New approaches to fluoromodifications of electron acceptor molecules for organic photovoltaics
Date
2021
Authors
Brook, Colin P., author
Strauss, Steven H., advisor
Prieto, Amy L., committee member
Kennan, Alan J., committee member
Gelfand, Martin P., committee member
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Abstract
The overall goal of this work is to advance fundamental science and applications of organic electron acceptors based on fluorinated fullerenes and polycyclic aromatic hydrocarbons. The synthetic part of the dissertation focused on the development of new synthetic approaches towards the fluoromodification of large conjugated organic molecules and on the improvements of existing methods for the preparation of high-performing fullerene-based n-type semiconductors. Chapter 1 describes development and application of a new configuration of the gradient-temperature gas-solid reactor for the efficient and high-yielding trifluoromethylation of fullerenes. Significant improvements were achieved in the yields and selectivity of bis-trifluoromethylated C60 and C70 fullerenes: 2-fold and 10-fold yield increase compared to prior state of the art, respectively. An approach to maintain a constant reactive gas pressure in the reactor has been introduced by creating a liquid-gas reservoir of CF3I by submerging the reservoir in one of several low-temperature slush baths available that resulted in improvements in both yields and selectivity for trifluoromethylfullerenes, while also solving a problem of previously unproductive use of the gaseous reagent. Chapter 2 presents the author's work in partnership with the National Renewable Energy Lab (NREL) aimed at investigation of the prominent stabilizing effect of perfluoroalkylated fullerenes on the rate of photobleaching of common high-performance donor-polymers used in OPV devices, compared to the pure polymer films and blends with prototypical non-fluorinated fullerene, PC60BM. It is demonstrated that rationalizing complex photobleaching behaviour ultimately required consideration of the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene blend. The ability of the bis-trifluoromethylfullerene and Fauxhawk fullerene to stabilize certain donor materials against photodegradation, to blend well with fluorinated (and even certain non-fluorinated) polymers, and to quench excited states efficiently was thoroughly studied and correlated with structure-property relationships amongst several polymer donor and fullerene acceptor combinations. Chapter 3 describes a new approach to the direct fluoromodification of polycyclic aromatic hydrocarbons based on replacing commonly used perfluoroalkyl groups (CnF2n+1, or RF) with perfluorobenzyl groups (C6F5CF2, or BnF). For the first time, solution-phase direct perfluorobenzylation of an electron-rich perylene (PERY) and electron-poor perylene diimide (PDI), has been achieved. Five new bay- and peri-substituted compounds of perylene, PERY-(BnF)n, where n = 1, 2 and 3; and three new bay-substituted compounds of perylene diimide, PDI-(BnF)n, where n = 1, 2; were synthesized and fully characterized, revealing that electron withdrawing BnF group is comparable to RF in increasing acceptor strength of new compounds. Chapter 4 deals with a new type of annulative pi-extension (APEX) reaction discovered in this work that occurs via reductive dehydrofluorination/aromatization reactions involving perfluorobenzylated PDI compounds, that afforded fluorinated benzoperylene and coronene-based derivatives with prodigious electron acceptor properties. Another type of annulation leading to the transition-metal free formation of new compounds with all-carbon seven-membered rings across the bay regions of PDIs, consequently forming as rare examples of a newly-recognized fundamental type of conformational isomers, named akamptisomers, is also reported here for the first time. Studies of the likely reaction pathways in both types of reactions and effects of varying reaction parameters on the preferred product formation are presented. Single crystal crystallographic studies of many of the new compounds prepared in this work provide rich and unique data for in-depth analysis of the solid-state packing motifs and influences of the type and position of fluorinated functional groups on the intermolecular interactions, and ultimately, charge transport in these new organic n-type semiconductor materials.
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Subject
fluorine
organic photovoltaics
perylene diimide
fullerene
annulation
perfluorobenzyl