Π-conjugated Backbone Research Articles

Mechanically strong, fluorescent hydrogels from zwitterionic, fully π-conjugated polymers.

Mechanically strong supramolecular hydrogels (up to 98.9% water content) were obtained by the combination of a rigid, fully π-conjugated polymer backbone and zwitterionic side chains. The gels were characterized by SAXS, SEM and rheology measurements and are fluorescent, stimuli responsive (temperature, salts) and bind DNA.

Resent Progress in Side-Chain Engineering of Organic Photovoltaic Conjugated Polymer

Photovoltaic conjugated polymer materials are composed of π-conjugated backbone and flexible side chains. Their potential advantages of cost-effective production, fabrication on flexible and light weight substrates by roll-to-roll solution processing, capability to be fabricated into flexible devices and reduced environment impact have made them receive consid- erable attention in both academia and industry. In recent years, most of the interest has been directed into the optimization of main chain, while research on side-chain engineering of polymer is relatively few. Initially, side chains have been primarily utilized as solubilizing groups in organic photovoltaic conjugated polymers. However, roles which side chains play are far beyond. Accordingly, it has been found that side chains of organic conjugated polymer have a different impact on polymer absorption, emission, energy level, carrier mobility, nanoscale morphology and interfacial contact. Certainly enough, these findings have profound guiding significance on molecular structure design and optimization, blend microstructure, interfacial morphology, preparation methods and processing technology of device and so on. Moreover, based on ameliorative prepara- tion and processing technology of device, side-chain engineering is deserved to be deeply researched due to their remarkably improved ability to regulate and control blend microstructure, interfacial morphology so as to obtain ideal photovoltaic proper- ties and device performance, the ultimate goal. Thus, as for resent progress in application, in this perspective article, we will present a review on side chain engineering and assess different side chain on the basis of molecular structure design and opti- mization, as well as summarize relevant issues to be solved and related work in our laboratory. Keywords side chain engineering; photovoltaic properties; polymer solar cells; nanoscale morphology

Studies on polythiophenes containing fused polyaromatic rings as side chains

The influence of fused polyaromatic and polyphenylene side chains on polythiophene absorption and photoluminescence was studied in detail. The above polymers were used as emissive layer in Light emitting diodes (LEDs) and the influence of structure and packing on current density, color and intensity of emission was studied in detail. The current density of the LEDs with poly 3-[4-(10-Phenyl-anthracen-9-yl)-phenyl]-thiophene (poly DPAT) and light emission intensity are high and there is a band narrowing of peaks at high voltages along with very high intensity of electroluminescence. The above properties are due to the presence of diphenyl anthracenyl group which produces molecular overcrowding and high spatial distribution off the π-Conjugated backbone.

Synthesis and Structural Analysis of Thiophene-Pyrrole-Based S,N-Heteroacenes

Fused S,N-heterohexacene 4 was synthesized by applying Pd-catalyzed tandem Buchwald-Hartwig coupling and further functionalized to corresponding acceptor-capped derivatives 5 and 6 showing bond length equalization in the π-conjugated backbone and intense optical transitions. Organic thin film transistors (OTFTs) based on a vacuum-deposited film of 6 exhibit p-channel charge-carrier mobilities as high as 0.021 cm(2) V(-1) s(-1) and current on/off ratios of 10(5).

Guiding Suprastructure Chirality of an Oligothiophene by a Single Amino Acid

Within the present work, synthesis and properties of three stereoisomeric hybrid materials consisting of a π-conjugated oligothiophene backbone and a single amino acid (proline), which is attached by click chemistry, are described. The hybrids were in particular investigated regarding their self-assembling behavior in solution. From optical investigations, including circular dichroism spectroscopy, the formation of chiral suprastructures could be deduced and correlated with the stereochemistry of the proline residue. In addition, an elaborate theoretical model of the compounds’ self-assembly into suprastructures was developed on the basis of the experimental findings.

