Photothermally Driven Chemistry
Harnessing the photothermal effect of nanoparticles to achieve nanometer and nanosecond scale control over heat, enabling chemical reactions at 700–1300 K with billion-fold rate enhancements.
Heat is one of chemistry’s (and humankind’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 (centimeters) and the molecular scale of the transformation (nanometers). By way of analogy, if all tools had this sort of mismatch in scale, hammers would be the size of the moon.
Nanoscale Heat Sources
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 suggests 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. When the object is on the nanoscale, however, the response to light is rapid, and the extent to which heat can disperse into the surroundings is short. Thus, it is the nanoscale dimension of the particles that provides this unprecedented local control.
Billions-Fold Reaction Accelerations
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 remarkably well to photothermal heating. Indeed, all of our efforts point to routinely running reactions at significantly elevated temperatures—frequently at 700 K, and reaching as high as 1300 K. These temperatures, in turn, are associated with billion-fold enhancements to the rate of reaction.
Clean Chemistry Far Above Degradation Thresholds
The ability to drive reactions at such extreme temperatures is one of the distinct benefits of using photothermal heating by nanoparticles. Though these temperatures are far above those typically used in organic chemistry—and far above those at which most organic molecules degrade—we find that reactions proceed cleanly.
For example, a polyurethane reaction cured photothermally at kinetic/thermodynamic temperatures near 700 K shows identical product selectivity to room-temperature synthesis, completely avoiding the degradation seen under bulk heating. This unique capability opens up entirely new frontiers for materials synthesis and chemical manufacturing.
