Tandem High-Pressure Crystallography-Optical Spectroscopy Unpacks Noncovalent Interactions of Piezochromic Fluorescent Molecular Rotors

To develop luminescent molecular materials with predictable and stimuli-responsive emission, it is necessary to correlate changes in their geometries, packing structures, and noncovalent interactions with the associated changes in their optical properties. Here, we demonstrate that high-pressure single-crystal X-ray diffraction can be combined with high-pressure UV-visible absorption and fluorescence emission spectroscopies to elucidate how subtle changes in structure influence optical outputs. A piezochromic aggregation-induced emitter, sym-heptaphenylcycloheptatriene (Ph7C7H), displays bathochromic shifts in its absorption and emission spectra at high pressure. Parallel X-ray measurements identify the pressure-induced changes in specific phenyl-phenyl interactions responsible for the piezochromism. Pairs of phenyl rings from neighboring molecules approach the geometry of a stable benzene dimer, while conformational changes alter intramolecular phenyl-phenyl interactions correlated with a relaxed excited state. This tandem crystallographic and spectroscopic analysis provides insights into how subtle structural changes relate to the photophysical properties of Ph7C7H and could be applied to a library of similar compounds to provide general structure-property relationships in fluorescent organic molecules with rotor-like geometries.

Ligand-protected metal nanoclusters as low-loss, highly polarized emitters for optical waveguides

Photoluminescent molecules and nanomaterials have potential applications as active waveguides, but such a use has often been limited by high optical losses and complex fabrication processes. We explored ligand-protected metal nanoclusters (LPMNCs), which can have strong, stable, and tunable emission, as waveguides. Two alloy LPMNCs, Pt1Ag18 and AuxAg19-x (7 ≤ x ≤ 9), were synthesized and structurally determined. Crystals of both exhibited excellent optical waveguide performance, with optical loss coefficients of 5.26 × 10-3 and 7.77 × 10-3 decibels per micrometer, respectively, lower than those demonstrated by most inorganic, organic, and hybrid materials. The crystal packing and molecular orientation of the Pt1Ag18 compound led to an extremely high polarization ratio of 0.91. Aggregation enhanced the quantum yields of Pt1Ag18 and AuxAg19-x LPMNCs by 115- and 1.5-fold, respectively. This photonic cluster with low loss and high polarization provides a generalizable and versatile platform for active waveguides and polarizable materials.

Hexaphenylbenzene-Based Deep Blue-Emissive Metallacages as Donors for Light-Harvesting Systems

We herein report the preparation of a series of hexaphenylbenzene (HPB)-based deep blue-emissive metallacages via multicomponent coordination-driven self-assembly. These metallacages feature prismatic structures with HPB derivatives as the faces and tetracarboxylic ligands as the pillars, as evidenced by NMR, mass spectrometry and X-ray diffraction analysis. Light-harvesting systems were further constructed by employing the metallacages as the donor and a naphthalimide derivative (NAP) as the acceptor, owing to their good spectral overlap. The judiciously chosen metallacage serves as the antenna, providing the suitable energy to excite the non-emissive NAP, and thus resulting in bright emission for NAP in the solid state. This study provides a type of HPB-based multicomponent emissive metallacage and explores their applications as energy donors to light up non-emissive fluorophores in the solid state, which will advance the development of emissive metallacages as useful luminescent materials.

Precise Molecular Design of a Pair of New Regioisomerized Fluorophores With Opposite Fluorescent Properties

Aggregation-induced emission (AIE) has attracted much attention in the past 2 decades. To develop novel AIE-active materials, ACQ-to-AIE transformation via regioisomerization is one of the most straightforward method. However, most of the reported ACQ-to-AIE transformations are achieved by migrating bulky units. In this work, a facile conversion was realized by migrating a small pyrrolidinyl group from para- to ortho-position on the rofecoxib scaffold. As a result, a pair of new isomers named MOX2 and MOX4 exhibited AIE behavior and ACQ activity, respectively. Moreover, MOX2 also showed solvatochromic, mechanochromic, and acidochromic properties with reversible multi-stimulus behavior. Single crystal X-ray analysis of MOX2 revealed that the molecular conformation and its packing mode were responsible for the AIE emission behavior. Further investigation indicated that MOX2 showed high lipid droplets staining selectivity. Taken together, the current work not only provides a new design philosophy for achieving ACQ-to-AIE conversion by migrating a small pyrrolidinyl group but also presents a promising candidate MOX2 for potential applications such as in security ink, optical recording and biological applications.

