Weak Cotton Effect Research Articles

Four-Component Ugi Reaction for Optical Chirality Sensing and Surface Nanoengineering of Chiral Self-Assemblies.

Using green chemistry to control chirality at hierarchical levels as well as chiroptical activities endows with new opportunities to the development of multiple functions. Here, the four-component Ugi reaction is introduced for the general and precise optical chirality sensing of amines as well as the surface nanoengineering of chiral soft self-assemblies. To overcome the relatively weak Cotton effects, direct synthesis of a folded peptide structure on a rotatable ferrocene core with axial chirality was accomplished from chiral amine, 1,1'-ferrocenyl dicarboxylic acid, formaldehyde and isocyanide. Enhanced Cotton effects benefiting from the folded structure allow for the precise and quantitative sensing of natural and synthetic chiral amines covering alkyl, aromatic amines and amino acid derivatives. In addition, aqueous reaction enables the modification of amine-bearing dye to microfibers self-assembled from π-conjugated amino acids. Surface dye-modification via Ugi reaction barely changes the pristine morphology, showing non-invasive properties in contrast to dye staining, which is applicable in soft nano/microarchitectures from self-assembly. This work which combines the four-component Ugi reaction to enable precise ee% detection and surface nanoengineering of soft chiral assemblies sheds light on the advanced application of green chemistry to chirality science.

Adaptive Chirality of an Achiral Cage: Chirality Transfer, Induction, and Circularly Polarized Luminescence through Aqueous Host–Guest Complexation

Chirality transfer, induction, and circularly polarized luminescence (CPL) using supramolecular hosts, such as macrocycles and cages, have been explored for wide-ranging applications in chiral reco...

Circularly polarized luminescence from axially chiral binaphthalene-bridged BODIPY

Two axially chiral BODIPY-binaphthalene dyads, (R)-/(S)-BDP1-2, were synthesized by tethering BODIPY to the 3- or 3,3′-position of chiral binaphthalene. (R)-/(S)-BDP1 show a very weak Cotton effect and circularly polarized luminescence (CPL), demonstrating the inefficient perturbation effect of binaphthalene on BODIPY in both ground and excited state. In contrast, (R)-/(S)-BDP2 show an intense Cotton effect and CPL. According to the theoretical results, this difference should be originated from the energy transfer between the two BODIPY units in (R)-/(S)-BDP2.

Chiral resolution and bioactivity of enantiomeric furofuran lignans from Juglans mandshurica Maxim

Enantiomers have generally been reported mostly for racemic mixtures with a 1:1 ratio, as in that case there were weak Cotton effects in the ECD spectrum and negligible optical rotations. A furofuran lignan (sesamin), with a remarkable rotation and significant Cotton effects, was isolated from Juglans mandshurica Maxim. Subsequently, sesamin was resolved by chiral HPLC to afford a pair of enantiomers, (+)-sesamin (a) and (−)-sesamin (b), in a ratio of approximately 1:3. Their absolute configurations were determined by computational analysis of their electronic circular dichroism (ECD) spectrum. In addition, the pair of enantiomers were evaluated for the inhibition of self-induced Aβ aggregation. Interestingly, (+)-sesamin (a) (67.7%) and (−)-sesamin (b) (80.6%) exhibited different degrees of anti-Aβ aggregation activity. The different inhibition profiles were further explained by molecular dynamics and docking simulation study.

Circular Dichroisms of Mono- and Dibromo[2.2]paracyclophanes: A Combined Experimental and Theoretical Study.

