Pulse-radiolysis Time-resolved Microwave Conductivity Technique Research Articles

Charge Transport along Coiled Conjugated Polymer Chains

The motion of charges on coiled polymer chains was studied using a combination of experimental and theoretical methods. The conductive properties of dilute solutions of polyfluorene and fluorene−binaphthyl copolymers were studied by the pulse-radiolysis time-resolved microwave conductivity technique. This technique enables measurement of the (high frequency, 34 GHz) mobility of charges on isolated polymer chains. The motion of positive charges on the coiled polymer chains was studied theoretically by charge transport simulations with parameters from density functional theory calculations. This combined experimental and theoretical study shows that the mobility of charges decreases with increasing degree of chain coiling. The mobility along (infinitely long) stretched polyfluorene is calculated to be as high as tens of centimeters2/volt·second. Our results imply that the performance of conjugated polymers in optoelectronic devices can be significantly improved by optimization of the organization on a molecular scale.

A Fluorine‐Substituted Hexakisdecyloxy‐hexa‐peri‐hexabenzocoronene.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A Fluorine-Substituted Hexakisdecyloxy- hexa-peri-hexabenzocoronene

[structure: see text] The first fluorine-substituted hexabenzocoronene has been synthesized and is much more readily reduced than its nonfluorinated analogues. Thin films of this material show columnar order at room temperature, and the charge-carrier mobility measured by the pulse-radiolysis time-resolved microwave conductivity technique is comparable to that of other hexabenzocoronene materials.

High Charge‐Carrier Mobility in π‐Deficient Discotic Mesogens: Design and Structure–Property Relationship

Hexaazatrinaphthylene (HATNA) derivatives with six alkylsulfanyl chains of different length (hexyl, octyl, decyl and dodecyl) have been designed to obtain new potential electron-carrier materials. The electron-deficient nature of these compounds has been demonstrated by cyclic voltammetry. Their thermotropic behaviour has been studied by means of differential scanning calorimetry and polarised optical microscopy. The supramolecular organisation of these discotic molecules has been explored by temperature-dependent X-ray diffraction on powders and oriented samples. In addition to various liquid crystalline columnar phases (Col(hd), Col(rd)), an anisotropic plastic crystal phase is demonstrated to exist. The charge-carrier mobilities have been measured with the pulse-radiolysis time-resolved microwave-conductivity technique. They are found to be higher in the crystalline than in the liquid crystalline phases, with maximum values of approximately 0.9 and 0.3 cm(2) V(-1) s(-1), respectively, for the decylsulfanyl derivative. Mobilities strongly depend on the nature of the side chains.

The Mobility and Decay Kinetics of Charge Carriers in Discotic Hexabenzocoronenes

AbstractThe temperature dependence of the mobility and decay kinetics of charge carriers in discotic hexa‐peri‐hexabonzocoronene derivatives has been measured using the pulse‐radiolysis time‐resolved microwave conductivity technique (PR‐TRMC). For both the racemic and chiral dimethyloctyl, and the isomeric n‐decyl substituted derivatives, the mobility decreases at the crystalline solid (K)–liquid crystalline (Col) transition from 0.5±0.1 to 0.30±0.05 cm2 V–1 s–1 with the transition temperature ca. 30 °C lower for the branched chain compounds. The charge recombination kinetics are similar for the branched chain isomers in the Col phase but a faster decay is found for the racemic compound in the K phase. The fact that the mobility values for an asymmetrically substituted butylanthraquinone (C4 AQ) derivative are a factor of 2–3 lower than for the fully hexakis‐alkyl substituted compounds is attributed to the similar intracore mobilities for holes and electrons in the latter materials and the electron localization on the AQ moiety in the former. Charge recombination is found to be orders of magnitude faster in the Col phase of the C4 AQ derivative than in the K phase, this is attributed to the motional freedom of the AQ– group in the liquid‐crystalline phase.

