Design and preparation of zwitterionic organic thin films: Self-assembled siloxane-based, thiophene-spaced N-benzylpyridinium dicyanomethanides as nonlinear optical materials

Regioselective alkylation of 1-(4-pyridyl)-2-[5-(dicyanomethanide)thien-2-yl]ethylene sodium salt (2) by a covalently surface-bound benzylhalide (4) produces a polar-ordered zwitterionic thin film (5). Chemisorption of 2 on the coupling layer 4 was monitored by UV-vis spectroscopy, contact-angle measurements, X-ray photoelectron spectroscopy, and second harmonic generation (SHG). Relative to carbanion 2, the film UV-vis spectrum exhibits a red-shifted (Δλ ∼ 170 nm) charge-transfer band, consistent with quaternization of the pyridyl group of 2 by 4. The nonlinear optical properties of the blue films of 5 were measured by SHG. The average molecular tilt angle of the chromophore was determined to be ∼46°, and χ_ZZZ⁽²⁾ ∼ 5.0 × 10^-8 esu (∼20 pm/V) was estimated at a fundamental wavelength of 1064 nm.