Pulse-radiolysis Time-resolved Microwave Conductivity Research Articles

Electronic mobility and crystal structures of 2,5-dimethylanilinium triiodide and tin-based organic-inorganic hybrid compounds

We synthesize single crystals of a new 2,5-dimethylanilinium tin iodide organic-inorganic hybrid compound and 2,5-dimethylanilinium triiodide. Single-crystal X-ray diffraction reveals that the hybrid grows as a unique rhombohedral structure consisting of one-dimensional chains of SnI6-octahedra that share corners and edges to build up a ribbon along the [111] direction. Notably, we find that hypophosphorous acid, H3PO2, is of central importance to the formation of this hybrid. In the absence of H3PO2, we synthesize 2,5-dimethylanilinium triiodide from the same starting compounds. We investigate the synthesis routes that drive the growth of these two compounds with distinct crystal structures, appearance and properties. Pulse-radiolysis time-resolved microwave conductivity measurements and density functional theory calculations reveal that both compounds have low charge carrier mobilities and very long lifetimes, consistent with their one-dimensional structural characteristics. Our findings give a better understanding of the relation between synthesis, crystal structures and charge carrier mobilities.

Mobility of Holes in Oligo- and Polyfluorenes of Defined Lengths

The high-frequency mobility of positive charges (holes) moving along the backbones of an extensive data set of 7 oligomers (n = 2–16) and 6 polymers (⟨n⟩ = 26–138) of fluorene was measured using pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) in benzene. As expected, at 8.9 GHz, the measured isotropic ac mobility, μac,measiso, was observed to be strongly dependent on the lengths of the chains due to the charges encountering chain ends during one microwave cycle. Values of the measured mobility, μac,measiso, ranged from 5 × 10–4 cm2/(V s) for an n = 2 repeat unit oligomer to 0.18 cm2/(V s) for a polymer with an average length of ⟨n⟩ = 86 repeat units. Global fits to the entire set of lengths extracted the chain-length-independent intramolecular mobility, μacintra, using the Kubo formula, assuming normal diffusion along the contour of the molecule with reflecting boundary conditions at the ends. The effects of chain conformation, chain defects, polymer length distributions, and a finite polaron length on μac,measiso were considered quantitatively. The best fit to the whole data set, taking into account the polymer length distributions, suggests μacintra = 1.1 cm2/(V s). The fit was improved slightly by implementing randomly spaced barriers to transport along the chain with an average spacing of ∼40 repeat units, although at this spacing, the estimate of μacintra was not affected. These barriers could represent defects in the polymer or dihedral angles between repeat units, giving poor electronic coupling that persists for times greater than the 50 ps period of the microwaves. In this defect model, the best fit for their average spacing depended strongly on the chain conformation used, while the predicted intramolecular mobility did not. This estimate of the frequency of defects, a new aspect of this work, is less accurate because coiling of chains and defects have similar effects on the measured mobilities, and each can mask the effect of the other. The present data set also shows that there are few, if any, traps having depths substantially greater than thermal energy for holes on polyfluorene. The value of 1.1 cm2/(V s) is tightly constrained by the mobility of charges on the oligomers. Should the diffusion model break down for short chains and only be applicable to longer polymers, the intrachain mobility may be able to take both larger and smaller values (∼0.7–3 cm2/(V s)), although the data points to this being reasonably unlikely. It is shown that measurements of the imaginary part of the conductivity could be used in the future to clarify the mobility if this is the case.

Chlorophyll J-Aggregates: From Bioinspired Dye Stacks to Nanotubes, Liquid Crystals, and Biosupramolecular Electronics

