Π-conjugated Chains Research Articles

Self-assembly and phase transformations of π-conjugated block copolymers that bend and twist: from rigid-rod nanowires to highly curvaceous gyroids

In this study, a series of π-conjugated block copolymers of regioregular poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT–P2VP) was synthesized and their self-assembly behavior and the detailed thermodynamic phase diagram were explored. By a combination of TEM, SAXS, WAXS, and UV-VIS measurements, it was found that the π-conjugated P3HT in their various self-assembled nanodomains could be a rigid rod, or a semi-rigid chain, or even a fully flexible chain. With the P2VP volume fraction, ϕ, = 0.20, the P3HT–P2VP displays a nanowire structure with a fully extended all-transP3HT rod structure across the width of the nanowires, indicating a prevailing rod–rod interaction between P3HT blocks over the microphase separation interaction between the constituent blocks. With ϕ = ∼0.30 to 0.6, the P3HT–P2VPs show a highly ordered lamellar structure with the P3HT block exhibiting as a semirod-like chain composed of shorter rods connected by twisted 3HT units. With ϕ > 0.68, the π-conjugated block copolymers display self-assembling nanostructures of hexagonal close packed cylinders and spheres, indicating that P3HT adopts a fully coil-like structure that favors interfacial curvatures. In particular, for the P3HT–P2VP with ϕ = 0.68, a gyroid phase, the first of its kind for π-conjugated block copolymers, was observed upon heating. For the nanowire structured P3HT–P2VP, a liquid crystalline phase transition from the smectic-like crystalline state to a nematic structure was observed at ∼200 °C. The observed microstructures and transformations reveal the importance of the semirigid nature of π-conjugated P3HT chains and provide new guidelines for the organization of π-conjugated block copolymers for future optoelectronic applications.

Bond Length Alternation of Conjugated Oligomers: Wave Function and DFT Benchmarks.

We have computed the bond length alternation (BLA) in a series of π-conjugated quasilinear chains containing from two to six unit cells. Several structures (eight oligomeric sets including three conformers of polyacetylene, polymethineimine, polysilaacetylene, etc.) have been considered to cover the possible evolutions of the BLA with increasing chain length. Three objectives have been tackled: (1) the computation of accurate reference values using the CCSD(T) theory; (2) an evaluation of the performances of other electron correlated wave function approaches (MPn, SCS-MP2, CCSD, etc.); (3) the benchmarking of several DFT functionals, including global, range-separated, and double hybrids. It turns out that the SCS-MP2 approach is, on average, an efficient scheme in terms of its accuracy/cost ratio. Among the selected DFT approaches, no single functional emerges as uniformly accurate for all oligomeric series and chain lengths, but BHHLYP, M06-2X, and CAM-B3LYP could be reasonable choices for long oligomers.

π-Conjugated Poly(aryleneethynylene)s Consisting of Salophen and Ni−Salophen Units in the π-Conjugated Main Chain: Preparation and Chemical Properties

π-Conjugated poly(aryleneethynylene)s consisting of N,N′-bis(salicylidene)-1,2-phenylenediamine (salophen) units (Poly-1−Poly-3) and salophen−nickel complex units (Poly-1−Ni) were prepared in high yields by the palladium-catalyzed polycondensation of H−≡−Ar−≡−H (Ar = fluorene or 2,5-dialkoxy-p-phenylene) with dibromo compounds of salophen (Br2−Salophen) and its nickel complex (Br2−Salophen−Ni), respectively. The Ni-free polymers (Poly-1−Poly-3) showed good solubility in CHCl3 and THF, had high thermal stability, and exhibited number-average molecular weights (Mn) of 9200−14000 in GPC analysis. The UV−vis spectra of the polymers showed π−π* transition peaks at about 420 nm, which was comparable to those of poly(p-phenyleneethynylene)s. The UV−vis spectrum of Poly-1−Ni exhibited additional intermediate and small peaks at about 480 and 600 nm, which were assigned to a charge-transfer (CT) electronic transition and a d−d transition, respectively. The complexation of Poly-1 with Ni2+ proceeded clearly and quan...

