Stilbene Units Research Articles

A [3]Rotaxane with Three Stable States That Responds to Multiple‐Inputs and Displays Dual Fluorescence Addresses

A [3]rotaxane molecular shuttle containing two alpha-cyclodextrin (alpha-CD) macrocycles, an azobenzene unit, a stilbene unit, and two different fluorescent naphthalimide units has been investigated. The azobenzene unit and the stilbene unit can be E/Z-photoisomerized separately by light excited at different wavelengths. Irradiation at 380 nm resulted in the photoisomerization of the azobenzene unit, leading to the formation of one stable state of the [3]rotaxane (Z1-NNAS-2CD); irradiation at 313 nm resulted in the photoisomerization of the stilbene unit, leading to the formation of another stable state of the [3]rotaxane (Z2-NNAS-2CD). The reversible conversion of the Z1 and Z2 isomers back to the E isomer by irradiation at 450 nm and 280 nm, respectively, is accompanied by recovery of the absorption and fluorescence spectra of the [3]rotaxane. The E isomer and the two Z isomers have been characterized by 1H NMR spectroscopy and by two-dimensional NMR spectroscopy. The light stimuli can induce shuttling motions of the two alpha-CD macrocycles on the molecular thread; concomitantly, the absorption and fluorescence spectra of the [3]rotaxane change in a regular way. When the alpha-CD macrocycle stays close to the fluorescent moiety, the fluorescence of the moiety become stronger due to the rigidity of the alpha-CD ring. As the photoisomerization processes are fully reversible, the photo-induced shuttling motions of the alpha-CD rings can be repeated, accompanied by dual reversible fluorescence signal outputs. The potential application of such light-induced mechanical motions at the molecular level could provide some insight into the workings of a molecular machine with entirely optical signals, and could provide a cheap, convenient interface for communication between micro- and macroworlds.

Studies on thermotropic liquid crystalline polyphosphates containing photoreactive dual mesogens

A series of thermotropic liquid crystalline polyphosphates, containing photochromic molecules of stilbene in the backbone and substituted azobenzene in the side chain, were synthesized respectively. The inherent viscosity measurements were determined for all the polymers. The stability and char yield, Tg, Tm and Ti were investigated by thermal analysis. Polarizing optical microscopy of all the polymers exhibited birefringent melts with liquid crystalline behaviour. Variable temperature powder X‐ray diffraction techniques were performed for confirmation of textures. UV‐visible photolysis studies investigated the simultaneous behaviour of reactivity rates of crosslinking of stilbene units and isomerization, caused by azobenzene units in the main chain/side chain LCPs. Photoisomerization kinetics demonstrated the switching time rates for the trans‐cis conversion of the azobenzene unit. A model polymer was synthesized and compared for the effect of stilbene in the main chain. Dipole moment values were calculated for the simulated low molecular mass of the pendant substituents to predict the polarity using MOPAC 3D Pro.

Ladder Bis-Silicon-Bridged Stilbenes as a New Building Unit for Fluorescent π-Conjugated Polymers

π-Conjugated polymers containing the bis-silicon-bridged stilbene (5,10-dihydro-5,10-disilaindeno[2,1-a]indene) skeleton in the main chain have been synthesized. Our synthesis uses 3,8-dimethoxy derivatives (2) and 3,8-difluoro derivatives (3) of the bis-silicon-bridged stilbene as the key precursors. Thus, their regioselective ortho-metalation reactions with appropriate bases (sec-butyllithium for 2 and LitBu2ZnTMP for 3) followed by quenching with electrophiles afford various 2,7-difunctionalized bis-silicon-bridged stilbenes. By using the 2,7-difunctionalized derivatives as starting materials, a series of π-conjugated polymers, including homopolymers of the bis-silicon-bridged stilbene skeleton and copolymers between the bis-silicon-bridged stilbene unit and other types of monomer units, such as bithiophene and diethynylbenzene, have been synthesized by cross-coupling methodologies. While all the homopolymers of the bis-silicon-bridged stilbene skeleton exhibit an intense blue to greenish-blue emission, the copolymers with the bithiophene and diethynylbenzene units show green and orange emissions, respectively, suggesting easy tuning of the emission colors over a wide range.

