The Effective Conjugation Length Is Responsible for the Red/Green Spectral Tuning in the Cyanobacteriochrome Slr1393g3

The origin of the spectral shift from a red‐ to a green‐absorbing form in a cyanobacteriochrome, Slr1393g3, was identified by combined quantum mechanics/molecular mechanics simulations. This protein, related to classical phytochromes, carries the open‐chain tetrapyrrole chromophore phycocyanobilin. Our calculations reveal that the effective conjugation length in the chromophore becomes shorter upon conversion from the red to the green form. This is related to the planarity of the entire chromophore. A large distortion was found for the terminal pyrrole rings A and D; however, the D ring contributes more strongly to the photoproduct tuning, despite a larger change in the twist of the A ring. Our findings implicate that the D ring twist can be exploited to regulate the absorption of the photoproduct. Hence, mutations that affect the D ring twist can lead to rational tuning of the photoproduct absorption, allowing the tailoring of cyanobacteriochromes for biotechnological applications such as optogenetics and bioimaging.     

Fourth-order nematic elasticity and modulated nematic phases: a poor man’s approach

We propose an extension of Frank-Oseen’s elastic energy for bulk nematic liquid crystals which is based on the hypothesis that the fundamental deformations allowed in nematic liquid crystals are splay, twist and bend. The extended elastic energy is a fourth-order form in the fundamental deformations. The existence of bulk spontaneous modulated or deformed nematic liquid crystal ground states is investigated. The analysis is limited to bulk nematic liquid crystals in the absence of limiting surfaces and/or external fields. The non deformed ground state is stable only when Frank-Oseen’s elastic constants are positive. In case where at least one of them is negative, the ground state becomes deformed. The analysis of the stability of the deformed states in the space of the elastic parameters allows to characterise different types of deformed nematic phases. Some of them are new nematic phases, for instance a twist – splay nematic phase is predicted. Inequalities between second-order elastic constants which govern the stability of the twist–bend, splay–bend, and splay–twist states are obtained. Their stability in respect to triple splay–bend–twist deformations is investigated.     

含氮给体结构对丙烯腈电荷转移光聚合的影响

<正> 对于正性烯类单体,主要是乙烯基咔唑的电荷转移聚合,诚田已作过详细综述。负性烯类单体的电荷转移聚合主要研究的单体是丙烯腈(AN)及甲基丙烯酸甲酯(MMA)。Barton等研究了芳烃为引发剂的MMA光聚合。我们研究了芳胺为引发剂的光聚合。 芳胺,尤其是芳叔胺,是较强的电子给体,与负性单体在光照下经激基复合物而引发聚合,我们的实验表明,吡啶及其同系物(喹啉与吖啶)并不象吡咯及其同系物(吲哚与咔唑)那样有效地引发AN等负性单体的光聚合(见表1)。从图1所示的结构看,它们有     

Electroclinic effects in a homogeneously aligned smectic A cell

The electroclinic effect, in a material having a first order S_A to S_C* transition, is studied using the half leaky guided mode geometry. Using an approximately 1 μm thick, homogeneously aligned cell, the voltage induced director twist is characterized at a few temperatures in the S_A phase. The mean field theory readily explains the data recorded at low fields where a linear dependence on voltage is found. However, at higher fields, an unexpected saturation occurs which is most likely caused by the influence of strong surface anchoring forces.     

A Conformational Study on Silacyclohexane. Comparison of ab initio (HF,MP2), DFT,and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI‐MP2, RI‐DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair‐to‐chair inversion. Each of them consists of two sofa‐like transition states, two twist forms with C₁ symmetry (twist‐C₁), two boat forms with Si in a gunnel position (C₁ symmetry), and one twist form with C₂ symmetry (twist‐C₂). All methods calculate the relative energy to increase in the order chair < twist‐C₂ < twist‐C₁ < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol^–1 for the twist‐C₂, twist‐C₁, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol^–1 from MM3 and RI‐DFT calculations, respectively. The boat form with Si at the prow (C_s symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI‐DFT optimization under C_s symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair‐to‐twist‐C₁ transition state, however, and it is clearly not a part of the chair‐to‐chair inversion.     

