Two-dimensional covalent organic frameworks (2D COFs) are crystalline porous polymers with highly tunable structural and electronic properties. Although single crystals of these materials are highly desirable for fundamental research and potential applications, their synthesis has so far been limited to only a few examples. We have developed a high-temperature double-modulator synthetic strategy that enables the growth of single crystals from fully dissolved precursors. We applied this approach to generate a series of perylene diimide (PDI)-based 2D COFs. These materials crystallize as platelets with lateral dimensions reaching up to 50 µm for the biphenyl-linked PDI(Me)₈-2P COF, providing a suitable platform for studying electronic processes via micro-spectroscopy. Photoluminescence (PL) measurements revealed ultrafast monomer-like emission together with a slower excimer-related component. The availability of COF single crystals further enabled polarization-dependent measurements, which revealed that the fast PL component is linearly polarized due to the parallel orientation of the PDI chromophores in the frameworks. These findings highlight the importance of COF single crystals for elucidating structure—photophysical property relationships of these intriguing materials.