Circular dichroisms (CDs) of planar chiral 4-bromo[2.2]paracyclophane (1) and three isomeric dibromo[2.2]paracyclophanes (p-2, m′-2, and o′-2) were investigated experimentally and theoretically. They all exhibited strong multisignate Cotton effects (CEs) at the 1Lb, 1La, and 1B transitions of the component (bromo)benzene chromophore and were comparable to each other. For all of the cyclophanes examined, the enantiomer that eluted earlier from a chiral high-performance liquid chromatography column (Chiralcel IA or IB) exhibited negative and positive CEs at the 1Lb and 1La bands, respectively, which were followed by a more complicated pattern of CDs at the higher-energy bands. These CD features were well reproduced by quantum chemical calculations, allowing us to unambiguously assign the absolute configurations of the first-eluted enantiomers as Rp in all of the cases examined. Interestingly, the CDs of 1 and 2, although largely comparable in shape, were still sensitive to the number and pattern of bromine substitution, showing closer resemblance between m′-2 and o′-2 and between p-2 and 1. The theoretical calculations also reproduced successfully these spectral resemblance between them. The anisotropy (g) factors for the 1Lb bands of these cyclophanes were considerably large (∼10–2), whereas those for the 1La band were conventional in the order of 10–3. In addition, a weak CE was observed in the low-energy region at around 320 nm, which turned out to originate from the interplanar interaction and is hence assigned to the “cyclophane band”. The experimental g factors of this band were fairly large in the order of 10–2, but the computation turned out to be quite challenging and were less well reproduced theoretically, ascribable to the forbidden nature of the transition.

Synthesis of novel optically active poly(thiophenyleneethynylenephenylene)s. Effects of chirality competition and cooperation at the side chains on higher order structures

Novel poly(thiophenyleneethynylenephenylene)s having optically inactive/active amide groups were synthesized by the Sonoghashira-Hagihara coupling polymerization of 3-substituted 2,5-dibromothiophene 1 with 1-substituted 3,5-diethynylbenzenes 2N, 2R, 2S derived from glycine and d-/ʟ-alanines. Poly(1–2S) exhibited an intense Cotton effect based on negative exciton chirality at 405 and 366 nm. On the other hand, poly(1–2N) exhibited a Cotton effect similar to that of poly(1–2S) but weaker, and poly(1–2R) exhibited a weak Cotton effect based on positive exciton chirality. The chiral secondary structures of the polymers were dependent on chirality cooperation and chirality competition of the side chains. The trend of the circular dichroism (CD) signs and intensities of the polymers can be explained by the concepts of “chiral cooperation” and “chiral competition” between the optically active thiophene unit and optically inactive/active phenylene unit. These polymers formed aggregate structures with increasing MeOH content of CHCl3/MeOH mixed solvents, with accompanying decrease of the UV–vis absorptions and photoluminescence (PL) intensities.

Combined Experimental and Theoretical Investigations on Optical Activities of Möbius Aromatic and Möbius Antiaromatic Hexaphyrin Phosphorus Complexes.

Intrinsically chiral Möbius aromatic [28]hexaphyrin monophosphorus(V) and Möbius antiaromatic [30]hexaphyrin bisphosphorus(V) complexes have been optically resolved and their absolute configurations (ACs) were determined by combined experimental and theoretical investigations on their circular dichroisms (CDs). First elutes in chiral HPLC exhibited strong positive Cotton effects (CEs) at the B-band, characteristic for the ML configurations in their Möbius strips. Weak CEs at the Q-band, if attainable, complemented their AC assignment. The whole CD pattern and intensity were well reproduced by time-dependent approximate coupled cluster theory using model systems that omit five outward meso-aryl substituents (inward-meso-retained model), providing a solid basis for AC assignment. The cost efficient TD-DFT method with appropriate functionals for fully substituted (nontruncated) complexes well reproduced CEs around the B-band (but less satisfactory at the Q-band), also allows the rapid AC estimation for their Möbius strips. Observed difference in CDs between aromatic and antiaromatic hexaphyrins were better interpreted by their shifts in energy levels and altered interactions of relevant molecular orbitals, rather than small differences in Möbius geometries nor aromatic/antiaromatic character, despite the correlations recently claimed in planar π-systems.

