Measuring the Thermal Properties of Organic Materials Using Time-Domain Thermotransmittance

Optical pump-probe measurements are routinely performed in the field of nanoscale heat transfer. The last three decades have seen much progress in the characterization of technologically relevant material systems and in furthering our understanding of the fundamental mechanisms of thermal transport. The vast majority of measurements are implemented in a reflection geometry to measure the thermoreflectance signal; however, some material systems or thermophysical properties can be easily characterized using a transmission geometry. Understanding the capabilities and limitations of thermoreflectance and thermotransmittance experiments is key to ensure good scientific practice, to push the boundaries of the experiment, and to developing new technologies. The work in this thesis represents a study on the adaptability of the time-domain thermotransmittance (TDTT) technique, particularly when applied to organic materials.
Control over the thermal conductance from excited molecules into an external environment is essential for the development of customized photothermal therapies and chemical processes. This control could be achieved through molecule tuning of the chemical moieties in fullerene derivatives. For example, the thermal transport properties in the fullerene derivatives indene-C60 monoadduct (ICMA), indene-C60 bisadduct (ICBA), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), [6,6]-phenyl C61 butyric acid butyl ester (PCBB), and [6,6]-phenyl C61 butyric acid octyl ester (PCBO) could be tuned by choosing a functional group such that its intrinsic vibrational density of states bridge that of the parent molecule and a liquid. However, this effect has never been experimentally realized for molecular interfaces in liquid suspensions. Using the pump−probe technique time domain thermotransmittance, we measure the vibrational relaxation times of photoexcited fullerene derivatives in solutions and calculate an effective thermal boundary conductance from the opto-thermally excited molecule into the liquid. We relate the thermal boundary conductance to the vibrational modes of the functional groups using density of states calculations from molecular dynamics. Our findings indicate that the attachment of an ester group to a C60 molecule, such as in PCBM, PCBB, and PCBO, provides low-frequency modes which facilitate thermal coupling with the liquid. This offers a channel for heat flow in addition to direct coupling between the buckyball and the liquid. In contrast, the attachment of indene rings to C60 does not supply the same low-frequency modes and, thus, does not generate the same enhancement in thermal boundary conductance. Understanding how chemical functionalization of C60 affects the vibrational thermal transport in molecule/liquid systems allows the thermal boundary conductance to be manipulated and adapted for medical and chemical applications.
We report the development of a novel use of the TDTT technique to characterize the nanoscale morphology of structural proteins. We also show theoretically that the thermotransmittance measurement is correlated to the protein crystallinity via the thermo-optic coefficient. As shown here, time-resolved changes in the refractive index of semi-crystalline proteins vary as a function of temperature, and the strength of this effect correlates with crystallinity. Ultimately, this allows us to quantify rapidly the crystallinity of a protein sample using TDTT by decoupling volumetric thermal expansion from its structural response at room temperature. This approach can potentially be used for screening an ultra-large number of proteins in vivo. The size of the library for these proteins is simply limited by the fluidic and electronic components of the sorting since the TDTT technique operates in the order of minutes to seconds. If this screening technique is achieved, we could answer many fundamental questions in protein research, such as the underlying sequence–structure relationship for structural proteins. Successful development of this technique for proteins will have a significant impact on multiple applications in various fields (e.g., materials science, synthetic biology, metabolic engineering, agriculture, prion based diseases) and open new avenues of protein research.