Tuning the charge transfer and band shape of donor–acceptor covalent organic frameworks for optoelectronics

Abstract

Donor–acceptor (D–A) covalent organic frameworks (COFs) have gained great attention in the fields of optoelectronics due to their ability to promote charge transfer (CT) and charge transport, leading to long-lived charge carriers and thus better device performances. The modularity of these materials makes them ideal for computational molecular design based on electronic structure calculations, able to predict their excited state and photoconductive properties in silico. However, the characterization of CT transitions in D–A COFs inherits the same difficulties as their analogous molecular systems. This poses a challenge for standard cost-effective electronic structure methods, whose reliability needs to be carefully assessed. Moreover, strong CT is usually associated with localized states, whereas band dispersion is ascribed to delocalized states. Thus, whether the CT and the in-plane photoconductivity can be enhanced simultaneously still needs to be addressed. In this work, we propose 12 chemical modifications for two families of 1,3,6,8-tetraphenylpyrene (Py) 2D-COFs with potential for light-induced CT and highly dispersive bands. Based on DFT/TD-DFT calculations, we characterize the low-lying excited state properties of the 26 monolayers and expose the limitations of the most common approximations to provide reliable data. Ultimately, we analyze the CT versus band shape correlation and identify two possible candidates with improved features for optoelectronic applications.