Molecular hyperpolarizabilities measurement

The quadratic NLO active molecules are mostly based on donor-acceptor substituted aromatics benzene, stil-benes, diaryl acetylenes, diacetylenes, and biaryls are commonly used frameworks. In addition to the above materials, many more organic and organometallic compounds were examined by the powder SHG method, or their molecular hyperpolarizabilities were measured using EFISHG/HRS techniques, or both.t >... 

SmCP phases exhibit high spontaneous polarization (Pg = 500-800 nC cm ) which indicates a high degree of polar order (>50%). In contrast, only a few percent of chromophores are aligned in poled polymers, or 20% in conventional FLCs. However, since the early bent-core molecules incorporated chromophores with small molecular hyperpolarizabilities, the measured NLO coefficients (1.9 < D (pmA ) < 8 1 < d (pmA ) <4, X. = 1.064 p,m) via SHG for bent-core compounds is insufficient for NLO applications [182-185]. 

In the case of the first hyperpolarizability, the measured quantity is the vector component of li in the direction of the permanent dipole moment ft, defining the molecular z axis. The relevant averages are given by ... 

In order to measure molecular hyperpolarizabilities the now standard D-C induced SHG experiment is used (12). Although it would be more suitable to work in the gas phase to minimize molecular interactions, high molecular weights (low vapour pressure) and chemical decomposition processes make it hardly feasible for the molecules of interest. 

Stark effect measurements for determination of molecular orientation and second-order molecular hyperpolarizability... 

The non-linear optical properties of a material are evaluated by measuring (using techniques from the field of physics) its molecular hyperpolarizability coefficient (3. 

Sulfur heterocycles, including those with more than two sulfur atoms, are used for optical materials <2000JPP2002040201>. The molecular third-order optical nonlinearity 7R (Second hyperpolarizability or nonlinear refractive index) was measured for pentathiepinethiafulvalene <1999PCA6930>. 

As mentioned above, the powder SHG method is a useful technique for the screening of second-order nonlinear materials. However, because of the sensitivity of the SHG coefficients of crystalline materials to the orientational aspects of the molecular packing and because the measurement is performed on an essentially random distribution of microcrystalline particles, the powder SHG method is not generally useful for obtaining information about molecular hyperpolarizabilities. 

For molecules containing several conjugated bonds yn becomes much larger than y°. Of course, y itself is a fourth rank tensor property (analogous to x(3)) and can be specified in the molecular or laboratory reference frames. For an isotropic medium one measures an orientational average of the hyperpolarizability... 

This paper summarizes the theoretical analysis of some new molecules with methylsulfonyl group as the electron acceptor group, describes the syntheses of new stilbene and azobenzene systems, and presents the measurements of their optical spectra, ground-state dipole-moments, and molecular hyperpolarizability coefficients, p. We compare theoretical and experimental results and comment on the potential usefulness of these chromophores as components for NLO materials. The incorporation of sulfonyl-containing chromophores into polymers, and the NLO properties of the resulting materials, will be discussed in our forthcoming paper (9). 

In this study we have described theoretical calculations, syntheses, optical spectra, ground-state dipole moment measurements, and measurements of molecular second-order hyperpolarizability coefficients (/J) for new stilbene and azobenzene derivatives containing a methylsulfonyl group as the electron acceptor. We have shown that theoretical calculations can be used to predict the ratio of molecular hyperpolarizabilities between similar compounds, and that these gas phase calculations underestimate /J, probably as a result of the valence basis set used in the calculations. 

EFISH measurements indicate that indoaniline dyes with very polarizable n systems have large molecular hyperpolarizabilities (P), in one case approaching three times that of DANS. 

A-priori, given that FLC phases possess polar order, and the DOBAMBC structure should possess a substantial molecular second order hyperpolarizability P, it seems reasonable that %(2) of a material such as DOBAMBC in the FLC phase might be large. Measured values of the second harmonic generation efficiencies of DOBAMBC and of several other FLC materials, however, indicate that in fact the dcff of FLCs is very small relative to LiNb03. 

The molecular hyperpolarizibilities, y, given in Table 1 were derived from the measured %(3) of the solutions using the expression ... 

---