Charge Mobilities in Organic Semiconducting Materials Determined by Pulse-Radiolysis Time-Resolved Microwave Conductivity: π-Bond-Conjugated Polymers versus π−π-Stacked Discotics

A review is given of the one-dimensional, intrachain and intracolumnar, charge mobilities, Σμ1D, determined for π-bond-conjugated polymeric and for π−π-stacked columnar discotic materials using the pulse-radiolysis time-resolved microwave conductivity technique. The largest values, on the order of 10 cm2/(V s), are found for single-crystal polydiacetylenes polymerized either thermally or with low doses of radiation. Much lower values of Σμ1D, covering the range from 0.009 to 0.125 cm2/(V s), are found for solution-synthesized conjugated polymers for which six different backbone structures have been investigated. This is attributed mainly to their complex morphology and the resulting static disorder in the backbone structure. The highest mobilities for this class of material, ca. 0.1 cm2/(V s), are found for liquid crystalline derivatives of polyfluorene and poly(phenylenevinylene). Larger mobilities are found for discotic materials, with maximum values close to 1 cm2/(V s) in both the crystalline solid and liquid crystalline phases. This is attributed to their self-organizing nature and hence higher degree of structural order, which compensates for the weaker electronic coupling between monomeric units in the discotics compared with the covalently bonded conjugated polymers.

The optical and charge transport properties of discotic materials with large aromatic hydrocarbon cores.

The optical absorption and charge transport properties of a series of discotic molecules consisting of peripherally alkyl-substituted polycyclic aromatic cores have been investigated for core sizes, n, of 24, 42, 60, 78, 96, and 132 carbon atoms. In dilute solution, the wavelength maximum of the first absorption band increases linearly with n according to lambda(max) = 280 + 2n and the spectral features become increasingly broadened. The two smallest core compounds display a slight red-shift and increased spectral broadening in spin-coated films. For derivatives with n = 24, 42, 60, and 96, the one-dimensional, intracolumnar charge mobility, Sigma mu(1D), was determined using the pulse-radiolysis time-resolved microwave conductivity technique. For the compounds which were crystalline solids at room temperature, Sigma mu(1D) lay within the range 0.4-1.0 cm(2)/Vs. In the discotic mesophases at ca. 100 degrees C, Sigma mu(1D) was somewhat lower and varied from 0.08 to 0.38 cm(2)/Vs. The mobility values in both phases are considerably larger than the maximum values found previously for discotic triphenylene derivatives. However, the recently proposed trend toward increasing mobility with increasing core size is not substantiated by the results on the present series of increasingly large aromatic core compounds.

Charge transport properties in discotic liquid crystals: a quantum-chemical insight into structure-property relationships.

We describe at the quantum-chemical level the main parameters that control charge transport at the molecular scale in discotic liquid crystals. The focus is on stacks made of triphenylene, hexaazatriphenylene, hexaazatrinaphthylene, and hexabenzocoronene molecules and derivatives thereof. It is found that a subtle interplay between the chemical structure of the molecules and their relative positions within the stacks determines the charge transport properties; the molecular features required to promote high charge mobilities in discotic materials are established on the basis of the calculated structure-property relationships. We predict a significant increase in the charge mobility when going from triphenylene to hexaazatrinaphthylene; this finding has been confirmed by measurements carried out with the pulse-radiolysis time-resolved microwave conductivity technique.

Charge mobility in discotic liquid crystalline porphyrins and phthalocyanines measured by PR-TRMC

The pulse-radiolysis time-resolved microwave conductivity technique, “PR-TRMC”, has been used to determine the charge carrier mobility within columnar stacks of mesomorphic discotic porphyrins and phthalocyanines. The influences of temperature, morphology and variations in the primary molecular structure are demonstrated and discussed. Both the mesomorphic and conductive properties are shown to be dramatically influenced by subtle changes in the peripheral alkyl chain structure or the core-to-chain coupling element. Mobilities close to 1 cm2.V−1.s−1 are found in crystalline solids, and well in excess of 0.1 cm2.V−1.s−1 in columnar, liquid crystalline phases. These values which are even larger than those determined by PR-TRMC for conjugated polymers and similar to values found for electrons and holes in organic single crystals.

