Metal-to-ligand Charge Transfer Excitation Research Articles

Observation of Raman components in resonant X-ray emission spectra of Fe cyanide

There were two theoretical calculations to reproduce the Fe 2p absorption spectrum of hexacyanoferrate [R. K. Hocking et al., J. Am. Chem. Soc. 128 (2006) 10442, and Y. Nanba and K. Okada: J. Phys. Soc. Jpn. 79 (2010) 114722], but there was a big difference in the parameters used in both. To confirm which of the theoretical calculations is more suitable, we have carried out the experiments of the resonant X-ray emission spectroscopy (RXES) around the Fe Kβ emission lines for K4Fe(CN)6 and K3Fe(CN)6, in order to acquire the electronic structure of Fe atom as a model compound of hexacyanoferrate. In the RXES results of K4Fe(CN)6, three Raman scattering peaks are observed at 6.5, 9.5, and 15.4 eV near the elastic peak, respectively, called Raman Peak (RP) 1, RP2, and RP3. By comparing theoretical calculations of Raman scattering peaks of hexacyanoferrate [Y. Nanba: Doctoral Thesis, Okayama University, 2012 (in Japanese)], the Raman shift energies of RP1 were close to those of the metal-to-ligand charge transfer (MLCT). On the other hand, the Raman shift energies of RP3 were close to those of the ligand-to-metal charge transfer (LMCT). A similar result was also obtained in those of K3Fe(CN)6. It was found that the features of the MLCT excitation and the LMCT one were observed using the RXES experiments.

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV2+) via bimolecular, dynamic electrostatic interactions. The effect of the sulfonate groups on the ground- and excited-state properties was evaluated by comparison with the unsulfonated analogues in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated complexes are similar to what we expect from previous literature, with the exception of the sulfonated heteroleptic complex whose metal-to-ligand charge-transfer (MLCT) lifetime in water has two components that are fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced MV+• in both solvent environments following MLCT excitation, but control measurements in 1:1 acetonitrile/water with the unsulfonated analogues showed no PET to MV2+, indicating that electrostatically driven supramolecular assemblies of the sulfonated complexes with MV2+ facilitate the observed PET. Additionally, the strength of the intermolecular interactions driving the formation of these assemblies changes drastically with the solvent environment. In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes with quantum yields (ΦET) of 2-3% but increased to a remarkable 98% for the sulfonated heteroleptic complex with a 3 μs CS-state lifetime in water.

Leveraging excited-state coherence for synthetic control of ultrafast dynamics.

Design-specific control over excited-state dynamics is necessary for any application seeking to convert light into chemical potential. Such control is especially desirable in iron(II)-based chromophores, which are an Earth-abundant option for a wide range of photo-induced electron-transfer applications including solar energy conversion1 and catalysis2. However, the sub-200-femtosecond lifetimes of the redox-active metal-to-ligand charge transfer (MLCT) excited states typically encountered in these compounds have largely precluded their widespread use3. Here we show that the MLCT lifetime of an iron(II) complex can be manipulated using information from excited-state quantum coherences as a guide to implementing synthetic modifications that can disrupt the reaction coordinate associated with MLCT decay. We developed a structurally tunable molecular platform in which vibronic coherences-that is, coherences reflecting a coupling of vibrational and electronic degrees of freedom-were observed in ultrafast time-resolved absorption measurements after MLCT excitation of the molecule. Following visualization of the vibrational modes associated with these coherences, we synthetically modified an iron(II) chromophore to interfere with these specific atomic motions. The redesigned compound exhibits a MLCT lifetime that is more than a factor of 20 longer than that of the parent compound, indicating that the structural modification at least partially decoupled these degrees of freedom from the population dynamics associated with the electronic-state evolution of the system. These results demonstrate that using excited-state coherence data may be used to tailor ultrafast excited-state dynamics through targeted synthetic design.

Bias induced spin state transition mediated by electron excitations.

