Molecular Twist Research Articles

Differences in thermal expansion and motion ability for herringbone and face-to-face π-stacked solids.

A series of aromatic organic molecules functionalized with different halogen atoms (I/ Br), motion-capable groups (olefin, azo or imine) and molecular length were designed and synthesized. The molecules self-assemble in the solid state through halogen bonding and exhibit molecular packing sustained by either herringbone or face-to-face π-stacking, two common motifs in organic semiconductor molecules. Interestingly, dynamic pedal motion is only achieved in solids with herringbone packing. On average, solids with herringbone packing exhibit larger thermal expansion within the halogen-bonded sheets due to motion occurrence and molecular twisting, whereas molecules with face-to-face π-stacking do not undergo motion or twisting. Thermal expansion along the π-stacked direction is surprisingly similar, but slightly larger for the face-to-face π-stacked solids due to larger changes in π-stacking distances with temperature changes. The results speak to the importance of crystal packing and intermolecular interaction strength when designing aromatic-based solids for organic electronics applications.

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

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.

A molecular twist on hydrophobicity.

A thorough exploration of the molecular basis for hydrophobicity with a comprehensive set of theoretical tools and an extensive set of organic solvent S/water binary systems is discussed in this work. Without a single exception, regardless of the nature or structure of S, all quantum descriptors of bonding yield stabilizing S⋯water interactions, therefore, there is no evidence of repulsion and thus no reason for etymological hydrophobicity at the molecular level. Our results provide molecular insight behind the exclusion of S molecules by water, customarily invoked to explain phase separation and the formation of interfaces, in favor of a complex interplay between entropic, enthalpic, and dynamic factors. S⋯water interfaces are not just thin films separating the two phases; instead, they are non-isotropic regions with density gradients for each component whose macroscopic stability is provided by a large number of very weak dihydrogen contacts. We offer a definition of interface as the region in which the density of the components in the A/B binary system is not constant. At a fundamental level, our results contribute to better current understanding of hydrophobicity.

The effect of molecular twisting on electronic and transport properties of Chitosan: Ab initio approach

Abstract Chitosan is a linear chain copolymer composed of D-glucosamine and N-acetyl-D-glucosamine and is the second most naturally discovered polymer on the planet. In the research perspective, this material has non-toxicity, biocompatibility, and biodegradable in nature as in turn it has extraordinary applications in biomedical, food packaging, sensors and electronic gadgets [1] . In this study, a Density Functional Theory implemented in Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) has performed intending to determine the influence of the degree of rotation between the monomer of Chitosan. In each degree of rotation in the gape of 15 degrees, the changes in the structural, electronic and transport properties of chitosan dimer are investigated. Before the study of the effect of twisting in various properties the 2-monomer units of chitosan is optimized using conventional geometrical optimization techniques were comparatively analyzed for the total energies and other parameters. We supply the voltage of −4.0 to 4.0 V in each rotation and found the current in the interval with different rotation up to 0.15 μA. The bandgap is in the range of 0.1 eV to 4 eV in different degree of rotations.

Effect of Cationic Substitution of Metal Species in Poly(vinylidene difluoride) (C2H2F2) by Ab Initio Calculations.

The stronger activity of poly(vinylidene difluoride) (PVDF; C2H2F2), related with its high dielectric constant and its ferromagnetic behavior, puts it into the kinds of materials with novel mechanical features to develop new devices with numerous advantages. Even though PVDF is a common piezoelectric polymer with promising multifunctionality, there still has been little progress in theoretically understanding its band structure and the addition of itinerant electrons because of its polymorphous and multiple phases. In this work, we present the structural and electronic properties of 25% doped C2H2F(X) (X = Al, Ni, and Cu) compounds, including high-frequency dielectric constant (ϵ∞) calculations. Our results show that doping induces structural changes, inducing molecular twists due to weak interactions between the different chains, besides increasing ϵ∞ with itinerant electrons and increasing the band gap with a transition insulator-metal with 25% Ni doping.

