Self-assembled monolayers for nonlinear optics

• Main Author: Roscoe, Stephen Barrie
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1996
• Subjects: Chemistry; Inorganic chemistry; Materials science
• ISBN: 9798209219484

NLO-active monolayers prepared by the nucleophilic substitution of pre-assembled films of the coupling agent (p-ClCH$\rm\sb2C\sb6H\sb4)(CH\sb2)\sb2SiCl\sb3$ (1a) with the chromophore precursor 4- (N,N-bis(3-hydroxypropyl)amino) styryl-4$\sp\prime$-pyridine (2a) display an optical second harmonic response, $\chi\sp{(2)}\ (\lambda\sb0$ = 1064 nm) of $3.0\times10\sp{-7}$ esu corresponding to a chromophore coverage of $\approx{2}\times10\sp{14}$ molecules.cm$\sp{-2}.$ Monolayers prepared with methylated coupling agents display reduced thickness and lower chromophore coverage, as do monolayers prepared with coupling agent 1a and the analogous alkynyl chromophore precursor 4- (N,N-bis(3-hydroxypropyl)amino) - phenylethynyl-4$\sp\prime$-pyridine (5a), and then capped with octachlorotrisiloxane (4). A substantial increase in monolayer electron density indicates penetration of the capping layer into the film microstructure. Approximately 60% of the densely packed benzyl chloride groups undergo reaction with 2a, in contrast to 4-(dimethylamino)pyridine which can substitute $\sim$90% of the benzylic chlorides. The kinetics of the reaction with 2a, studied in real time by in situ polarized second harmonic generation (SHG) techniques, can be fit to a biexponential rate equation with $\rm k\sp\prime\sb1\approx 2\times10\sp{-2}$ Lmol$\rm\sp{-1}s\sp{-1}$ and $\rm k\sp\prime\sb2\approx 2\times10\sp{-3}$ Lmol$\rm\sp{-1}s\sp{-1}$ and also to a coverage-dependent activation energy model (E$\rm\sb{A} = E\sb0 + E\sb{b}\theta),$ yielding a perturbative energy E$\sb{\rm b}$ of 6-8 kJmol$\sp{-1}$. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. Densely packed monolayers can be ion exchanged with various anions, a maximum of 40% of available chloride being exchanged with iodide. Introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films, however the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second harmonic generation efficiency which, for iodide and sulfanilate, is attributable to the incoming anion assuming a different position within the monolayer microstructure. Monolayers self-assembled on silicon (111) substrates, having oxide layers 2.5-860 nm thick, display clear three-fold symmetry when studied by in-plane SHG. This pattern is derived from interference between an isotropic component generated by the chromophoric monolayer, and an anisotropic component of 6-fold symmetry which originates from the Si(111) substrate.