Quadratic Hyperpolarizability Research Articles

Photostability of D–π–A nonlinear optical chromophores containing a benzothiazolium acceptor

AbstractFor the practical application of second‐order NLO materials, not only a high molecular quadratic hyperpolarizability β but also good thermal, chemical, and photochemical stabilities are required. Most of the state‐of‐the‐art chromophores with high NLO response cannot be put to use because they are photochemically highly unstable. Good thermal and photochemical stabilities with preserved high hyperpolarizabilities can be achieved by replacement of an aromatic ring with easily delocalizable heteroaromatics, e.g., with benzothiazole. Furthermore, desirable modifications of the benzothiazole fragment lead to improvement in β values. Here we report results of a comprehensive investigation of the photochemical stability of seven D–π–A push–pull molecules based on a N‐methylbenzothiazolium acceptor and a N,N‐dimethylaminophenyl donor with a different length of conjugated bridge and different acceptor strength. The quantum yield (Φ) and the kinetic parameters of photoreactions were determined for existing photodegradation pathways on irradiation at 300–850 nm in MeOH. Trans–cis photoisomerization is proposed as a fast but inefficient photobleaching mechanism for these irradiation wavelengths. Self‐sensitized photooxidation by 1O2 makes very slow parallel photodegradation pathway and, albeit to small value of Φ, plays a dominant role in the photodegradation of the compounds investigated. Both structural modifications (extension of conjugated bridge and an additional acceptor group bonded to heterocycle) resulting in an increase of NLO response led to a decrease in photostability due to the self‐sensitized 1O2 photooxidative attack. Thus a compromise should be found between an increase in NLO response and a decrease in photostability to make a choice of studied compounds for practical applications. Copyright © 2010 John Wiley & Sons, Ltd.

Optical Second Harmonic Generation of Single Metallic Nanoparticles Embedded in a Homogeneous Medium

We report the optical second harmonic generation from individual 150 nm diameter gold nanoparticles dispersed in gelatin. The quadratic hyperpolarizability of the particles is determined and the input polarization dependence of the second harmonic intensity obtained. These results are found in excellent agreement with ensemble measurements and finite element simulations. These results open up new perspectives for the investigation of the nonlinear optical properties of noble metal nanoparticles.

Effects of the Metal Center and Substituting Groups on the Linear and Nonlinear Optical Properties of Substituted Styryl-Bipyridine Metal(II) Dichloride Complexes: DFT and TDDFT Computational Investigations and Harmonic Light Scattering Measurements

UV-visible absorption spectroscopy and harmonic light scattering measurements coupled with density functional theory (DFT) calculations have been carried out for a series of 4,4'-bis(X-styryl)-2,2'-bipyridine M(II) dichloride complexes (M = Co, Ni, Cu, Zn; X = H, OMe, SMe, NMe(2), NEt(2), CN, NO(2)). The roles of the metal and the substituent X on their coordination geometries, absorption, and quadratic nonlinear optical properties have been investigated. We show that these complexes all exhibit a high-spin configuration and display a distorted tetrahedral metallic environment except the copper ones, which are distorted square-planar complexes. When X is a strong electron-donating group (X = NMe(2), NEt(2)), TDDFT calculations clearly demonstrate that, whereas the Zn complexes show an ILCT transition in the visible range, the Co, Ni, and Cu complexes exhibit additional MLCT and LLCT transitions. These latter transitions are vectorially opposed to the ILCT and could contribute to the decrease of the experimental quadratic hyperpolarizability beta values, in the order Zn > Ni approximately Cu > Co. The computation of the beta values using TDDFT for the whole series of the closed-shell Zn(II) complexes featuring different X substituents established that the NLO activity increases with the donating strength of X and more generally with the decrease of the HOMO-LUMO energy gap. When X is a strong withdrawing group, the drastic decrease of the NLO response is explained by the negligible participation of the HOMO-LUMO transitions.

