Geometry and quadratic nonlinearity of charge transfer complexes in solution: A theoretical study

In this paper, we have computed the quadratic nonlinear optical (NLO) properties of a class of weak charge transfer (CT) complexes. These weak complexes are formed when the methyl substituted benzenes (donors) are added to strong acceptors like chloranil (CHL) or di-chloro-di-cyano benzoquinone (DDQ) in chloroform or in dichloromethane. The formation of such complexes is manifested by the presence of a broad absorption maximum in the visible range of the spectrum where neither the donor nor the acceptor absorbs. The appearance of this visible band is due to CT interactions, which result in strong NLO responses. We have employed the semiempirical intermediate neglect of differential overlap (INDO/S) Hamiltonian to calculate the energy levels of these CT complexes using single and double configuration interaction (SDCI). The solvent effects are taken into account by using the self-consistent reaction field (SCRF) scheme. The geometry of the complex is obtained by exploring different relative molecular geometries by rotating the acceptor with respect to the fixed donor about three different axes. The theoretical geometry that best fits the experimental energy gaps, beta(HRS) and macroscopic depolarization ratios is taken to be the most probable geometry of the complex. Our studies show that the most probable geometry of these complexes in solution is the parallel displaced structure with a significant twist in some cases. (C) 2011 American Institute of Physics. doi:10.1063/1.3526748]

Electric Field Directed Growth of Molecular Wires of Charge Transfer Molecules on Prefabricated Metal Electrodes

Molecular wires of charge transfer molecules were formed by co-evaporating the 7 7 8 8-Tetracyanoquinodimethane [TCNQ] (acceptor) and Tetrathiafulvalene [TTF] (donor) molecules across prefabricated metal electrodes. Molecular wires of TTF TCNQ were also formed by evaporating single complex of TTF:TCNQ across prefabricated metal electrodes The prefabricated metal electrodes were made using electron beam lithography on SiO2 and glass cover slip substrates. Even though TTF: TCNQ wires grown from both co-evaporation and evaporation techniques show semiconductor like behavior in temperature dependence of resistance they show different activation energies due the difference in stoichiometry of TTF and TCNQ.

Heterogeneity in Dye-TiO2 Interactions Dictate Charge Transfer Efficiencies for Diketopyrrolopyrrole-Based Polymer Sensitized Solar Cells

Power conversion efficiency of a solar cell is a complex parameter which usually hides the molecular details of the charge generation process. For rationally tailoring the overall device efficiency of the dye-sensitized solar cell, detailed molecular understanding of photoinduced reactions at the dye-TiO2 interface has to be achieved. Recently, near-IR absorbing diketopyrrolopyrrole-based (DPP) low bandgap polymeric dyes with enhanced photostabilities have been used for TiO2 sensitization with moderate efficiencies. To improve the reported device performances, a critical analysis of the polymerTiO(2) interaction and electron transfer dynamics is imperative. Employing a combination of time-resolved optical measurements complemented by low temperature EPR and steady-state Raman spectroscopy on polymerTiO(2) conjugates, we provide direct evidence for photoinduced electron injection from the TDPP-BBT polymer singlet state into TiO2 through the C-O group of the DPP-core. A detailed excited state description of the electron transfer process in films reveals instrument response function (IRF) limited (<110 fs) charge injection from a minor polymer fraction followed by a picosecond recombination. The major fraction of photoexcited polymers, however, does not show injection indicating pronounced ground state heterogeneity induced due to nonspecific polymerTiO(2) interactions. Our work therefore underscores the importance of gathering molecular-level insight into the competitive pathways of ultrafast charge generation along with probing the chemical heterogeneity at the nanoscale within the polymerTiO2 films for optimizing photovoltaic device efficiencies.

Crystal structure and charge transfer energy of the vaterite-type orthoborate YBO3:Eu

Ligand-to-metal charge transfer energies of YBO3:Eu have been investigated from the chemical bond viewpoint. The chemical bond parameters, such as the covalency, the polarizability of the chemical bond volume, and the presented charge of the ligands in the chemical bond have been quantitatively determined based on the dielectric theory of complex crystal. We calculated the environmental factor (h(e)), which is the major factor influencing the charge transfer energy in the compounds. The calculated results show that the suitable group space of YBO3 is C2/c. The method provides us with a supplementary tool to judge the proper structure when the structure of the crystal has many uncertain space groups.