Luminescent Monomer and Poly(methacrylate) Containing 1,3,4‐Oxadiazole and Stilbene Units: Synthesis and Optical Properties

AbstractSummary: A monomer M as well as a poly(methacrylate) P carrying the same chromophore, that consists of 1,3,4‐oxadiazole and stilbene units, were efficiently synthesized. They dissolved in common organic solvents such as THF, chloroform, dichloromethane, 1,1,2,2‐tetrachloroethane and chlorobenzene. P showed a Tg value of 145 °C. Both M and P were stable up to approximately 300 °C and afforded anaerobic char yield of about 40% at 800 °C. Their optical properties were comparable. They emitted intense violet‐blue light in THF solution with a PL maximum at 413 nm and a PL quantum yield of 0.29 for M and 0.73 for P. Thin films of them displayed optical band gap of 3.03 eV and blue‐light emission with a PL maximum around 440 nm. The PL curves of both samples were progressively red‐shifted with increasing the solvent polarity. The influence of the annealing on the PL emission spectrum of M and P thin films was investigated. magnified image

Stereochemistry of isoplagiochin C, a macrocyclic bisbibenzyl from liverworts.

Cyclic bisbibenzyls, like isoplagiochins C (1) and D (2), are stereochemically intriguing molecules: Although not equipped with any of the traditional stereogenic elements that render molecules conformationally stable per se, they are sometimes isolated in an optically active form and are thus chiral at room temperature. The paper describes quantum chemical calculations, in particular investigations of the conformational space and molecular dynamics simulations, showing that the helicity is a property of the entire molecule, whose ring strain makes the molecule configurationally stable overall, with (formally) three stereogenic elements (two biaryl axes and one helical stilbene unit). Only one of the biaryl axes (the 'upper' one, joining C-12' and C-14) has a stable configuration, leading to a population of four interconverting diastereomers, yet without racemization at room temperature. On the basis of these conformational and dynamic calculations, the circular dichroism spectrum of isoplagiochin C (1) was calculated, leading to the first assignment of the absolute configuration of a cyclic bisbibenzyl. Accordingly, 1 has the P-configuration at the stereochemically stable biaryl axis and constitutes a mixture of diastereomers with respect to the other biaryl axis and the helical stilbene unit. From the temperature dependence of the racemization rates, an enantiomerization barrier of 101.6 kJ/mol was determined. Likewise, for the first time for cyclic bisbibenzyls, the enantiomeric ratio of this natural product was determined, by chromatography on a chiral phase with CD-coupling. Accordingly, 1 from Plagiochila deflexa is not enantiomerically pure, but occurs in a 85:15 ratio in favor of the enantiomer that has the P-configuration at the stereochemically stable axis.

Synthesis of Tetrastilbenylmethanes by Wittig−Horner Reactions

AbstractThe all‐(E)‐configured tetrastilbenylmethanes 3a−e and 5a,b can be obtained by fourfold Wittig−Horner reactions. The tetrahedral arrangement of these compounds guarantees independent stilbenoid chromophores with a high chromophore density. Apart from (E)/(Z) isomerization reactions, irradiation leads to a three‐dimensional network with isolated unchanged stilbene units. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Photochemical and photophysical properties of a poly(propylene amine) dendrimer functionalized with E-stilbene units.

