Synthesis, structure and mechanofluorochromic properties of phenothiazine-S-oxide and phenothiazine-S,S-dioxide derivatives

In this work, two novel luminescent molecules containing distorted phenothiazine-S-oxide and phenothiazine-S,S dioxide skeletons were synthesized by oxidation reactions using different oxidants (m-chloroperoxybenzoic acid, acetic acid /hydrogen peroxide). The target compounds were all confirmed by ¹H NMR, ¹³C NMR and EI-MS. Combined with the results of UV-vis absorption spectra and fluorescence emission spectra, we found that the different oxidation states of S-atom, from sulfide (+2) to sulfoxide (+4) and sulfone (+6), led to the blue, yellow-green and yellowish fluorescence of these compounds in the solid states. Subsequent studies showed that the molecule containing the phenothiazine-S-oxide skeleton exhibited obvious solvatochromism, and the increase of solvent polarity induced a red-shift in the emission wavelength. Moreover, this molecule also exhibited a rare self-recovery mechanochromatic behavior. In addition, these properties were further confirmed by theoretical calculations and X-ray single-crystal diffraction analysis.

Facile Synthesis of Tetraphenylethene (TPE)-Based Fluorophores Derived by π-Extended Systems: Opposite Mechanofluorochromism, Anti-Counterfeiting and Bioimaging

Although remarkable progresses are achieved in the design and development of the mono-shift in photoluminescence for mechanofluorochromic materials, it is still a severe challenge to explore the opposite mechanofluorochromic materials with both blue- and red-shifted photoluminescence. Herein, two unprecedented 4,5-bis(TPE)-1H-imidazole fused pyridine or quinoline-based fluorophores X-1 and X-2 were designed and synthesized, and X-1 and X-2, exhibit completely opposite mechanofluorochromic behavior. Under UV lamp, the color of pristine X-1 changed from blue to green with reversible redshifted 27 nm in fluorescence emission spectra after ground, while the color of pristine X-2 changed from red to yellow with reversible blue-shifted 74 nm after ground. The detailed characterizations (including PXRD, SEM and DSC) confirmed that this opposite mechanofluorochromism was attributed to the transformation of order-crystalline and amorphous states. The crystal structure analysis and theoretical calculation further explain that opposite mechanofluorochromic behavior take into account different π-π stacking mode by induced π-extended systems. In addition, these TPE-based fluorophores (X-1 and X-2) exhibited excellent bio-compatibility and fluorescence properties for bio-imaging, writable data storage and anti-counterfeiting materials.

Advances in Pure Organic Mechanoluminescence Materials

Mechanoluminescence (ML) is the production of light from materials in response to the external stimulus of mechanical action. For organic compounds, the production of ML is tightly associated with fracture and plastic deformation of materials with piezoelectric effect in crystal lattice, and the ML property is highly dependent on the molecular packing mode, spatial conformation, and intermolecular interaction in the solid state. In the past few years, our group focused on the molecular design of pure organic ML compounds, with an attempt to discover different features of ML in pursuing the inherent emission mechanism and potential practical applications. We successfully found polymorph-dependent ML, ML with a phosphorescent property, conformation-dependent ML, ML with odd-even effect, wearable ML devices applied in heartbeat and pressure detection, etc. In this Perspective, we aim to deepen the understanding of ML and provide some guidance for the molecular design of organic light-emitting materials through the combination of our contributions.

Multi-Directional Mechanofluorochromism of Acetyl Pyrenes and Pyrenyl Ynones

A series of acetyl pyrenes and pyrenyl ynones with and without tert-butyl groups showed distinct mechanofluorochromism (MFC). Four pairs of polymorphic solids were found out of six compounds and interestingly, each of them showed hypsochromic, bathochromic or off-to-on MFC. The MFC properties were rationalized by categorizing the packing schemes into herringbone, sandwich, beta and gamma motifs depending on the relative contributions of C⋅⋅⋅C (or π-π) against C⋅⋅⋅H contacts. The bulky tert-butyl and trimethylsilyl groups served not only to reduce the number of aggregation patterns but also to prohibit the complete back reactions in solid state. Our results suggest that the simple pyrene derivatives may be promising candidates for a novel group of mechanically-sensitive materials.

A Feasible Strategy of Fabricating Type I Photosensitizer for Photodynamic Therapy in Cancer Cells and Pathogens

The utilization of photochemical reaction channel based on radical process is rarely reported, which might be a very efficient and feasible strategy for improving generation of Type I reactive oxygen species (ROS). In this work, a double ionic-type aggregation-induced emission luminogen (AIEgen) of TIdBO was developed as a photosensitizer, of which the potential photocyclization characteristic involving an electron-transfer process had a positive effect on Type I ROS generation in aggregates under continuous light irradiation. Its noticeable photodynamic therapy (PDT) performance and self-monitoring of PDT process by the relationship between cellular morphology change and fluorescence intensity enhancement were achieved. In addition, it showed a good killing ability to microbes and specific interactions with microbes but not cells by regulating the incubation time. These intriguing results reveal a feasible design principle for the implementation of efficient PS preparation in clinical treatment under hypoxic conditions.

