Construction Of COF Crystals For Efficient Two-photon Absorption

Linking molecules into crystalline structures such as metal organic frameworks(MOFs)and covalent organic frameworks(COFs)represents a fast-developing branch in material science.COFs were constructed by covalent linkage of molecules and were useful in catalysis,molecular recognition and separation where the molecular feature was commonly accessed.On the other hand,the crystalline feature,involving the longrange ordering of molecular building blocks,was much less explored.This is critical,however,for their optical,electrical and magnetic properties.In this study,we show that,for the first time,the formation of long-range ordering coherent domain among two dimensional(2D)COFs can lead to drastic promotion in two-photon absorption performance,where both the molecular feature and crystalline feature of COFs are fully accessed.The 2PA performance of molecule-based materials is determined by their transition dipole moment,which is influenced by several key factors including the conformation of molecule,size of coherent domain,and the type of motifs in the structure.Although all three factors can be tuned in traditional polymers,the orientation of the compositional chromophore as building blocks are usually not well-aligned in the polymer structure,therefore the accumulation of transition dipole moment is restricted and the coherent domain size is limited due to the leak of long-range ordering.In contrast,the chromophores as molecular building blocks in COFs are orderly arranged across the entire crystal,forming large coherent domain size covering Avogadro number of chromophores.This combined with functional groups that can be varied in the design of different type of motifs,and further restriction of their conformation dynamics by the layered packing in the crystal lattice,leads to unprecedented enhancement in the transition dipole moment.Six tetra-topic molecular building blocks of different length and carrying different functional groups(L-H,L-Ph,L-2Me O,L-2F,L-4F and L-BT)were used to construct six 2D COFs with the identical topology,but different structural motifs,D-π-D for COF-601,COF-602,D-π-D-π-D for COF-603,D-π-A-π-D for COF-604,COF-605,and COF-606,respectively(Figure 2a,D represents electron donor while A represent electron acceptor).The crystallization of all these COFs leads to the optimization of their 2PA performance.Furthermore,the impact of coherent domain size was revealed experimentally,for the first time,by the synthesis of COFs with crystal size ranging from 100 nm to 1 μm.Drastic enhancement of 2PA performance was found as the crystal size increased,demonstrating the critical contribution from the crystalline feature of COFs.A micrometer-sized 2D imine-COF single crystal,COF-606-1μm,was synthesized for the first time and the unit cell of COF-606 was determined by 3D electron diffraction tomography(3D-EDT)unprecedentedly.The 2D COFs described here are distinguished from other COFs in that the packing of the molecular constituents between the layers are resolved experimentally for the first time and found to be serrated,which is distinctly different from the eclipsed and staggered scenarios commonly applied in structure simulations and refinements.The resolution of our COF crystals allowed us to uncover and assess the qualitative and quantitative impact of their structural features(geometry,spatial orientation,and packing)on their nonlinear optical performance as exemplified by 2PA.Such information is difficult to obtain from polymers and molecular crystals,but we find that it is extremely valuable for understanding the optical behavior.These structural features provide an extra handle in the design of organic materials that goes beyond the traditional tuning of functionality.