Charge Transfer Electronic Transitions Research Articles

Ultrafast activation of the double-exchange interaction in antiferromagnetic manganites

In doped manganite systems, strong electronic correlations result in rich phase diagrams where electron delocalization strongly affects the magnetic order. Here, we employ a femtosecond all-optical pump-probe scheme to impulsively photodope the antiferromagnetic parent manganite system CaMnO3 and unveil the formation dynamics of a long-range ferromagnetic state. We resonantly target intense charge transfer electronic transitions in CaMnO3 to photodope the system and probe the subsequent dynamics of both charges and spins using a unique combination of time-resolved terahertz spectroscopy and time-resolved magneto-optical Faraday measurements. We demonstrate that photodoping promotes a long-lived population of delocalized electrons and induces a net magnetization, effectively promoting ferromagnetism resulting from light-induced carrier-mediated short-range double-exchange interactions. The picosecond set time of the magnetization, much longer than the electron timescale, and the presence of an excitation threshold are consistent with the formation of ferromagnetic patches in an antiferromagnetic background.

Low-symmetry phenyl substituted pyrazine analogues of subphthalocyanine type dyes

Statistical template cyclotrimerization of 5,6-diphenylpyrazine-2,3-dicarbonitrile and tetrafluorophthalonitrile in the presence of boron trichloride leads to novel fluorinated low-symmetry subphthalocyanine azaanalogs containing phenyl substituted pyrazine rings ([F8Ph2N2sPc] and [F4Ph4N4sPc]), which were characterized by spectral methods (UV–Vis, IR, 1H, 11B, 13C and 19F NMR spectroscopy). The influence of pyrazine ring on the electronic structure of low-symmetry macrocycle has been analyzed using theoretical DFT methods, electronic absorption and emission spectroscopy and electrochemical measurements. Fused diphenylpyrazine and tetrafluorobenzene fragments have a similar influence on the electron affinity of the macrocycle facilitate its reversible reduction at −0.41 to −0.42 V vs Ag/AgCl. Introduction of pyrazine rings increases the HOMO-LUMO gap due to relative stabilization of HOMO and leads to hypsochromic shift of the Q band. The charge-transfer electronic transition from phenyl rings to the central subporphyrazine macrocycle appearing near 390 nm has a small influence on the excited state properties and all compounds have similar fluorescence quantum yields (0.25–0.28). Pyrazine containing subphthalocyanine analogues are stable in acid medium and their donor center (nitrogen atoms in pyrazine rings and in meso-positions) having very low basicity can be involved in protonation only in strong acids (CF3COOH, H2SO4).

Ground- and excited-state properties of Re(I) carbonyl complexes – Effect of triimine ligand core and appended heteroaromatic groups

In this work, a series of six rhenium(I) complexes bearing 2,2′:6′,2″-terpyridine (terpy), 2,6-di(thiazol-2-yl)pyridine (dtpy), and 2,6-di(pyrazin-2-yl)pyridine (dppy) with appended quinolin-2-yl and N-ethylcarbazol-3-yl groups were prepared and spectroscopically investigated to evaluate the photophysical consequences of both the trisheterocyclic core (terpy, dtpy and dppy) and the heterocyclic substituent. The [ReCl(CO)3(Ln-κ2N)] complexes are regarded as ideal candidates for getting structure–property relationships, while terpy-like framework represents an excellent structural backbone for structural modifications. The replacement of the peripheral pyridine rings of 2,2′:6′,2″-terpyridine by thiazoles and pyrazines resulted in a significant red-shift of the absorption and emission of [ReCl(CO)3(Ln-κ2N)] due to stabilization of the ligand-centred LUMO orbital. Both quinoline and N-ethylcarbazole are extended π-conjugation organic chromophores, but they differ in electron-donating abilities. The low-energy absorption band of Re(I) complexes with the triimine ligands bearing quinolin-2-yl group was contributed by the metal-to-ligand charge-transfer (MLCT) electronic transitions. The introduction of electron-donating N-ethylcarbazol-3-yl substituent into the triimine acceptor core resulted in the change of the character of the HOMO of Re(I) complexes and a significant increase of molar absorption coefficients of the long-wavelength absorption, which was assigned to a combination of 1MLCT and 1ILCT (intraligand charge-transfer) transitions. Regardless of the appended heteroaromatic group, the emitting excited state of Re(I) terpy-based complexes was demonstrated to have predominant 3MLCT character, as evidenced by comprehensive studies including static and time-resolved emission spectroscopy along with ultrafast transient absorption measurements. The diodes with Re(I) complexes dispersed molecularly in a PVK:PBD matrix were emissive and effects of the complex structure on colour of emitted light and its intensity was pronounced.

