Organic materials with hydrostatic pressure induced mechanochromic properties

Control of Fluorescence of Organic Dyes in the Solid-State by Supramolecular Interactions

Fluorescent organic dyes play an essential role in the creation of new "smart" materials. Fragments and functional groups capable of free rotation around single bonds can significantly change the fluorescent organic dye's electronic structure under analyte effects, phase state transitions, or changes in temperature, pressure, and media polarity. Dependencies between steric and electronic structures become highly important in transition from a solution to a solid-state. Such transitions are accompanied by a significant increase in the dye molecular structure's rigidity due to supramolecular associates' formation such as H-bonding, π···π and dipole-dipole interactions. Among those supramolecular effects, H-bonding interactions, first of all, lead to significant molecular packing changes between loose or rigid structures, thus affecting the fluorescent dye's electronic states' energy and configuration, its fluorescent signal's position and intensity. All the functional groups and heteroatoms that are met in the organic dyes seem to be involved in the control of fluorescence via H-bonding: C-H···N, C-H···π, S = O···H-C, P = O···H, C-H···O, NH···N, C - H···C, C - H···Se, N-H···O, C - H···F, C-F···H. Effects of molecular packing of fluorescent organic dyes are successfully used in developing mechano-, piezo-, thermo- fluorochromes materials for their applications in the optical recording of information, sensors, security items, memory elements, organic light-emitting diodes (OLEDs) technologies.

Mechanofluorochromism of 2-Biarylyl Cinchoninic Acids with High Sensitivity and Large Mechanochromic Shift

In recent years, organic mechanofluorochromism (MFC) materials have attracted wide attention in many fields. However, the exploration of MFC materials with high-contrast, high-sensitivity and high-responsiveness remains a challenge. Herein, a series of MFC materials with 2-biarylyl cinchoninic acid skeleton were successfully established, which are based on interconversion of classical/ frustrated Brönsted pairs. These compounds have the mechanochromic shift of up to 115 nm, as well as the property of stunning sensitivity and multiple responses to external mechanical force stimuli. The luminescence properties can be easily tuned by changing the substituents.

Mechanofluorochromism with Aggregation-Induced Emission (AIE) Characteristics: A Perspective Applying Isotropic and Anisotropic Force

Organic mechanofluorochromic (MFC) materials (that change their emission under anisotropic and isotropic pressure) have attracted a great attention in recent years due to their promising applications in sensing pressure, storage devices, security inks, three-dimensional (3D) printing, etc. Stimuli-responsive organic materials with aggregation-induced emission (AIE) characteristics would be an interesting class of materials to enrich the chemistry of MFC compounds. A diamond anvil cell (DAC) is a small tool that is employed to generate high and uniform pressure on materials over a small area. This article discusses the relationship between the chemical structure of AIE compounds and the change in emission properties under anisotropic (mechanical grinding) and isotropic (hydrostatic) pressure. The luminescent properties of such materials depend on the molecular rearrangement in the lattice, conformational changes, excited state transitions and weak intermolecular interactions. Hence, studying the change in luminescent property of these compounds under varying pressure will provide a deeper understanding of the excited-state properties of various emissive compounds with stress. The development of such materials and studies into the effect of pressure on their luminescence properties are summarized.

A synergy between the push-pull electronic effect and twisted conformation for high-contrast mechanochromic AIEgens

Mechanochromic (MC) luminogens in response to external stimulus have shown promising applications as pressure sensors and memory devices. Meanwhile, research on their underlying mechanism is still in the initial stage. Here, three pyridinium-functionalized tetraphenylethylenes bearing n-pentyloxy, hydrogen and nitro groups, namely TPE-OP, TPE-H and TPE-NO, are designed to systematically investigate the influence of the push-pull electronic effect and molecular conformation on MC luminescence. Upon anisotropic grinding and isotropic hydrostatic compression, TPE-OP with strong intramolecular charge transfer (ICT) affords the best MC behavior among them. Analysis of three polymorphs of TPE-H clearly indicates that planarization of the molecular conformation plays an important role in their bathochromic shifts under mechanical stimuli. Theoretical calculations also verify that high twisting stress of AIEgens can be released under high pressure. This study presents a mechanistic insight into MC behaviour and an effective strategy to achieve high-contrast MC luminescence.

