Physicochemical and adsorption properties of hypercross‐linked polystyrene with ultimate cross‐linking density

The paper describes unexpected properties of hypercross‐linked polystyrenes with ultimate cross‐linking degrees of 300, 400, and 500%, where three, four, or five methylene links, respectively, could bind each polystyrene phenyl ring to its spacious neighbors. The polymers exhibit a strong electron spin resonance signal, unusual spectra in IR, UV, and visible ranges, and they are not typical dielectrics. The nonfunctionalized hypercross‐linked polymers absorb significant amounts of inorganic acids, salts, and bases due to interactions of protons or other cations with electron‐donating fragments of the aromatic network with the high extent of mutual connectivity and also due to dispersion interactions of anions with the polymer matrix.     

Cholesteric mixture containing chiral‐photochromic and diarylethene dopants as novel material with dual photochromism

New cholesteric polymeric mixture containing nematic side‐chain homopolymer, 2 wt% of photochromic diarylethene dopant and 5 wt% of chiral‐photochromic dopant based on cinnamic acid and isosorbide was prepared. Upon UV irradiation of planarly‐oriented mixture films at room temperature, a transformation of open colorless form of diarylethene dopant into a closed colored form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region and decreasing selective light reflection intensity. This process is photo‐ and thermo‐reversible and many cycles ‘recording‐erasing’ can be realized. UV irradiation and subsequent annealing of the films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long‐wavelength region. This process is explained by the E‐Z isomerization of chiral photochromic groups of the dopant relative to a C?C bond accompanied by a decrease in their twisting ability. It has been shown, that the use of the material prepared in this work provides an opportunity to record two images at the same time: one image is due to a change in the helical pitch and another (second) image is due to the photochromism of diarylethene dopant. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under study shows advantage as new photosensitive material for optics, optoelectronics and data recording. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis and properties of photoalignable aromatic polyesters containing phenylenediacrylate units in their backbones and n‐alkyl moieties in their side groups

A series of poly[oxy(4‐n‐alkyl‐3,5‐benzoate)oxy‐1,4‐phenylenediacryloyl]s (PPDA‐CnBZ polymers) with high molecular weights was synthesized. These polymers exhibit excellent solubility in some common organic solvents and produce good quality films using conventional spin‐casting and drying processes. The polymers are thermally stable up to 357–362 °C in a nitrogen atmosphere; their glass transition temperatures are greater than 121 °C. The photoreactions and photoalignments of the polymers were investigated using ultraviolet‐visible and infrared spectroscopy, and their liquid crystal (LC) alignment properties were examined. The phenylenediacrylate (PDA) chromophores in the polyesters were found to mainly undergo photocyclization upon ultraviolet light irradiation. Irradiation of the polyester films with linearly polarized ultraviolet light (LPUVL) induces preferential orientation of the polymer main chains, while the unreacted PDA chromophores are aligned along the direction perpendicular to the electric vector of the LPUVL. All the films irradiated with LPUVL were found to align LCs in a direction perpendicular to the electric vector of the LPUVL. Moreover, these LC alignments persisted even on irradiated films annealed at temperatures up to 210 °C, which is much higher than the glass transition temperatures of the polyesters. These LC alignment characteristics are due to the anisotropic interactions of the LC molecules with the oriented polymer chains and with the unreacted PDA chromophores. LC alignments on the polyester film surfaces have homeotropic to homogeneous characteristics, depending on the length of the n‐alkyl side group, providing strong evidence that the n‐alkyl side groups of the polyesters play a critical role in determining the pretilt angles of the LCs. The LC pretilt angles were also found to be influenced by the thermal annealing history of the irradiated films. In summary, the excellent properties of the PPDA‐CnBZ polymers make them promising candidate materials for use as LC alignment layers in advanced LC display devices. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1322–1334, 2004     

Synthesis and characterization of fluorescently labeled core cross‐linked star polymers

