The design and synthesis of crystalline extended organic structures in which the building blocks are linked together by strong covalent bonds is an undeveloped area of research. It is widely believed that the required microscopic reversibility for crystallization of linked organic molecules into such solids is difficult if not impossible to achieve ('the crystallization problem').

The lack of crystalline cross-linked polymers is often cited as evidence in support of this view. Recently, we embarked on a program aimed at challenging this notion by constructing porous crystalline covalent organic frameworks (COFs) solely from light elements (H, B, C, N, and O) that are known to form strong covalent bonds in well established and useful materials such as diamond, graphite, and boron nitride.

The successful realization of COF materials through molecular building blocks would provide the first covalent frameworks which can be functionalized into light-weight materials optimized for gas storage, photonic, and catalytic applications. Here, we report a general design strategy and its implementation for synthesis and crystallization of micro- and mesoporous crystalline COFs. These materials have rigid structures, exceptional thermal stabilities (up to 600 °C), low densities and exhibit permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates. Furthermore, the synthesis of the first two members, COF-1 and COF-5, employs a simple 'one pot' procedure using mild reaction conditions that are efficient and high yielding.