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The surface-sensitive spectroscopic technique of optical second-harmonic generation (SHG) is applied to the in situ study of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant SHG from molecules which exhibit a large non-linear optical response at a specific wavelength can be used to measure the relative surface coverage of surfactants at the ITIES as a function of the external electrochemical parameters. In addition, the polarization dependence of the resonant surface SHG can be used to estimate the average molecular orientation of adsorbates at the liquid/liquid electrochemical interface. As an example, the adsorption of the surfactant 4-(4'-dodecyloxyazobenzene)benzoic acid at the water/l,2-dichloroethane interface is characterized as a function of applied potential, surfactant concentration and aqueous pH with in situ resonant molecular SHG measurements. An analysis of the SHG data results in the determination of the local potential and surface pH experienced by the surfactant.

The surface-sensitive spectroscopic technique of optical second harmonic generation (SHG) is employed to study the adsorption and photochemistry of surfactant molecules at the water/1,2-dichloroethane (DCE) interface. Resonant SHG measurements at 730 nm are used to monitor the adsorption of the azobenzene surfactant 4-[[(dodecyloxy)benz-4-yl]azo]benzoic acid (DBA) from DCE solution to the interface at an aqueous pH of 8 or greater. The concentration and pH dependence of the resonant SHG from the adsorbed monolayer indicates that the DBA exists in its anionic (carboxylate) form at the interface. In a series of combined photochemical/SHG experiments the trans/cis photoisomerization of the adsorbed DBA anions is examined. SHG measurements demonstrate that illumination of the surface with light at 365 nm converts the adsorbed trans-DBA molecules to the cis isomer. The cis-DBA anion is found to be unusually stable at the liquid/liquid interface, and the entire photoactive monolayer (total area 4 cm2) can be photochemically converted in approximately 450 s with an illumination spot of 0.12 cm2. This relatively short conversion time is attributed to surface-tension-induced convection effects that occur during the photochemical conversion of the monolayer. Illumination of the cis-DBA anions with light at 440 nm reconverts the entire surfactant monolayer back to the trans-DBA form in the same time frame.

Ultrathin noncentrosymmetric nonlinear optical films based on zirconium phosphonate (ZP) self-assembled multilayers that incorporate the asymmetric azobenzene chromophore [5-[4-[[4-[(6-hydroxyhexyl)sulfonyl]phenyl]azo]phenyl]pentoxy]phosphonic acid (HAPA) are synthesized and constructed. The ZP film structure and multilayer deposition chemistry are characterized by a combination of polarization/modulation Fourier transform infrared reflection absorption spectroscopy, surface plasmon resonance (SPR) measurements, and optical second harmonic generation (SHG). SPR measurements on the HAPA multilayer films yield an average monolayer thickness of 27 ( 0.5 Å. The resonant SHG at 365 nm from ultrathin HAPA ZP films on silica surfaces increases quadratically with the number of self-assembled HAPA monolayers, and an analysis of the polarization dependence of the surface SHG yields an orientation parameter D ) 0.79 ( 0.03 corresponding to an average tilt angle of 27 ( 2°for the azobenzene chromophores with respect to the surface normal. An electro-optic coefficient r 33 for the HAPA monolayers of 11 pm/V at 632.8 nm is obtained from SPR modulation experiments of ZP films on gold substrates that have been incorporated into airgap capacitors. SPR modulation experiments are then performed on a HAPA monolayer in an in situ electrochemical environment in order to determine that a modulation of (25 mV corresponds to a change in the electric field strength of 1 × 10 4 V/cm within the ultrathin organic film at the electrode surface.

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