EFISH method

In the EFISH method, the molecule of interest is dissolved in an appropriate solvent and put into a cell of the type shown in Figure 9. Electrodes above and below the cell provide the means for a D.C. electric field, which orients the solute (and solvent) molecules through its interaction with the molecular dipoles. Similar to the poled polymer approach, the average molecular orientation is increased along the field direction and an oriented gas model used to extract p. 

In contrast to the EFISH method, the hyperpolarizability P can be measured directly by means of the HRS method developed by Clays and Persoons [21, 22]. This method involves measuring the intensity of the incoherently scattered, frequency-doubled light from isotropic solutions. As shown in Fig. 3.2, an infrared laser beam is focused on the center of a cell containing a solution of the NLO-active compound. 

In liquids, the EFISH method is used to measure the total average microscopic cubic hyperpolarizability of a single molecule, y. The principle of this experiment has been described in a number of papers [35-37]. The nonlinearity y measured using EFISH contains contributions from electronic, vibrational, and rotational motions. This can be seen by considering the harmonic polarization Pf" for a single molecule [38]. 

The EFISH method (Singer and Garito, 1981) permitted for the first time the establishment of a correlation between molecular structure of organic chromophores and the first hyperpolarizability p. In this method an electric field is applied to a solution of the nonlinear optical materials, resulting in an alignment of the dipoles. A direct determination of P with the EFISH method is not possible the third-order polarization y is measured, the dipole moment p. must be known, and with these values the hyperpolarizability p can be calculated. The EFISH technique is not readily applied to salts as the solutions conduct electricity. 

Many other organic and some organometallic materials have been examined by the powder SHG method and/or tested by EFISH techniques. They will not be discussed in detail here but are listed by structural classes for completeness. Several compendia of materials responses have been published. (38,54,55) Clearly, the largest single class of second order materials consists of donor-acceptor substituted aromatics. The class has been extended to stilbenes and diarylacetylenes and... 

Both theoretical analysis and dipole moment measurements indicated that sulfonyl-substituted compounds may have ft coefficients similar in magnitude to their nitro analogues. Therefore, we have measured p for several sulfonyl- and nitro-substituted compounds using electric-field-induced second-harmonic generation method (EFISH) (11,25). In this experiment, one measures an effective third-order nonlinearity rEFISH for a solution containing the compound of interest, given by... 

A detailed investigation of the NLO properties of molecules of series 2 and 3 has been performed in the powder state (12) as well as in solution by the electric-field-induced-second-harmonic (EFISH) generation method (13-15). It has allowed the analysis of the influence of the molecular parameters on the NLO features (12-15). 

TDDFT methods have also been applied successfully to the description of the linear and nonlinear optical properties of heteroleptic sandwich complexes. The optical spectrum and the hyperpolarizability of Zr(OEP)(OEPz,) for which large first hyperpolarizabilities, /JSHG (SHG=second-harmonic generation) were measured in an electric field induced second-harmonic generation (EFISH) experiment [182], have been investigated by TDDFT methods [134]. The excitation energies and oscillator strengths calculated... 

The earlier experimental EFISH studies have often been linked to analysis of the results by the semi-empirical SOS method. An examination of the exact SOS formulae used to interpret the data can provide confirmatory evidence of the convention that has been adopted. The standard SOS procedure, for the electronic contribution to the first hyperpolarizability for frequency doubling, is summarized below. 

If the objective is to express both the macroscopic and molecular quantities involved in the analysis of the EFISH experiment in terms of the fundamental definitions of section 2, then eqn (4.15) can be regarded as the standard form of the EFISH equation (see for example Reiss ). In eqn (4.15) the superscript has been re-inserted to emphasise that the equation relates a susceptibility defined by the standard method of section (2) to h5q)erpolarizabilities in the T convention. 

