Dihydroazulene-Azobenzene-Dihydroazulene Triad Photoswitches

Photoswitch triads comprising two dihydroazulene (DHA) units in conjugation with a central trans-azobenzene (AZB) unit were prepared in stepwise protocols starting from meta- and para-disubstituted azobenzenes. The para-connected triad had significantly altered optical properties and lacked the photoactivity of the separate photochromes. In contrast, for the meta-connected triad, all three photochromes could be photoisomerized to generate an isomer with two vinylheptafulvene (VHF) units and a cis-azobenzene unit. Ultrafast spectroscopy of the photoisomerizations revealed a fast DHA-to-VHF photoisomerization and a slower trans-to-cis AZB photoisomerization. This meta triad underwent thermal VHF-to-DHA back-conversion with a similar rate of all VHFs, independent of the identity of the neighboring units, and in parallel thermal cis-to-trans AZB conversion. The experimental observations were supported by computation (excitation spectra and orbital analysis of the transitions).     

Controlling Two‐Step Multimode Switching of Dihydroazulene Photoswitches

We present the synthesis and switching studies of systems with two photochromic dihydroazulene (DHA) units connected by a phenylene bridge at either para or meta positions, which correspond to a linear or cross‐conjugated pathway between the photochromes. According to UV/Vis absorption and NMR spectroscopic measurements, the meta‐phenylene‐bridged DHA–DHA exhibited sequential light‐induced ring openings of the two DHA units to their corresponding vinylheptafulvenes (VHFs). Initially, the VHF–DHA species was generated, and, ultimately, after continued irradiation, the VHF–VHF species. Studies in different solvents and quantum chemical calculations indicate that the excitation of DHA–VHF is no longer a local DHA excitation but a charge‐transfer transition that involves the neighboring VHF unit. For the linearly conjugated para‐phenylene‐bridged dimer, electronic communication between the two units is so efficient that the photoactivity is reduced for both the DHA–DHA and DHA–VHF species, and DHA–DHA, DHA–VHF, and VHF–VHF were all present during irradiation. In all, by changing the bridging unit, we can control the degree of stepwise photoswitching.     

Dihydroazulene-Boron Subphthalocyanine Conjugates with Oligo(phenyleneethynylene) Bridging Unit: Photoswitchable Fluorophores

In this work, we have linked the dihydroazulene (DHA)/vinylheptafulvene (VHF) photo-/thermoswitch and the boron subphthalocyanine (BsubPc) fluorophore via an axial oligo(phenyleneethynylene) bridging unit into new DHA-BsubPc conjugates. The objectives were to elucidate the influence of BsubPc on the DHA/VHF switching reactions and the influence of DHA/VHF on the BsubPc fluorescence in these conjugates for which the entire axial substituent connected to boron comprises one large, conjugated scaffold. We present the synthesis and properties of DHA-BsubPc conjugates with varying peripheral substituents on the BsubPc core, being either unsubstituted (H₁₂BsubPc) or partially fluorinated (F₆BsubPc). Fluorination of the BsubPc core provided a remarkable increase in the reversibility of the DHA-VHF interconversions promoted by light and heat, respectively, and accompanied by on/off switching of the BsubPc fluorescence. Synthetically, the units were connected using Sonogashira coupling reactions of suitable acetylenic building blocks.     

Unexpected formation of a xanthone in the synthesis of a bisbenzoxazole

Recently we reported the synthesis in polyphosphoric acid of a graft copolymer consisting of a rigid-rod poly[benzo(1,2-d;4,5-d′)bisthiazole-2,6-diyl]-1,4-phenylene backbone and flexible poly(oxy-1,3-phenylenecarbonyl-1,4-phenylene) side-chains [1]. However, upon extension of this synthetic approach to the analogous graft copolymer with a poly[benzo(1,2-d:5,4-d′)bisoxazole-2,6-diyl]-1,4-phenylene backbone, we encountered some unexpected results due to methyl group migration. In order to better understand these results, we carried out the synthesis of selected benzothiazole and benzoxazole structures under appropriate reaction conditions. The results are reported in this article.     

