Restriction Of Intramolecular Motion Research Articles

A triple pH-responsive AIEgen: Synthesis, optical properties and applications

• A novel AIEgen with triple pH-responsiveness is synthesized. • Under strong alkaline conditions, the fluorescence is quenched and the micro-rods become nano-rods. • Under weak acidic conditions, the red fluorescence blue-shifts to yellow-green and the micro-rods transform to nano-spheres. • Under strong acidic conditions, the fluorescence is quenched and the nano-spheres are destroyed. • The AIEgen has been successfully applied in anti-counterfeit and dual encryption, photo-patterning and molecular logic gates. Stimuli-responsive luminogens with aggregation-induced emission (AIE) properties have attracted more and more attention for their intense emission in the aggregated state and tunable physicochemical performances. Nevertheless, it still remains challenging to design multifunctional AIE luminogens (AIEgens) with multi-responsive moieties and tunable fluorescence and morphologies. Here, a novel multifunctional AIEgen (DAS-BA) is elaborately designed based on the synergistic effect of twisted intramolecular charge-transfer (TICT), restriction of intramolecular motions (RIM), and excited-state intramolecular proton transfer (ESIPT). DAS-BA not only possesses triple pH-responsivenesses and long-wavelength emission, but also shows corresponding transition in its fluorescence and self-assembled morphologies. Under strong alkaline conditions, owing to the deprotonation of phenolic hydroxyl group, the red emission of DAS-BA is quenched and the molecular assemblies change from micro-rods to nano-rods. However, under weak acidic conditions, the red fluorescence dramatically shifts to yellow-green and the assemblies strikingly transform from micro-rods to nano-spheres due to the protonation of diethylamino moieties. Further acidification can hydrolyze the Schiff base, resulting in fluorescence quenching and destruction of the nano-spheres. Based on the reversible protonation process of diethylamino moiety, DAS-BA exhibits CO 2 -activated reversible fluorescence switching. Furthermore, DAS-BA has been successfully applied in anti-counterfeit and dual encryption, photo-patterning and molecular logic gates.

Remarkable mechanochromism and force-induced thermally activated delayed fluorescence enhancement from white-light-emitting organic luminogens with aggregation-induced emission

The development of organic materials with white-light emission and thermally activated delayed fluorescence (TADF) properties in the solid state remain a challenge. Herein, a series of white-light-emitting organic luminogens have been developed and are found to show aggregation-induced delayed fluorescence (AIDF) characteristics. The AIDF emitters present dual-emission consisted of prompt fluorescence and TADF in the crystalline state. Their white-light emissions can be easily tuned by altering the chemical structure and connecting position of the heterocyclic aromatic substituent. Under the stimuli of mechanical force and solvent vapor, the compounds exhibit remarkable and reversible mechanochromism, in which their emission colors are switchable between white and yellow. Upon grinding, they also display linearly tunable luminescence colors, as well as force-induced TADF enhancement, which may be associated with the more compact molecular packing and the restriction of intramolecular motions. The results from time-resolved emission scanning and theoretical calculation suggest that the dual-emission of the AIDF luminogens likely results from the twisted intramolecular charge transfer transitions of the molecules, and the reversible mechanochromism properties probably stem from the interconversion of the quasi-axial and the quasi-equatorial conformations.

Luminescent gold-peptide spheric aggregates: selective and effective cellular targeting

Although the restriction of intramolecular motion has been well recognized as the fundamental of aggregation induced emission enhancement (AIEE), the regulation mechanism of gold nanoclusters (AuNCs) based AIEE system are still unclear. In this paper, we have investigated the Zn2+-induced AIEE process of thiolate ligands (i.e., cysteine, glutathione and an 8-mer peptide) protected AuNCs, which shows a pH-dependent evolution from single AuNCs to spheric aggregates to irregular network. Using photoluminescent enhancement ratio as an index, the concept of “mid-pH” is proposed to indicate the optimal pH for the formation of spheric AuNCs aggregates. Importantly, the surface ligands allow the formation of spheric AuNCs aggregates at tunable mid-pH between 5.7 and 7.5. Owing to the appropriate size and surface peptide targetability, the spheric AuNCs aggregates can be successfully screened for targeted tumor cell uptake and imaging at physiological pH. The cell uptake mechanism study showed that AuNCs aggregates was specifically recognized by arginine-glycine-aspartic acid (RGD) sequence on the ligand and integrin αvβ3 on the cell surface, thus mainly through clathrin-mediated endocytosis. This work provides new sight to artificially regulate the construction of efficient cellular imaging probes.