Fluorinated β-Diketonate Diglyme Lanthanide Complexes as New Second-Order Nonlinear Optical Chromophores: The Role of f Electrons in the Dipolar and Octupolar Contribution to Quadratic Hyperpolarizability

The second-order nonlinear optical (NLO) properties of [Ln(hfac)(3)(diglyme)] (hfac = hexafluoroacetylacetonate; diglyme = bis(2-methoxyethyl)ether; Ln = La, Ce, Pr, Sm, Eu, Gd, Er, Lu) complexes have been investigated by a combination of electric-field second harmonic generation (EFISH) and harmonic light scattering (HLS) techniques, providing evidence for the relevant role of f electrons in tuning the second-order NLO response dominated by the octupolar contribution. These lanthanide NLO chromophores allow a clean valuation of the influence of f electrons on the quadratic hyperpolarizability and on its dipolar and octupolar contributions. Molecular quadratic hyperpolarizability values measured by the EFISH method, beta(EFISH), initially increase rapidly with the number of f electrons, the value for the Gd complex being 11 times that of the La complex, whereas this increase is much lower for the last seven f electrons, the beta(EFISH) value of the Lu complex being only 1.2 times that of the Gd complex. The increase of beta(HLS), which is dominated by an octupolar contribution, is much lower along the Ln series. Remarkably, the good beta(HLS) values of these simple systems, well known for their luminescence properties, are reached at no cost of transparency.

Symmetry Cancellations in the Quadratic Hyperpolarizability of Non-Centrosymmetric Gold Decahedra

We report the second harmonic response of non-centrosymmetric gold decahedra with four different sizes, (side length between 17 and 150 nm). We show that, for small sizes, symmetry cancellations occur, leading to quadratic hyperpolarizabilities similar to that of gold spherical nanoparticles despite the non-centrosymmetric shape of the particles. For larger sizes, the quadratic hyperpolarizabilities scale with the volume of the particles, as expected from the multipolar contribution arising from the field retardation effects.

Dyes for biological second harmonic generation imaging

Nonlinear optical imaging has revolutionized microscopy for the life sciences. Second harmonic generation (SHG), the younger sibling of two-photon excited fluorescence (2PF), is a technique that can produce high resolution images from deep inside biological tissues. Second harmonic light is generated by the coherent scattering of an ensemble of aligned chromophores in a focused, pulsed laser beam. SHG is only generated at the focal spot, reducing the background signal, and requires ordered chromophores, so is highly structure-specific. In contrast to two-photon fluorescence, the physical process that creates the signal does not require the formation of excited states, allowing elimination of harmful photochemistry. While the SHG of native proteins and biopolymers is well known, the use of exogenous dyes can provide SHG contrast from areas without a sufficiently high intrinsic quadratic hyperpolarizability, β. Dyes for SHG primarily target lipid bilayers; a trait that, combined with sensitivity to transmembrane potential, allows monitoring of action potentials in a variety of excitable cells, most importantly mammalian neurons. This article summarizes the principles of SHG imaging and explores approaches for maximizing the SHG signal from a biological specimen. We survey methods of optimizing the optical set-up, enhancing the β of the dye and achieving biological compatibility. In conclusion, we examine novel applications of SHG imaging and highlight promising directions for the development of the field.

Solvent effect on quadratic hyperpolarizability of 4-(dimethylamino-4′-stilbazole)tungsten pentacarbonyl: A revisit of mechanism for second-order response

The quadratic hyperpolarizability ( β) of 4-(dimethylamino-4′-stilbazole)tungsten pentacarbonyl (W(CO) 5DAS) organometallic chromophore has been studied by DFT/TDDFT methods. The intrinsical mechanism for the second-order response has been ascribed to the contribution from metal-to-ligand charge transfer. The consideration of the solvent effect quantitatively improves the consistency of the calculated data with the experimental data, and switches the β vec sign. The β vec value is remarkably sensitive to the polarized solvent and even exhibits the different mechanisms for the second-order response in gas and solution phases. The possibilities of using this intriguing effect in the perspective of a molecular switch device are evaluated.

Second-Order Nonlinear Optical (NLO) Properties of a Multichromophoric System Based on an Ensemble of Four Organic NLO Chromophores Nanoorganized on a Cyclotetrasiloxane Architecture

