Incorporation of phenoxy groups in self-assembled monolayers of trichlorosilane derivatives. Effects on film thickness, wettability, and molecular orientation

Abstract

To evaluate the possibilities for preparing monolayer films from trichlorosilane derivatives containing aromatic functional groups, model studies of monolayer-forming trichlorosilanes which incorporate a phenoxy moiety in various positions of a long alkyl chain were undertaken. Self-assembled monolayers of several such compounds could be successfully formed. The properties of these monolayers have been evaluated by utilizing ellipsometry, wettability measurements, and FTIR and compared with analogous alkyltrichlorosilanes. Monolayers prepared from 1 -(trichlorosilyl)- 1 1 -(p-n-nony1phenoxy)undecane (1) show that the phenoxy group can be introduced into such monolayers with no loss of oleophobicity, as compared with octadecyltrichlorosilane (OTS), and with spectral features which indicate a high level of monolayer order and close packing. Linear dichroism measurements using FTIR-ATR are used to estimate the orientation of the alkyl chains and the phenoxy group in 1, and a model is proposed to explain the results. Shortening of the alkyl chain lengths in several examples led to more disordered, liquidlike monolayers. The orientation of the phenoxy group did not vary significantly with position in the alkyl chain. Interest in the formation of monolayers at a liquid-solid in- terface by spontaneous self-assembly has grown considerably in recent years with the discovery of methods that result in the formation of close-packed monolayers chemically bonded to the surface. These include the use of derivatives of trichlorosilane on hydroxylated surfaces, including glass, quartz, aluminum, and silicon (pioneered by the Sagiv group'), and of thiol and disulfide monolayers on gold.'9* A particular advantage of the use of trichlorosilane systems is the application to the formation of ordered, cross-linked, multilayer structures. Such multilayer films can be constructed from molecules that terminate in a functional group that can be activated (e.g., a C=C bond or an ester group) to form a fresh hydroxylated surface, upon which a new monolayer can be ads~rbed.~~~-'' This technique, therefore, has great po- tential utility in the construction of macromolecular structures, or artificial lattices, with order at the molecular level and with electronic, electrooptic, or other bulk properties. We are interested in the eventual fabrication of multilayer structures that contain useful aromatic and other functional groups for eventual optical or electronic uses. However, most of the fundamental work on self-assembled monolayers, to date, has involved the study of monolayers of octadecyltrichlorosilane (OTS) and various similar, functional-group-terminated alkyltrichloro- silanes. Many potentially useful structures that may be built into such macromolecular structures contain aromatic or hetero- aromatic rings and possess symmetries different from cylindrical, long-chain alkyl groups. Hence incorporation of such groups into the alkyl chain will be accompanied by the introduction of disorder into the monolayer. The effect of this expected disorder on the packing of the monolayer, the orientation of the aromatic group and the alkyl chains, and the resultant quality of the monolayer and its surface properties are very important issues. Also im- portant are the effects of the position in the alkyl chain of the chromophore of interest on monolayer orientation, packing, and ~~~ ~~~~ ~~~