Charge Mobility in Discotic Materials Studied by Pr-Trmc

The application of the pulse-radiolysis time-resolved microwave conductivity technique, "PR-TRMC", to the determination of intracolumnar charge carrier mobilities within mesomorphic discotic materials is described. A review is given of the mobility values obtained for the large variety of materials which have been studied since the first results were reported in 1989. This includes peripherally substituted derivatives of triphenylene, phthalocyanine, porphyrin and hexabenzo-coronene. The influences of temperature, morphology and variations in the primary molecular structure are demonstrated and discussed. Both the mesomorphic and conductive properties are shown to be dramatically influenced by subtle changes in the peripheral alkyl chain structure or the core-to-chain coupling element, in addition to changes in the nature of the aromatic core itself. Mobilities as high as 1 cm 2 /Vs are found in crystalline solids, and well in excess of 0.1 cm 2 /Vs in columnar, liquid crystalline phases; values which approach those for electrons and holes in organic single crystals. The PR-TRMC mobility values are in reasonable agreement with values determined by the time-of-flight method (TOF) for the mesophase of triphenylene derivatives; the only class of discotic compounds for which the TOF method has been applied with success. In addition to its more general applicability to a range of core structures, the PR-TRMC method has the advantage that it does not require homeotropic alignment of the sample and even polycrystalline materials can be studied. A disadvantage is that only the sum of the charge carrier mobilities is measured; the sign of the major carrier cannot therefore be determined.

Charge-carrier mobilities in binary mixtures of discotic triphenylene derivatives as a function of temperature

The pulse-radiolysis time-resolved microwave conductivity technique was used to study charge transport in three binary mixtures of triphenylene derivatives and in three of these components separately. In the liquid-crystalline mesophase the hole mobilities in the mixtures are found to be an order of magnitude higher than those in the separate components. This is partly due to the more stable columnar structure in the mixtures and partly due to efficient ``alloy band formation.'' In the crystalline phase the mobility in the mixtures is only a few times lower than that of the separate components. The ionization energies of the components as measured by cyclic voltammetry are up to \ensuremath{\sim}0.5 eV apart. It turns out that this energy difference is easily compensated by the relatively large charge-transfer integrals for hole transport in the binary mixtures, which was obtained from ab initio Hartree Fock calculations. The mobilities estimated theoretically for ordered systems largely exceed the experimental values. This is most likely due to structural disorder along the columns in the material. Theoretical estimations of the hole mobility suggest that mobilities in excess of 1 ${\mathrm{cm}}^{2}$/V s could be attainable in well-ordered crystalline triphenylene samples.

Charge carrier dynamics in bulk poly(3-hexylthiophene) as a function of temperature

The charge carrier mobility in bulk samples of regioregular poly-3-hexylthiophene is determined from -100 to +140°C with the pulse-radiolysis time-resolved microwave conductivity technique. The lower limit to the mobility obtained increases only slightly with temperature in this range from 0.003 cm 2 /Vs to 0.01 cm 2 /Vs. However, the decay kinetics of the charge carriers is strongly dependent on temperature. This is attributed to changes in the effective domain size in which the charge carriers are able to move.

The influence of chain-to-core coupling on the charge transport and mesomorphic properties of discotic materials

The charge carrier mobilities in discotic liquid crystalline materials consisting of peripherally alkyl chain substituted triphenylene (T), phthalocyanine (Pc), and peri-hexabenzocoronene (HBC) cores have been measured using the pulse-radiolysis time-resolved microwave conductivity technique (PR-TRMC). The use of more bulky coupling units joining the mesogenic alkyl chains to the core, e.g. sulphur or para-phenylene, results in charge transport and mesomorphic properties more favorable for device applications than the frequently used coupling via an oxygen atom.

Liquid crystalline coronene derivatives

The synthesis of liquid crystalline coronenediimides 9 and coronenemonoimide 10 is described in detail and the mesophases are investigated. A large intracolumnar charge carrier mobility of ca. 0.2 cm2 Vs−1 has been found for the discotic mesophase of the coronenemonoimide 10 using the pulse-radiolysis time-resolved microwave conductivity technique. The large mobility, together with the room temperature liquid crystallinity, make this compound an attractive candidate for applications as the electron transport layer in molecular electronic devices.

