Realization of expeditious layer-by-layer siloxane-based self-assembly as an efficient route to structurally regular acentric superlattices with large electro-optic responses

A new, expeditious siloxane-based layer-by-layer assembly process for the formation of intrinsically polar organic electro-optic thin films is described using highly reactive-SiCl₂I- functionalized, silyl-protected donor-acceptor azo-benzene chromophore derivatives and octachlorotrisiloxane as a deprotecting reagent/interlayer precursor. This all-"wet-chemical" two-step process can be efficiently implemented in a vertical dipping procedure to yield polar films consisting of 40 alternating chromophore and capping layers. Each nanoscale bilayer (chromophore + polysiloxane layer ≈ 3.26-nm thick) can be grown in ≈40 min - at least 1 order of magnitude more rapidly than previous siloxane-based solution deposition methodologies. Chromophore monolayer deposition from solution reaches completion in ≈15 min at 55°C. The adherent, structurally regular assemblies exhibit appreciable electro-optic responses (χ⁽²⁾ ∼ 180 pm/V and r₃₃ ∼ 65 pm/V determined by SHG measurements at 1064 nm) and high chromophore surface densities (≈40 Ų/chromophore) and have been characterized by a full complement of physicochemical techniques: optical spectroscopy, aqueous contact-angle measurements, specular X-ray reflectivity, atomic force microscopy, and angle-dependent polarized second-harmonic generation.