Recent experiments reported that spin state transitions were realized by applying bias voltages. However, these bias-induced transitions are not fully understood, especially the mechanism. It is well established in experiments that the metal-to-ligand charge transfer (MLCT) excitation activated by light radiation can lead to the transition from low spin (LS) to high spin (HS), and the transition from HS to LS can be achieved by light due to the metal-centered (MC) excitation. Moreover, electronic excitations are accessible by inelastic cotunneling in molecular junctions under bias voltages. Based on these two facts, we propose that the MLCT excitation is responsible for the bias-induced transition from LS to HS, and the bias-induced transition from HS to LS is attributed to the MC excitation. The rationality of our proposed mechanism is demonstrated by comparing first-principles results and experimental observations. Threshold voltages of MLCT and MC excitations predicted in theory are consistent with bias voltages used to reach the transition from LS to HS and that from HS to LS in the experiment [Miyamachi et al., Nat. Commun. 3, 938 (2012)]. Activation of MLCT or MC excitation depends on the bias polarity, which can explain the bias-polarity dependence of the transition in the experiment. Our study is important for further design of molecular spintronic devices working on the bias-controlled transition.

Photoselective MLCT to d-d pathways for light-induced excited spin state trapping.

We use femtosecond optical pump-probe spectroscopy to study the Light Induced Excited Spin State Trapping (LIESST) dynamics in an FeII spin-crossover material. In these systems, LIESST derives from fast molecular switching induced by light from low (LS, S = 0) to high spin (HS, S = 2) states, as reported for molecules in solution as well as in the solid state. Since the direct LS-to-HS conversion is forbidden by selection rules, the switching dynamics involves intermediate electronic states such as metal-to-ligand charge transfer (MLCT) or ligand-field excited states of singlet or triplet nature. In addition, the HS state is structurally trapped by the elongation of the metal-ligand bond, which is accompanied by the coherent activation and damping of the molecular breathing mode. The ultrafast LIESST dynamics was mainly investigated in FeN6 ligand field systems with almost octahedral symmetry, under MLCT excitation. Our recent study on the FeII(pap-5NO2)2 spin-crossover material, with a FeIIN4O2 ligand field of C2 symmetry, has shown that in addition to MLCT bands, optical excitation, through quite intense and low-energy shifted d-d bands, can also drive LIESST. Compared to MLCT, d-d excitation involves shorter-lived intermediates, drives faster LS-to-HS switching, and enhances the coherent structural dynamics. In this paper, we present an ultrafast study of the pump wavelength dependence of LIESST and we evidence a photoselective crossover from the MLCT to the d-d pathways.

Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States.

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)2]+ (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)2]+, are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (ε = 16 500 M-1 cm-1 at 491 nm). Strong red photoluminescence (Φ = 2.7%, λmax = 687 nm) was observed from [Cu(sbmpep)2]+, which featured an average excited-state lifetime of 1.4 μs in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed ∼300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the π-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)2]+ was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)2]+ from initial photoexcitation to the formation of the lowest-energy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.

Transient Absorption Dynamics of Sterically Congested Cu(I) MLCT Excited States

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

Ultrafast excited-state dynamics of copper(I) complexes.

