Realization of Expeditious Layer-by-Layer Siloxane-Based Self-assembly as an Efficient Route to Structurally Regular Acentric Superlattices with Large Electro-optic Responses

Abstract

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 minat 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 Å2/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.