Reversible Mechanofluorochromism Research Articles

Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms.

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3 - for CF3 -groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.

One-Step Transformations from ACQ Luminogens to DSEgens via the Boc Protection Process.

Dual-state emission luminogens (DSEgens), as a new type of luminescent materials that can effectively emit light in solution and solid state, have attracted tremendous attention due to their potential application in chemical sensing, biological imaging, organic electronic devices, etc. In this study, two new rofecoxib derivatives ROIN and ROIN-B have been synthesized, and their photophysical properties are fully investigated by experimental studies and theoretical calculations. The key intermediate ROIN, resulting from one-step conjugation of rofecoxib with an indole unit, shows the classical aggregation-caused quenching (ACQ) effect. Meanwhile, by introducing a tert-butoxycarbonyl (Boc) group on the basis of ROIN without enlarging the π conjugation system, ROIN-B was successfully developed with an obvious DSE property. In addition, both fluorescent behaviors and their transformation from ACQ to DSE were elucidated clearly by going through the analysis of their single X-ray data. Moreover, the target ROIN-B, as a new DSEgens, also displays reversible mechanofluorochromism and lipid droplet-specific imaging ability in HeLa cells. Taken together, this work proposes a precise molecular design strategy to afford a new DSEgens, which may provide guidance for the future exploration of new DSEgens.

A water-stable Schiff base fluorophore: AIEE behavior, reversible mechanofluorochromism, detection of water content and viscosity of automobile brake fluid, and cell imaging

Imine compounds (Schiff base) have been reported with aggregation-induced emission (AIE)/aggregation-induced emission enhancement (AIEE) characteristics: they were weakly or non-emissive when dissolved well in organic solvents while the fluorescence emission turned on drastically in the presence of water. However, for some of the Schiff bases, the enhanced emission upon aggregation is derived from their parent aldehydes rather than the Schiff base molecule themselves due to the hydrolysis. The susceptibility to hydrolysis has thus limited the wide application of Schiff bases in aqueous media. In this work, a water-stable Schiff base fluorophore DBSFCN was synthesized in which the triphenylamine (TPA) electron donor and benzonitrile electron acceptor were joined together by the imine structure. The hydrolytic stability of DBSFCN was confirmed by structural and spectral analysis. Meanwhile, DBSFCN displayed significant fluorescence emission enhancement in a water/tetrahydrofuran (THF) mixture with high water content and relatively good fluorescence quantum yield at the solid state (0.48), which exhibited a typical AIEE feature. Based on the water stability and AIEE characteristic of DBSFCN, a quantitative method for the determination of water content and kinematic viscosity of the automobile brake fluid was established. In addition, DBSFCN was available in living cell imaging and reversible mechanofluorochromism.

Synthesis of ESIPT fluorophores with two intramolecular H-bonding functionalities: Reversible mechanofluorochromism and conformation controlled solid state fluorescence efficiency

We have synthesized two new ESIPT fluorophores (1-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl) naphthalen-2-ol (Naph-2-OH) and 5-(diphenylamino)-2-((E)-((E)-((2-hydroxyphenyl)(phenyl) methylene)hydrazono)methyl) phenol (TPA-2-OH)) with two unsymmetrical intramolecular H-bonding functionalities and investigated the solid-state structural assembly and fluorescence properties. The crystal structure analysis confirmed the formation of strong intramolecular H-bonding and twisted molecular conformation. The nonplanar twisted molecular conformation produced well separated molecules in the crystal lattice that lead to strongly enhanced emission ((λmax = 538 nm, Φf = 5.2% (Naph-2-OH), λmax = 562 nm, Φf = 13.4% (TPA-2-OH)). Density functional theoretical (DFT) calculation supported the fluorescence tuning and intramolecular charge transfer (ICT) from TPA donor to imine acceptor. Both Naph-2-OH and TPA-2-OH showed mechanical crushing and heating induced reversible fluorescence switching due to the reversible phase transformation between crystalline and amorphous state.

Conjugating Coumarin with Tetraphenylethylene to Achieve Dual-State Emission for Reversible Mechanofluorochromism and Live Cell Imaging.

