EFISH Measurements Research Articles

Electric charge build-up and dissipation at pulsed streamer corona by EFISH and probe measurements

This work examines the dynamics of volumetric electric charge deposited by a single pin electrode streamer corona in atmospheric air at ambient conditions. Recent studies show that, at pulse-periodic operation, electric charge remains in surrounding air after the first discharge. This residual charge significantly affects the electric field distribution and morphology of discharges at subsequent pulses. In this study, the residual electrical potential/field was measured during and after dissipation of a pulsed 80 kV streamer corona of alternating polarity. This was accomplished using electrostatic probes and the electric field induced second harmonic method (EFISH generation). The residual volumetric charge of a positive pulse polarity streamer corona occupies a zone of up to 80 mm from the electrode. The subsequent application of a negative polarity pulse leads to partial neutralization of the previous charge and the generation of a region with a high-amplitude electric field. The propagation of following positive polarity pulses is limited by the residual positive volumetric charge. The EFISH method reveals the magnitude of the electric field at discharge propagation and electrostatic probes inform the polarity. Intensified imaging acquisition was used to evaluate the spatial distribution and dynamics of corona discharge during the alternating polarity waveform.

Direct Observation of Surface Hole Accumulation Under Water Oxidation Conditions By Efish and Impedance Measurements

Water oxidation reaction is considered the bottleneck and rate-determining step in photoelectrochemical water-splitting process. To achieve efficient water oxidation on photoanode, sufficient built-in potential on semiconductors should be maintained. Herein, we introduced in-situ Electric Field Induced Second Harmonic Generation (EFISH) technique to probe the change of built-in potential under water oxidation reaction process of a single-crystal rutile TiO2 photoanode. The band flattening effect caused by the reduction of built-in potential on photoanode was investigated by EFISH. Up to 600 mV built-in potential screening on TiO2 was observed at 1 V vs Ag/AgCl applied bias under 15mW/cm2 on TiO2 photoanode, in accordance with the anodic shift of flat-band potential. This phenomenon can be explained by the contribution of hole accumulation at surface states and local pH variation in an insufficiently buffered electrolytes, indicating that surface OH· is the rate determining step in water oxidation process. Built-in potential under excitation can be well maintained in a sufficiently buffered electrolytes, suggesting a different rate determining step in water oxidation process.

Thioethyl Porphyrazines: Attractive Chromophores for Second-Order Nonlinear Optics and DSSCs

The first study on the second-order NLO properties of unconventional push–pull systems constituted by asymmetrically monoaryl-substituted thioalkyl porphyrazines, carried out by EFISH measurements and DFT calculations, is here reported. The results obtained show that the porphyrazine macrocycle behaves as a better electron donor and acceptor with respect to structurally similar porphyrin and phthalocyanine tetrapyrroles. Noteworthy, the highest quadratic hyperpolarizability (β1907) values are displayed by the pyrene-substituted thioalkyl porphyrazines, both as free-base and as PdII complex, without the presence of either electron-withdrawing or electron-donating groups. In these cases, the pyrene moiety behaves as an electron-donating group, giving rise to charge transfer HOMO–LUMO electronic transitions. These outcomes show, for the first time, the great potential of the thioalkyl porphyrazine macrocycles for second-order NLO applications. Moreover, the hydroxyphenyl-substituted porphyrazine 2 dye has been also tested as a sensitizer in a TiO2–DSSC, representing the second example reported so far of a DSSC with thioalkyl porphyrazines.

A new donor-acceptor crosslinkable l-shape chromophore for NLO applications

A new nonlinear-optical (NLO) chromophore containing L-Shape 2-benzofuranyldiazene in π-electron bridge has been synthetized and its chemical and thermal properties examined. Evaluation of the second order NLO properties was performed by EFISH measurements of μβ on chloroform solution and by SHG measurements on amorphous spin-coated thin films obtained from a blend of polymer and chromophores crosslinked “in situ”. The value of 830∙10−48 esu for μβ measured on the dye and of 10 pm/V for the d33 for the poled network are in medium range for similar systems, confirming the proper employ of the molecule to build NLO active networks. The crystal structures dye and its precursor examined by single-crystal analysis show a planar geometry and bond lengths pattern confirm the push pull L-shape nature of the D-π-A systems.

