Electronic Charge Delocalization Research Articles

Charge-Transfer Complexes: Fundamentals and Advances in Catalysis, Sensing, and Optoelectronic Applications.

Supramolecular assemblies, formed through electronic charge transfer between two or more entities, represent a rich class of compounds dubbed as charge-transfer complexes (CTCs). Their distinctive formation pathway, rooted in charge-transfer processes at the interface of CTC-forming components, results in the delocalization of electronic charge along molecular stacks, rendering CTCs intrinsic molecular conductors. Since the discovery of CTCs, intensive research has explored their unique properties including magnetism, conductivity, and superconductivity. Their more recently recognized semiconducting functionality has inspired recent developments in applications requiring organic semiconductors. In this context, CTCs offer a tuneable energy gap, unique charge-transport properties, tailorable physicochemical interactions, photoresponsiveness, and the potential for scalable manufacturing. Here, an updated viewpoint on CTCs is provided, presenting them as emerging organic semiconductors. To this end, their electronic and chemical properties alongside their synthesis methods are reviewed. The unique properties of CTCs that benefit various related applications in the realms of organic optoelectronics, catalysts, and gas sensors are discussed. Insights for future developments and existing limitations are described.

Investigation of nonlinear optical properties of a group of transition metal-decorated B12N12 nanocages: A DFT study

BN nanostructures are a promising group of materials with different technical applications. In this study, the electronic characteristics and NLO response of a group of B12N12 nanocages modified by transition metals have been studied using the DFT approach. Population analysis has been performed to investigate the role of transition metal in the delocalization of electronic charge on the surface of nanostructures. In this regard, molecular electrostatic potential (MEP) maps and density of state (DOS) plots have been surveyed. Based on frontier molecular orbitals (FMOs) energies, the reactivity of considered nanostructures has been discussed in the framework of global reactivity descriptors. NLO properties of considered nanostructures in terms of dipole moment, the mean polarizability, the anisotropy of the polarizability, the first-order hyperpolarizability, and the mean second-order hyperpolarizability have been studied in the gas phase and in the presence of Ethanol as solvent. The correlation between the NLO response of the nanocages and their different electronic characteristics has been studied. Results show that the presence of solvent makes an apparent increase in their NLO response. Electronic transitions responsible for absorption spectra of B12N12 nanocages have been considered, too.

Testing of Exchange-Correlation Functionals of DFT for a Reliable Description of the Electron Density Distribution in Organic Molecules

Researchers carrying out calculations using the DFT method face the problem of the correct choice of the exchange-correlation functional to describe the quantities they are interested in. This article deals with benchmark calculations aimed at testing various exchange-correlation functionals in terms of a reliable description of the electron density distribution in molecules. For this purpose, 30 functionals representing all rungs of Jacob's Ladder are selected and then the values of some QTAIM-based parameters are compared with their reference equivalents obtained at the CCSD/aug-cc-pVTZ level of theory. The presented results show that the DFT method undoubtedly has the greatest problems with a reliable description of the electron density distribution in multiple strongly polar bonds, such as C=O, and bonds associated with large electron charge delocalization. The performance of the tested functionals turned out to be unsystematic. Nevertheless, in terms of a reliable general description of QTAIM-based parameters, the M11, SVWN, BHHLYP, M06-HF, and, to a slightly lesser extent, also BLYP, B3LYP, and X3LYP functionals turned out to be the worst. It is alarming to find the most popular B3LYP functional in this group. On the other hand, in the case of the electron density at the bond critical point, being the most important QTAIM-based parameter, the M06-HF functional is especially discouraged due to the very poor description of the C=O bond. On the contrary, the VSXC, M06-L, SOGGA11-X, M06-2X, MN12-SX, and, to a slightly lesser extent, also TPSS, TPSSh, and B1B95 perform well in this respect. Particularly noteworthy is the overwhelming performance of double hybrids in terms of reliable values of bond delocalization indices. The results show that there is no clear improvement in the reliability of describing the electron density distribution with climbing Jacob's Ladder, as top-ranked double hybrids are also, in some cases, able to produce poor values compared to CCSD.