An unusual twist conformation of 2-O-methyl-1,3,4,5-tetrakis-O-tert-butyldiphenylsilyl-myo-inositol

The ring conformation of 2-O-methyl-1,3,4,5-tetrakis-O-tert-butyldiphenylsilyl-myo-inositol was in a twist form both in solid state and in solution. This is the first observation of a stable twist conformer induced by the introduction of bulky silyl protecting groups.     

Computational studies on the ground and excited states of BrOOBr

In this work, theoretical computations for the ground and excited states of BrOOBr have been performed at high-level ab initio molecular orbital theories. The ground-state geometries of BrOOBr in different forms (trans, cis, and twist form) have been optimized at the couple-cluster CCSD(T) level of theory with cc-pVTZ and aug-cc-pVTZ basis sets, which indicates that at CCSD(T)/cc-pVTZ level of theory, the twist form is 4.96 kcal/mol more stable than the trans form and 10.67 kcal/mol more stable than the cis form; at the CCSD(T)/aug-cc-pVTZ basis set the twist form is 4.33 kcal/mol more stable than the trans form and 9.54 kcal/mol more stable than the cis form. The vertical excitation energies and potential-energy curves for the singlet and triplet low-lying excited states of BrOOBr were calculated at both the complete active space self-consistent-field (CASSCF) level of theory and the multireference internally contracted configuration interaction (MRCI) level of theory. The differences of potential-energy curves at CASSCF and MRCI levels of theory are found for the BrOOBr excited states. At CASSCF level of theory, none of the BrOOBr excited states are bound. However, at MRCI level of theory, all the BrOOBr states studied in this work are bound or slightly bound at the Frank-Condon region. In addition, the scalar relativistic effect and the spin-orbital coupling effect on the vertical excitation energies of the electronic states of BrOOBr were estimated.     

Vibrational spectra and conformational isomerism of 2-phenoxy-5,6-benz-1,3,2-dioxaphosphepins

Conclusions Using vibrational spectroscopy methods in different aggregate states and solutions with variation of the polarity of the medium, we have studied 2-phenoxy-5,6-benz-1,3,2-dioxaphosphepin and its oxo, thio, and seleno analogs. In the liquid and solutions, they exist as a conformational equilibrium of three forms: two chair forms, differing in the orientation of the phenoxyl radical, and a twist form. The population of the twist form is no more than 25%. The content of the chair conformers is determined by the solvent. In the crystal, all the compounds have the chair form with axial phenoxyl group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 844–849, April, 1989.     

Die Konformeren des 5,5-Dimethyl-3,7-dithia-1,2-benzocycloheptens-(1) und ihre Umwandlungen Konformative Beweglichkeit flexibler Ringsysteme—XV. Untersuchungen mit Hilfe der Protonenresonanzspektroskopie

The temperature dependence of the NMR spectrum of 5,5-dimethyl-3,7-dithia-1,2-benzocyclo-heptene-( 1 ) is described and discussed. This compound occurs in two conformers with different topographies of the seven membered ring. From the chemical shift of the 2 geminal methyl groups (obtained by low-temperature spectra) it can be shown that the ring occurs in one case in the chair form and in the other case in the twist form. The free conformational energy of the twist form is only about 20 cal/mole. Two conformational changes can be distinguished: the conversion between chair and twist forms and the pseudorotation of the twist forms. The free activation enthalpies of the conversion (ΔG_V^≠) and the pseudorotation (ΔG_P^≠) are 12·6 ± 0·1 Kcal/mole (at ?12°C) and 8·3 ± 0·3 Kcal/mole (at ?95°C) respectively.     

