Abstract

Covalent organic frameworks (COFs) formed by connecting multidentate organic building blocks through covalent bonds provide a platform for designing multifunctional porous materials with atomic precision. In particular, the integration of π-conjugated building blocks, such as thiophenes or porphyrins, into the networks enables the formation of structurally defined organic semiconductors.

We have developed a synthetic concept to allow consecutive COF sheets to lock in position during crystal growth, and thus minimize the occurrence of stacking faults and dislocations.[1] Hereby, the three-dimensional conformation of propeller-shaped molecular building units was used to generate well-defined periodic docking sites, which guided the attachment of successive building blocks that, in turn, promoted long-range order during COF formation. This approach enables us to achieve a very high crystallinity for a series of COFs that comprise tri- and tetradentate central building blocks.

The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration (Figure 1).[2] Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor−acceptor heterojunctions, which can drive the photogeneration of free charge carriers. We have constructed a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer.

We anticipate these concepts to be applicable to a broad variety of functional building blocks and to promote the establishment of COFs in catalysis and organic electronics.

[1] 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, DOI 10 .1038/NCHEM.2444. [2] M. Calik, F. Auras, L. M. Salonen, K. Bader, I. Grill, M. Handloser, D. D. Medina, M. Dogru, F. Löbermann, D. Trauner, A. Hartschuh, T. Bein, J. Am. Chem. Soc. 2014, 136, 17802.