A Quantum Chemical Screening of Two Imidazole-Chalcone Hybrid Ligands and Their Pd, Pt and Zn Complexes for Charge Transport and Nonlinear Optical (NLO) Properties: A DFT Study

A quantum chemical screening of two imidazole-based chalcone ligands: 2- [1-(3-(1H-imidazol-1-yl)propylimino)-3-(phenylallyl)]phenol and 2-[1-(3-(1H- imidazol-1-yl)propylimino)-3-4-nitrophenylallyl]phenol (hereinafter referred to as HL1 and HL2 respectively) and their Pd, Pt and Zn chelates for charge transport and nonlinear optical (NLO) properties, is reported via dispersion- corrected density functional theory (DFT-D3) and time-dependent DFT (TD- DFT) methods. From our results, Pd and Pt complexes have been observed to show excellent hole-transport properties, owing to their very small reorganization energies. The light extraction efficiency of the HL1-Pt complex was deduced to be particularly impressive, thus suitable for the manufacture of hole transport layer in violet light emitting diodes (LEDs). Moreover, redox potentials and chemical stability studies have enabled us to validate the greater stability in moisture (towards oxidation), of HL2 complexes compared to their HL1 counterparts. The first and second hyperpolarizabilities of both ligands and their complexes have been found to be outstandingly higher than those of the push-pull prototypical, para-nitroaniline by factors of up to 12 in the case of HL2. These compounds, with the exception of the HL2-Pt complex, are thus interesting candidates having wide transparency tradeoffs for NLO efficiency in the manufacture of optoelectronic and photonic devices capable of second and third-order NLO response. Finally, metal chelation has been established to enhance the NLO response of all the chalcone-based imidazole ligands investigated as a result of metal-ligand charge transfer and ligand- metal charge transfer electronic transitions identified in the resulting complexes with the exception of the zinc complexes.

Synthetic Co-Existing Wadsleyite β-(Mg,Fe)2SiO4 and Ringwoodite γ-(Mg,Fe)2SiO4: an Optical Absorption Spectroscopy Study

The synthetic high-pressure α- and β-modification of (Mg1–xFex)2SiO4, wadsleyite and ringwoodite, respectively, were studied by optical absorption spectroscopy at ambient and hydrostatic high-pressure conditions. In addition, the effects of thermal annealing on the crystals were investigated. Under hydrostatic compression up to ~13 GPa and then consequent released to atmospheric pressure there were changes in the spectra and related changes in the crystal color. This is a clear indication that some Fe2+ was oxidized to Fe3+. The spectra of both ringwoodite and wadsleyite change after annealing in air at temperatures up to 300 °C. The intensities of electronic spin-allowed bands of Fe2+ decrease and the intensity of the charge-transfer electronic transition O2– → Fe3+, as given by the low-energy absorption edge in the UV region, increases. These crystal-chemical changes are shown by a weakening of the blue (ringwoodite) and green (wadsleyite) colors and a concomitant increase in yellowish tints. The effects of Fe2+ oxidation to Fe3+, upon decompression from high pressures as well as through annealing at relatively low temperatures, can cause the disintegration of both phases. Thus, both minerals have not yet been reliably identified at near surface Earth conditions after originating from deep-seated volcanism or deep subduction zone processes.

Correlation between the C-C Cross-Coupling Activity and C-to-Ni Charge Transfer Transition of High-Valent Ni Complexes.

