Charge Transfer Pattern Research Articles

Carborane tuning on iridium complexes: redox-switchable second-order NLO responses.

Much effort has been devoted to investigating the molecular geometries, electronic structures, redox properties and nonlinear optical (NLO) properties of Ir complexes involving o-, m- or p-carborane groups by density functional theory (DFT) methods. Switchable second-order NLO properties were induced by redox processes involving these complexes, and it was found that mainly the coordination bonds of Ir complexes changed during the oxidation process. Our calculations revealed that oxidation reactions have a significant influence on the second-order NLO response owing to the change in charge transfer pattern. The β tot values of oxidized species are at least ∼9 times larger for set I and ∼5 times larger for set II than those of the corresponding parent complexes. Introduction of carborane groups into ppy (phenylpyridine) ligands can enhance the second-order NLO response by 1.2- 1.6 times by a metal-to-ligand charge transfer (MLCT) transition between the Ir atom and carborane. The β tot of complex 2 [(ppy)2Ir(phen)](+) (phen = phenanthroline) is 3.3 times larger than that of complex 1 (ppy)2Ir(acce) (acce = acetylacetonate), which is caused by ligand-to-ligand charge transfer (LLCT) between ppy ligands and the ancillary ligand. Therefore, it can be concluded that the second-order NLO response can be effectively enhanced by oxidation reactions.

Large Nonlinear Optical Responses of Dimers Bearing a Donor and Acceptor: Long, Intradimer Multicenter Bonding

Unusual long, multicenter dimers bearing a large electron donor and acceptor have been the subject of great interest over the last decades due to their better conducting, superconducting, magnetic, or other physical properties. Two-electron, multicenter bonding between two interplanar fragments has been recently recognized as a novel and important bonding interaction. Herein, the [TTF][TCNE], [TTF][TCNQ], [TTF][TCNP], and [TTF][TCNB] dimeric species have been studied by quantum mechanics methods with the view of assessing their interactions and first hyperpolarizabilities. It is found that the stabilities of the dimers primarily originate from the electrostatic bonding component. Although to a lesser extent, long, multicenter interactions due to the overlap of the molecular orbitals of the monomers are important also in the stability of these systems. Significantly, a severe hyperpolarizability decrease with changing the acceptor monomers of the dimeric species is qualitatively explained in terms of change in their charge transfer patterns. It indicates that the first hyperpolarizabilities of these dimers can be optimized by controlling their relative acceptor monomers. We believe that these results shall provide important information for further exploration of long, multicenter dimers with versatile and fascinating nonlinear optical properties.

Mechanistic insight into the second-order nonlinear optical properties of Ru-coordinated DTE complexes: Photoisomerization, redox, and protonation switches

In this paper, the switchable second-order nonlinear optical (NLO) responses of Ru dithienylperfluorocyclopentene (DTE) complexes controlled by photoisomerization, oxidation and protonation have been systematically investigated by the density functional theory. Theoretical results reveal some important structural changes upon these three pathways. It can also be found that the protonation and oxidation processes have some infuence on the charge transfer (CT) pattern but slightly affect the first hyperpolarizability (βvec). In striking contrast, in the photoisomerization process, the difference on the βvec values between 2o and 2c is the largest with respect to the protonation and oxidation processes with a βvec(2c)/βvec(2o) = 52.7 ratio. Time-dependent density functional theory calculation predicts that the DTE intramolecular π–π* charge transfer is helpful for the large NLO response. It confirms that the photoirradiation evokes the photoisomerization character to display dramatic difference in βvec values, which paves the way for the applications of switchable NLO materials.

Insights into graphene functionalization by single atom doping

Chemical modification of graphene is a common approach to control its electronic properties and hence fabricate electronic devices with new or improved functionalities. In this work we analyze, with density functional based calculations, the effect of chemical adsorption of fluorine atoms at different coverage levels on the electronic structure of graphene. We suggest a simple and general model for the shift of the Fermi level with coverage level and show the trends of the band gap and the Fermi level shift with coverage. We then show that the same model can be applied to explain the Fermi level shift in a different system of nitrogen substitution in graphene. Finally, we analyze the resulting charge transfer patterns and show that they are consistent with the model for the Fermi level shift.

