Self-assembled chromophoric NLO-active monolayers. X-ray reflectivity and second-harmonic generation as complementary probes of building block-film microstructure relationships

The microstructural changes in chromophoric self-assembled monolayers induced by modifications in the architecture of the molecular components are studied by X-ray reflectivity and polarized second-harmonic generation (SHG) spectroscopy. Monolayers prepared by chemisorption/quaternization of the chromophore precursor 4-[N,N-bis(3-hydroxypropyl)amino]styryl-4′-pyridine (2a) on preassembled films of the coupling agents (p-ClCH₂C₆H₄)(CH₂) ₂SiCl₃ (1a), (p-ClCH₂C₆H₄)(CH₂) ₂SiCl₂CH₃ (1b), and (p-ClCH₂C₆H₂)(CH₂) ₂SiCl(CH₂)₂ (1c) display a progressive reduction in measured film thickness from 26.4 to 21.8 to 19.6 Å and in optical second-harmonic response, χ⁽²⁾ (λ₀ = 1064 nm), from 3.0 ×10^-7 to 1.7× 10^-7 to 0.8 × 10^-7 esu, respectively, indicating reduced chromophore surface density with increasing methyl substitution on the silicon of the coupling agent. The chromophoric groups in the film also experience a slight increase in tilt angle with respect to the surface normal, from 37° to 41° to 43°, with increased methylation of Si, indicating that the reduction in thickness is primarily due to microstructural changes in other regions of the film. Monolayers prepared under slightly different conditions with coupling agent 1a and chromophore precursor 2a or the analogous alkynyl chromophore precursor [4-[N,N-bis(3-hydroxypropyl)amino]phenyl]ethynyl-4′-pyridine (2b), and then capped with octachlorotrisiloxane, display a decrease in measured film thickness from 20.5 to 15.1 Å, an increase in monolayer-air interface width, and a reduction in χ⁽²⁾ from 2.0 × 10^-7 to 0.6 ×10^-7 esu when 2b is used in place of 2a. Penetration of the capping layer into the film microstructure is suggested by a substantial increase in film electron density to ∼70% of silicon and an increased measured interface width.