Side-chain effect on the structural evolution and properties of poly(9,9-dihexylfluorene-alt-2,5-dialkoxybenzene) copolymers

The structural evolution and properties of poly(9,9-dihexylfluorene-alt-2,5-dialkoxybenzene) with different lengths of alkoxy side chains on phenylene have been systematically investigated by means of thermogravimetric analysis (TGA), X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarizing light microscopy (PLM), atomic force microscopy (AFM), and cyclic voltammetry (CV) techniques. The polymer self-organizes into a lamellar structure consisting of both two- and one-layer packing, and the two-layer packing style is the dominant structure. In addition, the two-layer and one-layer packing structures also accompany the presence of planar stacking and/or crystalline and noncrystalline structures, thus maintaining the stability of the packing. PF6OC6 shows three ordered phases (two crystalline phases and one nematic phase) during the heating process. With further increase of the length of alkoxy side chains, only two ordered phases (one crystalline phase and one nematic phase) are observed and the polymers show a melting-recrystallization phenomenon, which is steadily inhibited as the length of the alkoxy side chains increases. The optical and electrochemical properties of the polymers do not exhibit noticeable dependence on the length of the alkoxy side chains. However, the thermal stability, the vibronic structures, and the full width at half-maximum (fwhm) in photoluminescence spectra of the films gradually decrease, and the oxidation onset potentials and the corresponding HOMO energy levels slightly increase with increasing length of alkoxy side chains on phenylene. These results indicate that the length variation of alkoxy side chains does not change the electronic structure of the polymer backbones, but remarkably affects the microphase separation between the flexible side chains and the conjugated backbones.     

Stacked conjugated oligomers as molecular models to examine interchain interactions in conjugated materials

Previous studies of the redox states of linear conjugated oligomers as models for polarons and bipolarons in conjugated polymers do not fully address the influence of intermolecular interactions on the electronic structure of conjugated systems in the solid state. Fusion of oligothiophenes onto a bicyclo[4.4.1]undecane core holds the conjugated oligomers in a permanent cofacial stack. One- and two-electron oxidation of the stacked oligomers affords mono(radical cation)s and dications that serve as models for polarons and bipolarons in p-doped conjugated polymers and demonstrates the effect of pi-stacking on the electronic structure of these species.     

From star-shaped to dendritic poly(ethylene oxide)s: Toward increasingly branched architectures by anionic polymerization

Polymers with dendritic structure are a category of macromolecular architectures that has received considerable attention in the last decade. These polymers, also referred to as dendrimers, exhibit a degree of branching equal to unity. Interest in dendrimers whose branching points are linked to each other by generations of macromolecular size is in contrast quite new. This paper describes a new synthetic strategy which allows access to poly(ethylene oxide) (PEO) with dendritic structure. PEO dendrimers with different degree of compactness have been synthesized upon modifying the size of successive generations.     

Theoretical and experimental study of low band gap polymers for organic solar cells

A combined theoretical and experimental investigation of the electronic structure and optical properties of poly(3-hexylthiophene) (P3HT), poly[3-(4-octylphenyl)thiophene] (POPT) and poly[3-(4-octylphenoxy)thiophene] (POPOT) is reported. In comparison with P3HT, POPT and POPOT exhibit better stabilities and the presence of an oxygen atom and/or a phenyl ring in the side chains enhances conjugation. Quantum chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT to the new derivative POPOT. The knowledge of the structure of these polymers is of utmost importance in understanding their optical properties in different phases (solution and condensed phase). The calculations indicate that, in opposition to P3HT and POPT polymers where the introduction of alkyl chains and the pendant phenyl disturbs the planarity of the backbone of the conjugated segment, POPOT has a better degree of organization in both states: the conjugated chain remains planar even in the presence of the phenoxy groups. Finally, the exciton binding energy is evaluated for these polymers and allows us to conclude that the POPOT is a promising polymer for photovoltaic applications when compared to P3HT and POPT.     