Intriguing Influence of -COOH-Driven Intermolecular Aggregation and Acid-Base Interactions with N,N-Dimethylformamide on the Second-Order Nonlinear-Optical Response of 5,15 Push-Pull Diarylzinc(II) Porphyrinates.

A series of 5,15 push-pull meso-diarylzinc(II) porphyrinates, carrying one or two -COOH or -COOCH3 acceptor groups and a -OCH3 or a -N(CH3)2 donor group, show in N,N-dimethylformamide and CHCl3 solutions a negative and solvent-dependent second-order nonlinear-optical (NLO) response measured by the electric-field-induced second-harmonic generation (EFISH) technique, different from the structurally related zinc(II) porphyrinate carrying a -N(CH3)2 donor group and a -NO2 acceptor group, where a still solvent-dependent but positive EFISH second-order response was previously reported. Moreover, when a -N(CH3)2 donor group and a -COOH acceptor group are part of a sterically hindered 2,12 push-pull β-pyrrolic-substituted tetraarylzinc(II) porphyrinate, the EFISH response is positive and solvent-independent. In order to rationalize these rather intriguing series of observations, EFISH measurements have been integrated by electronic absorption and IR spectroscopic investigations and by density functional theory (DFT) and coupled-perturbed DFT theoretical and 1H pulsed-gradient spin-echo NMR investigations, which prompt that the significant concentration effects and the strong influence of the solvent nature on the NLO response are originated by a complex whole of different aggregation processes induced by the -COOH group.

An acido-triggered reversible luminescent and nonlinear optical switch based on a substituted styrylpyridine: EFISH measurements as an unusual method to reveal a protonation–deprotonation NLO contrast

Diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (DPVPA) constitutes a novel acido-triggered reversible luminescent and nonlinear optical switch. Remarkably, for the first time the Electric-Field Induced Second Harmonic generation (EFISH) technique is used to reveal a protonation-deprotonation NLO contrast.

Switching of emissive and NLO properties in push–pull chromophores with crescent PPV-like structures

We report on a series of novel homologous push-pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, bearing, respectively, 1, 2 and 3 p-phenylenevinylene units in direct linear conjugation (compounds 7-9). The three compounds exhibit striking differences in their linear and nonlinear optical properties. The shorter compound 7 exhibits aggregation-induced emission with a strong luminescence in the solid state (blue emission, photoluminescence quantum yield 38%), and it is nonemissive in solution; the more extended conjugated systems 8 and 9 show classical aggregation-caused quenching in the solid state, while high quantum yield photoluminescence (21 and 93% in toluene) is restored in diluted solutions, through mechanisms involving intramolecular charge transfer in the excited states. EFISH measurements in solutions demonstrate a strong solvent and concentration dependence. As rationalized with the aid of molecular modelling, compounds 8 and, more markedly, 9 aggregate in stable centrosymmetric dimers in solution.

Photoswitching of the Second Harmonic Generation from Poled Phenyl-Substituted Dithienylethene Thin Films and EFISH Measurements

The photoswitching of the second harmonic generation signal of poled phenyl-substituted dithienylethene/polystyrene thin films evidence a different behavior in function by their molecular structure. The three nonzero χXXZ(2), χZXX(2), and χZZZ(2) coefficients of second order susceptibility at 1064 nm have been measured for both isomeric forms of substituted diarylethene in polymer films. The relationship between χXXZ(2), χZXX(2), and χZZZ(2) for symmetrical diarylethene (OMe–OMe and CN–CN) differs from that found for the asymmetrical OMe–CN chromophore, suggesting for the two symmetrically substituted molecules a two-dimensional first hyperpolarizability. The two-dimensional first hyperpolarizability model has been checked by a statistical analysis from the correlation between susceptibility and first hyperpolarizability. Analysis of βxxz, βzxx, and βzzz values confirm for OMe–OMe and CN–CN a two-dimensional charge flow pattern, which is different from the monodimensional OMe–CN. Moreover, the goodness of...