Electrochemical reduction potential of n-type π-conjugated polymers

The ease of reduction of π-conjugated poly(p-phenylene)-, poly(naphthalene-2,6-diyl)-, and poly(naphthalene-1,4-diyl)-type polymers is determined by the electron accepting properties of the recurring aromatic units. Linear relationships hold between reduction potentials (E_red) of the π-conjugated poly(arylene)s and electron affinities (EA) of the recurring arylene units: E_red = ρEA + a. A ρ value of 0.85 ± 0.02 is found for the three types of polymers investigated. © 1996 John Wiley & Sons, Inc.     

Photoswitchable Tetraethynylethene‐Dihydroazulene Chromophores

The synthesis, characterization, and photophysical as well as electrochemical properties of the photochromic hybrid systems 11 – 16 and 18 , which contain photoswitchable tetraethynylethene (TEE; 3,4‐diethynylhex‐3‐ene‐1,5‐diyne) and dihydroazulene (DHA) moieties, are presented. The molecular photoswitches were synthesized by a Sonogashira cross‐coupling reaction between an appropriate TEE precursor ( 6 – 10 and 17 ) and an iodinated DHA 1 or its vinylheptafulvene (VHF) isomer ( 4 ) (Schemes 5 – 7). X‐Ray crystal structures of five DHA derivatives ( 1 , trans‐ 11a , cis‐ 11a , 12 , and 13 ) are discussed (Figs. 2 – 5). In all compounds, the cyclohexatriene moiety of the DHA chromophore adopts a clear boat conformation (Table 1). Presumably due to crystal‐packing effects, the arylated TEE moieties in the hybrid systems show substantial distortions from planarity, with the dihedral angles between the planes of the central TEE core and the adjacent aryl substituents amounting to 44°. The switching properties were investigated by electronic absorption spectroscopy. Upon light absorption, DHAs 1 , 12 – 16 , and 18 underwent retro‐electrocyclization in solution to give the corresponding VHFs (Figs. 6, 11, and 12). The reaction is thermally reversible, with half‐lives τ_1/2 between 3.9 and 5.8 h at 25° in CH₂Cl₂ (Figs. 7 and 13 and Table 3). A comparatively slower (E)→(Z) isomerization process about the central C=C bond of the TEE moiety was also observed. The N,N‐dimethylanilino‐(DMA) substituted TEE‐DHA hybrid systems trans‐ 11a and cis‐ 11a did not react to the corresponding VHFs upon irradiation (Scheme 9). Instead, only the reversible (E)→(Z) photoisomerization of the TEE core occurred (Fig. 16 and Table 4). This process was further investigated for photofatigue by electronic‐emission spectroscopy (Fig. 17). After protonation of the DMA group, the usual DHA→VHF photoreaction took place. Compound 11 represents a three‐way chromophoric molecular switch with three addressable sub‐units (TEE core, DHA/VHF moiety, and proton sensitive DMA group) that can undergo individual, reversible switching cycles (Scheme 9). A process modeling the function of an `AND' logic gate (Fig. 19) and three write/erase processes could be performed with this system. Cyclic and linear sweep‐voltammetry studies in CH₂Cl₂ (+Bu₄NPF₆) revealed the occurrence of characteristic first‐reduction steps in the TEE‐DHA hybrid systems between −1.6 and −1.8 V vs. Fc/Fc⁺ (ferrocene/ferricinium couple) (Table 5). Oxidations occur at ca. +1.10 V. After photoisomerization to the VHF derivatives, reduction steps at more positive and oxidation steps at more negative potentials were recorded. No DHA→VHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).     