Spectroelectrochemical characterisation of poly[Ni(saltMe)]-modified electrodes

Electrogenerated polymers based on the nickel(II) complex 2,3-dimethyl-N,N'-bis(salicylidene)butane-2,3-diaminatonickel(II), poly[Ni(saltMe)], were characterised by in situ FTIR and UV/Vis spectroscopy and ex-situ EPR spectroscopy in order to gain insights into film structure, electronic states and charge conduction. The role of the nickel ions during film oxidation was probed by using EPR to study naturally abundant Ni and 61Ni-enriched polymers. The data from all the spectroscopic techniques are consistent, and clearly indicate that polymerisation and redox switching are associated with oxidative ligand based processes; coulometry suggests that one positive charge was delocalised through each monomer unit. EPR provided evidence for the non-direct involvement of the metal in polymer oxidation: the polymer is best described as a polyphenylene-type compound (conducting polymer), rather than an aggregation of nickel complexes (redox polymer), and the main charge carriers are identified as polarons. An explanation for the high electrochemical stability and conductivity of poly[Ni(saltMe)] with respect to that of poly[Ni(salen)] is proposed. based on stereochemical repulsion between monomeric units; this can impose a less compact supramolecular structure on polymers with bulkier substituents.     

Low band-gap oligomers and polymers

The relationship between electronic structure and chemical stability is discussed for three different classes of low band-gap oligomers and polymers. In oligo- and poly[n]acenes as well as in poly-(arylenemethide)s, the low band-gap character is connected with a high chemical reactivity due to an energetically favored rearomatization of olefinic or quinonoid substructures, respectively. For double-stranded oligo- and poly(rylene)s, the stability is dramatically increased. Therefore, rylenes appear especially promising in the field of low band-gap materials.     

Preparation of Information‐Containing Macromolecules by Ligation of Dyad‐Encoded Oligomers

A simplified strategy for preparing non‐natural information‐containing polymers is reported. The concept relies on the successive ligation of oligomers that contain minimal sequence motifs. It was applied here to the synthesis of digitally‐encoded poly(triazole amide)s, in which propyl and 2‐methyl propyl motifs are used to code 0 and 1, respectively. A library of four oligo(triazole amide)s containing the information dyads 00, 01, 10, and 11 was prepared. These oligomers contain two reactive functions, that is, an alkyne and a carboxylic acid. Thus, they can be linked to another with the help of a reactive spacer containing azide and amine functions. Using two successive chemoselective steps, that is, azide‐alkyne Huisgen cycloaddition and carboxylic acid‐amine coupling, monodisperse polymers can be obtained. In particular, the library of dyads permits the implementation of any desired sequence using a small number of steps. As a proof‐of‐concept, the synthesis of molecular bytes 00000000 and 00000110 is described.     

Effect of ammonium groups on the properties and alkaline stability of poly(arylene ether)‐based anion exchange membranes

Five kinds of ammonium groups functionalized partially fluorinated poly(arylene ether) block copolymer membranes were prepared for investigating the structure–property relationship as anion exchange membranes (AEMs). Consequently, the pyridine (PYR)‐modified membrane showed the highest alkaline and hydrazine stability in terms of the conductivity, water uptake, and dry weight. The chloromethylated precursor block copolymers were reacted with amines, such as trimethylamine, N‐butyldimethylamine, 1‐methylimidazole, 1,2‐dimethylimidazole, and PYR to provide the target quaternized poly(arylene ether)s. The structures of the polymers, as well as model compounds and oligomers were well characterized by ¹H NMR spectra. The obtained AEMs were subjected to water uptake and hydroxide ion conductivity measurements and stabilities in aqueous alkaline and hydrazine media. The pyridinium‐functionalized quaternized polymers membrane showed the highest alkaline and hydrazine stability with minor losses in the conductivity, water uptake, and dry weight. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 383–389     

Theoretical investigation of the tunable behavior of p-n copolymers based on oligothiophenes and 1,4-bis(oxadiazolyl)-benzene