In this paper we report the synthesis, the photophysical, conductometric, and second-order nonlinear optical (NLO) characterization of an ensamble of four NLO active organic tails nanoorganized on a cyclotetrasiloxane ring, to produce various macrocyclic NLO chromophores. The second-order NLO response of the macrocyclic NLO chromophores measured by the EFISH technique as μβ1.91, where μ is the dipole moment and β1.91 the projection along the dipole moment axis of the vectorial component of the quadratic hyperpolarizability working with an incident wavelength of 1.907 μm, increases, compared to the reference monomeric NLO chromophores, from 2.8 up to 3.5 times for nonionic macrocyclic NLO chromophores. The increase is mainly due to an increase of the dipole moment when compared to reference monomeric NLO chromophores while the β1.91 values remain almost unchanged. These macrocyclic NLO chromophores can be also considered as a simple model of a monolayer of organic NLO chromophores on a chemically engineered silica surface. Since the factor controlling their second-order NLO response is the orientation toward the dipole moment axis of the single organic NLO active tails, this kind of model confirms that the second-order NLO response of a monolayer of organic NLO chromophores on a chemically engineered silica surface is controlled by the topology of the binding sites on the surface, as suggested by previous investigations on multilayers on chemically engineered silica surface. The macrocyclic NLO chromophore with a ionic organic NLO active tail shows a strong concentration dependence of its second-order NLO response to be ascribed to a larger increase of its ionic dissociation by dilution, when compared to that of the reference ionic monomeric NLO chromophore. This behavior is evidence of a cooperative effect due to the nanoorganization, to be ascribed to a more facile ionic dissociation of the single ionic organic tail originated by a local increase of the solvent polarity favored by the closeness of the positive charges of the four organic tails. Finally it was shown that the Si(OSiMe3)3 group behaves in a classical organic push−pull NLO chromophore as a pull group as strong as the nitro group.

Second-order nonlinear optical properties of transition metal clusters [MoS4Cu4X2Py2] (M = Mo, W; X = Br, I)

We present in this paper the second-order nonlinear optical properties of a series of penta-nuclear metal clusters [MS4Cu4X2Py6] (M=Mo, W; X=Br, I) on the basis of the hyper-Rayleigh scattering experiments and the first-principle calculations (TDDFT). The measurements obtain the notably large dynamic quadratic hyperpolarizabilities at 1064 nm [beta(-2omega, omega, omega) values are around 200x10(-30) esu] and, by extrapolation, a large static values around 60x10(-30) esu. The computational results of the electronic excitation energies and quadratic hyperpolarizabilities by TDDFT method are in good agreement with the experimental values and by careful examination they are both dependent on the nature of the metals. The in-depth analysis of the mechanism for the second-order response unambiguously shows the evidence of the contribution of direct metal-metal interaction charge transfers. This provides a new tool to tune nonlinear optical properties in exploiting metal cluster materials and molecular devices.

Bottom-up approach to nonlinear optical imaging of biomaterials

Biological tissues are complex systems. Conventional microscopy, ultimately limited to a spatial resolution of roughly half the wavelength of light, is often unable to see molecular details unless sophisticated and invasive methods are employed. However, recent advances in nonlinear optical-imaging techniques for biological tissues have resulted in both higher spatial resolution and better tissue contrast than achieved before.1 As a result, a great deal of focus has been placed on engineering nonlinear optical probes for use with two-photon excitedfluorescence and second-harmonic-generation (SHG) imaging. Both employ femtosecond-laser microscopy to measure nonlinear optical responses. In SHG, two photons at a fundamental frequency are converted into one at once or twice the harmonic frequency. In addition to the numerous organic and inorganic optical probes that have been developed for labeling tissues and cell membranes, certain cellular components, such as collagen, have intrinsic SHG properties.2 This makes SHG a favourable method for noninvasive imaging of large structures and cellular processes. However, it still lacks resolution at the molecular level. In addition, for smaller components the relationship between the intensity and polarization state of the nonlinear optical response and the molecule’s conformation remains elusive. Here, we provide preliminary insights into this dependence. In addressing this problem, the focus is usually placed on the imaging potential of the techniques. Very few studies have tackled the problem in a bottom-up approach, starting from the elementary bricks of the proteins—the individual amino acids— and working up to the protein itself. The nonlinear optical properties of these molecules can be observed using hyper-Rayleigh Figure 1. Quadratic hyperpolarizability of tryptophan (W)-rich peptides as a function of the number of these amino acids. Dots: Experimental data. Empty circles and squares: coherent and incoherent models, respectively.

Preparation and characterization of second order non-linear optical properties of new “push–pull” platinum complexes

In this contribution, we report the synthesis and characterization of the second order non-linear properties of a series of unconventional terpyridyl and dipyridylphenyl platinum complexes. These new platinum complexes consist of a tridentate ligand substituted by a high electron acceptor, and a chloro or an arylacetylide ligand. The hyper-Rayleigh scattering measurements reveal that these complexes display large quadratic hyperpolarizabilities reaching a dynamic value of 1460 x 10(-30) at 1064 nm esu for the best complex. This investigation also demonstrates that cyclometalated dipyridylphenyl platinum complexes greatly exhibit enhanced first hyperpolarizability with respect to analogous terpyridyl complexes.