Non-additive electronic polarizabilities of ionic liquids: Charge delocalization effects

Electronic charge delocalization on the molecular backbones of ionic liquid-forming ions substantially impacts their molecular polarizabilities. Density functional theory calculations of polarizabilities and volumes of many cations and anions are reported and applied to yield refractive indices of 1216 ionic liquids. A novel expression for the precise estimation of the molecular volumes of the ionic liquids from simulation data is also introduced, adding quadratic corrections to the usual sum of atomic volumes. Our significant findings include i) that the usual assumption of uniform, additive atomic polarizabilities is challenged when highly mobile electrons in conjugated systems are present, and ii) that cations with conjugated large carbon chains can be used together with anions for the design of ionic liquids with very high refractive indices. A novel relation for the polarizability volume is reported together with a refractive index map made up of the studied ionic liquids.

Ferrocenyl Migrations and Molecular Rearrangements: The Significance of Electronic Charge Delocalization

The enhanced stabilization of a carbocationic site adjacent to a ferrocenyl moiety was recognized within a few years of the discovery of sandwich compounds. While a detailed understanding of the phenomenon was the subject of some early debate, researchers soon took advantage of it to control the ease and direction of a wide range of molecular rearrangements. We, here, discuss the progress in this area from the pioneering studies of the 1960s, to more recent applications in chromatography and analytical detection techniques, and currently in the realm of bioactive organometallic complexes. Several classic reactions involving ferrocenyl migrations, such as the pinacol, Wolff, Beckmann, and Curtius, are discussed, as well as the influence of the ferrocenyl substituent on the mechanisms of the Nazarov, Meyer-Schuster, benzoin, and Stevens rearrangements. The preparation and isomerizations of ferrocenyl-stabilized vinyl cations and vinylcyclopropenes, together with the specific cyclization of acetylcyclopentadienyl-metal derivatives to form 1,3,5-substituted benzenes, demonstrate the versatility and generality of this approach.

DFT zwitterion model for vibrational and electronic structure of unnatural 3-amino-3-(4-fluorophenyl)propionic acid, aided by IR and Raman spectroscopy

3-Amino-3-(4-fluorophenyl)propionic acid - a fluorinated building block in synthesis and a non-proteinogenic amino acid - belongs to the class of β-amino acids. We present here ab initio and DFT computed zwitterionic (ZW) monomer and dimer structures of 3-Amino-3-(4-fluorophenyl)propionic acid. These structures comprise intra- and inter-H-bonds (dimer), namely, N–H⋯O bonds formed between NH3+ and COO¯ moieties and the inter-N−H⋯O bond vibrational modes are correlated to the characteristic absorptions in the experimental IR spectrum. We propose two structures for 3-Amino-3-(4-fluorophenyl)propionic acid: a ZW monomer resulting from the explicit solvation model with four water molecules and a ZW dimer resulting from the implicit solvation model in water. By varying the bond length, rH … O around 1.70 Å, we get stable degenerate structures, getting the most stable ZW dimer mentioned above at 1.72 Å. Computed vibrational frequencies of NH3+ and COO¯ moieties for the ZW cluster and dimer complement each other in so far as good agreement with experimental IR and Raman spectra. We also have characterized both H-bonds using electronic properties computed from NBO, AIM and NCI methods. The natural bond analysis has shown that in both intra- and inter- N–H⋯O bonding, there is delocalization of electronic charge from n orbital on the O atom to σ∗ orbital on the N atom. The bond critical points are present between the cationic NH3+ and anionic COO¯ moieties along with atomic bond path confirming the N–H⋯O bond and is shown to be medium strong by NCI.

Matrix isolation infrared spectroscopic study of 4-Pyridinecarboxaldehyde and of its UV-induced photochemistry

The structure, infrared spectrum, barrier to internal rotation, and photochemistry of 4-pyridinecarboxaldehyde (4PCA) were studied by low-temperature (10K) matrix isolation infrared spectroscopy and quantum chemical calculations undertaken at both Moller-Plesset to second order (MP2) and density functional theory (DFT/B3LYP) levels of approximation. The molecule has a planar structure (Cs point group), with MP2/6-311++G(d,p) predicted internal rotation barrier of 26.6kJmol−1, which is slightly smaller than that of benzaldehyde (~30kJmol−1), thus indicating a less important electron charge delocalization from the aromatic ring to the aldehyde moiety in 4PCA than in benzaldehyde. A complete assignment of the infrared spectrum of 4PCA isolated in an argon matrix has been done for the whole 4000–400cm−1 spectral range, improving over previously reported data. Both the geometric parameters and vibrational frequencies of the aldehyde group reveal the relevance in this molecule of the electronic charge back-donation effect from the oxygen trans lone electron pair to the aldehyde CH anti-bonding orbital. Upon in situ UV irradiation of the matrix-isolated compound, prompt decarbonylation was observed, leading to formation of pyridine.