Triboluminescence of a new family of CuI-NHC complexes in crystalline solid and in amorphous polymer films

Triboluminescent compounds that generate emission of light in response to mechanical stimulus are promising targets in the development of “smart materials” and damage sensors. Among triboluminescent metal complexes, rare-earth europium and terbium complexes are most widely used, while there is no systematic data on more readily available and inexpensive Cu complexes. We report a new family of photoluminescent Cu–NHC complexes that show bright triboluminescence (TL) in the crystal state visible in ambient indoor light under air. Moreover, when these complexes are blended into amorphous polymer films even at small concentrations, TL is easily observed. Observation of TL in polymer films overcomes the limitation of using crystals and opens up possibilities for the development of mechanoresponsive coatings and materials based on inexpensive metals such as Cu. Our results may also have implications for the understanding of the TL effect's origin in polymer films.

Triboluminescent compounds that generate emission of light in response to mechanical stimulus are promising targets in the development of “smart materials” and damage sensors.

Deep insight into reasons for the mechanoluminescence phenomenon of triphenylamine-substituted imidazoles and their distinct mechano-fluorochromic behaviours

Three imidazoles with different numbers of fused aromatic rings have been prepared by the respective introduction of triphenylamine and 4-cyanophenyl at the N1 and C2 positions in the imidazole ring. Each imidazole effectively exhibits positive solvatochromism, and that of benzo[d]imidazole is the most significant. Although these imidazole crystals have centrosymmetric space groups, they are all ML-active. It was verified by DFT calculations based on X-ray crystallography that some molecular couples with strong intermolecular interactions possess large net dipole moments that should be dominantly responsible for the ML behaviours of these crystals. Moreover, the considerably high molecular dipole moments of the three imidazoles also make a great contribution to good ML effects. Based on this triphenylamine-substituted imidazole system, the relationships among space groups, molecular dipole moments, polar molecular couples and the ML phenomenon are made clear for the first time. Unlike the remarkable MFC activities on imidazole and benzo[d]midazole crystals, phenanthro[9,10-d]imidazole is MFC-inert, and this may well be attributed to strong intramolecular C-H⋯π interactions, which make the rotation of triphenylamine nearly impossible under force stimuli.

Molecular Packing: Another Key Point for the Performance of Organic and Polymeric Optoelectronic Materials

ConspectusOptoelectronic material properties are governed by the whole collective of organic moieties, and these aggregate states present the characteristic performance of extended assemblies with different molecular packing, not only of single molecules themselves. Thus, controlling molecular packing is an essential issue for obtaining the optimized optical and electronic properties. It is also a great challenge because of the unclear structures and complicated intermolecular interactions, including dispersion forces, electrostatic interactions and hydrogen bonding. Moreover, upon the introduction of some external force as the stimulus source, dynamic optical properties can be achieved with the transformation of molecular packing in some cases, such as the photoinduced room temperature phosphorescence (RTP) effect, mechanochromic luminescence, the thermal treatment-dependent mechanoluminescence effect, and the optimized nonlinear optical (NLO) property achieved after electric poling. Therefore, it is essential to understand the relation between characteristics of molecular packing and the resultant optoelectronic performance at the molecular level, which becomes increasingly demanding for the further development of functional materials for their applications in organic light-emitting diodes (OLEDs), chemo- and biosensors, organic solar cells, data storage, and anticounterfeiting devices.This Account gives a summary of our research on the molecular design of optoelectronic materials, with the consideration of a molecular uniting effect in different aggregated states, such as crystalline states, thin films, and nanoparticles. Through the systematical investigation of structure-packing-performance relationships, some strategies are afforded to partially control the molecular packing via the tunable size, shape, and configuration of aromatic moieties with different electronic and steric effects, together with different types of substituents as functional units to adjust the intermolecular interactions. The utilization of π-π interactions and hydrogen bonding by rational molecular design is considered as the key point to achieve the bright emission of organic materials, including the RTP and mechanoluminescence effects. Also, the dynamic optoelectronic properties are highlighted with different kinds of stimuli, including light irradiation, mechanical force, thermal treatment, and electric field, which are mainly related to the subtle molecular motions under external force and the changeable molecular packing as the metastable state. These selected examples will not only open a window for further development of organic and polymeric optoelectronic materials by the adjustable molecular packing and noncovalent interactions, but also prompt further advances for more interesting and exciting properties.