High-valent Ni complexes have proven to be good platforms for diverse cross-coupling reactions that are otherwise difficult to be achieved with conventional low-valent catalysts. However, their reductive elimination (RE) activities are still significantly variable by up to 5 orders of magnitude, depending on the supporting ligand and oxidation state of the Ni center. To elucidate frontier molecular orbitals (FMOs) that determine the RE activity of the Ni center, the electronic structures of cycloneophyl (CH2C(CH3)2-o-C6H4) NiIII and NiIV complexes have been characterized by utilizing various transition metal-based spectroscopic techniques such as electronic absorption, magnetic circular dichroism, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopies. In combination with density functional theory computations, the spectroscopic analyses have shown that the energies of the C-to-Ni charge-transfer (CT) electronic transitions are strongly correlated to the rates of C-C bond-forming RE reaction. This correlation suggests that the kinetic barrier of the RE reaction is determined by energy cost for internal CT (ICT) from the coordinated carbon moiety to the Ni center, and that FMOs involved in the RE reaction and the C-to-Ni CT electronic transitions are essentially identical. This FMO determination has led us to discover that photoexcitation to the C-to-Ni CT excited states accelerates the C-C cross-coupling reaction by up to 105 times, as the CT electronic transition can substitute for the rate-determining ICT step of the RE reaction at the ground electronic state.

Impact of Phase Transitions on Temperature Evolution of Absorption Spectra and Electron–Phonon Interactions in [N(C2H5)4]2CoCl2Br2 Crystals

Absorption spectra and structural parameters of [N(C2H5)4]2CoCl2Br2 (TEACCB-2) crystals were investigated. Absorption bands corresponding to internal electronic transitions of Co2+ were identified. The impact of phase transitions on the temperature evolution of absorption spectra was studied. The low-energy tail of the absorption edge formed by ligand–metal charge-transfer electronic transitions was shown to be exponential and obeyed the empirical Urbach rule in the two high-temperature crystal phases. The temperatures and types of phase transitions at T1 = 244 K and T2 = 228 K (in cooling mode) were refined by analyzing the temperature changes of the main characteristics of the obtained spectra. An anomaly at T3 = 200 K was detected for the first time in TEACCB-2 bulk crystals and appeared earlier in the absorption spectrum of TEACCB-2 nanocrystals. It could be attributed to a phase transition.

Crystallographic elucidations of indium(III) porphyrin conformations, morphology and aggregation behaviour: Comparative optical study of free base porphyrins and their indium(III) derivatives at varying pH

The supramolecular structural architectures of 5, 10, 15, 20-Tetrakis-(4-methoxyphenylporphyrinato) indium (III) chloride (I) and 5, 10, 15, 20-Tetrakis-(4-methylphenylporphyrinato) indium (III) chloride (II) have been determined from their three dimensional X-ray diffraction data. Complex I crystallizes in the orthorhombic space group, Pbca with a = 23.0083 (4) Å, b = 15.2447 (3) Å, c = 23.3737 (4) Å, α = β = γ = 90°, V = 8198.5 (3) Å3 and Z = 8, whereas complex II crystallizes in the triclinic space group, P-1 with a = 12.0737 (5) Å, b = 12.1000 (5) Å, c = 14.1682 (6) Å, α = 88.560 (3)°, β = 78.730 (3)°, γ = 78.280 (3)° V = 1987.40 (14) Å3 and Z = 2. The geometries around In+3 ions in both the complexes are square pyramidal giving dome shaped architectures to the metalloporphyrins. In their respective crystal lattices, the monomeric units of complex I and II are packed in J-type (head-to-tail) supramolecular aggregation. These monomers are linked into their three dimensional supramolecular networks by various non covalent forces. Further the packing of monomers of each complex in their respective unit cells leaves significant void volumes (porosities). The optical absorption spectra of complex I and II and their free base analogues i.e. 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP and 5, 10, 15, 20-Tetrakis-(4-methylphenyl) porphyrin, H2TTP have also been investigated in CH2Cl2 at different ionic strengths of the media. The investigations revealed that both the free bases exist as simple monomers, j-aggregates and dianionic monomers at neutral (pH = 7), acidic (pH = 4 and lower) and basic (pH = 10) media conditions. Also at high ionic strength of the media, the free bases undergo charge transfer electronic transitions. The two In3+ porphyrin exists as simple monomers at pH = 7. However at pH = 5 and lower, the In3+ ions in complex II is remarkably easily replaced by the protons of the acid to generate the corresponding protonated free base monomers which simultaneously aggregate in J-type stacking mode. In contrast to complex II that exhibits low stability in acidic media, the complex I remain intact in such media reflecting extremely high stability of the complex. The insertion of In3+ ions into the porphyrinic cores of free bases causes bathochromic shift followed by increase in the intensities of absorption bands. The appearance of two set of doublets for ortho phenyl protons in the 1H NMR spectra of each complex is attributed to the magnetic inequivalence of their ortho phenyl protons caused by the pulling of In+3 ions above the mean porphyrin planes.