ChemInform Abstract: Semiempirical Calculations of Hyperpolarizabilities for Donor-Acceptor Molecules: Comparison to Experiment

The dipole moments (μ), polarizabilities (α), hyperpolarizabilities (β), and second-order hyperpolarizabilities (γ) of benzene, biphenyls, fluorene, styrene, stilbenes, and tolans have been calculated by a finite field method with the PM-3 parametrization of the MNDO Hamiltonian. These results were compared to experimental value obtained from EFISH and THG measurements. Good correlations were obtained, and coefficients between the measured and calculated value (calculated/measured) were 1.11, 0.75, 1.00, and 0.94 for μ, α, β, and γ, respectively

An EFISH, Theoretical, and PGSE NMR Investigation on the Relevant Role of Aggregation on the Second Order Response in CHCl3 of the Push−Pull Chromophores [5-[[4′-(Dimethylamino)phenyl]ethynyl]-15-[(4′′-nitrophenyl)ethynyl]-10,20-diphenylporphyrinate] M(II) (M = Zn, Ni)

This work has produced experimental and theoretical evidence for the independence, on the nature of the metal, of the second order NLO response of [5-[[4′-(dimethylamino)phenyl]ethynyl]-15-[(4′′-nitrophenyl)ethynyl]-10,20-diphenylporphyrinate]M(II) NLO chromophores (M = Zn, Ni). EFISH measurements, carried out at a nonresonant 1.907 μm incident wavelength and at variable concentrations in CHCl3 or in a polar and donor solvent such as DMF or by addition of an excess of pyridine to a CHCl3 solution, have shown, together with a PGSE NMR investigation, that the different second order NLO response obtained in CHCl3 for Zn(II) and Ni(II) NLO chromophores is due to a different aggregation in CHCl3 solution. Theoretical DFT/TDDFT calculations on the NLO properties of dimeric aggregates of the NLO chromophores containing the Zn(II) center have suggested, in agreement with EFISH measurements and PGSE NMR investigation, a J aggregation which induces a doubling of the second order NLO response. The PGSE NMR investigation has also suggested a much weaker dipolar interaction for the dimerization process of the NLO chromophores containing the Ni(II) center. In this latter case the antiparallel alignment of the dipole moments of the two chromophores produces a lower second order NLO response, as experimentally observed in CHCl3 solution by increasing concentration.

“Push–pull” 2-ferrocenyl-4-hydroxythiazoles: A novel method of the construction of the thiazole ring

The title compounds were obtained by cyclization of ferrocenyl thioimidates in the presence of sodium ethoxide. They were transformed into the corresponding 4-acetoxy-derivatives in reaction with acetic anhydride/pyridine. The structure of 4-acetoxy-5-(ethoxycarbonyl)-2-ferrocenylthiazole was determined by X-ray diffraction. The EFISH measurements revealed significant second-order nonlinear properties of 4-acetoxy-2-ferrocenyl-5-(4-nitrophenyl)ferrocenylthiazole ( μβ = 250 × 10 −48 esu at 1907 nm).

Electronic, Optical, and Vibrational Properties of Bridged Dithienylethylene-Based NLO Chromophores

The vibrational, optical, and nonlinear optical (NLO) properties of a series of push−pull chromophores built around dithienylethylene-based π-conjugating spacers have been investigated by UV−vis, IR, and Raman spectroscopies and electric field-induced second harmonic generation (EFISH) measurements. The effects of the strength of the acceptor group on the molecular electronic properties of these conjugated NLO chromophores have been addressed. The magnitude of the intramolecular charge transfer has been tested as a function of the acceptor strength and of the bridging of the spacer. Density functional theory (DFT) calculations have been performed to help the assignment of the main electronic and vibrational features of the NLO chromophores and to derive useful information about their molecular structures. EFISH measurements show that push−pull systems that contain strong electron-acceptor groups connected via a rigidified dithienylene spacer exhibit large values of μβ0. Theoretical NLO calculations are in...