Synthesis of New Thiophene-Based π-Conjugated Organic Compounds and Polymer

Abstract p-Dibromobenzenes and p-phenylenediboronate with π-conjugated bithiophene and terthiophene arms were synthesized. They showed UV–vis absorption peaks in the range of 376–417 nm. p-Phenylenediboronate with bithiophene arms were used for synthesis of a new π-conjugated polymer consisting of isothianaphthene and the p-phenylene units with π-conjugated bithiophene side chains.

Insulated Molecular Wire with Highly Conductive π-Conjugated Polymer Core

We have recently developed a new method for synthesizing polyrotaxanes with a high covering ratio, rigidity, photoluminescence efficiency, and solubility in a variety of organic solvents through the polymerization of structurally defined rotaxane monomers. The rigid rodlike structure of the pi-conjugated core polymers in these polyrotaxanes is thought to facilitate the effective transport of charge carriers. Here we applied this method to the synthesis of a polyrotaxane having a poly(phenylene ethynylene) backbone by the Sonogashira copolymerization of a structurally defined rotaxane with a linker molecule. According to time-resolved microwave conductivity and transient absorption spectroscopy measurements, the hole mobility along the pi-conjugated polymer chain of the polyrotaxane thus formed was extremely high and comparable to that in amorphous silicon.

Synthesis of Organoboron Quinoline-8-thiolate and Quinoline-8-selenolate Complexes and Their Incorporation into the π-Conjugated Polymer Main-Chain

Low-molecular-mass organoboron quinoline-8-thiolate and -selenolate complexes as model compounds, and organoboron polymers incorporated their complex structures into the poly(p-phenylene-ethynylene) main chain were prepared. Tetracoordination states of boron atoms in the obtained compounds were confirmed by 11B NMR spectroscopy, and the detailed structures of the model compounds were determined by single-crystal X-ray diffraction analysis. The polymers were synthesized by Sonogashira-Hagihara coupling reaction of organoboron quinolate-based monomers having diiodo groups with 1,4-diethynylbenzene derivatives bearing electron-donating or -withdrawing groups in moderate yields. Their optical properties were studied by UV−vis absorption and photoluminescence spectroscopies. Increasing atomic number of the 16 Group atom adjacent to the boron atom caused emission shift to longer wavelength and decreasing of absolute quantum yields for both the model compounds and the polymers. There are no differences between the polymers with the donating π-conjugated linker and with the accepting one in the photoluminescence property, resulting from efficient energy transfer from π-conjugated main chain to Q ligand. Furthermore, the obtained polymers showed high refractive indices (nd > 1.66).

THEORETICAL STUDY ON OPEN-SHELL NONLINEAR OPTICAL MOLECULAR SYSTEMS AND A DEVELOPMENT OF A NOVEL COMPUTATIONAL SCHEME OF EXCITON DYNAMICS

This contribution firstly elucidates a structure–property relationship in third-order nonlinear optical molecular systems with singlet diradical characters. It turns out that the second hyperpolarizabilities (γ) of the singlet open-shell molecules with intermediate diradical characters are significantly enhanced as compared with those of closed-shell and pure diradical molecules. The hybrid density functional theory method, i.e. UBHandHLYP, is applied to the calculations of γ of dimer models composed of singlet diradical diphenalenyl molecules, which show a remarkable enhancement of γ per monomer as decreasing the intermolecular distance. The second contribution is concerned with a development of ab initio molecular orbital configuration-interaction-based quantum master equation (QME) approach. This is found to provide both coherent processes, e.g. dynamic polarization and exciton (electron–hole pair) recurrence motion, and incoherent processes, e.g. exciton migration, in molecular systems. Using this approach, the electron/hole dynamics for dynamic polarizabilities α(ω) are examined for several π-conjugated linear chain systems, and the structural dependences of α(ω) are elucidated.

A Scheme for the Evaluation of Electron Delocalization and Conjugation Efficiency in Linearly π-Conjugated Systems.