Development of a New Type of Multi-Functional Mechanochromic Luminescence Materials by Infusing a Phenyl Rotator into the Structure of 3,4-diphenylmaleic anhydride

Mechanochromic luminescence (MCL) materials have attracted increasing attention due to their versatile functions in many fields. In this work, based on the structure of 3,4-diphenylmaleic anhydride (BPMA, Mei et. al,...

Hexaarylbenzene based high-performance p-channel molecules for electronic applications

Hexaarylbenzene-based molecules find potential applications in organic electronics due to wider energy gap, high HOMO level, higher photoconductivity, electron-rich nature, and high hole-transporting property. Due to the unique propeller structure, these molecules show low susceptibility towards self-aggregation. This property can be tailored by proper molecular engineering by the incorporation of appropriate groups. Therefore, hexaarylbenzene chromophores are widely used as the materials for high-efficiency light-emitting materials, charge transport materials, host materials, redox materials, photochemical switches, and molecular receptors. This review highlights the diverse structural modification techniques used for the synthesis of symmetrical and unsymmetrical structures. Also, the potential applications of these molecules in organic light-emitting diodes, organic field-effect transistors, organic photovoltaics, organic memory devices, and logic circuits are discussed.

Tetraphenylethylene-functionalized hexaphenylbenzene with unique conformation-driven selectivity for gas chromatographic separations

Tetraphenylethylene-functionalized hexaphenylbenzene composed of a neat aromatic hydrocarbon with unique conformation-driven selectivity for gas chromatographic separations.

Mechanochromic Fluorescent Polymers with Aggregation-Induced Emission Features

Mechanochromic polymers are defined as materials that are able to detect a mechanical stress through an optical output. This feature has evoked a growing interest in the last decades, thanks to the progress of chromogenic molecules whose optical characteristics and chemical functionalities allow their effective insertion in many thermoplastic and thermoset matrices. Among the different types of fluorogenic probes able to detect mechanical solicitations, those with aggregation-induced emission (i.e., AIEgens) have attracted tremendous interest since their discovery in 2001. In the present review, the main principles behind the AIEgens working behavior are introduced along with the current state of knowledge concerning the design and preparation of the derived mechanochromic fluorescent polymers. Examples are provided concerning the most ingenious solution for the preparation of chromogenic materials, starting from different types of commodity plastics or synthetic polymers and combined with the latest AIE technology to provide the most sensitive response to mechanical stress.

A Dual Associated-Functional Fluorescent Switch: From Alternate Detection Cycle for Fe(III) and pH to Molecular Logic Operations

With the expansion and deepening of scientific research, dual-functional or multifunctional materials are urgently needed to replace those for single application. Herein, a fluorescence sensing system based on an In(III)-organic complex with in situ Lewis acid sites has been constructed, exhibiting high sensitivity for the detection of Fe(III) ions with a low detection limit of 3.95 μM and a short response time of within 10 s. It is noteworthy that the quenched fluorescence of the Fe(III)-incorporated sample could be reopened linearly with an increase of alkalinity, followed by the reactivation of its functionality to identify Fe(III) ions, forming an alternate detection cycle for Fe(III) and pH with off-on-off fluorescent switch characteristics. Considering its unique molecular recognition capability, an advanced three-input (Fe(III), EDTA, and OH^-) and two-output (B440 and G489) Boolean logic operation comprising BUFF, NOT, OR, and AND logic gates was integrated, possessing potential applications in intelligent multianalyte sensing systems.