Electronic excitations and optical properties of YbFe2O4 thin films

We present the structural, optical properties, and electronic excitation spectra of the YbFe2O4 thin film deposited on (0001) sapphire substrates. The optical spectra of a YbFe2O4 thin film show several electronic transitions, dominated by Fe d to d on-site electronic transitions as well as O 2p to Fe 3d, Yb 6s, and Yb 5d charge-transfer electronic transitions. The direct energy band gap of YbFe2O4 has been found to be 1.40 ± 0.01 eV at 300 K. Moreover, the electronic transitions centered at 1.75 eV and 2.41 eV exhibit strong temperature dependence with a discontinuity at ∼190 ± 10 K, indicating evidence for a structural instability in the system. The optical spectra of YbFe2O4 have been analyzed and also compared with that of isostructural YFe2O4 and LuFe2O4.

Correlating thermochromic and mechanochromic phosphorescence with polymorphs of a complex gold(i) double salt with infinite aurophilicity.

A complex [Au(NHC)2][Au(C[triple bond, length as m-dash]N)2] double salt exhibits polymorphic crystalline structures with distinct phosphorescence and variations at AuAu distances and ligand-ligand torsion angles. This serves as a model system correlating aurophilicity and low-energy charge-transfer electronic transitions and helps to understand the thermochromism and mechanochromism of the gold(i) complexes.

The solvation inhomogeneity of sulfur dioxide in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid probed by Raman spectroscopy

AbstractThe different solvation states of SO2 in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid were probed by the detailed analysis of the SO2 symmetric stretching mode, νs(SO2). The band position and linewidth were monitored in function of gas molar fraction, excitation wavelength, and temperature. As expected, the decreasing of the SO2 molar fraction, XSO2, from approximately 0.8 to 0.2, leads to a gradual downshift of the νs(SO2) band, attributed to a more effective charge transfer interaction from thiocyanate anion to SO2. Interestingly, at high gas concentrations (XSO2 = 0.8), a gradual downshift of the νs(SO2) band was noticed as the Raman excitation energy increases, an unexpected behavior because the change in the excitation wavelength in Raman spectroscopy should affect only the band intensities. Such νs(SO2) Raman dispersion can be understood as a consequence of the distinct charge transfer electronic transition energies associated to different SO2 solvation states, whose relative band intensities are modulated by resonance Raman conditions. Moreover, the lowering from room temperature to 100 K showed an increasing in the Raman dispersion effect. The temperature dependence was attributed to the strengthening of the cation–anion interaction, responsible for the increasing of SO2 solvation states distribution. The vibrational spectroscopic data regarding the SO2 solvation inhomogeneity in ionic liquids are showed for the first time, and it may contribute in the understanding of why such liquids are promising solvents for SO2 capture.

