58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Self-assembled monolayers provide exquisite control of over surface chemistry and, as a result, have been employed to solve a wide variety of problems in diverse areas from biointerface science to corrosion resistance and microelectronics.  Uniform monolayers prepared form either long chain alkane monomers or aryl monomers form highly ordered molecular assemblies on a wide variety of substrates depending on the reactive headgroup used to anchor the monomers to the surface.  A variety of contact and non-contact patterning techniques, including microcontact printing, dip pen nanolithography, and photolithographic methods, have been developed to create well-defined patterns with features as small as a few hundred nanometers.  Mixed monolayers have been used extensively to control functional group density within these surfaces, however the reactive functional groups in these systems are not patterned at the nanometer scale and, at best, are stochastically distributed in the monolayer.  Our project is focused on developing methods for controlling and patterning reactive functional groups within self-assembled monolayers at the nanometer scale with the long-term goal of developing molecular assemblies capable of organizing proteins, small molecules, and nanostructures into well-defined macroscopic structures for biological studies and device production.  To accomplish this goal, we are using macromolecular synthons, which were developed in the area of organic crystal engineering, to precisely array functional groups within mixed monolayers. 

During the first reporting period, we have focused on building functional group arrays using the robust guanidinium-sulfonate macromolecular synthon and a phosphonate analogue of this sython.  The guanidinium-sulfonate motif is one of the best-characterized motifs in crystal engineering consisting of a triangular array of hydrogen bonded guanidinium cations and sulfonate anions.  We have examined self-assembled mixed monolayers consisting of long-chain alkanethiols and 10-mecaptodecanesulfonic acid on atomically flat gold substrates prepared by template stripping.  Substrates were prepared with different length long-chain alkanethiols, ranging from octanethiol to dodecanethiol, and with and without a guanidium ion template.  In the absence of the templating ligand, we observed clear phase separation of the methyl-terminated and sulfonate terminated alkanethiols by scanning probe microscopy.  This phase separation disappeared for mixed monolayers assembled with a templating guanidium ion and decanethiol.  Due to the minimal height differences between the monomers, the phase separation is not observed in height, but is clearly visible using quantitative nanomechanical mapping.  Significant adhesion, deformation, and dissipation differences are observed for the sulfonate terminated and the methyl terminated phases of the monolayer.  We are currently characterizing these surfaces at atomic resolution using scanning tunneling microscopy.  Additionally, similar experiments with the phsophonate analogue are ongoing to determine the optimal background long-chain alkanethiol monomer and the appropriate methyl terminated to phosphonate terminated monomer ratio.

We are also synthesizing extended guanidium analogues to increase the spacing between functional groups.  These templating ligands are a series of planar conjugated tris(amidine) ligands, which will provide an identical hydrogen bonding motif to the classic guanidium-sulfonate motif.  The conjugated planar nature of these ligands makes them ideal substitutes for guanidium in lattices with spacing upto tens of nanometers, but also makes them synthetically challenging.

From a student training prospective, this grant has provided partial support for a number of graduate students and summer support for an undergraduate research student.  This work has helped students to developed proficiency in organic, physical, and materials chemistry.  Specifically, students have developed skills in complex multi-step organic synthesis, atomically flat substrate preparation by electron beam deposition and template striping, a broad array of scanning probe microscopy techniques, and other conventional surface characterization methods. The interdisciplinary nature of this project and other work going on in our research group is critical for preparing students to function within modern highly collaborative scientific environments.