SYNTHESIS AND CHARACTERIZATION OF THIOPHENE-CONTAINING CHROMOPHORES FOR NONLINEAR OPTICAL (NLO) MATERIALS

A series of chromophores based on benzene and thiophene moieties as π-conjugating spacers were designed and synthesized by Horner-Emmons-Wadsworth reaction and Vilsmeier-Haack reaction, followed by Knoevenagel reaction with different electron acceptors. Their structures were confirmed by 1HNMR, FT-IR, HPLC-Ms, UV-visible spectra, and the thermal properties were characterized by differential scanning calorimetry (DSC). Solvatochromic method was used to measure their nonlinear optical properties. The variation of chromophoric structures on NLO properties was investigated, indicating that the cyano substitution on the vinylene bridge of the chromophore produces a considerable red-shift of the absorption maximum, and would greatly enhance the molecular quadratic hyperpolarizability (βμ).

Donor–π-acceptor benzothiazole-derived dyes with an extended heteroaryl-containing conjugated system: synthesis, DFT study and antimicrobial activity

A series of novel derivatives containing an electron-donating N, N-dimethylaminophenyl ring connected to an electron-withdrawing benzothiazole or benzothiazolium moiety via a heteroaryl system (furan, thiophene or N-methylpyrrole) and up to two ethenylene groups have been synthesized and characterized. Furthermore, their nonlinear optical (NLO) properties have been investigated at the theoretical level using DFT and time-dependent DFT methods, and their antimicrobial activities were evaluated against a standard set of unicellular organisms. Both benzothiazole and benzothiazolium systems are predicted to exhibit large NLO responses, based on the calculated static molecular quadratic hyperpolarizabilities β 0 as well as intramolecular charge transfer (ICT) transition characteristics. Moreover, the 3-alkyl-benzothiazolium salts were found to display high toxicity against several tested microbes.

Synthesis of organometallic Ru(II) and Fe(II) complexes containing fused rings hemi-helical ligands as chromophores. Evaluation of non-linear optical properties by HRS

A new family of three-legged piano stool structured organometallic compounds containing the fragment η 5-cyclopentadienyl-ruthenium(II)/iron(II) has been synthesized to evaluate the existence of electronic metal to ligand charge transfer upon coordination of the novel benzodithiophene ligands ( BDT), benzo[1,2- b;4,3- b′]dithiophen-2-carbonitrile ( L1) and benzo[1,2- b;4,3- b′]dithiophen-2′nitro-2-carbonitrile ( L2). All the compounds were characterized by 1H, 13C, 31P NMR, IR and UV–Vis. spectroscopies and their electrochemistry studied by cyclic voltammetry. The X-ray structures of [Ru(η 5-C 5H 5)(PPh 3) 2(NCC 10H 5S 2)][PF 6] ( 1Ru), [Ru(η 5-C 5H 5)(PPh 3) 2(NCC 10H 5S 2)][CF 3SO 3] ( 1′ Ru), [Ru(η 5-C 5H 5)(DPPE)(NCC 10H 5S 2)][PF 6] 2Ru and [Fe(η 5-C 5H 5)(DPPE)(NCC 10H 5S 2)][PF 6] ( 2Fe) were determined by X-ray diffraction showing centric crystallization on groups P 1 ¯ and P2 1/ n, respectively. Quadratic hyperpolarizabilities ( β) of some of the complexes ( 2Fe, 2Ru and 3Fe) have been determined by hyper-Rayleigh scattering (HRS) measurements at a fundamental wavelength of 1500 nm, to minimize the probability of fluorescence due to two-photon absorption and to reduce the effect of resonance enhancement, in order to estimate static β values.

Direct Metal−Metal Interaction Contributions to Quadratic Hyperpolarizability: A Study on Dirhenium Complexes

We present a comparative study of the metal-metal interaction effect on the static quadratic hyperpolarizabilities of two typical dinuclear rhenium clusters. The electronic structures, excitation spectra, dipolar moments, static polarizabilities, and quadratic hyperpolarizabilities of the two complexes with direct metal-metal interactions have been computed and analyzed with the use of high-level DFT/TDDFT methods. The geometries and the first intense excitations agree with the relevant reported measurements. The orbital decomposition scheme ( J. Phys. Chem. A 2006, 110, 1014-1021) has been applied to analyze the relationship between the electronic structures and nonlinear optical (NLO) properties of these two complexes. We propose an unprecedented NLO response mechanism featuring the contribution of the direct metal-metal interaction transition process in these dinuclear rhenium complexes. This contribution positively enhances the quadratic hyperpolarizability and relates to the intensity of the metal-metal interactions of the complexes. The results are helpful to the development of NLO chromophores in polynuclear metal clusters through the molecular design technique.