Switchable Nonlinear Optical Properties of η5-Monocyclopentadienylmetal Complexes: A DFT Approach

Density functional theory (DFT) calculations have been carried out to investigate the switching of the second-order nonlinear optical (NLO) properties of η(5)-monocyclopentadienyliron(II) and ruthenium(II) model complexes presenting 5-(3-(thiophen-2-yl)benzo[c]thiophen-1-yl)thiophene-2-carbonitrile as a ligand. The switching properties were induced by redox means. Both oxidation and reduction stimulus have been considered, and calculations have been performed both for the complexes and for the free benzo[c]thiophene derivative ligand in order to elucidate the role played by the organometallic fragment on the second-order NLO properties of these complexes. B3LYP, CAM-B3LYP, and M06 functionals were used for our calculations. The results show some important structural changes upon oxidation/reduction that are accompanied by significant differences on the corresponding second-order NLO properties. TD-DFT calculations show that these differences on the second-order NLO response upon oxidation/reduction are due to a change in the charge transfer pattern, in which the organometallic iron and ruthenium moieties play an important role. The calculated static hyperpolarizabilities were found to be strongly functional dependent. CAM-B3LYP, however, seems to predict more reliable structural and optical data as well as hyperpolarizabilities when compared to experimental data. The use of this functional predicts that the studied complexes can be viewed as acting as redox second-order NLO switches, in particular using oxidation stimulus. The β(tot) value of one-electron oxidized species is at least ~8.3 times (for Ru complex) and ~5.5 times (for Fe complex) as large as that of its nonoxidized counterparts.

Dispersion Overwhelms Charge Transfer in Determining the Magnitude of the First Hyperpolarizability in Triindole Octupoles

A comprehensive series of systematically functionalized C3-symmetric hexakis (para-substituted) triindoles has been studied for its linear and second-order nonlinear optical properties. The carbazole-derived triindole central core is electron rich and electron donating. The peripheric substitution pattern, resulting in a donor–donor or a donor–acceptor charge transfer, is reflected in the UV–vis absorption spectra, where a lower energy charge-transfer band is observed for the donor–acceptor pattern. On-resonance all compounds exhibit a strongly enhanced second-order nonlinear optical response, critically dependent on the particular wavelength but showing no clear correlation with the charge-transfer character imparted by the peripheral substituents. Nonetheless, extremely large values are obtained: we measured the highest value ever reported for octupolar compounds in transparent conditions on-resonance. Off-resonance significantly smaller values are found, which are very similar for all compounds and show no correlation with the charge-transfer character as well. Both observations have been unambiguously confirmed by (linear and nonlinear) spectroelectrochemistry on a donor–donor structure, effectively transforming this to an acceptor–donor structure (by oxidizing the donor triindole core to an electron-accepting triindole-based cation radical). The strong wavelength dependence of the first hyperpolarizability values around resonance is clearly shown to be overwhelmed by dispersion effects and not to be determined by the charge-transfer pattern in these octupolar materials. This finding provides insight for independent tuning of the linear absorptive properties, determined by the charge-transfer pattern, and the second-order nonlinear polarizability, not determined by this pattern but strongly dispersive.

Theoretical investigation on redox‐switchable second‐order nonlinear optical responses of push–pull Cp*CoEt2C2B4H3‐expanded (metallo)porphyrins

The second-order nonlinear optical (NLO) properties of the Cp*Co(C(2)H(5))(2)C(2)B(4)H(3)-expanded (metallo)porphyrins (Cp* = C(5)Me(5)) have been investigated by using ab inito RHF and density functional theory (DFT) methods. The investigation shows that the compound with expand porphyrin possesses remarkable large molecular hyperpolarizability β(tot) value, ~414.1 × 10(-30) esu (at LC-ωPBE level), and might be an excellent second-order NLO material. From the character of charge transfer (CT) transition, it indicates that the -Cp*Co(C(2)H(5))(2)C(2)B(4)H(3) acts as an electron donor in this kind of systems. As a result of the redox behavior on expanded (metallo)porphyrin, the redox switching character of the NLO responses for the systems 2a-4a has also been studied. The results show that the β(tot) values of reduced forms are larger than that of neutral ones. Furthermore, the time-dependent DFT calculation illustrates that reduced forms have a significant difference on the CT patterns versus neutral ones. The present investigation provides insight into the comparison with DFT results on estimating first hyperpolarizability and the NLO properties of the series of push-pull compounds.