A versatile color tuning strategy for iridium(III) and platinum(II) electrophosphors by shifting the charge-transfer states with an electron-deficient core

By fusing an electron-deficient ring system with the phenyl ring of a 2-phenylpyridine (ppy)-type ligand, a new and synthetically versatile strategy for the phosphorescence color tuning of cyclometalated iridium(III) and platinum(II) metallophosphors has been established. Two robust red electrophosphors with enhanced electron-injection/electron-transporting features were prepared by using an electron-trapping fluoren-9-one chromophore in the ligand design. The thermal, photophysical, redox and electrophosphorescent properties of these complexes are reported. These exciting results can be attributed to a switch of the metal-to-ligand charge-transfer (MLCT) character of the transition from the pyridyl groups in the traditional Ir-III or Pt-II ppy-type complexes to the electron-deficient ring core, and the spectral assignments corroborate well with the electrochemical data as well as the timedependent density functional theory (TD-DFT) calculations. The electron-withdrawing character of the fused ring results in much more stable MLCT states, inducing a substantial red-shift of the triplet emission energy from yellow to red for the Ir-III complex and even green to red for the PtII counterpart.

Investigation on relationship between charge transfer position and dielectric definition of average energy gap in Eu3+-doped compounds

The dielectric definition of average energy gap E-g of the chemical bond has been calculated quantitatively in Eu3+-doped 30 lanthanide compounds based on the dielectric theory of chemical bond for complex structure crystals. The relationship between the experimental charge transfer (CT) energy of Eu3+ and the corresponding average energy gap E-g has been studied. The results show that the CT energy increases linearly with increasing of the average energy gap E-g. The linear model is obtained. It allows us to predict the CT position of Eu3+-doped lanthanide compounds with knowledge of the crystal structure and index of refraction. Applied to the Ca4GdO(BO3)(3):Eu and Li2Lu5O4(BO3)(3):Eu crystals, the predicted results of CT energies are in good agreement with the experimental values, and it can be concluded that the lowest CT energy in Li2Lu5O4(BO3)(3):Eu originates from the site of Lu1.

Dependence of charge transfer energy on crystal structure and composition in Eu3+-doped compounds

We report a method for estimating the positions of charge transfer (CT) bands in Eu3+-doped complex crystals. The environmental factor ( he) influencing the CT energy is presented. he consists of four chemical bond parameters: the covalency, the bond volume polarization, the presented charge of the ligand in the chemical bond, and the coordination number of the central ion. These parameters are calculated with the dielectric theory of complex crystals. The relationship between the experimental CT energies and calculated environmental factors was established by an empirical formula. The calculated values are in good agreement with the experimental results. Such a relationship was confirmed by detailed analysis. In addition, our method is also useful to predict the charge-transfer position of any other rare earth ion.

Synthesis, characterization and crystal structure of a charge transfer salt based on 1.3-cyclopentadiene tetrabutylammoniumdodecamolybdosilicoate

A charge transfer salt, (Bu4N)(4) (C5H6)[(HSiMo11MoO40)-Mo-VI-O-V] has been photochemically synthesized from (Bu4N)(4)SiMo12O40 and 1.3-cyclopentadiene and Characterized, by elemental analysis, IR spectra, solid diffusion reflectance electronic spectra, CV and ESR. The X-ray crystal structure revealed that the title complex crystal data are as follows: triclinic, space group P (1) over bar, a = 14.347(3), b = 14.423(3), c = 27.158(5) Angstrom, alpha = 96.90(3), beta = 104.18(3), gamma = 98.20(3)degrees, V = 5322(2) Angstrom (3), Z = 2, M-r = 2855. 30, D-c = 1.782g.cm(-3), F(000) = 2860, R = 0.0719, wR = 0.198. The title compound is composed of 1.3-cyclopentadiene, four tetrabutylammonium and [(SiMo11MoO40)-Mo-VI-O-V](4-) anion.

Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene: Contributions from intra-ligand charge transfer states

The photophysical properties of Ru(II) and Re(I) polypyridyl complexes including a bis-bipyridyl pyrene ligand are presented. The complexes ([(bpy)(2)Ru](2)bpb)(4+) and [(CO)(3)ReCl(bpb)] (bpy = 2,2'-bipyridine, bpb = 1,6-bis-(4-(2,2'-bipyrid-yl)-pyrene) were designed with the intent of examining intramolecular energy migration between MLCT states localized on the metal complexes and pyrene-localized (3)(pi-pi) states. Absorption spectroscopy of both complexes containing the bpb ligand reveals that in addition to the MLCT and the pyrene-centered (1)(pi-pi) transitions, a new absorption band is observed near 400 nm for both complexes. Absorption spectral data for the Re(I) complex strongly suggest the presence of a pyrene(pi) to bpy(pi) intraligand charge transfer (ILCT) transition. Emission spectra at room temperature and at 77 K are almost identical for the Ru(II) and Re(I) complexes containing the bpb ligand. The (3)MLCT emission of related bipyridyl compounds lacking the pyrene is observed at higher energy than for the pyrene-containing complexes, ([(bpy)(2)Ru](2)bpb)(4+) and [(CO(3)ReCl(bpb)]. The Ru(II) complex emits at room temperature with a remarkably long lifetime (130 micros in degassed DMSO). This emission is also strongly sensitive to oxygen and is almost entirely quenched in an aerated solution. In addition, excited-state absorption spectra exhibit features not consistent with (3)MLCT or (3)(pi-pi) states of the parent chromophores. The combined characteristics suggest the emission arises from either (3)(pi-pi) or (3)ILCT states or a state with mixed parentage.

Ultracold, radiative charge transfer in hybrid Yb ion – Rb atom traps

Ultracold hybrid ion–atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the ${\rm Y}{{{\rm b}}^{+}}$ ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes; ${\rm Y}{{{\rm b}}^{+}}(6{\rm s}{{\;}^{2}}{\rm S})+{\rm Rb}(5{\rm s}{{\;}^{2}}{\rm S})\to {\rm Yb}(6{{{\rm s}}^{2}}{{\;}^{1}}{\rm S})+{\rm R}{{{\rm b}}^{+}}(4{{{\rm p}}^{6}}{{\;}^{1}}{\rm S})+h\nu $ and ${\rm Y}{{{\rm b}}^{+}}(6{\rm s}{{\;}^{2}}{\rm S})+{\rm Rb}(5{\rm s}{{\;}^{2}}{\rm S})\to {\rm YbR}{{{\rm b}}^{+}}({{X}^{1}}{{\Sigma }^{+}})+h\nu $. Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion–atom collisions dominates cold ion–atom collisions. For spin-dependent processes [1] the anisotropic magnetic dipole–dipole interaction and the second-order spin–orbit coupling can play important roles, inducing coupling between the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately five orders of magnitude higher than the charge-exchange collision rate [1]. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question.

The role of dynamical polarization of the ligand-to-metal charge transfer excitations in the ab initio determination of effective exchange parameters

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits us to suggest a variant of the difference dedicated configuration interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center.

A hexadecanuclear copper(I)-copper(II) mixed-valence compound: structure, magnetic properties, intervalence charge transfer, EPR, and NMR

Hexadecanuclear copper mixed-valence complex 2 containing 10 Cu-II, centers and 6 Cu-I centers was isolated with N,O donor ligands. From the X-ray crystal structure, 2 was found to contain a centrosymmetric dimeric cation - each monomeric unit composed of eight copper centers. It displays a very broad and weak intervalence charge-transfer band around 1100 nm at room temperature in the solid state. Variable-temperature magnetic susceptibility measurements indicate an S = 1/2 ground state for half of 2, explicitly, each Cu-8 moiety has a g value around 2.26. Complex 2 was examined by NMR spectroscopy at room temperature in solution and by EPR at low temperature; the data indicates that the valence is delocalized in 2 at room temperature but localized at low temperature. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Electron spectroscopy of novel charge transfer systems based on polycyclic aromatic hydrocarbons