Reversible Shrinkage of Self-Assembled Two-Component Organogels by Lithium Salts: Synthesis of Gelation Property and Lithium Salt Response Using Bidomain Helicene Oligomer

A bidomain oligomer containing a (P)-amidohelicene tetramer domain and a (P)-ethynylhelicene tetramer domain formed self-assembled organogels in the presence of an (M)-ethynylhelicene pentamer. Strong Cotton effects were observed for gels in pyridine, piperidine, cyclohexanone, and 1,2-dimethoxybenzene (Type A solvents), and weak Cotton effects in anisole, ethyl benzoate, and toluene gels (Type B solvents). The amido domain was in the random-coil state in the gels of the Type A solvents, and in the helix-dimer state in the gels of the type B solvents. The pyridine gels shrank upon the addition of lithium perchlorate to 25% of their original volume, and the volume recovered when the supernatant was removed, the solvent was added, and the mixture was briefly heated and cooled. The shrinkage process could be reproducibly repeated five times. In contrast, the gels of anisole and toluene did not shrink in the presence of lithium perchlorate. The gelation property and lithium salt response were induced using th...

Synthesis of Optically Active Regioregular Polythiophenes and Their Self-Organization at the Air–Water Interface

Regioregular polythiophenes containing an optically active substituent in the third position of the thiophene ring, head-to-tail poly(3-[2-((S)-1-methyloctyloxy)ethyl]thiophene)s (HT-P(S)MOETs), were synthesized using highly reactive zinc. For comparison, HT-P(R)MOET and achiral HT-P(±)MOET also were synthesized from R-type monomers and racemic monomers, respectively. The HT-PMOET possessed greater than 95% head-to-tail coupling with a weight-average molecular weight (Mw) between 1.96 × 10(4) and 2.94 × 10(4). The polymers were characterized using (1)H and (13)C NMR, optical rotatory power measurements, circular dichroism (CD), and UV-vis spectroscopy. X-ray diffraction patterns of the cast films demonstrated that regioregular HT-PMOET possessed a strong tendency to self-assemble into highly ordered, crystalline structures. The HT-P(S)MOET and HT-P(R)MOET showed strong Cotton effects, while HT-P(±)MOET showed very weak Cotton effects. The presence of a circular dichroism effect indicated that the side chain chirality induced optical activity in poly(thiophene) main chains. The monolayer formation of HT-PMOET spread on the water surface was characterized using a pressure-area (π-A) isotherm. The molecular areas of HT-P(S)MOET and HT-P(R)MOET molecules on the water surface were 33.5 and 32.9 Å(2), respectively, at 10 °C, which were larger than that of HT-P(±)MOET (27.9 Å(2)), suggesting that optically active HT-PMOET expanded because of the chiral repulsion between side chains. Multilayer films of HT-PMOET were prepared by repeating horizontal deposition of the monolayer on the water surface. The multilayer films of optically active HT-PMOET obtained showed stronger Cotton effects than did the cast films. In addition, electrical conductivities of HT-PMOET multilayer films were superior to those of spin-coated films. Head-to-tail poly(3-[2-((S)-1-methylpropyloxy)ethyl]thiophene) (HT-P(S)MPET), which contained shorter side chain lengths compared to HT-P(S)MOET, also was synthesized. The CD intensities of HT-P(S)MPET multilayer films were smaller than those of HT-P(S)MOET multilayer films, suggesting that the optically active side-chain length is critically important to the optically active self-assembly.

Theoretical and Experimental Studies of Circular Dichroism of Mono- and Diazonia[6]helicenes