Charge transport in mesomorphic derivatives of perylene

Charge carrier mobilities have been measured in the solid and liquid crystalline phases of peripherally alkyl-chain substituted perylene derivatives using the pulse-radiolysis time-resolved microwave conductivity technique, PR-TRMC. By varying the alkyl substituents, the structure of the materials can be changed from a two-dimensional layered arrangement of the perylene units to one-dimensional, hexagonally packed columnar stacks. In all cases the pseudo-isotropic mobility within organised domains, σμ TRMC, is found to lie within the range 0.01 to 0.1 cm 2/V/s and is only slightly dependent on temperature. In contrast to other discotic materials previously studied, no pronounced sharp decrease in σμ TRMC is found at the crystalline solid to liquid crystal phase transition.

Mobile charge carriers in pulse-irradiated poly- and oligothiophenes

Lower limits of the intrinsic charge carrier mobility in the solid phase of a series of oligothiophene compounds were determined with the pulse-radiolysis time-resolved microwave conductivity technique, PR-TRMC. The mobility values fall roughly into two regimes and show no correlation with the number of conjugated thiophene units. Relatively low mobilities (in the range of 3–6 10−4 cm2/Vs) were found for a series of cyclohexyl-endcapped thiophenes, while significantly higher values of 0.01–0.02 cm2/Vs were obtained for several n-hexyl and n-dodecyl substituted compounds and for sexithiophene. Interestingly, these latter values are similar to those of n-alkyl substituted polythiophenes measured earlier.

Radiation-induced conductivity in poly(phenylene vinylene) derivatives

Using the pulse-radiolysis time-resolved microwave conductivity technique the mobility and decay kinetics of radiation-induced charge carriers is studied in a series of poly(2,5-dialkoxy-phenylene vinylene) derivatives. The lower limit to the sum of the mobilities of the positive and negative charge carriers, Σμmin, depends strongly on the alkoxy functionalization and ranges from 1.2·10−7 to 1.4·10−6 m2/V·s at room temperature. Σμmin increases with the degree of order in the material. The after-pulse conductivity decay kinetics are disperse and are controlled by a combination of charge recombination and trapping.

Charge Transport in the Mesomorphic Free-Radical Compound Bis(octakis(dodecyloxy)phthalocyaninato)lutetium(III)

We have used the pulse-radiolysis time-resolved microwave conductivity technique (PR-TRMC) to study the charge transport properties of the free-radical sandwich complex bis(octakis(dodecyloxy)phthalocyaninato)lutetium(III), [(C12O)8Pc]2Lu. This compound displays multimesomorphic thermotropic behavior with two columnar liquid crystalline phases, Dx and Dh, between the freshly precipitated crystalline solid at room temperature, Kv, and the isotropic liquid, I, at 196 °C. In contrast with all other discotic materials previously studied using the PR-TRMC technique, the mobility of charge carriers is found to increase markedly at the transition from the Kv phase to the first columnar mesophase, Dx. The mobility displays a further substantial increase on entering the higher temperature hexagonal columnar phase, Dh. The intracolumnar mobility determined just above the Dx → Dh transition temperature of 90 °C is 0.17 × 10-4 m2/(V s), which is the highest yet determined for a columnar liquid crystalline material. T...

Structural, Photophysical, and Conductive Properties of n-Hexyl Substituted Hybrid Polysilylene−Polysilyne Networks

Structural, photophysical, and charge transport properties of n-hexyl substituted polysilylenes containing 5, 33 or 67% n-hexylsilyne branching points were investigated. According to WAXD, DSC, and vibrational studies, introduction of branching points partially reduces the high degree of crystallinity of linear poly(di-n-hexylsilylene). A thermal transition occurs in all polymers, which leads to solid state thermochromic behavior. Solution thermochromism is also observed. It is shown that in the partially branched polymers the narrow band gap of polysilynes is accompanied by an exciton delocalization approaching that of linear polysilylenes. In the solid state, charge carrier mobilities, determined with the pulse-radiolysis time-resolved microwave conductivity technique, are high but decrease with increasing branching. They range from 1.5 × 10-5 for the linear polymer to 7.6 × 10-7 m2 V-1 s-1 for the polymer with 67% branching sites. At the thermal transition an abrupt drop in the mobility is observed, of which the magnitude decreases with increasing branching. The branching points appear to act as scattering points for both exciton and charge carrier migration.