Tuning optoelectronic and redox properties of covalent organic frameworks for energy applications

Guntermann, Roman Andreas Ferdinand (2025): Tuning optoelectronic and redox properties of covalent organic frameworks for energy applications. Dissertation, LMU München: Faculty of Chemistry and Pharmacy

Abstract

Covalent organic frameworks (COFs) represent a class of crystalline porous polymers that are emerging as promising active materials in catalysis, energy storage, chemical sensing and optoelectronics. The extended frameworks, formed by organic building units connected via strong yet reversible covalent bonds, offer suitable platforms for tuning optical, electronic and structural properties through a pre-designed wide range of functionalities or post-synthetic modifications, while at the same time – and depending on linkage chemistry – providing remarkable stability even under harsh chemical conditions. Moreover, the potential for extended π-conjugation and the possible incorporation of redox-active moieties within the backbone can endow efficient charge transport, charge storage and light absorption. By applying suitable film preparation techniques and leveraging the aforementionend benefits, COFs can be utilized as absorber layer and catalyst for the light-driven photoelectrochemical (PEC) water splitting reaction. Furthermore, the incorporation of redox-active units, such as carbonyl and nitrogen-containing groups, into COFs has been identified as promising approach for electrode materials in supercapacitors and batteries, demonstrating high specific capacitances, excellent rate capabilities, and long-term stability. However, further research is required to fully understand the structure-property relationships and the capabilities of COFs, as these promising semiconductors offer a powerful basis for designing suitable application-oriented materials. A promising structural motif for COFs in the field of energy storage is the redox-active Wurster-type molecular building block based on the twisted tetragonal N,N,N',N'-tetraphenyl-1,4-phenylenediamine (W) node. Condensing the W unit with terephthalaldehyde (TA) or benzodithiophene dialdehyde (BDT) yielded highly crystalline, imine-linked 2D COFs, namely WTA and WBDT, with a dual-pore kagome-type structure, large specific surface areas and mesoporosity. The experimentally determined high conduction band energies made both COFs suitable for oxidative doping, leading to a massive increase of radical cation density. Van der Pauw measurements were used to determine the anisotropic electrical conductivity on oriented COF films, while the isotropic conductivity was determined using COF powder pressed pellets. Different dopants, including F4TCNQ, antimony pentachloride and iodine significantly increased the radical cation density (up to 0.5 radicals per unit cell with F4TCNQ) and achieved long-term stable anisotropic conductivities as high as 3.67 S m−1 in an oriented film. Notably, both COFs demonstrated comparable results in terms of their respective isotropic and anisotropic charge transport in films and pressed pellets. These properties qualify the Wurster-type building units for electrically conducting COFs in electrochemical applications, transitioning from planar systems to twisted geometries. To apply this structural redox-active Wurster-type motif to an energy storage device, three highly crystalline COFs were synthesized by either combining the aldehyde of the N,N,N',N'-tetraphenyl-1,4-phenylenediamine node with the corresponding amine, yielding the WW COF, or a pyrene connecting node, resulting in WPy-I COF and WPy-II COF. These COFs exhibit distinctive topologies and small pore sizes, with the pyrene-based COFs demonstrating a temperature-dependent shift in their intralayer bonding and interlayer stacking arrangement, thereby resulting in the formation of COFs at temperatures as low as 4 °C. Significantly, the COFs were subsequently grown directly on glass and stainless steel mesh (SSM) substrates with various surface coatings, eliminating the need for conductive additives such as carbon to enable the direct electrochemical characterization of the COFs. The electrochemical characterization of the COF-coated SSMs, which served as effective current collectors, revealed a high reversibility of their redox processes and promising specific capacitances, reaching up to 48.9 F g−1 for the WW COF. Based on the electrode construction principle developed, symmetrical supercapacitor devices were fabricated using COF-coated SSMs as electrodes and an ionic liquid