Low-energy Charge-transfer Transitions Research Articles

Significant nonlinear‐optical switching capacity in atomic clusters built from silicon and lithium: A combined ab initio and density functional study

Starting from a hypothetical but fundamental charge/discharge sequence, the topic of nonlinear optical switching in atomic clusters built from silicon and alkali metals is opened up. The outcomes presented in this work, obtained with ab initio methods of exceptional predictive capabilities, offer strong evidences that sizable hyperpolarizability contrasts between neutral and charged alkali metal doped cluster forms might be simultaneously accomplished. The observed switching procedure involves redox polyatomic clusters formed by Si atoms. These centers function as electron acceptors at the ground state and as electron donors at the excited states facilitating low energy charge transfer transitions upon electronic excitation. © 2014 Wiley Periodicals, Inc.

Ligand-Localized Triplet-State Photophysics in a Platinum(II) Terpyridyl Perylenediimideacetylide

The synthesis, electrochemistry, and photophysical behavior of a Pt(II) terpyridyl perylenediimide (PDI) acetylide (1) charge-transfer complex is reported. The title compound exhibits strong (ε ≈ 5 × 10(4) M(-1)cm(-1)) low-energy PDI acetylide-based π-π* absorption bands in the visible range extending to 600 nm, producing highly quenched singlet fluorescence (Φ = 0.014 ± 0.001, τ = 109 ps) with respect to a nonmetalated PDI model chromophore. Nanosecond transient absorption spectroscopy revealed the presence of a long excited-state lifetime (372 ns in 2-methyltetrahydrofuran) with transient features consistent with the PDI-acetylide triplet state, ascertained by direct comparison to a model Pt(II) PDI-acetylide complex lacking low-energy charge-transfer transitions. For the first time, time-resolved step-scan FT-IR spectroscopy was used to characterize the triplet excited state of the PDI-acetylide sensitized in the title compound and its associated model complex. The observed red shifts (∼30-50 cm(-1)) in the C═O and C≡C vibrations of the two Pt(II) complexes in the long-lived excited state are consistent with formation of the (3)PDI acetylide state and found to be in excellent agreement with the expected change in the relevant DFT-calculated IR frequencies in the nonmetalated PDI model chromophore (ground singlet state and lowest triplet excited state). Formation of the PDI triplet excited state in the title chromophore was also supported by sensitization of the singlet oxygen photoluminescence centered at ∼1275 nm in air-saturated acetonitrile solution, Φ((1)O(2)) = 0.52. In terms of light emission, only residual PDI-based red fluorescence could be detected and no corresponding PDI-based phosphorescence was observed in the visible or NIR region at 298 or 77 K in the Pt(II) terpyridyl perylenediimideacetylide.

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

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

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.

Metal–thiolate bonds in bioinorganic chemistry

Metal-thiolate active sites play major roles in bioinorganic chemistry. The M--S(thiolate) bonds can be very covalent, and involve different orbital interactions. Spectroscopic features of these active sites (intense, low-energy charge transfer transitions) reflect the high covalency of the M--S(thiolate) bonds. The energy of the metal-thiolate bond is fairly insensitive to its ionic/covalent and pi/sigma nature as increasing M--S covalency reduces the charge distribution, hence the ionic term, and these contributions can compensate. Thus, trends observed in stability constants (i.e., the Irving-Williams series) mostly reflect the dominantly ionic contribution to bonding of the innocent ligand being replaced by the thiolate. Due to high effective nuclear charges of the Cu(II) and Fe(III) ions, the cupric- and ferric-thiolate bonds are very covalent, with the former having strong pi and the latter having more sigma character. For the blue copper site, the high pi covalency couples the metal ion into the protein for rapid directional long range electron transfer. For rubredoxins, because the redox active molecular orbital is pi in nature, electron transfer tends to be more localized in the vicinity of the active site. Although the energy of hydrogen bonding of the protein environment to the thiolate ligands tends to be fairly small, H-bonding can significantly affect the covalency of the metal-thiolate bond and contribute to redox tuning by the protein environment.

