCl 2 · 2CH 3CH 2OH · H 2O]Cl was synthesized by the approach of “complexes as ligands”. Data showed that the emission quenching was observed in the solid state when the Ir III–Yb III complex was compared with the Ir III complex, which implied that energy transfer might occur from Ir III complex-ligand to Yb III ion. Upon irradiation of the MLCT (metal-to-ligand charge transfer) absorption of Ir(pdt) 2(phen5f), the characteristic emission of Yb III was obtained with the peak around 978 nm.

Study of the correlations of the MLCT Vis absorption maxima of 4‐pentacyanoferrate‐ 4′‐arylsubstituted bispyridinium complexes with the Hammett substituent parameters and the solvent polarity parameters E and AN

AbstractIn this work six new 4‐aryl substituted bispyridinium salts have been synthesized using 2,4‐dinitrophenyl‐ bispyridinium chloride as the starting material, which was arenamine exchanged via the Zincke reaction. These aromatic heterocyclic salts were used as ligands, acting as Lewis bases to form a series of 4‐pentacyanoferrate‐4′‐aryl substituted bispyridinium complex salts in high yields. The complex salts were characterized using several spectroscopic techniques including 1H as well as 13C NMR spectroscopy, UV–Vis and FTIR spectrophotometry. These materials present a number of interesting electronic and optical properties. Herein we mainly focused on the electronic absorption of these materials. The visible metal‐to‐ligand‐charge‐transfer (MLCT) band of these materials assigned as dπ(FeII) → π*(L) was proved to be affected by substituent changes (of the benzene ring of the complexes) but mainly by solvent polarity changes. Band shifts of even 4000 cm−1 were induced by small solvent polarity changes (e.g., water to methanol). The Vis absorption spectra were investigated in four protic solvents (water, 2,2,2‐trifluoroethanol, ethylene glycol, and methanol). The substituent effects were then quantified using the Hammett equation which correlates the MLCT absorption wavenumbers with the Hammett constant (σx). Furthermore, two of the most successful solvent polarity parameters (the acceptor number AN and the normalized solvent polarity scale E) were used to quantify the solvent polarity effects on the MLCT absorption wavenumbers. The correlations obtained in all cases proved to be satisfactory. The dominant interaction responsible for the solvent polarity effects proved to be the hydrogen‐bond formation between the cyano groups of the complex salts and the protic solvents. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Synthesis and properties of electrochemiluminescent dinuclear Ru(II) complexes assembled with ester-bridged bis(bipyridine) ligands

New bridging ligands, such as bpy-(COOCH 2)-bpy (BL1), mbpy-(CH 2) 3COOCH 2-bpy (BL2), bpy-COO(CH 2) 6OOC-bpy (BL3), and bpy-COOCH 2PhPhCH 2OOC-bpy (BL4), have been synthesized and coordinated to [RuL 2(acetone) 2](PF 6) 2 for various [Ru(L) 2(BL)Ru(L) 2](PF 6) 4-type dinuclear ruthenium complexes (where BL = BL1, BL2, BL3, BL4, and L = bpy, o-phen, DTDP). Their electrochemical redox potentials, spectroscopic properties and relative electrochemiluminescence were investigated in detail. All dinuclear Ru complexes exhibit MLCT (metal-to-ligand charge transfer) absorption and luminescence bands in the visible region. ECL intensities of dinuclear ruthenium(II) complexes were affected not only by the kind of the ligand, but also by the nature of the bridging ligand. Among the synthesized dinuclear Ru complexes, [(DTDP) 2Ru(mbpy)-(CH 2) 3COOCH 2-(bpy)Ru(DTDP) 2](PF 6) 4 exhibited enhanced ECL intensities as high as 2.9 times greater than that of the reference, [Ru( o-phen) 3](PF 6) 2.

Sensitised near-infrared emission from lanthanides using an iridium complex as a ligand in heteronuclear Ir2Ln arrays

The iridium(III) complex Ir(ppy)2(phen5f) [ppy = 2-phenylpyridinato-N,C2′, and phen5f = 4,4,5,5,5-pentafluoro-1-(1′,10′-phenanthrolin-2′-yl)-pentane-1,3-dionate] has been synthesised and used as “complex ligands” to make heteronuclear d–f complexes by the attachment of Ln(NO3)3·xH2O at the vacant coordination sites in the bridging ligand phen5f. The microanalyses and crystal structure characteristics confirmed the formation of the heteronuclear Ir2Ln arrays. The measurement of the lowest triplet state energy level of Ir(ppy)2(phen5f) indicates that it is suitable for the NIR (near-infrared) lanthanide ions, NdIII, YbIIIand ErIII. Upon irradiation of the MLCT (metal-to-ligand charge transfer) absorption of Ir(ppy)2(phen5f) at an excitation wavelength from 380–490 nm, the characteristic emission spectra of the three Ir2Ln arrays (Ln = Nd, Yb, Er) in both the solid state and in CH3CN solution were measured. According to the results, more IrIII complexes will be designed for lanthanide NIR emission by the proper combination between the cyclometalated ligand and the tetradentate ancillary ligand.