Study of the mechanoluminescence and ‘aggregation-induced emission enhancement’ properties of a new conjugated oligomer containing tetraphenylethylene in the backbone: application in the selective and sensitive detection of explosive

D–π-A based ‘Aggregation-Induced Emission Enhancement (AIEE)’ active conjugated oligomer (oTPETP) exhibits an abnormal blue shifting mechanoluminescence and selective and sensitive detection of explosive.

Excited-State Aromatic Interactions in the Aggregation-Induced Emission of Molecular Rotors

Small, apolar aromatic groups, such as phenyl rings, are commonly included in the structures of fluorophores to impart hindered intramolecular rotations, leading to desirable solid-state luminescence properties. However, they are not normally considered to take part in through-space interactions that influence the fluorescent output. Here, we report on the photoluminescence properties of a series of phenyl-ring molecular rotors bearing three, five, six, and seven phenyl groups. The fluorescent emissions from two of the rotors are found to originate, not from the localized excited state as one might expect, but from unanticipated through-space aromatic-dimer states. We demonstrate that these relaxed dimer states can form as a result of intra- or intermolecular interactions across a range of environments in solution and solid samples, including conditions that promote aggregation-induced emission. Computational modeling also suggests that the formation of aromatic-dimer excited states may account for the photophysical properties of a previously reported luminogen. These results imply, therefore, that this is a general phenomenon that should be taken into account when designing and interpreting the fluorescent outputs of luminescent probes and optoelectronic devices based on fluorescent molecular rotors.

T-Shaped donor–acceptor–donor type tetraphenylethylene substituted quinoxaline derivatives: aggregation-induced emission and mechanochromism

T-Shaped D–A–D type tetraphenylethylene (TPE) substituted acenapthene-quinoxaline1and TPE substituted phenanthrene quinoxaline2were synthesized by the Suzuki cross-coupling reaction and exhibits highly reversible mechanochromic behavior.

Fluorene-containing tetraphenylethylene molecules as lasing materials

A series of star‐shaped oligofluorene molecules, each containing a TPE core, have been specifically designed and produced to show effective aggregation‐induced emission (AIE). Each molecule differs either in the number of fluorene units within the arms (e.g., 1 or 4, compounds 4 and 5), or the terminal group positioned at the end of each arm (e.g., H, TMS, or TPA, compounds 4, 6, and 7). Although they are all poor emitters in solution phase they become efficient yellow‐green luminogens in the condensed state. Their AIE properties were investigated in THF/H₂O mixtures, with each molecule exhibiting a clear emission enhancement at specific water contents. An all‐organic distributed feedback (DFB) laser was fabricated using compound 4 as the gain material and exhibited an average threshold energy fluence of 60 ± 6 μJ/cm² and emission in the green region. Furthermore, piezofluorochromism studies on a thin film of this material displayed a linear dependence of the amplified spontaneous emission (ASE) peak position on applied pressure, indicating potential applications as lasing‐based pressure sensors. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 734–746

Rational design of asymmetric red fluorescent probes for live cell imaging with high AIE effects and large two-photon absorption cross sections using tunable terminal groups

In this work, we report the synthesis of a family of donor-acceptor (D-A) π-conjugated aggregation-induced red emission materials (TPABT, DTPABT, TPEBT and DTPEBT) with the same core 2,2-(2,2-diphenylethene-1,1-diyl)dithiophene (DPDT) and different amounts and different strengths of electron-donating terminal moieties. Interestingly, TPABT and TPEBT, which have asymmetric structures, give obviously higher solid fluorescence quantum efficiencies in comparison with those of the corresponding symmetric structures, DTPABT and DTPEBT, respectively. In particular, the thin film of TPEBT exhibited the highest fluorescence quantum efficiency of ca. 38% with the highest αAIE. Moreover, TPEBT and DTPEBT with TPE groups showed two-photon absorption cross-sections of (δ) 1.75 × 103 GM and 1.94 × 103 GM at 780 nm, respectively, which are obviously higher than the other two red fluorescent materials with triphenylamine groups. Then, the one-photon and two-photon fluorescence imaging of MCF-7 breast cancer cells and Hela cells, and cytotoxicity experiments, were carried out with these red fluorescent materials. Intense intracellular red fluorescence was observed for all the molecules using one-photon excitation and for TPABT using two-photon excitation in the cell cytoplasm. Finally, TPEBT is biocompatible and functions well in mouse brain blood vascular visualization. It is indicated that these materials can be used as a specific stain fluorescent probe for live cell imaging.

