Oriented Z-Type Langmuir−Blodgett Films from a Soluble Asymmetrically Substituted Polydiacetylene

Abstract

A soluble, asymmetric polydiacetylene has been shown to form stable monolayers at the air−water interface of a Langmuir trough. The monolayers of this unconventional amphiphilic polymer could be repeatedly deposited onto hydrophobic substrates during every upstroke cycle of the vertical dipping method to form uniform Z-type multilayers. Characterization of the precise orientation of the polymer main chain has been carried out to determine the spatial correlation between the nonlinear optical chromophore responsible for second-order nonlinear optical properties and the backbone. Polarized UV/vis and IR spectroscopic studies indicate a preferential orientation of the main chain along the dipping direction with a mean fluctuation of ±30° between director axis and chain trajectories. Polarization-dependent second-harmonic generation (SHG) in multilayers indicates a high degree of orientational anisotropy of the backbone. As expected, SHG intensities increase with the number of layers due to the noncentrosymmetric Z-type structure. The second-order nonlinear coefficient (d 33) was estimated to be 1.52 pm/V at 1064 nm of Nd:YAG laser after correcting for absorption. Surface characterization and alignment of the extended chain backbones is further established using atomic force microscopy (AFM).

New Processable, Functionalizable Polydiacetylenes

“…Replacement of one of the urethane groups on a diacetylene such as 4-BCMU with a functional chromophore leads to numerous interesting possibilities. Recently, our center has reported studies on a novel asymmetric diacetylene, 1-(5-pyrimidyl)-1,3-octadiyn-8-ol (BPOD), containing pyrimidyl and urethane groups.

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“…However, corona poled films of PDA−BPOD did not show significant improvement of the d -coefficient. A few polymers obtained from other similar urethane-substituted DAs, containing nitrobenzene or azobenzene dyes, showed second-order NLO properties similar to PDA−BPOD, but most of them were insoluble in common organic solvents 3d. Nevertheless, the attractive properties of PDA − BPOD suggest a design strategy that merits elaboration.…”

Formation and Orientation Studies of a Z-Type Polymeric Langmuir–Blodgett Film

“…Thin films formed in this manner have numerous applications (1), and much research to date has focused on the formation of polymeric LB films (2)(3)(4)(5)(6)(7). Polymer LB films can be mechanically and thermally more stable than LB films formed from simple surfactants.…”

“…Polymer LB films can be mechanically and thermally more stable than LB films formed from simple surfactants. Recent interest in polymeric LB films for nonlinear optical applications has stimulated research in the area of z-type polymeric LB films constructed from preformed monolayers on water (2,8). While the most common route of deposition is y-type, in which the surfactants deposit in a head-to-tail manner, this orientation precludes the film from nonlinear optical applications unless optically inactive "spacer" molecules are inserted in between the layers of optically active monolayers.…”

“…Few reports of materials capable of forming stable z-type LB films have been noted (2,8,9). With simple amphiphilic molecules, molecular rearrangement in a z-type LB film commonly occurs, resulting in the formation of a y-type film over time.…”