Synthesis and properties of 6,6′-dithienyl-4,4′-bipyrimidine and its hetero- and homo-leptic Ru(II) complexes

Hetero- and homo-leptic Ru(II) complexes of a new 4,4′-bipyrimidine ligand, th 2bpm (6,6′-di(2″-thienyl)-4,4′-bipyrimidine), have been synthesized and characterized. The parent ligand th 2bpm has electron rich thiophene units on the periphery of a bidentate ligand which is capable of binding to metal ions. The heteroleptic complex of th 2bpm [Ru(bpy) 2th 2bpm] 2+ (bpy = 2,2′-bipyridine) exhibits a Ru-to-bpm metal-to-ligand charge transfer (MLCT) absorption centered at 547 nm and a Ru-to-bpy MLCT absorption centered at 438 nm. The assignment of the low energy absorption is supported by the relative ease of electrochemical reduction of the new complex as compared to [Ru(bpy) 3] 2+. The homoleptic complex, [Ru(th 2bpm) 3] 2+, exhibits a Ru-to-bpm MLCT absorption at slightly higher energy (544 nm). Both complexes are emissive at room temperature in fluid solution and 5 is one of the lowest energy emitters based on tris-bidentate Ru(II) complexes known ( λ max = 770 nm). The luminescence spectra is red-shifted compared to [Ru(bpy) 3] 2+ and this effect is ascribed to the delocalization in the acceptor ligand.

Spectroscopy of Ru(II) polypyridyl complexes used as intercalators in DNA: Towards a theoretical study of the light switch effect

The absorption spectroscopy of [Ru(phen) 2dppz] 2+ and [Ru(tap) 2dppz] 2+ (phen = 1,10-phenanthroline, tap = 1,4,5,8-tetraazaphenanthrene; dppz = dipyridophenazine) complexes used as molecular light switches by intercalation in DNA has been analysed by means of Time-Dependent Density Functional Theory (TD-DFT). The electronic ground state structures have been optimized at the DFT (B3LYP) level of theory. The absorption spectra are characterized by a high density of excited states between 500 nm and 250 nm. The absorption spectroscopy of [Ru (phen) 2dppz] 2+ in vacuum is characterized by metal-to-ligand-charge-transfer (MLCT) transitions corresponding to charge transfer from Ru(II) either to the phen ligands or to the dppz ligand with a strong MLCT ( d Ru → π dppz * ) absorption at 411 nm. In contrast, the main feature of the lowest part of the vacuum theoretical spectrum of [Ru(tap) 2dppz] 2+ between 522 nm and 400 nm is the presence of various excited states such as MLCT ( d Ru → π TAP * ), ligand-to-ligand-charge-transfer LLCT ( π dppz → π TAP * ) or intra-ligand IL ( π dppz → π dppz * ) states. When taking into account solvent corrections within the polarizable continuum model (PCM) approach (H 2O, CH 3CN) the absorption spectrum of [Ru(tap) 2dppz] 2+ is dominated by a strong absorption at 388 nm (CH 3CN) or 390 nm (H 2O) assigned to a 1IL ( π dppz → π dppz * ) corresponding to a charge transfer from the outside end of the dppz ligand to the site of coordination to Ru(II). These differences in the absorption spectra of the two Ru(II) complexes have dramatic effects on the mechanism of deactivation of these molecules after irradiation at about 400 nm. In particular, the electronic deficiency at the outside end of the dppz ligand created by absorption to the 1IL state will favour electron transfer from the guanine to the Ru(II) complex when it is intercalated in DNA.