Rational design of anthracene-based deep-blue emissive materials for highly efficient deep-blue organic light-emitting diodes with CIEy ≤ 0.05

By using molecular twisting 2,2′-dimethylbiphenyl core and different side group substitutions, three anthracene-based deep blue emitters, 10,10'-(2,2′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)bis(9-phenylanthracene) (PABP), 10,10'-(2,2′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)bis(9-(naphthalen-2-yl)anthracene) (βNABP) and 10,10'-(2,2′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)bis(9-(naphthalen-1-yl)anthracene) (αNABP), were effectively developed. These three twisted anthracene derivatives exhibited strong deep blue photoluminescence which remarkably surpassed the standard blue for the high-definition television (HDTV). Particularly, PABP with high hole mobility of 1.67 × 10−5 cm2 V−1 s−1 delivered a high OLED device performance with external quantum efficiency (EQE) of 4.04% and deep blue emission colour with CIE coordinates of (0.15, 0.05).

"Watching" a Molecular Twist in a Protein by Raman Optical Activity.

Light-absorbing chromophores in photoreceptors contain a π-electron system and are intrinsically planar molecules. However, within a protein environment these cofactors often become nonplanar and chiral in a manner that is widely believed to be functionally important. When the same chromophore is out-of-plane distorted in opposite directions in different members of a protein family, such conformers become a set of enantiomers. In techniques using chiral optical spectroscopy such as Raman optical activity (ROA), such proteins are expected to show opposite signs in their spectra. Here we use two microbial rhodopsins, Gloeobacter rhodopsin and sodium ion pump rhodopsin (NaR), to provide the first experimental and theoretical evidence that the twist direction of the retinal chromophore indeed determines the sign of the ROA spectrum. We disrupt the hydrogen bond responsible for the distortion of the retinal in NaR and show that the sign of the ROA signals of this nonfunctional mutant is flipped. The reported ROA spectra are monosignate, a property that has been seen for a variety of photoreceptors, which we attribute to an energetically favorable gradual curvature of the chromophore.

An Insight of Molecular Twisting of Coumarin Dyes

AbstractThis article highlights the recent understanding of twisted intramolecular charge transfer state (TICT) of coumarin dyes on TiO2 and Au NP surface. The natural origin of coumarin dye in several plants makes it attractive to solar cell and fluorescence sensing applications. Two different classes of D‐π‐A coumarin dyes, having cyano‐acrylic acid as acceptor and 7‐amino moiety as a donor, have been chosen: one of them, having 7‐amino moiety in the ring, shows only intramolecular charge transfer (ICT) states in the excited state. While due to twisting of the free 7‐amino moiety, the second class of coumarin shows both ICT and TICT states in their photo‐excited states in the polar environment, in the form of a strong dual‐fluorescence. On TiO2 surface such coumarin dyes found to couple with carboxylic moiety, however on Au NP surface they bind with amino moiety. As a result, on TiO2 surface, the 7‐amino moiety is free to rotate, thus exhibiting TICT state which is a charge‐separated state and leads to higher electron injection yield for the second class of coumarins. The applicability of such twisted conformers of coumarin molecules in the solar cell has been demonstrated. Furthermore, due to molecular coupling with the 7‐amino moiety on Au NP surface, a restriction imposed on the molecular twisting of the free 7‐amino group. This constrains the formation of the TICT state for the second class of coumarins on the Au NP surface. Such ICT‐TICT fluorescence modulation character of the coumarin dyes may lead them to use for fluorescence sensing application, which is also demonstrated to detect specific ions and biomolecules. Thus the present review acmes very fundamental photophysical properties of coumarin dyes based on their molecular twisting on different solvents, semiconductors, and metal nanoparticles which have great consequences of real life applications.

Acetone Sensing Based on Transmittance of Hydroxyethyl Methacrylate-Liquid Crystal Materials

Poly hydroxyethyl methacrylate (p-HEMA) has a natural property that is very easy to absorb a liquid or solution and become a hydrogel when absorbing water. In this work, by combining p-HEMA material with a cholesteric liquid crystal (CLC), the material can absorb the solution and the optical properties of the liquid crystal will change depending on the solution it absorbs. The solution used in this work is acetone. During the absorption process the p-HEMA material expanded so that the orientation direction of liquid crystal molecular twist with a helical axis along can freely move and change. In this work, we observed the study of transmittance caused by thickness effect and the mixture concentration effect of the poly hydroxyethyl methacrylate liquid crystals (PHM-LC).