Semiempirical calculations were carried out on several model oligomers to investigate the tunable behavior of p-n copolymers with the repeating units constructed by oligothiophenes as the pi-excessive type blocks and 1,4-bis(oxadiazolyl) benzene as the pi-deficient type block. The calculated evolutions of the HOMO and LUMO of the model oligomers were in good agreement with the experimental oxidation and reduction potentials of the corresponding polymers. The effect of the length of the oligothiophene on the electronic structures and optical properties was elucidated by analyzing the HOMO and LUMO spatial distribution patterns of the model oligomers. When the number of thiophene rings increases, the HOMO and LUMO are contributed mostly from the oligothiophene segments and either the introduced single positive or negative charge focuses on the oligothiophene segments. The absorption spectra of polymers were also simulated by performing calculations on the corresponding oligomers. Good matches were observed between the calculated absorption spectra and the experimental UV-vis spectra of the corresponding polymers. The study shows that the backbone modification of the p-n copolymer, that is, changing the number of thiophene unit in the p-n diblock copolymer, greatly modifies the optical properties of the polymer.     

Recent advances in acetylene-based helical oligomers and polymers: Synthesis,structures, and properties

In this review, we describe the recent advances in the chemistry of helical polymers and oligomers containing acetylene units in the main chain. Owing to their great benefits such as high availability and handleability, good reactivity, rigidity, linearity, and low bulkiness, acetylene units have been utilized and incorporated in helical folding oligomers and polymers such as oligo- and poly(m-phenylene ethynylene)s. General synthetic methods as well as the structures, functions, and properties of acetylene-based helical oligomers and polymers are discussed by focusing on recent examples from 2009 to 2017.     

Thieno[3,4-b]-1,4-oxathiane: an unsymmetrical sulfur analogue of 3,4-ethylenedioxythiophene (EDOT) as a building block for linear pi-conjugated systems

[reaction: see text] An unsymmetrical analogue of 3,4-ethylenedioxythiophene (EDOT) has been synthesized by transetherification of 3,4-dimethoxythiophene. Electropolymerization leads to a stable electroactive polymer with electrochemical and electronic properties intermediate between those of the two symmetrical parent polymers poly(EDOT) and poly(3,4-ethylenedithiathiophene). Experimental work shows that the 2- and 5-positions possess a different reactivity, thus opening the possibility of synthesizing regioregular oligomers or polymers.     

Spectroscopy and photophysics of methylphenylsiloxaneand diphenylsiloxane-based molecules and polymers

Spectroscopy and photophysics of various types of methylphenylsiloxane- and diphenylsiloxane-based oligomers and polymers are reviewed. The molecules treated here include homopolymers such as poly(methylphenylsiloxane) and copolymers such as poly(dimethylsiloxane-codiphenylsiloxane) as well as related oligomers or molecules such as diphenyltetramethyldisiloxane. These polymers and oligomers normally exhibit monomer fluorescence in fluid solution at temperatures near room temperature, and the monomer fluorescence and phosphorescence in rigid matrices at 77 K. In addition to these emissions, the excimer fluorescence is often observed depending on the molecular structure of the siloxanes. These emission properties are rationalized based on the molecular structure and kinetics of the excimer formation processes as well as on the flexibility of the Si-O-Si bonds.     

A NEW APPROACH TO ELECTROACTIVE POLYMERS VIA WELL-DEFINED OLIGOMERRS WITH FURTHER POLYMERIZABLE END-GROUPS

Among the inherent drawbacks of conducting polymers are the limited processibility, uneven polydispersity inmolecular weigh and the existence of structure defects, which become the obstacles for many electronic, optical andbiological applications that demand the materials to have well-defined structures and high chemical purity. To solve theseproblems, our research in the last decade or so has focused on the synthesis of electroactive oligomers of well-definedstructures, controllable molecular weighs, narrow or uniform polydispersity. We have developed a general strategy for thesynthesis of such oligomers based on the theory of non-classical or reactivation chain polymerization. The aniline oligomerswith minimum 4 nitrogen atoms and 3 phenylene rings exhibit similar characteristic redox behavior and electroactivity aspolyaniline. Electronic conductivity of the oligomers of 7 or 8 aniline units approaches that of polyaniline. Solubility of theoligomers is much improved over that of conventional polyaniline. Various functional groups can be introduced into theoligomers either by proper selection of starting materials or by post-synthesis modifications via common organic reactions.The functionalized oligomers undergo further polymerizations to afford a variety of new electroactive materials, includingpolyamides, polyimides, polyureas, polyurethanes, polyacrylamides and epoxy polymers. Numerous potential applications,particularly as anticorrosion materials, are discussed for the oligomers and their polymeric derivatives.     

