Timothy R. Cook, Assistant Professor of the University at Buffalo, spoke about his research on supramolecules last week to the Chemistry Department.
His talk, entitled “Exploring Coordination-Driven Self-Assembly for Energy Conversion,” delved into the synthetic methods of creating complex molecules using metal-ligand bond formations.
Cook states that the basis for his work is the concept of self-assembly. Self-assembly is the spontaneous formation of a structure from a body containing all the necessary components. For Cook, this concept extends beyond the chemical level to include larger-scale structures.
Voicing his appreciation for self-assembly, Cook stated, “You have something that really jumps over like truly sixteen orders of magnitude on the length scale.” He gave several examples of self-assembled structures, ranging from simple molecules and DNA to Norwegian stone circles and space nebula.
Previous work on self-assembly was done by George Whitesides, who postulated five factors that govern self-assembly. These five factors are components, interactions, reversibility, environment and mass transport. In essence, these five factors describe the criteria a mixture of starting material must meet in order to form an energetically stable product.
The interactions that lead to self-assembly can include hydrogen bonding, pi-pi stacking and hydrophobic-hydrophilic effects. Speaking of this multitude of factors, Cook stated: “All of these can take place in concert to generate some large overall structure.”
“We also self-assemble,” mentioned Cook. He described people in an elevator who react to the entrance of another person. Two people who are strangers to each other will occupy the farthest corners of the elevator, while two people who know each other well will stand near each other. The addition of a third person changes this dynamic and invites rearrangement. It is the attractive or repulsive force we feel for other people that determines where we stand in the elevator.
On the chemical level, the attractive and repulsive forces of matter were in part explored by Alfred Werner in his work on coordination compounds, which won him the 1913 Nobel Prize. Coordination compounds are structures formed by atoms or ligands arranging themselves geometrically around a central metal atom. The ligands order themselves according to a metal center’s d-orbitals. These compounds can exist as single entities, or can become a part of an extended framework known as a coordination polymer through the use of bridging ligands, which as their name suggests bridge metal centers.
Mixing bridging ligands with other ligands can create the structures that Cook’s group focuses on: metallacycles. These metallacycles form discrete shapes such as triangles or squares.
Cook’s group uses 3-D printed models in order to illustrate how these structures can self-assemble. While molecules may be governed by electrostatic forces, the models are governed by magnetic forces. By simply shaking the different pieces, self-assembly occurs as the magnetic pieces align into different shapes.
The metallacycles and supramolecules Cook’s research group works with have applications in many fields. They can be used for light harvesting, for redox-flow batteries, as cofacial catalyst platforms and for organic pollutant encapsulation.
Cook and his team’s interest in light harvesting comes from their belief in solar energy as “extremely scalable to meet energy demands.” The metallacycles employed by Cook’s team in light harvesting can present issues for fluorescence, an important process for solar materials due to the heavy metal content.
To fix this, Cook focused on triplet emitters, which excite electrons into the triplet state. Cook’s triplet emitters, which contain platinum centers and carbene and pyridine ligands, may be used as fluorophores within solar cells. To solubulize this metallacycle, Cook’s team added more alkyl groups.
However, it was unclear what shape these metallacycles were forming because of distortions due to molecular interactions. To determine the structure, Cook’s group uses electrospray ionization mass spectrometry (ESI-MS), which concluded that they material were triangles.
Changing the ligands coordinated with the platinum metal center affected the absorbance and fluorescence characteristics of the material.
Partnering with chemists like Peter Stang at the University of Utah and Feihe Huang at Zhejiang University, Cook hopes to further explore the uses of metallacycles and to characterize them using crystallography.