(Invited) Optoelectronic Processes in Covalent Organic Frameworks

Abstract

New opportunities emerge by spatially integrating photoactive molecular building blocks into the crystalline lattice of covalent organic frameworks (COFs), thus creating models for organic bulk heterojunctions and porous electrodes for photoelectrochemical systems. In this presentation, we will address means of controlling the morphology and packing order of COFs in thin films (1) and with spatially locked-in building blocks.(2) Regarding the latter, the design of well-defined periodic docking sites enables us to achieve remarkably high crystallinity with several multidentate building blocks and a series of linear bridging units. We will discuss different strategies aimed at creating electroactive networks capable of light-induced charge transfer. For example, we have developed a COF containing stacked thienothiophene-based building blocks acting as electron donors with a 3 nm open pore system, which showed light-induced charge transfer to an intercalated fullerene acceptor phase.(3) Contrasting this approach, we have designed a COF integrated heterojunction consisting of alternating columns of stacked donor and acceptor molecules, promoting the photo-induced generation of mobile charge carriers inside the COF network.(4) Moreover, additional synthetic efforts have led to several COFs integrating extended chromophores capable of efficient harvesting of visible light, for example (5). Extending newly developed thin film growth methodology to a solvent-stable oriented 2D COF photoabsorber structure, we have recently established the capability of COF films to serve in photoelectrochemical water splitting systems.(6) The great structural diversity and morphological precision that can be achieved with COFs make these materials excellent model systems for organic optoelectronic systems. (1) D. D. Medina, J. M. Rotter, Y. H. Hu, M. Dogru, V. Werner, F. Auras, J. T. Markiewicz, P. Knochel, T. Bein, J. Am. Chem. Soc. 2015, 137, 1016. (2) L. Ascherl, T. Sick, J. T. Margraf, S. H. Lapidus, M. Calik, C. Hettstedt, K. Karaghiosoff, M. Döblinger, T. Clark, K. W. Chapman, F. Auras, T. Bein, Nature Chem. 2016, 8, 310. (3) M. Dogru, M. Handloser, F. Auras, T. Kunz, D. Medina, A. Hartschuh, P. Knochel, T. Bein, Angew. Chem. Int. Ed. 2013, 52, 2920. (4) M. Calik, F. Auras, L. M. Salonen, K. Bader, I. Grill, M. Handloser, D. D. Medina, M. Dogru, F. Lobermann, D. Trauner, A. Hartschuh, T. Bein, J. Am. Chem. Soc. 2014, 136, 17802. (5) N. Keller, D. Bessinger, S. Reuter, M. Calik, L. Ascherl, F. C. Hanusch, F. Auras, T. Bein, J. Am. Chem. Soc. 2017, 139, 8194. (6) T. Sick, A. G. Hufnagel, J. Kampmann, I. Kondofersky, M. Calik, J. M. Rotter, A. Evans, M. Döblinger, S. Herbert, K. Peters, D. Böhm, P. Knochel, D. D. Medina, D. Fattakhova-Rohlfing, T. Bein, J. Am. Chem. Soc. 2018, 140, 2085.