Konformative Beweglichkeit Flexibler Ringsysteme—XIII: Untersuchungen mit Hilfe der Protonenresonanzspektroskopie die Konformation des Ungesättigten Siebenringes im Benzocyclohepten und in symmetrisch substituierten derivaten

The confromations of the unsaturated seven membered ring in 4,4,6,6-tetradeuterium-1,2-benzocycloheptene-(1) ( 1 ) and five benzocycloheptene derivatives were determined by NMR spectroscopy. For all investigated compounds at ?80°C only one conformer was present in detectable quantity. By analysis of the NMR data – molecular symmetry, coupling constants and chemical shift – it can be shown that the conformation is always the chair form. The free conformational enthalpy of both the other conformations with boat or twist form of the ring is for all six compounds more than 1.8 kcal/mole. The experimental results agree with those from model calculations: thus for benzocycloheptene, the 5,5-dimethyl derivative ( 2 ) and the 4,4,6,6-tetramethyl derivative ( 4 ) the lowest energy was found for the chair conformation; the second most stable conformations were found to be the boat for 1 and 4 , and the twist form for 2 .     

Controlled conjugated backbone twisting for an increased open-circuit voltage while having a high short-circuit current in poly(hexylthiophene) derivatives

Conjugated polymers with nearly planar backbones have been the most commonly investigated materials for organic-based electronic devices. More twisted polymer backbones have been shown to achieve larger open-circuit voltages in solar cells, though with decreased short-circuit current densities. We systematically impose twists within a family of poly(hexylthiophene)s and examine their influence on the performance of polymer:fullerene bulk heterojunction (BHJ) solar cells. A simple chemical modification concerning the number and placement of alkyl side chains along the conjugated backbone is used to control the degree of backbone twisting. Density functional theory calculations were carried out on a series of oligothiophene structures to provide insights on how the sterically induced twisting influences the geometric, electronic, and optical properties. Grazing incidence X-ray scattering measurements were performed to investigate how the thin-film packing structure was affected. The open-circuit voltage and charge-transfer state energy of the polymer:fullerene BHJ solar cells increased substantially with the degree of twist induced within the conjugated backbone--due to an increase in the polymer ionization potential--while the short-circuit current decreased as a result of a larger optical gap and lower hole mobility. A controlled, moderate degree of twist along the poly(3,4-dihexyl-2,2':5',2'-terthiophene) (PDHTT) conjugated backbone led to a 19% enhancement in the open-circuit voltage (0.735 V) vs poly(3-hexylthiophene)-based devices, while similar short-circuit current densities, fill factors, and hole-carrier mobilities were maintained. These factors resulted in a power conversion efficiency of 4.2% for a PDHTT:[6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) blend solar cell without thermal annealing. This simple approach reveals a molecular design avenue to increase open-circuit voltage while retaining the short-circuit current.     

Designed Synthesis of a 1D Polymer in Twist‐Stacked Topology via Single‐Crystal‐to‐Single‐Crystal Polymerization

To synthesize a fully organic 1D polymer in a novel twist‐stacked topology, we designed a peptide monomer HC≡CCH₂‐NH‐Ile‐Leu‐N₃, which crystallizes with its molecules H‐bonded along a six‐fold screw axis. These H‐bonded columns pack parallelly such that molecules arrange head‐to‐tail, forming linear non‐covalent chains in planes perpendicular to the screw axis. The chains arrange parallelly to form molecular layers which twist‐stack along the screw axis. Crystals of this monomer, on heating, undergo single‐crystal‐to‐single‐crystal (SCSC) topochemical azide–alkyne cycloaddition (TAAC) polymerization to yield an exclusively 1,4‐triazole‐linked polymer in a twist‐stacked layered topology. This topologically defined polymer shows better mechanical strength and thermal stability than its unordered form, as evidenced by nanoindentation studies and thermogravimetric analysis, respectively. This work illustrates the scope of topochemical polymerizations for synthesizing polymers in pre‐decided topologies.     

In search of the Bailar and Rây-Dutt twist mechanisms that racemize chiral trischelates: a computational study of Sc(III), Ti(IV), Co(III), Zn(II), Ga(III), and Ge(IV) complexes of a ligand analogue of acetylacetonate

Two nondissociative processes, a Bailar twist that proceeds through a transition state of D3h symmetry and a Ray-Dutt twist mechanism that proceeds through a transition state of C(2v) symmetry, as well as dissociative/associative processes are potential mechanisms by which the enantiomeric forms of chiral metal trischelates can be interconverted. We have applied density functional theory to locate the stationary points for metal trischelates of a beta-diketonate ligand analogue that interconvert Delta and Lambda forms via one or both of these nondissociative pathways. Although many two-dimensional static representations of the Bailar and Ray-Dutt twist mechanisms can be found in the chemical literature (of the type shown in Figure 1), in this communication, we present our results in the form of interactive three-dimensional animations as a means of enhancing the scientific perception of these fluxional processes.     

