Source: Dissertation Abstracts International, Volume: 77-06(E), Section: B.

Adviser: Victor S. Batista.

Due to their small sizes and abilities to tune electronic and optical properties, the field of molecular electronics is gaining attention rapidly. Recent advancements in both computational and experimental work enable researchers to further study the electron transport properties in single molecules. This thesis focuses' on the study and design of intrinsic molecular rectifiers, investigating the rectification mechanism and searching for more improved devices. It proves the concept and feasibility of implementing molecular rectifiers and, maybe in the future, other molecular electronics into various assemblies.

Linkers that favor rectification of electron transfer are likely to be required for efficient photo-driven catalysis of multi-electron reactions at electrode surfaces. The linkers have a terpyridyl group that can covalently bind Mn as in a well-known water oxidation catalyst and an acetylacetonate group that allows attachment to TiO2 surfaces. The appropriate choice of the sense of the amide linkage yields directionality of interfacial electron transfer, essential to enhance electron injection and slow back-electron transfer. Support comes from electron paramagnetic resonance and terahertz spectroscopic measurements, as well as computational modeling characterizing the asymmetry of electron transfer properties.

With the experimental current-voltage (I-V) characteristic curves at hand, we observe the usual uncertainty of the correlation between calculated and experimental transport properties of single molecules. Here, we implement the s-Extended Huckel non-equilibrium Green's function (Sigma-NEGF/EH) method to analyze the histogram of I-V curves of 4,4'-diaminostilbene probed by break junction experiments. We elucidate the nature of the molecular conformations with a widespread distribution of I-V curves, typically probed under experimental conditions. We find maximum conductance for molecules that are not at the minimum energy configuration but rather aligned almost parallel to the transport direction. These findings provide valuable guidelines for the design of anchoring groups that stabilize conformations of molecular assemblies with optimal charge transport properties. With this Sigma-NEGF/EH method, we have studied photoinduced electron injection efficiencies from modular assemblies of a Zn-porphyrin dye and a series of linker molecules which are axially bound to the Zn-porphyrin complex and covalently bound to TiO2 nanopartides. Experimental measurements based on terahertz spectroscopy are compared to the calculated molecular conductance of the linker molecules. We find a linear relationship between measured electron injection efficiency and calculated single-molecule conductance of the linker employed. These results suggest that the linker single-molecule conductance is a key factor which should be optimized for maximum electron injection efficiencies in DSSCs.

A mechanism for electronic rectification under low bias potentials is elucidated for the prototype molecule HS-phenylene-amide-phenylene-SH, which is the simplified linker compared to the terpyridyl linkers. We find that a single frontier orbital, the closest to the Fermi level, provides the dominant contribution to the overall transmission and determines the current. The asymmetric distribution of electron density in that orbital leads to rectification in charge transport due to its asymmetric response, shifting towards (or away from) the Fermi level under forward (or reverse) applied bias voltage. These findings provide a simple design principle to suppress recombination in molecular assemblies of dye-sensitized solar cells (DSSCs) where interfacial electron transfer is mediated by frontier orbitals with asymmetric character.

Utilizing the understanding of low bias single molecule rectification mechanism, a theoretical and experimental study is presented here to predict and measure low-bias intrinsic electronic rectification in molecular diode designs. A computational screening methodology was developed and applied to a broad range of synthetically plausible small molecules for single-molecule electronic rectification. In the screening process, small molecule candidates were proposed on the basis of their distinctive chemical properties, and a rapid, computationally efficient method was employed to systematically compute the conductance and I-V curves. The best candidates identified were measured by performing single-molecule break junction experiments and the trends in rectification were in excellent agreement with the trends identified by the screening process. We found that, for the best candidates, electronic rectification can be enhanced by shifting the transmission function peak closer to the Fermi Level.

With a simple tight-binding method, we were able to make direct comparison between calculated and experimental rectification ratios. We obtained remarkable agreement in not only trends, but also absolute values. The results suggest that the rectification property of a certain molecular motif can be qualitatively transferred among different molecular assemblies. However, different ways of attachment may enhance or decrease the level of rectification. Nonetheless, it supports the assumption that molecular rectifiers would function similarly in different environments.