Self‐assembly into Polymorphic 2D Nanosheets with Crystallization‐Induced Emission Enhancement

Comprehensive Summary Organic micro/nanocrystals with polymorphic tailorability and perfect platform to modulate decode the complex relationship among molecular structures, supramolecular/noncovalent interactions, aggregates and exciton behaviors. Herein, we precisely tune the polymorphic two‐dimensional organic nanosheets (2DONs) of cyano‐spirocyclic aromatic hydrocarbon (2'‐CN‐SFX), where the crystal growth processes are dominated by precursor concentration. The low concentration ( ca. 2 mmol/L) affords the rhombus‐like nanosheets with monoclinic P 2 1 / c space group, in which molecules adopt antiparallel and slipped stacking mode. In contrast, the high concentration ( ca. 4 mmol/L) favors herringbone‐like molecular packing pattern and then generates shuttle‐like nanosheets with monoclinic C 2/ c space group. Both above polymorphic nanosheets exhibit unprecedented crystallization‐induced emission enhancement (CIEE) feature that increases the photoluminescence quantum yields (PLQYs) from 16% in solution, 21% in amorphous film to 37%—39% in crystalline states. This result will offer promising opportunities for nanophotonics and organic intelligent semiconductors.