Wavelength-Dependent Excitonic Properties of Covalent Organic Frameworks Explored by Theory and Experiments

Many aspects of the correlation between the physicalstructure,light harvesting, and excitonic properties of covalent organic frameworks(COFs) remain unclear despite being key properties determining theirphotocatalytic function. One area of COF research that could bringclarity is using both electronic structure theory and time-resolvedspectroscopic analysis over a series of systematically varied COFs.Here, we show structure-property relationships between fourimine COFs built from a combination of ditopic and tritopic monomersusing transient absorption spectroscopy together with time-dependentdensity functional theory. We find that monomer selection only moderatelyaffects the charge transfer (CT) behavior of the COFs. Instead, weinfer that imine chemistry profoundly impacts CT by acting as a CTmediator. Moreover, we discover two distinct valence bands arisingfrom varying degrees of locally excited/CT mixing, which is responsiblefor energy-dependent exciton dynamics. Finally, we use theory to hypothesizethat interlayer interactions can modify excitonic properties thatwe correlate with tail states commonly observed but rarely investigatedin COFs. These results reveal that imine chemistry should be recognizedas a very important factor to consider in the development of COF photocatalystsand the correlation of their structural environment with light-harvestingand CT properties that should ultimately determine their photocatalyticfunction.