Pathways to fluorescence via restriction of intramolecular motion in substituted tetraphenylethylenes

Functionalization is used to restrict the torsional, excited-state proton transfer and cyclization modes in UV-excited tetraphenylethylenes.

Triplet exciton dynamics of pure organics with halogen substitution boosted two photon absorption and room temperature phosphorescence: A theoretical perspective

Organic room temperature phosphorescence (RTP) molecules have shown promising applications in organic light emitting diodes and vivo imaging. Thus, triplet exciton dynamics in solid phase should be revealed and the molecule should possess large two photon absorption (TPA) cross sections under near-infrared excitation. The effects of halogen substitution and intermolecular interaction on RTP and TPA properties are studied at molecular level for a series of derivatives. Surrounding environment in solid phase is considered by combined quantum mechanics and molecular mechanics method. Intermolecular interactions are evaluated by the independent gradient model and calculated through the molecular force field energy decomposition method. Minimum energy crossing point, Huang-Rhys factor and reorganization energy are discussed, triplet exciton dynamics are investigated by thermal vibration correlation function method. Results indicate that the largest TPA cross sections are found for molecule in water. The halogen substitution can enlarge the proportion of (π, π*) and facilitate the intersystem crossing process. Restricted intramolecular rotation motions of dihedral angle in low frequency regions are found for Br-Np-Cz-BF2 in solid phase. While enhanced vibrations of bond length and bond angle in high frequency regions are detected for I-Np-Cz-BF2. Effects of halogen substitution and intermolecular interaction on triplet exciton dynamics are highlighted.

Ruthenium(II) Polypyridine Complex-Based Aggregation-Induced Emission Luminogen for Rapid and Selective Detection of Phosgene in Solution and in the Gas Phase.

A bis-heteroleptic ruthenium(II) complex, Ru-1, of 4,7-bis(2-aminoethylamino)-1,10-phenanthroline for selective "turn-on" detection of highly toxic chemical warfare agent phosgene is presented. Probe Ru-1 exhibits aggregation-induced emission (AIE), and the restricted intramolecular motion is responsible for the AIE activity. In a CHCl3/CH3CN [95:5 (v/v)] solvent mixture, a unique self-assembled vesicular structure was formed after aggregation, which was supported by transmission electron microscopy, field emission scanning electron microscopy, and atmoic force microscopy studies. Probe Ru-1 showed a rapid and highly selective luminescence turn-on response for phosgene over other competitive chemical warfare agents with a low detection limit (13.9 nM) in CH3CN. The 2-aminoethylamino groups in Ru-1 act as a reacting site for nucleophilic addition to the carbonyl center of phosgene and undergo intramolecular cyclization. The final product of the phosgene-mediated reaction, Ru-1-Phos, contains 2-imidazolidinone groups, which has been confirmed by electrospray ionization mass spectometry and 1H nuclear magnetic resonance (NMR) spectroscopy. 1H NMR titration of Ru-1 with phosgene supported the reaction mechanism and also pointed to the simultaneous reaction of phosgene at two 2-aminoethylamino sites. For the first time, the crystal structure of the phosgene reaction product, Ru-1-Phos, containing the cyclized 2-imidazolidinone group was confirmed by single-crystal X-ray diffraction, which indubitably validates the reaction mechanism. Triplet state time-dependent density functional theory calculations showed that the weak luminescence of Ru-1 was mostly due to the population of the non-emissive 3MC state. The cyclization reaction with phosgene and the corresponding 2-imidazolidinone product formation populated the emissive 3MLCT state in Ru-1-Phos and is the key reason for the enhanced luminescence. Furthermore, a low-cost portable test paper strip has been fabricated with Ru-1 for the real-time selective monitoring of phosgene gas at the nanomolar level.

