Virginia Tech researchers are using the building block approach to synthesis to create supramolecular complexes with multiple capabilities. They will present their research at the 220th national meeting of American Chemical Society Aug. 20-24 in Washington, D.C.
The researchers are using molecule groups known as "ligands" -- specifically, polyazine ligands -- to connect two metal-based molecules forming a bridge. "We are varying the ligands and metals to get the effect we want," says graduate student Elizabeth Bullock.
At the national meeting, chemistry professor Karen Brewer will discuss coupling a light absorber to other metal based subunits to create photochemical molecular devices or complexes. In one project "we link a unit that can collect light and a unit that collects electrons," says Bullock. "The point is to bind sub-units with different properties for an end goal of solar energy conversion -- from light to energy. The energy levels of the units 'tune' or adjust the system to determine the direction of electron movement, for instance, from a high energy ligand to a lower energy metal, or in the other direction when the ligand's energy is less than that of the metal.
"What is different about our approach is the huge array of systems possible by varying the ligands and molecules." Bullock says. "Recently, we have modified part of our molecular design to couple metal centers capable of binding DNA into our supramolecular structures. The ability to change one sub-unit while leaving the others alone and performing their separate function allows for the construction of diverse sets of systems. This is particularly appealing in biological applications such as metal-DNA binding since the relationship between structure and function is critical to many biological processes.
The paper, "Mixed-metal supramolecular complexes as photochemical molecular devices and DNA binding agents (INOR 158)," by Brewer, Bullock, and chemistry graduate students R. Lee Williams, Zhenglai, Fang, Lynette Jackson, and Leslie Dove, will be presented at 11:30 a.m. Monday, Aug. 21, in the Renaissance Washington Hotel, Renaissance West A room, as part of the Metals in Medicine symposium. The research is supported by the Virginia Tech Optical Sciences and Engineering Research Center (OSER) and the National Science Foundation.
There will also be a poster by Bullock highlighting one sub-system of the molecular design process introduced by Brewer. As these types of supramolecular complexes are relatively new, complexes are often prepared with new and unique properties. Bullock's system designed for a solar activated processes has yielded some unexpected properties which allow it to perform electron collection based on a metal center.
A tri-metallic supramolecular complex made up of two ruthenium (Ru) and one rhodium (Rh) atoms bridged by polyazine ligands has created a light absorber-electron collector-light absorber system. Study of this novel supramolecular complex has involved looking at the properties of this system as electrons are added.
"The complex behaves as desired when electrons are added in the presence of water," Bullock says. That is, the central metal -- the positively charged, electron-hungry rhodium (Rh) -- collects the electrons and the complex kicks off two chlorides. But if we add the electrons in a dry solution, we get an unexpected result," she reports. "The tri-metallic complex breaks down into a one ruthenium, rhodium complex and a separate ruthenium."
Bullock explains that without the water, the chlorides are not solubilized and the added electrons make the Rh unstable, so the tri-metals break up.
"This is the first system where the electrons are collected on the central metal (Rh), making them easily available to be picked up in a solar energy conversion system," Bullock explains.
The poster, "{[(bpy)2Ru(dpp)] 2Rhcl2}(PF6)5: A device for photo-initiated electron collection (INOR 183)," by Brewer and Bullock, will be presented at 9 a.m. Monday, Aug. 21, in the Convention Center Exhibit Hall B. The research is supported by the NSF CHE-9632713 and by OSER/Carilion Biomedical Institute.
PR Contact: Susan Trulove
540-231-5646 strulove@vt.edu