Complexes of ruthenium(III) and chromium(III) with a new tetradentate N 2O 2-donor ligand: crystal structures, redox properties and spectroelectrochemistry

Reaction of 3-(2-methoxyphenyl)pyrazole with 1,2-bis(bromomethyl)benzene under phase-transfer conditions, followed by demethylation of the methoxyphenyl units with BBr 3, affords the new ligand H 2L in which two bidentate chelating units (pyrazolyl–phenol) are separated by a o-xylyl spacer. Reaction of H 2L with [M(acac) 3] (M=Cr, Ru) afforded in each case two mononuclear products [M(L)(acac)] and [M(L)(HL)]. Crystal structure determinations of [M(L)(acac)] for both M=Cr and Ru show that the complexes have similar structures, with the ligand L 2− folded such that it coordinates as a tetradentate chelate in a cis-N 2, cis-O 2 manner with the remaining two coordination sites at the metal occupied by the acac ligand. Neither complex [M(L)(HL)] formed X-ray quality crystals but we assume a similar mononuclear structure in which one L 2− is coordinated as a tetradentate chelate, and the second (HL) − is coordinated via one bidentate N, O-binding site with the second site pendant and remaining protonated. [Ru(L)(acac)] undergoes reversible oxidation to Ru(IV) and an irreversible reduction to Ru(II); in contrast, for [Ru(L)(HL)] the Ru(III)–Ru(IV) and Ru(II)–Ru(III) couples are both reversible. A UV–Vis–NIR spectroelectrochemical study of [Ru(L)(HL)] was accordingly carried out, and shows that in the Ru(IV) state the spectrum develops intense, low-energy charge-transfer transitions in the near-IR region.

On the Question of Mixed-Valent States in Ligand-Bridged Dinuclear Organoplatinum Compounds [RkPt(μ-L)PtRk]n,k= 2 or 4

Symmetrically dinuclear complexes between the bis-bidentate bridging ligands μ-L (μ-L = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(1-phenyliminoethyl)pyrazine (bpip)) and the organoplatinum fragments PtMes2 (Mes = mesityl), PtMe2, or PtMe4 were synthesized as deeply colored compounds. Low-energy charge-transfer transitions from metal d orbitals (PtII) or metal−carbon σ bond combinations (PtIV) to low-lying π* orbitals of the π acceptor ligands are responsible for long-wavelength absorption maxima λmax(CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes indicate the formation of [PtIV]2(μ-L•-) and [PtII]2(μ-L•-) species, however, the latter exhibit a significant metal contribution according to a PtII/PtI formulation. Cyclic voltammetry reveals that the remarkable system [Mes2Pt(μ-bptz)PtMes2]n forms an enormously stabilized radical anion (n = 1−) with ΔE1/2 = 1250 V and Kc = 1021.2 and a PtIII/PtII mixed-valent state (n = 1+) with ΔE1/2 = 80 mV and Kc = 23. This small Kc value is attributed to the predominantly dσ orbital character of the redox orbitals on the Pt(II) centers.

Reactions of New Organoplatinum(II) and -(IV) Complexes of 1,4-Diaza-1,3-butadienes with Light and Electrons. Emission vs Photochemistry and the Electronic Structures of Ground, Reduced, Oxidized, and Low-Lying Charge-Transfer Excited States

Complexes between the 1,4-disubstituted 1,4-diaza-1,3-butadiene chelate ligands RNCHCHNR (R-DAB; R = alkyl, aryl) and the organoplatinum fragments PtMe2, PtMe4, and PtMes2 (Mes = mesityl) were prepared and characterized with respect to their electronic structures. All compounds are distinguished by low-energy charge-transfer transitions to low-lying π* orbitals of the R-DAB ligands, either from metal d orbitals (PtII) or from metal−carbon σ bond combinations (PtIV). These spectral assignments are supported by DFT calculations on model complexes between HNCHCHNH and PtMe2 or PtMe4. The calculations also reproduce the structural results for the complex between CyNCHCHNCy and PtMe4, which exhibits significantly longer Pt−C bonds to the axial methyl groups. The distinct solvatochromism of the long-wavelength transitions is described, as are the UV/vis spectroelectrochemical results for reversible reduction to PtIV(R-DAB•-) or PtII(R-DAB•-) species (no evidence for a PtI state). In contrast, the oxidation is electrochemically irreversible except for the dimesitylplatinum compounds. The electrochemical potentials of corresponding PtMe2 and PtMe4 compounds are very similar, demonstrating that the binding of two additional methyl carbanions compensates for the effect of the higher metal oxidation state. While the organoplatinum(II) species are emissive, the tetramethylplatinum(IV) complexes are photoreactive and undergo metal-to-ligand methyl transfer reactionsin agreement with the structurally confirmed weaker bonding to the axial methyl groups.

EPR Studies of Oxo-Molybdenum(V) Complexes with Sulfur Donor Ligands: Implications for the Molybdenum Center of Sulfite Oxidase.