A series of fluorescently labeled core cross‐linked star (CCS) polymers were synthesized via the “arm‐first” approach, employing atom transfer radical polymerization (ATRP) to control the resulting architecture. The initiator p‐toluenesulfonyl chloride (TsCl) was used to synthesize “living” poly(methyl methacrylate) (PMMA) macroinitiator, which was subsequently cross‐linked to generate the CCS polymers. Divinylbenzene (DVB) was used as the cross‐linker and 7‐[4‐(trifluoromethyl)coumarin] methacrylamide ( F₁ , λ_ex = 343 nm) was added as a fluorescent labeling monomer. A range of PMMA/DVB/ F₁ based CCS polymers were synthesized with the core domain made selectively fluorescent by using varying amounts of monomer F₁ . The core functionalized stars were characterized using gel permeation chromatography (GPC) equipped with multi‐angle laser light scattering (MALLS), refractive index (RI), and UV–visible detectors. The fluorescence quantum yield (Φ_F) and the amount of fluorescent monomer incorporated into the core were quantified by UV–visible and fluorescence spectrophotometry. It was recognized that the overall molecular weights of the stars produced, along with their core molecular weight, decreased as the mol % of monomer F₁ was increased relative to cross‐linker. Visual confirmation of F₁ incorporation was obtained by fluorescence microscopy of thin polymer films cast on glass substrates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2422–2432, 2008     

Switching Diarylethenes Reliably in Both Directions with Visible Light

A diarylethene photoswitch was covalently connected to two small triplet sensitizer moieties in a conjugated and nonconjugated fashion and the photochromic performance of the resulting compounds was investigated. In comparison with the parent diarylethene (without sensitizers) and one featuring saturated linkages, the conjugated photoswitch offers superior fatigue resistance upon visible‐light excitation due to effective triplet energy transfer from the biacetyl termini to the diarylethene core. Our design makes it possible to switch diarylethenes with visible light in both directions in a highly efficient and robust fashion based on extending π‐conjugation and by‐product‐free ring‐closure via the triplet manifold.     

Versatile Supramolecular Cross‐Linker: A Rotaxane Cross‐Linker That Directly Endows Vinyl Polymers with Movable Cross‐Links

A supramolecular cross‐linked cross‐linker, capable of introducing rotaxane cross‐links to vinyl polymers, has been developed for the rational synthesis of polyrotaxane networks. The experimental results reveal that the combination of an oligocyclodextrin (OCD) and a terminal bulky group‐tethering macromonomer (TBM) forms a polymer‐network structure having polymerizable moieties through supramolecular cross‐linking. Radical polymerization of a variety of typical vinyl monomers in the presence of the vinylic supramolecular cross‐linker (VSC) afforded the corresponding vinyl polymers cross‐linked through the rotaxane cross‐links (RCP) as transparent stable films in high yields under both photoinitiated and thermal polymerization conditions. A poly(N,N‐dimethylacrylamide)‐based hydrogel synthesized by using VSC, RCP_DMAAm, displayed a unique mechanical property. The small‐angle X‐ray scattering (SAXS) results, indicating patterns characteristic of a polyrotaxane network, clearly suggested the presence and role of the rotaxane cross‐links. The confirmation of the introduction of rotaxane‐cross‐links into vinyl polymers strongly reveals the significant usefulness of VSC.     

Light‐Controllable Ionic Conductivity in a Polymeric Ionic Liquid

Polymeric ionic liquids (PILs) have attracted considerable attention as electrolytes with high stability and mechanical durability. Light‐responsive materials are enabling for a variety of future technologies owing to their remote and noninvasive manipulation, spatiotemporal control, and low environmental impact. To address this potential, responsive PIL materials based on diarylethene units were designed to undergo light‐mediated conductivity changes. Key to this modulation is tuning of the cationic character of the imidazolium bridging unit upon photoswitching. Irradiation of these materials with UV light triggers a circa 70 % drop in conductivity in the solid state that can be recovered upon subsequent irradiation with visible light. This light‐responsive ionic conductivity enables spatiotemporal and reversible patterning of PIL films using light. This modulation of ionic conductivity allows for the development of light‐controlled electrical circuits and wearable photodetectors.     

