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A new series of nonlinear optical molecules are described where the ground state polarization is predominantly zwitterionic when the molecules are dissolved in solution. The molecules, which are derived in general from facile reactions between tertiary amines and tetracyano-p-quinodimethane (TCNQ), are of a type where the stabilization of the charge-separated ground state is favored by an increase in aromaticity over the neutral, quinoidal forms of the molecules. The measured second-order optical nonlinearity of one in the series has been measured by hyper-Rayleigh scattering and a figure of merit value, µβ(0), being the product of the dipole moment and static first hyperpolarizability, is found to be 9500 × 10 -48 esu. This value, which is higher than most other reported values, is taken from studies in chlorinated solvents of relatively low polarity, but the discussion emphasizes the evolution of µβ(0) with solvent polarity, showing that even higher values could be expected with only modest increases in the polarity of the surrounding medium. The analysis of experimental data taken during dipole moment studies is thoroughly examined, and it is concluded that full account must be taken of the molecular shape to correlate the results with theoretical calculations. An ellipsoidal reaction field model is preferred for these highly one-dimensional molecules having strongly anisotropic polarizabilities.

A method for analysing thin films using a dual-waveguide interferometric technique is described. Alternate dual polarization addressing of the interferometer sensor using a ferroelectric liquid crystal polarization switch allowed the opto-geometrical properties (density and thickness) of adsorbed layers at a solid–liquid interface to be determined. Differences in the waveguide mode dispersion between the transverse electric and transverse magnetic modes allowed unique combinations of layer thickness and refractive index to be determined at all stages of the layer formation process. The technique has been verified by comparing the analysis of the surface adsorption of surfactants with data obtained using neutron scattering techniques, observing their behaviour on trimethylsilane coated silicon oxynitride surfaces. The data obtained were found to be in excellent agreement with analogous neutron scattering experiments and the precision of the measurements taken to be of the order of 40 pm with respect to adsorbed layer thicknesses. The study was extended to a series of surfactants whose layer morphology could be correlated with their hydrophilicity/lipophilicity balance. Those in the series with longer alkyl chains were observed to form thinner, denser layers at the hydrophobic solid/aqueous liquid interface and the degree of order attained at sub-critical micelle concentrations to be correlated with molecular fluidity.The technique is expected to find utility with those interested in thin film analysis. An important and growing area of application is within the life sciences, especially in the field of protein structure and function.

The wavelength-dependent molecular first hyperpolarizability β of the zwitterionic nonlinear optical (NLO) chromophore picolinium quinodimethane (PQDM) is determined by hyper-Rayleigh scattering (HRS) and used to test and improve theoretical β dispersion models. Experimental HRS data are obtained over a very wide fundamental wavelength range (780−1730 nm), spanning the entire range of two-photon resonance with the intramolecular charge-transfer (ICT) transition and reaching the onset of a higher energy resonance. Reliable calibration against the pure solvent (dimethylformamide, DMF, and DMF-d 7 at the longest wavelengths) was achieved over the full spectral range as a result of the high sensitivity of the HRS setup. Extremely high resonant β values are obtained (up to 4560 × 10-30 esu at 1360 nm) and also away from resonance β remains very large (1210 × 10-30 esu at 1730 nm). The two-photon resonance with the ICT band shows a pronounced red shift (∼33 nm in second-harmonic wavelength) relative to the absorption maximum. The various two-level β dispersion models available in the literature are considered, and some important improvements are introduced. Furthermore, a vibronic model including a single vibrational mode and incorporating inhomogeneous broadening is developed and contrasted to the other extreme of a continuum of vibronic lines without inhomogeneous broadening. The red shift of the β maximum can be largely explained by either an improved inhomogeneous broadening model or by vibronic coupling. However, the vibronic models are physically more realistic and lead to a better description of the observed β dispersion. In general, models with more inhomogeneous broadening result in a narrower β resonance, whereas incorporating more homogeneous broadening yields a broader resonance. Hence, the derived static electronic hyperpolarizability β0 depends very critically on the precise modeling of the broadening mechanisms. Upper and lower bounds to the true β0 are estimated from the two limiting vibronic models.

Table 1, footnote (b). FAB should read electron impact. Left-hand column. Ref. 1 1 should be ref. 12(a).

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A novel method for the analysis of thin biological films, called dual polarization interferometry (DPI), is described. This high resolution (<1 Å), laboratory-based technique allows the thickness and refractive index (density) of biological molecules adsorbing or reacting at the solid–liquid interface to be measured in real time (up to 10 measurements per second). Results from the adsorption of bovine serum albumin (BSA) on to a silicon oxynitride chip surface are presented to demonstrate how time dependent molecular behaviour can be examined using DPI. Mechanistic and structural information relating to the adsorption process is obtained as a function of the solution pH.

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