interactions (e.g., hydrogen bonds, π-π interactions, van der Waals forces) in the 2D plane. [5] Compared with their inorganic counterparts (i.e., 2D atomic crystals, such as graphene, transition metal dichalcogenides, and black phosphorus), the building blocks (i.e., the organic molecules) are soluble, enabling low temperature solution process on plastics for flexible electronics. [6][7][8] More importantly, the organic molecules can be designed with tailored electronic, optical, and magnetic properties, providing an unlimited number of perfect structures for both academic investigations and technical applications. [9,10] 2DMCs can be produced by both the top-down (TD) and the bottom-up (BU) approaches. The TD approach thins bulk organic single crystals down to nanometer scale by mechanical exfoliation. [11] As for inorganic crystals such as highly oriented pyrolytic graphite (HOPG), strong covalent bonds appear within one plane and the interplane interactions are weak van der Waals (vdW) forces. Due to the strong anisotropic interactions, such crystals can be exfoliated layer by layer to produce 2D atomic crystals (e.g., graphene). [12] However, in contrast to layered inorganic crystals, the bondings in organic crystals are typically weakly anisotropic, thus layer-by-layer exfoliation is inherently challenging. As a result, successful cases by the TD approach are scarce and the BU approach is considered more practical. The BU approach takes advantage of the solution processability of organic molecules and produces 2DMCs by solution self-assembly. [13,14] In this strategy, the structure of the organic semiconductor is the critical factor, because the interactions in the solids determine the morphologies and dimensions of the crystals. For example, Jiang and co-workers reported millimeter-sized 2DMCs by simple drop-casting, [15] a method widely used for bulk polycrystalline thin film preparation. Although the exact reason was not clear, the success for the growth of the molecularly thin 2DMC was ascribed to the structure of the semiconductor used, i.e., 1,4-bis((5ʹ-hexyl-2,2ʹ-bithiophen-5-yl)ethynyl)benzene (HTEB). [15] However, up to now, a precise molecular design towards 2DMCs is missing and little is known about the relationship between 2D self-assembly and molecular structure, hindering the practical application of the BU approach. [13] 2D molecular crystals (2DMCs) have attracted considerable attention because of their unique optoelectronic properties and potential applications. Taking advantage of the solution processability of organic semiconductors, solution self-assembly is considered an effective way to grow large-area 2DMCs. However, this route is largely blocked because a precise molecular design towards 2DMCs is missing and little is known about the relationship between 2D solution self-assembly and molecular structure. A "phase separation" molecular design strategy towards 2DMCs is proposed and layer-by-layer growth of millimeter-sized monolayer or few-layer 2DMCs is realized. High-performance orga...