Spectroscopic, electrochemical and X-ray crystallographic properties of a novel palladium(II) complex of thioamide deprotonated di-2-pyridyl ketone thiosemicarbazone (dpktsc-H)−

The reaction between di-2-pyridyl ketone thiosemicarbazone (dpktsc) and PdCl2(CH3CN)2, generated in situ from the reaction between PdCl2 and CH3CN, gave the unprecedented [Pd2Cl3(κ5-Npy,Nim,S,Npy,Nam-dpktsc-H)]·2CH3CN (1) complex (py = pyridine, im = imine and am = amide). The identity of 1 was confirmed via its elemental analysis and spectroscopic properties. Infrared and 1H-NMR spectra confirmed the coordination of (dpktsc-H)− to the palladium ions. The electronic absorption spectra measured in dmso and dmf and density functional theory (DFT) calculations revealed metal-to-ligand charge-transfer (MLCT), d–d and intra-ligand charge-transfer (ILCT) electronic transitions. X-ray structural analysis on a crystal of [Pd2Cl3(κ5-Npy,Nim,S,Npy,Nam-dpktsc-H)]·H2O (2) grown from dmf solution of 1 confirmed its formulation and showed the solid-state structure contains a web of molecules locked via a network of non-covalent interactions. Electrochemical measurements on 1 in dmf revealed metal- and ligand-based redox processes. In contrast to the electrochemical decomposition of uncoordinated dpktsc, coordinated (dpktsc-H)− in 1 does not undergo electrochemical decomposition. Electrochemical titrations of 1 with p-toluenesulfonic acid monohydrate (p-TSOH) revealed electro-catalytic proton reduction. Over-potential (η) of 180 mV for the H2 evolution was observed and is comparable to several molecular electro-catalysts for proton reduction. Controlled-potential electrolysis confirmed the electro-catalytic proton reduction by the Pd-complex. Electrochemical reactions of CO2 in the presence of 1 exhibited a proton dependence, and metal- and ligand-based electrochemical reaction.

Frontier orbitals and ligand-to-metal charge transfer electronic transitions in d 0-metal complexes

Properties of frontier orbitals and low-energy electronic transitions in a d 0-organometallic complex have been studied by TDDFT and DFT methods using B3LYP hybrid functional and 3-21G*, 6-31G**, SDD, CEP-121G, and DGDZVP basis sets. It has been shown that the electronic transition between frontier orbitals in the excitation and absorption spectra is associated with charge transfer mainly from π-type ligands to a central metal d 0-ion. The good agreement of the data (the shape and band position of the spectra of electronic absorption and excitation, energy of electronic transitions, and strength of the harmonic oscillator) of quantum-chemical and photophysical studies is demonstrated.

Bonding and Electronic Properties of Linear Diethynyl Oligothienoacene-Bridged Diruthenium Complexes and Their Oxidized Forms.

A series of five diruthenium diethynyl complexes based on α,β-fused oligothienoacenes in the core of the bridging ligands [{Ru(dppe)Cp*}2(μ-C≡C-L-C≡C)] [dppe = 1,2-bis(diphenylphosphino)ethane, Cp* = η5-C5Me5; L = thieno[3,2-b]thiophene (4), thieno[2,3-b]thiophene (5), 3,4-dimethylthieno[2,3-b]thiophene (6), dithieno[3,2-b:2',3'-d]thiophene (7), and thieno[3,2-b]thieno[2',3':4,5]thieno[2,3-d]thiophene (8)] have been synthesized and fully characterized electrochemically and spectroscopically. Elongation of the redox noninnocent oligothienoacene bridge core causes a smaller potential difference between the initial two anodic steps, not seen for free dialkyl oligothienoacenes, and increased positive charge delocalization over the conjugated bridge backbone. The highest occupied molecular orbital of the parent complexes resides predominantly on the oligothienoacene core, with strong participation of the ethynyl linkers and slightly smaller contribution from the metallic termini. This bonding character makes the initial one-electron oxidation symmetrical, as revealed by combined voltammetric and spectroscopic (IR, UV-vis-near-IR, and electron paramagnetic resonance) methods as well as density functional theory (DFT) and time-dependent DFT calculations of truncated and selected nontruncated models of the studied series. The remarkable gradual appearance of two C≡C stretching absorptions in the IR spectra of the monocationic diethynyl complexes is ascribed to increasing vibronic coupling of the IR-forbidden νs(C≡C) mode of the oxidized -[C≡C-core-C≡C]+- bridge with a low-lying π-π*(intrabridge)/metal-to-ligand charge-transfer electronic transition in the near-to-mid-IR spectral region.