Copper(II) Complexes of salen Analogues with Two Differently Substituted (Push−Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear Optical Properties

This paper presents some copper(II) complexes of salen analogues in which the two salicylaldehyde moieties carry different (electron donor, D, and acceptor, A) substituents in position 5, producing a push-pull charge asymmetry. The X-ray structures of some compounds show the presence of pairs of stacked molecules with head-to-tail intermolecular associations. The geometries of all complexes have been optimized through density functional theory (DFT) studies, which have shown that a major influence on the coordination bond lengths is given by the presence of the electron acceptor NO2 group. Such an influence operates mainly on the Cu-phenolato bonds: elongation of the Cu-O distance of the 5-nitrosalicylaldehyde moiety, with a concomitant decrease of the other Cu-O distance; the Cu-N bonds are less affected. The D groups have only a minor influence. The nonlinear optical responses, microgBetavec, of some molecules have been determined by EFISH measurements, and the Beta-vec values have been obtained using the DFT-calculated microg values because solubility problems hampered the experimental measurements of microg of some derivatives; the former, however, have been found to be in agreement with the experimental values that could be obtained. Deconvolution of the absorption bands in the near-UV region has allowed recognition of the charge-transfer (CT) transition, assigned to a ligand-to-metal CT (LMCT) by time-dependent DFT computations; we have then used the solvatochromism of this transition to obtain Beta0 and BetaCT values using the two-state model. These values were compared with those obtained by computational studies, which have also allowed evaluation of the influence of the substituents on the directions of microg and Betatot.

Synthesis, Crystal Structures, Linear and Nonlinear Optical Properties, and Theoretical Studies of (p‐R‐Phenyl)‐, (p‐R‐Phenylethynyl)‐, and (E)‐[2‐(p‐R‐Phenyl)ethenyl]dimesitylboranes and Related Compounds

The (p-R-phenyl)dimesitylboranes (R=Me(2)N, MeO, MeS, Br, I), (p-R-phenylethynyl)dimesitylboranes (R=Me(2)N, MeO, MeS, H), (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes (R=Me(2)N, H(2)N, MeO, MeS, H, CN, NO(2)), (E)-[2-(2-thienyl)ethenyl]dimesitylborane, and (E)-[2-(o-carboranyl)ethenyl]dimesitylborane have been prepared through the reaction of the appropriate p-R-phenyl- and p-R-phenylethynyllithium reagents with dimesitylboron fluoride and by hydroboration of the appropriate p-R-phenylacetylene, 2-ethynylthiophene, and o-ethynylcarborane with dimesitylborane. Their UV/Vis absorption and emission spectra have been recorded in a range of solvents with the fluorescence maxima of the donor-substituted compounds in particular exhibiting large bathochromic shifts in highly polar solvents, indicative of charge transfer leading to large dipole moments in the excited state. The molecular structures of the (p-R-phenyl)dimesitylboranes (R=Me(2)N, MeO, MeS, Br, I), the (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes (R=Me(2)N, H(2)N MeO, MeS, H), (p-R-phenylethynyl)dimesitylborane (R=Me(2)N), and (E)-[2-(2-thienyl)ethenyl]dimesitylborane, which have been determined from single-crystal X-ray diffraction measurements, offer evidence of increased conjugation in the ground state with increased donor strength of the R substituent. Their first- and second-order molecular hyperpolarizabilities have been obtained from EFISH and THG measurements, the first-order hyperpolarizabilities being largest for the strongest R-substituent donors. AM1 calculations have been performed on these compounds, showing reasonable agreement with the experimentally obtained bond lengths and hyperpolarizabilities, as well as on several related hypothetical compounds containing multiple C==C bonds, most of which are proposed to have even larger hyperpolarizabilities.

On the Rigidity of Polynorbornenes with Dipolar Pendant Groups

A range of polynorbornenes (PNBs) with fused dipolar pendant groups at C-5,6 positions was synthesized by ring-opening metathesis polymerization catalyzed by a ruthenium carbene complex (Grubbs I). Photophysical studies, EFISH measurements, and atomic force microscopy images have been used to investigate the structures and morphology of these polymers. These results suggest that the polymers may adopt rigid rod-like structures. The presence of the double bonds in PNBs appeared to be indispensable for the rigidity of the polymers. Interaction between unsaturated pendant groups may result in coherent alignment leading to a rod-like structure.