Bis-diimine Cu(I) complexes exhibit strong absorption in the visible region owing to the metal-to-ligand charge transfer (MLCT) transitions, and the triplet MLCT ((3)MLCT) states have long lifetimes. Because these characteristics are highly suitable for photosensitizers and photocatalysts, bis-diimine Cu(I) complexes have been attracting much interest. An intriguing feature of the Cu(I) complexes is the photoinduced structural change called "flattening". Bis-diimine Cu(I) complexes usually have tetrahedron-like D2d structures in the ground (S0) state, in which two ligands are perpendicularly attached to the Cu(I) ion. With MLCT excitation, the central Cu(I) ion is formally oxidized to Cu(II), which induces the structural change to the "flattened" square-planar-like structure that is seen for usual Cu(II) complexes. In this Account, we review our recent studies on ultrafast excited-state dynamics of bis-diimine Cu(I) complexes carried out using femtosecond time-resolved optical spectroscopy. Focusing on three prototypical bis-diimine Cu(I) complexes that have 1,10-phenanthroline ligands with different substituents at the 2,9-positions, i.e., [Cu(phen)2](+) (phen = 1,10-phenanthroline), [Cu(dmphen)2](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), and [Cu(dpphen)2](+) (dpphen = 2,9-diphenyl-1,10-phenanthroline), we examined their excited-state dynamics by time-resolved emission and absorption spectroscopies with 200 fs time resolution, observed the excited-state coherent nuclear motion with 30 fs time resolution and performed complementary theoretical calculations. This combined approach vividly visualizes excited-state processes in the MLCT state of bis-diimine Cu(I) complexes. It was demonstrated that flattening distortion, internal conversion, and intersystem crossing occur on the femtosecond-early picosecond time scale, and their dynamics is clearly identified separately. The flattening distortion predominantly occurs in the S1 state on the subpicosecond time-scale, and the precursor S1 state retaining the initial undistorted structure appears as a metastable state before the structural change. This observation indicates that the traditional understanding based on the Jahn-Teller effect appears irrelevant for realistically discussing the photoinduced structural change of bis-diimine Cu(I) complexes. The lifetime of the precursor S1 state significantly depends on the substituents in the three complexes, indicating that the flattering distortion requires a longer time as the substituents at 2,9-positions of the ligands become bulkier. It is suggested that the substituents are rotated to avoid steric repulsions to achieve the flattened structure at the global minimum of the S1 state, implying the necessity of discussion based on a multidimensional potential energy surface to properly consider this excited-state structural change. After the flattening distortion, the S1 states of [Cu(dmphen)2](+) and [Cu(dpphen)2](+), which have bulky substituents, relax to the T1 state by intersystem crossing on the ∼10 ps time scale, while the flattened S1 state of [Cu(phen)2](+) relaxes directly to the S0 state on the ∼2 ps time scale. This difference is rationalized in terms of the different magnitude of the flattening distortion and relevant changes in the potential energy surfaces. Clear understanding of the ultrafast excited-state process provides a solid basis for designing and using Cu(I) complexes, such as controlling the structural change to efficiently utilize the energy of the MLCT state in solar energy conversion.

Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Preparation and electrochemical and photoelectrochemical properties of a covalently self-assembled monolayer film based on a bis-terpyridyl ruthenium(II) complex

A bis-terpyridyl Ru(II) complex of RuII(IPTP)2(ClO4)2 (in which IPTP=4′-(4-(imidazol-1-yl)phenyl)-2,2′:6′,2″-terpyridine) has been synthesized using a microwave irradiation method, and characterized by elementary analysis, and proton nuclear magnetic resonance, and mass spectroscopy. A RuII(IPTP)2(ClO4)2 based covalently self-assembled monolayer film was prepared and characterized by UV–Vis spectroscopy and cyclic voltammetry. The cyclic voltammograms demonstrated that RuII(IPTP)2(ClO4)2 assembled in the film was redox active with surface-confined characteristics. The photoelectrochemical properties and electron-transfer mechanism of the film were studied. It was found that the RuII(IPTP)2(ClO4)2/ITO electrode film exhibited a large cathodic photocurrent density of 2.72μA/cm2 while irradiated with polychromatic light (325nm<λ<730nm) at an applied potential of −0.4V versus saturated calomel electrode. The photocurrent action spectrum was in agreement with the metal-to-ligand charge-transfer (MLCT) absorption band of RuII(IPTP)2(ClO4)2, indicating that the photocurrent was generated based on MLCT excitation of the RuII(IPTP)2(ClO4)2 in the film. An incident monochromatic photon-to-current conversion efficiency of this monolayer film was calculated to be 1.8% at 500nm.