Roles of Flexible Chains in Organic Semiconducting Materials

In the past couple of years, remarkable progress has been made in solution-processable organic semiconducting materials for optoelectronics. The development of novel π-conjugated backbones has always been the central issue in this field. In contrast, flexible side chains are less developed and usually used only as solubilizing groups. In this Perspective, we highlight the effects of the flexible chains in organic semiconductors, including the influences of length, odd–even effect, substitution position, terminal groups, branching position, and chirality of alkyl chains, as well as some significant features of oligo(ethylene glycol) and fluoroalkyl chains. Although the roles of flexible chains in organic semiconducting materials are complex and differ when corresponding conjugated skeleton changes, in this Perspective, we emphasize the synergy of conjugated backbones and flexible side chains, which might significantly facilitate the understanding of the roles of flexible chains in structure–property relationship and promote the development of high-performance organic semiconductors.

Cell imaging and DNA delivery in fibroblastic cells by conjugated polyelectrolytes

This study concentrates on the potential application of conjugated polyelectrolytes (CPEs) to cell imaging and DNA delivery. Four different types of polyfluorene copolymers, namely, PAHFP-Br, PAEFP-Br, PAHFbT-Br, and PSBFP-Na, which have the same π-conjugated backbone but different side chains, were synthesized. For cytotoxicity testing, L-929 fibroblastic cells were treated with increasing concentrations (0-50 µM) of each CPE and then cell viability was determined by WST-8 assay. Cellular uptake of CPEs into cultured L-929 cells was observed by fluorescence microscopy. To examine DNA delivery by CPEs, the cells were incubated for 1 H with PAHFP-Br/fluorescein (Fl)-labeled single-stranded DNA (ssDNA-Fl) complex and then visualized by fluorescence microscopy. Cytotoxicity of CPEs was increased in a dose-dependent manner but at lower than 10 µM, PAHFP-Br, PAEFP-Br, and PSBFP-Na did not show any cytotoxic effects on the cells. When added to cell cultures at 1 µM, PAHFP-Br/ssDNA-Fl complex was delivered and then dissociated into PAHFP-Br and ssDNA-Fl within the cells. This result implies that PAHFP-Br can enable cell imaging and DNA delivery into fibroblastic cells. Therefore, it is suggested that PAHFP-Br with various advantages such as low cytotoxicity and high fluorescence efficiency can be extensively used as a potential agent for cell imaging and gene delivery.

Charge Delocalization through Benzene, Naphthalene, and Anthracene Bridges in π-Conjugated Oligomers: An Experimental and Quantum Chemical Study

To understand the influence of orthogonal conjugation pathways fused directly to π-conjugated polymer backbones, we synthesized and studied three series of thiophene-based model compounds containing benzene, naphthalene, and anthracene peri-substituted central cores as representative acenes. These models were functionalized with methyl groups at the reactive thiophene positions in order to generate and observe oxidized species without complications from follow-up polymerization. The neutral monomers and their oxidized charged counterparts were subjected to cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy as appropriate, and these results were further corroborated with thorough density functional theory studies. This joint experimental and theoretical analysis allowed us to determine that benzene-based conjugated linkers led to more delocalized charge carriers on account of the quinoidal character maintained within the benzene core. In contrast, anthracene-based linkers displayed very localized carriers due to torsional strain between the adjacent aryl groups and to the local evolution of formal aromatic sextets on the benzo-fused rings orthogonal to the backbone in the quinoidal state. In some cases, the electronics of the thiophene-based substituent dominated the electronic properties of the oxidized species regardless of the nature of the central acene linker. These results highlight the dramatic influence that orthogonal conjugation pathways can exert on the electronic properties of π-conjugated materials.

Solvatochromic fluorescence behavior of 2-amino-6-hydroxy-4-(3,4-dimethoxyphenyl)-pyrimidine-5-carbonitrile: A sensitive fluorescent probe for detection of pH and water composition in binary aqueous solutions