Nondestructive Photoluminescence Read‐Out by Intramolecular Electron Transfer in a Perylene Bisimide‐Diarylethene Dyad

A novel, highly stable photochromic dyad 3 based on a perylene bisimide (PBI) fluorophore and a diarylethene (DAE) photochrome was synthesized and the optical and photophysical properties of this dyad were studied in detail by steady‐state and time‐resolved ultrafast spectroscopy. This photochromic dyad can be switched reversibly by UV‐light irradiation of its ring‐open form 3 o leading to the ring‐closed form 3 c , and back reaction of 3 c to 3 o by irradiation with visible light. Solvent‐dependent fluorescence studies revealed that the emission of ring‐closed form 3 c is drastically quenched in solvents of medium (e.g., chloroform) to high (e.g., acetone) polarities, while the emission of the ring‐open form 3 o is appreciably quenched only in highly polar solvents like DMF. The strong fluorescence quenching of 3 c is attributed to a photoinduced electron‐transfer (PET) process from the excited PBI unit to ring‐closed DAE moiety, as this process is thermodynamically highly favorable with a Gibbs free energy value of ?0.34 eV in dichloromethane. The electron‐transfer mechanism for the fluorescence quenching of ring‐closed 3 c is substantiated by ultrafast transient measurements in dichloromethane and acetone, revealing stabilization of charge‐separated states of 3 c in these solvents. Our results reported here show that the new photochromic dyad 3 has potential for nondestructive read‐out in write/read/erase fluorescent memory systems.     

An All‐Inorganic Colloidal Nanocrystal Flexible Polarizer

Inorganic single crystals with anisotropic structures usually suffer from high brittleness and stiffness. Flexible polymers are used to replace inorganic crystals, but the hot‐stretching‐induced orientation process is tedious, and oriented molecular chains tend to revert to random coils during aging. To overcome these obstacles and using the similarities between sub‐1 nm nanowires (NWs) and linear polymers, we successfully fabricated anisotropic, transparent, flexible, and stable (ATFS) NW films with great potential for optical applications through a wet‐spinning method. The NW films show birefringence, and their birefractive index is higher than that of many polymers. They also showed polarized absorption of UV light and anisotropic scattering of visible light. The integrated films composed of NWs and quantum dots showed good fluorescence polarization. The tedious synthesis of quantum rods and fabrication of oriented polymer films can thus be avoided.     

Bio‐Based Polyketones by Selective Ring‐Opening Radical Polymerization of α‐Pinene‐Derived Pinocarvone

The most abundant naturally occurring terpene, α‐pinene, which cannot be directly polymerized into high polymers by any polymerization method, was quantitatively converted under visible‐light irradiation into pinocarvone, which possesses a reactive exo methylene group. The bicyclic vinyl ketone was quantitatively polymerized in fluoroalcohols by selective (99 %) ring‐opening radical polymerization of the four‐membered ring, which results in unique polymers containing chiral six‐membered rings with conjugated ketone units in the main chain. These polymers display good thermal properties, optical activities, and contain reactive conjugated ketone units. Reversible addition fragmentation chain transfer (RAFT) polymerization was successfully accomplished by using appropriate trithiocarbonate RAFT agents, enabling the synthesis of thermoplastic elastomers based on controlled macromolecular architectures.     

Photochemically-induced refractive index and fluoresence patterning on polymer films

The molecular design for large photo-induced refractive index changes in transparent visible light region was proposed and realized with norbornadiene polymers and poly(vinyl cinnamate). The patterning of pure refractive-index contract on their transparent films was made with near-field scanning optical microscopy (NSOM). Reversible fluorescence patterning on polymer films is also presented by using controlled energy transfer from a fluorescent pyromethene to a photochromic diarylethene.     

Thermally curable organic/inorganic hybrid polymers as gate dielectrics for organic thin‐film transistors

New low‐temperature curable organic/inorganic hybrid polymers were designed and synthesized as gate dielectrics for organic thin‐film transistors (OTFTs). Allyl alcohols were introduced to polyhedral oligomeric silsesquioxane (POSS) via hydrosilyation to produce an alcohol‐functionalized POSS derivative (POSS‐OH). POSS‐OH was then reacted with hexamethoxymethylmelamine at carrying molar ratios at 80 °C in the presence of a catalytic amount of p‐toluenesulfonic acid to give highly cross‐linked network polymers (POSS‐MM). The prepared thin films were smooth and hard after the thermal cross‐linking reaction and had very low leakage currents (<10^?8 A/cm²) with no significant absorption over the visible spectral range. Pentacene‐based OTFTs using the synthesized insulators as gate dielectric layers had higher hole mobilities (up to 0.36 cm²/Vs) than a device using thermally cross‐linked poly(vinyl phenol) and melamine as the gate dielectric layer (0.18 cm²/Vs). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3260–3268     

Visible Light‐Triggered Photoswitchable Diarylethene‐Based Iridium(III) Complexes for Imaging Living Cells

A novel diarylethene‐based iridium(III) complex was synthesized as a phosphorescence probe for monitoring living cells. The switchable phosphorescence complex in solution and within living cells was controlled by two distinguishable visible‐light irradiations, which suggests that this complex can be developed as a promising probe with weak photodamage for biological samples.     

