This Research award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports work by Professors Theodore Burkey and Charles Edwin Webster at the University of Memphis to develop photochromic compounds based on the a migration a transition metal between functional groups. The synthesis of organometallic compounds with functional groups located at key positions relative to the metal provides a means to investigate how a metal can migrate between the functional groups as a mechanism for color change. Studies of new organometallic photochromic complexes reveal how structural changes 1) improve the yield per photon, 2) increase rates of the color change caused by the metal migration, and 3) reduce side reactions such as recombination or solvation that interfere with the metal migration. The structural changes include functional groups, the bridge between the metal and functional group, and the bridge between functional groups. This project trains students and postdoctoral fellows important to the infrastructure of academic, government, and industrial institutions in collaborative experimental and computational research. This project increases the participation of underrepresented groups in chemistry in our Minority Undergraduate Mentoring and Immersion in Research, American Chemical Society Project SEED (Summer Research Internship Program for Economically Disadvantaged High School Students), and National Organization of Black Chemists and Chemical Engineers at the University of Memphis. Faster, more stable, and more efficient photochromic organometallics are required in practical applications of photochromic materials, which have established or potential use in data storage, optical switches, microfluidic devices, photoacuators, and smart windows.
The goal of this project is the development of metal based photochromes for molecular devices controlled by light. A photochrome is a material that reversibly changes color when irradiated with light of different colors (Figure 1). Upon irradiation with light of one color, the bond between the metal M and group X in compound 1 is broken allowing the formation of 2 where a new bond forms between the metal and group Y. By changing the group attached to the metal the environment about the metal changes and therefore the color. The reverse process is accomplished with a different color photon allowing independent control of forward and reverse reactions. Reactions with the environment can delay or destroy the color change, and the photochromic response is lost. One method to avoid environmental interference is to make the photochromic reactions very fast. This can require that the tether and bridge hold X and Y in very specific positions. Our research has found that a specific bridge or tether may work well for one X,Y pair but not another. We have determined what types of groups are likely to work with simple tethers and bridges. Our research also suggests how to construct more complex bridges and tethers that will work with other groups. In other cases, a pair of groups can exchange but the colors are not very different. A typical method to enhance color change is to use one large group that has many electron states that can interact with the metal. Unfortunately such large groups can compete with the metal for light absorption rendering the photochrome inefficient. We have discovered that small groups containing an NO2 fragment can produce a photochrome with a very different color. Future work will require examining what tethers and bridges will be compatible with these groups.