A new class of fluorescence sensor for detection of water in organic solvents based on photo-induced electron transfer (PET) of substituted pyrimidine appended with dimethoxyphenyl system has been designed and characterized. The probe 2-amino-6-hydroxy-4-(3,4-dimethoxyphenyl)-pyrimidine-5-carbonitrile AHDPC (I) was synthesized by one step three-component condensation reaction. The solvatochromic fluorescence behavior of AHDPC featuring electron withdrawing CN group, electron donating ―OCH3, NH2, OH groups and various π-conjugated backbones in varying polar and non-polar solvents has been investigated. The fluorescence of the compound exhibits red shift from its absorption spectra and correlated with solvent polarity. AHDPC exhibited significant spectral shifts in λem as a function of water composition in binary aqueous solvent systems. The changes are due to the specific interactions of AHDPC by intramolecular hydrogen bonding with water, photoinduced electron transfer as well as general solvent effect. The observed solvatochromic fluorescence characteristic of AHDPC could used as a new probe for micro-environmental polarity changes as well as a sensitive sensor for the determination of water composition in binary aqueous solutions. This study shows that due to the solvent sensitive emission, pyrimidine derivatives are good candidates to be used as fluorescent probes.

Synthesis of a Novel Low‐Bandgap Polymer Based on a Ladder‐Type Heptacyclic Arene Consisting of Outer Thieno[3,2‐b]thiophene Units for Efficient Photovoltaic Application

A novel conjugated polymer PIDTT-quinoxaline (Qx) based on the coplanar thieno[3,2-b]thiophene-phenylene-thieno[3,2-b]thiophene structure is synthesized and evaluated as an electron-donor material for bulk-heterojunction polymer solar cells (BHJ PSCs). The absorption spectra, electrochemical, charge transport, and film morphology properties as well as theoretical modeling of PIDTT-Qx are investigated to understand its intrinsic structure-property relationship. As expected, this polymer with an extended π-conjugated backbone exhibits a narrow-bandgap and board absorption spectrum for enhanced light harvesting. BHJ PSCs (ITO/PEDOT:PSS/polymer:PC71 BM/interlayer/Al) afford a maximum power conversion efficiency of 5.05% with an open-circuit voltage of 0.84 V, a short-circuit current density of 11.26 mA cm(-2) , and a fill factor of 53.4%. These results demonstrate the potential of PIDTT-Qx as an efficient electron-donor material for BHJ PSCs.

Structural study of helical polyfluorene under high quasihydrostatic pressure

We report on an x-ray diffraction (XRD) study of helical poly[9,9-bis(2-ethylhexyl)fluorene] (PF2/6) under high quasihydrostatic pressure and show an effect of pressure on the torsion angle (dihedral angle) between adjunct repeat units and on the hexagonal unit cell. A model for helical backbone conformation is constructed. The theoretical position for the most prominent 00l x-ray reflection is calculated as a function of torsion angle. The XRD of high molecular weight PF2/6 (M(n)=30 kg/mol) is measured through a diamond anvil cell upon pressure increase from 1 to 10 GPa. The theoretically considered 00l reflection is experimentally identified, and its shift with the increasing pressure is found to be consistent with the decreasing torsion angle between 2 and 6 GPa. This indicates partial backbone planarization towards a more open helical structure. The h00 peak is identified, and its shift together with the broadening of 00l implies impairment of the ambient hexagonal order, which begins at or below 2 GPa. Previously collected high-pressure photoluminescence data are reanalyzed and are found to be qualitatively consistent with the XRD data. This paper provides an example of how the helical π-conjugated backbone structure can be controlled by applying high quasihydrostatic pressure without modifications in its chemical structure. Moreover, it paves the way for wider use of high-pressure x-ray scattering in the research of π-conjugated polymers.

Density functional study of mono-branched and di-branched di-anchoring triphenylamine cyanoacrylic dyes for dye-sensitized solar cells

Electronic structures and absorption spectra of mono-branched and di-branched di-anchoring triphenylamine cyanoacrylic dyes were calculated by density functional theory. Our result suggested that one can develop organic dyes focusing on the preferred higher absorptivity or the wider absorption range, based on the symmetric or asymmetric di-branched architectures, respectively. The calculated absorption spectra for mono-branched dyes are in good agreement with the experiment. Different types of aromatic rings in the π-conjugated backbone of mono-branched dyes have varying effect on the absorption wavelength in the order of pyrrole < furan < thiophene. Among the mono-branched dyes studied, dye 6 with the extended π-conjugated structure of a thiophene–pentacyclodithiophene–thiophene showed the highest molar absorption coefficient, also at the longest absorption wavelength (∼570 nm). Our study gave an insight into how the absorption properties of di-branched dyes relate to their mono-branched analogs. The symmetric di-branched di-anchoring dye can take an advantage of a higher molar absorptivity than its mono-branched analog due to the near degeneracy of LUMO and LUMO+1 orbitals, which facilitates two transitions at the similar energy. For the asymmetric di-branched dianchoring dyes, the combination of two π-spacer units that absorb light at different wavelength regions leads to a wider absorption wavelength range, which can benefit DSSCs differently from the previous symmetric di-branched dyes. Asymmetric di-branched dyes need to be explored further for dye-sensitized solar cell applications.