Amplification of Electronic Circular Dichroism-A Tool to Follow Self-Assembly of Chiral Molecular Capsules.

Electronic circular dichroism (ECD) can be used to study various aspects of self-assembly (definition of stoichiometric ratios, chirality amplification during self-assembly, host-guest complexation). In this work, we show that ECD is a valuable tool for monitoring the self-assembly of chiral peptide-based capsules. By analyzing the signs, intensities, and temperature dependences of ECD bands, the effects of the non-specific processes can be separated from the restriction of intramolecular motion (RIM) caused by discrete self-assembly. Analysis of experimental and theoretical ECD spectra show that the differences between assembled and non-assembled species originate from induction of inherently chiral conformation and restriction of conformational freedom that leads to amplification of ECD signals during self-assembly of discrete species.

Excited-State Modulation of Aggregation-Induced Emission Molecules for High-Efficiency Triplet Exciton Generation

Restriction of intramolecular motion (RIM) is commonly adopted to achieve high luminescence for aggregation-induced emission (AIE) materials. However, recent studies found that RIM cannot fully exp...

Aggregation-Induced Fluorogens in Bio-Detection, Tumor Imaging, and Therapy: A Review

Aggregation-Induced Fluorogens in Bio-Detection, Tumor Imaging, and Therapy: A Review

Emerging Designs of Aggregation-Induced Emission Agents for Enhanced Phototherapy Applications

Emerging Designs of Aggregation-Induced Emission Agents for Enhanced Phototherapy Applications

Frontier luminous strategy of functional silica nanohybrids in sensing and bioimaging: From ACQ to AIE

AbstractFluorescent silica organic–inorganic nanohybrids which combine designable luminescence performance of organic fluorescent dyes and various outstanding advantages of silica nanomaterials have attracted increasing research interests in these fascinating areas. Optical transparency and facile functional modification properties of silica material provide great opportunities to integrate desired fluorescent molecules for various frontier luminous applications. However, conventional organic dyes are typically subject to aggregation‐caused quenching due to their aggregation in silica matrix, which could be detrimental for their performance in sensing and biomedical applications. The appearance of aggregation‐induced emission luminogens (AIEgens) paves a new way for developing highly efficient fluorescent silica nanohybrids (FSNs). FSNs with intensive luminescence could be obtained due to the formation of aggregates and the restricted intramolecular motion of AIEgens in silica inorganic matrix. In this review, the reported fabrication methodologies of various FSNs based on colloidal silica nanoparticles (SNs) and mesoporous SNs including physical entrapment and covalent strategies are summarized. Especially, the AIEgens‐functionalized silica hybrid nanomaterials are introduced in detail. Furthermore, chemical sensing, biosensing, and bioimaging applications of resultant FSNs are also discussed.

Aggregation‐induced emission of copper nanoclusters

AbstractCopper nanoclusters (Cu NCs) have recently emerged as promising luminophores, featuring ultra‐small size, reasonable photostability, large Stokes shift, and long emission lifetimes. Aggregation‐induced emission (AIE) has been often used to further improve both the emission intensity and stability of these clusters, with plenty of potential applications in the fields of chemical sensing and bioimaging. This review starts with a summary of the current understanding of emission mechanisms of Cu NCs and proceeds with the analysis of contributions from the Cu metal core and the organic ligands. We summarize the recent research progress on the design of ligands, and the ways on how to induce aggregation of the Cu NCs through electrostatic charge neutralization, host‐guest interactions, and the use of templates. We also discuss the current understanding of emission mechanisms of Cu NCs experiencing AIE, such as the often‐cited restriction of intramolecular motion and contributions from Cu(I) molecular complexes. We finish this review by providing concluding remarks and offering our own perspective on the active field of AIE of Cu NCs, with a hope to further promote the research on the fundamental aspects of this useful phenomenon.

