Less is More: Tunable Polymorphs with Packing-Dependent Deep-Red to Near-Infrared Emission Based on a Geometrically Simple Molecule

Abstract

Deep-red (DR) and near-infrared (NIR) luminogens have witnessed powerful applications in various aspects in recent decades. However, their inevitably complicated structures not only bring tedious and time-consuming synthesis, but also lead to inherent difficult in crystallization which hinder the understanding of structure-emission relationship. Polymorphism represents an ingenious strategy for revealing unambiguous structure-emission relationships. Therefore, designing simple molecules to construct DR and NIR polymorphic emission is challenging yet highly desirable. Herein, multiple DR and NIR emission ranges from 624 to 742 nm is achieved in polymorphisms of an extremely simple π-conjugated luminogen. Multiple packing modes are simultaneously integrated into this compound. Its unique packing-dependent emission facilitates the establishment of clear packing-emission relationship. Increased intermolecular exciton coupling between adjacent molecules leads to gradually red-shifted emission from orange to DR and finally to NIR. The aggregation-induced emission feature of the molecule renders the bulk crystals with visible crystal-to-crystal polymorphic transformation under external stimuli. This work not only breaks the limitations of the research of organic polymorphism materials with NIR emission, but also provides a reliable strategy to reveal the influence of molecular packing on NIR emission.

Recent advances in tunable solid-state emission based on α-cyanodiarylethenes: from molecular packing regulation to functional development

“…4(a) and (b)). 49 The fluorescence color of these crystals ranged from orange to red, and finally to near-infrared. It was noted that the maximum twist angle of the four single crystals is only 4.141, and the minimum is 0.031 (Fig.…”

Molecular and Aggregate Synergistic Engineering of Aggregation-Induced Emission Luminogens to Manipulate Optical/Electronic Properties for Efficient and Diversified Functions

“…Polymorphic crystals of π-conjugated fluorophores with well-defined aggregation structures furnish an ideal visualized model to clarify molecular conformation and intermolecular interactions and thus precisely reveal their correlation with the optical properties of aggregates. − Although a number of organic polymorphs have been reported, development of such systems still remains a challenge because of the lack of clear and effective mechanisms that allow organic molecules to aggregate to form specific polymorphs . Besides, in most of the reported cases fluorophores show polymorphism in the blue to orange region, while those with efficient near-infrared (NIR) (650–900 nm) emission are rarely reported due to the inherent difficulty in molecular design and crystallization of their complex structures. − For example, NIR emission could be achieved by introducing strong D–A strength or/and extension of π-conjugation, while the organic materials with these features usually suffer from the ACQ effect, leading to decreased or quenched emission in the aggregate state . Thus, endowing molecules with AIE features is highly desirable for developing NIR fluorescent polymorphic materials.…”

Diphenylamine substituted 5,6,12,13-tetraazaperopyrene based polymorphic microcrystals versatile in multi-directional isotropic and anisotropic photon transport

“…[24][25][26][27][28] Notably, polymorphism with two or more crystalline phases stemming from one organic molecule can greatly enrich the types of micro/nano-crystals for excellent photonic properties; meanwhile it provides a sophisticated platform to shed more light on the relationship between molecular stacking patterns and solid-state optical properties. [29][30][31][32][33][34][35] Apart from the polymorphic control of multimodal and low loss optical waveguides, 2D crystals capable of anisotropic propagation of photons, with multiple channels showing different polarization and emission properties, are also in high demand due to their considerable potential in optical planar diodes, logic devices, etc. 2,[36][37][38][39][40][41][42] Although it has been recognized that the fabrication of 2D organic crystals with anisotropic features is pertinent to the molecular-packing modes with controlled transition dipole of molecules in crystals, most reported 2D organic crystals exhibited isotropic photon propagation behaviors.…”

“…24–28 Notably, polymorphism with two or more crystalline phases stemming from one organic molecule can greatly enrich the types of micro/nano-crystals for excellent photonic properties; meanwhile it provides a sophisticated platform to shed more light on the relationship between molecular stacking patterns and solid-state optical properties. 29–35…”