Concept of Organic Photovoltaics: Operational Principles and Materials

Organic molecules possessing a π-conjugated (hetero)aromatic backbone are capable of transporting charge and interact efficiently with light. Therefore, these systems can act as semiconductors in opto-electronic devices similar to inorganic materials. However, organic chemistry offers tools for tailoring materials’ functional properties via modifications of the molecular units, opening new possibilities for inexpensive device manufacturing. In this presentation, we discuss exploitation of such organic molecular systems in photovoltaics; the operation, advantages, and limitations of molecular donor-acceptor heterojunction structures.

Circle chain embracing donor–acceptor organic dye: simultaneous improvement of photocurrent and photovoltage for dye-sensitized solar cells

We demonstrate for the first time that employing a circle chain embracing π-conjugated backbone is a promising strategy to construct superior organic sensitizers for dye-sensitized solar cells (DSCs), with simultaneous improvement of photocurrent and photovoltage. A DSC based on one circle chain embracing dye produced a high conversion efficiency of 8.34%.

Effect of Solvent-Induced Coil to Helix Conformational Change on the Two-Photon Absorption Spectrum of Poly(3,6-phenanthrene)

This paper investigates the effect of solvent-induced conformational changes of poly(3,6-phenanthrene) on their two-photon absorption (2PA). Such effect was studied employing the wavelength-tunable femtosecond Z-scan technique and modeled using the sum-over-essential states approach. We observed a strong reduction of the 2PA cross-section when the sample was prepared in hexane (poor solvent) in comparison to chloroform (good solvent), which is related to the conformation adopted by the polymer in each case. In chloroform it adopts a random coil conformation, as opposed to the one-handed helix conformation in hexane. Our results pointed out that the coil to helix conformation change decreases the degree of molecular planarity of the polymer π-conjugated backbone, which is primarily responsible for their optical nonlinearity, contributing to diminishing the effective transition dipole moments and, consequently, the 2PA cross-section. Moreover, by studying the nonlinear response with different light polarization, we showed that, although the solvent-induced conformational change does not alter the molecular symmetry of the polymer, it modifies considerably the direction of the transition dipole moments between the excited states.

Experimental and Theoretical Study on the One- and Two-Photon Absorption Properties of Novel Organic Molecules Based on Phenylacetylene and Azoaromatic Moieties

This Article reports a combined experimental and theoretical analysis on the one and two-photon absorption properties of a novel class of organic molecules with a π-conjugated backbone based on phenylacetylene (JCM874, FD43, and FD48) and azoaromatic (YB3p25) moieties. Linear optical properties show that the phenylacetylene-based compounds exhibit strong molar absorptivity in the UV and high fluorescence quantum yield with lifetimes of approximately 2.0 ns, while the azoaromatic-compound has a strong absorption in the visible region with very low fluorescence quantum yield. The two-photon absorption was investigated employing nonlinear optical techniques and quantum chemical calculations based on the response functions formalism within the density functional theory framework. The experimental data revealed well-defined 2PA spectra with reasonable cross-section values in the visible and IR. Along the nonlinear spectra we observed two 2PA allowed bands, as well as the resonance enhancement effect due to the presence of one intermediate one-photon allowed state. Quantum chemical calculations revealed that the 2PA allowed bands correspond to transitions to states that are also one-photon allowed, indicating the relaxation of the electric-dipole selection rules. Moreover, using the theoretical results, we were able to interpret the experimental trends of the 2PA spectra. Finally, using a few-energy-level diagram, within the sum-over-essential states approach, we observed strong qualitative and quantitative correlation between experimental and theoretical results.

