Heat is one of chemistries (and mankind's) oldest tools. It is valued for its efficacy and generality. In contrast to the design of a catalyst, which requires detailed molecular-level insight into the nature of the transformation, using heat to drive a reaction only requires knowledge of the energy of activation. However, the fact that we do not need molecular-level insight allows one to glaze over the shockingly large difference between the scale at which heat is typically applied in chemistry (cm) and the molecular scale of the transformation (nm). By way of analogy, if all tools had this sort of mis-match in scale, hammers would be the size of the moon.

Our lab seeks to understand what, if any, advantage could come from applying heat on a more molecularly-relevant scale. Consideration of the elementary steps of reaction suggest that control over heat distribution should approach the nanometer and picosecond scales. Though we have yet to attain this temporal control, we have realized both nanometer and nanosecond scale control over heat, using the photothermal effect of nanoparticles. The photothermal effect arises anytime an object absorbs electromagnetic energy, followed by non-radiative decay, whereby the energy lost to this decay is converted to thermal energy and the object heats up. This process is familiar to anyone who has sat in a car on a sunny day. When the object is on the nanoscale, however, the response to the light is rapid, and the extent to which heat can disperse into the surroundings is short. Thus, it is the size of the particles that give us this control.

Using these nanoscale heat sources, we have examined a number of thermally activated reactions. To date, we have explored the ability of this heat to break bonds for molecules in solution as well as for polymers. We have also explored the ability of this heat to drive the formation of bonds in solution and in the solid state. In all of these trials, we have found that thermally activated reactions respond well to photothermal heating. Indeed, all of our efforts point to routinely running reactions at significantly elevated temperatures. Often we run at 700K, but we have observed temperatures as large as 1300K. These temperatures, in turn, are associated with billion-fold enhancements to the rate of reaction.

The ability to drive reactions at such high temperatures is one of the benefits of using photothermal heating by nanoparticles. Though these temperatures are far above those typically used in organic chemistry---indeed, they are far above those at which most organic molecules degrade---we find that we can drive the reactions quite cleanly. To the right, we have one example, where a polyurethane reaction was cured photothermally at a temperature that both kinetic and thermodynamic data suggested as roughly 700K. This temperature is above that associated with thermal degradation of the polymer---at least when bulk-scale heat is applied. However, when we compare the products attained upon photothermal heating to those attained at room temperature, we observe no differences. This ability to drive organic reactions cleanly at temperature that are far above those typically accessible to organic chemistry is one advantage of the photothermal approach to chemistry.

Metallic nanoparticles are known for their remarkable tunability in terms of size, shape, and ligand. This tunability has led to exciting applications in catalysis, sensing, and medicine. These applications, in turn, rest upon the electronic properties of the metallic nanoparticles. However, when people talk about controlling the electronic properties of these nanoparticles, the focus is invariably falls on how the size and shape impact the position of the localized plasmon resonance (LSPR). While the LSPR is, without a doubt, a useful property, this view of electronic properties and tunability is quite different from the traditional chemical view.

A conventional approach to controlling electronic properties focuses on the electronic structure of the system, typically described using orbital diagrams. This is by far the dominant paradigm for molecular systems. Even for molecules that have heavy metals that have open shells, chemists find the molecular orbital diagrams a familiar and powerful tool for understanding the behavior of the systems. Moreover, these diagrams emphasize that the natural way to adjust the orbital structure is via changing the ligands attached to the metal center.

A similar understanding can be applied to the electronic structure of metallic nanoparticles. The electronic structure of the core is fundamentally no different: consisting still of a series of molecular orbitals holding electrons. The different is simply that the number of electrons involved is large, so that the number of orbitals involved is large enough that the separation between them in energy is effectively zero. When the orbitals run together like this, we call them a band. The nature of these bands arise from the atoms that form them. For instance gold has filled 5d orbitals and a half-filled 6s orbital. When bands are formed from these atoms, we arrive at a filled 5d band and a half-filled 6s band. The energy at which change from filled to empty states (at 0K) is known as the the Fermi energy.

These partially-filled bands are a defining characteristic of metallic systems, and we can exploit this property to probe the nature of the electronic structure near the Fermi energy of the system. To understand this, we must acknowledge that there are actually two band structures, one for spin up electrons and one for spin down electrons. This is analogous to the spin up and spin down orbitals shown for our molecular system. Placing the system in a magnetic field induces Zeeman splitting, which will stabilize one spin state over the other, shifting the spin bands relative to one another. This results in a situation where filled orbitals in one spin band are higher in energy than empty orbitals in the other and electrons will be transferred between the spin bands until a constant Fermi level is regained. This places unpaired electron spin directly at the Fermi energy, and we can probe this spin to understand the nature of these electronic states, and how they change with ligand identity.

To date, we have demonstrated this approach for metallic systems made from silver, gold, palladium, platinum, and iridium. We have also show that ligands can influence the electronic structure for gold nanoparticles. We have used linear alkanethiolates and aromatic thiolates. From these measurements, we have deduced that it is the nature of the Au-S interface---specifically the Au-S bond---that seems to dominate the properties of the system. Future work seeks to understand the size dependence of these effects, as well as explore other binding motifs beyond thiolates on gold.