Linear and non-linear optical response properties of β-enamino ketones

β-Enamino ketone (I) is experimentally shown to have a strong red shifted absorption maximum in its electronic spectrum in polar solvents and theoretically predicted to have high quadratic hyperpolarizability. Electron releasing and withdrawing substituents in the para position of the phenyl ring affect the NLO response indicating ICT as one of the controlling processes.

Multipolar Contributions to the Second Harmonic Response from Mixed DiA−SDS Molecular Aggregates

Polarization-resolved hyper-Rayleigh scattering experiments are reported for mixed suspensions of a constant concentration of 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA), a cationic amphiphilic compound with a strong quadratic hyperpolarizability, and varying concentrations of sodium dodecyl sulfate (SDS), an anionic surfactant with a vanishing quadratic hyperpolarizability. It is observed that the electric dipole contribution of the HRS signal intensity attributed to the free DiA molecules in the solution decreases with an increasing concentration of SDS whereas the electric quadrupole contribution increases simultaneously. These changes are direct indicators of the molecular changes undergone in the solution, and in particular the appearance of centrosymmetric mixed DiA-SDS micelles. A detailed analysis of the HRS signal intensity is performed, allowing the monitoring of the evolution of the organization of these mixed molecular systems. These experimental results are described within a general model based on the centrosymmetrical spatial organization of the DiA probe compounds in the micelles. These results underline the potential use of polarization-resolved hyper-Rayleigh scattering as a noninvasive technique to investigate the molecular organization at the nanometer scale in liquid suspensions.

On the Sensitivity of f Electrons to Their Chemical Environment

Density functional calculations have been carried out on three families of lanthanide complexes of D3 or C4 symmetry, namely [Ln(H2O)9]3+, [Ln(DPA)3]3-, and [Ln(DOTAM)]3+ (Ln = Y, La, Lu; DPA = pyridine-2,6-dicarboxylate; DOTAM = 1,4,7,10-tetracarbamoylmethyl-1,4,7,10-tetraazacyclododecane), to get some insights concerning the sensitivity of 4f electrons to the surrounding ligands. We show that the electron density accumulations found within 0.7 A of the metal center, that precisely give the opposite image of the coordination sphere as they are located trans with respect to the Ln-ligand bonds, are almost exclusively due the f electrons. This polarization of the 4f electrons in lanthanides complexes has therefore to be considered as a general feature that plays a crucial role in some experimentally observed phenomenons such as the dependency of quadratic hyperpolarizability to the number of f electrons in [Ln(DPA)3]3- complexes that we have evidenced.

Size-dependent linear and non-linear optical response of single carrier two-dimensional quantum dots

We explore the pattern of size dependence of linear and non-linear optical (NLO) responses of one-electron quantum dots in two dimensions with or without anharmonicity in the confinement potential. For some fixed values of transverse magnetic field strength ( ω c ) and harmonic confinement potential ( ω 0 ), the influence of the size of the dot on the linear ( α ), the first ( β ) and the second ( γ ) NLO responses of the system computed through a finite field linear variational route is analysed. Size-dependent maximization is predicted to be feasible for the quadratic hyperpolarizability.

Alkynyl Expanded Donor–Acceptor Calixarenes: Geometry and Second‐Order Nonlinear Optical Properties

A number of wide- and narrow-rimmed functionalized alkynylcalix[4]arenes have been synthesized by Sonogashira coupling. With respect to their optical properties, these donor-acceptor systems are treated as ensembles of covalently linked, electronically independent tolane subchromophores. Linear UV/visible and fluorescence spectroscopic investigations revealed that the charge-transfer character of the electronic transitions in calixarenes, and also the second-order nonlinear optical (NLO) properties depend on the electron-withdrawing nature of the terminal ethynylphenyl substituent (NO(2), CF(3), H). The nitro derivatives display high values of the quadratic hyperpolarizability beta. Not only do the (nonlinear) optical properties of the target compounds depend on the number and relative disposition of the subchromophores, but also on the geometry of the calixarenes. In particular, the opening angle of the calixarene cavity can be determined by the substitution pattern of the calixarene scaffold (wide- versus narrow-rim substitution) and the number of the acetylene functions introduced. Both the NLO properties and the conformational issues are conveniently assessed by using hyper-Rayleigh scattering (HRS) in solution, and supported by X-ray crystallography in the solid state.