Dual-state emission luminogens (DSEgens) are receiving research interest in the construction of multifunctional materials due to their inherent advantage of high emission efficiency in both the molecularly dispersed solution state and the solid state. However, it remains challenging in synthesizing DSEgens via a delicate manipulation of the molecular structures. This work presents an example of bright DSEgen synthesis by tuning the molecular electronic structures and conformations. Three coumarin-tetraphenylethylene (TPE) molecules with a donor-acceptor electronic structure and highly twisting conformations have been synthesized. While compound resulting from direct conjugation of coumarin with a TPE unit shows aggregation-induced emission, compound with an N,N-diaminoethyl modification on the 7-position of coumarin and compound with a further phenyl linker between coumarin and TPE units feature strong dual-state emission. Benefiting from their strong solid emission and twisting conformations, these fluorophores display reversible mechanofluorochromism. Finally, applications for rewritable information storage in the solid state and live-cell imaging in the solution state were demonstrated.

Solution and Solid-State Emissive Organophosphonates with High-Contrast Reversible Mechanofluorochromism: Beyond the Classical Frameworks

Solution and Solid-State Emissive Organophosphonates with High-Contrast Reversible Mechanofluorochromism: Beyond the Classical Frameworks

Aggregation induced emission and reversible mechanofluorochromism active carbazole–anthracene conjugated cyanostilbenes with different terminal substitutions

Three novel carbazole–anthracene–cyanostilbene based D–A conjugates were designed and synthesized with slight structural modification of the acceptor unit to understand the effect of the change of the optical properties.

Multi Stimuli Responsive Non‐Doped Red Emitting AIEE Active Phenothiazine‐Based Chalcones: Crystal Structure, Solvatochromism, Turn‐on Mechanofluorochromism and Acidochromism

AbstractWe report on a series of phenothiazine‐based D‐π‐A‐π‐D type cross‐conjugated non‐doped red‐emitting molecules synthesized by simple Claisen‐Schmidt condensation, which exhibit excellent AIEE phenomena through the formation of large‐sized nano‐aggregates. These thermally stable red fluorescent organic materials have low band gap energies (Eg=1.84–2.15 eV) and also display reversible mechanofluorochromism. The CIE chromaticity coordinates of red emissive ground powders were closer to the standard red‐light chromaticity coordinates. These chalcones also exhibited positive solvatochromism, and large Stokes shifts. Finally, these compounds were utilized as reversible volatile acid sensors to detect TFA in solution as well as in solid state, by virtue of the presence of electron withdrawing carbonyl oxygen, and they could detect TFA at a concentration as low as 15.8 ppm in the presence of C4.

Tuning Electronic Structures of Carbazole‐Cyanostyrene Molecules to Achieve Dual‐State Emission for Trace Water Analysis, Picric Acid Sensing, and Reversible Mechanofluorochromism

AbstractDual‐state emissive (DSE) materials with efficient emission both in solution and in the solid state can be used in complex environments. In this work, we demonstrate the synthesis of DSE materials by finely tuning the electronic structures of luminogens. Four carbazole‐cyanostyrene compounds with different terminal groups, i. e., CZ‐Ph‐H without additional substituent, CZ‐Ph‐TA with an N,N‐dimethylamino functional group, CZ‐Ph‐CN with a cyano group, and CZ‐Ph‐TB with a t‐butyl group, have been synthesized and their photophysical properties are fully investigated by spectroscopic studies, theoretical calculations, and crystallographic analysis. Interestingly, while CZ‐Ph‐H, CZ‐Ph‐CN, and CZ‐Ph‐TB are aggregation‐induced emissive (AIE) molecules only showing strong emission in the aggregated states (aggregate suspension and solid powder), CZ‐Ph‐TA is a DSE molecule showing strong emission in both dilute solution and aggregated states. In addition, CZ‐Ph‐TA could serve as a fluorescent probe for trace water analysis in organic solvents and picric acid (PA) detection in both dilute solution and aggregate suspension. CZ‐Ph‐TA also exhibits reversible mechanofluorochromism with a spectral shift of 21 nm. The synthesis of the N,N‐dimethylamino‐substituted molecule with excellent photophysical properties represents an important attempt to achieve DSE by finely tuning the electronic structures of molecules.