Second order molecular nonlinearities in new orthopalladated push–pull chromophores

Three new orthopalladated chromophores based on push–pull π conjugated ligands have been prepared and characterized. The second order molecular NLO activity has been determined by EFISH measurements giving μgβ values up to 610×10−48 esu at an incident laser wavelength of 1.907 μm. The X-ray structures of two different polymorphs of one chromophore are also reported and polymorphism is discussed in terms of conformational changes.

Synthesis, structure, and second-order nonlinear optical properties of copper(II) and palladium(II) acentric complexes with N-salicylidene-N'-aroylhydrazine tridentate ligands.

Two new N-salicylidene-N'-aroylhydrazines ligands have been prepared: N-4-diethylaminosalicylidene-N'-4-nitrobenzoyl-hydrazine (L(1)) and N-4-diethylaminosalicylidene-N'-4-(4-nitrophenylethylidene)-benzoyl-hydrazine (L(2)). The ligands are properly functionalized with strong electron donor-acceptor groups and are of potential interest in second-order nonlinear optics (NLO). Dimeric copper(II) and palladium(II) complexes with L(1) and L(2) have been prepared, and, starting from these, mononuclear acentric adducts with pyridine as a further ligand have been prepared and characterized. The X-ray structures of three adducts are also reported. The NLO activity of the adducts has been determined by EFISH measurements giving mubeta values up to 1500 x 10(-48) esu for an incident wavelength of 1.907 microm.

The influence of the solute/solvent interactions on the first-order hyperpolarizability in urea molecule. A quantum chemical study

The results of quantum chemical computations of first-order hyperpolarizability ( β) of urea molecule in the gas phase and in polar solvents are presented. The environmental effects were included via the quantum-mechanical Langevin dipoles/Monte Carlo (QM/LD/MC) method as well as supermolecule approach. The results of calculations confirm the existense of the substantial solvent effect on β value for urea molecule. The calculated β values, including solvent effect, are compared with experimental data determined in solution phase by EFISH measurements.

Electric field induced second harmonic generation (EFISH) measurements on absorbing compounds: push–pull substituted anilines

This paper deals with the influence of various electron acceptors, which contain at least two cyano groups (obtained from reactions with malononitrile, tetracyanoethene, tetracyanoquinodimethane, dimeric malononitrile), on the generation of second harmonic light of donor–acceptor-substituted anilines. For EFISH measurements, which were performed on solutions in 1,4-dioxane and acetone a Yag–Nd laser was used. The influence of the solvent on the SHG efficiency is discussed shortly and compared to solvatochromic effects. The measurement data were corrected for absorptive effects so that absorbing compounds can be compared to non-absorbing ones. Increasing the acceptor strength not only bathochromically shifted the longest wavelength UV–vis absorption band but also leaded to an enhancement of μβ-values obtained by the EFISH measurements. Trends of hyperpolarizabilities calculated by semi-empirical molecular orbital methods by and large conformed those found experimentally.

Quantum Chemical Calculations of the First- and Second-Order Hyperpolarizabilities of Molecules in Solutions

A study of the effect of solute/solvent interactions on static and dynamic molecular hyperpolarizabilities (β and γ) of series of prototypical π-conjugated donor−acceptor chromophores is presented. The solvent effect was included via a recently proposed discrete quantum-mechanical Langevin dipoles/Monte Carlo method. The nonlinear optical properties (NLO) were computed using the INDO-like Hamiltonian based finite-field (FF) and sum-over-states (SOS) methods implemented in the GRINDOL code. The calculated β(γ) values are compared with experimental data determined in solution phase EFISH(THG) measurements and other published theoretical works. Generally, a reasonable agreement between calculated and experimental data was obtained. Moreover, our results indicate that the QM/LD/MC model gives a correct description of the solvent effect on the nonlinear optical response of molecules. It suggests that this level of theory can be used as an effective tool for investigation of NLO properties in condensed phases.