Closed-form solution of the finite torsion problem by use of the λ form of the strain-energy function

A closed-form solution for the stress distribution in a cylindrical elastic rod under finite torsion is given involving use of the logarithmic λ form of the strain-energy function proposed by Valanis and Landel. Experimental torque data on natural rubber which give support to the present solution over a large range of angles of twist are shown. It is also shown how torsion data may be used to determine the λ form for deformations that lie outside the range of applicability of the logarithmic function if certain conditions apply. A solution, also in closed form, is given by using another λ form proposed by Dickie and Smith on the basis of experiments on styrene—butadiene rubber.     

Molecular and crystal structure of sophoridine N-oxide

The spatial structure of natural sophoridine N-oxide has been studied by x-ray structural analysis. It has been shown that when sophoridine is oxidized, rings B and C, which have the boat form, are deformed in the direction of the twist form.     

Reversible photoinduced twisting of molecular crystal microribbons

9-Anthracenecarboxylic acid, a molecule that undergoes a reversible [4 + 4] photodimerization, is prepared in the form of oriented crystalline microribbons. When exposed to spatially uniform light irradiation, these photoreactive ribbons rapidly twist. After the light is turned off, they relax back to their original shape over the course of minutes. This photoinduced motion can be repeated for multiple cycles. The final twist period and cross-sectional dimensions of individual microribbons are measured using a combination of atomic force and optical microscopies. Analysis of this data suggests that the reversible twisting involves the generation of interfacial strain within the ribbons between unreacted monomer and photoreacted dimer regions, with an interaction energy on the order of 3.4 kJ/mol. The demonstration of reversible twisting without the need for specialized irradiation conditions represents a new type of photoinduced motion in molecular crystals and may provide new modes of operation for photomechanical actuators.     

Spherulite morphology in liquid crystals of polybenzylglutamates. Optical rotatory power and effect of magnetic field

The stepwise growth and emergence of spherulitic structures as isolated entities from lyotropic solutions of polybenzylglutamate liquid crystals is described. Spherulites that form ringed internal structures indicative of a cholesteric organization of macromolecules have large optical rotatory powers. The sense of the rotation is solvent dependent. The morphology of the spherulite is distorted in the presence of a magnetic field. The pitch of the cumulative twist of “onion-shell” molecular layers was measured as a function of field strength and found to follow the theoretical form shown previously to hold for continuous-phase polypeptide liquid crystals.     

Lanthanide-Base Helical Chain Constructed by In Situ Schiff Base Reaction: Structures and Magnetic Properties

Journal of Cluster Science - Schiff base ligands with multiple chelating coordination sites are usually able to quickly capture lanthanide metal ions to form complexes, and it is difficult to twist...     

A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGB_C*) phase. In the UTGB_C* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGB_A-TGB_C-SmC* sequence. As the chiral strength is increased this goes to the sequence TGB_A-UTGB_C*-SmC*. Such sequences have been observed experimentally.     

Optical diffraction in twisted liquid-crystalline media-phase grating mode

We have calculated the diffraction of light perpendicular to the twist axis in a chiral smectic C liquid crystal. In contrast to a cholesteric liquid crystal, in a chiral smectic C liquid crystal we find extra orders which form the odd orders in the diffraction pattern. For an incident linearly polarized light, at a general azimuth, these odd orders are linearly polarized and the even orders are elliptically polarized. The intensities of the odd orders are always independent of the azimuth of the incident light, while this is possible for even orders only at a particular tilt angle of the chiral smectic C liquid crystal. Also, for the incident vibration parallel or perpendicular to the twist axis the odd orders are polarized in the orthogonal linear state with respect to incident vibration, while the even orders are in the same linear state.