Polymorphic and mechanochromic luminescence modulation in the highly emissive dicyanodistyrylbenzene crystal: secondary bonding interaction in molecular stacking assembly

Understanding the role of self-assembly and electronic interactions of constituent molecules in determining optoelectronic properties of a molecular solid is a fundamental and essential issue in material science. Particularly, the correlation between the molecular stacking mode and modulated optical properties has barely been established in spite of its scientific and technological importance. Herein, we present dicyanodistyrylbenzene-based highly luminescent crystals which uniquely exhibit polymorphism and mechanochromism. In these materials, various secondary bonding interactions (local dipole interaction, C-H[small pi] interaction, and C-HN hydrogen bond) play key roles in molecular stacking assembly, as well as in polymorphic and mechanochromic behaviors. The two different polymorphic phases of dicyanodistyrylbenzene crystal were correlated to the different modes of local dipole coupling, which caused a substantial alternation of [small pi]-[small pi] overlap and excited state delocalization to give differently colored fluorescence emission. Most uniquely, the phase transformation between those crystalline phases was effected through thermal and mechanical processes. We have comprehensively carried out in-depth and systematic optical, structural, and photophysical investigations to establish unambiguous structure-property relationships.