Two-Dimensional Infrared Spectroscopy of Heterogeneous Systems: On the Path to Measuring Charge Transfer in Solution Processed Organic Electronic Thin Films

Abstract

Entering graduate studies, I had a clear vision: renewable, tunable, biodegradable organic electronics. Renewable technologies and luxuries enjoyed by society might prevent catastrophe due to energy shortage as demand increases. With this vision in mind, my focus was to work towards applications of organic electronics. Specifically, work towards the goal of biodegradable, tunable, renewable organic electronics was focused on heterogeneous systems. Scientists and engineers most often work with composites, mixtures, and amorphous materials and Nature has produced optimized structures that are heterogeneous. Recognizing this and knowing that energy is the world’s primary concern motivated the study of electronic processes in heterogeneous systems. Being a physical chemist with a background in automotive repair meant having an interest in both fundamental science and its applications. I believed that the farthest reaches of computation could meet the farthest reaches of experiment on challenging but applicable systems. Applicable implies commercially and industrially viable systems. This also implies ease of processing which is often associated with an increase in heterogeneity. Ideally, measuring charge transfer in heterogeneous organic electronics on a mechanical level would enable their rational design. Mechanical here refers to ultrafast time scales or molecular scales. For example, the measurement of the rate of individual charges hopping would be the mechanical level as opposed to a thermodynamic bulk level measurement like charge mobility or conductivity. Researching the connection between mechanical properties like ultrafast vibrational dynamics or molecular charge hopping and then correlating thermodynamic bulk properties of heat capacity, charge mobility, or conductivity is still an enticing challenge. Bulk properties that have been historically observed are catalysis reaction rates, charge mobility, large-scale morphological changes like glass transition or annealing, coefficient of thermal expansion, etc. Their mechanical corollaries would be molecules reaching transition states, charge hopping between molecular sites or distinct subensembles as opposed to bulk mobility, or ultrafast changes in vibrational correlation as these processes occur. Catalysis is an example of why this might be of interest. For a rate of catalysis with a given concentration, one could have 100% of the catalytic species facilitating 10 times the uncatalyzed reaction rate. Alternatively, one could have 1% of the catalytic species facilitating 1000 times the uncatalyzed reaction rate. Both scenarios would appear the same on the thermodynamic level but would differ on the mechanical level. Rational design of chemical systems begins at the mechanical level, but the effects of interest are often observed on the bulk scale. This connection between the quantum mechanical and the thermodynamic is an open area of research both theoretically and experimentally. These connections are almost completely obfuscated for disordered systems and in many respects are only approachable with phenomenological analysis. Affirming or negating hypotheses that specific vibrational dynamics unique to each molecular configuration give rise to protein function, catalytic mechanisms, or are the source of the apparent charge mobility ceiling in organic electronics (OEs) would change the direction of a tremendous amount of research effort. To illustrate with another example, if it were known that charge mobility were limited byvi charge hopping rates, and charge hopping was limited by molecular motions modulating wavefunction overlap, then the applications of OEs would be limited. If this were known, it would prevent wasted effort on applications that were practiallly beyond reach. Similarly, if a catalytic species had only a small population of extremely active conformers it would be necessary to know this in order to improve their function in an efficient rational way. Given the challenges in applying theory to heterogeneous systems, comparative experiments are one way to connect 2D-IR data to physical processes on both the thermodynamic and mechanical level. This approach involves changing one variable within the molecule or system, observing changes in spectra, and then connecting these to system properties to enable rational design of systems or their moieties. The ultrafast time scales of 2D-IR and its multidimensional nature allow one to decompose ensembles within heterogeneous systems experimentally. Then the efficient application of theory to a subset of a system is approachable. This is opposed to the currently intractable task of modeling an entire heterogeneous system and all its properties at once. Ultrafast 2D-IR is a technique capable of decomposing ensemble measurements and monitoring their dynamics on femtosecond time scales. The capabilities of this technique are manifest in the results below: measurement of temperature dependent intramolecular proton exchange rates, gas adsorption and exchange within heterogeneous systems, solvent effects on complex formation, and morphological changes in the surfaces of nanoparticles