Intramolecular Rotation Research Articles

Dynamics of a Molecular Rotor Exhibiting Local Directional Rotational Preference within Each Enantiomer

Directional internal rotation in molecular systems, generally controlled by chirality, is known to occur in natural and artificial systems driven by light or fueled chemically, but spontaneous directional molecular rotation is believed to be forbidden. We have designed a molecular rotor, whereby ferrocene and triptycene linked by a methylene bridge provide two rotational degrees of freedom. On the basis of experimental observations, in conjunction with computational data, we show that the two different modes of rotation are strongly coupled and the spatial orientation of the bistable ferrocene moiety controls the barrier to its own rotation about the triptycene axis. It is proposed that the barrier to clockwise 120° rotation across each individual triptycene blade is lower in the M-enantiomer and for counterclockwise 120° rotation, it is lower in its P-counterpart. These findings demonstrate the possibility of locally preferred thermal directional intramolecular rotation for each dynamically interconverting enantiomer.

Development of a near-infrared fluorescent ligand that visualizes and stabilizes G-quadruplexes by decorating the triphenylamine scaffold

Abstract G-quadruplex (G4) structures are often enriched in cancer-driving genes with elevated transcriptional activity, which are now treated as potential targets for cancer diagnosis and therapy. Thus, development of dual-function ligands that can simultaneously visualize and stabilize G4s are urgently needed. In this study, two π-extended compounds were designed and synthesized by decorating the triphenylamine scaffold with positively charged indolium moieties, which were then revealed as NIR fluorescent probes possessing AIE characteristics. Next, the more promising compound TAIN-2 was further investigated, demonstrating that TAIN-2 could selectively bind to G4s through an end-stacking mode, which thereby triggered its fluorescence by restricting the intramolecular rotations, and largely stabilized the G4 structures. Finally, we also explored the applications of TAIN-2 in the triple-negative breast cancer (TNBC) cells, and the results suggested that TAIN-2 may have the ability to visualize and stabilize the intracellular G4s, holding great potential to develop into feasible diagnostic/therapeutic dual-function agents for TNBC. This study provides new insights into the development of novel theranostic agents for cancers.

Front Cover: Structure and Properties of Fumaryl Fluoride (Z. Anorg. Allg. Chem. 4/2021)

The cover picture shows a rotating laser wheel of different colors. The three planar rotational isomers of fumaryl fluoride (trans-trans, cis-cis and cis-trans) are placed on a blue circle of light in the picture. The arrows illustrate the intramolecular rotation of fumaryl fluoride, also exemplified by the spinning laser wheel. The molecular structure and the conformational composition of fumaryl fluoride were determined by low-temperature vibrational spectroscopy and single-crystal X-ray structure analysis. More details can be found in the article by Marie C. Bayer, Christoph Jessen, and Andreas J. Kornath on page 258.

Structure–Property Relationship on Aggregation-Induced Enhanced Emission of the Spirobifluorene Derivatives including Herringbone, T, and Helical Aggregation Mode in Solid-Phase: Synthesis, Crystallography, Density Functional Theory, Optical, and Thermal Properties

The development of efficient luminescent materials in the solid state attracts great attention for their potential applications. Based on this importance, we presented the structure–property relationship of the spirobifluorene (SBF) derivative molecules by photophysical studies and single-crystal X-ray crystallography and very important luminogens having aggregation-induced emission behavior in the solid state. Also, cyclic voltammetry (CV), thermogravimetric analysis (TGA), spectroscopic, and theoretical studies of these molecules are presented. We show that the molecular structure, conformational twisting, structural rigidity, and supramolecular packing play significant roles in the SBF molecules’ photoluminescence behavior, especially in determining whether they are aggregation-induced emission -active. These molecules demonstrated enhanced emission in the supramolecular aggregate state created by herringbone-, helical-, and T-type π···π stacking interactions compared to that of the solution phase. Moreover, the restricted intramolecular rotation has been demonstrated to play a significant role in the aggregation-induced enhanced emission properties of the SBF molecules. The most increasing in PL intensity was observed in the molecule SBF1 and SBF3 with the T- and helical-type stacking interactions, respectively. We concluded that the helical- and T-type stacking modes with a small perpendicular distance result in high fluorescence intensity in the SBF molecules’ aggregate state. This study will provide an overview of supramolecular arrangements on the solid-state optoelectronic properties of SBF derivatives. Also, it would suggest new pathways toward targeted molecular design strategies.