Synthesis and characterization of functional photoactive organic-inorganic block copolymers of poly(methylphenylsilane) and disperse red 1 methacrylate and study of their optical and photophysical properties

ABSTRACTFunctional photoactive organic-inorganic block copolymers of poly(methylphenylsilane) (PMPS) and disperse red 1 methacrylate (DR1MA) were synthesized in a quartz tube using UV-technique. The synthesized block copolymers were characterized by FTIR, NMR, GPC and thermal analyses and studied for their optical and photoluminescence properties. The weight average and number average molecular weights of such a synthesized block copolymer are 2.47 × 103 and 2.27 × 103, respectively. The appearance of two glass transition temperatures indicated the synthesized polymers as block copolymers. The functional organic-inorganic block copolymers exhibited optical absorbance at 276 nm due to aromatic ring associated with both the blocks and at 325 nm due to σ-electron delocalization of Si-Si chain of PMPS block. Also, the optical absorbance appeared at 472 nm is due to combining the contribution of n-π* and first π-π* charge transfer electronic transition of the azobenzene chromophore of DR1MA unit. Two photoemissions were observed at 307 nm and 415 nm when such a polymer was excited at 275 nm. The photoluminescence was also observed at 415 nm when excited by 325 nm. The multi-emission spectra appeared between 510 nm to 580 nm are presumed to be due to exciton coupling between azobenzene chromophore of DR1MA and and Si-Si σ-conjugation of PMPS block. The synthesized copolymers are thermally stable up to 260°C. Such functional photoactive block copolymers may find novel optoelectronic application.

Optical Properties of DMA-π-DCV Derivatives: A Theoretical Inspection under the DFT Microscope

The optical properties of a series of donor-acceptor N,N-dimethylaniline-π-dicyanovinylene (DMA-π-DCV) chromophores have been investigated under the density functional theory framework. Focus has been made on the low-lying charge-transfer (CT) electronic transitions for which experimental data is available. The effect of theπ-conjugated bridge length and type was analysed between the families of oligoene and oligoyne derivatives of increasing size. Theoretical calculations demonstrate that the ethylene bridge is a betterπ-communicator and allows for more delocalized frontier molecular orbitals compared to the acetylene spacer. TheΛdiagnostic test allowed rationalization of the orbital spatial overlap in the main CT excitations. The performance of different density functional rungs was assessed in the prediction of the lowest-lying CT electronic transition. Surprisingly, most modern long-range corrected functionals demonstrated to provide among the largest errors, whereas hybrid functionals showed the best performance. Solvatochromism was confirmed in both oligoene and oligoyne compounds. A donor-acceptor-donor triad based on tetrathiafulvalene was utilised as a test system for the prediction of its two CT bands of different nature, energy, and intensity. The hybrid PBE0 (or a similar hybrid analogue) consolidates as the best choice for the prediction of CT excitations in the DMA-π-DCV push-pull family.

Synthesis and Luminescent Properties of Eu3+ Doped CaGd4O7 Phosphors by Solvothermal Reaction Method

Eu(3+) doped CaGd4O7 phosphors have been newly synthesized using a solvothermal reaction method and sintered at 1400 °C. The phase, composition, morphologies, and photoluminescent properties of the phosphors have been well characterized by means of the X-ray diffraction (XRD) patterns, energy dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FE-SEM), photoluminescence (PL) spectroscopy, and decay curves, respectively. The XRD patterns of the as-prepared phosphors confirm their monoclinic structure and the FE-SEM images reveal flower-like morphology, formed through agglomeration. The calculated size of the crystallites was approximately 83 nm. The photoluminescence excitation (PLE) spectra of CaGd4O7:Eu(3+) phosphors consist of a broad band due to the charge transfer (CT) electronic transition, and several sharp peaks that can be attributed to the f-f transitions of Eu(3+) and Gd(3+). The PL spectra exhibited a stronger red emission corresponding to the (5)D0 --> (7)F2 transition. The CIE chromaticity coordinates of the phosphors were calculated and all the chromaticity coordinates have been placed in the red spectral region. These luminescent powders are expected to have potential applications for white light-emitting diodes (WLEDs) and optical display systems.