Organische Ladungstransfersysteme weisen eine Vielfalt von konkurrierenden Wechselwirkungen zwischen Ladungs-, Spin- und Gitterfreiheitsgraden auf. Dies führt zu interessanten physikalischen Eigenschaften, wie metallische Leitfähigkeit, Supraleitung und Magnetismus. Diese Dissertation beschäftigt sich mit der elektronischen Struktur von organischen Ladungstransfersalzen aus drei Material-Familien. Dabei kamen unterschiedliche Photoemissions- und Röntgenspektroskopietechniken zum Einsatz. Die untersuchten Moleküle wurden z.T. im MPI für Polymerforschung synthetisiert. Sie stammen aus der Familie der Coronene (Donor Hexamethoxycoronen HMC und Akzeptor Coronen-hexaon COHON) und Pyrene (Donor Tetra- und Hexamethoxypyren TMP und HMP) im Komplex mit dem klassischen starken Akzeptor Tetracyanoquinodimethan (TCNQ). Als dritte Familie wurden Ladungstransfersalze der k-(BEDT-TTF)2X Familie (X ist ein monovalentes Anion) untersucht. Diese Materialien liegen nahe bei einem Bandbreite-kontrollierten Mottübergang im Phasendiagramm.rnFür Untersuchungen mittels Ultraviolett-Photoelektronenspektroskopie (UPS) wurden UHV-deponierte dünne Filme erzeugt. Dabei kam ein neuer Doppelverdampfer zum Einsatz, welcher speziell für Milligramm-Materialmengen entwickelt wurde. Diese Methode wies im Ladungstransferkomplex im Vergleich mit der reinen Donor- und Akzeptorspezies energetische Verschiebungen von Valenzzuständen im Bereich weniger 100meV nach. Ein wichtiger Aspekt der UPS-Messungen lag im direkten Vergleich mit ab-initio Rechnungen.rnDas Problem der unvermeidbaren Oberflächenverunreinigungen von lösungsgezüchteten 3D-Kristallen wurde durch die Methode Hard-X-ray Photoelectron Spectroscopy (HAXPES) bei Photonenenergien um 6 keV (am Elektronenspeicherring PETRA III in Hamburg) überwunden. Die große mittlere freie Weglänge der Photoelektronen im Bereich von 15 nm resultiert in echter Volumensensitivität. Die ersten HAXPES Experimente an Ladungstransferkomplexen weltweit zeigten große chemische Verschiebungen (mehrere eV). In der Verbindung HMPx-TCNQy ist die N1s-Linie ein Fingerabdruck der Cyanogruppe im TCNQ und zeigt eine Aufspaltung und einen Shift zu höheren Bindungsenergien von bis zu 6 eV mit zunehmendem HMP-Gehalt. Umgekehrt ist die O1s-Linie ein Fingerabdruck der Methoxygruppe in HMP und zeigt eine markante Aufspaltung und eine Verschiebung zu geringeren Bindungsenergien (bis zu etwa 2,5eV chemischer Verschiebung), d.h. eine Größenordnung größer als die im Valenzbereich.rnAls weitere synchrotronstrahlungsbasierte Technik wurde Near-Edge-X-ray-Absorption Fine Structure (NEXAFS) Spektroskopie am Speicherring ANKA Karlsruhe intensiv genutzt. Die mittlere freie Weglänge der niederenergetischen Sekundärelektronen (um 5 nm). Starke Intensitätsvariationen von bestimmten Vorkanten-Resonanzen (als Signatur der unbesetzte Zustandsdichte) zeigen unmittelbar die Änderung der Besetzungszahlen der beteiligten Orbitale in der unmittelbaren Umgebung des angeregten Atoms. Damit war es möglich, präzise die Beteiligung spezifischer Orbitale im Ladungstransfermechanismus nachzuweisen. Im genannten Komplex wird Ladung von den Methoxy-Orbitalen 2e(Pi*) und 6a1(σ*) zu den Cyano-Orbitalen b3g und au(Pi*) und – in geringerem Maße – zum b1g und b2u(σ*) der Cyanogruppe transferiert. Zusätzlich treten kleine energetische Shifts mit unterschiedlichem Vorzeichen für die Donor- und Akzeptor-Resonanzen auf, vergleichbar mit den in UPS beobachteten Shifts.rn

Intermolecular and Intramolecular Charge Transfer in Functional Molecular Materials