Synthesis and characterization of poly(ethylene oxide-co-ethylene sulfone)s and their precursors: Poly(ethylene oxide-co-ethylene sulfide)s

Three poly(ethylene oxide-co-ethylene sulfide)s with oxygen to sulfur ratios of 2/1, 2/2, and 1/2 were prepared by phase-transfer catalyzed polycondensations of (1) sodium sulfide and 1,2-bis (2-chloroethoxy)ethane, (2) 1,2-ethanedithiol and 1,2-bis(2-chloroethoxy)ethane, and (3) 1,2-ethanedithiol and 2-chloroethyl ether, respectively. A buffered solution with pH between the pK_a of the monothiol (RSH) and the pK_a2 of the dithiol (HS–R–SH), or H₂S, was needed to obtain high molecular weight polymers, which suggests that nucleophiles transfer and react as monoanions rather than dianions. These poly(ethylene oxide-co-ethylene sulfide)s were oxidized completely to poly(ethylene oxide-co-ethylene sulfone)s using 3-chloroperoxybenzoic acid as oxidant. Both the final polymers and the precursors have regular sequenced structures and are semicrystalline. As expected, their glass transition temperatures and melting points increase and solubilities decrease with the decrease of ether oxygen to sulfur ratio. © 1994 John Wiley & Sons, Inc.     

Tuning of electronic structures of poly(p-phenylenevinylene) analogues of phenyl, thienyl, furyl, and pyrrolyl by double-bond linkages of group 14 and 15 elements

We investigated electronic structures of four sets of monomers and polymers comprising of phenyl rings and five-membered hetero(aromatic) moieties connected with double-bond -X=X- linkages (X = CH, SiH, GeH, N, P, As) by density functional theory, time-dependent density functional theory, and periodic boundary condition calculations with B3LYP functional. Electronic structures of poly(p-phenylenevinylene) (PPV) analogues are primarily dominated by central double-bond moieties. The introduction of ethylene homologues with group 14 and 15 elements was demonstrated to be a promising approach to modify electronic structures of conjugated oligomers and polymers. Excitation energies of monomers with double-bond linkages were reduced by around 13-50% with respect to corresponding dimers of phenyl, thienyl, furyl, and pyrrolyl rings. Similarly, band gaps of poly(p-phenylene) and polythiophene were decreased by 0.3-0.9 eV upon the insertion of double-bond linkages. Furthermore, excitation energies of monomers presented decreasing trends when descending through groups 14 and 15. For group 14 ethylene homologues, the decreasing trend in the lowest excitation energies was rationalized by a progressively favoring of pi-sigma* interactions as descending X = CH, SiH, and GeH. Increasing p contents of central bonds along X = N, P, and As accounted for geometry features and the lowest excitation energies of group 15 species. A decrease in the extent of electronic communications between aromatic rings and -X=X- linkages within higher congeners was also revealed.     

Optimizing the self-assembly of conjugated polymers and small molecules through structurally programmed non-covalent control

Organic conjugated polymers and oligomers are key electronic materials for applications such as transistors, photovoltaics, and light emitting devices due to their potential for solution processability, mechanical flexibility, and precise structure-based tuning compared to inorganic materials. In dilute environments, the optoelectronic properties of conjugated polymers are largely governed by their constitutional structure and, to a lesser degree, their solution-state intramolecular configuration. In the solid state, intramolecular conformation and intermolecular electronic coupling impact these properties substantially, especially in relation to device performance. Therefore, an increasingly important area of research concerning conjugated materials is developing design strategies aimed at optimizing the solid-state packing for electronic applications. Programming solid-state packing arrangements through discrete non-covalent interactions is an emerging strategy within the context of conjugated polymers. This review focuses on the use of the two most prevalent discrete and directional interactions used to dictate the self-assembly of conjugated polymers and oligomers—hydrogen bonds and chalcogen bonds. We also discuss how these design motifs can imbue conjugated materials with appealing physical properties while simultaneously retaining or improving electronic capabilities.     