Quantum molecule in the low-electron limit

Coupled few-electron dots with incorporated quantum point-contact charge detectors have been successfully fabricated and studied both experimentally and theoretically. In this paper we focus on charge detection measurements for the regimes most relevant to spin and charge qubit applications. We first characterize the degree of coherence of a single electron trapped in the double dot by measuring the average occupation of one of the dots at equilibrium. We find that the spin degree of freedom dramatically affects the charge states when a second electron is added to the system, resulting in novel charge transfer patterns. The patterns evolve as a function of tunnel coupling and magnetic field. A nonequilibrium occupation of ground and excited states as a possible origin of the observed phenomena is investigated theoretically through extensive numerical and CI calculations of 1–3 electron states for realistic confining potentials.

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M n + (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and vm, the maximum cross-section and the corresponding velocity. For small size clusters ( ), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and vm parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures.

13C-Nmr Spectroscopy of β-Nitrostyrenes. II. Mono-, Bi- and Tri-Methoxy Phenyl-Substitutions and Long Distance Electronic Effects

By means of 13C-NMR spectroscopy and AM1 molecular orbital calculations of mono-, bi- and tri-methoxy-β-nitrostyrenes at the meta and para positions, we have characterized a long distance electronic charge transfer pattern on the ethylenic bridge (CH=CH) and on the aromatic ring (Ph) carbon centers, determined by the electron-donor nature of the methoxy-substituent groups. After a complete spectral assignment of the 13C-NMR signals, we have found a functional dependence of the chemical shifts on the C1 and Cβ centers respect to the C4 and C3 methoxy subtitution sites on the aromatic ring, while in the same molecular series Calfa-chemical shifts are practically constants. on the other hand, the 13C-NMR chemical shifts of the C3 and C4 centers plus the analysis of the AM1 electronic charge density have permitted us determine the long distance charge transfer effect induced by the C4 methoxy substitutions as well as the attenuation of this effect due to the C3 methoxy substitutions.

HSAB Analysis of Charge Transfer in the Gas-Phase Acid−Base Equilibria of Alkyl-Substituted Alcohols

A simple model to analyze charge redistribution associated with proton transfer (PT) reaction is derived from a classical ion transport model. The model is applied to the gas-phase acid-base equilibria of alkyl alcohols. Proton transfer is simulated as the motion of a charged particle in an applied external potential defined by the chemical environment of the proton, and represented by the difference in proton affinity (PA) of the conjugated bases ROand CH3O; the latter is taken as reference. The electronic chemical potential of transfer accounts for both the amount and direction of charge transfer (CT). The relative acidity for a short series of alkyl alcohols is determined by the difference in proton affinity (∆PA ) PA(RO-) PA(CH3O)) of the conjugated bases. The predicted charge transfer is in agreement with the CT pattern obtained from the group hardness and electronegativity analysis.

Long distance electronic effects of para-substituted β-nitrostyrenes by 13C-NMR spectroscopy

By means of 13C-NMR spectroscopy and AM1 molecular orbital calculations of para-β-nitrostyrenes, we have found a characteristic long distance charge transfer pattern, where the olefinic bridge (CHCH) and the aromatic ring (Ph) carbon centres are perturbed according to the donor-nature of the para-substituent groups. After a complete spectral assignment of the 13C-NMR signals, the chemical shifts ( δ) of the C 1, C 3 and C β centres show a linear functional dependence with the charge densities ( q AM1), while in the same molecular series C 2 and C α are practically constants. On the other hand, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C 4 centre, has permitted us to find a good correlation between the C 4 chemical shift and the electronegativity of this vicinal centre.

13C-NMR Spectroscopy of para-Substituted Benzylideneacetones.: I. Long Distance Electronic Effects

para-benzylideneacetones present a characteristic long distance charge transfer pattern, where the olefinic bridge (CH=CH) and the aromatic ring (Ph) carbon centers are perturbed according to the nature of the para-substituent groups. By means of 13C-NMR spectroscopy and AMl molecular orbital calculations we have found that in this molecular series the chemical shifts (Δ) and the charge densities (qAMI) corresponding to the C3, C1 and Cβ centers follow a functional dependence of the type: Δ = a qAMl + Δ°, while C2, Cα and CCO are practically constants. On the other hand, after a complete spectral assignment of the 13C-NMR signals, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C4 center, has permitted us to find a good correlation between the C4 spectral shift and the electronegativity of this vicinal center.