Synthesis, crystal structures and solid-state acidochromism of multiaryl-substituted pyridine derivatives with aggregation-induced emission property

Pyridine derivatives have been widely reported as fluorescent sensors in solution phase, but they are rarely used in solid state due to aggregation-caused quenching effect. Herein, two new multiaryl-substituted pyridine derivatives with benzofuran (ABFP) and benzothiophene (ABTP) at 6-position were synthesized to study solid-state emissions and fluorescence stimulus-responsive properties. These two compounds have highly twisted molecular conformations, as revealed by the single-crystal structure analyses. They emit slight fluorescence in pure organic solutions due to the free rotations of peripheral aromatic rings, whereas show strong blue/cyan emissions in aggregated state because of the restriction of intramolecular rotation, revealing typical aggregation-induced emission (AIE) characteristic. These compounds adopt loose stacking arrangements and have weak intermolecular interactions, which are advantageous to strong solid-state emissions. Upon the stimulation of trifluoroacetic acid-ethylenediamine, the ABFP and ABTP solid-state samples exhibit a transformation in fluorescence colour between blue and green/yellow-green, revealing reversible acidochromic activities. Furthermore, the protonation of the nitrogen atom on the pyridine ring is unambiguously confirmed to be responsible for the acidochromic activities in solid state by the single-crystal structure analyses. The work provides important information for the design of the fluorescent molecules with AIE and solid-state acidochromism based on the traditional pyridine skeleton.

Copper phthalocyanine/fluorapatite nano composite and its pH- and photo-responsive properties

An efficient drug delivery system has been prepared using copper phthalocyanine (CuPc) and fluorapatite (CuPc-FAP). This composite showed excellent dispersibility in water compared with pure CuPc. Interestingly, unlike the CuPc, the CuPc-FAP composite exhibited strong fluorescence emission around 480 nm, which should be attributed to the restriction of intramolecular rotation of CuPc. Drug delivery experiments were carried out using glycine (Gly), histidine (His) and adenine (Aden) as the drug markers. For Gly and His, the suitable adsorption arises from electrostatic interactions between COO− groups of the drug markers and Ca+ ions of CuPc-FAP. Besides, the hydrogen bonds could be form between NH3+ of Gly and His and surface oxygen atoms of the PO4 group of CuPc-FAP. However, FTIR results showed that the CC group from the six-membered ring of Aden and the NH2 group are the suitable sites for the interaction of Aden with CuPc-FAP. The composite showed interesting properties including pH- and photo-responsive drug release, due to the co-existence of fluorapatite and CuPc, respectively. We hope that CuPc-FAP be a novel promising candidate as water dispersible drug carrier.

Microcrystal Electron Diffraction Elucidates Water-Specific Polymorphism-Induced Emission Enhancement of Bis-arylacylhydrazone.

Aggregation-induced emission (AIE) phenomena have gained intense interest over the last decades because of its importance in solid-state emission. However, the elucidation of a working mechanism is difficult owing to the limited characterization methods on solid-state molecules, further complicated if dynamic structural changes occur. Here, a series of bis-arylacylhydrazones (BAHs) were synthesized, for which their AIE properties are only turned on by the reversible adsorption of water molecules. We used microcrystal electron diffraction (MicroED) to determine the molecular structures of two BAHs directly from bulk powders (without attempting to grow crystals) prepared in the absence or presence of water adsorption. This study reveals the unambiguous characterization of the dependence of crystal packing on the specific cocrystallization with hydrates. The structural analysis demonstrates that water molecules form strong hydrogen bonds with three neighboring BAH-1, resulting in the almost complete planarization and restriction of the intramolecular rotation of the molecule. MicroED plays an important role in providing a decisive clue for the reversible polymorphism changes induced by the adsorption of water molecules, regulating emissive properties.

Green fluorescent waterborne polyurethane polyols

In this paper, we reported a series of green fluorescent flame-retardant waterborne polyurethane polyols. The polyurethane polyols were uniformly sized and possessed decent colloidal stability. Comparing to other reported products, our aqueous dispersions showed much stronger fluorescence intensity and greatly improved long-term stability, which were desired for practical applications. Small molecule conjugation units were gated into polyurethane chains via reaction between −OH of naphthalene and −NCO of diisocyanate, which restricts intramolecular rotation of naphthalene chromophores and non-radiative transition to enhance fluorescence intensity. The non-radiative transition and chromophore aggregation are weakened to hinder aggregation-caused quenching of chromophores to improve fluorescence by weak collision. This study has demonstrated a facile method to prepare waterborne polyurethane polyols, providing detailed explanation and guidance for related work in the future.

InVivo Brain Imaging of Amyloid-β Aggregates in Alzheimer's Disease with a Near-Infrared Fluorescent Probe.

