Interrupted-growth studies of the self-assembly of intrinsically acentric siloxane-derived monolayers

The evolution of an intrinsically acentric monolayer formed by self-assembly of a trimethoxysilane-functionalized, high-β azobenzene-based chromophore from solution onto hydrophilic silicon oxide surfaces shows two distinctly different adsorption steps. The kinetics of the monolayer formation on sodium lime glass and polished silicon were monitored ex situ by a combination of aqueous contact angle measurements, optical spectroscopy, transmission second-harmonic generation (at λ₀ = 1064 nm), and synchrotron X-ray reflectivity. The initial adsorption step from toluene onto the substrate surface at 80°C is completed within minutes, k₁ ≈ 1 × 10^-2 s^-1, and results in an acentric material having ∼65% of the properties of a fully formed, dense monolayer (i.e., thickness, coverage, optical absorption, wettability, and nonlinear optical response). The second assembly stage is about 2 orders of magnitude slower, k₂ ≈ 1.3 × 10^-4 s^-1, and reaches completion after about 6 h.