Thermally stable silarylene-1,3,4-oxadiazole polymers

The effects of incorporating a p-phenylene- (or m-phenylene)-1,3,4-oxadiazole fragment into the backbone of poly[1,4-phenylene(diphenylsilyl)-1,4-phenylene-2,5-(1,3,4-oxadiazole)], which was developed by the authors, was investigated. Bis[(p-carbohydrazidophenyl)]diphenylsilane was copolymerized with dipentachlorophenyl terephthalate or isophthalate to produce the prepolymers poly[N-(p-diphenylsilylbenzoyl)-N′N″-(terephthaloyl)-N″′-(p-benzoyl)dihydrazide] and poly[N-(p-diphenylsilylbenzoyl)-N′,-N″-(isophthaloyl)-N″′-p-(benzoyl) dihydrazide], respectively. The polyhydrazides were converted by thermal dehydration into poly[1,4-phenylene(diphenylsilyl)-1,4-phenylene-(1,3,4-oxadiazole-2,5-diyl)-1,4-phenylene-2,5-(1,3,4-oxadiazole)] and poly[1,4-phenyl-ene(diphenylsilyl)-1,4-phenylene-(1,3,4-oxadiazole-2,5-diyl)-1,3,4-(oxadiazole)]. The new polymers were soluble in organic solvents. Films cast from these solutions exhibited good adhesion to glass and metal surfaces. Thermal analysis showed that the heat stability of all these polymers was about the same and that they were resistant to decomposition when heated in air to about 400°C. The results also indicated that these polymers were somewhat less heat-resistant than samples of poly-[1,4-phenylene(diphenylsilyl)-1,4-phenylene-2,5-]1,3,4-(oxadiazole) synthesized from bis(p-carbohydrazidophenyl)diphenylsilane and bis-(p-carbopentachlorophenoxy-phenyl)diphenylsilane.     

Poly[(3,3′-dimethyl-1,1′-biphenyl-4,4′-diyl)ethynediyl(2,5-dioctyl-1,4-phenylene)ethynediyl] and a related copolymer – new blue EL polymers

Electroluminescence (EL) properties of a new poly(aryleneethynylene) and a related copolymer were studied. They are poly[(3,3′-dimethyl-1,1′-biphenyl-4,4′-diyl)ethynediyl(2,5-dioctyl-1,4-phenylene)ethynediyl] (PPEBE) and related copolymer (PPEBE-co-mP) containing 20 mole % of 1,3-phenylene units. Both polymers are blue-light emitters; the former was found to perform better than the latter when the light-emitting diode (LED) device had the configuration of ITO/PEDOT/polymer/Li:Al. The device constructed with the former polymer exhibited the external quantum efficiency of 0.05 and the maximum brightness higher than 400 cd/m² with its EL spectrum showing maxima at λ = 445 and 472 nm. The performance of the device constructed with the copolymer was about one fifth of the device fabricated with the homopolymer.     

Highly conductive meta derivatives of poly(phenylene sulfide)

Changes in the backbone structure of several meta derivatives of polyphenylene sulfide upon doping with AsF₅ have been investigated by IR spectroscopy. Poly(m-phenylene sulfide) does not form a conducting complex with AsF₅ unless reacted under conditions where carbon-carbon bonds form intramolecularly between phenyl rings. This assertion has been verified by comparison of the IR spectra of poly(m-phenylene sulfide) and a newly synthesized derivative, poly(thio-3,7-dibenzothiophendiyl), after doping with AsF₅. This new derivative forms a complex with AsF₅ which exhibits a conductivity of 18.5 S/cm. A sulfone-containing derivative has also been synthesized, poly[thio-3,7-(dibenzothiophene-5,5-dioxide)-diyl]. With this polymer, a much lower conductivity, 10^?3 S/cm, was obtained after exposure to AsF₅.     

Solar Thermal Energy Storage in a Photochromic Macrocycle

The conversion and efficient storage of solar energy is recognized to hold significant potential with regard to future energy solutions. Molecular solar thermal batteries based on photochromic systems exemplify one possible technology able to harness and apply this potential. Herein is described the synthesis of a macrocycle based on a dimer of the dihydroazulene/vinylheptafulvene (DHA/VHF) photo/thermal couple. By taking advantage of conformational strain, this DHA–DHA macrocycle presents an improved ability to absorb and store incident light energy in chemical bonds (VHF–VHF). A stepwise energy release over two sequential ring‐closing reactions (VHF→DHA) combines the advantages of an initially fast discharge, hypothetically addressing immediate energy consumption needs, followed by a slow process for consistent, long‐term use. This exemplifies another step forward in the molecular engineering and design of functional organic materials towards solar thermal energy storage and release.     

