Self-Assembling Organic Superlattices
Professor Fred Wudl
University of California at Los Angeles
Department of Chemistry & Bio-Chemistry
4505 A Molecular Science Building
Los Angeles, CA 90095-1569
(310) 206-0941
Duncan McBranch
Los Alamos National Laboratory
CMS, MS G755
Los Alamos, NM 87545
505-665-4836
505-665-4817
We are developing an interesting new class of solid-state materials based on layered organic superlattices built from conducting polymers, fullerenes, and other electronically- and optically-active building blocks. These complex superlattices are constructed molecular layer-by-layer using self-assembly of charged (ionic) molecular components. In our ongoing research into novel optical properties of these materials, we have discovered several exciting phenomena that fit extremely well into the theme of complex adaptive materials. These include: 1) evidence for long-range charge and energy transfer between molecular layers, controllable by tuning structure and commensurate electronic communication between layers; 2) evidence for bulk changes in structural conformation in the solid-state driven by complex electrostatic interactions; 3) evidence for electronic/excitonic delocalization within and across molecular layers. These phenomena suggest the tantalizing possibility of being able to design and construct organic and mixed organic/inorganic strongly correlated electronic materials with an unprecedented level of control and tunability.
Our immediate (i.e. currently funded) interests include exploring nonlinear optical effects driven by inter-layer charge separation in these solids, as well as device applications from photo-voltaics to lasers and displays based on controlled charge and energy flow. However, the opportunities for making new connections within the complex materials community are particularly exciting, and could lead to dramatic conceptual breakthroughs in both hard and soft condensed-matter physics, in addition to pushing the boundaries of materials chemistry. We point out here two such possibilities which are far beyond our current programs, but which could be fostered through ICAM by catalyzing new interactions. 1) It is attractive to make an explicit connection to recent advances in understanding layered inorganic correlated-electronic systems such as the cuprate superconductors; this would require more systematic investigations of electronic and magnetic interactions within self-assembled organic superlattices, using a wider array of structural, electronic, and magnetic probes as well as theoretical tools. 2) Since we are using building blocks, such as conducting polymers, which closely approximate the charge density of DNA, it is attractive to explore ways in which binding of specific agents could dramatically alter the electronic or optical properties of the polymers. These materials offer a controlled way for investigating binding mechanisms and models for DNA, as well as a route to new biochemical sensors.
The nonlinearities in these materials exist on several levels, and the interplay of nonlinearities on different length scales makes these materials particularly interesting. On the molecular level, the p-conjugated electronic structures of the individual building blocks lead to strong nonlinear electronic-lattice coupling. Hence, excitations on individual molecules (excitons, polarons) involve structural deformations coupled to electronic wavefunctions. These excitations can couple among adjacent molecules to form collective nonlinear excitations in the solid-state (responsible, e.g., for superconductivity in a regular array of fullerenes). We believe that understanding and controlling the length-scale for nonlinear coupling in organic layered systems using self-assembly can be a frontier area for complex materials research.