DFT/TDDFT Investigation of the Modulation of Photochromic Properties in an Organoboron‐Based Diarylethene by Fluoride Ions

The diarylethene derivative 1,2‐bis‐(5′‐dimesitylboryl‐2′‐methylthieny‐3′‐yl)‐cyclopentene ( 1 ) containing dimesitylboryl groups is an interesting photochromic material. The dimesitylboryl groups can bind to F^?, which tunes the optical and electronic properties of the diarylethene compound. Hence, the diarylethene derivative 1 containing dimesitylboryl groups is sensitive to both light and F^?, and its photochromic properties can be tuned by a fluoride ion. Herein, we studied the substituent effect of dimesitylboron groups on the optical properties of both the closed‐ring and open‐ring isomers of the diarylethene molecule by DFT/TDDFT calculations and found that these methods are reliable for the determination of the lowest singlet excitation energies of diarylethene compounds. The introduction of dimesitylboron groups to the diarylethene compound can elongate its conjugation length and change the excited‐state properties from π→π* transition to a charge‐transfer state. This explains the modulation of photochromic properties through the introduction of dimesitylboron groups. Furthermore, the photochromic properties can be tuned through the binding of F^? to a boron center and the excited state of the diarylethene compound is changed from a charge‐transfer state to a π→π* transition. Hence, a subtle control of the photochromic spectroscopic properties was realized. In addition, the changes of electronic characteristics by the isomerization reaction of diarylethene compounds were also investigated with theoretical calculations. For the model compound 2 without dimesitylboryl groups, the closed‐ring isomer has better hole‐ and electron‐injection abilities, as well as higher charge‐transport rates, than the open‐ring isomer. The introduction of dimesitylboron groups to diarylethene can dramatically improve the charge‐injection and ‐transport abilities. The closed isomer of compound 1 ( 1 C ) has the best hole‐ and electron‐injection abilities, whereas the charge‐transport rates of the open isomer of compound 1 ( 1 O ) are higher than those of 1 C . Importantly, 1 O is an electron‐accepting and ‐transport material. These results show that the diarylethene compound containing dimesitylboryl groups has promising potential to be applied in optoelectronic devices and thus is worth to be further investigated.     

Digital photoswitching of fluorescence based on the photochromism of diarylethene derivatives at a single-molecule level

Photochromic reactions of diarylethene derivatives were detected at a single-molecule level by using a fluorescence technique. Fluorescent photoswitching molecules in which photochromic diarylethene and fluorescent bis(phenylethynyl)anthracene units are linked through an adamantyl spacer were synthesized, and switching of fluorescence upon irradiation with UV and visible light was followed in solution as well as on polymer films at the single-molecule level. Although in solution the fluorescence intensity gradually changed upon irradiation with UV and visible light, digital on/off switching between two discrete states was observed at the single-molecule level. The "on"- and "off"-times were dependent on the power of UV and visible light. When the power of UV and visible light was increased, the average on- and off-times became short in proportion to the reciprocal power of the light. The response-times were found to show distribution. The distribution of the on- and off-times is considered to reflect the difference in the micro-environment as well as conformation of the molecules.     

Nanoscopic Visualization of Cross‐Linking Density in Polymer Networks with Diarylethene Photoswitches

The in situ nanoscopic imaging of soft matter polymer structures is of importance to gain knowledge of the relationship between structure, properties, and functionality on the nanoscopic scale. Cross‐linking of polymer chains effects the viscoelastic properties of gels. The correlation of mechanical properties with the distribution and amount of cross‐linkers is relevant for applications and for a detailed understanding of polymers on the molecular scale. We introduce a super‐resolution fluorescence‐microscopy‐based method for visualizing and quantifying cross‐linker points in polymer systems. A novel diarylethene‐based photoswitch with a highly fluorescent closed and a non‐fluorescent open form is used as a photoswitchable cross‐linker in a polymer network. As an example for its capability to nanoscopically visualize cross‐linking, we investigate pNIPAM microgels as a system known with variations in internal cross‐linking density.     