Smaller molecules and inert gas atoms have been extensively studied using EFISH in the gas phase (see for example Miller and Ward, Ward and Miller, Shelton ). Shelton and Rice provide a comprehensive list of gaseous EFISH measurements on small molecules up to 1994. The only such result reported for molecules with donor/ acceptor substitution on a benzene ring appears to be that obtained by Kaatz et alP for pNA in 1998. In this experiment a gas mixture containing 0.075 mole fraction of pNA was used to obtain an EFISH measurement at 1064 nm at one temperature. The y (—2(B (B,a),0) of eqn (4.15) was estimated from a THG experiment and taken as the intercept on a two point plot of y versus /T. The value of was calculated from the slope. The linearity of such plots has been confirmed in the work on smaller molecules. The gas phase method differs from that used for solutions in that the extrapolation to infinite dilution is not made since the molecular density in the gas is very much smaller. Also the internal field factors are close to unity. It is usually possible to make measurements over a sufficiently wide range of temperatures to obtain the quantity (jifi/k) from the plot of F versus l/F. In the case of pNA the value of the dipole was chosen as 6.87 D. 

The high first hyperpolarizability of pNA was first identified in EFISH measurements in the 1970s. The experimental method itself was developed by Levine and Bethea,Oudar, Chemla and others and the inherent difficulties of the procedure and the devices used to overcome them are well documented in the some of the papers. Measurements have been made on a large range of molecules (see for example Cheng et where full account of the method is given and Zyss and... 

The first experimental method for determining 3 measurements of neutral dipolar molecules is the electric field induced second harmonic generation (EFISH). This method consists of measuring the light intensity at a frequency twice the fundamental frequency to of the infrared nanosecond laser generated by a solution submitted to a static electric field E°. In the case where the static electric field is applied along the z-axis in the laboratory framework, and the polarization of the laser is also along the same axis, the... 

A second experimental method, Hyper-Rayleigh Scattering (HRS), allows (3 measurements for nondipolar and/or ionic molecules, which cannot be measured with the standard EFISH, The intensity of the scattered light is proportional to the square of the incident intensity and can be written as... 

The chemical structure of the polyimide polymers (named PI-1 and PI-2) studied by Sekkat et al. is shown in Figure 12.12. They prepared the polymer samples by spin-casting onto glass substrates. PTl was cast from a cyclohexanone solution and PI-2 from 1,1,2,2- tetrachloroethane. The Tg values of PI-1 and PI-2 were determined to be 350°C and 252 C, respectively, by scanning calorimetry method. The thicknesses of the PI-1 and PI-2 films were, respectively, approximately 0.72 im and 0.14 im, and their respective optical densities were approximately 0.79 and 0.3 at 543.5 nm. Details of the preparation and characterization of the samples can be found in References 3 and 20. In their EFISH experiment, a typical corona poling technique was used to pole the samples, with a dc electric field about 2-3 MV/cm across a 1-2 lm thick polymer film. They used the SHG output from the EFISH experiment to in situ monitor the photochemical change in the third-order susceptibility of the PI-1 and PI-2 polymers. 

The first common method for molecular first hyperpolarizability determination is the electric field-induced second harmonic generation (EFISH) technique in solution [6-10]. This technique can be applied only to dipolar molecules. Under an applied external electric field, molecules in solution orient approximately in the direction of the field giving rise to second harmonic generation. The measured third-order nonlinear optical susceptibility is given by the following expression ... 

The influence of the substituent at tlie periphery of the macrocycle on the NLO properties was studied in solution. Thus, various subphthalocyanines bearing representative electron-donor or acceptor groups were synthesized and their NLO properties were measured employing HRS, EFISH and Third Harmonic Generation (THG) methods [62, 63]. 

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. 

Zemer et have used the ZINDO/SOS method in conjunction with AMI structural optimizations to investigate dicyanomethylene-derived heteroaromatic dyes. The effect of E/Z conformational isomerism and the introduction of various combinations of end groups on (the part of the tensor that is detected in EFISH experiments) is elucidated. Solvent effects are included through the self-consistent reaction field approximation. 

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