Abnormal accumulation of amyloid-β (Aβ) has been determined to be a critical factor for the progression of Alzheimer's disease (AD), which has motivated the development of new chemical approaches for early sensing and imaging of these Aβ aggregates. Herein, we report a new near-infrared (NIR) fluorescent probe for the selective monitoring of Aβ aggregates in vivo. This novel fluorophore, named CAQ, was based on the curcumin scaffold and was designed by introducing an intramolecular rotation donor and a quinoline functional group. CAQ was an environment-sensitive fluorescent probe that can be used as a reliable chemical tool for NIR imaging of amyloid plaques in a live Caenorhabditis elegans model of AD and in 5× FAD transgenic mice of early amyloid deposition. Our observations indicate that CAQ is promising for providing comprehensive information on neurodegenerative research, thereby promoting a deeper understanding of Alzheimer's pathological processes.

Aggregation induced emission (AIE)‐active N‐arylated ferrocenyl pyrazole‐based push–pull chromophores: Structural, photophysical, theoretical and effect of substitution on second‐order non‐linear optical properties

In this study, we report the synthesis and structural characterization of N‐arylated ferrocenyl pyrazole [Fc‐Pz‐(CF3)‐C6H5‐R] (R = CH3, 1; OCH3 2), where Pz = pyrazole and Fc = ferrocenyl = Fe(η5‐C5H5)(η5‐C5H4) chromophores with D–π–A–π–D system. The solvatochromic behaviour of compounds 1 and 2 was investigated in various solvents from non‐polar to polar. When solvent polarity increases, the charge transfer process produces the more polarized excited state, and consequently, positive solvatochromism was observed. The emission performance of the compounds is weak luminescence in CH3CN solution but becomes high emissive in a mixture of water/CH3CN fraction at (fw) 90%, demonstrating aggregation‐induced emission (AIE) characteristics, and the mechanism of AIE has been attributed to restricted intramolecular rotation (RIR) process. The time‐resolved fluorescence lifetime studies have been demonstrated in water/CH3CN mixture at 90%, as it shows three relaxation pathways in their fluorescence decay. In addition, density functional theory calculations have been employed to optimize the geometry of the molecules, followed by computation of dipole moment (μ), first hyperpolarizability (β) using various functionals (B3LYP, long‐range corrected CAM‐B3LYP and LC‐BLYP) and frontier molecular orbital levels for understanding the charge transfer, electrochemical and non‐linear optical (NLO) properties. The second‐order non‐linear optical properties were studied by hyper‐Rayleigh scattering (HRS) technique using an incident wavelength at 1064 nm. The chromophores exhibited the first hyperpolarizability (βHRS = 38.6 × 10−30 e.s.u. for 1 and βHRS = 116.8 × 10−30 e.s.u. for 2) and were found to be 6.8 times greater than that of the reference material, p‐nitroaniline (pNA) (βHRS = 17.1 × 10−30 e.s.u.). Also, compound 2 shows higher NLO effect because the presence of –OCH3 group makes effective charge transfer process by mesomeric effect (+M).

Luminescence properties of cyclometalated platinum(II) complexes in a dichloromethane/n-hexane system

The luminescence properties of three neutral cyclometalated Pt(II) complexes were investigated in a CH2Cl2/n-hexane system. The diphenylamino and trifluoromethyl modified Pt2 exhibits aggregation-induced phosphorescent emission (AIPE) feature. The crystal stacking of Pt2 shows that the intermolecular hydrogen bonding restricts both intramolecular rotation (RIR) and vibration (RIV), which weakens the non-radiative decay ways and leads to an AIPE performance. TEM images reveal that Pt2 aggregates into nanosheets.

Heat Capacity and Bond Dissociation Energy Calculations of Some Fluorinated Ethanol’s and its Radicals: CH<sub>3-x</sub>CH<sub>2</sub>F<sub>x</sub>OH, CH<sub>3</sub>CH<sub>2-x</sub>F<sub>x</sub>OH