Mechanism of Different Piezoresponsive Luminescence of 2,3,4,5-Tetraphenylthiophene and 2,3,4,5-Tetraphenylfuran: A Strategy for Designing Pressure-Induced Emission Enhancement Materials

Mechanoresponsive luminescent materials have attracted widespread attention for their potential applications, especially for these behaving pressure-induced emission enhancement (PIEE). Designing and seeking systems with high-efficiency PIEE are desirable and crucial for material science. Here, the mechanisms of different piezoresponsive luminescence of 2,3,4,5-tetraphenylthiophene (TPT) and 2,3,4,5-tetraphenylfuran (TPF) crystals are explored. The experimental results combined with density functional theory (DFT) theory calculation indicate that the PIEE phenomenon is possibly exhibited in V-shape arrangement for the reason of the weak π-π interactions. This study not only gains deep insight into the relationship between optical properties and structural evolution but also puts forward a strategy for designing PIEE materials from the point of molecular arrangement.

A mechanochromic luminescent material with aggregation-induced emission: Application for pressure sensing and mapping

In this study, we report a new compound, (E)-4-(((2-hydroxynaphthalen-1-yl)methylene)amino)-3-methylbenzoic acid (HNMB), which shows aggregation-induced emission property as well as intramolecular charge transfer (ICT) nature. In addition, it exhibits unique mechanochromic luminescence (MCL). The HNMB solid powder emits strong emission but shows quenching effect together with bathochromic-shift after grinding, suggesting a high contrast ratio up to 1420%. Through crystallographic analysis, the relationship between MCL nature and molecular packing mode is verified: Molecules in crystalline phase adopt the J-type coupling based on less overlapped π⋯π stacking, in which multiple intermolecular interactions mainly including C-H⋯π, C-H⋯O and hydrogen bonding, help to stabilize such packing mode. When these interactions are destructed by mechanical force, the packing would be disassembled, activating the MCL behavior. Such working mechanism only needs weak external force capable of destructing intermolecular interactions, rendering the MCL material highly sensitive to pressure. As a practical application, a film sensor for pressure detection is designed based on HNMB, which gives a linear relation between the emission intensity and the external pressure in a lower range. The detection limit of the film sensor is 27.24 Mpa, suggesting high sensitivity. In addition, pressure mapping with high contrast ratio is obtained by surface plot, making this pressure sensor a reliable candidate to be instrumented for various applications.

Toward a simple way for a mechanochromic luminescent material with high contrast ratio and fatigue resistance: Implication for information storage

In this work, we present the synthesis and photoluminescence (PL) behaviour of a new compound, DHNC. The molecular design includes twisted conformation and the incorporation of electron donor (D) and acceptor (A) pairs, which endows the compound with both twisted intramolecular charge transfer (ICT) and aggregation-induced emission (AIE) properties. Importantly, the compound exhibits mechanochromic luminescence (MCL): The emission of the crystalline powder shows strong green emission but turns into orange-red with an obvious quenching effect after grinding, demonstrating a high contrast ratio. The emission of the ground sample can be rejuvenated though recrystallization by either immersion or fumigation in common organic solvents. The emission can be reversibly switched between two states for more than 10 cycles, showing fatigue resistance. In a quantitative mechanical experiment, the DHNC-loaded film has a remarkable emission loss with the external force up to 67.9 Mpa, showing high sensitivity. An archetype of information storage is developed based on this MCL material, which uses mechanical force to write information and organic vapour to erase. Letters and cartoon pictures can be written and erased repeatedly on the DHNC-loaded film, indicating high contrast ratio and fatigue resistance.