Characterization of Hidden Chirality: Two-Fold Helicity in β-Strands

A β-strand is a component of a β-sheet and is an important structural motif in biomolecules. An α-helix has clear helicity, while chirality of a β-strand had been discussed on the basis of molecular twists generated by forming hydrogen bonds in parallel or non-parallel β-sheets. Herein we describe handedness determination of two-fold helicity in a zig-zag β-strand structure. Left- (M) and right-handedness (P) of the two-fold helicity was defined by application of two concepts: tilt-chirality and multi-point approximation. We call the two-fold helicity in a β-strand, whose handedness has been unrecognized and unclarified, as hidden chirality. Such hidden chirality enables us to clarify precise chiral characteristics of biopolymers. It is also noteworthy that characterization of chirality of high dimensional structures like a β-strand and α-helix, referred to as high dimensional chirality (HDC) in the present study, will contribute to elucidation of the possible origins of chirality and homochirality in nature because such HDC originates from not only asymmetric centers but also conformations in a polypeptide chain.

Sine-Gordon Solitons and Breathers in Rod-like Magnetic Liquid Crystals under External Magnetic Field**Supported by the Basic Scientific Research Project of Provincial Colleges and Universities in Heilongjiang Province under Grant No. KJCXZD201723, the Youth Science Foundations of Heilongjiang University under Grant No. QL201606

To study the nonlinear phenomena in rod-like magnetic liquid crystals (RMLCs), this paper establishes the dynamic model of molecular motion when giving a twisting disturbance to the molecules under external magnetic field. We find the twist of the molecules under magnetic field can be propagated in the form of a traveling wave. The dynamic equation of the molecular twisting we derived satisfies the form of Sine-Gordon equation. We obtain two solutions of the Sine-Gordon equation by theoretical calculation: the kink and anti-kink solitons and breathers. The characteristics of those solitons and breathers are discussed.

Compound Shape Effects in Minor Groove Binding Affinity and Specificity for Mixed Sequence DNA.

AT specific heterocyclic cations that bind in the DNA duplex minor groove have had major successes as cell and nuclear stains and as therapeutic agents which can effectively enter human cells. Expanding the DNA sequence recognition capability of the minor groove compounds could also expand their therapeutic targets and have an impact in many areas, such as modulation of transcription factor biological activity. Success in the design of mixed sequence binding compounds has been achieved with N-methylbenzimidazole ( N-MeBI) thiophenes which are preorganized to fit the shape of the DNA minor groove and H-bond to the -NH of G·C base pairs that project into the minor groove. Initial compounds bind strongly to a single G·C base pair in an AT context with a specificity ratio of 50 ( KD AT-GC/ KD AT) or less and this is somewhat low for biological use. We felt that modifications of compound shape could be used to probe local DNA microstructure in target mixed base pair sequences of DNA and potentially improve the compound binding selectivity. Modifications were made by increasing the size of the benzimidazole N-substituent, for example, by using N-isobutyl instead of N-Me, and by changing the molecular twist by introducing substitutions at specific positions on the aromatic core of the compounds. In both cases, we have been able to achieve a dramatic increase in binding specificity, including no detectible binding to pure AT sequences, without a significant loss in affinity to mixed base pair target sequences.

The Sine-Gordon solitons in nematic liquid crystals under the external electric field

Abstract In this paper, we study the dynamic equation of molecular motion in nematic liquid crystals when there exists the twisting disturbance of the molecular arrangement under the external electric field. After a series of derivation, we find that the evolution equation satisfies the form of Sine-Gordon equation which indicates that the molecular twisting deformation may propagate in the form of solitary waves under external electric field, and the kink and anti-kink Sine-Gordon solitary solutions are given then. The existence of a couple of kink and anti-kink Sine-Gordon solitons in nematic liquid crystals is proved. And we also discuss the influence of the velocity of the traveling wave on the solitary propagation.