Synthesis of Polydiacetylenes with Pyridyl Groups Directly Bound to the Main Chain

N-Substituted carbamic acid esters of 8-pyridyl-5,7-octadiyn-1-ol were synthesized and all the monomers showed solid-state polymerizabilities by UV irradiation to give polydiacetylenes with pyridyl groups directly bound to the π-conjugated backbone. Hydrogen-bond complexes of these monomers with dodecanoic acid (DA) or perfluorododecanoic acid (PFDA), in which the pyridyl nitrogen atom formed a hydrogen bond with the hydrogen atom of the carboxyl group, were also prepared. Although complex formability of DA with the monomers in this study was not so high, many PFDA complexes were obtained because of the higher acidity. All complexes could be polymerized in the solid state and the conversion increased by complexation compared with the original monomers. More favorable monomer stacking for solid-state polymerization could be achieved by combination of intermolecular hydrogen bonds between urethane groups and alkyl (or perfluoroalkyl) chain packing.

Charge Injection and Transport in Metal-Containing Conducting Polymers: Spectroelectrochemical Mapping of Redox Activities

Electropolymerization of tris(dioximate) cage complexes furnished metal-containing conducting polymers (MCPs) that deposit directly onto the electrode surface as uniform films. The injection of electrons into, or removal of electrons from, these electroactive materials proceeds via different pathways with different rates, the underlying molecular mechanisms of which were investigated by a combination of electrochemical, spectroscopic, and focused-ion-beam–scanning electron microscopy (FIB-SEM) cross-section analysis studies. For cobalt-containing polymers, both the metal centers and π-conjugated organic backbone work cooperatively as hopping stations for migrating holes, whereas the reduced polymer utilizes less-efficient self-exchange between cobalt(II) and cobalt(I) centers for electron transport. A small molecule model of such reductively doped polymer was prepared independently, which provided compelling electrochemical and spectroelectrochemical evidence to support the structural integrity of the metal centers upon redox switching. A well-defined metal-to-ligand charge transfer (MLCT) band of the n-doped polymer was exploited further as a straightforward spectroscopic tool to quantify the number of redox-active metal centers directly and to estimate the lower distance limit of diffusional charge transport across the bulk material.

Chiral Homobimetallic Palladium(II) Complexes Composed of Chirality-Organized Quinonediimines Bearing Amino Acid Moieties

The chirality-organized quinonediimine derivatives bearing amino acid moieties were demonstrated to react with 2 molar equiv. of Pd(OAc)2, resulting in the formation of the chiral homobimetallic palladium(II) complexes. The crystal structures of the chiral conjugated complexes revealed the coordination of the quinonediimine nitrogen to a palladium center and a chiral propeller twist conformation of the π-conjugated backbone. The mirror image relationship of the CD signals around the quinonediimine moieties in acetonitrile was observed between l- and d-derivatives, indicating the preservation of the chirality-organized structures even in a solution.

Pd complexes with trans-chelating ligands composed of two pyridyl groups and rigid π-conjugated backbone

1,2-Bis(2-pyridylethynyl)benzene derivatives, having two pyridyl groups and π-conjugated backbone, were prepared from the 1:2 Sonogashira reaction of 1,2-diiodobenzene with 2-alkynyl pyridines. The obtained ligands react with the palladium(II) complexes such as [PdCl2(cod)] and [PdCl2(MeCN)2] to form the complexes with the ligands coordinated in a trans-chelating bidentate mode. The ligand obtained from 1,3-diiodobenzene and 2-alkynyl pyridine bridges two Pd(II) centers, yielding a dipalladium complex. A dichloropalladium complex with the trans-chelating ligand, containing two methoxy groups in the central arylene group, promotes coupling of phenyl lithium and of phenyl acetylene to yield the respective homo-coupling products. The ligand displacement reactions of {bis(2-pyridylethynyl)benzene}palladium(II) complex with phosphine and dinitrogen ligands, such as PPh3, dppe, dppb, en, tmeda, bpy and phen, takes place smoothly to release the trans-chelating ligand.