! 11I11!llltll 111I11111 JIIII 1m IIl11t till III/11m /Ill EJ52-1995-659 Journal of the Chinese ChemicalSociety, 1995,42,659-666 Invited Lecture Structural Tailoring of Monomolecular Films on Evaporated Silver Surface Yu-Tai Tao* ( ), Shun-Chi Chang ( ) and Li-Fang Ma ( ) Institute of Chemistry; Academia Sinica, Taipei, Taiwan, Republic of China Monomolecular films with well-defined structure were prepared by self-assembling of n-alkanoic acids on an evaporated silver surface. Here we report the structural studies of thin films of acids containing aro- matic chromophores of varied size and polarity. Information regarding the packing, orientation and confor- mation of the structural units within the assembly were deduced from ellipsometnc thickness measurement, polarized reflection absorption infrared spectra and wetting property measurement. Implications of results to the design of functional thin films are discussed. 659 INTRODUCTION Well-ordered organic thin ftlms with thickness in the range from a few nanometers to several hundred nanometers have considerable potential in technological applications, such as in optical or electronic devices, etectroanalytical chemistry and biological interfaces. 1 Persuit in this area continues and increases rapidly in recent years because of advances in techniques for surface characterization. Among various.approaches to prepare thin organic films, the Lang- muir-niodgett (L-B) technique" is a typical method to ob- "tain fihns of well-defined conformation, orientation, fnnc- andthickness. Nevertheless, the sophisticated ma- nipulation involved in preparation of L·B films and their metast?ble nature will limit the scope of practical applica- tions. In tbepast decade, self-assembled monolayers (SAMs) have attracted much attention because this ap- proach provides a much simpler way to prepare highly Of- deredrnonolayerftlms on various surfaces.i" Throughjudi- __ cial combination of the substrate and adsorbate, stable films -- of-controlled structure can be formed. - While most ofthe detailed characterizations of $AMs were done onadsorbates with linear hydrocarbon tails, it is _importallt to know the scope of structural units that allows formationof a highly ordered assembly since many opto- or electronic applications require films containing complicated chromophores. Thus understanding structural details and _principlesthat dictate formation of ordered SAMs is of great Importance in molecular engineering for device applications based this technique, Recently we addressed effects of severaI1U"0matic cbromophores on structures of otherwise linear alkanoic acid monolayers on silver and copper sur- faces Of alkanethiol monolayers on gold and silver sur- faces. HI It was suggested that a 4,4'-drsubstituted biphenyl group or a 2,6-disubstituted naphthyl group introduced near the head group perturbs the structure of the hydrocarbon matrix above the aromatic cbromophore in the assembly to only a small extent (mainly in the degree of molecular chain twist) but still allows formation of an ordered assembly of molecules, presumably because of commensurability be- tween the favorable herringbone packing arrangement of the aromatic cnromophore, the packing arrangement of hydro- carbon chains and the favorable binding lattice that is adopted by anchoring sulfur head groups (for example, d3 x -Y3)R30' on Au). It is also suggested that the packing in- teraction and size constraint of the aromatic chromophore dominate the option of an adsorption scheme so that the same scheme was adopted on both Au and Ag, even though for the less sterically demanding n-afkanethiols the binding energy of sulfur with the surface predominates, so that ad- sorption scheme adopted on Ag is different from that on A 12 u. In this paper we compared the film-forming properties of carboxylic acid derivatives containing diverse chromo- phores. As shown in Scheme It cbromophores of varied size and polarity were incorporated between the long alkyl chain and the carboxyl group. The phenyl group in I is the small- est aromatic cbromophore (van der Waals dimension 0.64 nm x 0.34 nm) considered here, It affords a smaller packing interaction than the biphenyl moiety in V, 13 but occupies the same lateral space as a biphenyl group if both orient verti- cally in a monolayer assembly. A naphthyl group substi- tuted at the 2,6-positions as in II have different lateral di- mension (0.74 nm x 0.34 om) from the one substituted at I A-positions (0,86 om x 034 nm) as in III, if a symmetric binding of tile head group is assumed. The anthracene unit in IV, has the largest lateral dimension among the molecules considered here, i.e. 1.14 nm x 0.34 nm, The biphenyl, to- lane (in VI) and stilbene (in VII) moieties, differing mainly in the vertical dimension, are commonly used tt-conjugated chromophores in the design of non-linear optical materi- alS. 14 Compound VIII contains an electron-deficient