Structures and thermochemical properties of these species were determined by the gaussian M-062x/6-31 + g (d, p) calculation enthalpies of formation for 19 fluorinated ethanol and some radicals were calculated with a popular Ab initio and density functional theory methods: The gaussian M-062x/6-31 + g (d, p) via several series of isodesmic reactions. Entropies (S298°K in Cal·Mol-1 K-1) were estimated using the M-062x/6-31 + g (d, p) computed frequencies and geometries. Contributions of entropy, S298°K, and heat capacities, Cp(T) due to vibration, translation, and external rotation of the molecules were calculated based on the vibration frequencies and structures obtained from the M-062x/6-31 + g (d, p) Density Functional Method. Potential barriers are calculated using M-062x/6-31 + g (d, p) density functional method and are used to calculate rotor contributions to entropy and heat capacity using integration over energy levels of rotational potential. Rotational barriers were determined and hindered internal rotational contributions for S298° - 1500°K, and Cp (T) were calculated using the rigid rotor harmonic oscillator approximation, with direct integration over energy levels of the intramolecular rotation potential energy curves. Thermochemical properties of fluorinated alcohols are needed for understanding their stability and reactions in the environment and in thermal process

Construction of fluorescent rotors with multiple intramolecular rotation sites for visualization of cellular viscous compartments with elevated fidelity.

Viscosity-sensitive fluorescent dyes are widely utilized to image viscous intracellular compartments with high fidelity. However, the sensitivity of many fluorescent rotors needs improvement for bioimaging applications. Herein, we proposed to construct a fluorescent rotor with multiple intramolecular rotation sites to elevate its sensitivity to environmental viscosity. The fabricated fluorescent rotor showed evidently increased sensitivity to viscosity and had the potential to image intracellular viscous compartments with improved fidelity. By decorating the rotor with sidechains of different lengths, we successfully fabricated two fluorescent probes, TAPI-6 and TAPI-16, to visualize the mitochondria and plasma membrane, respectively, with high fidelity. The two probes were also successfully utilized to clearly visualize the mitochondria and plasma membranes in skeletal muscle tissue, cardiac muscle tissue, and liver tissue, demonstrating the potential of the fluorescent rotor for bioimaging applications. We believe that the strategy of increasing the sensitivity to viscosity using multiple rotation sites is valuable for the construction of fluorescent rotors, and the presented fluorescent probes in this work can serve as powerful tools for biological research.

An unprecedented pyridine-based dinuclear mixed-valent ReI/VII oxo-bridged complex: a solvatochromic and AIE-active probe for nanomolar detection of picric acid and trinitrotoluene.

This paper describes the synthesis of an unprecedented oxo-bridged rheniumI/VII (Re) complex by treating Re2(CO)10 with a pyridyl-linked anthracene-based twisted π-conjugated ligand. The molecular structures of both the ligand and the complex are determined by analyzing IR, NMR, and HR-MS spectra and unequivocally determined using single-crystal X-ray diffraction studies. Unlike previous observations, the complexation occurs uniquely to yield an unprecedented oxo-bridged ReI/VII complex. Such a complex is uncommon, and in most cases, Re(vii) appears as the ReO4- counter ion. The aggregation-induced emission (AIE) feature could have been achieved from this conformationally twisted ligand, but the emission of the ligand was quenched in the aggregated state. The complex exhibited solvatofluorochromic properties with a faint emission. The emission intensity significantly (∼6 times) increased in DMF after the addition of a water fraction of 90%, resulting in a bright orange emission. The AIE is mainly caused by restricted intramolecular rotation (RIR) and is supported by the polarity and viscosity effects. The nanoaggregate formation is captured by SEM, and DLS studies were used to determine the average particle size. After the complexation, the ligand becomes more rigid, and the RIR effect becomes prominent facilitating the AIE effect. The electron-rich aggregate's intense orange emission was used for the selective and sensitive detection of picric acid (PA) and 2,4,6-trinitrotoluene (TNT) at nanomolar levels amongst other nitroaromatics through emission quenching. The detailed mechanistic studies reveal the active role of dynamic quenching and complementary photo-induced electron transfer between the probe and TNT or PA. The easy electron transfer process from the electron-rich to the electron-poor system is confirmed by calculating the lowest unoccupied molecular orbital energy of the associated levels. The application is further extended for on-site PA and TNT detection by permeating the probe on a paper and detected at 10-3 M concentration with the naked eye. The PA/TNT detection efficiency is also confirmed by mixing PA or TNT with soil.

Carbazole-modified thiazolo[3,2-c][1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism.

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability and comparatively high photoluminescence quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxadiazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggregation-induced emission properties of the compounds: in THF/water mixtures with a large water percentage, they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole moiety induced significant mechanofluorochromic properties of the compounds.

