Doping‐Induced Absorption Bands in P3HT: Polarons and Bipolarons

In this work, we focus on the formation of different kinds of charge carriers such as polarons and bipolarons upon p‐type doping (oxidation) of the organic semiconductor poly(3‐ hexylthiophene‐2,5‐diyl) (P3HT). We elucidate the cyclic voltammogram during oxidation of this polymer and present spectroscopic changes upon doping in the UV/Vis/near‐IR range as well as in the mid‐IR range. In the low‐oxidation regime, two absorption bands related to sub‐gap transitions appear, one in the UV/Vis range and another one in the mid‐IR range. The UV/Vis absorption gradually decreases upon further doping while the mid‐IR absorption shifts to lower energy. Additionally, electron paramagnetic resonance (EPR) measurements are performed, showing an increase of the EPR signal up to a certain doping level, which significantly decreases upon further doping. Furthermore, the absorption spectra in the UV/Vis range are analyzed in relation to the morphology (crystalline vs. amorphous) by using theoretical models. Finally, the calculated charge carriers from cyclic voltammogram are linked together with optical transitions as well as with the EPR signals upon p‐type doping. We stress that our results indicate the formation of polarons at low doping levels and the existence of bipolarons at high doping levels. The presented spectroscopic data are an experimental evidence of the formation of bipolarons in P3HT.     

Magnetic properties of conducting polymer nanostructures

Magnetic susceptibility measurements on conducting polyaniline and polypyrrole nanostructures with different dopant type and doping level as functions of temperature and magnetic field are reported. The susceptibility data cannot be simply described as Curie-like susceptibility at lower temperatures and temperature-independent Pauli-like susceptibility at higher temperatures; some unusual transitions are observed in the temperature dependence of susceptibility, for example, paramagnetic susceptibility decreases gradually with lowering temperature, which suggests the coexistence of polarons and spinless bipolarons and possible formation of bipolarons with changing temperature or doping level. In particular, it is found that the direct current magnetic susceptibilities are strongly dependent on applied magnetic field, dopant type, and doping level.     

Raman spectroelectrochemical study of sodium intercalation into poly(p-phenylene)

The structural defects formed along the polymer chain during the electrochemical intercalation of sodium into polyparaphenylene (PPP) were investigated by in situ FT Raman measurements with an excitation wavelength of 1064 nm. Our Raman data are compared with those obtained for PPP and its oligomers intercalated with Na(+) ions by chemical ways. The coexistence of polarons and bipolarons is observed in the early stages of the intercalation process. When the doping level increases polarons are transformed to bipolarons, then in the highly intercalated sample (corresponding to a composition close to Na(0.5)(C(6)H(4))), bipolarons are the main structural defects. This evolution, which is reversible, is in accordance with the electron spin resonance (ESR) data of PPP intercalated with sodium ions in the same experimental conditions.     

Raman spectra of carriers in ionic-liquid-gated transistors fabricated with poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)

We observed the Raman spectra of carriers, positive polarons and bipolarons, generated in a poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14) film by FeCl₃ vapor doping. Electrical conductivity and Raman measurements indicate that the dominant carriers in the conducting state were bipolarons. We identified positive polarons and bipolarons generated in an ionic-liquid-gated transistor (ILGT) fabricated with PBTTT-C14 as an active semiconductor and an ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [BMIM][TFSI] as a gate dielectric using Raman spectroscopy. The relationship between the source−drain current (I_D) at a constant source−drain voltage (V_D) and the gate voltage (V_G) was measured. I_D increased above −V_G = 1.1 V and showed a maximum at −V_G = 2.0 V. Positive polarons were formed at the initial stage of electrochemical doping (−V_G = 0.8 V). As I_D increased, positive bipolarons were formed. Above V_G = −2.0 V, bipolarons were dominant. The charge density (n), the doping level (x), and the mobility of the bipolarons were calculated from the electrochemical measurements. The highest mobility (μ) of bipolarons was 0.72 cm² V⁻¹ s⁻¹ at x = 110 mol%/repeating unit (−V_G = 2.0 V), whereas the highest μ of polarons was 4.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ at x = 10 mol%.     