A second generation poly(propylene amine) dendrimer functionalized at the periphery with eight E-stilbene and eight 4-tert-butylbenzenesulfonyl units has been prepared. The absorption spectrum, fluorescence spectrum and decay, E<==>Z photoisomerization, and photocyclization of the Z-isomer of the stilbene units have been investigated in air equilibrated acetonitrile solutions. For comparison purposes, a reference compound of the peripheral dendrimer units, namely 4-tert-butyl-N-propyl-N-(4-styryl-benzyl)-benzenesulfonamide, has also been studied. The quantum yield of the E-->Z photoisomerization reaction (0.30) and the fluorescence quantum yield of the E isomer (0.014) are substantially smaller for the units appended to the dendrimer compared to those of the reference compound (0.50 and 0.046, respectively). The presence of a red tail and the biexponential decay of the emission band of the dendrimer indicate formation of excimers between the stilbene units appended at the poly(propylene amine) dendritic structure. Under the experimental conditions used (lambda(exc)= 313 nm), a Z/E photostationary state (around 9 : 1 for both reference compound and dendrimer ) is reached in the time scale of minutes. On continuing irradiation, other photoreactions take place in the time scale of hours: the stilbene moiety of compound undergoes photocyclization to phenanthrene (quantum yield 0.015), whereas in dendrimer photocyclization to phenanthrene is accompanied by other processes, including a photoreaction involving the internal amine groups.

Photocapture of dynamic hydrogen‐bonded assemblies

Recent advances in the continuing study of [2+2] photodimerization reactions in supramolecular, non‐covalent systems are presented. The covalent photocapture of small dynamic assemblies which are formed using weak hydrogen‐bonding interactions between two different complementary units, barbiturates and melamines, is discussed. One unit serves as a photo‐inert supramolecular template capable of bringing two photoactive units together using multiple hydrogen‐bonds. The second type of unit unites the corresponding, complementary hydrogen‐bonding motif with a photoactive unit. Irradiation of the supramolecular assemblies leads to photodimerization of adjacent units and generation of an imprinted site for the template. Moieties which are adapted to participate in photodimerization reactions are styrene, cinnamate, stilbene and fullerene units. The results are interpreted on the basis of topochemical reaction control.

Synthesis and Properties of Novel T-Type Polyurethanes Containing 2,5-Dioxynitrostilbenyl Group as a Nonlinear Optical Chromophore

(2-hydroxyethoxy)-4-nitrostilbene (2) was prepared by reaction of 2-iodoethanol with 1. Polymer 3 was prepared by polyaddition reaction between a diol 2 and 2,4-toluenediisocyanate (TDI) in a dry DMF solvent. Synthetic method of polymer 3 is summarized in Scheme 1. The chemical structures of the compounds were confirmed by 1 H NMR, IR spectra and elemental analysis. The signal at 8.32-8.43 ppm of the 1 H NMR spectra, assigned to the amide proton, indicates the formation of urethane linkage. The IR spectra of the same polymer samples also show a strong carbonyl peak near 1713 cm −1 , indicating the presence of urethane bond. Two strong absorption bands due to the nitro group in the NLO-chromophore of the stilbene unit appeared near 1520 cm −1 and 1350 cm −1 . These results are consistent with the proposed structures, indicating that the NLO-chromophore remained intact during the polymerization. We now have a novel T-type polyurethane (3) and investigate its property. The polymer 3 was soluble in common solvents such as acetone, DMF and DMSO, but were not soluble in methanol

Synthesis and fluorescence of polyurethane cationomers N-modified with a stilbene chromophore

Preparation of a polyurethane cationomer with stilbene chromophore on urethane nitrogen atoms was performed by stepwise modification of a neutral polymer (PU-S1, stilbene units: 13.6 wt.%), followed by the quaternization with benzyl chloride (PUC-S1, 34.8 meq. ionic groups/100 g polymer). A linear polyesterurethane based on poly(ethylene adipate) diol ( M n: 2000), 4,4′-methylene-bis(phenyl isocyanate), 1,4-butanediol and N-methyldiethanolamine (1:6:3:2 molar ratio) was used as precursor. The trans–cis photoisomerism and fluorescent emission of the stilbene in polycation was studied and compared with the non-ionomeric form and the urethane-stilbene derivative. Another cationomer with fluorophore introduced via quaternization of the same precursor with urethane-stilbene halide was also taken in study (PUC-S2, 39.7 meq. ionic groups/100 g polymer). All polymers absorb in the UV region ( λ A: 313–332 nm) and reemit the light as violet–blue fluorescence in the region 420–470 nm. The significant red shift of the emission band in both polymer solution and film was assigned to the formation of excimers between fluorophores.