A TD-DFT study on the electronic spectrum of Ru(II)L 2 [L = bis(5′-methyl-2,2′-bipyridine-6-carboxylato)] in the gas phase and DMF solution

Ruthenium polypyridine and polypyridined-derivative complexes are used in dye-sensitized solar cell as a light to current conversion sensitizers, study on the electronic absorption spectroscopy of which are very important for the understanding of their photovoltaic character. A theoretical research of UV–Vis spectrum on the Ru(II)L 2[L = bis(5′-methyl-2,2′-bipyridine-6-carboxylato)], a polypyridined-derivative mononuclear ruthenium complex, is presented here. Transition excited states of Ru(II)L 2 in the gas phase have been studied by the time-dependent density functional theory (TD-DFT) using a hybrid functional, B3LYP, as well as the 3-21G ∗ basis set. Two maximum absorptions are dominated by the metal-to-ligand-charge-transfer (MLCT) transitions, from Ru 4d orbitals to the bipyridine rings π ∗. Taking account of solvent effect, the electronic absorption spectrum of Ru(II)L 2 in N, N′-dimethyl formamide (DMF) solution has also been investigated on the basis of Polarized Continuum Model (PCM) in connection with TD-DFT. Calculation results indicate that the two maximum MLCT absorption peaks are blue-shifted in DMF solution with respect to those in the gas phase, which is in agreement with the observed phenomenon of other ruthenium polypyridine-type complexes.

Synthesis of transition-metal-containing poly(pyrazabole)s

Abstract Novel organoboron polymers containing transition metals (Ni, Pd, Pt) and pyrazaboles in the main chain were prepared by Cu-catalyzed coupling polymerization. The Ni- and Pt-containing polymers showed MLCT (metal-to-ligand charge transfer) absorption, whereas the Pd-containing polymer did not demonstrate the absorption. Only the Pd-containing polymer exhibited the emission spectrum with a vibronic structure. These differences in the optical properties were studied in detail.

Interligand Electron Transfer Dynamics in [Os(bpy)3]2+: Exploring the Excited State Potential Surfaces with Femtosecond Spectroscopy

Femtosecond transient absorption studies of [Os(bpy)3]2+ have been performed to gain new insights into the excited-state dynamics. Experiments were performed for a series of excitation wavelengths throughout both the singlet and triplet metal-to-ligand charge transfer (MLCT) bands. The dynamics are probed via the π−π* transition on the bpy- anion at 370 nm. Time scales for interligand electron transfer (ILET) and vibrational cooling are extracted from the amplitude and anisotropy of the excited state absorptions. The ILET rate has been measured as a function of temperature and, from these data, estimates for the electron transfer activation energy and ligand−ligand coupling are made. Our data also provide insight into the vibrational cooling dynamics within these large complexes, suggesting that vibrational cooling occurs only when exciting into the singlet MLCT state, after intersystem crossing. It appears that photoexcitation within the triplet MLCT absorption produces molecules that are vibrationally cold, even for excitation at the blue edge of the absorption band.

Syntheses, spectroscopic and molecular quadratic nonlinear optical properties of dipolar ruthenium(II) complexes of the ligand 1,2-phenylenebis(dimethylarsine).

Six new complex salts trans-[Ru(II)Cl(pdma)2L][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); L = (E,E,E)-1,6-bis(4-pyridyl)hexa-1,3,5-triene (bph), n= 1, 5; L =N-methyl-4-[(E)-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+), n= 2, 7; L =N-methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]pyridinium (Mebpb+), n= 2, 8; L =N-methyl-4-[(E,E,E)-6-(4-pyridyl)hexa-1,3,5-trienyl]pyridinium (Mebph+), n= 2, 9; L = bis(4-pyridyl)acetylene (bpa), n= 1, 10; L =N-methyl-4-[2-(4-pyridyl)ethynyl]pyridinium (Mebpa+), n= 2, 11] have been prepared. The electronic absorption spectra of 5 and 7-11 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with lambdamax values in the range 434-492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible Ru(III/II) waves with E(1/2) values in the range 1.06-1.15 V vs. Ag-AgCl, together with irreversible L-based reduction processes. Along with a number of previously reported related compounds (B. J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797), salts 5 and 7-11 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes Deltamu12 for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities beta0 according to the two-state equation beta0= 3Deltamu12(mu12)2/(Emax)2 (mu12 = transition dipole moment, Emax = MLCT energy). MLCT absorption and electrochemical data show that a trans-[Ru(II)Cl(pdma)2]+ centre is considerably less electron-rich than a [Ru(II)(NH3)5]2+ unit. Although the beta0 responses of the pdma complexes are only a little smaller than those of their [Ru(II)(NH3)5]2+ analogues, this result is partly attributable to unexpected changes in the relative mu12 values on freezing. Thus, substantial increases in mu12 for the arsine compounds act to partially offset the beta0-decreasing influence of their higher Emax values when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained for the salts 1(.)2.5MeCN, 4(.)2MeCN, 7 and 11, but only 1(.)2.5MeCN adopts a non-centrosymmetric space group (Fdd2) such as may show bulk NLO effects.