Among the natural light-harvesting (LH) systems, those of green sulfur and nonsulfur photosynthetic bacteria are exceptional because they lack the support of a protein matrix. Instead, these so-called chlorosomes are based solely on "pigments". These are self-assembled bacteriochlorophyll c, d, and e derivatives, which consist of a chlorophyll skeleton bearing a 3(1)-hydroxy functional group. Chemists consider the latter as an essential structural unit to direct the formation of light-harvesting self-assembled dye aggregates with J-type excitonic coupling. The intriguing properties of chlorosomal J-type aggregates, particularly narrow red-shifted absorption bands, compared with monomers and their ability to delocalize and migrate excitons, have inspired intense research activities toward synthetic analogues in this field. The ultimate goal of this research field is the development of (opto-)electronic devices based on the architectural principle of chlorosomal LH systems. In this regard, the challenge is to develop small, functional building blocks with appropriate substituents that are preprogrammed to self-assemble across different length scales and to emulate functions of natural LH systems or to realize entirely new functions beyond those found in nature. In this Account, we highlight our achievements in the past decade with semisynthetic zinc chlorins (ZnChls) as model compounds of bacteriochlorophylls obtained from the naturally most abundant chlorin precursor: chlorophyll a. To begin, we explore how supramolecular strategies involving π-stacking, hydrogen bonding, and metal-oxygen coordination can be used to design ZnChl-based molecular stack, tube, and liquid crystalline assemblies conducive to charge and energy transport. Our design principle is based on the bioinspired functionalization of the 3(1)-position of ZnChl with a hydroxy or methoxy group; the former gives rise to tubular assemblies, whereas the latter induces stack assemblies. Functionalization of the 17(2)-position with esterified hydrophilic or hydrophobic chains, dendron-wedge substituents, and chromophores having complementary optical properties such as naphthalene bisimides (NBIs) is used to modulate the self-assembly of ZnChl dyes. The resulting assemblies exhibit enhanced charge transport and energy transfer abilities. We have used UV/vis, circular dichroism (CD), fluorescence spectroscopy, and dynamic light scattering (DLS) for the characterization of these assemblies in solution. In addition, we have studied assembly morphologies by atomic force microscopy (AFM), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), and cryogenic-TEM. Crystallographic techniques such as powder X-ray and solid-state NMR have been used to explain the precise long- and short-range packing of dyes in these assemblies. Finally, functional properties such as charge and energy transport have been explored by pulse radiolysis time-resolved microwave conductivity (PR-TRMC), conductive AFM, and time-resolved fluorescence spectroscopy. The design principles discussed in this Account are important steps toward the utilization of these materials in biosupramolecular electronics and photonics in the future.

Effects of the Environment on Charge Transport in Molecular Wires

Supramolecular engineering offers opportunities for creating polymer-based materials with tailored conductive properties. However, this requires an understanding of intermolecular interaction effects on intramolecular charge transport. We present a study of hole transport along molecular wires consisting of fluorene–p-biphenyl or Zn–porphyrin monomer units, in dilute solutions. The intramolecular hole mobility was studied by pulse radiolysis–time-resolved microwave conductivity. Experiments were supplemented by charge transport simulations employing a quantum-mechanical description of the hole and a classical description of the polymer and solvent dynamics. The model was parametrized using ab initio and molecular dynamics calculations. It was found that the solvent-induced energy disorder along a polymer chain in common solvents (benzene, cyclohexane, acetonitrile, water) is ∼1 eV, significantly greater than the values of 0.05–0.2 eV commonly cited in the literature. Environment-induced disorder of this m...

Charge Mobilities in Conjugated Polymers Measured by Pulse Radiolysis Time-Resolved Microwave Conductivity: From Single Chains to Solids

We discuss how the mobility of a charge moving along a conjugated polymer chain is affected by different types of disorder. The intrachain mobility has been determined by using pulse radiolysis in combination with microwave conductivity measurements. The intrachain mobility of charges on isolated planar ladder-type poly(p-phenylene) chains in solution is as high as 600 cm2/(V s). In solid samples, the intrachain mobility is only 30 cm2/(V s) due to disorder caused by interactions between different polymer chains. Interestingly, this mobility is 4 orders of magnitude higher than the DC device mobility, which is limited by charge hopping between different chains. Torsional disorder along poly(p-phenylene vinylene) chains limits the intrachain mobility to 60 cm2/(V s), while in polyfluorenes, coiling of the chains strongly reduces the mobility. The results imply that higher charge mobilities can be realized in devices if the polymer chains directly interconnect the electrodes and adopt a straight, planar structure in a homogeneous environment.

Columnar Mesophases Based on Zinc Chlorophyll Derivatives Functionalized with Peripheral Dendron Wedges

A chlorophyll derivative with a central zinc ion, a methoxy functionality at its 3(1)-position, and functionalized with a second-generation dendron (3,4-3,4,5)12G2-CH(2)OH at its 17(2)-position was synthesized starting from natural chlorophyll a (Chl a). This compound exhibits liquid crystalline (LC) behavior and its mesomorphic properties have been characterized by differential scanning calorimetry (DSC), polarisation optical microscopy (POM), powder X-ray diffraction (XRD), and scanning probe microscopy (SPM). A combination of powder XRD, high resolution scanning tunneling microscopy (STM), and atomic force microscopy (AFM) experiments revealed the formation of nano-segregated well-ordered columnar tubular superstructures consisting of about five molecules in the column stratum. These self-assembled columns are further self-organized into a two-dimensional oblique unit cell lattice. Semiconducting behavior of this compound has been studied by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) method and charge carrier mobility values of ∼10(-2) cm(2) V(-1) s(-1) are observed. Such organized columnar superstructures constructed from semisynthetic zinc chlorins are reminiscent of the tubular organization of the bacteriochlorophyll dyes in the light-harvesting chlorosomal antennae of green sulphur bacteria.