A series of six-coordinate compounds containing a chelating dithiolate coordinated to the [LMo(V)O](2+) unit (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) have been characterized by EPR spectroscopy as models for the molybdenum centers of pterin-containing molybdenum enzymes. The structure of LMoO(bdt) (1) (bdt = 1,2-benzenedithiolate) has been determined by X-ray crystallography; the space group is P2(1)/n with a = 10.727(1) Å, b = 14.673(2) Å, c = 15.887(2) Å, beta = 100.317(4) degrees and Z = 4. Compound 1 exhibits distorted octahedral stereochemistry; the terminal oxo group and the sulfur atoms are mutually cis to one another. The Mo=O distance is 1.678(4) Å, and the average Mo-S distance is 2.373(2) Å. The EPR parameters for 1, determined from simulation of the frozen-solution spectrum, are g(1) = 2.004, g(2) = 1.972, g(3) = 1.934 and A(1)((95,97)Mo) = 50.0 x 10(-)(4), A(2) = 11.4 x 10(-)(4), A(3) = 49.7 x 10(-)(4) cm(-)(1). The EPR parameters for several LMo(V)O{S(CH(2))(x)()S} compounds (x = 2-4) with saturated chelate skeletons are similar to those of 1, indicating that it is the coordinated S atoms and not unsaturation of the chelate skeleton that gives rise to the large g values for 1. The presence of g components larger than the free-electron value is ascribed to low-energy charge transfer transitions from the filled sulfur pi orbitals to half-filled Mo d orbitals. The EPR spectrum of [LMo(V)O{S(2)P(OEt)(2)}](+) shows an unusually large isotropic (31)P hyperfine splitting of 66.1 x 10(-)(4) cm(-)(1) from the noncoordinated phosphorus atom. The frozen-solution EPR spectra of the low-pH and high-pH forms of sulfite oxidase have been reinvestigated in D(2)O and the anisotropic g and A((95,97)Mo) parameters determined by simulation of the spectrum arising from the naturally abundant Mo isotopes (75% I = 0, 25% I = (5)/(2)). The EPR parameters for the low-pH form are g(1) = 2.007, g(2) = 1.974, g(3) = 1.968 and A(1) = 56.7 x 10(-)(4), A(2) = 25.0 x 10(-)(4), A(3) = 16.7 x 10(-)(4) cm(-)(1). The EPR parameters for the high-pH form are g(1) = 1.990, g(2) = 1.966, g(3) = 1.954 and A(1) = 54.4 x 10(-)(4), A(2) = 21.0 x 10(-)(4), A(3) = 11.3 x 10(-)(4) cm(-)(1). These are the first determinations of the complete A((95,97)Mo) hyperfine components for an enzyme that possesses an [Mo(VI)O(2)](2+) core in its fully oxidized state.

Raman characterization of charge-transfer transitions in ligand-bridged binuclear polypyridyl complexes of ruthenium(II)

In the ligand-bridged complexes [(bipy)2Ru(dpp)Ru(biq)2]4+ and [(biq)2Ru(dpp)Ru(biq)2]4+[dpp = 2,3-bis(2′-pyridyl)pyrazine, bipy = 2,2′-bipyridine and biq = 2,2′-biquinoline], the photophysical and redox properties do not allow differentiation of the low-energy charge-transfer transitions Ru → biq and Ru → dpp. Based on the selective resonance enhancement of the intensities of the Raman bands of the ligand biq by excitation within the lowest-energy absorption band, the lowest excited states in the dinuclear complexes have been assigned unambiguously to Ru → biq charge transfer.

Theoretical evaluation of the frequency and intensity of low-energy charge-transfer transitions in aromatic hydrocarbon crystals. I. Anthracene

Calculations are reported for the low-energy part of the charge-transfer (CT) spectrum of anthracene. It is shown that the CT states previously obtained from an electrostatic model are split by intracell and intercell electron and hole transfer integrals. The model chosen for detailed calculations consists of a quasilinear crystal with two molecules per unit cell and only nearest-neighbour interactions. Parameter values are derived from earlier electrostatic and quantum-chemical calculations and from spectroscopic observations. The resulting energy levels and band intensities are compared directly with the experimental electromodulation spectra of anthracene. It is shown that, contrary to an earlier analysis, these spectra show evidence for four rather than three CT bands in the range 3.50–3.80 eV, located at 3.528, 3.590, 3.658 and 3.760 eV. Calculated CT energies and relative intensities compare favourably with these experimental results, leading to unambiguous assignments of the bands as components of the ( 1 2 1 2 0) and (010) CT states. These assignments imply predictions about the polarization of these bands which remain to be tested experimentally.