可溶性聚[3-(2,2,3,3-四氟丙氧基)噻吩]的光谱性质

研究了中性和经FeCI3掺杂后聚[3-(2, 2, 3, 3-四氟丙氧基)噻吩](P3FT)溶液的光谱性质。中性P3FT具有特殊的溶剂色效应, 其甲醇 溶液的溶剂化吸收带强度随P3FT浓度增大而增强, 该吸收带是高分子链与链之间的相互作用(即聚合物的聚集态结构)的反映。用电子光谱跟踪FeCI3掺杂P3FT的过程, 结果表明, P3DT随着掺杂程度的增大, 载流子由极化子转变成双极化子, 且双极化子的能级位于禁带中间, 与孤子相似。     

A facile preparation method for synthesis of silica sulfuric acid/poly(o‐methoxyaniline) core–shell nanocomposite

A new nanocomposite of poly(o‐methoxyaniline) (POMA) is introduced by overlayer formation of POMA on silica. The key appealing feature of the synthesis is the role of silica sulfuric acid (SSA) both as solid acid dopant and template in overlayer self‐assembly of POMA on silica surface. Hereon siloxide group (Si―O^?) of silica surface is replaced with dopant anion of SSA (≡Si―O―SO₃^?), which leads to formation of a overlayer of POMA on the silica surface. The composite particles are spherical in the nanoscale range of 50 nm without application of any external template (no‐template synthesis). Nanocomposite was fully characterized by various instrumentation methods: Fourier transform infrared (FT‐IR), ultraviolet–visible (UV–vis), thermogravimetric analysis (TGA), diffrential thermal analysis (DTA), elemental analysis (CHNS), energy dispersive X‐ray (EDX), X‐ray photoelectron spectroscopy (XPS) and X‐ray difraction (XRD). Based on XPS and CHNS results, it is demonstrated that the doping level of POMA is as high as 50% and for the first time the ratio of 4:2:2 is obtained for ―NH― (amine): ―HN^.+― (polarons): ?HN⁺― (bipolarons), respectively. In fact, bipolarons may also coexist with polarons with a 1:1 ratio of them. Moreover, the synthesis benefits from the perspective of green chemistry which is preparation under solid‐state (solvent‐free) condition. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and optoelectrochemical properties of novel phenothiazinophanes

Phenothiazinophanes, a novel class of phenothiazine based stilbenophanes are synthesized and characterized. All the phenothiazinophanes exhibited excellent photoluminescence properties. The cyclic voltammetry studies of the phenothiazinophanes demonstrated multiple absorption peaks due to the formation of radical cations (polarons) and dications (bipolarons).     

Bipolarons or polaron pairs in conducting polymers: Equilibrium and kinetics

Equilibrium and kinetics are compared for conjugated polymers, supposing the charged states are either polarons (Ps) and bipolarons (BPs) or polarons and polaron pairs (PPs). For low concentrations equilibrium and kinetics are virtually indistinguishable. Both are essentially different for high concentrations due to the fact that the extension of the bipolaron is almost the same as that of one polaron whereas the polaron pair has the extension of two polarons. As a result, the polaron concentration, i.e. the spin concentration, shows for a system with bipolarons a maximum and decreases for continuing increase of the potential. This dependence corresponds to results of ESR spectroelectrochemistry. In contrast, for the system with PPs both P and PP concentrations saturate at high potentials. Thus, the PP model is only reasonable if an additional subsequent second oxidation step is possible. The kinetics of the formation and dissociation of PPs or BPs, is essentially the same for concentrations that can be achieved in accumulation layers in organic MOS devices. The estimated relaxation times point to the formation and dissociation of either BPs or PPs as processes that can either cause or influence hysteresis effects in polymeric MOS capacitors, which can be described by an interface charge, depending on the sweep direction in the capacitance-voltage measurements. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1293–1301. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Nature and energies of electrons and holes in a conjugated polymer, polyfluorene