In this study, we present a scheme for the evaluation of electron delocalization and conjugation efficiency in lineraly π-conjugated systems. The scheme, based on the natural bond orbital theory, allows monitoring the evolution of electron delocalization along an extended conjugation path as well as its response to chemical modification. The scheme presented is evaluated and illustrated by means of a computational investigation of π-conjugation in all-trans polyacetylene [PA; H(-CH═CH)n-H], polydiacetylene [PDA, H(-C≡C-CH═CH)n-H], and polytriacetylene [PTA, H(-C≡C-CH═CH-C≡C)n-H] with up to 180 carbon atoms, all related by the number of ethynyl units incorporated in the chain. We are able to show that for short oligomers the incorporation of ethynyl spacers into the PA chain increases the π-delocalization energy, but, on the other hand, reduces the efficiency with which π-electron delocalization is promoted along the backbone. This explains the generally shorter effective conjugation lengths observed for the properties of the polyeneynes (PDA and PTA) relative to the polyenes (PA). It will also be shown that the reduced conjugation efficiency, within the NBO-based model presented in this work, can be related to the orbital interaction pattern along the π-conjugated chain. We will show that the orbital interaction energy pattern is characteristic for the type and the length of the backbone and may therefore serve as a descriptor for linearly π-conjugated chains.

Nonlinear optics and surface relief gratings in alkynyl–ruthenium complexes

Generally, the organometallic complexes rich in carbon and containing π-conjugated chains are materials interesting for the study of electronic transfer processes. Currently, the ruthenium–acetylide organometallic complexes form part of the most studied organometallic compounds in nonlinear optics. In this work, we highlight the nonlinear optical properties and the photo-induced structure of new organometallic complexes containing a ruthenium–acetylide donor fragment able to compete with the strongest organic donors. We determine, using various experimental techniques (degenerate four-wave mixing, second and third harmonic generation), the influence of the functionalization of these molecular structures on the improvement of their second-and third-order nonlinear optical properties exploiting in particular the nature of the acceptor fragment and the π-conjugated transmitter. Also we present the dynamics of formation of photo-induced surface relief gratings using a transmission holographic technique and the atomic force microscopy on ruthenium–acetylide complexes containing an azobenzene fragment.

Fabrication of Stretch-Oriented Regioregular Poly(3-Hexylthiophene) film and Its Application to Organic Field-Effect Transistors

We report a fabrication process for organic field-effect transistors (OFETs) based on stretch-oriented regioregular poly(3-hexylthiophene) (rr-P3HT) film and the anisotropic carrier transport properties in the OFETs. The optical absorption spectra and XRD results indicate that π-conjugated rr-P3HT chains are aligned in the stretching direction with the π-stacking polymer planes parallel to the substrate. The field-effect hole mobility in a current flow parallel to the stretching direction is larger by a factor of 3.4 compared to that perpendicular to the stretching direction. These results indicate that charge transport via the π-conjugated rr-P3HT chains is advantageous in the stretch-oriented rr-P3HT film.

A Template‐Free Method Towards Conducting Polymer Nanostructures

AbstractConducting polymer nanostructures have recently received special attention in nanoscience and nanotechnology because of their highly π‐conjugated polymeric chains and metal‐like conductivity, such that they can be regarded not only as excellent molecular wires, but also as basic units for the formation of nanodevices. Although various approaches, such as hard‐template methods, soft‐template methods, electrospinning technology, and so on are widely employed to synthesize or fabricate conducting polymer nanostructures and their composite nanostructures, each of the currently used methods possess disadvantages. Therefore, finding a facile, efficient, and controlled method of forming conducting polymer nanostructures is desirable. Similar to other nanomaterials, the effect of size (in these cases 1–100 nm) on the properties of the conducting polymer nanostructures must be considered. Electrical measurements of single nanotubes or nanowires are desirable in order to be able to understand the pure electrical properties of conducting polymer nanostructures. Compared with bulk conducting polymers, conducting polymer nanostructures are expected to display improved performance in technological applications because of the unique properties arising from their nanometer‐scaled size: high conductivity, large surface area, and light weight. Thus, it is also desirable to develop promising applications for conducting polymer nanostructures. In accordance with the issues described above, our research focuses on a new synthesis method to form conducting polymer nanostructures and on the related formation mechanism of the resultant nanostructures. The electrical and transport properties of single nanotubes of conducting polymer, measured by a four‐probe method, and promising applications of such template‐free‐synthesized conducting polymer nanostructures as new microwave absorbing materials and sensors guided by a reversible wettability are also of interest. This article reports some of our main results and reviews some important contributions of others.magnified image