Tuning the electronic structures and related properties of Ruthenium-based dye sensitizers by ligands: A theoretical study and design

The geometries, electronic structures, and absorption properties of three heteroleptic ruthenium complexes for dye-sensitized solar cells (DSCs), including Ru(L)(L1)(NCS)2 [L=4,4′-dicarboxylic acid-2,2′-bipyridine, Ru-T1: L1=(E)-2-(4′- methyl-2,2′-bipyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile, Ru-T2: L2=(E)-3-(5′- hexyl-2,2′-bithiophen-5-yl)-2-(4′-methyl-2,2′-bipyridin-4-yl)acrylonitrile, and Ru-T3: L3=(E)-3-(5″-hexyl-2,2′:5′, 2″-terthiophen-5-yl)-2-(4′-methyl-2,2′-bipyridin-4-yl) acrylonitrile)], were investigated by using density functional theory (DFT) and time dependent DFT. The absorption of Ru-T1 in visible region mixed the character of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), while the absorption of Ru-T2 and Ru-T3 mixed MLCT, LLCT, and local excitation. Through the introduction of the 3-(5-hexyloligothiophen-5-yl) acrylonitrile group extended the conjugation length of the bipyridine donor ligand of Ru-complexes, the overlap between the ground state and excited states were enhanced, and their molar absorption coefficients were improved. However, the smaller shorter-circuit current densities of Ru-T2 and Ru-T3 were resulted from the excited states with the character of not-effective charge transfer. The 11 kinds of Ru-complexes were designed by removing CN group, substituting of COOH with cyanoacetic acid, and increasing thiophene units. The analysis of excited states and oxidation potential suggest that several designed dyes may have good performance in DSCs.

Restricted Photochemistry in the Molecular Solid State: Structural Changes on Photoexcitation of Cu(I) Phenanthroline Metal-to-Ligand Charge Transfer (MLCT) Complexes by Time-Resolved Diffraction

The excited-state structure of [Cu(I)[(1,10-phenanthroline-N,N') bis(triphenylphosphine)] cations in their crystalline [BF(4)] salt has been determined at both 180 and 90 K by single-pulse time-resolved synchrotron experiments with the modified polychromatic Laue method. The two independent molecules in the crystal show distortions on MLCT excitation that differ in magnitude and direction, a difference attributed to a pronounced difference in the molecular environment of the two complexes. As the excited states differ, the decay of the emission is biexponential with two strongly different lifetimes, the longer lifetime, assigned to the more restricted molecule, becoming more prevalent as the temperature increases. Standard deviations in the current Laue study are very much lower than those achieved in a previous monochromatic study of a Cu(I) 2,9-dimethylphenanthroline substituted complex ( J. Am. Chem. Soc. 2009 , 131 , 6566 ), but the magnitudes of the shifts on excitation are similar, indicating that lattice restrictions dominate over the steric effect of the methyl substitution. Above all, the study illustrates emphatically that molecules in solids have physical properties different from those of isolated molecules and that their properties depend on the specific molecular environment. This conclusion is relevant for the understanding of the properties of molecular solid-state devices, which are increasingly used in current technology.

Competing Pathways in the photo-Proton-Coupled Electron Transfer Reduction of fac-[Re(bpy)(CO)3(4,4′-bpy]+* by Hydroquinone

The emitting metal-to-ligand charge transfer (MLCT) excited state of fac-[ReI(bpy)(CO)3(4,4′-bpy)]+ (1) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [ReII(bpy–•)(CO)3(4,4′-bpy)]+*, is reductively quenched by 1,4-hydroquinone (H2Q) in CH3CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give ReI(bpy–•)(CO)3(4,4′-bpy)]0 with k = (1.8 ± 0.2) × 109 M–1 s–1, followed by proton transfer from H2Q to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+. Protonation triggers intramolecular bpy•– → 4,4′-bpyH+ electron transfer. In the second pathway, preassociation occurs between the ground state and H2Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H2Q in concert with intramolecular bpy•– → 4,4′-bpyH+ electron transfer to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+ with k = (1.0 ± 0.4) × 109 s–1 in 3:1 CH3CN/H2O.