Multiple-color aggregation-induced emission (AIE) molecules as chemodosimeters for pH sensing

A series of 4-N,N-dimethylaminoaniline salicylaldehyde Schiff-base derivatives (DAS) were facilely prepared. They exhibit typical AIE properties with various fluorescence emissions and high fluorescence quantum yields in an aggregated state. DAS exhibit unique pH-dependent optical properties, which indicated their potential applications in pH sensing.

Exploring the Intrinsic Piezofluorochromic Mechanism of TPE-An by STS Technique

9,10-bis(4-(1,2,2-triphenylvinyl)styryl)anthracene (TPE-An) materials have attracted considerable attention in recent years because they have high luminescence efficiency and excellent piezofluorochromic properties, which have potential applications in organic light-emitting display (OLED) area. Scanning tunneling spectroscopy (STS) technique was used to study the piezofluorochromic mechanism of aggregation-induced emission (AIE) materials for the first time. Photoluminescence (PL) experiments revealed that the emission peak of TPE-An is observed to exhibit a red-shift with the increase of the grinding time. A theoretical calculation was carried out to find the relationship between the bandgap of TPE-An and the external force by combination of the classical tunneling theory and STS results. It is found that when the pressure variation on the surface of TPE-An film was increased to be over 4.38 × 10(4) Pa, the shrink of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap can arrive at 1.1 eV. It is concluded that the piezofluorochromic behaviors of TPE-An should originate from the shrinking effect of the bandgap under external force. Moreover, this research method may shed light on comprehending and adjusting the piezofluorochromic characters of other AIE materials.

A Suite of Tetraphenylethylene-Based Discrete Organoplatinum(II) Metallacycles: Controllable Structure and Stoichiometry, Aggregation-Induced Emission, and Nitroaromatics Sensing

Materials that organize multiple functionally active sites, especially those with aggregation-induced emission (AIE) properties, are of growing interest due to their widespread applications. Despite promising early architectures, the fabrication and preparation of multiple AIEgens, such as multiple tetraphenylethylene (multi-TPE) units, in a single entity remain a big challenge due to the tedious covalent synthetic procedures often accompanying such preparations. Coordination-driven self-assembly is an alternative synthetic methodology with the potential to deliver multi-TPE architectures with light-emitting characteristics. Herein, we report the preparation of a new family of discrete multi-TPE metallacycles in which two pendant phenyl rings of the TPE units remain unused as a structural element, representing novel AIE-active metal-organic materials based on supramolecular coordination complex platforms. These metallacycles possess relatively high molar absorption coefficients but weak fluorescent emission under dilute conditions because of the ability of the untethered phenyl rings to undergo torsional motion as a non-radiative decay pathway. Upon molecular aggregation, the multi-TPE metallacycles show AIE-activity with markedly enhanced quantum yields. Moreover, on account of their AIE characteristics in the condensed state and ability to interact with electron-deficient substrates, the photophysics of these metallacycles is sensitive to the presence of nitroaromatics, motivating their use as sensors. This work represents a unification of themes including molecular self-assembly, AIE, and fluorescence sensing and establishes structure-property-application relationships of multi-TPE scaffolds. The fundamental knowledge obtained from the current research facilitates progress in the field of metal-organic materials, metal-coordination-induced emission, and fluorescent sensing.

Aggregation-induced emission of a star-shape luminogen based on cyclohexanehexone substituted with AIE active tetraphenylethene functionality

A rigid star-shaped luminogen (HTCA) exhibits aggregation-induced emission (AIE) characteristics.