Singlet-triplet splittings and their relevance to the spin-dependent exciton formation in light-emitting polymers: an EOM/CCSD study

By employing the coupled-cluster equation of motion method (EOM/CCSD) for excited-state structures, we have investigated the structure dependence of the singlet and triplet exciton splittings, through extensive calculations for polythiophene (PT), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(thienylenevinylene) (PTV), polyparaphenylene vinylene (PPV), MEHPPV, polyparaphenylene ethylene (PPE), polyfluorene (PFO), and ladder-type polyparaphenylene (mLPPP). The results for the polymer are extrapolated through computations for the oligomers with increasing length. Recent investigations have been quite controversial about whether the internal quantum efficiency of electroluminescence could be higher than the 25% spin statistics limit or not in polymeric materials. Using a simple relationship between the exciton formation rate and the excitation energy level, we have discussed the material-dependent ratios of singlet and triplet exciton formation, which are in good agreement with the magnetic-field resonance detected transient spectroscopy measurement by Wohlgenannt et al. for a series of electronic polymers. This provides another piece of evidence to support the view that the internal quantum efficiency for conjugated polymers can exceed the 25% limit.     

Molecularly defined caprolactone oligomers and polymers: synthesis and characterization

The synthesis of molecularly defined epsilon-caprolactone oligomers and polymers up to the 64-mer, via an exponential growth strategy, is described. By careful selection of orthogonal protecting groups, t-butyldimethylsilyl (TBDMS) ether for the hydroxyl group and benzyl (Bn) ester for the carboxylic acid group, a highly efficient synthetic strategy was developed with yields for both deprotection steps being essentially quantitative and for the coupling reactions using 1,3-dicyclohexylcarbodiimide (DCC), yields of 80-95% were obtained even at high molecular weights. This allows monodisperse dimers, tetramers, octamers, 16-mers, 32-mers and 64-mers to be prepared in gram quantities and fully characterized using mass spectroscopy, size exclusion chromatography (SEC), and IR and NMR spectroscopy. Thermal and physical properties were measured using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). These results conclusively show a distinct structure/property relationship with a close correlation between the number of repeat units and physical properties. In addition, a number of marked differences were observed on comparison with the parent poly(caprolactone) polymer.     

Excitonic effects in a time-dependent density functional theory

Excited state properties of one-dimensional molecular materials are dominated by many-body interactions resulting in strongly bound confined excitons. These effects cannot be neglected or treated as a small perturbation and should be appropriately accounted for by electronic structure methodologies. We use adiabatic time-dependent density functional theory to investigate the electronic structure of one-dimensional organic semiconductors, conjugated polymers. Various commonly used functionals are applied to calculate the lowest singlet and triplet state energies and oscillator strengths of the poly(phenylenevinylene) and ladder-type (poly)(para-phenylene) oligomers. Local density approximations and gradient-corrected functionals cannot describe bound excitonic states due to lack of an effective attractive Coulomb interaction between photoexcited electrons and holes. In contrast, hybrid density functionals, which include long-range nonlocal and nonadiabatic corrections in a form of a fraction of Hartree-Fock exchange, are able to reproduce the excitonic effects. The resulting finite exciton sizes are strongly dependent on the amount of the orbital exchange included in the functional.     

Bandgap Engineering through Controlled Oxidation of Polythiophenes

The use of Rozen’s reagent (HOF?CH₃CN) to convert polythiophenes to polymers containing thiophene‐1,1‐dioxide (TDO) is described. The oxidation of polythiophenes can be controlled with this potent, yet orthogonal reagent under mild conditions. The oxidation of poly(3‐alkylthiophenes) proceeds at room temperature in a matter of minutes, introducing up to 60 % TDO moieties in the polymer backbone. The resulting polymers have a markedly low‐lying lowest unoccupied molecular orbital (LUMO), consequently exhibiting a small bandgap. This approach demonstrates that modulating the backbone electronic structure of well‐defined polymers, rather than varying the monomers, is an efficient means of tuning the electronic properties of conjugated polymers.