Electric and magnetic characteristics of metal-free polymers containing azaporphine macrocycles

The electric and magnetic characteristics of metal-free polymers with azaporphine macrocycles in varying regular series were compared with the structural parameters of macromolecules, such as the macromolecule size, the length of conjugation regions, and the concentration of free peripheral CN groups. It was found that the array of the test polymers in general exhibits an average statistical tendency characteristic of organic semiconductors: as the molecular mass of macromolecules and, correspondingly, the conjugation chain length increase, the conductivity increases and the ESR signal narrows. For the revealed case of an abnormal drop in conductivity with an increase in the molecular mass, unequivocal evidence was obtained as to which of two possible factors responsible for the abnormality (breaking the continuous π-conjugation chain in the macromolecule or the emergence of intermolecular barriers) takes place in each particular series of such polymers.

Synthesis of new polyconjugated molecules with biphenyl, dibenzothiophene, carbazole and phenanthrene units

The simple methods of synthesis of hardly accessible substituted biphenyl, dibenzothiophene, carbazole and phenanthrene derivatives were elaborated starting from dimethyl 4,4’biphenyldicarboxylate. The series of new luminophores with extended π-conjugated chains based on combinations of biphenyl, carbazole, dibenzothiophene, phenanthrene fragments and alternating phenyl, vinyl or heterocyclic units were synthesized by the Wittig and the Knoevenagel reactions of corresponding aromatic dialdehydes and different CH-acids or phosphonium salts. Investigation of the effect of various substituents on the luminescent properties has been presented. The new luminophores could be used as emissive or charge transport layers in organic light emitting diodes (OLEDs).

Coordinating Tectons: Bipyridyl Terminated Allenylidene Complexes

A series of complexes with π-conjugated carbon chains terminated by bipyridyl moieties has been prepared. These allenylidene complexes were derived from 9-hydroxy-9-ethynyl-4,5-diazafluorene, the preparation of which is reported; the new allenylidene complexes are highly colored with the cumulated carbon chain terminating in a bipyridyl unit providing a site for further coordination. The synthesis, characterization, and X-ray structure determination of trans-[MCl(P∩P)2═C═C═(4,5-diazafluoren-9-yl)]PF6 (M = Ru, P∩P = bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,2-bis(dimethylphosphino)ethane (dmpe); M = Os, P∩P = dppm) are described. The effect of the variation in metal and ligand on electronic and electrochemical characteristics of these complexes has been investigated by using UV–vis, solution electrochemistry, and a combination of these techniques in spectroelectrochemical experiments. DFT calculations have been performed on trans-[RuCl(P∩P)2═C═C═(4,5-diazafluoren-9-yl)]q (P∩P = dppm, bis(dimethylphosphino)methane (dmpm); q = −1, 0, +1, +2) and subsequently solvent-corrected calculations with use of COSMO were also undertaken to examine the nature of electronic transitions in various oxidation states.

Cryptand-like Porphyrinoid Assembled with Three Dipyrrylpyridine Chains: Synthesis, Structure, and Homotropic Positive Allosteric Binding of Carboxylic Acids

Cryptand-like porphyrinoid assembled with three 2,6-dipyrrylpyridine chains binds alcohol at each molecular crevice in the C3h symmetric conformation through three hydrogen bondings. This receptor binds dichloroacetic acid at each crevice in the D3 symmetric conformation, where strong positive allosteric effect was observed on the basis of the Hill plot with a Hill coefficient (n = 2.7 ± 0.2), and the association constant (log K = 13.6 ± 1.8). Ligand binding at the first crevice enhances polarization of the π-conjugated chain to make pyrrole more acidic at the second crevice and pyridine more basic at the third crevice.

Mono- and Dipalladium Movement on the π-Conjugated Five-Carbon Chain

We prepared new mono- and dipalladium complexes containing a highly conjugated pentadiynyl ligand where the palladium movement to the C5 position with the bonding rearrangement provided the first cumulenyl-type complexes.