Aromatic and rigid rod polyelectrolytes based on sulfonated poly(benzobisthiazoles)

Aromatic polyelectrolytes based on sulfonated poly(benzobisthiazoles) (PBTs) have been synthesized by a polycondensation reaction of sulfo-containing aromatic dicarboxylic acids with 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) in freshly prepared polyphosphoric acid (PPA). Several sulfonated PBTs, poly[(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl)-2-sulfo-1,4-phenylene] sodium salt (p-sulfo PBT), poly[(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl)-5-sulfo-1,3-phenylene] sodium salt (m-sulfo PBT), their copolymers, and poly[(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl)-4,6-disulfo-1,3-phenylene] potassium salt (m-disulfo PBT), have been targeted and the polymers obtained characterized by ¹³C-NMR, FT-IR, elemental analysis, thermal analysis, and solution viscosity measurements. Structural analyses confirm the structures of p-sulfo PBT and m-disulfo PBT, but suggest that the sulfonate is cleaved from the chain during synthesis of m-sulfo PBT. m-Disulfo PBT dissolves in water as well as strong acids, while p-sulfo PBT dissolves well in strong acids, certain solvent mixtures containing strong acids, and hot DMSO. TGA indicates that these sulfonated PBTs are thermally stable to over 500°C. Free-standing films of p-sulfo PBT, cast from dilute neutral DMSO solutions, are transparent, tough, and orange in color. Films cast from basic DMSO are also free standing, while being opaque and yellow-green. p-Sulfo PBT was incorporated as the dopant ion in polypyrrole, producing conductive films with conductivities as high as 3 S/cm and electrical anisotropies as high as 10. © 1996 John Wiley & Sons, Inc.     

Palladium-catalyzed amination in the synthesis of macrocyclic compounds containing 1,3-disubstituted adamantane fragments

Palladium-catalyzed amination of 1,3-dibromobenzene, 2,6-dibromopyridine, 3,3′-dibromobiphenyl, 2,7-dibromonaphthalene, and 1,8-dichloroanthracene with an equimolar amount of 2,2′-(adamantane-1,3-diyl)diethanamine resulted in the formation of macrocyclic compounds containing one or several adamantane and one or several aromatic fragments. The reactions of 2,2′-(adamantane-1,3-diyl)diethanamine with excess 1,3-dibromobenzene, 2,6-dibromopyridine, 1,8-dichloroanthracene, and 1,8-dichloroanthraquinone gave the corresponding N,N′-diaryl derivatives. Polyaza macrocycles incorporating adamantane, aromatic, and 4,7,10-trioxatridecane-1,13-diamine fragments, were obtained by palladium-catalyzed amination of the N,N′-diaryl derivatives with 4,7,10-trioxatridecane-1,13-diamine.     

Dimer liquid crystals with bent mesogenic units

Three series of dimer liquid crystals containing an aromatic and a cholesteryl mesogenic unit were synthesized and investigated The compounds within the series differ in the length of the spacer connecting the mesogenic groups. Two of the three series contain an aromatic mesogenic group that is connected to the spacer at a meta-position instead of the para-position. Due to the presence of this bent mesogenic group, the odd-even effect in the transitional properties that is normally observed for dimer liquid crystals is reversed as compared with that of the third series, which contains a para-substituted aromatic mesogenic group. The reversed odd-even effect is found for the isotropization temperatures, the associated enthalpy changes and also for the optical properties.     

Synthesis and photoisomerization of dithienylethene-bridged diporphyrins

Dithienylethene-bridged diporphyrins 1-6 were prepared as photochemical switching molecules. Porphyrin and dithienylethene are directly linked in 1, and linked, respectively, through a 1,4-phenylene spacer in 2, through a 4-ethynylphenylene spacer in 3, and through a di-4-phenylethynylene spacer in 4, while meso-ethynylated porphyrin and dithienylethene are directly connected in 5 and linked through a 1,4-phenylene spacer in 6. Compounds 1, 2, and 5 do not undergo any photochemical isomerization, probably due to efficient quenching of the excited dithienylethene by the attached porphyrin moiety via intramolecular energy transfer. Compounds 4 and 6 undergo open-to-closed and closed-to-open photoisomerizations in quantum yields of 4.3 x 10(-)(2) and 1.8 x 10(-)(3), and 2.6 x 10(-)(3) and 7.5 x 10(-)(4), respectively, by irradiation with 313 and 625 nm light, which are considerably smaller than quantum yields of 0.52 and 3.8 x 10(-)(3) for reference dithienylethene molecule 7. The fluorescence of 4 was regulated in a reversible manner by the photoisomerization of the dithienylethene moiety. In addition, the absorption properties of the porphyrin in 6 changed in response to the photochromic reaction of the dithienylethene bridge.     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units: I. Synthesis and characterization

