Understanding odd-even effects in organic self-assembled monolayers

The molecular chain length odd-even effect is an important phenomenon in physical chemistry and materials science and closely related to surface structures and functions of these materials. A wide spectrum of molecular self-assembled monolayers exhibit numerous odd-even effects in structure and property. All the odd-even effects revealed in the self-assembled monolayers on solid surfaces have a common structural feature of self-assembled molecules: the odd-even number of CH2 units. This review has mainly discussed odd-even effects on structure and property revealed in organic self-assembled monolayers on solid substrates. Two categories of organic self-assembled monolayers bound to substrates via weak van der Waals interactions and strong chemical bonds, respectively, were reviewed. In general, the scope of self-assem bled monolayers exhibiting odd-even effects could be extended by replacing the molecular moiety/group/atom and using a different spacer unit. A simple approach is replacement of the terminal moiety/group, some portion of the head moiety, or the tethered group of the head moiety. Notably, simple replacement of the molecular functional group does not produce an odd-even effect for the first class of self-assembled monolayers of organic molecules. However, it is an effective way for introduction of an odd-even effect for the second category of self-assembled monolayer. For the second class of se lf-assembled monolayer, a number of new substrates including various metal and semiconductor single-crystal surfaces which have 2D lattice ordering, homogeneous structure, and evenly distributed reactive sites can be prepared easily under vacuum conditions. For example, Si(100)-2 × 1,232-234 Si(111)-7 × 7,235 Ge(100),236 and metal surfaces237 have a homogeneous surface structure and reactive sites as well as high chemical reactivity for various organic molecules. Thus, future study using these substrates will possibly significantly extend the scope of odd-even effects of organic self-assembled monolayers on solid surfaces. To develop the self-assembled monolayer formed by weak van der Waals interactions between organic molecules and the substrate surface, Au(111) is definitely a good substrate since the adsorption energy of each CH2 unit on this surface is ∼6.0 kJ/mol.207 Although for Au(111) there is not the same lattice match as that between the carbon skeleton of the all-trans alkyl chain and the zigzag surface lattice of HOPG, formation of self-assembled monolayers via weak van der Waals interactions could be possible. Whether these proposed monolayers can exhibit an odd-even effect on packing structure and even chirality on the Au(111) surface or not is an interesting topic to study. On the other hand, observation of an odd-even effect for a molecular self-assembled monolayer on some substrate may provide an analytical method for examining the structural and chemical homogeneities of the solid surface since the homogeneity is a main factor determining whether the odd-even effect in the structure and property of the organic self-assembled monolayers can be observed or not. © 2007 American Chemical Society.