Quadratic hyperpolarizability

Nonlinear optical properties depend upon the molecular environment as well as the individual molecular prop erties. In particular, at the molecular level, second harmonic generation depends upon the magnitude of p (the quadratic hyperpolarizability), which is the coefficient... 

LeBozec and co-workers have reported nonlinear behavior in a series ofterpyri-dyl and dipicolinic acid complexes, with further studies on these complexes by Maury and co-workers [83, 84]. Their research was on new molecular materials for optoelectronics, with studies based on octupolar nonlinear optical molecules showing that molecular quadratic hyperpolarizability values were strongly influenced by the symmetry of the complexes [85]. Other studies on organic-lanthanide complexes with nonlinear optics have also reported second- and third-harmonic generation behavior with simultaneous multiphoton absorption properties [50]. Such studies have shown the importance of coordination chemistry as a versatile tool in the design of nonlinear materials. 

Tancrez, N., Eeuvrie, C., Ledoux, I., Zyss, J., Toupet, L., Le Bozec, H. et al. (2005) Lanthanide complexes for second order nonlinear optics evidence for the direct contribution of /-electrons to the quadratic hyperpolarizability. Journal of the American Chemical Society, 127 (39), 13474-13475. 

Recently, the quadratic hyperpolarizabilities of heterobimetallic metallocenes have also been studied using the HRS technique [116-118]. The addition of ametal-pentacarbonyl function to a pyridyl acceptor group appeared to substantially increase the NLO efficiencies of the ferrocene-pyridyl compound [117, 118]. Notwith-... 

Silicon can be incorporated as a donor, bridge, or even as an acceptor into an organic system [159-161]. Studies report rather moderate quadratic hyperpolarizabilities, the largest efficiency being found for the silicon donor. The main advantage of these systems compared with other metallic complexes is their transparency in the visible region. 

In this expression, po is the permanent dipole moment, a the linear polarizability and (3 the quadratic hyperpolarizability (origin of the NLO behavior), E being the electric field component of the light. The... 

Di Bella. S.. Marks, T.J., Ratner, M.A. Environmental elects on nonlinear optical chromophore performance. Calculation of molecular quadratic hyperpolarizabilities in solvating media. J. Am. Chem. Soc. 116, 4440-4445 (1994)... 

Therefore, prediction of the p values for a given chemical structure is essential in the second-order NLO field in order to prepare appropriate systems for a specific application and constitutes the basis for the optimization of the microscopic NLO performance. The most widely used theoretical model to discuss the NLO performance of organic molecules consists of a summation over the electronic states of the system (SOS sum over states). The general SOS expressions for the components of the p tensors are well-known [30]. In many cases, only a few electronic levels contribute significantly to the NLO response and the general expressions simplify. For ID charge-transfer molecules (such as p-nitroaniline) a two-level model has yielded satisfactory results for the quadratic hyperpolarizability. For planar (2D) systems, as most phthalocyanines and analogous compounds, three... 

Table 2. Calculated and experimental quadratic hyperpolarizability, jSjjrs values...

Keywords Organometallics, Alkynyl, Vinylidene, Quadratic Hyperpolarizability, Cubic Hyper-... 

The studies summarized above involve complexes with 18 valence electron metal centres. We also examined related 14 electron gold alkynyl complexes. Molecular quadratic hyperpolarizabilities of gold alkynyl complexes by HRS measurements at 1.064 xm are given in Table 3, and the most efficient gold-containing complex is depicted in Figure 6. 

Roberto, D., Tessore, F., Ugo, R., Bruni, S., Manfredi, A., Quid, S. Terpyridine Zn(ll), Ru(lll) and Ir(lll) complexes as new asymmetric chromophores for nonhnear optics first evidence for a shift from positive to negative value of the quadratic hyperpolarizability of a ligand carrying an electron donor substituent upon coordination to different metal centres. Chem. Commun. 846-847 (2002)... 