Combined experimental and theoretical studies revealed the characteristic circular dichroism (CD) spectral profiles of mono- and diazonia[6]helicenes, which were distinctly different from those reported for parent [6]helicene and neutral (di)aza-analogues. Aza[6]helicenes and [6]helicene showed bisignate Cotton effects (CEs) at the (1)Ba and (1)Bb bands, along with a weak CE at the (1)Lb band, where the signs of the former bands are responsible for the helical chirality of the helicenes while the sign of the latter is susceptive to the various factors such as electronic and steric effects. Protonation to monoaza[6]helicenes produces azonia[6]helicenes, showing dramatic changes in the CE pattern from the two bisignate to a three positive, two negative CE extremum series of comparable magnitudes, while dual protonation to diaza[6]helicenes forming diazonia[6]helicenes led to only nominal changes (slightly different rotational strength and excitation energy) in the CE pattern. Such rather complicated and contrasting CE behaviors of mono- versus diazoniahelicenes are derived mostly from the electronic effects of (unsymmetrical) protonation because the structures of neutral, mono-, and dicationic species are essentially identical to each other. Compared with those of neutral (di)aza[6]helicenes, the experimental CD spectra of (di)azonia[6]helicenes were less satisfactorily reproduced by the theoretical calculations at the state-of-the-art RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level, most probably due to the inadequate incorporation of the effects of solvation. Nevertheless, the bytheoretical predictions were reasonably accurate and highly valuable in assigning the observed CE and elucidating the origin of the elaborate CD spectral behaviors upon protonation through inspection of the molecular orbital configuration of each transition, encouraging the extended use of the present protocol for analyzing the CD spectral behavior of aza- and other heteroatom-incorporated helicenes upon protonation. The CD spectral behavior upon metal ligation will also be explained through further theoretical and experimental studies.

Chiroptical properties of ajugol investigated by quantum chemical calculation using time-dependent density functional theory

The chiroptical properties of an iridoid glycoside ajugol were fully investigated by quantum chemical calculations of specific optical rotation at the sodium D line ([α]D value), optical rotatory dispersion (ORD), and electronic circular dichroism (ECD) using time-dependent density functional theory (TDDFT). TDDFT calculations of the [α]D value and ORD of ajugol over the range of 365–633 nm were in good agreement with the experimental data. The predicted ECD spectrum of ajugol was also consistent with the experiment, showing a strong negative Cotton effect (CE) at around 190 nm and a weak positive CE at around 220 nm. Our results unambiguously determined the absolute configuration (AC) of the aglycone part of ajugol as (1S, 5R, 6R, 8S, 9S) and supported the generally accepted AC assignments of iridoid glycosides. Semi-empirical rule for the enol ether chromophore, basis set selection, and effect of glucose group on ECD spectra were also discussed in the case of ajugol.

Acidification and Assembly of Porphyrin at an Interface: Counterion Matching, Selectivity, and Supramolecular Chirality

The interfacial diprotonation and assemblies of a free-base achiral porphyrin, 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine, on various acidic subphases were investigated. It has been shown that the compound could be diprotonated in situ on an acidic subphase and can form assemblies. The interfacially organized supramolecular assemblies were transferred onto a solid substrate, and the assemblies showed supramolecular chirality. Interestingly, the supramolecular chirality of the assemblies of the diprotonated species showed a counterion-dependent behavior. For the assemblies fabricated from the aqueous HCl subphases, a strong Cotton effect (CE) could be observed, although the porphyrin itself is achiral. When an aqueous HBr solution was used as the subphase, the assemblies showed a weak CE, whereas no CE could be detected for the assemblies formulated from the HNO3 or HI subphase. Interestingly, when a mixture of HBr and NaCl, or HNO3 and NaCl, was employed as the subphase, the formed assemblies displayed chiral features similar to those fabricated on the HCl subphase, suggesting that the Cl(-) could be preferentially visualized in terms of supramolecular chirality, although the system itself is composed of achiral species. On the basis of the experimental facts and a theoretical calculation, an explanation with regard to the different sizes of the counterions and the distinct binding affinities of the counteranions to the diprotonated porphyrin species has been proposed. Our findings provide new insights into the assembly of the diprotonated porphyrins as well as the interfacially occurring symmetry breaking.