A series of novel wholly aromatic polyamide-hydrazides was synthesized by a low temperature solution polycondensation reaction of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl chloride (TCl), isophthaloyl chloride (ICl), or mixtures of various molar ratios of TCl and ICl in anhydrous N,N-dimethylacetamide (DMAc) as a solvent. Polymer structures were identified by elemental analysis and infrared spectroscopy. All the polymers have the same structural formula except the way of linking phenylene units inside the polymer chain. The content of para- and meta-phenylene moieties was varied within this series so that the changes in the latter were 10 mol % from polymer to polymer, starting from an overall content of 0-100 mol %. The prepared polymers were characterized for their properties in order to acquire clear understanding of the influence exerted by controlled structural variations in these polymers upon some of important properties, such as solubility, intrinsic viscosity, moisture regain, mechanical properties and thermal as well as thermo-oxidative stability. The polymers were readily soluble in several organic polar solvents such as DMAc, N,N-dimethylformamide, dimethyl sulphoxide, N-methyl-2-pyrrolidone and hexamethyl phosphoramide and could be cast into flexible films. Their solubilities were found to increase remarkably with introduction of meta-phenylene moieties into the polymer chains. Their intrinsic viscosities ranged from 0.73 to 4.83 dl g⁻¹ in DMAc at 30 °C and increased with the increase of para-phenylene units content. Mechanical properties of the films produced from these polymers are improved markedly by substitution of para-phenylene units for meta-phenylene units. Thermogravimetric studies revealed that the completely para-oriented type of polymer has better thermal and thermo-oxidative stability relative to that of the other polymers. Moreover, the results reveal that the prepared polymers have a great affinity to water sorption. The hydrophilic character increases as a function of meta-oriented phenylene rings incorporated into the polymer chains.     

Melt-processable oligoamides having internal acetylene groups in the backbone,and thermally stable polyamides therefrom

Aromatic oligoamides of DP = 5 and 11 that have all meta-phenylene linkages were prepared by controlling the ratio of isophthaloyl chloride and diamines [m-phenylenediamine and bis(3-aminophenyl)acetylene], and then end-capped with aniline or 2-aminobiphenylene. Aromatic oligoamides having para-phenylene linkages were also prepared similarly using terephthaloyl chloride as a monomer. The oligoamides that have all m-phenylene linkages were soluble in organic solvents such as N-methyl-2-pyrrolidone and N,N-dimethylacetamide, though the oligoamides having p-phenylene linkages were much less soluble. The oligoamides having acetylene units in the backbone showed exotherm due to crosslinking. The onset of the exotherm appeared at 310-340°C for the oligoamides having all m-phenylene linkages, and 330-370°C for the oligoamides having p-phenylene linkages. They were melt-processed at 350 or 380°C for 1 h, giving tough and insoluble films from oligoamides having all m-phenylene linkages and brittle films from oligoamides having p-phenylene linkages. The films showed excellent thermal properties. For example, viscoelastic analyses showed little decrease of mechanical property up to 370°C, and T_g was not observed below the temperature. Thermogravimetric analyses also showed that thermal stability of the melt-processed films were excellent. © 1992 John Wiley & Sons, Inc.     

Phenylene‐bridged Porphyrin meso‐Oxy Radical Dimers

Stable meta‐ and para‐phenylene bridged porphyrin meso‐oxy radical dimers and their Ni^II and Zn^II complexes were synthesized. All the dimers exhibited optical and electrochemical properties similar to the corresponding porphyrin meso‐oxy radical monomers, indicating small electronic interaction between the two spins. Intramolecular spin‐spin interaction through the π‐spacer was determined to be J/k_B=?15.9 K for m‐phenylene bridged Zn^II porphyrin dimer. The observed weak antiferromagnetic interaction has been attributed to less effective conjugation between the porphyrin radical and linking π‐spacer due to large dihedral angle. In the case of Zn^II complexes, both para‐ and meta‐phenylene bridged dimers formed 1D‐chain in solutions and in the solid states through Zn‐O coordination.     