Z-Isomers of (4α→6″, 2α→O→1″)-phenylflavan substituted with R′=R=OH. Conformational properties, electronic structure and aqueous solvent effects

Procyanidins are highly hydroxylated polymers known as antioxidant compounds, thereby exhibiting beneficial effects. These compounds are protective agents against oxidative stress and the damage induced by free radicals in membranes and nucleic acids. This paper describes a study of the conformational space of (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=OH as part of a larger study of similar structures with different substitutions. The relationships between aqueous solution-vacuum variations of some properties were studied, as well as the stabilization and reactivity of (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=H, R'=H, R=OH, R'=R=OH, and (+)-catechin. The variations in geometric parameters and electronic properties due to conformational changes, as well as the effects of substituents and polar solvents, were evaluated and analyzed. Bader's theory of atoms in molecules was applied to characterize intramolecular interactions, along with a natural bond orbital analysis for each conformer described. The molecular electrostatic potential was rationalized by charge delocalization mechanisms and interatomic intramolecular interactions, relating them to the structural changes and topological properties of the electron charge density. Molecular polarizability and permanent electric dipole moment values were estimated. The results show the importance of a knowledge of the conformational space, and values for each conformer. Based on our previous results, we showed the existence of electron charge delocalization mechanisms acting cooperatively as "delocalization routes", showing interactions between different rings not even sharing the same plane. These "delocalization routes" were more effective for (4α→6″, 2α→O→1″)-phenylflavan substituted with R'=R=OH than for (+)-catechin, and are proposed as adding insight into the structure-antioxidant activity relationship of flavans.

What Makes Hydroxamate a Promising Anchoring Group in Dye-Sensitized Solar Cells? Insights from Theoretical Investigation.

We report, from a theoretical point of view, the first comparative study between the highly water-stable hydroxamate and the widely used carboxylate, in addition to the robust phosphate anchors. Theoretical calculations reveal that hydroxamate would be better for photoabsorption. A quantum dynamics description of the interfacial electron transfer (IET), including the underlying nuclear motion effect, is presented. We find that both hydroxamate and carboxylate would have efficient IET character; for phosphate the injection time is significantly longer (several hundred femtoseconds). We also verified that the symmetry of the geometry of the anchoring group plays important roles in the electronic charge delocalization. We conclude that hydroxamate can be a promising anchoring group, as compared to carboxylate and phosphate, due to its better photoabsorption and comparable IET time scale as well as the experimental advantage of water stability. We expect the implications of these findings to be relevant for the design of more efficient anchoring groups for dye-sensitized solar cell (DSSC) application.

One- and two-photon absorption and emission properties of an oligo(phenylenethienylene)s series.

The photophysical and nonlinear absorption properties of an oligo(phenylenethienylene)s series (nTBT) are investigated in this article. The length of the chromophore is gradually increased from one to four phenylenethienylene repeating units in order to evaluate the effects of the electronic delocalization on the two-photon absorption cross sections (δ). According to the excitation anisotropy measurements and quantum chemical calculations, two electronic transitions with distinctive symmetries, 1Ag → 1Bu and 1Ag → 2Ag, are present in the low energy region of the linear absorption spectrum. The lowest-energy transition 1Ag → 1Bu is one-photon allowed but two-photon forbidden and implies an electronic charge delocalization all along the oligomer segment whereas the weakly-allowed 1Ag → 2Ag transition exhibits a transition moment perpendicular to the average plane of the chromophore. The latter transition mainly contributes to the two-photon absorption ability of the oligomers. All derivatives are poorly solvatochromic and the breakdown of the mirror symmetry rule observed between absorption and fluorescence spectra at room temperature has been attributed to a photoinduced geometrical relaxation leading to a very efficient planarization process of the oligomer irrespective of its size. Increasing the oligomer length results in a slight shift of the two-photon absorption band (∼1300 cm(-1)) and in a drastic increase of δ from 2 ± 1 GM up to 802 ± 160 GM for 1TBT and 4TBT respectively. Based on a three-level model, it was found that main contributions to the strong increase of δ stem from the transition moments Mge and Mee' which are multiplied by a factor of 2.8 and 5 when going from 1TBT to 4TBT.