Optically Active Phthalocyaninato−Polysiloxane Constructed from Achiral Monomers: From Noncovalent Assembly to Covalent Polymer

In this paper, we reported that optically active supramolecular assemblies of an achiral phthalocyanine derivative, silicon 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine dihydroxide (Pc 1), could be easily achieved through the Langmuir and Langmuir−Schaefer (LS) technique. We have found that the LS film of Pc 1 showed weak Cotton effect (CE) when it was subjected to the circular dichroism (CD) measurements. The chirality of the LS film was suggested to be due to the spontaneous symmetry breaking occurred at the air/water interface. More interestingly, such assemblies connected through the noncovalent bond could be further converted into covalent bond under heating at 180 °C in a high vacuum for 10 h. UV−vis spectra and FT-IR have confirmed the polymerization of Pc 1 in the LS film during the heating, which resulted in the formation of the corresponding phthalocyaninato−polysiloxane (PcPs 2) polymer. Stronger Cotton effect was detected for the thin solid film of PcPs 2 compared with that of the Pc 1 LS film, suggesting that more efficient exciton coupling existed between the covalently linked phthalocyanine chromophore in the polymer PcPs 2. The investigation provided an easy way of constructing optically active polymeric PcPs films exclusively from achiral monomer unit by the preassembly of monomer through interfacial organization.

Chiral Organic Radical Cation and Dication. A Reversible Chiroptical Redox Switch Based on Stepwise Transformation of Optically Active Tetrakis(p-alkoxyphenyl)ethylenes to Radical Cations and Dications

Optically active tetrakis(p-alkoxyphenyl)ethylenes were found to function as reversible chiroptical switches upon redox transformations. Successive one-electron oxidations of chirally modified tetraarylethylene to the corresponding radical cation and then to the dication led to dramatic changes in the electronic absorption and circular dichroism (CD) spectra. The neutral species showed no color or CD in the visible region, while the radical ion was blue in color and exhibited a weak Cotton effect, with the dication green and giving an intense Cotton effect and a sign opposite that observed for the radical cation, at a longer wavelength. Molecular orbital calculations and X-ray crystallographic studies clearly indicate that the olefinic C=C bond is significantly twisted in the dication to minimize the electrostatic and steric repulsions. By lowering the temperature of the dication, the twist around the double bond is more firmly fixed in either P or M chirality to give a stronger Cotton effect and a larger anisotropy (g) factor. Since the spectral changes are completely reversible and reproducible for multiple redox cycles, this chiral redox system can be used in novel redox-driven chiroptical applications, such as molecular switches and memory devices, in which the information is written/read chiroptically in the ternary mode, giving zero CD signal in the neutral form, positive CD for the radical cation, and negative CD for the dication at a given wavelength.

Electrochemical Synthesis and Optical Properties of a Chiral Poly[1,4‐bis(3′,4′‐ethylenedioxythienyl)‐phenylene] Derivative

AbstractSummary: Optically active poly[(R)‐ or (S)‐1,4‐bis(2‐(3′,4′‐ethylenedioxy)thienyl)‐2‐benzoic acid 1‐methylheptyl ester] was prepared by an electrochemical technique and characterized by circular dichroism measurements. It was found that the optical activity and optical rotation of the film could be controlled by adjusting the electronic state of the electrochemical process. Polymer films prepared in the oxidized state exhibit a weak Cotton effect, while the reduced polymer film exhibits the expected mirror‐image bisignate Cotton effect in the region of the π–π* transition of the polymer main chain. These results indicate that the main chain itself is chiral in the film state. This procedure has great potential for the preparation of functional electrochromic devices and the improved preparation of durable electrochromic devices based on the good film‐forming properties of the chiral polymer.Cyclic voltammogram and CD spectra of the chiral polymer thin film produced here.imageCyclic voltammogram and CD spectra of the chiral polymer thin film produced here.

Transfer of chirality by dithiophosphate ligands and chiral discrimination in the stereoselective formation of square-planar Ni(II) complexes.