Excited-state metal-to-ligand charge transfer dynamics of a ruthenium(II) dye in solution and adsorbed on TiO2 nanoparticles from resonance Raman spectroscopy.

The dynamics of metal-to-ligand charge transfer (MLCT) in a cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II) dye (N3) are compared for the free dye in solution and the dye adsorbed on the surface of the TiO(2) nanoparticles from resonance Raman spectroscopy. The 544-nm MLCT absorption band of N3 adsorbed on TiO(2) is slightly blue-shifted from that of the free N3, indicating a weak electronic coupling between N3 and TiO(2). The resonance Raman spectra of N3 and the N3|TiO(2) complex obtained upon excitation within the lowest-lying MLCT singlet state of the dye are similar except for slight shifts in band positions. Resonance Raman cross sections have been obtained for the vibrational modes of both N3 and N3|TiO(2) with excitation frequencies spanning the 544-nm MLCT band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism over two electronic states yields mode-specific vibrational and solvent reorganization energies. Despite the weak electronic coupling between N3 and TiO(2) in N3|TiO(2), adsorption strongly affects the reorganization energies of N3 in the intramolecular MLCT state. Adsorption of N3 onto TiO(2) increases the absolute Raman cross section of each mode by a factor of ca. 1.6 and decreases the vibrational and solvent reorganization energies by factors of 2 and 6, respectively. The excited-state dynamics of N3 adsorbed on the surface of TiO(2) nanoparticles were observed to be independent of the number of N3 molecules adsorbed per TiO(2) nanoparticle. The effect of TiO(2) on the dynamics of the adsorbed N3 is primarily due to both mode-specific vibrational and electronic pure dephasing, with the dominant contribution from the latter process.

Excited state interfacial electron transfer from a compound with a single pyridine ligand.

The coordination compound Ru(NH(3))(5)(eina)(PF(6))(2), where eina is ethyl isonicotinate, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO(2) films, abbreviated Ru(NH(3))(5)(eina)/TiO(2). The metal-to-ligand-charge-transfer (MLCT) absorption was found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 486 nm in acetonitrile and 528 nm in dimethylformamide for Ru(NH(3))(5)(eina)(PF(6))(2) and at 512 and 555 nm for Ru(NH(3))(5)(eina)/TiO(2), respectively. The compound was found to be nonemissive with an excited state lifetime <10 ns under all conditions studied. Light excitation in fluid solution and when attached to insulating ZrO(2) films resulted in a loss of the MLCT absorption, consistent with ligand field photochemistry. Pulsed light excitation of Ru(NH(3))(5)(eina)/TiO(2) yields an absorption difference spectrum consistent with an interfacial charge separated state, Ru(III)(NH(3))(5)(eina)/TiO(2)(e(-)). This state forms within 10 ns and returns cleanly to ground state product within milliseconds. The injection quantum yields were determined by comparative actinometry and were found to be excitation wavelength dependent: phi(inj)(417 nm) = 0.30 +/- 0.05 and phi(inj)(532.5 nm) = 0.15 +/- 0.03. Regenerative solar cells based on Ru(NH(3))(5)(eina)/TiO(2) with 0.5 M TBAI, where TBA is tetrabutylammonium, and 0.05 M I(2) in acetonitrile were very inefficient. Sluggish iodide oxidation is expected, on the basis of the negative E degrees (Ru(III/II)) = +0.17 (V vs Ag/AgCl) reduction potential, and this presumably allows a greater fraction of the injected electrons to recombine with the oxidized compound thereby lowering the solar cell efficiency.

Role of Siloxy Substituents in the Activation of Zirconocene Polymerization Catalysts with Methylaluminoxane. A Combined Spectroscopic and Theoretical Study

The activation of 1-siloxy-substituted zirconocene with MAO occurs slowly, in 4-6 h. The slow activation process involves three distinct ligand to metal charge transfer (LMCT) band shifts in the UV/vis spectrum before the maximum LMCT absorption energy and intensity of the third appearing band is reached. In addition, at very high MAO concentrations, a fourth band at 460 nm begins to form. On the contrary, the corresponding 2-siloxy-substituted as well as the unsubstituted zirconocene react with MAO in less than a minute, immediately producing the final species, as confirmed by only one observed LMCT band shift. The 1-silyl-substituted zirconocene reacts with MAO, producing the final active species in 1 h. UV/vis spectroscopic observations were interpreted with the aid of molecular modeling studies, demonstrating that the interactions of the oxygen donor atom of the siloxy substituent with electron deficient Zr center and coordinatively unsaturated Al in MAO play a prominent role in the activation process.