An Electronic Nose for the Detection of Carbonyl Species

Conjugated polymers are an important class of materials due to their luminescent and electrical properties which result from electronic charge delocalization. Consequently, these polymers provide a suitable platform for new sensors since electronic and conformational changes that arise from the interaction with analytes translate into measurable changes in electrical conductivity or luminescence. The main advantage of this platform is based on the ability to partially tune the response of the sensor by changing the chemical structure of the polymer. The present work illustrates proof-of-concept chemical sensors that incorporate a molecular recognition site for the detection of common indoor air polluting carbonyl species such as aldehydes and ketones. Emphasis is placed on the detection of formaldehyde which poses major concerns in Indoor Air Quality (IAQ). By taking advantage of the differential response of various polymers towards analytes in question, sensor elements can be implemented in an array format to give an electronic nose

Tailoring opto‐electronic properties of small InAsN and InN quantum dots

AbstractElectronic energy level structure (ELS), charge and spin density distributions (CDD and SDD, respectively) of small indium nitride clusters have been studied by means of the first‐principle, many‐body field theoretical methods. Electron charge delocalization due to charge accumulation on nitrogen and dopant nickel atoms, and resulting violation of the octet rule of the valence theory, have been observed. This charge delocalization mechanism provides for minimization of the total cluster energy, and thus is necessary to stabilize small non‐stoichiometric indium nitride systems. It may be used to manipulate opto‐electronic properties of indium nitride clusters, and also to synthesize novel, indium nitride‐based nanostructures with desirable properties (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical Study of the Interactions of In+ and In3+ with Stone−Wales Defect Single-Walled Carbon Nanotubes

We have performed a density functional study on the binding of indium ions (In+ and In3+) to the surface of pristine and defective armchair single-wall carbon nanotubes (SWNT). Among the various structural isomers that are studied, the position above the hexagon is found to be the most stable site for absorption, while the binding is enhanced in the defective SWNT compared to that in the pristine SWNT. The computed Mulliken charges, HOMO−LUMO gap energies, interactions, and interaction energies of the systems reveal fascinating electronic charge delocalization phenomena explaining the observed electromigration.

The influence of pH in nonresonant third-order nonlinearities of amino acid solutions

Abstract We report on the use of the Z -scan technique with femtosecond pulses at 775 nm to investigate the electronic charge delocalization in l -arginine, l -proline and l -serine amino acids. We have observed a variation in the nonresonant nonlinear transmittance with the pH, which reflects the variation in the relative concentration of the cationic and anionic forms and, consequently, the electronic charge delocalization. For l -serine, for instance, at a pH around 6.0 the n 2 rises to its maximum value corresponding to the one expected for the maximum concentration of the zwitterionic form. A good correspondence between the measured nonlinear transmittance variation and the titration curve was observed for the three amino acid samples, in the pH range studied. The n 2 values measured range from 10 −17 to 10 −16 cm 2 /W for pH varying from about 2 to 9.2.

Magnetic resonance studies of chemically intercalated LixV2O5 aerogels

Li 7 , V51 solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements have been performed upon chemically lithiated LixV2O5 aerogels, with compositions of 1.00<x<5.84. These compounds can intercalate reversibly large amounts of Li+ and, therefore, are of interest as battery cathodes. Still, the mechanism regarding the electron transfer from an inserted lithium metal to a host aerogel V2O5 and details regarding the lithium cation environments are not fully understood. LixV2O5 crystals are known to exhibit various structural phase changes and, when multiple phases are present, the capability of the material to intercalate reversibly appears to be adversely affected. On the other hand, aerogels have no such multiphase behavior and aerogel based cathodes exhibit greater stability upon cycling. NMR shows that neither the structure nor the dynamics vary greatly with the amount of lithium content, and that the lithiated aerogel is best described as a single-phase material. Characterization of lithium and vanadium sites is performed through analysis of both NMR and EPR spectra. Li7 line shapes are affected by first-order quadrupolar, magnetic dipolar interactions and motional narrowing. At and above room temperature, relaxation is governed primarily by a quadrupolar mechanism. NMR derived activation energies and diffusion coefficients are different from those of bronzes and electrochemically intercalated V2O5. V51 NMR lines, indicative of the presence of V5+ at all compositions, undergo diamagnetic shifts of up to about 50 ppm with an increase in lithium content. These results imply the presence of oxidized impurities or electronic charge delocalization. Additionally, EPR measurements provide evidence of VO2+ impurities and indirect evidence of nonbridging oxygen at high lithium contents.