Insights into Optical Probes Based on Aggregation‐Induced Emission: from Restriction of Intramolecular Motions to Dark State

AbstractSince the exposure of the concept of aggregation‐induced emission (AIE) to the scientific community in 2001, AIE fluorophores attract much attention for biological imaging, detection, and advanced material fabrications. Some efforts have been devoted to the mechanism studies on the luminescence nature of AIE fluorophores. Restriction of intramolecular motions (RIM) can explain the luminescence mechanism at a relatively large level and has been recognized as the general working mechanism of AIE. To disclose the connotation of the RIM mechanism, an in‐depth study to elucidate the dark state of AIE molecules is needed. In this review, the recent progress of luminescence quenching mechanisms for AIE molecules is summarized and compendiously categorized into four parts, which are: restriction of access to dark state (RADS), restricted access to a conical intersection (RACI), restrict molecules from entering twisted intramolecular charge transfer (TICT) state, and excited state intramolecular proton transfer (ESIPT) coupled with TICT state, respectively. These insights and systematical elucidation of the emission quenching in the view of dark state are of great significance for fundamental understanding. It is hoped that this feature article will attract more researcher's interests and open up new vistas for the development of aggregate science.

A water-soluble AIEgen for ultrafast and wash-free imaging of plasma membranes in biosystems

Visualizing the plasma membrane (PM) in biological systems is in a front field of life science research. Herein, a rotor-based compound Car-py with aggregation-induced emission (AIE) activity has been designed and synthesized. Car-py can selectively light-up the PM through binding to the lipid bilayer structure owing to a restriction of intramolecular motions (RIM). Importantly, compared with commercial PM dyes, Car-py exhibits many advantages, including wash-free procedure, good photostability, ultrafast staining process, long-term tracking, excellent plasma membrane specificity, and favorable biocompatibility, which were confirmed by the fluorescence image in vitro and in vivo. Such PM-specific probe may extend the arsenal of currently available fluorescent agents to visualize the plasma membrane.

Mechanistic connotations of restriction of intramolecular motions (RIM).

Restriction of intramolecular motion (RIM) is the widely-accpeted mechanism of aggregation-induced emission (AIE). In this concise and comprehensive perspective, four mechanistic models related to different nonradiative pathways are summarized with examples to disclose the connotation of RIM, and meaningful mechanistic topics for future researches are advised.

Monitoring of the decreased mitochondrial viscosity during heat stroke with a mitochondrial AIE probe.

Heat stroke is a fatal condition which usually results in central nervous system dysfunction, organism damage and even death. The relationship between heat stroke and mitochondria is still relatively unknown due to a lack of suitable tools. Herein, an aggregation-induced emission (AIE) probe CSP, by introducing a pyridinium cation as the mitochondria-targeted group to an AIE active core cyanostilbene skeleton, is highly sensitive to viscosity changes due to the restriction of intramolecular motion (RIM) and inhibition of twisted intramolecular charge transfer (TICT) in high-viscosity systems. As expected, with the viscosity increasing from 0.903cP (0% glycerol) to 965cP (99% glycerol), CSP exhibited a significant enhancement (more than 117-fold) in fluorescence intensity at 625nm, with an excellent linear relationship between log I 625 nm and log η (R2 = 0.9869, slope as high as 0.6727). More importantly, using CSP we have successfully monitored the decreased mitochondrial viscosity during heat stroke for the first time. All these features render the probe a promising candidate for further understanding the mechanism underlying mitochondria-associated heat stroke.

Twisted Intramolecular Charge Transfer—Aggregation-Induced Emission Fluorogen with Polymer Encapsulation-Enhanced Near-Infrared Emission for Bioimaging

Fluorescence probes with strong near-infrared (NIR) emission and water solubility are considered useful visualization tools for localization marking as well as investigating cell migration and tran...