In the formation reaction of Ni(2+) with the chiral racemic ligand, (R)(R)bdtp(-)/(S)(S)bdtp(-), bdtp(-) = [SSPOCH)CH(3))CH(CH(3))O](-), cyclo- O,O'-[1,2-dimethylethylene] dithiophosphato ion, the meso-complex Ni[(R)(R)(lambda)bdtp][(S)(S)(delta)-bdtp] is stereoselectively produced. The meso-complex was compared with the enantiopure crystals of (+)(589)Ni[(R)(R)(lambda)bdtp](2) or (-)(589)Ni[(S)(S)(delta)bdtp](2), as well as racemic crystals, rac-(+/-)Ni[bdtp](2), which were prepared from the solution containing the two enantiomers in a 1:1 ratio. Dissociation constants in solutions indicate different stability of the meso and enantiopure complexes depending on the solvent, whereas a more efficient crystal packing, weak H-bonding, and nonbonding interactions contribute to stabilization of the meso-species over the racemic one. Molecular structures show that the outer five-membered ligand ring adopts the half-chair conformation C(2) with either the lambda or the delta chirality and the methyl groups are in equatorial (e) positions. Enantiopure ligands of (+)(589)Ni[(R)(R)(lambda)bdtp](2) and (-)(589)Ni[(S)(S)(delta)bdtp](2) induce chirality into the symmetric SSNiSS chromophore with slightly helical distortion. Thus, their CD spectra exhibit weak negative or positive Cotton effects at 662 nm. CD spectra in L(+)- and D(-)diethyltartrate of the meso-complex and racemic crystal, rac-(+/-)Ni[bdtp](2), exhibit different weak Cotton effects of opposite sign. Complexes dissociate in methanol; rac-(+/-)Ni[bdtp](2) in methanol undergoes a crystallization-induced second-order asymmetric transformation which finally yields crystals of the meso-Ni[(R)(R)(lambda)bdtp][(S)(S)(delta)bdtp] complex.

Synthesis of 11-cis-locked bicyclo[5.1.0]octanyl retinal and an enantioselective binding to bovine opsin.

Both enantiomers of 13-(E) and 13-(Z) isomers of 11-cis-locked bicyclo[5.1.0]octanyl retinal were prepared by an improved synthesis and incubated with bovine opsin. The synthesis also establishes the absolute configuration of the enantiomers. Only one of the enantiomers binds to opsin, thus showing the steric restrictions regarding the middle polyene moiety of the retinoid molecule; this is in sharp contrast to the known leniency of the ring moiety binding site of retinoids. However, although one enantiomer is incorporated into the pigment, the circular dichroic spectrum of the pigment incorporating the bound enantiomer yields only a very weak Cotton effect, showing that, once incorporated, the bicyclo[5.1.0]octanyl chromophore is flattened by the opsin binding site. The titled retinoid was synthesized for study of the absolute conformation of the retinal pigment in rhodopsin.

Model investigations for vanadium-protein interactions: vanadium(III) compounds with dipeptides and their oxovanadium(IV) analogues

The reaction of VCl(3) with 1,10-phenanthroline and a series of dipeptides (H(2)dip), having aliphatic as well as aromatic side chains, in methyl alcohol and in the presence of triethylamine affords vanadium(III) compounds of the general formula [V(III)(dip)(MeOH)(phen)]Cl. Aerial oxidation/hydrolysis of the vanadium(III) species gives their oxovanadium(IV) analogues of the general formula [V(IV)O(dip)(phen)]. X-ray crystallographic characterization of the [V(IV)O(dip)(phen)] compounds (where dip(2-)=Gly- L-Ala, Gly- L-Val and Gly- L-Phe) revealed that the vanadium atom possesses a severely distorted octahedral coordination and is ligated to a tridentate dip(2-) ligand at the N(amine) atom, the deprotonated N(peptide) atom and one of the O(carboxylate) atoms, as well as an oxo group and two phenanthroline nitrogen atoms. Circular dichroism characterization of the V(III)/V(IV)O(2+)-dipeptide compounds revealed a strong signal for the V(IV)O(2+) species in the visible range of the spectrum, with a characteristic pattern which may be exploited to identify the N(am), N(pep) and O(car) ligation of a peptide or a protein to V(IV)O(2+) center, and a weak Cotton effect of opposite sign to their vanadium(III) analogues. The visible spectra of the V(III)-dipeptide compounds revealed two d-d bands with high intensity, thus indicating that the covalency of the metal-donor atoms is significant, i.e. the vanadium d orbitals are significantly mixed with the ligand orbitals, and this is confirmed by the low values of their Racah B parameters. The high-intensity band of the V(IV)O(2+)-dipeptide compounds at approximately 460 nm implies also a strong covalency of the metal with the equatorial donor atoms and this was supported by the EPR spectra of these compounds. Moreover, the V(III)/V(IV)O(2+)-dipeptide complexes were characterized by EPR and IR spectroscopies as well as conductivity and magnetic susceptibility measurements.

