Constitutional isomerism of the linkages in donor acceptor covalent organic frameworks and its impact on photocatalysis

When new covalent organic frameworks COFs are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage imine, boroxine, vinyl, etc. between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine linked donor acceptor D A COFs are synthesized, which are different in the orientation of the imine bonds D C N A DCNA and D N C A DNCA . The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H2 evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COF

Protonated Imine Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

Covalent organic frameworks COFs have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction HER from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor acceptor D A type imine linked COFs can produce hydrogen with a rate as high as 20.7 mmol g 1 h 1 under visible light irradiation, upon protonation of their imine linkages. A significant red shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff base moieties in the imine linked COFs, are responsible for the improved photocatalytic performanc

Programmable photocatalytic activity of multivariate covalent organic frameworks used as metallaphotocatalysts

The multivariate approach allows to incorporate several functionalities into a single covalent organic framework (COF) and consequently allows the construction of bifunctional materials for cooperative catalysis. The well-defined structure of such multivariate COFs is furthermore ideally suited for structure-activity relationship studies. We report a series of multivariate COFs that contain acridine- and 2,2’-bipyridine linkers connected through 1,3,5-benzenetrialdehyde derivatives. The acridine motif is responsible for broad light absorption, while the bipyridine unit enables complexation of nickel catalysts. These features enable usage of the framework materials as catalysts for light-mediated carbon–heteroatom cross-couplings. Variation of the node units shows that the catalytic activity correlates to the keto-enamine tautomer isomerism. This allows switching between high charge-carrier mobility and persistent, localized charge-separated species depending on the nodes, a tool to tailor the materials for specific reactions. Moreover, nickel-loaded COFs are recyclable and catalyze cross-couplings even using red light irradiation

Einfluss von Verzahnungsabweichungen auf das Passungs- und Laufverhalten von Zahnwellen-Verbindungen und Zahnkupplungen. Funktionsgerechtes Toleranzsystem und betriebsnahe Mess- und Lehrenverfahren Wissenschaftliche Zuarbeit zur Normung von Toleranzen und Messverfahren auf europaeischer Basis. Abschlussbericht

SIGLEAvailable from TIB Hannover: F98B1814 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekStiftung Volkswagenwerk, Hannover (Germany)DEGerman