Naulty, R.H., McDonagh, A.M., Whittall, I.R., Cifuentes, M.P., Humphrey, M.G., Houbrechts, S., Maes, J., Persoons, A., Heath, G.A., Hockless, D.C.R. Organometalhc complexes for nonlinear optics. 15. Molecular quadratic hyperpolarizabilities of trans-bis bis(diphenylphosphino)methane]ruthenium cr-aryl- and cr-pyridyl-acetyhdes x-ray crystal structure of trans-[Ru(2-C=CC5H3N-5-NO2)Cl(dppm)2]. J. Organomet. Chem. 563, 137-146 (1998)... 

I. D. L. Albert, S. di Bella, D. R. Kanis, T. J. Marks, and M. A. Ratner, in Polymers for Second-Order Nonlinear Optics, American Chemical Society, Washington, DC, 1995, pp. 57-65. Solvent Effects on the Molecular Quadratic Hyperpolarizabilities. 

The second-order NLO properties are of interest for a variety of NLO processes [1-3]. One of the most relevant is the SHG, originated by the mixing of three waves two incident waves with frequency co interact with the molecule or the bulk material with NLO properties, defined by a given value of the quadratic hyperpolarizability, fi, or of the second-order electrical susceptibility, respectively, to produce a new electrical wave, named SH, of frequency 2co. Another important second-order NLO process is the electrooptic Pockels effect which requires the presence of an external d.c. electric field, E(0), in addition to the optical E co) electrical field. This effect produces a change in the refractive index of a material proportional to the applied electric field, and can be exploited in devices such as optical switches and modulators [1-3]. 

To obtain molecular or bulk materials displaying significant second-order NLO effects, high values of jS or of respectively, are required. In the case of molecules, in 1977 Oudar gave a theoretical interpretation of the electronic factors controlling [20,21]. The quadratic hyperpolarizability of a molecule is originated... 

Experimentally, mainly two techniques - the electric field induced second harmonic generation (EFISH) and hyper-Rayleigh scattering (HRS, also termed harmonic light scattering method) - are used in order to determine in solution the experimental value of the quadratic hyperpolarizability of molecular NLO chromophores. 

The HRS technique [25-27] involves the detection of the incoherently scattered second harmonic generated by the molecule in solution under irradiation with a laser of wavelength 2, leading to the mean value of the x tensor product. By analysis of the polarization dependence of the second harmonic signal, which can be evaluated selecting the polarization of the incident and scattered radiation, it is possible to obtain information about the single components of the quadratic hyperpolarizability tensor jS. Unlike EFISH, HRS can also be used for ionic molecular species and for nondipolar molecules such as ocmpolar molecules. In this chapter, the quadratic hyperpolarizability measured with an incident wavelength 2 by the EFISH and HRS techniques will be indicated as /l i(EFISH) and / (HRS), respectively. 

Since the two level model (Eq. 4) applies well to NLO chromophores characterized by a major CT transition, the solvatochromic method may afford a way to evaluate the quadratic hyperpolarizability, but only the component along the major CT direction, by recording electronic absorption spectra of this absorption... 

The static hyperpolarizability Po can also be evaluated by means of Stark (electroabsorption) spectroscopy, which affords the value of A/igg by analyzing the effects of an applied electric field on the shapes of the major absorption bands of CT character [29-31]. The contribution of each transition to the quadratic hyperpolarizability value is thus obtained according to the two level model (Eq. 4). However, the estimated pc, are generally approximate, especially when dynamic P data are resonantly enhanced, or when many excited states contribute to the NLO response [32]. 

NLO response due to the positive charge, produced by the facile solvolysis of the triflate ligand, as confirmed by electrical conductivity measurements which evidenced a sharp conductivity increase at concentrations below 10 " M for the triflate complexes [64, 65]. A similar behavior of the quadratic hyperpolarizability in CHCI3 solution by dilution was observed for Zn complexes with the same stilba-zoles and with ancillary ligands such as the methansulfonate or para-toluensulfo-nate anions [66],... 

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