Light-driven molecular switching of atropisomeric polymers containing azo-binaphthyl groups in their side chains

Light-driven atropisomeric polymers containing azo and binaphthyl units with a conjugated structure were designed and synthesized. The polymers exhibit a glass transition temperature higher than 75 °C with thermal stability above 280 °C and form uniform and smooth thin films without grain boundaries. The trans-cis isomerization is efficiently reversible upon alternating photoirradiation with UV and visible light or with heat. The photoisomerization of the chiral polymer results in photomodulation of the ellipticity and optical rotation ( $$[\alpha ]_{\mathrm{D}}^{25}$$ ), owing to a chiroptical switching behavior. The maximum changes in the dihedral angle and $$[\alpha ]_{\mathrm{D}}^{25}$$ are 21% and 700°, respectively, from the initial state owing to a photoinduced molecular twisting motion of the binaphthyl moiety. The polymer also displays photoswitchable fluorescence with a maximum at 409 nm. A photoinduced change in the refractive index of the formed film is also observed after irradiation with linearly polarized light at 532 nm according to the anisotropic molecular orientation. These results suggest that atropisomeric polymers are potential candidates for light-driven chiroptical switches. Photoinduced molecular switches were achieved by atropisomeric polymers containing conjugated structures of azo and binaphthyl units. trans-cis Photoisomerization of the azo unit led to molecular twisting motion of the binaphthyl unit, yielding chiroptical switching in a neat film. The atropisomeric polymers also displayed photoswitchable fluorescence by repeated irradiation of UV and visible light. Irradiation of the neat film with linearly polarized light caused selective excitation of the atropisomeric polymer, leading to anisotropic molecular orientation.

Conductance Switching in Expanded Porphyrins through Aromaticity and Topology Changes.

Expanded porphyrins are flexible enough to switch between different π-conjugation topologies, namely Möbius, Hückel and twisted-Hückel, each with distinct electronic properties and aromaticity. Since these switches can be induced by different external stimuli, expanded porphyrins represent a promising platform to develop a novel type of molecular switch for molecular electronic devices. In this work, the feasibility of conductance switches based on topology and/or aromaticity changes in expanded porphyrins is assessed for the first time. In particular, the electron transport properties of penta-, hexa- and heptaphyrins with different π-conjugation topologies and aromaticity were carefully investigated using the nonequilibrium Green's function formalism in combination with density functional theory for various configurations of the gold contacts. Our results highlight the importance of the macrocyclic aromaticity and connectivity and, to a lesser extent, the molecular topology, in determining the transmission functions and local currents. When the electrodes are connected along the longitudinal axis of the macrocycle, we found that aromaticity of Hückel expanded porphyrins increases single-molecule junction conductance, contrary to the negative relationship between conductance and aromaticity found in single five-membered rings. For this particular connectivity, antiaromatic Hückel structures with [4n] π-electrons exhibit a sharp reduction in transmission near the Fermi level due to destructive quantum interference between the HOMO and LUMO. Belt-shaped Möbius aromatic structures exhibit a lower conductance as compared to the Hückel aromatic structures, and the current flow avoids the molecular twist. Importantly, we show that expanded porphyrins, upon redox and topology interconversions, could act as efficient three-level molecular switches with high ON/OFF ratio, up to 103 at low bias voltage.

Unusual fluorescent photoswitching of imidazole derivatives: the role of molecular conformation and twist angle controlled organic solid state fluorescence.

Molecular photoswitching, light induced reversible color/fluorescence modulation, has mostly been realized in organic molecules via E/Z isomerization of azobenzenes and stilbenes and ring opening/closing reactions of spiropyrans and diarylethenes. We report here new fluorescent molecular photoswitches based on triphenylamine (TPA)-imidazole derivatives, N-phenyl-N-(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)benzenamine (NTPB) and N-phenyl-N-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)benzenamine (NPPB), that exhibited light induced reversible fluorescence switching via conformational change from a twisted molecular structure to more planar. NTPB and NPPB in CHCl3 showed red shift of absorption and fluorescence upon UV light irradiation whereas white light exposure reversed both absorption as well as fluorescence. The role of the TPA-imidazole twisted molecular structure in photoswitching was established based on structure property, computational and photophysical studies. The isobestic point observed in time dependent fluorescence change under UV light irradiation clearly demonstrated the presence of two different conformational isomers. Interestingly, polymorphism and torsion angle (τ) dependent fluorescence of NTPB and NPPB in the solid state also supported the role of the twisted molecular structure of TPA-imidazole in fluorescence switching/tuning. Interestingly, NTPB showed fluorescence photoswitching in the solid state also whereas rigid phenanthrene based NPPB did not show fluorescence photoswitching. Thus the present studies provide structural insight for designing a new type of fluorescent organic molecular photoswitches based on conformational modulation that could be of potential interest in optoelectronic devices.