Molecular Switching: A Fully Reversible, Optically Active Photochemical Switch Based on a Tetraethynylethene‐1,1′‐Binaphthalene Hybrid System

The synthesis, characterization, and physical properties of a novel, fully reversible, light-driven molecular switch, (R,R)-1/(R,R)-2, based on a tetraethynylethene-1,1′-binaphthalene hybrid system are presented. trans-Configured (R,R)-1 was synthesized in 57% yield by Stille cross-coupling between stannylated tetraethynylethene 3 and 3-iodo-1,1′-binaphthalene derivative (R)-4 (cf. Scheme 2). The cis-isomer (R,R)-2 was prepared from (R,R)-1 by photoisomerization. X-Ray crystal-structure analyses were obtained for both cis- and trans-forms of the photoswitch (Figs. 1 and 2). In the crystalline state, molecules of the cis-isomer (R,R)-2 exhibit intramolecular edge-to-face (C−H⋅⋅⋅π) interactions between naphthalene rings of the two 1,1-binaphthalene moieties (Fig. 3). The switching properties were investigated by electronic absorption spectroscopy (Table and Fig. 4): irradiation at λ=398 nm converts trans-isomer (R,R)-1 into cis-isomer (R,R)-2, whereas switching occurs in the opposite direction upon irradiation at λ=323 nm. No thermal interconversion between the two isomers was observed in CH2Cl2 at room temperature over a period of 2 – 3 months, and the system possesses good resistance against photofatigue (Fig. 5). Investigations of the circular dichroism of (R,R)-1 and (R,R)-2 in CH2Cl2 solution showed that the chiral binaphthalene moieties induce a weak Cotton effect in the achiral tetraethynylethene core (Fig. 6). System (R,R)-1/(R,R)-2 represents one of the rare switches allowing two-way photochemical interconversions, not perturbed by thermal-isomerization pathways.

Chirality induction in cyclopolymerization: 4. Using three diastereomeric templates, methyl, 4,6- O-isopropylidene-α- d-gluco-, manno- and altropyranoside in the cyclopolymerization of bis(4-vinylbenzoate) with styrene

Three distyryl monomers, methyl 4,6- O-isopropylidine-,2,3-bis- O-(4-vinylbenzoyl)-α- d-gluco- ( 1a), altro- ( 1b) and mannopyranoside ( 1c), were prepared. These monomers possess the same configuration except for the torsion angle between the two (4-vinylbenzoyl) groups, which is 60°, −60° and 180° for 1a, 1b and 1c respectively. Their cyclocopolymerizations with styrene ( 2) were carried out using α,α′-azobisisobutyronitrile in toluene at 60°C. The resulting copolymers were hydrolysed using potassium hydroxide in methanol/tetrahydrofuran under reflux and then treated with diazomethane to yield poly(methyl 4-vinylbenzoate- co-styrene)s ( 4a, 4b and 4c). The copolymer 4a showed optical activity with a maximum specific rotation ([ α] 435 23, c = 1.0 in CHCl 3) of −6.0°. Copolymers 4b and 4c had optical rotations of −1.9° and −2.0° respectively, which were smaller than that of 4a. The chirality was negative for 1a and positive for 1b, and was positive for 4a and 4b. On the other hand, 1c showed a very weak Cotton effect, and the chirality was positive for 4c.