Vibration and photoelectron spectroscopies of iodine-doped poly( p-diethynylbenzene)

The doping mechanism of poly( p-diethynylbenzene), chemically doped with iodine, was investigated. Absorption, infrared, far infrared, Raman, X-ray photoelectron and electron spin resonance spectroscopies were used to determine the nature of the dopant in doped polymer. The experimental results suggest that the charge transfer reaction between the polymer chain and the dopant results in the formation of a relatively stable I 5 − species, the π electron charge delocalization along the polymeric chain and the reduction of π− π ∗ transition energy.

An approach based on quantum chemistry calculations and structural analysis of a [2Fe-2S] ferredoxin that reveal a redox-linked switch in the electron-transfer process to the Fd-NADP+ reductase.

[2Fe-2S] ferredoxins act as electron carriers in photosynthesis by mediating the transfer of electrons from photosystem I to various enzymes such as ferredoxin:NADP(+):reductase (FNR). We have analyzed by density functional theory the possible variations of the electronic properties of the [2Fe-2S] ferredoxin, from the cyanobacterium Anabaena, depending on the redox-linked structural changes observed by X-ray diffraction at atomic resolution (Morales, R.; et al. Biochemistry 1999, 38, 15764-15773). The present results point out a specific and concerted role of Ser47, Phe65, and Glu94 located at the molecule surface, close to the iron-sulfur cluster. These residues were already known to be crucial for efficient electron transfer to FNR (e.g., Hurley, J. K.; et al. Biochemistry 1997, 36, 11100-11117). Our calculations suggest that the Glu94 carboxylate negative charge regulates the electron charge delocalization between the Ser47 CO group and the Phe65 aromatic ring, depending on the redox state. The Glu94 carboxylate is stabilized by a strong hydrogen bond implicating a hydroxyl-containing side chain (i.e., Ser or Thr) at location 47. We propose that the Phe65 ring acts as an intermediary carrier receiving the reducing electron prior to its transfer from the reduced Fd to FNR, in view of its central role in the Fd-FNR interaction.

Vibration and X-ray photoelectron spectroscopies of FeCl 3-doped poly( p-diethynylbenzene)

The doping mechanism of poly( p-diethynylbenzene), chemically doped with FeCl 3, was investigated. Absorption, infrared, far infrared, Raman, X-ray photoelectron spectroscopies were used to determine the nature of the dopant in doped polymer. The experimental results suggest that the charge transfer reaction between the polymer chain and the dopant results in the formation of FeCl 4 − species, the π electron charge delocalization along the polymeric chain and the reduction of π–π * transition energy.

On the Relevance of Tunneling for the Isotope Effect in KDP

First-principles calculations for KH 2 PO 4 (KDP) show that hydrogen off-center (OC) ordering in the ferroelectric phase is accompanied by electronic charge delocalization from the acceptor and localization at the donor oxygen within the O-H···O bonds. This induces P atoms off-centering in the PO 4 tetrahedrons and pairing with K + ions along the tetragonal axis, thus developing macroscopic polarization. Centered H are collectivelly unstable. However, if the protons are centered, the P and K atoms are stable in their centered positions. In addition, off-centering of a single proton is not energetically favorable. Only correlated proton motions, involving also displacements of heavier atoms, exhibit significant double wells, thus questioning the role of tunneling in isotope effects.

Star-shaped polyferrocenes based on thiophene and triphenylamine: synthesis, spectroscopy and electrochemistry

Star-shaped tri- and tetra-ferrocenes anchored on triphenylamine or thiophene cores were obtained by palladium-catalyzed cross-coupling reactions in moderate to good yields. These polymetallic systems were characterized by NMR, UV–vis and mass spectral methods, elemental analyses and by electrochemical studies. As observed earlier for tris(ferrocenyl)benzenes, these complexes also lack electronic communication, however, a thorough analysis indicates an existence of electronic charge delocalization between the ferrocenyl moiety and the central core.