Intramolecular Singlet Fission in Acenes

In 2017, 98 gigawatts of solar capacity were added globally, outpacing new contributions from coal, gas and nuclear plants combined, based on 161 billion dollars of investment. Solar is the leading contributor to the clean energy revolution and continues to grow in market share and drop in price every year as economy of scale advances the technology. Within this market, silicon and cadmium telluride solar cells dominate nearly all of market share, converting roughly 20% of incident solar power into electricity. It is worth noting that the gains from a 1% increase in power conversion efficiency of the typical 20% solar cell to 21% would be measured, annually, in billions of dollars. If the solar cells installed last year had 1% more power conversion efficiency and the power displaced coal power generation, this enhancement in efficiency would now save roughly 8,000,000 pounds of carbon dioxide emission per hour every hour for the ~220,000-hour (~25 year) lifetime of the solar cells. Within this context, enhancing the power conversion efficiency of solar cells is crucial economically and environmentally. Because sunlight is incident on the earth as a broad spectrum of different colors, the energy of the photons spans a wide range. Unfortunately, the spectral range limits power conversion efficiency. For example, solar cells are transparent to photons with insufficient energy, while photons with excess energy relax to the band edge of the solar material, losing the excess energy as heat. This thesis focuses on improving the utilization of high energy photons currently lost to this thermalization process. In Chapter 1, we introduce the photophysical process of singlet exciton fission and give an overview of the field, with a focus on its potential for incorporation into photovoltaic devices. In Chapter 2-8, we discuss our results realizing singlet exciton fission in molecular systems, specifically bipentacenes. This chapter includes the synthesis of these materials, theoretical calculations predicting and rationalizing their photophysical behavior, and the spectroscopic characterization used to demonstrate the singlet fission process. In Chapter 3, we detail a modular synthetic approach to oligomers and even the first polymer of pentacene. We also discuss some basic properties of these materials using techniques such as linear absorption, cyclic voltammetry, and grazing incidence wide angle X-ray scattering spectroscopy. In Chapter 4, we investigate the photophysics of these materials. Photoluminescence upconversion spectroscopy reveals the decay of the singlet exciton on ultrafast timescales, while transient absorption spectroscopy is used to assign the singlet fission timescale, as well as to characterize the triplet absorption spectra. Chapter 5 discusses the synthesis and photophysics of homoconjugated and non-conjugated pentacene dimers, where singlet fission occurs through sigma bonds. Again, transient absorption spectroscopy is crucial to the assignment of the photophysics at play, but continuous wave time resolved electron spin resonance measurements yield additional insights into interaction between the resulting triplet pair excitons. Chapter 6 provides further detail into the formation of strongly exchange coupled triplet pair states. Continuous wave time resolved electron spin resonance spectroscopy is used to determine the quintet character of these states, and pulsed electron spin resonance measurements nutate the spin of these states to confirm this assignment. Chapter 7 provides the first demonstration that singlet exciton fission is also possible in heterodimer systems. Finally, Chapter 8 delves more deeply into the exciton correlations in these materials with a special focus on the pentacene-tetracene dimer system

Preparing Undergraduates for Research Careers: Using Astrobites in the Classroom

Because undergraduate participation in research is a longstanding and increasingly important aspect of the career path for future scientists, students can benefit from additional resources to introduce them to the culture and process of research. We suggest the adoption of the web resource Astrobites as a classroom tool to increase the preparation of undergraduate physics and astronomy students for careers in research. We describe the content and development of the website, discuss previous university courses that have made use of Astrobites, and suggest additional strategies for using Astrobites in the classroom.Comment: Published in the Astronomy Education Revie

Simulating Quantum Dynamics On A Quantum Computer

We present efficient quantum algorithms for simulating time-dependent Hamiltonian evolution of general input states using an oracular model of a quantum computer. Our algorithms use either constant or adaptively chosen time steps and are significant because they are the first to have time-complexities that are comparable to the best known methods for simulating time-independent Hamiltonian evolution, given appropriate smoothness criteria on the Hamiltonian are satisfied. We provide a thorough cost analysis of these algorithms that considers discretizion errors in both the time and the representation of the Hamiltonian. In addition, we provide the first upper bounds for the error in Lie-Trotter-Suzuki approximations to unitary evolution operators, that use adaptively chosen time steps.Comment: Paper modified from previous version to enhance clarity. Comments are welcom

AI hype in the classroom

Artificial intelligence (AI) tools, especially generative tools based on large language models (LLMs), such as ChatGPT, raise critical concerns for academic integrity, for ensuring genuine assessment of student learning, and for equity. Public understanding of these tools is clouded by hype about their capabilities, as they are often treated as knowledgeable and even sentient, and thus suitable for any human task. Of particular concern for instructors is how, and how much, students rely on these tools to complete their coursework. We address some of these issues in our classrooms by reporting on a recent pedagogical initiative within the Department of Linguistics at the University of Toronto during Summer 2023. As part of the initiative, we highlight the crucial role that linguistics can play in these discussions, by shifting the focus to LLMs as objects of study that are directly relevant to the linguistics classroom and to educate students on what linguistic tasks they are and are not good at. We offer strategies and sample assignment questions to help instructors deflate AI hype and facilitate greater AI literacy by demystifying the technology

Model Averaging with AIC Weights for Hypothesis Testing of Hormesis at Low Doses

For many dose–response studies, large samples are not available. Particularly, when the outcome of interest is binary rather than continuous, a large sample size is required to provide evidence for hormesis at low doses. In a small or moderate sample, we can gain statistical power by the use of a parametric model. It is an efficient approach when it is correctly specified, but it can be misleading otherwise. This research is motivated by the fact that data points at high experimental doses have too much contribution in the hypothesis testing when a parametric model is misspecified. In dose–response analyses, to account for model uncertainty and to reduce the impact of model misspecification, averaging multiple models have been widely discussed in the literature. In this article, we propose to average semiparametric models when we test for hormesis at low doses. We show the different characteristics of averaging parametric models and averaging semiparametric models by simulation. We apply the proposed method to real data, and we show that P values from averaged semiparametric models are more credible than P values from averaged parametric methods. When the true dose–response relationship does not follow a parametric assumption, the proposed method can be an alternative robust approach