Visible‐Light‐Triggered Cross‐Linking of DNA Duplexes by Reversible [2+2] Photocycloaddition of Styrylpyrene

Reversible photo‐cross‐linking of a DNA duplex through the [2+2] photocycloaddition of styrylpyrene is reported. Styrylpyrene moieties on d ‐threoninol linkers were introduced into complementary positions on DNA strands. Irradiation of the styrylpyrene pair in the duplex with visible light at λ=455 nm induced a [2+2] photocycloaddition between styrylpyrenes that cross‐linked the two strands of the duplex. Two diastereomers were formed after [2+2] photocycloaddition as a result of rotation of the styrylpyrene residues. Also, the cycloreversion reaction was induced by UV light at λ=340 nm, which reversibly yielded the uncross‐linked strands.     

Synthesis and Characterization of Cross‐Linked Poly(p‐phenylene ethynylene)s

Summary: Conjugated poly(p‐phenylene ethynylene) networks with interesting optoelectronic properties were synthesized by the palladium‐catalyzed polycondensation of 2,5‐diiodo‐4‐[(2‐ethylhexyl)oxy]methoxybenzene, and 1,4‐diethynyl‐2,5‐bis‐(octyloxy)benzene, with 1,2,4‐tribromobenzene as cross‐linker. The cross‐linker concentration was varied and materials with different cross‐link densities were prepared. The materials were processed into films by simultaneous polymerization and shaping. An alternative approach is to synthesize these cross‐linked polymers in the form of spherical particles, which can be processed from dispersions.

Schematic representation of the cross‐linking process.     

Reversible Photomodulation of Electronic Communication in a π‐Conjugated Photoswitch‐Fluorophore Molecular Dyad

The extent of electronic coupling between a boron dipyrromethene (BODIPY) fluorophore and a diarylethene (DAE) photoswitch has been modulated in a covalently linked molecular dyad by irradiation with either UV or visible light. In the open isomer, both moieties can be regarded as individual chromophores, while in the closed form the lowest electronic (S₀→S₁) transition of the dyad is slightly shifted, enabling photomodulation of its fluorescence. Transient spectroscopy confirms that the dyad behaves dramatically different in the two switching states: while in the open isomer it resembles an undisturbed BODIPY fluorophore, in the closed isomer no fluorescence occurs and instead a red‐shifted DAE behavior prevails.     

Synthesis and properties of nitrogen‐linked poly(2,7‐carbazole)s as hole‐transport material for organic light emitting diodes

A novel class of carbazole polymers, nitrogen‐linked poly(2,7‐carbazole)s, was synthesized by polycondensation between two bifunctional monomers using the palladium‐catalyzed amination reaction. The polymers were characterized by ¹H NMR, Infrared, Gel permeation chromatography, and MALDI‐TOF MS and it was revealed that the combination of the monomer structures is important for producing high molecular weight polymers. Thermal analysis indicated a good thermal stability with high glass transition temperatures, e.g., 138 °C for the higher molecular weight polymer P2 . To pursue the application possibilities of these polymers, their optical properties and energy levels were investigated by UV‐Vis absorption and fluorescence spectra as well as their electrochemical characteristics. Although the blue light emission was indeed observed for all polymers in solution, the quantum yields were very low and the solid films were not fluorescent. On the other hand, the HOMO levels of the polymers estimated from the onset potentials for the first oxidation in the solid thin films were relatively high in the range of ?5.12 to ?5.20 eV. Therefore, light emitting diodes employing these polymers as a hole‐transport layer and iridium(III) complex as a triplet emitter were fabricated. The device of the nitrogen‐linked poly(2,7‐carbazole) P3 with p,p′‐biphenyl spacer, which has a higher HOMO level and a higher molecular weight, showed a much better performance than the device of P2 with m‐phenylene spacer. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3880–3891, 2009