Aromaticity‐Controlled Energy Storage Capacity of the Dihydroazulene‐Vinylheptafulvene Photochromic System

Photochemical conversion of molecules into high‐energy isomers that, after a stimulus, return to the original isomer presents a closed‐cycle of light‐harvesting, energy storage, and release. One challenge is to achieve a sufficiently high energy storage capacity. Here, we present efforts to tune the dihydroazulene/vinylheptafulvene (DHA/VHF) couple through loss/gain of aromaticity. Two derivatives were prepared, one with aromatic stabilization of DHA and the second of VHF. The consequences for the switching properties were elucidated. For the first type, sigmatropic rearrangements of DHA occurred upon irradiation. Formation of a VHF complex could be induced by a Lewis acid, but addition of H₂O resulted in immediate regeneration of DHA. For the second type, the VHF was too stable to convert into DHA. Calculations support the results and provide new targets. We predict that by removing one of the two CN groups at C‐1 of the aromatic DHA, the heat storage capacity will be further increased, as will the life‐time of the VHF. Calculations also reveal that a CN group at the fulvene ring retards the back‐reaction, and we show synthetically that it can be introduced regioselectively.     

Synthesis,Structures, and Complexation with Phenolic Guests of Acridone-Incorporated Arylene–Ethynylene Macrocyclic Compounds

Acridone units were incorporated into the arylene–ethynylene structure as polar arene units. Cyclic trimers consisting of three acridone-2,7-diyl units and three 1,3-phenylene units were synthesized by Sonogashira couplings via stepwise or direct route. X-ray analysis revealed that the trimer had a nearly planar macrocyclic framework with a cavity surrounded by three carbonyl groups. In contrast, the corresponding tetramer had a nonplanar macrocyclic framework. ¹H NMR measurements showed that the trimer formed a 1 : 1 complex as a macrocyclic host with dihydric phenol guests, and the association constants were determined to be ca. 1.0×10³ L mol⁻¹ for hydroquinone or resorcinol guests in CDCl₃ at 298 K. The calculated structures of these complexes by the DFT method supported the presence of two sets of OH⋅⋅⋅O=C hydrogen bonds between the host and guest molecules. The spectroscopic data of the cyclic trimers and tetramers are compared with those of reference acridone compounds.     

Two reaction routes for the preparation of aromatic polyoxadiazoles and polytriazoles: Syntheses and properties

Two reaction routes for the preparation of aromatic poly-1,3,4-oxadiazoles and poly-1,2,4-triazoles are studied and their influence on the physical properties, i.e., inherent viscosity, glass transition, degradation temperature, and film integrity of the final products are discussed. Aromatic poly-1,3,4-oxadiazoles are prepared by means of a polycondensation reaction of terephthaloyl chloride and isophthalic dihydrazide yielding a precursor polymer, poly(p, m-phenylene) hydrazide, which is converted into the corresponding poly-1,3,4-oxadiazole by means of a cyclodehydration reaction. Poly-1,3,4-oxadiazoles are also prepared by means of a polycondensation reaction between terephthalic and isophthalic acid and hydrazine yielding poly-1,3,4-oxadiazoles with higher inherent viscosities. Flexible poly-1,3,4-oxadiazole films are obtained only if the inherent viscosities of the polymers used are higher than 2.7 dL/g. The thermal stability is found to increase with increasing content of p-phenylene groups in the polymer backbone. Aromatic poly-1,2,4-triazoles are prepared using polyhydrazides with alternating para- and meta-phenylene groups and poly-1,3,4-oxadiazoles with a random incorporation of para- and meta-phenylene groups in the main chain as precursor polymers. The glass transition temperatures are found to increase with increasing content of p-phenylene groups in the main chain of these polymers. Cold crystallization is observed only for the alternating polymer. © 1994 John Wiley & Sons, Inc.