Photothermally Driven Transformations
Why molecular scale heat?
Nearly every educated person in the world can instantly recognize the basic tools of synthetic chemistry: flasks, beakers, graduated cylinders, and so on. These are so strongly associated with chemistry, that it can be easy to overlook the mis-match in scale that they represent. Namely, we are using tools that exist on the centimeter scale, to effect changes on the scale of Angstroms. This extreme mismatch in scale inspired us to seek ways to generate heat on a more molecular scale.
How can one generate molecular scale heat?
Until recently, the use of centimeter scale tools might have been the best option for applications of heat. However, the recently developed ability to purposefuly prepare uniform nanoscale materials has opened the door for much finer control over the distribution of heat. Using light of wavelength the materials will absorb, we can use the nanomaterials to convert the light to thermal energy, thereby heating the local environment. In our laboratory, we use materials with dimensions on the scale of single nanometers to tens of nanometers—providing precision in the distribution of heat on the same scale. By the very nature of the heat source, this also brings us nanosecond precision over heat, in terms of time. Combined, this approach brings us much closer to applying heat on a truly molecular scale.
Can molecular scale heat drive molecular transformations?
To date, our laboratory has demonstrated that, despite it’s tight localized nature, molecular scale heat, we can effectively and efficiently drive many chemical transformations. For instance, we have shown that we can drive the formation of polymers (polyurethane and PDMS), we have shown that we can drive the decomposition of polymers (polypropylene carbonate), and we have shown we can drive thermal reactions of small molecules in solution-phase (retro Diels-Alder reactions). Notably, we have shown that we can drive these reactions billions of times faster than they would otherwise occur. Together, these results show that molecular scale heat retains its generality, but also brings immense power for driving chemical reactions.
Are there advantages for using molecular scale heat to drive molecular transformations?
The billion-fold increases in reaction rate imply very large temperatures. From both a kinetic and thermodynamic perspective, the chemical reactions are experiencing temperatures of up to 1000 C! Despite these extreme temperatures, we still retain specificity in our products, and also are able to drive reactions at these temperatures in standard solvents, like chloroform. Thus, it is clear that molecular-scale heat provides a new and effective approach to one of mankind’s oldest tools: heat.
Ligand control over metallic nanoparticle electronic structure
Why focus on ligands?
In molecular inorganic chemistry, the dominant paradigm is to use ligands to control the behavior of metal centers. If you have a metal center that absorbs light, catalyzes a reaction, mediates charge transfer, then one uses ligands to control these behaviors. In part, this is because the ligands provide a much greater flexibility in composition and properties than do the metal centers. By changing ligands around the metal center, one can impart this flexibility to the metal itself.
Why ligands on metallic nanoparticles?
Metal nanoparticles are valued for their light-absorbing, catalytic, and charge transfer mediating properties. To date, there are many ways known to control these properties, such as changes in particle size, shape, and composition. However, it is also true that nearly all of these particles have ligands attached at the surface. Historically, these have been used to impart solubility to the particles, passivate the surface, or protect the particles from unwanted reactions. However, the effects of changes to ligands on the metallic particle’s electronic properties is largely unexplored, despite this being the dominant paradigm in molecular inorganic chemistry. Thus, our laboratory thought that these ligands offered a new and powerful means to fine-tune the behavior of nanoparticles.
How to think of the effects of ligands on metallic nanoparticles?
In molecular chemistry, one often frames the effects of ligands in the context of a molecular orbital diagram, where changes in ligands change the energy and character of the molecular orbitals. Often times, the orbitals of most interest are those that are near the HOMO and LUMO of the complex. Even nanoscale materials systems have a large number of these orbitals, and so we must consider an electronic band, formed of these many orbitals. But again, we will be concerned with orbitals that lie near the transition between filled and empty orbitals—a position that corresponds to the Fermi energy for metallic systems. The most basic questions we can ask are if the nature, number, or energy of these orbtials are changing with changes in ligand.
How to measure the changes in electronic structure?
The very nature of metallic systems provides a convenient means to measure the change in electronic structure near the Fermi energy. The bands that hold the electrons can be thought of having two identical manifolds: one for spin up electrons and one for spin down electrons. Since metallic systems have partially filled electronic bands, placing a metal in a magnetic field will lift any energetic degeneracy between electron spins. In response, electrons are transferred between spin manifolds, produces an excess population of one spin. In other words, the metal becomes paramagnetic. Because the unpaired spin resides near the Fermi energy, we can use probes of these spins to probe the nature of the orbitals near the Fermi energy. In our group, we perform electron spin resonance (ESR) on the electrons to understand the nature and energy of the orbitals in which they reside. We measure the magnetic susceptibility to determine the density of electronic states at the Fermi energy.
What control do ligands offer?
To date, work has focused primarily on thiolate-protected gold nanoparticles—one of the most widely used systems in nanoscience. We have found that these ligands influence the energy and density of states near the Fermi energy, primarily acting via charge donation through the Au-S bond. This occurs for both aromatic and aliphatic thiolates. Currently, we are working to examine the effects of other ligands and the response of other metallic systems.