Molecular stacking mode-directed mechanical compliance and room-temperature phosphorescence achieved by polymorphic 4-cyanobenzamide crystals

Abstract

Organic molecular crystals with both excellent room-temperature phosphorescence (RTP) and adaptive mechanical compliance are of particular importance in flexible photonic materials and devices but remain challenging because of the complexity and sensitivity of molecular stacking modes driven by weak intermolecular interactions. To understand the underlying mechanism for the dual-functional integration on a molecular level, two polymorphs of 4-cyanobenzamide (CN-BZM) with the same space group but quite different mechanical responses and RTP emissions have been identified through well-organizing intermolecular hydrogen-bonding interactions. Naturally, straight acicular Form I, constructed from an interlocked alignment of π-stacked {CN-BZM}₂ columns, exhibits reversibly elastic deformations and unusual time-dependent multicolor afterglow from orange to green over 3 s. However, flaky Form II with a parallel displaced arrangement of the π-stacked {CN-BZM} arrays is brittle, emitting only orange RTP with a long-lived lifetime of up to 148.3 ms. Further structural comparisons and theoretical calculations demonstrated that strong hydrogen-bond coupled {CN-BZM}₂ dimer, co-planar inclined H-aggregation and interlocked π-stacked arrays in the crystal matrix are collaboratively responsible for the elastic flexibility and multicolor afterglow of Form I. Thus, well-organizing intermolecular secondary forces by polymorphic engineering has become one of the powerful methods for elaborating the mechanism of dual-functional organic crystals with adaptive mechanical compliance and persistent luminescence.