Authors: Thomas Bein
Source: ECS Meeting Abstracts (article, 2025-07-11)
Field: Materials Science — Polymers and Plastics

Impact of Structure on Optoelectronic Processes in Covalent Organic Frameworks Heteroaromatic molecular building blocks can be assembled in the highly ordered spatial environment of covalent organic frameworks (COFs), forming two-dimensional (2D) and 3D crystalline architectures. For example, photoactive molecular moieties such as porphyrins and other chromophores can be spatially integrated into their crystalline lattice, allowing us to create models for organic bulk heterojunctions, chemical sensors and porous electrodes for photoelectrochemical systems. Here, we will discuss different strategies aimed at creating electroactive networks capable of light-induced and electrochemical charge transfer. In earlier work, we have developed COF-based heterojunctions containing stacked thienothiophene-based building blocks acting as electron donors with a 3 nm open pore system, which show light-induced charge transfer to an intercalated fullerene acceptor phase.[1] We now create interpenetrated donor-acceptor COF phases with novel dibenzochrysene-based building blocks that can be viewed as constrained propeller-shaped tetraphenylethylene with reduced curvature engaging in very tight π-stacking.[2] Thienothiophene- (TT) and benzodithiophene-2,6-dicarboxaldehyde based 2D kagome COFs were synthesized in situ with a fullerene derivative to create interpenetrated electron-donor/acceptor host-guest systems showing efficient charge transfer within ps from the COF to the fullerene guest. Contrasting the above approach, one can design COF integrated heterojunctions consisting of alternating columns of stacked donor and acceptor molecules, promoting the photo-induced generation of mobile charge carriers inside the COF network.[3] Synthetic efforts have led to several COFs integrating extended chromophores capable of efficient harvesting of visible and near infrared light, for example.[4] Notably, heterocyclic regioisomers that can be embedded in the same COF crystal structure allow for fine-tuning of optical absorption and luminescence.[5] Extending thin film growth methodology to create a solvent-stable oriented 2D COF photoabsorber structure, COF films can serve in photoelectrochemical water splitting systems.[6] The detailed mechanism of excited state dynamics in light-harvesting conjugated COFs has been revealed by means of transient absorption spectroscopy,[7] while ongoing work establishes efficient excited state diffusion – even across grain boundaries – in 2D COF thin films. Many optoelectronic applications of COFs depend on significant electrical conductivity. Here, Wurster-type structural motifs are attractive building blocks for imparting high conductivity in the corresponding COFs,[8] which feature tunable optical properties upon integrating donor-acceptor moieties. COF films can also act as ultrafast solvatochromic chemical sensors,[9] as photodetectors,[10] and show very efficient electrochromic response.[11] While guest-responsive (breathing) MOFs have been studied for some time, to date few guest-responsive COF structures have been described. We have now developed dynamic two-dimensional COFs that can open and close their pores upon uptake or removal of guests while fully retaining their crystalline long-range order.[12] Here, a wine rack design based on rigid, π-stacked columns of perylene diimides (PDIs) interconnected by non-stacked, flexible bridges shows stepwise phase transformations between contracted pore and open pore forms with up to 40% increase in unit cell volume. By reversibly tuning π-interactions via guest sorption, we modulate excitonic coupling within the COFs, where the PDI moieties can be switched from excimer-forming H-aggregates to null-aggregates with monomer-like absorption and emission characteristics. These findings open new pathways towards designing stimuli-responsive optical, electronic, or spintronic materials. Ongoing work focuses on the design of ultra-large pore donor-acceptor COFs with extended light-harvesting abilities and optimized charge separation, illustrating their intriguing structural diversity leading to enhanced optoelectronic functionality.[13] References Dogru et al., Chem . Int. Ed. 2013, 52 , 2920. Xue et al., to be submitted Calik et al., J. Am. Chem. Soc . 2014, 136 , 17802. Keller et al., J. Am. Chem. Soc . 2017, 139 , 8194. Guntermann et al., 2024, in revision. Sick et al., J. Am. Chem. Soc . 2018, 140 , 2085. Jakowetz et al., J. Am. Chem. Soc . 2019, 141 , 11565. Rotter et al., Chem. Sci . 2020, 11 , 12843. Ascherl et al., Nature Commun . 2018, 9 , 3802. Bag et al., J. Am. Chem. Soc. 2023, 145 , 1649. Muggli et al., ACS Nanosci. Au 2023, 3 , 153. Auras et al., Nature Chem. 2024 , 16, 1373. Blätte, Ortmann, Bein, J. Am. Chem. Soc. , 2024, 146, 32161. Covalent bond ; Materials science ; Optoelectronics ; Nanotechnology ; Chemistry ; Organic chemistry Conducting polymers and applications ; Advanced Memory and Neural Computing ; Gas Sensing Nanomaterials and Sensors