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.

Preparation, Characterization, and Photoelectric Properties of an Electrostatically Self-Assembled Film Based on Tungstophosphoric Acid and a Binuclear Ru(II) Complex

An electrostatically self-assembled film based on tungstophosphoric acid (PTA) and a binuclear Ru(II) complex RuL was prepared and characterized by UV-vis absorption spectroscopy and cyclic voltammetry. A linear increase in the UV-vis absorbance with the number of deposited layers indicated that the film deposition was uniform and reproducible. The cyclic voltammogram demonstrated that PTA and the binuclear Ru(II) complex assembled in the film were redox active with surface confined characteristics. The photoelectrochemical properties and mechanism of photocurrent generation have also been studied. A five-layer (PTA/RuL)5 film on the ITO was found to exhibit a large cathodic photocurrent density of 5.32 microA/cm2 while irradiated with polychromatic light (730 nm > lambda > 325 nm) at an applied potential of -0.3 V versus saturated calomel electrode. The photocurrent action spectrum was in agreement with metal-to-ligand charge-transfer (MLCT) band of RuL in the absorption spectrum, which indicated that the photocurrent was generated based on MLCT excitation of the RuL in the film. It is noteworthy that the incident monochromatic photon-to-current conversion efficiency was calculated to be 1.1% at a wavelength of 450 nm.

Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

Solution-phase photoinduced low-spin to high-spin conversion in the Fe(II) polypyridyl complex [Fe(tren(py)(3))](2+) (where tren(py)(3) is tris(2-pyridylmethyliminoethyl)amine) has been studied via picosecond soft X-ray spectroscopy. Following (1)A(1) --> (1)MLCT (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L(2)- and L(3)-edges were observed concomitant with formation of the transient high-spin (5)T(2) state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of approximately 1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 A increase in Fe-N bond length upon formation of the high-spin state. The overall occupancy of the Fe-3d orbitals remains constant upon spin crossover, suggesting that the reduction in sigma-donation is compensated by significant attenuation of pi-back-bonding in the metal-ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft X-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations.

Characterization of charge transfer excitations in hexacyanomanganate(III) with Mn K-edge resonant inelastic x-ray scattering

We use hard x-ray resonant inelastic x-ray scattering (RIXS) and density functional theory (DFT) calculations to characterize charge transfer excitations in K(3)Mn(CN)(6). The combination of RIXS measurements and DFT calculations allows us to characterize the strength of the ligand-metal electronic interaction and assign the Raman resonances in the RIXS spectra to charge transfer excitations. With x-ray excitation energies resonant with the T(2g) and E(g) pre-edge peaks derived predominantly from the Mn 3d orbitals, we observe Raman resonances in the energy transfer range from 2 to 12 eV, which results from the filling of the 1s core-hole from T(1u)-symmetry occupied orbitals. DFT calculations indicate that these orbitals exhibit primarily ligand character, supporting the assignment of the energy transfer resonances to ligand-to-metal charge transfer excitations. Our RIXS measurements and DFT calculations also indicate that the E(g)-orbital spin-splits by roughly 0.8 eV, though we do not cleanly resolve the two absorption peaks in the RIXS spectra. We also see evidence for a metal-to-ligand charge transfer (MLCT) excitation when exciting with a 6545.0 eV incident photon, roughly 4 eV above the E(g) absorption peaks. The 6545.0 eV resonant emission spectrum shows a 6.0 eV energy transfer resonance, which corresponds to a final state hole in the T(2g) partially occupied orbital. DFT calculations indicate that excitation at 6545.0 eV populates an unoccupied T(1u)-symmetry orbital of primarily ligand character. Given the predominantly metal character of the final state hole, we assign the 6.0 eV Raman resonance to a MLCT excitation. These measurements demonstrate the ability of hard x-ray RIXS to characterize the valence electronic structure of coordination compounds.