Synthesis and Characterization of Nonlinear Optical Polymers Having Quinoline-based Chromophores

We synthesized three kinds of chromophores incorporating aromatic quinoline unit as a π-conjugated bridge in order to prepare more thermally stable nonlinear optical (NLO) chromophores than general stilbene unit. The NLO poly(methylmethacrylate) copolymer, polyimides, and polyester were successfully synthesized by these corresponding quinoline-based monomers. Their physical and optical properties were investigated by thermogravimetry, gel permeation chromatography, ultraviolet-visible spectroscopy, second harmonic generation (SHG) and electro-optic (EO) measurement. All the polymers exhibited better thermal stability, however their NLO activity was a little lower than that of general stilbene-based NLO polymers. Among three kinds of polymers, the PMMA copolymer with quinoline chromophores had the largest SHG coefficient d 3 3 value of 27 pm/V (at 1.064 μm) and EO coefficient r 3 3 value of 6.8 pm/V (at 1.3 μm).

Tetraphenylmethane based tectons in dendrimer synthesis: preparation and energy transfer properties of a dendritic stilbene–anthracene dyad

A topologically new stilbene–anthracene dendrimer has been prepared using a highly branched tristilbenyl dendron derived from tetraphenylmethane. Steady-state photophysical studies on this dendrimer showed energy transfer from the peripheral stilbene units to the anthracene core.

Synthesis of dendritic stilbenoid compounds: Heck reactions for the periphery and the core

A convergent synthetic strategy for stilbenoid dendrimers is described in which multifold Heck reactions have been used to construct both the peripheral and the core stilbene units. Solution optical properties of one of the dendrimers indicated some energy transfer having taken place from the periphery to the core.

Synthesis and characterization of novel polyimides containing stilbene unit in the side chain and their controllability of nematic liquid crystal alignment on the rubbed surfaces

AbstractFour polyimides containing hexylene spacer and a fluorostilbene unit in the side chains were prepared in thin‐film form by two‐step condensation of 3,3′‐bis[(4′‐fluoro‐4‐stilbenyl)oxyhexyloxy]‐4,4′‐biphenyldiamine (FS6B) with pyromellitic dianhydride (PMDA), benzophenone‐3,3′,4,4′‐tetracarboxylic dianydride (BTDA), 4,4′‐oxydi(phthalic anhydride) (ODPA), and 4,4′‐hexafluoroisopropylidenedi(phthalic anhydride) (6FDA), respectively, and their controllability of liquid crystal (LC) alignment on rubbed surfaces was investigated. Pretilt angles of LCs were achieved in the 2–9° range, depending on the rubbing density and backbone structures. The effect of the mesogenic stilbene group on the pretilting of LCs was distinctive in FS6B‐PMDA. Contact‐angle measurements on thin films annealed at 120 °C revealed that FS6B‐PMDA potentially had the better alignment stability than FS6B‐6FDA. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3622–3632, 2001

Synthesis and thermal polymerization of a novel stilbene-maleimide AB-monomer

A novel AB-monomer, 3-maleimidostilbene (ST-MAI), was synthesized. DSC investigation indicated that the ST-MAI monomer melted at 127°C and thermally polymerized in the temperature range of 180 ∼ 300°C. IR investigation on the thermal polymerization processes proved that the thermal polymerization included not only copolymerizaiton between stilbene and maleimide, but also homopolymerization of maleimide. The largest reaction conversion of maleimide and stilbene unit in a ST-MAI monomer was about 82% and 50% respectively. The glass transition temperature of cured ST-MAI resin was 234°C, determined by DSC. The decomposition temperatures for 10% weight loss was above 430°C in both air and nitrogen atmospheres.

