Research and Education Summary

[There sections that follow briefly review the most important areas of my ongoing research (and teaching) program.]

Current projects:

(1) Conformational isomerism and chain packing in $\pi$-conjugated polymers: Rational design of new conjugated polymers and optimized processing of existing ones requires an improved understanding of the underlying structure-property relationships. This project seeks to 1) better address, at the molecular level, the origins of the enormous diversity of structure and structural phase behavior in functionized CPs and then 2) relate this microscopic molecular structure to structural anisotropy at larger length scales. This is to be accomplished through a combination of modeling and polyfluorene experimental studies. The first area is intended to provide a more fundamental focus and context while the second has greater overlap with research aimed at near-term applications.

(2) Studies of hyperconjugation and conformational isomerism in $\sigma$-conjugated polymers Polysilanes are prime examples of extended chain molecules in which electron delocalization occurs through resonance interactions of $\sigma$ orbitals along an all-silicon atom skeletal backbone. An original motivation for studying these polymers was based on simple analogies to the better known and, at present far better understood, $\pi$-conjugated hydrocarbons. Although polysilanes have optical transitions that appear superficially similar to polyacetylene and other $\pi$-conjugated systems, the presence of a saturated yet chromophoric backbone is highly unusual. Polysilanes are characterized by an intense $\sigma$-$\sigma ^*$ transition at strikingly low energies; even polysilanes with saturated side chains are strongly absorbing at wavelengths ranging from 300 - 400 nm.

Early on it was recognized that conformational changes (rotation about a single bond) had a large effect on optical properties and that conformation-dependent delocalization of $\sigma$ electrons was the underlying cause. The exact nature this conformational dependence was obscure; e.g., there was a lengthy debate as to just what conformational defect might cause backbone segmentation into individual weakly interacting chromophores. Many polysilanes are intensely thermochromic, showing that polymer conformation can change drastically with temperature. For certain polysilanes other factors, such as pressure, electric or magnetic fields, and solvents, can affect the conformation and UV absorption properties leading to other forms of chromotropism. Investigations of polysilanes have extended to nearly all aspects of their chemistry and physics, including their synthesis, electronic structure properties, linear and nonlinear spectroscopy, photochemistry and practical applicability. Examples of the latter include photoresists, polymerization photoinitiators, dopants for conductive and optical devices (particularly those active in the UV portion of the spectrum), and even use in pre-ceramic materials technology.

Regardless of these material issues, it is the nature of the $\sigma$ bond itself, including the electronic nature of its conformational properties, that stands out as the unique central issue. Understanding the nature of valency and chemical bonding stands as a fundamental pillar of the chemical science research. Here the crucial questions involve the balance of skeletal (through-bond) hyperconjugative interactions and side-group steric interactions that dictate the conformational energy landscape. Manipulation of this landscape (e.g., by altering side-group steric bulk) can in turn alter the hyperconjugative delocalizations that underlie electrical and optical properties of the polymer. Sigma electron delocalization itself is a ubiquitous yet poorly understood phenomenon in both saturated and unsaturated molecules; it is dominant in the former and often couples to stronger $\pi$-conjugative interactions in the latter. This project involves a combination of synthesis (in collaboration), theory (collaboration), modeling and experimental activities. Both spectroscopy and structural studies are being pursued.

(3) High-Performance Computer Modeling of Conducting Polymer Microstructures: This new research initiative will bring to bear state-of-the-art computational modeling techniques (already well developed and implemented in the study of conventional non-conjugated polymers) to specific issues of molecular ordering and orientation in prototypical conjugated polymer films. In terms of this proposal the first step will be to assess chain structure and packing at nanometer length scales. The second step will then be to incorporate the influence of substrate interactions. Achieving a better understanding of the polymer microstructure in juxtaposition with interactions by an orienting field (i.e., a substrate generated pinning potential) will both address fundamental questions of science and provide salient information for systematically improving organic electronic polymer device performance.

(4) Fluorescence of Conjugated Polymers under Conditions of High Shear: This project is a spin off from the shear cells developed previously (seed funding from the Petroleum Research Fund.) One of the major thrusts in the conducting polymer field is to understand how systematic change in polymer molecule structure, packing and conformation alter the resulting photophysics of energy absorption, migration and recombination. Because of the high shear developed by this cell (up to 10$^6$ sec$^{-1}$), various polymers necessarily undergo partial extension in the flow field. This extension implies changes in the chain conformation and so would be expected to alter the electronic properties of the conjugated polymer. These variations are systematically controllable and so should allow for very detailed studies.

(5) Introductory Laboratory Modernization and Instructional Efforts: Since 1996 I have been an active participant in the Physics Department's effort to revitalize the Physics 201/207 and 202.208 laboratories by incorporating new equipment and computers in the laboratory. Ugo Camerini was a major contributor for similar activities in the 103/104 lab sequence. Other faculty have contributed as well. Rather than continue to introduce new computer based experiments which replace the existing ones, I undertook the process of computerizing the entire laboratory presentation by developing a Web-based laboratory and laboratory manual. In this way we can now better structure the individual lab modules, use the computers for our existing instrumentation and allow for world access while we add new experiments in which we hopefully enhance the understanding of physics concepts.