This thesis is devoted to the investigation of inter and intramolecular charge transfer (CT) in molecular functional materials and specifically organic dyes and CT crystals. An integrated approach encompassing quantum-chemical calculations, semiempirical tools, theoretical models and spectroscopic measurements is applied to understand structure-property relationships governing the low-energy physics of these materials. Four main topics were addressed: 1) Spectral properties of organic dyes. Charge-transfer dyes are constituted by electron donor (D) and electron acceptor (A) units linked through bridge(s) to form molecules with different symmetry and dimensionality. Their low-energy physics is governed by the charge resonance between D and A groups and is effectively described by a family of parametric Hamiltonians known as essential-state models. These models account for few electronic states, corresponding to the main resonance structures of the relevant dye, leading to a simple picture that is completed introducing the coupling of the electronic system to molecular vibrations, treated in a non-adiabatic way, and an effective classical coordinate, describing polar solvation. In this work a specific essential-state model was proposed and parametrized for the dye Brilliant Green. The central issue in this work has been the definition of the diabatic states, a not trivial task for a multi-branched chromophore. In a second effort, we have used essential-state models for the description of the early-stage dynamics of excited states after ultrafast excitation. Crucial to this work is the fully non-adiabatic treatment of the coupled electronic and vibrational motion, allowing for a reliable description of the dynamics of systems showing a multistable, broken-symmetry excited state. 2) Mixed-stack CT salts. Mixed-stack (MS) CT crystals are an interesting class of multifunctional molecular materials, where D and A molecules arrange themselves to form stacks, leading to delocalized electrons in one dimension. The interplay between the intermolecular CT, electrostatic interactions, lattice phonons and molecular vibrations leads to intriguing physical properties that include (photoinduced) phase transitions, multistability, antiferromagnetism, ferroelectricity and potential multiferroicity. The standard microscopic model to describe this family of materials is the Modified Hubbard model accounting for electron-phonon coupling (Peierls coupling), electron-molecular vibrations coupling (Holstein coupling) and electrostatic interactions. We adopt and validate a method, based on DFT calculations on dimeric DA structures, to extract relevant model parameters. The approach offers a powerful tool to shed light on the complex physics of MS-CT salts. 3) Charge transfer in organic radical dipolar dyes. In collaboration with the group of Prof. Jaume Veciana (ICMAB- Barcellona), we have studied spectral properties of a special class of CT dyes with D-bridge-A structure where the acceptor group is a stable radical (of the perchlorotriphenylmethyl, PTM, family), leading to an open-shell CT dyes. These materials are of interest since they associate the electronic and optical properties of CT dyes with magnetic properties from the unpaired electron. The first effort was devoted to the parametrization of the relevant essential-state model. Two strategies were adopted, one based on the calculation of the low-energy spectral properties, the other based on the variation of ground state properties with an applied electric field. 4) The spectral properties of organic nanoparticles based on radical species are investigated in collaboration with Dr. I. Ratera (ICMAB- Barcellona). Intriguing spectroscopic behavior was observed pointing to the presence of excimer states. In an attempt to rationalize these findings, extensive calculations (TD-DFT and ZINDO) were performed. The results for the isolated dyes are validated against experimental spectra in solution. To address intermolecular interactions we studied dimeric structures in the gas phase, but the preliminary results obtained do not support excimer formation.

Time-resolved spectroscopy of the metal-to-metal charge transfer excited state in dinuclear cyano-bridged mixed-valence complexes

Visible pump-probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [(LCoNCMII)-N-III(CN)(5)](-), where L is a pentadentate macrocyclic pentaamine (L-14) or triamine-dithiaether (L-14S) and M is Fe or Ru. Nanosecond pump-probe spectroscopy on frozen solutions of [(LCoNCFeII)-Co-14-N-III(CN)(5)](-) and [(LCoNCFeII)-Co-14S-N-III(CN)(5)](-) at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350-400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [(LCoNCRuII)-Co-14-N-III(CN)(5)](-) was too short-lived to allow its detection. Femtosecond pump-probe spectroscopy on aqueous solutions of [(LCoNCFeII)-Co-14-N-III(CN)(5)](-) and [(LCoNCFeII)-Co-14S-N-III(CN)(5)](-) at room temperature enabled the lifetimes of their Co-II-Fe-III MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.