Electrons and holes were injected selectively into poly-2,7-(9,9-dihexylfluorene) (pF) dissolved in a tetrahydrofuran (THF) and a 1,2-dichloroethane (DCE) solution, respectively, using pulse radiolysis. Transient absorption spectra of monoions of both signs revealed two bands attributable to formation of polarons, one in the visible region (pF+* at 580 nm, pF-* at 600 nm) and another in the near-IR region. Additional confirmation for the identification of pF+* and pF-* comes from bimolecular charge-transfer reactions, such as bithiophene-* + pF --> pF-* or pF+* + TTA --> +TTA+* (TTA = tri-p-tolylamine), in which known radical ions transfer charge to pF or from pF. Difference absorption spectra of pF chemically reduced by sodium in THF provided a ratio of absorbance of anions formed to bleaching of the neutral band at 380 nm. In conjunction with pulse-radiolysis results, the data show that each polaron occupies 4.5 +/- 0.5 fluorene units, most probably contiguous units. Extensive reduction of pF by sodium also revealed resistance to formation of bipolarons: excess electrons reside as separate polarons when two or more electrons were injected. Redox equilibria with pyrene and terthiophene by pulse radiolysis established reversible one-electron redox potentials of E0(pF+/0) = +0.66 V and E0(pF0/-) = -2.65 V vs Fc+/0. Together with the excited-state energy, these results predict a singlet exciton binding energy of 0.2 eV for pF in the presence of 0.1 M tetrabutylammonium tetrafluoroborate. This binding energy would increase substantially without an electrolyte.     

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.     

Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers

Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaron. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory.     

Polarons, bipolarons, and side-by-side polarons in reduction of oligofluorenes

The nature of charge carriers in conjugated polymers was elucidated through optical spectroscopy following single- and multielectron reduction of 2,7-(9,9-dihexylfluorene) oligomers, F(n), n = 1-10, yielding spectra with the two bands typical of polarons upon single reduction. For short oligomers addition of a second electron gave a single band demonstrating the classic polaron-bipolaron transition. However, for long oligomers double reductions yielded spectra with two bands, better described as two polarons, possibly residing side-by-side in the F(n) chains. The singly reduced anions do not appear to delocalize over the entire length of the longer conjugated systems; instead they are polarons occupying approximately four fluorene repeat units. The polarons of F(3) and F(4) display sharp absorption bands, but for longer oligomers the bands broaden, possibly due to fluctuations of the lengths of these unconfined polarons. DFT calculations with long-range-corrected functionals were fully consistent with the experiments describing polarons in anions, bipolarons in dianions of short oligomers, and side-by-side polarons in dianions of long oligomers, while results from standard functionals were not compatible with the experimental results. The computations found F(10)(2-), for example, to be an open-shell singlet ( ≈ 1), with electrons in two side-by-side orbitals, while dianions of shorter oligomers experienced a gradual transition to bipolarons with states of intermediate character at intermediate lengths. The energies and extinction coefficients of each anionic species were measured by ultraviolet-visible-near-infrared absorption spectroscopy with chemical reduction and pulse radiolysis. Reduction potentials determined from equilibria mirrored oxidation potentials reported by Chi and Wegner. Anions of oligomers four or more units in length contained vestigial neutral (VN) absorption bands that arise from neutral parts of the chain. Energies of the VN bands correspond to those of oligomers shorter by four units.     

ESR and ESE studies on polymer semiconductors of weakly doped poly(tetrathiafulvalene)

In the studied poly(tetrathiafulvalenes), PTTF, the TTF units are linked via phenyl groups or via tetrahydroanthracene bridges. The PTTF are doped with oxygen and with iodine respectively. The electrical conductivity strongly depends on temperature as it is typical of semiconducting materials. The existence of various types of paramagnetic centres is confirmed by ESR saturation experiments, by spectra simulation and by electron spin echo (ESE) measurements in the temperature region of 2 K to 400 K. At room temperature the doped PTTF have a dc conductivity between 10⁻⁹ Scm⁻¹ and 10⁻⁴ Scm⁻¹. Relations between the concentration of various radicals and the electrical conductivity of the polymers are discussed by means of a model based on mobile and immobile radicals, polarons and bipolarons. There are experimental indications for temperature-dependent formation and break-up of bipolarons which are discussed in the paper.     