Twisted intramolecular motion arrested in aggregated state emission and the nonlinear optical properties of pyrene pyrazoline derivatives

Heterocyclic pyrene pyrazoline moieties containing similar structures but with differences in thiophene (PPT), furan (PPF) and pyridine (PPP) substitutions at the terminal molecules were synthesized. Their aggregation behaviour in THF-water mixtures was investigated and results demonstrated that PPT and PPP exhibited aggregation-induced emission (AIE), whereas PPF exhibited aggregation-induced blue-shifted emission (AIBSE). PPT and PPP provided red-shifted emission, while PPF had observed blue-shifted emission at high water fractions of 70-90%, confirming that aggregation effects played a major role in the molecular structure. Two emission peaks from locally excited and twisted intramolecular charge transfer confirmed the twisted nature from the dihedral angle values of the free reorganized molecules that were completely restricted in high water fractions due to molecular aggregation. This was further confirmed from colour Commission Internationale de l'Eclairage values as well as dynamic light scattering analysis. Third-order nonlinear optical properties were studied using a Nd:Yag laser beam Z-scan technique at 532 nm. The open aperture Z-scan revealed that PPT and PPF towards the peak point endured strong saturable absorption, whereas PPP indicated a strong reverse saturable absorption process. The AIE and AIBSE mechanisms from undergoing restricted twisting intramolecular motion in the aggregated luminogens provide great insight into new developments in AIEgen materials for these optoelectronic materials.

Adjusting the photophysical properties of AIE-active TADF emitters from through-bond to through-space charge transfer for high-performance solution-processed OLEDs

Intramolecular through-space charge transfer (TSCT) excited state has been exploited for developing thermally activated delayed fluorescence (TADF) emitters, while the adjustment of excited state dynamics by conformational engineering is still rare. Herein, we designed a series of AIE-active TADF emitters, bearing triphenylamine as donor and phenyl ketone as acceptor unit, and the evolution of charge transfer from through-bond to through-space is realized by controlling the relative position of the donor and acceptor units. Upon transforming the charge transfer categories from through-bond to through-space, the reverse intersystem crossing (RISC) rate is progressively improved by virtue of the narrower energy split between singlet and triplet states (ΔΕST), meanwhile, the restricted intramolecular motion suppresses the non-radiative decay effectively, resulting in obviously improved luminescence efficiency. Appointing a bipolar blue TADF material as host to sensitize these emitters, solution-processed organic light-emitting diodes (OLEDs) based on TSCT emitter 1TCPM-Cz achieve maximum current efficiency (CE) of 35.5 cd A−1 and external quantum efficiency (EQE) of 13.3%, respectively, which is higher than that of through-bond charge transfer (TBCT) analogues. The work demonstrates a convenient design strategy to achieve high-performance TSCT emitters by regulating the conformation of the donor and acceptor.

Restriction of Intramolecular Motion(RIM): Investigating AIE Mechanism from Experimental and Theoretical Studies

Mechanistic studies promote scientific development from phenomena to theories. Aggregation-induced emission(AIE), as an unusual photophysical phenomenon, builds the bridge between molecular science and aggregate mesoscience. With the twenty-year development of AIE, restriction of intramolecular motion(RIM) has been verified as the working mechanism of AIE effect. In this review, these mechanistic works about RIM are summarized from experimental and theoretical perspectives. Thereinto, the experimental studies are introduced from three parts: external rigidification, structural modification and structural characterization. In the theoretical part, calculations on the low-frequency motion of AIEgens have been performed to prove the RIM mechanism. By virtue of the theoretical calculations, some new mechanisms are proposed to supplement the RIM, such as restriction of access to conical intersection, suppression of Kasha transition, restriction of access to dark state, etc. It is foreseeable that the RIM mechanism will unify the photophysical theories for both molecules and aggregates, and inspire more progress in aggregate science.