Reflection of Molecular Twist in Unoccupied Molecular Orbitals in PTCDI Derivatives: A Density Functional Study

In this work, the structural and electronic properties of perylene tetracarboxylic diimide (PTCDI) derivative molecules have been calculated using density functional theory simulations. Here, we have obtained the equilibrium geometry for certain PTCDI derivatives and calculated their occupied and unoccupied density of states separately for molecular orbitals lying in-plane (σ type) and orthogonal to the plane (π type) of the molecules. We have also simulated the X-ray absorption spectroscopy (XAS) spectra for these molecules separately for π- and σ-type orbitals. A comparison between the unoccupied density of states and XAS data has been made because both provide a description of the molecular orbitals above the Fermi level. We have observed the presence of shallow-lying σ orbitals in twisted molecules and have obtained an almost linear relationship between the abundance of these orbitals and the degree of molecular twist. Additionally, we have shown the possibility of an experimentally viable stereoisomerism in PTCDI-C3.

Characterization of the toxin Plx2A, a RhoA-targeting ADP-ribosyltransferase produced by the honey bee pathogen Paenibacillus larvae.

The toxin Plx2A is an important virulence factor of Paenibacillus larvae, the etiological agent of American Foulbrood, the most destructive bacterial disease of honey bees. Biochemical and functional analyses as well as the crystal structure of Plx2A revealed that it belongs to the C3 mono-ADP-ribosylating toxin subgroup. RhoA was identified as the cellular target of Plx2A activity. The kinetic parameters (KM , kcat ) were established for both the transferase and glycohydrolase reactions. When expressed in yeast, Plx2A was cytotoxic for eukaryotic cells and catalytic variants confirmed that the cytotoxicity of Plx2A depends on its enzymatic activity. The crystal structure of Plx2A was solved to 1.65 Å and confirmed that it is a C3-like toxin, although with a new molecular twist, it has a B-domain. A molecular model of the 'active' enzyme conformation in complex with NAD+ was produced by computational methods based on the recent structure of C3bot1 with RhoA. In murine macrophages, Plx2A induced actin cytoskeleton reorganization while in insect cells, vacuolization and the occurrence of bi-nucleated cells was observed. The latter is indicative of an inhibition of cytokinesis. All these cellular effects are consistent with Plx2A inhibiting the activity of RhoA by covalent modification.

Tren-Capped Hexaphyrin Zinc Complexes: Interplaying Molecular Recognition, Möbius Aromaticity, and Chirality.

Over the past decade, the hexaphyrin skeleton has emerged as a multifaceted frame exhibiting strong interplay between topology, aromaticity, and metal coordination, opening new research areas beyond porphyrins. However, molecular recognition with hexaphyrins has been underexplored, mainly because of the lack of general synthetic strategies leading to sophisticated molecular hosts. Here we have developed a straightforward approach for capping the heteroannulene frame with tripodal units (e.g., tris(2-aminoethyl)amine [tren]) through postsynthetic modification of a readily accessible meso-(2-aminophenyl) tris-substituted platform. The resulting tren-capped hexaphyrins, obtained in three steps from a 5-(aryl)dipyrromethane precursor, display remarkable features: (i) Considering the 28π-conjugated system, instantaneous and site-selective Zn(II) metalation at the level of a dipyrrin versus tren unit triggers a planar-to-singly twisted conformational change and hence a Hückel antiaromatic-to-Möbius aromatic transformation. In spite of the tripodal linkage, a smooth twist and efficient π overlap are preserved. (ii) Selective and cooperative binding of both an acetato ligand and an amino ligand to zinc occurs in distinct confined environments, reminiscent of substrate discrimination at the buried metal centers of metalloenzymes. The ligand binding pockets are allosterically tuned by monoprotonation of the tren unit. (iii) Substantial chiral induction of the molecular twist is achieved using chiral amino ligands (diastereomeric excess up to 77%, the highest reported to date for a Möbius compound), to which is associated a strong chiroptical signature in circular dichroism. These results provide unprecedented insights into molecular recognition with hexaphyrins, paving the way to innovative Möbius-type molecular hosts for sensing and catalysis.