Chain-Length Effects on the Self-Assembly of Short 1-Bromoalkane and n-Alkane Monolayers on Graphite

The structural properties of self-assembled monolayers of short 1-bromoalkanes and n-alkanes on graphite were investigated by a combination of ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at 80 K and theoretical methods. STM images of 1-bromohexane reveal a lamellar packing structure in which the molecules form a 57 degrees +/- 3 degrees lamella-molecular backbone angle and a head-to-head assembly of the bromine atoms (Muller, et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5315). STM images of 1-bromoheptane also show a head-to-head 60 degrees +/- 3 degrees lamella-molecular backbone pattern; however, the molecules pack in a herringbone structure. The odd/even chain-length alternation in the monolayer morphologies of 1-bromoalkanes is similar to that observed for the self-assembly of short n-alkanes on graphite, suggesting that the bromine atom acts effectively as an extension of the carbon backbone. The analogy, however, is incomplete. Odd and even short n-alkanes (hexane, heptane, octane) display 60 degrees herringbone and rectangular (not 60 degrees) lamella-molecular backbone configurations, respectively. The balance of intermolecular forces and packing considerations responsible for this odd/even alternation in monolayer morphology for short 1-bromoalkanes on graphite is examined here according to classical molecular dynamics simulations and in light of the structural properties of analogous n-alkane assemblies.