Covalent organic frameworks can be

Covalent organic frameworks can be designed to contain inter- connected chromophore units that absorb visible light. Changing the size of the chromophore or introducing donor–acceptor features into the framework can enhance the absorption at lower energies. In addition, the degree of conjugation between the linked building units and the rotation of bonds within and between the linker molecules play an important role when it comes to absorb- ing visible light. With COFs, it is possible to tune these features and to synthesize strong absorbers not only in the visible but also in the (near) infrared (IR). The photoexcited states in the COF are then able to relax by emission or non-radiative decay. The emis- sion can be of fluorescent or phosphorescent nature, depending on the spin relaxation pathways designed through the choice of linker molecules.