Formation Constants of Complexes of Monovalent Cations with Benzocrown Ethers in Nitromethane

The complexation reactions between the macrocyclic polyethers dibenzo-18-crown-6, benzo-18-crown-6, benzo-15-crown-5 and polyethers bearing a stilbene unit with alkali metal and silver cations have been studied conductometrically in nitromethane. The formation constants of 1 : 1 and 1 : 2 (metal : ligand) complexes were determined and found to decrease with increasing cation diameter. The stability of the stilbene crown – metal cation complexes is lower than for complexes of other investigated crown ethers with analogous cations. There seem to be some effects of double bond-silver ion interactions.

Five new stilbene dimers from the lianas of Gnetum hainanense.

Five new stilbene dimers, gnetuhainins A-E (1-5), were isolated together with resveratrol trans-dehydrodimer (6), resveratrol, oxyresveratrol, and (-)-epsilon-viniferin from the lianas of Gnetum hainanense. Their structures and stereochemistry were determined on the basis of their chemical and spectral data. Compounds 1-5 are dimers formed by a resveratrol unit and an oxyresveratrol unit and belong to a new type of oligostilbenes polymerized from two different stilbene units.

A Tandem Hexaannulation Reaction of Benzyllithium Involving Base-Induced Cyclodehydrogenation

The reaction of the tribenzo[3.3.3]propellane ketone 1 (triptindan-9-one) with benzyllithium/TMEDA affords an efficient one-pot peri annulation of a dihydronaphthalene across two wings of the propellane framework. The key step of this surprising tandem reaction was determined to be a nucleophilic cyclization of (Z)-9-benzylidenetriptindane (4) with concomitant hydride elimination. The formation of the novel centrotetracyclo[5.5.5.6] framework bearing a distorted (E)-stilbene unit was confirmed by single-crystal X-ray structure analysis.

A Tandem Hexaannulation Reaction of Benzyllithium Involving Base-Induced Cyclodehydrogenation

The reaction of the tribenzo[3.3.3]propellane ketone 1 (triptindan-9-one) with benzyllithium/TMEDA affords an efficient one-pot peri annulation of a dihydronaphthalene across two wings of the propellane framework. The key step of this surprising tandem reaction was determined to be a nucleophilic cyclization of (Z)-9-benzylidenetriptindane (4) with concomitant hydride elimination. The formation of the novel centrotetracyclo[5.5.5.6] framework bearing a distorted (E)-stilbene unit was confirmed by single-crystal X-ray structure analysis.

Phenanthro[1.10]-Annelated [3.3.3]Propellanes by Cyclodehydrogenation Reactions of Mono-, Di-, and Tribenzylidenetriptindanes

The synthesis of [3.3.3]propellanes 18, 19, and 3 bearing one, two, or three [1,10]-fused phenanthrene units instead of a simple benzene ring has been performed. Starting from tryptindan-9-one 4, the hitherto unknown tryptindane-9,10-dione 5 and tryptindane-9,10,11-trione 10, the corresponding mono-, di-, and tribenzylidenetriptindanes 7, 9, and 13 have been prepared. In all three cases. the stereoisomers bearing cis-oriented benzylidene groups. i.e., (Z)-stilbene units. are formed predominantly. The monostilbene (Z)-7 and the tristilbene (Z,Z,Z)-13 were obtained in pure form and their stereochemistry was fully characterized. Photocyclodehydrogenation of 7 under Mallory-Katz conditions gave dibenzo(phenanthro-[1,10])[3.3.3]propellane 18 in an almost quantitative yield, irrespective of the stereoisomeric composition of 7. In contrast, photocyclodehydrogenation of distilbene 9 gave the unusual propellane-fused elassovalene 20 in a moderate yield, and the expected benzodi-(phenanthro-[1,10])[3.3.3]propellane 19 was only obtained in a low yield. As an extreme case, tristilbene (Z,Z,Z)-13 eluded the desired threefold photolytic cyclodehydrogenation; however, catalytic cyclodehydrogenation by Pt/Al2O3/Ti at 310 degrees C furnished the threefold acephenanthrylene 3 in a yield of 18%.