α-Cyanostilbene functionalized carbazole derivatives exhibiting dual-state emission and multi-stimuli responsive fluorescent switching

Interest in organic luminescent materials that show strong dual state emission (DSE) in both solution and the solid state is rapidly growing due to their widespread applications. The development of DSE luminogens (DSEgens) with multi-stimuli responsive properties capable of achieving multi-parameter detection in complicated environments remains challenging due to the lack of an explicit molecular design strategy. In this work, we devised and prepared two D-π-A-π-D-structured carbazole derivatives, PY-CZ-Pa and PY-CZ-Ta, with intramolecular charge transfer (ICT) characteristics and excellent DSE properties. Both the compounds as fluorescent probes are highly sensitive to trace amount of water and picric acid (PA) in organic solvents. The limits of detection are determined to be 0.016% (PY-CZ-Pa) or 0.040% (PY-CZ-Ta) for water and 320 nM (PY-CZ-Pa) or 538 nM (PY-CZ-Ta) for PA. Moreover, the two compounds exhibit reversible mechanofluorochromism, with spectral shifts of over 24 nm. The results presented in this work suggest that bright DSEgens with multi-stimuli responsive properties are promising materials for applications at both the molecular and aggregated levels.

Aggregation-Induced Emission, Mechanofluorochromism, and Selective Fluoride Detection by a Tripodal Salicylaldimine.

Fluoride ions are indispensable in biology and environmental science and hence the selective and sensitive detection of fluoride is important. This work reports the design and synthesis of a tripodal Schiff's base 1 through a simple condensation reaction between a commercially available aldehyde and an amine. Single crystal X-ray crystallography revealed that compound 1 is a planar entity with the three salicylidene derivatives on the three arms of the central phenyl moiety linked by imine groups. Compound 1 forms a molecular dimer that resembles a six-petal flower and is stabilized through multiple intermolecular interactions such as C-H.π and π.π interactions. Compound 1 exhibited moderately good emission in the solid state with aggregation induced emission and reversible mechanofluorochromic properties. Moreover, 1 was observed to selectively detect fluoride among various anions with a limit of detection of ∼9 ppm. Compound 1 was also capable of detecting fluoride under a variety of conditions such as in thin films and under cellular conditions.

π-Bridge dependent efficient aggregation-induced emission characteristic and significant mechanofluorochromic behavior

Two novel luminophores both with triphenylamine donor and diaminomaleonitrile acceptor but with differing phenyl (TAPD) and thiophene (TATD) π-bridges were designed and synthesized. The effect of the π-bridge on fluorescent properties, such as aggregation-induced emission (AIE) and mechanofluorochromism (MFC) was investigated. In comparison with TATD, the phenyl bridge-based TAPD exhibits typical AIE phenomenon and excellent reversible MFC behaviors. The efficient emission and remarkable MFC performance in the solid state of TAPD result in its potential application in the fields of light-emitting devices (LEDs) and rewritable information storage media.

Reversible mechanofluorochromism by simple phenyl and mesitylene appended solid state emitters via crystal to amorphous transitions

Developing a variety of organic compounds with bright solid-state emission is highly advantageous, provided they exhibit stimuli-responsive properties. Such ‘smart’ materials offer a remarkable role in sensing, digital machinery, material science, and the biomedical field. Taking this into account, two phenothiazines (PT) appended vinyl-linked aryl unit (mesitylene/phenyl) two based D–π–A structures have been synthesized. The derivatives have exhibited remarkable luminescence characteristics in a diverse set of solvents due to the twisted intramolecular charge transfer (TICT) state and aggregation-induced emission enhancement properties. Surprisingly, between them, only the mesitylene derivative displays significant and highly reversible mechanofluorochromic (MFC) activity by exhibiting an emission shift from sky blue (467 nm) to cyan (482 nm). The mechanistic insight obtained from P-XRD analysis suggests crystal to amorphous transition causes bathochromic shift upon grinding. SEM analysis reveals two discrete morphologies of the MFC compound in its pristine and ground states. Additionally, DFT and TD-DFT calculations provide insight into their electronic energy levels. Furthermore, results from the electrochemical experiment indicate that they are potential candidates for developing optoelectronic devices.

