Design and Theoretical Characterization of Novel Molecules for Clean Energy Applications [electronic resource].

9781088316078

Ann Arbor : ProQuest Dissertations & Theses, 2019.

1 online resource (97 p.)

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With global energy demand continuing to rise year over year, the need for clean, sustainable energy sources has never been greater. Solar energy in particular represents an underutilized resource, though fundamental research is needed to improve the efficiency of solar light harvesting, conversion, and storage. Accordingly, it is important to understand the fundamental processes underpinning solar energy conversion to allow for the rational design of improved light-harvesting systems. Here computational methods are used to obtain molecular-level insights into the directional charge transport induced by intrinsic molecular rectifiers, the process of interfacial electron transfer from dyes to semiconductor surfaces, and the absorption properties of panchromatic dye-sensitizers. Much of the work presented here was produced in collaboration with experimentalists, and care is taken to validate the methods used against experimental results whenever possible. In many cases the insights presented here allow for the design of novel classes of molecules with improved performance characteristics.In the design of molecular rectifiers, or molecules that act as one-way conductors of electrical current, there is often a trade-off between the rectification ratio and the overall conductance of a given molecule. Here a new class of low-bias intrinsic molecular rectifiers based off of a zwitterionic phosphonium ylide backbone with high rectification ratios (up to 8.79) is presented. Charge transport through these molecules is found to be a result of highly-asymmetric HOMO orbitals that couple more strongly to one electrode than other due to their spatial distribution. One of the screened molecules demonstrates charge transport at low bias voltages through two closely-spaced states, corresponding to the molecular HOMO and HOMO-1. Two-state transport as a design principle for molecular rectifiers is shown to be of great promise because of its potential to overcome the aforementioned rectification-conductance trade-off at low bias voltages. The study of molecular rectifiers presented here is of broad interest in the field of molecular electronics and is potentially valuable in solar energy conversion applications where it is desirable to minimize nongeminate processes and move charges as efficiently as possible.The interfacial electron transfer process between dye sensitizers and semiconductor surfaces is also studied here. It is shown that density functional theory calculations and tight-binding quantum dynamics simulations can broadly reproduce the electron injection dynamics obtained via spectroscopic techniques across a wide variety of dye and linker group combinations. Most of the systems studied here show through-linker injection dynamics, but in one study it is shown that the linker π* orbitals are energetically inaccessible to the excited state of the dye, and as a result through-space injection is likely the dominant mechanism. The insights provided by the calculations presented here suggest that it is possible to predict the dominant mechanism for interfacial electron transfer in a given system, allowing for the fine-tuning of molecular assemblies for favorable charge injection properties.Finally, a new class of panchromatic light-sensitizers based off of a Zn-porphyrin core coupled to an accessory dye is proposed. A density functional theory method for predicting the absorption spectra of Zn-porphyrin species is benchmarked against a previously reported class of dyes and is found to be able to reproduce the experimental spectra well. A variety of different linker groups and accessory dyes were screened, and two dyes making use of a phenazine as the accessory dye and an ethynyl linker group were selected for synthesis and experimental characterization. It is shown that panchromatic absorption can be induced via a variety of accessory dyes, regardless of their electron-donating or electron-accepting natures, provided that the frontier molecular orbitals of each dyad component are of appropriate energies for orbital mixing. This result opens up the design space for panchromatic light-harvesters and suggests that panchromatic photosensitizers can be designed on a case by case basis to suit a given application.

Dissertations & Theses @ Yale University.

Books / Online / Dissertations & Theses

English

January 17, 2020

Thesis (Ph.D.)--Yale University, 2019.

Yale University. Chemistry.