Chromophoric self-assembled NLO multilayer materials. Real time observation of monolayer growth and microstructural evolution by in situ second harmonic generation techniques

The self-assembling adsorption/quaternization of a (dialkylamino)stilbazole nonlinear optical chromophore precursor onto benzyl chloride-prefunctionalized surfaces is studied in situ by polarized second harmonic generation techniques. The measurements reveal that the self-assembly kinetics are approximately biexponential (I_2ω^1/2 ∼ (1 - α₁e^-k1t - α₂^-k2t)) with a rapid initial phase, followed by a slower second phase. The in situ measurements also indicate that the average tilt angle of the chromophore dipole moment direction with respect to the surface normal increases significantly (∼20° → 42°) in the initial phases of chromophore adsorption and then remains nearly constant. These results suggest substantial chromophore-chromophore electrostatic and steric repulsion as monolayer growth proceeds. The maximum chromophore coverage observed corresponds to ∼49 Ų/ molecule and χ⁽²⁾ = 3 × 10^-7 esu at λ = 1064 nm.