导电聚吡咯膜电极的现场电化学-ESR研究

本文报导聚吡咯(PP)膜电极在水溶液中的电化学─ESR行为, 定性地探讨了电位,电量对PP膜中极化学形成与转化过程的影响, 并在PP膜/溶液界面存在电化学反应的情形下, 考察了PP膜中可能的电荷传输机理。     

Intramolecular transport of charge carriers in trimeric aniline upon a three-step acid doping process

The "acid doping" of a methyl-capped aniline trimer, N-[4-(dimethylamino)phenyl]-N-(4-{[4-(dimethylamino)phenyl]imino}-2,5- cyclohexadien-1-ylidene)-amine (TANI), was performed stoichiometrically to study the nature of charge carriers induced by the acid protonation process. The redox centers in TANI were found to undergo a reversible three-step protonation with 1 equiv, 2 equiv and a large molar excess of dodecylbenzenesulfonic acid (DBSA) in chloroform, as evidenced by three different chromophores (doping levels I, II and III) observed using UV-vis-NIR. Acidity of the dopants and solvent polarity were controlling factors. As revealed by electron paramagnetic resonance spectroscopy (EPR), the doping levels I, II, and III achieved by doping 0.1 mM TANI/chloroform solutions with different amounts of DBSA exhibited relative spin densities of 1:1.2:2.2. Since the expected maximum spin population of TANI through acid doping is two spins per molecule, the reduced paramagnetism given by the doubly protonated TANI (doping level II) indicated partially coupled unpaired spins. The third protonation step (doping level III) produced almost double the unpaired spin concentration compared to the lower doping levels and a multiline EPR spectrum likely comprising two overlapping signals of similar overall line width. The hyperfine couplings contributing to the splittings in this signal were estimated by simulation incorporating 6-H and 1-N nuclei most likely from one highly localized unpaired spin on a terminal dimethylamino group, with an underlying apparent singlet arising from a delocalized unpaired spin; the diradical proposed as the species exhibiting the multiplet EPR signal is isolated by the bridging ammonium cation created by the third doping step. The phenomena suggested the stepwise evolution of partly formed diamagnetic bipolarons from polaron interactions at doping level II and the transformation to the more isolated unsymmetrical system we label "two polarons on a chain" in a triplet state at doping level III. The results provide the characterization of novel doping behaviors for a trimeric aniline molecule in organic solution.     

Poly(phenylenevinylene)s - synthesis and redoxchemistry of electroactive polymers

The development of PPV-systems has moved from intractable powders to solution processible film-forming materials by introducing solubilizing phenyl groups. This opens new uses ranging from thin-film insulators, over photoconductive electrophotographic recording materials, to novel electroactive battery electrodes. In addition, the phenylated PPV may be utilized as a promising thin-film precursor for microstructured carbon patterns with a stable electrical conductivity (σ = 40 S/cm). PPV is not a metal-like conductor, but rather a high-ohmic photoconductor with a band gap of 2.4 eV. It can be made highly conducting upon appropriate chemical or electrochemical treatment (σ= 10−⁴ − 10³ S/cm). The formation of coexisting polymeric ion radicals (polarons) and diions (bipolarons) is well understood in terms of the concept of chain segment redox reactions (ECS concept). A wide variety of PPV derivatives has been synthesized and characterized by their electrochemical oxidation and reduction potentials. Substituents like phenyl, methoxy and cyano groups have a significant influence on the redox potentials of PPV as the standard polymer (E^Ox = 0.8 V, E^Red = −1.7 V). Thus, for poly(1,4-phenylene-1,2-diphenylvinylene) a broadening of the band gap up to 2.9 eV along with an increase of the oxidation potential up to 1.2 V is estimated. The knowledge of the redox potentials which reflect fundamental properties of conjugated polymers is essential for theoretical and practical reasons.     