Measuring Coherently Coupled Intramolecular Vibrational and Charge-Transfer Dynamics with Two-Dimensional Vibrational-Electronic Spectroscopy.

We demonstrate Fourier transform (FT) 2D vibrational-electronic (2D VE) spectroscopy employing a novel mid-IR and optical pulse sequence. This new femtosecond third-order nonlinear spectroscopy provides the high time and frequency resolutions of existing 2D FT techniques; however, resulting 2D VE spectra contain IR and electronic dipole moment cross terms. We use 2D VE spectroscopy to help understand the vibrational-electronic couplings in the cyanide-bridged transition-metal mixed valence complex [(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide. The amplitudes of the cross-peaks in the 2D VE spectra reveal that three of the intramolecular cyanide stretching vibrations lying along the charge-transfer axis are coherently coupled to the metal-to-metal charge-transfer electronic transition with differing strengths. Analysis of the 2D VE line shapes reveals positive and negative correlations of the cyanide stretching modes with the charge-transfer transition depending on the physical orientation of the vibration in the molecule and its interaction with the solvent. The insights found thus far into the vibronic couplings in the mixed valence model system indicate that the 2D VE technique will be a valuable addition to the existing multidimensional spectroscopy toolbox.

DFT and TD-DFT assessment of the structural and optoelectronic properties of an organic-Ag14 nanocluster.

An extensive benchmark of exchange-correlation functionals on the structure of the X-ray resolved phosphine and thiolate-protected Ag14-based nanocluster, named XMC1, is reported. Calculations were performed both on simplified model systems, with the complexity of the ligands greatly reduced, and on the complete XMC1 particle. Most of the density functionals that yielded good relaxed structures on analogous calculations on gold nanoclusters (viz. those employing the generalized gradient approximation) significantly deform the structure of XMC1. On the contrary, some of the exchange-correlation functionals including part of the exact Hartree-Fock exchange (hybrid functionals) reproduce the experimental geometry with minimal errors. In particular, the widely adopted B3LYP yields fairly accurate structures for XMC1, whereas it is outperformed by many other functionals (both hybrids and generalized gradient corrected) in similar calculations on analogous gold-based systems. Time-dependent density functional calculations have been employed to recover the experimental UV-vis spectrum. The present investigation shows that to correctly reproduce the optical feature of XMC1 the ligands cannot be omitted, because they interact with the metal core at energies much closer to the optical gap than in the case of gold-based nanoclusters of similar sizes. Due to this fact, a functional that accurately describes charge-transfer electronic transitions (such as the long-range corrected CAM-B3LYP) has to be adopted.

Theoretical insight on novel donor-acceptor exTTF-based dyes for dye-sensitized solar cells

A thorough density functional theory study is performed for the three carboxyl-based derivatives of the exTTF-TCF chromophore, where the π-extended tetrathiafulvalene (exTTF) electron-donor is linked to the tricyanofuran (TCF) electron-acceptor through an ethylene bridge, as dyes for dye-sensitized solar cells. Calculations predict that the carboxyl group in the acceptor moiety adopts an adequate orientation for an efficient anchoring on the semiconductor TiO₂ surface. The carboxylic acid group holds a negative charge twice larger than the cyano moiety that favors the electron injection to the semiconductor. Time-dependent calculations allow for the assignment of the absorption bands in the UV-vis spectrum of exTTF-TCF and confirm the presence of two low-lying charge-transfer electronic transitions that account for the moderately-intense absorption in the 450-800 nm range. The striking optical absorption properties of exTTF-TCF are preserved for the carboxylic analogues. Finally, periodic calculations show relevant topological differences between the carboxylic derivatives anchored on the TiO₂ surface, which would notably influence in the power conversion efficiency of a dye-sensitized solar cell.