Molecular quadratic non-linear optical properties of dipolar trans-tetraammineruthenium(ii) complexes with pyridinium and thiocyanate ligands

Seven new complex salts trans-[RuII(NCS)(NH3)4(LA)][PF6]2 [LA = N-methyl-4,4′-bipyridinium (MeQ+) 1, N-methyl-2,7-diazapyrenium (Medap+) 2, N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+) 3, N-phenyl-4,4′-bipyridinium (PhQ+) 4, N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe+) 5, N-(4-acetylphenyl)-4,4′-bipyridinium (4-AcPhQ+) 6, or N-(2-pyrimidyl)-4,4′-bipyridinium (2-PymQ+) 7] have been prepared. The electronic absorption spectra of 1–7 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, the energies (Emax) of which decrease in the order 2 > 1 > 3 > 4 > 6 > 5 > 7, as the electron-accepting ability of LA increases. The Emax values generally correlate with cyclic voltammetric and 1H NMR data. Comparisons of the MLCT absorption and electrochemical data for 1–7 show that a trans-{RuII(NCS)(NH3)4}+ centre is a stronger electron donor than analogous groups containing only neutral ligands. Molecular first hyperpolarizabilities β have been measured by using the hyper-Rayleigh scattering technique with acetonitrile solutions and a 1064 nm laser, and static first hyperpolarizabilities β0[H] were obtained by application of the two-state model. Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K have afforded dipole moment changes Δμ12 for the MLCT transitions which have been used to calculate β0[S] values according to the two-state equation β0 = 3Δμ12(μ12)2/(Emax)2 (μ12 = transition dipole moment). Both the β0[H] and β0[S] data confirm that the increased electron donor ability of a trans-{RuII(NCS)(NH3)4}+ centre affords enhanced β0 values, when compared with related complexes containing the neutral trans ligands NH3, 4-(dimethylamino)pyridine, N-methylimidazole (mim) or pyridine. However, the magnitude of the observed β0 increase with respect to the analogous mim complexes varies over a range of ca. 25–120%, depending upon LA.

Divalent osmium complexes: synthesis, characterization, strong red phosphorescence, and electrophosphorescence.

We report new divalent osmium complexes that feature strong red metal-to-ligand-charge-transfer (MLCT) phosphorescence and electrophosphorescence. The general formula of the complexes is Os(II)(N-N)(2)L-L, where N-N is either a bipyridine or a phenanthroline and L-L is either a phosphine or an arsine. New polypyridyl ligands synthesized are 4,4'-di(biphenyl)-2,2'-bipyridine (15) and 4,4'-di(diphenyl ether)-2,2'-bipyridine (16), and the 1,10-phenanthroline derivatives synthesized are 4,7-bis(p-methoxyphenyl)-1,10-phenanthroline (17), 4,7-bis(p-bromophenyl)-1,10-phenanthroline (18), 4,7-bis(4'-phenoxybiphen-4-yl)-1,10-phenanthroline (19), and 4,7-bis(4-naphth-2-ylphenyl)-1,10-phenanthroline (20). 4,4'-Diphenyl-2,2'-bipyridine (21) and 4,7-diphenyl-1,10-phenanthroline (22) were also used in these studies. Strong pi-acid ligands used were 1,2-bis(diphenylarseno)ethane (23), cis-1,2-bis(diphenylphosphino)ethylene (24), and cis-1,2-vinylenebis(diphenylarsine) (25). Ligand 25 is used for the first time in these types of luminescent osmium complexes. These compounds feature strong MLCT absorption bands in the visible region and strong red phosphorescent emission ranging from 611 to 651 nm, with quantum efficiency up to 45% in ethanol solution at room temperature. Red organic light-emitting diodes (OLEDs) were successfully fabricated by doping the Os(II) complexes into blend of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD). Brightness over 1400 cd/m(2) for a double-layer device has been reached, with a turn-on voltage of 8 V. The maximum external quantum efficiency was 0.64%. Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (x, y) of the red electrophosphorescence from the complexes are (0.65, 0.34), which indicates pure red emission.