Fluorescence Emissions and Torsional Conformations in π-Conjugated Chains of PolyDCHD-HS

The fluorescence emission of polyDCHD-HS in solution at room and low temperature and as an amorphous film has been investigated. Extended segments along the polymer backbone are responsible for the observed fluorescence spectrum of polyDCHD-HS in benzene solution. Spectra of the polymer in cyclohexane solution and as an amorphous film give evidence of an additional red-shifted fluorescence, assigned to local substructures behaving as emission centers. Model calculations with the TD-DFT theoretical framework on planar and distorted enyne oligomers have allowed correlation of the S0 → S1 transition energies to chain segments with different torsional angles around the C−C⋮C−C axis. In the ensuing structural model, benzene molecules interact attractively with carbazolyl side groups of polyDCHD-HS, thus enhancing the chain rigidity. The interaction energy of carbazolyl groups with cyclohexane is smaller than that with benzene. This leads to more folded chains and, plausibly, also to clusters of few chain segments.

Ground state of π-conjugated polymer chains forming an intermolecular charge-transfer complex as probed by Raman spectroscopy

The intermolecular charge-transfer complex (CTC) between the conjugated polymer MEH-PPV and the low-molecular organic acceptor trinitrofluorenone (TNF) has been studied by Raman and optical absorption spectroscopy. On mixing MEH-PPV with TNF, an absorption band due to the CTC appeared in the optical gap of the polymer, whereas, in the Raman spectra, characteristic bands of the polymer are shifted and their widths and intensities change. The low-frequency shift of the strongest band at 1580 cm−1 in the Raman spectrum of the polymer, assigned to the symmetric stretching vibration of the phenyl group, is shown to be due to electron density transfer from the π-conjugated system of the polymer to the acceptor and is as large as 5 cm−1, which corresponds to a charge transfer on the order of 0.1e−1. Even at a low acceptor concentration (one TNF molecule per 10 monomer units of the polymer), most Raman-active conjugated chains are involved in the CTC. It is suggested that conjugated segments of the polymer can form a CTC of variable composition MEH-PPV: TNF = 1: X, where 0.1 ≤ X ≤ 0.5 (for each monomer unit of the polymer), and one TNF molecule can thereby interact with two conjugated segments of MEH-PPV. The conjugated polymer chains involved in the CTC can become more planar, and their interaction with the local environment can noticeably change; however, their conjugation length, most likely, remains unaltered.

Photoinduced Birefringence and Optical Rotation on Achiral Azobenzene Copolymer

Photoinduced birefringence and photoinduced optical rotation has been investigated on poly{4-[2-(methacryloyloxy)ethyloxy-4′-cyanoazobenzene]-co-4-[[2-(methacryloyloxy) ethyloxy]-2-methyl-4-(phenylazo)]azobenzene} (PCDY50). Originating in a good optical response of cyanoazobenzene side-chain and large birefringence of long π-conjugated side chains, a fairly large photoinduced birefringence (0.28) and photoinduced optical rotation (41°/μm) could be observed. Some applications utilizing these superior photoresponsive properties of PCDY50 have been suggested and demonstrated.

Synthesis and through-bond spin interaction of stable 1,3-phenylene-linked polyradical carrying aminoxyls in the π-conjugated main chain

A stable polyradical carrying aminoxyls in the π-conjugated main chain was synthesized and its through-bond spin interaction was investigated. The polyradical is constructed with 1,3-phenylene as a spin coupler and 9,9-di-n-propyl-9,10-dihydroacridin-10-yloxy as a stable spin source. The synthetic method of the spin source moiety was modified by using conventional organic reactions from a previously reported method. The polymeric structure was synthesized by Suzuki–Miyaura coupling. The degree of polymerization and the spin concentration of the polyradical were ca. 11 and 0.71 spins per repeating unit, respectively, indicating that there were ca. 8 spins per molecule. The average S value of the polyradical was 3/2, therefore, 3 spins out of 8 spins were aligned in the polyradical. This showed that the through-bond spin interaction occurs more effectively than in our previous polyradical carrying aminoxyls in the side chain.