Highly Crystalline and Oriented Thin Films of Fully Conjugated 3D-Covalent Organic Frameworks

Fully conjugated 3D covalent organic frameworks (COFs) are a newly emerged class of materials that expands reticular chemistry to extended electron delocalization for optoelectronic applications. To overcome the limitations of sp³-connected 3D frameworks, the pseudo-tetrahedral motif cyclooctatetrathiophene (COTh) has gained attention for forming fully conjugated 3D COFs. We report on a novel COTh building block, featuring functional formyl groups directly attached to the core's conjugated thiophenes. The modulation synthesis approach with mono-functionalized inhibitors enables the formation of COTh-1P COF, which exhibited remarkable crystallinity and permanent porosity. By following this approach and by optimizing the synthesis conditions for the solvothermal growth of thin films, we fabricated the first preferentially oriented conjugated 3D COF films on various substrates without pre-functionalization. With these thin films, optical pump terahertz probe studies allowed us, for the first time with 3D-fully conjugated COFs, to provide insights into the excited state and charge-carrier dynamics of these unique organic frameworks. Low effective masses are discovered for valence and conduction bands by density functional theory simulations. The ability to create crystalline and oriented films of fully π-conjugated 3D COTh-based COFs on non-modified substrates is expected to open the way for integration of such frameworks into diverse optoelectronic applications.