James A. Rego

Overview of Self-Organizing Organic Materials for Optoelectronics



Ferroelectric Liquid Crystals

My first major contribution to the field of materials chemistry came during my Graduate work at the University of Colorado.  Working with Professor David Walba, I synthesized and characterized a family of smectic liquid crystals that were designed to exhibit nonlinear optical (NLO) behavior.

These chiral nonracemic molecules all form ferroelectric phases, meaning that the bulk material possesses a macroscopic dipole moment that can be reversed 180° with an externally applied electric field.  Moreover, this bulk dipole remains aligned indefinitely in either direction when the field is remove; thus, the phase is bistable. Using the Boulder Model -- a unique molecular level model describing the origins of the polar order occurring in the ferroelectric LC phase -- we were able to create materials that contained a high concentration of NLO-active chromophores aligned along the polar axis of the noncentrosymmetric materials.  These materials are easily processed into thin films and wave guide configurations.  Figure 1 shows four typical synthetic targets from my thesis work.

acetylenic LCs

Figure 1. Some NLO-Active LC Derivatives

The work I did in the Walba group was multidisciplinary in nature.  While the bulk of my efforts were aimed at designing and executing the synthesis and purification of dozens of new mesogens on the multigram level, I also studied their phase behavior and measured their ferroelectric polarization.  Each measurement required the fabrication of a single pixel LC device and the polarization reversal current was recorded as a function of temperature.  For these studies, I spent a great deal of time in the Condensed Matter Physics laboratory of Professor Noel Clark. There I learned a great deal about device physics and liquid crystal display fabrication.

Organic Photoconductors

In my first post doctoral position with Professor Helmut Ringsdorf at the University of Mainz, Germany, I did extensive work in the field of organic photoconductors. Discotic liquid crystals (DLCs) are another class of self-organizing molecules that form highly ordered "plastic crystals" capable of remarkably efficient intermolecular energy migration. Starting with symmetric hexaalokoxytriphenylene DLCs, I developed a synthetic methodology for the creation of asymmetric triphenylene DLCs functionalized directly on the polyaromatic core. Figure 2 shows the parent compound HPT and two subsequent variations.

discotic LCs
Figure 2. Triphenylene Discotic Liquid Crystals

We discovered that reducing the symmetry (i.e.. C1 vs. C3) and attaching electron withdrawing groups to the core induced broader, more ordered mesophases. Several of my publications from Mainz are available on my home page.

We also explored the fabrication of ordered monolayers of these photoconductive discotic liquid crystals. In particular, we were able to self-assemble triphenylene discotics bearing a terminal thiol group on gold surfaces. Figure 3 shows the compound employed and an atomic force micrograph (AFM) of the monolayer surface. The spacing of the lateral rows correspond precisely to the intercolumnar spacing observed in the bulk mesophase of HPT.
 

AFM image of discotics on gold
Figure 3. AFM of a Self-assembled Discotic Monolayer on Gold

We also discovered that discotic liquid crystal possessing long alkyl tails will self-assemble on graphite surfaces. Figure 4 show a scanning tunneling electron micrograph (STM) of a DLC on graphite.
 

stm image of TCQ on graphite tcq 10
Figure 4. STM of a Self-assembled Discotic Monolayer on Graphite

I believe that spatial control of these photoconductive liquid crystals at the surface of electrodes will prove to be critical for the development of DLC-based optoelectronic devices. These experiments in Mainz represent a promising start toward the molecular engineering of highly ordered photoconductive thin films.

Reflective Displays

Upon returning to the United States, I took another postdoc position at Sandia National Laboratories. Working with Dr. Paul Cahill, I designed and synthesized a number of novel monomers for the fabrication of chiral polymer networks. These polyacrylate and polyvinylether networks were used to create thin film liquid crystal gels that served as low power, multicolor display elements.

Chiral nematic liquid crystals (or "Cholesterics") are yet another class of self-organizing molecules that have proven invaluable for the fabrication of color displays. Indeed, the vast majority of laptop computer displays employ cholesteric or "twisted nematic" liquid crystals. Nematic LC phases comprised of chiral molecules posses a spontaneous helical ordering with the helical sense directly related to the absolute molecular configuration of the molecules. This supermolecular helix can act as an optical grating; when the helical pitch is comparable to visible light, selective reflection (Bragg diffraction) of specific colors occurs.

Typically in the industry an achiral nematic phase is doped with a chiral "twist agent" molecule that induces helical ordering in the phase; in fact, the entire phase is rendered chiral. Additionally, achiral monomers are added and are lightly crosslinked (2-5%) to stabilize the phase and influence the electrooptic response. These are referred to as Polymer Stabilized Cholesteric Text (PSCT) displays.

The novelty of our efforts lie in the fact that we achieved both helix induction and phase stabilization with the polymer network (i.e. no added twist agent). Moreover, we designed the network in such a way that helical pitch (and thus color of reflection) was addressable by an externally applied field.