Electrochemical and Photoelectrochemical Investigation of New Electrostatic Self-Assembled Films Based on Prussian Blue and a Binuclear Ru(II) Complex

New hybrid multilayer films containing Prussian blue (PB) and a binuclear Ru(II) complex were successfully fabricated using the electrostatic layer-by-layer self-assembly technique. The process was carefully monitored by ultraviolet-visible (UV-vis) spectroscopy. A linear increase in the UV-vis absorbance with the number of deposited layers indicated that the film deposition was uniform and reproducible. The electrochemical and photoelectrochemical properties of these hybrid multilayer films were investigated in an aqueous Na2SO4 solution. PB and the binuclear Ru(II) complex assembled in the film were redox active with surface confined characteristics. The surface concentration and molecular area of RuL in the film were calculated according to a UV-vis absorption spectrum and cyclic voltammetry, respectively. The photocurrent generated by switching on the light irradiation of the film increased linearly as the deposited bilayers increased. The photocurrent action spectrum was in agreement with metal-to-ligand charge-transfer (MLCT) band of RuL in the absorption spectrum, which indicated that the photocurrent was generated based on MLCT excitation of the RuL in the (PB/RuL) film.

Using Internal Coordinates to Describe Photoinduced Geometry Changes in MLCT Excited States

A resonance Raman intensity analysis of the metal-to-ligand charge-transfer (MLCT) transition for the rhenium compound Re(2-(2'-pyridyl)quinoxaline)(CO)(3)Cl (RePQX) is presented. Photoinduced geometry changes are calculated, and the results are presented using the vibrational normal modes and the redundant internal coordinates. A density functional theory calculation is used to determine the ground-state nonresonant Raman spectrum and a transformation matrix that transforms the redundant internal coordinates into the normal modes. The normal modes nu(37) (rhenium coordination sphere distortion) and nu(75) (ligand skeletal stretch) show the largest photoinduced geometry change (Delta = 1.0 and 0.7, respectively). A single carbonyl mode is enhanced in the resonance Raman spectra. Time-dependent density functional theory is used to calculate excited-state geometry changes, which are subsequently used to determine the signs of the photoinduced normal mode displacements. Transforming to internal coordinates reveals that all the CO bond lengths are displaced in the excited state. The Re-C and C-C ligand bond lengths are also displaced in the excited state. The results are discussed in terms of a simple one-electron picture for the electronic transition. Many bond angles and torsional coordinates are also displaced by the metal-to-ligand charge transfer, and most of these are associated with the rhenium coordination sphere. It is demonstrated that using internal coordinates presents a clear picture of the geometry changes associated with photoinduced electron transfer in metal polypyridyl systems.

Efficient Energy Conversion in Photochromic Ruthenium DMSO Complexes

The photochromic compounds trans- and cis-[Ru(tpy)(Mepic)(dmso)](OSO2CF3) (2 and 3, respectively; tpy is 2,2':6',2"-terpyridine; Mepic is 6-methyl-2-pyridinecarboxylate; dmso is dimethyl sulfoxide) and cis-[Ru(tpy)(Brpic)(dmso)](PF6) (4; Brpic is 6-bromo-2-pyridinecarboxylate) were prepared and characterized by single-crystal X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. The geometry labels refer to the relationship between the carboxylate oxygen of the picolinate ligand and dmso. Electrochemical studies reveal that only the trans isomer shows S-to-O isomerization following oxidation of Ru(II) and O-to-S isomerization following reduction of Ru(III). The cis isomers of both complexes feature reversible one-electron Ru(III/II) couples. All complexes undergo phototriggered S-to-O isomerization following MLCT (metal-to-ligand charge transfer) excitation with quantum yields (Phi(S-->O)) of 0.79 (2), 0.011 (3), and 0.014 (4). The methyl group in 2 promotes isomerization by hindering rotation of the dmso ligand about the Ru-S bond. Computational results support this role for the methyl group. Relative energy calculations show that the barrier to rotation is approximately 8 kcal mol(-1). These results suggest that rotation is an important vibration for isomerization in photochromic ruthenium-dmso complexes.