ORCID
https://orcid.org/0000-0003-0563-0127
Date of Award
Summer 2024
Language
English
Embargo Period
9-9-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
College/School/Department
Department of Chemistry
Program
Chemistry
First Advisor
Marina Petrukhina
Committee Members
Renana Gershoni-Poranne, Evgeny Dikarev, Michael Yeung
Keywords
Molecular Nanographene, Chemical Reduction, X-ray diffraction, Alkali Metal Intercalation, Cyclooctatetraene, Pyracylene
Subject Categories
Chemistry | Inorganic Chemistry
Abstract
The promising lithium storage capacity and fast charge transport rate make graphene a great candidate for energy storage applications. However, the harsh synthetic conditions lead to uneven size distribution and poor solubility, posing limitations on further applications and modifications of graphene and its derivatives. Molecular nanographenes, as graphene cutouts, possess outstanding chemical and physical properties stemming from their parent graphene but also demonstrate such unique features as controllable synthetic conditions, defined molecular structures, uniform size, and relatively good solubility. The improved solubility enables the use of solution chemistry methods for design modifications of molecular nanographenes and for systematical investigation of their electron transfer processes. To understand the charge transfer and electron distribution of molecular nanographenes as potential anode materials, their redox properties have to be disclosed. However, the most common assessment, electrochemical reduction, is highly dependent on the measurement conditions, generally underestimating the charge accepting ability of nanographenes.
In our laboratory, the chemical reduction with alkali metals has been broadly utilized to unravel redox properties of molecular nanocarbons, revealing their reduction limits, alkali metal binding trends, product stability and reactivity, as well solid-state packing and supramolecular assembly. In this work, we continued this investigation and focused on exploring the chemical reduction behavior of topologically different molecular nanographenes with negative and positive types of curvature. We aimed at comprehensive studies of structural and electronic responses of selected nanographenes to multi-electron addition.
In Chapter 2, we systematically investigated for the first time the chemical reduction behavior of a series of cyclooctatetraene (COT) derivatives with gradually expanded π-surfaces and different core conjugation/functionalization. We revealed that the central COT core in dibenzo[a,e]cyclooctatetraene (DBCOT) follows the same tub-to-planar conversion upon two-fold reduction as the parent COT; however, the fused benzene rings offer additional binding sites and enrich metal coordination chemistry of the doubly-reduced DBCOT. The two-fold reduction of π-expanded tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT) and its highly flexible derivative, octaphenyltetrabenzo[a,c,e,g]cyclooctatetraene (OPTBCOT), reveals a reversible core rearrangement accompanied by the formation of a new C–C bond and conversion of the COT ring to two fused five-membered rings. Remarkably, in contrast to DBCOT and TBCOT serving as two-electron acceptors, OPTBCOT can uptake four electrons to afford the tetra-reduced product with a contorted eight-membered core and unique alkali metal intercalation pattern. Lastly, a fully conjugated COT derivative, octabenzo[8]circulene (OB8C), exhibits exceptional multi-electron accepting properties and can undergo up to six-fold reduction reversibly. Importantly, its polymerized form POB8C functions as an anode material in lithium-ion batteries, demonstrating a maximum discharge capacity better than the theoretical value of graphene.
In Chapter 3, the chemical reduction of two pyracylene nanographene hybrids has been investigated with alkali metals for the first time. Particularly, the chemical reduction of an asymmetric pyracylene hybrid (TPP), fused by one hexa-peri-hexabenzocoronene (HBC) unit and one hexaphenylbenzene (HPB) fragment, showed the core flexibility and the charge-dependent curvature changes, enabling its potential application as a molecular switch. In contrast, the symmetrical pyracylene hybrid (HPH) with two fused HBC units exhibits advanced electron accepting properties and can reversibly uptake up to six electrons, as clearly demonstrated by stepwise UV-vis absorption changes. Moreover, the diamagnetic products are all detected by 1H NMR spectroscopy, while the paramagnetic radical products are characterized with EPR spectroscopy. The stepwise reduced products were isolated as single crystalline materials and provided experimental evidence for a remarkable core conformational change. Specifically, the outcomes of one- and two-electron acquisition include the curvature reduction of HPH and better π-conjugation over its curved carbon backbone. In contrast, the higher reduction to the tetra- and penta-reduced states leads to a significant geometry change of the HPH core from the boat conformation to a recliner-chair shaped structure.
In summary, we demonstrated the outstanding redox properties of selected molecular nanographenes with different shapes and curvatures. The resulting products were isolated as single crystalline materials and fully characterized with X-ray crystallographic and spectroscopic tools. This revealed unique structural and geometrical rearrangement pathways for both COT and pyracylene derivatives, along with the exceptional alkali metal binding patterns that are framework- and charge-dependent. DFT calculations offered in-depth investigation of both charge acquisition and core deformation processes. The observed multi-electron uptake and fully reversible structural rearrangement facilitate the development of practical applications of novel molecular nanographenes for charge transfer and energy storage.
License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Recommended Citation
Zhu, Yikun, "Chemical Reduction of Curved Molecular Nanographenes: Metal Intercalation and Core Transformation" (2024). Electronic Theses & Dissertations (2024 - present). 41.
https://scholarsarchive.library.albany.edu/etd/41