electrolyte. These devices demonstrated specific capacitances reaching up to 8.85 F g−1, highlighting the potential of the redox-active Wurster motif for high-perfomance energy storage materials. In line with the aim of developing materials capable of energy transformations, COFs can be adapted for use in energy storage applications and their characteristic properties make them promising candidates for photoelectrochemical (PEC) water splitting. In the fifth part of the thesis, the light-absorbing and charge-transporting Wurster moiety and derivatives were utilized to form a series of isostructural, photoactive COFs. Here, a series of donor–acceptor–donor (D–A–D) Wurster building blocks with varying acceptor strengths were synthesized by incorporating central diverse heteroaromatic acceptors between the triphenylamine donor groups, enabling a precise control over the electronic properties of the building units and, subsequently, of the resulting COFs. As central accepting units, moieties with increasingly electron-deficient character, namely phenyl (P), the isomeric pyrimidine (Pm) and pyridazine (Pd), and [1,2,5]thiadiazolo[3,4-c]pyridine (Tz) were selected. The COFs synthesized with these building blocks exhibited high crystallinity and uniform morphologies, that were even retained in the synthesis of homogeneous and oriented thin films. The introduction of stronger electron-withdrawing acceptors resulted in a significant red-shift in the photoluminescence spectra, demonstrating the successful tailoring of the optical properties and an enhanced photoinduced intramolecular charge transfer, particularly by the strong electron-withdrawing Tz unit. To utilize these tailored structures in energy-conversion applications, the pristine COF films were examined for photoelectrochemical water splitting, revealing a two-fold increase in current density (8.1 μA cm–2 at 0.2 VRHE) obtained for COF films containing the strongest acceptor unit, namely Tz-BTPA W COF. These findings illustrate the potential of molecular design in tailoring the properties of COFs, thereby paving the way for the development of advanced photoactive materials for energy conversion. Since molecular pre-design of the building blocks offers only limited control over the characteristics of the resulting COFs, a way to predict and systematically tune the optical properties of COFs by rational design is of high interest. In the case of inorganic materials, band-gap engineering represents a powerful tool for tailoring the optical and electronic properties through methods such as crystal downsizing or elemental doping, yet in the context of organic solids, systematic band-gap tuning remains a significant challenge. Accordingly, in its final chapter this thesis introduces a novel approach towards controlling the optical properties of COFs by utilizing isomeric thienothiophene-based (TT) building blocks. The combination of these isomeric building blocks with three different tetratopic amines i.e., Wurster, Etta and Pyrene, resulted in three exceptionally crystalline pairs of isomeric COF bulk materials termed WTT, EttaTT and PyTT, where each pair possessed almost identical crystal structure. Furthermore, highly oriented, crystalline thin films of all isomeric COFs were successfully grown on different substrates. Interestingly, all isomeric COF pair films exhibit light absorption within a 300-700 nm spectral range, wherein the respective absorption onsets as well as photoluminescence (PL) emission profiles are significantly affected by the respective isomeric TT structure implemented. By chemically doping a pure isomeric TT COF with its corresponding isomer, the PL emission maxima as well as the respective absorption onset of the COF-blended framework can be continuously and precisely tuned. The extent of π-conjugation within the framework, as revealed by density functional theory (DFT) calculations, is therefore crucial for tuning the optical properties. The achievement of band-gap engineering by altering the isomer blending ratios in the COF synthesis expands the paradigm of COF modulation concepts, allowing for adjusting them precisely for an efficient use in future organic (opto-)electronic devices.

• Item Type: Theses (Dissertation, LMU Munich)
• Subjects: 500 Natural sciences and mathematics; 500 Natural sciences and mathematics > 540 Chemistry and allied sciences
• Faculties: Faculty of Chemistry and Pharmacy
• Language: English
• Date of oral examination: 11. March 2025
• 1. Referee: Bein, Thomas
• Signature of the printed copy: 0001/UMC 31968