Reversible Chiral Optical Switching Based on Co-Assembled Spiropyran Gels

Recent Advances of Organic Cocrystals in Emerging Cutting-Edge Properties and Applications

Organic cocrystals, representing one type of new functional materials, have gathered significant interest in various engineering areas. Owing to their diverse stacking modes, rich intermolecular interactions and abundant functional components, the physicochemical properties of organic cocrystals can be tailored to meet different requirements and exhibit novel characteristics. The past few years have witnessed the rapid development of organic cocrystals in both fundamental characteristics and various applications. Beyond the typical properties like ambipolarity, emission tuning ability, ferroelectricity, etc. that are previously well demonstrated, many novel, impressive and cutting-edge properties and applications of cocrystals are also emerged and advanced recently. Especially during the nearest five years, photothermal conversion, room-temperature phosphorescence, thermally activated delay fluorescence, circularly polarized luminescence, organic solid-state lasers, near-infrared sensing, photocatalysis, batteries, and stimuli responses have been reported. In this review, these new properties are carefully summarized. Besides, some neoteric architecture and methodologies, such as host-guest structures and machine learning-based screening, are introduced. Finally, the potential future developments and expectations for organic cocrystals are discussed for further investigations on multiple functions and applications.

Excitation-Dependent Circularly Polarized Luminescence Inversion Driven by Dichroic Competition of Achiral Dyes in Cholesteric Liquid Crystals

The development of stimuli-responsive chiral cholesteric liquid crystals (CLCs) materials holds significant potential for achieving three-dimensional (3D) anti-counterfeiting and multi-level information encryption. However, constructing phototunable CLCs systems with easy fabrication and fast response remains a great challenge. Herein, we exploit an excitation-dependent CLCs (ExD-CLCs) material by establishing dynamically photoresponsive dichroic competition between two achiral dyes: a negative dichroic dye (SP-COOH) and a positive dichroic dye (Nile Red, NR) within a CLCs medium. The ExD-CLCs exhibits a negative circularly polarized luminescence (CPL) signal (glum=-0.16) at 625 nm when excited at 365 nm. Remarkably, under excitation at 430 nm, the CPL signal is inverted, and the glum value increases to +0.26. Notably, the helical superstructure and handedness of the ExD-CLCs remain unchanged during this reversal process. The CPL signal reversal is driven by the dichroic competition between the SP-COOH dimer, which displays strong negative dichroism in its open-ring isomer form and silent negative dichroism in its closed-ring isomer form, and the NR dye, which exhibits static positive dichroism. Leveraging these excitation-dependent CPL properties, the quadruplex numerical anti-counterfeiting using ExD-CLCs is achieved.

Disassembly of spherical structures into nanohelices by good solvent dilution

Supramolecular self-assembly of low molecular weight molecules into various organic nanostructures has attracted considerable research interest. However, preparing organic nanostructures through a top-down method, such as the disassembly of one large structure into many smaller nanoscale nanostructures, still remains a big challenge. Here, we make use of anti-solvent method to regulate the hierarchical self-assembly of an achiral C3-symmetric molecule in THF/water to prepare various nanostructures, including spherical structures, nanofibers, nanoribbons and nanotwists. Interestingly, the spherical structures could disassemble into nanohelices through good solvent dilution, providing a nanoscale top-down method to prepare organic nanostructures.

Transparent Organogels as a Medium for the Light-Induced Conversion from Spiropyran to Merocyanine

Low-molecular-weight peptide gelators are a versatile class of compounds able to form gels under a variety of conditions, even via simple ultrasound sonication. In this paper, the ability of Boc-L-Phe-D-Oxd-L-Phe-OBn to gelate three organic solvents (toluene, tert-butyl methyl ether, and ethanol) was evaluated. The rheological behaviour of the materials was assessed via strain sweep analysis, while the fibrous network was analysed via optical microscopy on the wet gels. The gel obtained from toluene is a highly transparent material, and the one from ethanol appears translucent, while the one from tert-butyl methyl ether is opaque. These gels were used to study the reversible light-induced transformation from spyropiran (SP) to merocyanine (MC) and back, as a model system to check the effect of the gel medium onto the rection kinetic. We observed that the solvent used to form the organogels has a crucial effect on the reaction, as gels from aprotic solvents stabilize the SP form, while the ones from protic solvents stabilize the MC form. We thus obtained a solid support to stabilize the two photochromic species just by changing the solvent polarity. Moreover, we could demonstrate that the self-assembled gels do not interfere with the light-driven conversion process, either starting from SP or MC, thus representing a valid and economical photochromic material.

Flexible Azo-Polyimide-Based Smart Surface with Photoregulatable Surface Micropatterns: Toward Rewritable Information Storage and Wrinkle-Free Device Fabrication

Stimulus-sensitive materials are of great fascination in surface and interface science owing to their dynamically tunable surface properties and/or morphologies. Herein, we have synthesized an azobenzene-containing polyimide (azo-PI) with enhanced chain flexibility for the fabrication of photosensitive surface patterns on a film/substrate wrinkle system or wrinkle-free devices. The phototriggered cis-trans isomerization kinetics of azobenzene groups in the novel azo-PI with various chain structures were systematically investigated. On the basis of the characteristics of stress relaxation that azobenzene reversible cis-trans isomerization induces in the wrinkled azo-PI film/substrate system, a variety of rewritable visual surface patterns with high resolution and a long legibility time (>30 days) could be easily constructed via visible-light irradiation, enabling the wrinkled azo-PI surfaces to be used as rewritable information storage media. Meanwhile, because of the visible-light irradiation strategy, these photoresponsive surfaces could find potential application in the fabrication of wrinkle-free flexible devices. This study not only sheds light on the influence of the azo-polymer chain structure on its photoresponsive behavior but also provides a versatile strategy for realizing tailor-made smart surface patterns on multilayer functional devices.

Induced CPL-Active Materials Based on Chiral Supramolecular Co-Assemblies

Circularly polarized luminescence (CPL) has attracted much interest due to its potential applications on chiral photonic techniques and optoelectronic materials science. As known, dissymmetry factor (g_em ) of CPL is one essential factor for evaluating the features of CPL-active materials. Much attention has focused on how to increase the g_em value, which is one of the most important issues for CPL practical applications. Recently, more and more works have demonstrated that chiral supramolecular could provide the significant strategy to improve the g_em value through the orderly helical superstructure of chiral building blocks. Normally, this kind of chiral supramolecular assembly process can be accompanied by chirality transfer and induction mechanism, which can promote the amplification effect on the induced CPL of achiral dyes. In this review, we fully summarized recent advances on the induced CPL-active materials of chiral supramolecular co-assemblies, their applications in circularly polarized organic light-emitting diodes (CP-OLEDs) and current challenges.