Pressure-induced emission band separation of the hybridized local and charge transfer excited state in a TPE-based crystal

We herein report a newly synthesized simple molecule, named TPE[double bond, length as m-dash]C4, with twisted D-A structure. TPE[double bond, length as m-dash]C4 showed two intrinsic emission bands ascribed to the locally excited (LE) state and the intramolecular charge transfer (ICT) state, respectively. In the crystal state, the LE emission band is usually observed. However, by applying hydrostatic pressure to the powder sample and the single crystal sample of TPE[double bond, length as m-dash]C4, dual-fluorescence (445 nm and 532 nm) was emerged under high pressure, owing to the pressure-induced emission band separation of the hybridized local and charge transfer excited state (HLCT). It is found that the emission of TPE[double bond, length as m-dash]C4 is generally determined by the ratio of the LE state to the ICT state. The ICT emission band is much more sensitive to the external pressure than the LE emission band. The HLCT state leads to a sample with different responsiveness to grinding and hydrostatic pressure. This study is of significance in the molecular design of such D-A type molecules and in the control of photoluminescence features by molecular structure. Such results are expected to pave a new way to further understand the relationship between the D-A molecular structure and stimuli-responsive properties.

An organoboron compound with a wide absorption spectrum for solar cell applications

Organoboron compounds offer new approaches to tune the electronic structures of π-conjugated molecules. In this work, an electron acceptor (M-BNBP4P-1) is developed by endcapping an organoboron core unit with two strong electron-withdrawing groups. M-BNBP4P-1 exhibits a unique wide absorption spectrum with two strong absorption bands in the long wavelength region (λ_max = 771 nm) and the short wavelength region (λ_max = 502 nm), which indicate superior sunlight harvesting capability. This is due to its special electronic structure, i.e. a delocalized LUMO and a localized HOMO. Prototype solution-processed organic solar cells based on M-BNBP4P-1 show a power conversion efficiency of 7.06% and a wide photoresponse from 350 nm to 880 nm.

Multi-Stimuli-Responsive Fluorescence Switching from a Pyridine-Functionalized Tetraphenylethene AIEgen

The discovery of the striking aggregation-induced emission (AIE) phenomenon has opened a new avenue for smart light-emitting materials. Herein, a new AIE luminogen (AIEgen), 1,1,2,2-tetrakis(4-((E)-2-(pyridin-2-yl)vinyl)phenyl)ethene (TP2VPE), has been designed and synthesized by introducing the vinylpyridine motifs into the tetraphenylethene backbone. The emission spectrum of the new obtained AIEgen crystalline material can be switched in response to not only mechanical grinding and hydrostatic compression but also the protonation effect with excellent reversibility and reproducibility. Single-crystal X-ray structural analysis disclosed the supramolecular porous channel structure, which provides a shrinkable volume to maintain the fluorescence emission upon high pressure. Furthermore, protonation-deprotonation of the pyridine moieties in TP2VPE has a significant effect on the frontier molecular orbitals as well as very distinctive emission characteristics upon acid and base stimuli. The dual-response performance and the ease of its preparation and renewal endow the material with potential applications in pressure and acid/alkali fluorescence sensing.

Pressure-Induced Emission Enhancement of Carbazole: The Restriction of Intramolecular Vibration

Pressure-induced emission enhancement (PIEE), a novel phenomenon in the enhancement of the solid-state emission efficiency of fluorophores, has been arousing wide attention in recent years. However, research on PIEE is still in the early stage. To further pursue more enhanced efficiency, discovering and designing more PIEE systems would be urgently desirable and of great importance. In this Letter, we found that carbazole presented a conspicuous emission enhancement under high pressure up to 1.0 GPa. In situ high-pressure infrared spectroscopy and angle-dispersive X-ray diffraction analysis combined with Hirshfeld surface theory calculation indicated that the PIEE of carbazole was attributed to the decrease of the nonradiation vibration process. This phenomenon mainly resulted from restriction of the N-H stretching vibration by increased N-H···π interactions under high pressure. Our study puts forward a mechanism of PIEE related to the restriction of intramolecular vibration, which provided deep insight into the essential role of intermolecular interaction in fluorescence emission properties.

Emission behaviours of novel V- and X-shaped fluorophores in response to pH and force stimuli

The solid-state emission behaviours of a series of V- and X-shaped fluorophores exposed to mechanical force and pH stimuli are dependent on the ICT effect as well as the size- and morphology-effect.