Oligothiophene bipyridine alternate copolymers and their ruthenium metalated analogues: in situ ESR and UV-vis investigations of metal-chain interactions

In situ electron spin resonance (ESR) and UV-vis spectro-electrochemical studies have been performed on two copolymers consisting of alternating subunits of regioregular head to tail (HT) coupled 3-octylthiophene tetramer and 2,2'-bipyridine subunits (P4) or 3-octylthiophene hexamer subunits of the same regioregularity and 2,2'-bipyridine subunits (P6). Both P4 and P6 have been investigated in their metal-free form as well as in the ruthenium(II) metalated form (P4-Ru and P6-Ru). P4 and P6 in the p-doped state exhibit a clear ESR signal characteristic of the presence of polarons in the oligothienylene subunits. In the case of P4, no recombination of polarons into bipolarons is observed, whereas the recombination process takes place in P6. The formation of bipolarons is well-rationalized in terms of the conjugation length, and it seems clear that the higher length of the oligothiophene subunit in P6( )()stabilizes bipolarons(.)() The same effect, is induced by the coordination of -Ru(bpy)(2)(2+) to the bipyridine unit in the metalated form of both polymers, which results in an increase of the conjugation length. Important information is gained from the analysis of the ESR spectra of both nonmetalated and metalated in the oxidized (p-doped) and reduced (n-doped) forms. In the p-doped state both nonmetalated and metalated polymers reveal the presence of a narrow ESR line characteristic of the mobile spin carriers in the polymer matrix. The oxidation of the metal center occurs at higher potentials and leads to an irreversible destruction of the system. To the contrary, in the reduced (n-doped) state the ESR lines of the nonmetalated and metalated polymers markedly differ. A significant line broadening with simultaneous change of the g-value is caused by spin-orbit coupling phenomenon induced by the presence of the coordinating metal. Finally, the observation of a clear polaronic band in the UV-vis spectrum of p-doped P4 and its strong dependence on the applied potential can be clearly correlated with the potential induced changes in the ESR spin density. The same applies to P4-Ru, where the changes in the polaronic and bipolaronic bands can also be correlated with the ESR spin density changes.     

Vibronic polarons: Self‐trapping,local rotation,and band features

We revisit basic theoretical concepts of local and itinerant vibronic polarons in crystals. The following results may be regarded as novel: (1) The electron self‐trapping rate to a small polaron is calculated via the reaction rate method; subsequently, self‐trapped on‐center small polarons relax to an off‐center vibronic polaron state. (2) The general vibronic Hamiltonian is redefined so as to incorporate both local and itinerant behavior and pairing into bipolarons or Cooper pairs. (3) The planar rotation and diametral tunneling of an off‐center polaron around and across its centrosymmetrical site are dealt with to adiabatic approximation. (4) Variational calculations are made for vibronic polarons itinerant along 1‐D chains by means of a two‐band extension of Merrifield's ansatz. This investigation of vibronic polarons is undertaken in view of their presumed role in high‐temperature superconductivity and colossal magnetoresistance. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

聚苯胺热掺杂十二烷基苯磺酸的反应过程及其结构

与传统的质子酸掺杂相比，采用具有功能性的两亲大分子对聚苯胺（PAN）掺杂，可以使产物的加工性和稳定性大大提高［1～3]．十二烷基苯磺酸（DBSA）是研究得比较多的掺杂酸之一，其掺杂过程一般都采用辅助溶剂，由此而引起PAN与DBSA结合程度低、掺杂效率低、溶剂需后处理?..     

Antiferromagnetism of poly(3-dodecylthiophene) irradiated by γ-beams

The magnetic properties of poly(3-dodecyl-thiophene) (PDDT) irradiated by -beams have been studied by measuring the magnetic susceptibility and EPR spectra. The irradiated PDDT is characterized, similarly as the non-irradiated one, by an antiferromagnetic course of magnetic susceptibility with a Néel temperature of 237 K. It is suggested that the antiferromagnetic properties of PDDT are caused by transformation of polarons to bipolarons, which takes place at lower temperatures.