Aggregation induced emission and mechanochromism: Multi stimuli responsive fluorescence switching of assistant acceptor modulated phenanthroimidazoles

We have reported four mechanochromic materials namely, 4-(6,9-di(anthracen-9-yl)-2-phenyl-1H-phenanthro[9,10-d]imidazol-1-yl)-N,N-diphenylaniline (PIDPA-An), 4-(6,9-di (anthracen-9-yl)-1-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl) benzonitrile (CNPIDPA-An), N,N-diphenyl-4-(2-phenyl-6,9-di(pyren-1-yl)-1H-phenanthro [9,10-d]imidazol-1-yl)aniline (PIDPA-Py) and 4-(1-(4-(diphenylamino)phenyl)-6,9-di(pyren-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)benzonitrile (CNPIDPA-Py) consist of triphenylamine and phenyl or 4-cyanophenyl at N1 and C2 positions, respectively and bulky fused aromatics at C6/C9 positions. These materials show excellent thermal stability and exhibit strong aggregation induced emission enhancement (AIEE). These materials exhibit high quantum yield in film and show reversible mechanofluorochromism under external pressure. The pristine PIDPA-An (422 nm), CNPIDPA-An (418 nm), PIDPA-Py (440 nm) and CNPIDPA-Py (448 nm) emits blue light, on grinding of PIDPA-An (466 nm), CNPIDPA-An (452 nm), PIDPA-Py (496 nm) and CNPIDPA-Py (490 nm) show bluish-green emission and annealing of these materials reverts to blue emission. The quantum yield (%) of PIDPA-An/CNPIDPA-An/PIDPA-Py/CNPIDPA-Py are found to be 0.55/0.58/0.60/0.65 (pristine), 0.51/0.54/0.57/0.61 (ground) and 0.48/0.50/0.51/0.57 (annealing) under external face. The adopted design strategy opens up new way to develop efficient mechanochromic materials.

Highly Emissive Multipurpose Organoplatinum(II) Metallacycles with Contrasting Mechanoresponsive Features.

The development of supramolecular coordination complexes (SCCs) with a bright aggregate state or mechanical-stimuli-responsive luminescence is very significant and challenging. Herein, we report the synthesis of three different supramolecular platinum(II) metallacycles via coordination-driven self-assembly of a diplatinum(II) acceptor and organic donors with a triphenylamine, carbazole, or tetraphenylethylene moiety. The triphenylamine-modified SCC exhibits aggregation-induced emission enhancement (AIEE) but no mechanofluorochromism. The carbazole and tetraphenylethylene-based SCCs exhibit changes in aggregate fluorescence and also exhibit reversible mechanofluorochromism. This work not only reports three rare metallacycles with AIEE, aggregate fluorescence change, or mechanofluorochromic nature but also explores their potential applications in cell imaging and solid-state lighting.

AIE and reversible mechanofluorochromism characteristics of new imidazole-based donor-π-acceptor dyes.