Spiro-Structure: A Good Approach to Achieve Mechanoluminescence Property

Following the development of organic mechanoluminescence (ML) materials, molecular packing was proved as the key point to the emission process under an external force. In this text, with the introduction of spiro-(fluorene-9-9'-xanthene) (SFX) unit as the building block, the molecular packing of the resultant compound (BSFXA) was optimized with the interlaced mode, directly leading to the efficient ML effect. The key role of SFX with a spiro-structure can be further confirmed by the ML inactivity of reference compound BFA with the replacement of SFX unit by dimethyl fluorene (MeF), which provided a novel strategy to construct organic ML luminogens.

Side chain effects on the solid-state emission behaviours and mechano-fluorochromic activities of 10H-phenothiazinylbenzo[d]imidazoles

A family of 4-cyanophenyl-substituted 10H-phenothiazinylbenzo[d]imidazoles with different side chains at the 10-position are prepared and their physical properties are studied. The detailed structure-property research demonstrates that the cold crystallization temperature of ground samples and the emission wavelengths of pristine samples are in good accordance with the packing density, conformation distortion and intermolecular interactions, but emission wavelengths of ground samples are slightly chain-dependent. For benzimidazoles with alkyl chains, longer and more branched chains can produce looser packings, which cause pristine samples to display red-shifted emission and reduced MFC activity. For benzimidazoles with a phenyl chain, the emission wavelengths of both the pristine and the ground samples are remarkably red-shifted. Moreover, the degree of conformation distortion is larger, and the cold crystallization temperature is higher. Interestingly, the homologue with the n-hexyl chain displays an intense ML effect that is mainly attributed to the heavy discharge quantity on largely enhanced discharge areas under force stimuli due to the great fragility of this crystal.

The solid-state emission behaviours and MFC performances of 10H-phenothiazinylbenzo[d]imidazoles with different side chains are investigated.

Triboluminescence of Centrosymmetric Lanthanide β-Diketonate Complexes with Aggregation-Induced Emission

Triboluminescence (TL) is a form of light emission induced upon mechanical forces on the material. However, our understanding of this phenomenon is still unclear and more examples are therefore needed in order to elucidate its mechanism. In this work, two types of TL complexes, [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen], which also displays aggregation-induced emission (AIE) were synthesized and investigated for its photo-physical and crystal structural properties. These complexes were crystallized in a centro-symmetric space group P2₁/n, and remarkably, displayed TL upon grinding that may be due to the presence of extensive π···π, C-H···π and C-H···Cl-C interactions in the close molecular packing of its structure. This rare example deviates from the widely accepted mechanism of TL, hence widening the scope of our understanding in the area.

Aggregation-induced emission enhancement (AIEE)-active tetraphenylethene (TPE)-based chemosensor for Hg2+ with solvatochromism and cell imaging characteristics

An aggregation-induced emission enhancement (AIEE)-active fluorescent sensor based on a tetraphenylethene (TPE) unit has been successfully designed and synthesized. Interestingly, the luminogen could detect Hg²⁺ with high selectivity in an acetonitrile solution without interference from other competitive metal ions, and the detection limit was 7.46 × 10⁻⁶ mol L⁻¹. Furthermore, the luminogen also showed interesting solvatochromic behavior and superior cell imaging performance.

A TPE-based AIEE-active fluorescent sensor for Hg²⁺ was synthesized. Furthermore, it showed solvatochromism and cell imaging characteristics.

Red-emitting dyes based on phenothiazine-modified 2-hydroxychalcone analogues: mechanofluorochromism and gelation-induced emission enhancement

Red-emitting mechanofluorochromic dyes based on phenothiazine-modified 2-hydroxychalcone analogues.

A nitro-capped tetraaniline derivative with AIE features for BSA detection and the selective imaging of Gram-positive bacteria

A nitro-capped tetraaniline derivative (NO2-B3-Ani4-NO2) with AIE feature was synthesized and characterized, which presented an ultrasensitive turn-on response towards bovine serum albumin and selective imaging of Gram-positive bacteria based on AIE mechanism.

Bromine-Substituted Fluorene: Molecular Structure, Br-Br Interactions, Room-Temperature Phosphorescence, and Tricolor Triboluminescence

Organic tribophosphorescence materials are rarely reported and the introduction of Br atoms may be a practical way to design such materials. Here four bromine-substituted fluorene-based derivatives are presented and BrFlu-CBr, having fluorescence-phosphorescence dual-emission induced not only by UV light but also by mechanical stimulus, manifests the highest phosphorescence efficiency of 4.56 % upon photoirradiation. During the grinding process, three different triboluminescent spectra were identified. Upon introduction of a mechanical stimulus, the triboluminescence emission is cyan, whereas after an extended period it changed to blue. After removing the mechanical stimulus, green-white phosphorescent emission was observed. Careful research on single-crystal structures and theoretical calculations demonstrate that strong Br⋅⋅⋅Br interactions are vital to facilitate spin-orbit coupling and promote intersystem crossing, thus generating the unique properties.