Terahertz time-domain spectroscopy (THz-TDS) probes inter-molecular interactions within solid materials. THz-TDS covers the spectral region of 0.1-3 THz or 5-100 cm-1 which is a low energy and non-ionizing region of the electromagnetic spectrum. Spectra are collected in a time-domain configuration where a coherent broad-band pulse of THz electromagnetic radiation passes through a sample and gated-detection is used to monitor the electric-field vector of the transmitted THz radiation. This methodology permits a direct measure of the time-of-flight of THz radiation through the sample and, as a result, provides a direct means to measure interactions between the propagating THz electromagnetic wave and the sample material. Time of interaction between the THz wave and the sample gives a measure of the optical and electronic properties of the material and attenuation of the propagating THz wave gives information pertaining to both absorption and scattering properties of the sample. The analytical utility of these features of THz-TDS is explored in this dissertation.
Cocrystals represent a novel class of supra-molecular materials composed of two or more inorganic or organic units (molecules, ions or atoms) configured within a crystalline structure. The components interact by hydrogen bonding, π-π stacking or weak Vander Waals interactions to create ordered structures with unique chemical and physical properties. The potential of such unique properties has spurred efforts to design cocrystal materials specifically direct toward long-standing problems within the fields of pharmaceutics and electronics. Research findings presented in this dissertation demonstrate the potential of THz-TDS as an analytical tool for characterizing fundamental chemical and physical properties of cocrystal materials, thereby providing the means to advance the rational design of cocrystal materials for selected applications.
The analytical utility of THz-TDS is established through a series of transmission measurements through samples composed of the cocrystal of interest embedded within a polymeric matrix. Both high-density polyethylene and polytetrafluoroethylene are used to form compressed pellets containing cocrystals for analysis. Initial efforts demonstrate quantitation of cocrystalline materials within such pellets through a Beer-Lambert relationship where the magnitude of selected absorption bands is related to the amount of a cocrystal within a given pellet. In addition, time-domain THz spectra are used to determine the refractive index of sample pellets and this information is shown to provide dielectric spectra at THz frequencies for the cocrystal components of the sample pellets. The so-called LLL model is applied to generate accurate dielectric information for the cocrystal component of these pellets on the basis of volume fraction. The ability to measure the polarizability of cocrystals is also demonstrated by applying the Clausius-Mossitti relationship between polarizability and dielectric spectra.
The utility of the established analytical features of THz-TDS is demonstrated in a series of preliminary experiments designed to: 1) follow the single-crystal-single-crystal (SCSC)[2+2] photodimerization reaction of 2(5-cyano-resorcinol)∙2(trans-1,2-bis(4-pyridyl)ethylene) to produce rctt-tetrakis(4-pyridyl) cyclobutane; 2) establish the relationship between polarizability and hardness for a series of cocrystals; and 3) determine differences in polarization of cocrystals produced by different synthetic methods. Results support the following conclusions: 1)kinetics of SCSC reactions can be followed through dielectric measurements, but concentration measurements are confounded by unique spectroscopic features observed for partially reacted cocrystals; 2) polarizability at THz frequencies are inversely related to hardness of the tested cocrystals; 3) differences are observed in the polarizability of macro versus nano size cocrystals; and 4) polarizability of these cocrystals is independent of synthesis method.