Four new imidazole-based donor–π–acceptor 2a–2d dyes have been synthesized, and their solvatochromism, aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated. The new dyes 2a–2d were designed to have 1,4,5-triphenyl-1H-imidazole as an electron donor (D) and 1-indanone, 1,3-indandione, 2-phenylacetonitrile and 2-thiopheneacetonitrile as electron acceptors (A) linked through a phenyl bridge. The maximum absorption wavelength of 2a–2d dyes in DCM solution appeared at 376, 437, 368, and 375 nm, respectively. The dyes exhibit a high molar extinction coefficient (ε) and large Stokes shift, making them useful in optoelectronic applications. Solvatochromic properties of dyes 2a–2d have been studied and showed bathochromic changes in emission wavelengths, from 449 to 550 nm for 2a, 476 to 599 nm for 2b, 438 to 520 nm for 2c, and from 439 to 529 nm for 2d, as the solvent polarity increased from n-hexane to acetonitrile. Moreover, in dioxane/water mixture systems, AIE behaviors were observed, and the emission intensity of 2b–2d dyes increased by around 5, 3, and 3 times in the mixed solvent (dioxane : water = 10 : 90) in contrast to pure dioxane. In addition, the XRD data of the 2a–2d dyes in pristine, ground, and fumed states illustrate that the transition between the ordered crystalline and disordered amorphous phases is the primary cause of MFC behaviors mechanism. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) showed that the highest occupied molecular orbital (HOMO) of dyes is distributed on the donor unit. In contrast, the lowest unoccupied molecular orbital (LUMO) is mainly placed on the acceptor unit to reveal that the HOMO–LUMO transition has a great ICT character.

D–A–D structured triphenylamine fluorophore with bright dual-state emission for reversible mechanofluorochromism and trace water detection

A donor–acceptor–donor-structured triphenylamine chromophore showing bright dual-state emission (DSE) has been synthesized and demonstrated multi-stimuli responsive properties, including water response, mechanochromism, and solvatochromism.

Excellent and reversible mechanofluorochromism in donor–acceptor π-systems based on bisarylic methanone derivatives

Mechanofluorochromic (MFC) materials characterized by high-contrast fluorescent colors and/or high intensities have rarely been reported. This can be attributed to the complex mechanism and molecular design associated with MFC phenomenon. Herein, the route followed for the preparation of two D–A type bisarylic methanone fluorescent molecules CAR-BZ-POZ and CAR-BZ-PTZ, has been presented. Results obtained from systematic photophysical experiments reveal that CAR-BZ-POZ and CAR-BZ-PTZ are characterized by twisted intramolecular charge-transfer (TICT) states, highly distorted molecular conformations, good solid-state emission properties, and high-contrast reversible mechanofluorochromism. The as-prepared CAR-BZ-POZ and CAR-BZ-PTZ solids emitted intense yellow-green fluorescence (531 and 550 nm, respectively), and the solid-state luminescence efficiencies recorded were 32.54% and 24.92%, respectively. When the samples were ground, orange-red fluorescence was emitted (602 and 594 nm, for CAR-BZ-POZ and CAR-BZ-PTZ, respectively). Results obtained by conducting mechanistic studies suggest that the MFC phenomenon can be attributed to the phase transition (from crystalline to amorphous phase) of the molecules. The red-shift in the PL spectral profiles, achieved following the process of grinding, could be attributed to the extension of the molecular conjugation length and the planar intramolecular charge transfer (PICT) properties.

Pyromellitic diimide-based luminophors: Tunable aggregation-induced emission (AIE) and reversible mechanofluorochromism characteristics

Four newly synthesized pyromellitic diimide-based fluorescent dyes with different aryl groups were first reported. All pyromellitic diimide (PMDI) derivatives were firstly displayed prominent aggregation-induced emission (AIE) properties. Additionally, the tunable fluorescence of PMDI derivatives were achieved by the varied substituent groups linked on the PMDI skeleton. With the enlarged sizes of isolation groups, aggregation-induced emission migration in the direction of red waves, as the result of the efficient inhibition of π-π stacking by the larger substituent groups. Furthermore, the mechanofluorochromic (MFC) performance of these PMDI derivatives could be tuned by different sized of the substituent groups. Interestingly, PMDI-ESCz, PMDI-PSCz and PMDI-DSCz exhibited reversible mechanofluorochromic performances, but no mechanochromism phenomenon was found for PMDI-In. These results exhibited that substituent groups had significant influence on their AIE and MFC properties.

Investigating the structure–fluorescence properties of tetraphenylethylene fused imidazole AIEgens: reversible mechanofluorochromism and polymer matrix controlled fluorescence tuning

A series of stimuli-responsive AIEgens of tetraphenylethylene (TPE) fused imidazole derivatives (1–7) were synthesized, and their substituent controlled fluorescence properties in the solid state were explored.