Examination of aggregation-induced enhanced emission in a propeller-shaped chiral nonconjugated blue emitter from restricted intramolecular rotation and J-type π⋯π stacking interactions

Herein, to get rid of aggregation-caused quenching (ACQ) in the solid phase, which is generally observed in π-conjugated planar molecules, a new chiral nonconjugated molecule with the propeller-shaped blue emitter property was prepared and characterized, and its photoluminescence behavior in the solid and solution phases were investigated through single crystal X-ray crystallography and photophysical studies. To further understand the aggregation-induced enhanced emission process (AIEE), concentration effect and solvent effect on the fluorescence properties of the solution phase and fluorescence quantum yield measurements in the solid and solution states were performed. In addition, thermal gravimetric analysis (TGA), cyclic voltammogram measurements (CV) and theoretical analysis of the molecule were carried out. According to the results of the experimental work done, the molecule showed aggregation-induced enhanced blue emission (AIEE) in the crystal state, which arose from the strong J-type ππ stacking interactions between the adjacent rings of the molecules, and the restricted intramolecular rotation process (RIR). The solvent polarity-dependent emission also verified the AIEE process. Therefore, combining the advantages of nonconjugation and chirality properties in the synthesis of the molecule afforded a new, unusual AIEE material and provided a new way for the design of a nonconjugated chiral fluorescent optoelectronic material.

An aggregation-induced phosphorescent emission-active iridium(III) complex for fluoride anion imaging in living cells

Fluoride anion plays a crucial role in bone and tooth growth. However, overdose of fluoride anion can also lead to many chronic diseases and there is no specific medicine for fluorosis so far. Hence, the early detection and accurate estimation of fluoride anion intake are important. In this work, an aggregation-induced phosphorescent emission (AIPE)-active iridium(III) complex as fluoride anion probe (Ir-AF) has been developed by incorporating N-(4-hydroxylphenyl)-rhodol protected by tert-butyldiphenylsilyl onto cyclometalating ligand. As Ir-AF reacts with fluoride anion, the nonradiative transitions of Ir-AF would be promoted by enhanced photoinduced electron transfer effect and intramolecular rotations, leading to its phosphorescence quenching dramatically. Thus, Ir-AF has been used for fluoride anion measurement in the aqueous solution by ratiometric readout. Furthermore, with the utilization of the long-lived phosphorescence from Ir-AF, fluoride anion sensing in living cells with high signal-to-noise ratio has been achieved by time-resolved photoluminescence imaging. This is the first report of AIPE-active probe for fluoride anion.

A novel aggregation induced emission (AIE) fluorescence probe by combining tetraphenylethylene and 2′,3′-O-isopropylideneadenosine for localizing Golgi apparatus

Golgi apparatus plays an important role in the final processing and packaging of cell secretions. Herein, we first present a new strategy using 2′,3′-O-isopropylideneadenosine (Ade) acting as active site of many molecules in the Golgi and tetraphenylethylene (TPE) with aggregation-induced emission (AIE) characteristics to design and synthesize a fluorescence probe termed as TPE-Ade. The fluorescence intensity of TPE-Ade in aqueous solution is enhanced by 160 times and thus AIE effect has been obviously confirmed. Furthermore, the theoretical calculation results demonstrate that aggregation enhanced fluorescence of TPE-Ade could be attributed to the suppression of non-radiative decay channel due to the intramolecular rotation hinder. Subsequently, TPE-Ade performs excellent in co-localization imaging experiment in targeting Golgi apparatus, with the Pearson’s co-localization coefficient of 0.86. Moreover, TPE-Ade shows better photostability in contrast to Golgi-Tracker Red. This work may develop into a fluorescence imaging tool based on a new Golgi apparatus localization group.

A viscosity-dependent carbon dots with anti-VEGF properties for monitoring and promoting apoptosis in cancerous cell

Carbon dots (CDs) as a fluorescent nanomaterial plays an important role in bioimaging and light-emitting devices, and its multifunctionalization will be a promising evolution. Here, by a freely rotatable single bond to connect materials with complementary functions together, a dual-function aggregation-induced emission (AIE) CDs (V-CDs) is obtained to visually monitor the dead of cancerous cells and inhibit the production of vascular endothelial growth factor (VEGF). The fluorescence of V-CDs will change between blue and green as the changes of environment viscosity based on the restriction of intramolecular rotation (RIR) effect. For another, the VEGF content decreased by about 3 times compared with the blank group after V-CDs was introduced. The co-administration group shows the best anti-cancer effect among the rest of control groups after Taxol was involved, and the same result are achieved in vitro and vivo. The performances of V-CDs make it possible to realize the potential of visualizing anti-cancer process.

Double-Stranded DNA Matrix for Photosensitization Switching

Photosensitization, originated from the activation of triplet states, is the basis of many photodynamic applications, but often competes with a series of nonradiative processes. Herein, we communic...