SELF-ASSEMBLED CHROMOPHORIC NLO-ACTIVE MONOLAYERS - X-RAY REFLECTIVITY AND 2ND-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 componen ts are studied by X-ray reflectivity and polarized second-harmonic gen eration (SHG) spectroscopy. Monolayers prepared by chemisorption/quate rnization of the chromophore precursor -[N,N-bis(3-hydroxypropyl)amino ]styryl-4'-pyridine (2a) on preassembled films of the coupling agents (p-ClCH2C6H4)(CH2)(2)SiCl3 (1a), (p-ClCH2C6He4)(CH2)(2)SiCl2CH3 (1b): and (p-ClCH2C6H4)(CH2)(2)SiCl( CH3)(2) (1c) display a progressive redu ction in measured film thickness from 26.4 to 21.8 to 19.6 Angstrom an d in optical second-harmonic response, chi((2)) (lambda = 1064 nm), fr om 3.0 x 10(-7) to 1.7 x 10(-7) to 0.8 x 10(-7) esu, respectively, ind icating reduced chromophore surface density with increasing methyl sub stitution on the silicon of the coupling agent. The chromophoric group s in the film also experience a slight increase in tilt angle with res pect to the surface normal, from 37 degrees to 41 degrees to 43 degree s, 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 a lkynyl chromophore precursor s(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 Angstrom, an inc rease in monolayer-air interface width, and a reduction in chi((2)) fr om 2.0 x 10(-7) to 0.6 x 10(-7) esu when 2b is used in place of 2a. Pe netration of the capping layer into the film microstructure is suggest ed by a substantial increase in film electron density to similar to 70 % of silicon and an increased measured interface width.