Substituent Effects on the Photocatalytic Properties of a Symmetric Covalent Organic Framework

Symmetric covalent organic framework (COF) photocatalysts generally suffer from inefficient charge separation and short-lived photoexcited states. By performing density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we find that partial substitution with one or two substituents (N or NH\begin{document}$_2$\end{document}) in the linkage of the representative symmetric COF (N\begin{document}$_0$\end{document}-COF) gives rise to the separation of charge carriers in the resulting COFs (\emph{i.e}., N\begin{document}$_1$\end{document}-COF, N\begin{document}$_2$\end{document}-COF, (NH\begin{document}$_2$\end{document})\begin{document}$_1$\end{document}-N\begin{document}$_0$\end{document}-COF, and (NH\begin{document}$_2$\end{document})\begin{document}$_2$\end{document}-N\begin{document}$_0$\end{document}-COF). Moreover, we also find that the energy levels of the highest occupied crystal orbital (HOCO) and the lowest unoccupied crystal orbital (LUCO) of the N\begin{document}$_0$\end{document}-COF can shift away from or toward the vacuum level, depending on the electron-withdrawing or electron-donating characters of the substituent. Therefore, we propose that partial substitution with carefully chosen electron-withdrawing or electron-donating substituents in the linkages of symmetric COFs can lead to efficient charge separation as well as appropriate HOCO and LUCO positions of the generated COFs for specific photocatalytic reactions. The proposed rule can be utilized to further boost the photocatalytic performance of many symmetric COFs.