Chemical modifications of covalent organic frameworks for tunable optoelectronic properties, surface chemistry, and photocatalytic performance

Paliušytė, Klaudija (2026): Chemical modifications of covalent organic frameworks for tunable optoelectronic properties, surface chemistry, and photocatalytic performance. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Covalent organic frameworks (COFs) are crystalline, porous polymers constructed from organic building blocks linked by covalent bonds, providing remarkable stability, low density, and tunable porosity. Initially introduced in 2005 by Yaghi and colleagues, COFs merge structural accuracy with chemical adaptability, allowing for the customization of pore dimensions, shapes, and electronic characteristics for uses in gas storage, catalysis, sensing, separation, and drug delivery. A range of bonding chemistries, including boronate esters, imines, β-ketoenamines, hydrazones, polyimides, amides, and triazines, permit meticulous control over stability and functionality, while synthetic techniques such as solvothermal, microwave-assisted, ionothermal, and mechanochemical methods enhance their availability. Among these, imine-linked COFs, formed via Schiff base condensation, continue to attract significant attention due to their structural flexibility and straightforward synthesis. Promising strategies to tailor the structure, properties, and performance of COFs include pre-synthetic, post-synthetic and hybrid modifications. Pre-synthetic modification involves altering the monomers or adjusting reaction conditions prior to framework formation, enabling control over topology, porosity, and stability. Post-synthetic modification enables the incorporation of new chemical functionalities into pre-formed COFs, offering access to features that are difficult or impossible to achieve through direct synthesis. Hybrid modifications involve one-pot approaches in which guest components are incorporated into the COF structure during synthesis, enabling simultaneous tuning of framework composition and functionality. This thesis presents a balanced investigation of pre-synthetic, post-synthetic and hybrid modification strategies. The first part of the thesis (Chapters 3 and 4) investigates the pre-synthetic functionalization of N,N,N′,N′-tetraphenyl-1,4-phenylen (Wurster, W)-anthracene(A)-based COFs, denoted as W-A-X (X = H, Cl, Br, I). The second part (Chapters 5) explores post-synthetic modifications of a thiophene-based COF, constructed from benzo[1,2- b:4,5-b′]-dithiophene-2,6-dicarboxaldehyde (BDT) and 4-fold amine functionalized tetraphenylethylene (1,1′,2,2′-tetra-p-aminophenylethylene) (ETTA), referred to as BDT-ETTA, aimed at enhancing its catalytic performance. The third part (Chapter 6) explores hybrid modification of the BDT-ETTA COF, aimed at controlling its charge- and energy-transfer behavior. Following this conceptual line of research, Chapter 3 focuses on the effect of pre-synthetic single-atom functionalization of W-A-X COFs. Anthracene is an electron-rich building block with versatile photophysical characteristics, making it an ideal candidate for the construction of optoelectronically active COFs. Introducing a single substituent at the 2-position of the anthracene core prior to framework formation yielded a series of highly crystalline COFs, enabling systematic investigations of how subtle chemical modifications influence crystallinity, domain size, and optoelectronic behavior. Comprehensive experimental characterization, complemented by theoretical modeling, revealed clear correlations between substituent identity, framework morphology, and optical response, demonstrating the potential of pre-synthetic single-atom modification as a strategy for tuning COF properties. These findings provide a fundamental understanding of structure-property relationships in functional COFs and offer a platform for their rational design toward applications in optoelectronics and photocatalysis. Chapter 4 builds on the previous work and examines how pre-synthetic halogen functionalization affects host-guest interactions in W-A-X COFs. Single-atom modifications on the peripheral anthracene rings allow subtle control over structural features and local electrostatics, influencing CO2 adsorption behavior. Experimental CO2 sorption measurements showed that all halogen-functionalized COFs exhibit significantly higher CO2 uptake and increased isosteric heats of adsorption compared to the non-functionalized W-A-H COF. Computational modeling identified two main adsorption regions: the imine linkage and the trigonal pores between anthracene units. Halogenation reduces nitrogen basicity and porosity but introduces σ-hole regions that enhance CO2 binding, with interaction strength increasing in the order I > Br > Cl. The combination of favorable electronic interactions and framework crystallinity results in W-A-Br achieving the highest CO2 uptake. These findings highlight how single-atom pre-synthetic functionalization can tailor electrostatics over the polymeric network, fine-tune adsorption sites and optimize COFs for host-guest interactions. Chapter 5 presents a post-synthetic modification strategy of the COF in which linkage conversion is employed to tune the interfacial and photocatalytic properties of the (BDT-ETTA). Conversion of imine to amide linkages alters the COF surface charge due to their distinct protonation behaviors, as confirmed by zeta potential measurements, which in turn directs in-situ Pt photodeposition. The imine-linked COF yields uniformly small Pt nanoparticles, while the amide-linked COF forms larger Pt particles and acts as an electron-transport antenna, enhancing charge separation, proton adsorption, and overall hydrogen evolution, achieving a 300 % increase in photocatalytic activity compared to the imine form. Section 5.6 in this chapter describes complementary mechanistic studies on Pt photodeposition in the imine-based COF BDT-ETTA. Transmission electron microscopy images confirm the homogeneous distribution of Pt particles within the polymeric matrix and reveal that the deposited particles replicate the COF structure, evidencing their incorporation into the COF pores. Experimental observations suggest that Pt formation within the COF pores is a multistep process requiring both photoexcitation and a sacrificial electron donor, with sulfur atoms in the BDT unit serving as active sites for hole trapping and oxidation, and the imine bonds coordinating and reducing the Pt precursor. Although these studies were preliminary, they provided valuable mechanistic insight and informed the subsequent investigation of linkage conversion as a strategy to modulate COF photocatalytic behavior. These results highlight that post-synthetic modulation of surface charge and a mechanistic understanding of active sites together offer a powerful approach to control interfacial processes and expand the functional versatility of COFs in photocatalysis. Finally, Chapter 6 explores a hybrid modification strategy of the BDT-ETTA COF, focusing on doping with carbon dots (CDs). Microwave-derived, photoluminescent, metal-free CDs were incorporated into the COF in a one-pot synthesis, enabling precise control over the COF:CDs ratio. Structural and optoelectronic characterization revealed that the CD loading governs the interaction mechanism between the two components. At low loadings (up to 20 wt%), strong interfacial contact promotes charge transfer, evident from photoluminescence quenching of both COF and CDs and accelerated exciton decay kinetics. At higher loadings (30 wt% and above), reduced contact favors Förster resonance energy transfer from CDs to COF, enhancing COF photoluminescence. The study shows how controlled incorporation of CDs allows tuning of charge- and energy-transfer pathways. These findings provide fundamental insights into structure-property relationships in the BDT-ETTA COF/CDs composites, informing the design of multifunctional materials for photocatalysis, optoelectronics, and sensing. In conclusion, this thesis has been focused on the functionalization strategies of imine-linked COFs employing pre-synthetic, post-synthetic and hybrid modification strategies to tailor structural, optoelectronic, and interfacial properties for enhanced gas sorption and photocatalysis. Single-atom halogenation, linkage conversion, Pt and carbon dot incorporation were employed to systematically investigate and control local electrostatics, adsorption behavior, photodeposition mechanisms, and charge- and energy-transfer pathways. These studies provide fundamental insights into structure-property relationships in COFs and demonstrate versatile strategies for tuning their functionality, highlighting their potential for applications in gas capture, photocatalysis, and optoelectronic applications.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: covalent organic frameworks; chemical modifications; photocatalysis; optoelectronics
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 23. Februar 2026
• 1. Bericht­erstatter:in: Bein, Thomas