Relationship between supramolecular assembly and charge-carrier mobility in perylenediimide derivatives: The impact of side chains

Discotic mesophases are known for their ability to self-assemble into columnar structures which serve as semiconducting molecular wires. Charge-carrier mobility along these wires strongly depends on molecular packing which is controlled by intermolecular interactions. Using solid-state NMR and molecular dynamics simulations we relate how conformations of alkyl and glycol side chains affect helical pitch and angular distribution of molecules within the columnar structures of perylenediimide derivatives. Using the high-temperature limit of Marcus theory we then establish a link between the secondary structure and charge-carrier mobility. Simulation results are compared to pulse-radiolysis time-resolved microwave conductivity measurements. We conclude that for achieving high charge-carrier mobilities in discotics, side chains with specific interactions are required in order to minimize the translational and orientational molecular disorder in the columns.

Self-assembly and semiconductivity of an oligothiophene supergelator

A bis(trialkoxybenzamide)-functionalized quaterthiophene derivative was synthesized and its self-assembly properties in solution were studied. In non-polar solvents such as cyclohexane, this quaterthiophene π-system formed fibril aggregates with an H-type molecular arrangement due to synergistic effect of hydrogen bonding and π-stacking. The self-assembled fibres were found to gelate numerous organic solvents of diverse polarity. The charge transport ability of such elongated fibres of quaterthiophene π-system was explored by the pulse radiolysis time resolved microwave conductivity (PR-TRMC) technique and moderate mobility values were obtained. Furthermore, initial AFM and UV-vis spectroscopic studies of a mixture of our electron-rich quaterthiophene derivative with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) revealed a nanoscale segregated assembly of the individual building blocks in the blend.

Temperature‐Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers

AbstractPrevious investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW:Mn = 7 200 g mol−1; high‐MW:Mn = 27 000 g mol−1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10−2cm2V−1s−1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.

Understanding Structure−Mobility Relations for Perylene Tetracarboxydiimide Derivatives

Discotic mesophases are known for their ability to self-assemble into columnar structures and can serve as semiconducting molecular wires. Charge carrier mobility along these wires strongly depends on molecular packing, which is controlled by intermolecular interactions. By combining wide-angle X-ray scattering experiments with molecular dynamics simulations, we elucidate packing motifs of a perylene tetracarboxdiimide derivative, a task which is hard to achieve by using a single experimental or theoretical technique. We then relate the charge mobility to the molecular arrangement, both by pulse-radiolysis time-resolved microwave conductivity experiments and simulations based on the non-adiabatic Marcus charge transfer theory. Our results indicate that the helical molecular arrangement with the 45 degrees twist angle between the neighboring molecules favors hole transport in a compound normally considered as an n-type semiconductor. Statistical analysis shows that the transport is strongly suppressed by structural defects. By linking molecular packing and mobility, we eventually provide a pathway to the rational design of perylenediimide derivatives with high charge mobilities.

Structure–charge mobility relation for hexabenzocoronene derivatives

AbstractCharge mobilities of several derivatives of discotic liquid crystals have been determined by combining three methods into one scheme: (i) quantum chemical methods for the calculation of molecular electronic structures and reorganization energies (ii) molecular dynamics for simulation of the relative positions and orientations of molecules in a columnar mesophase, and (iii) kinetic Monte Carlo simulations and Master Equation approach to simulate charge transport. Applying this scheme to differently substituted hexabenzocoronene derivatives we reproduce the trends and magnitudes of mobilities as measured by pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and connect mobility directly to the microscopic morphology of the columns. Our study also shows that it is possible to understand and reproduce experimental charge transport parameters, and, in some cases, accurately predict them. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogen-bond stabilized columnar discotic benzenetrisamides with pendant triphenylene groups

A series of 1,3,5-benzenetrisamide derivatives with three hexaalkoxytriphenylene pendant groups were prepared, in which the triphenylene groups are connected to the central 1,3,5-benzenetrisamide core through a flexible spacer. The length of this spacer, as well as the size of the ortho-substituent at the triphenylene core, influences the columnar packing of these molecules. All compounds show liquid crystalline behavior with high isotropization temperatures. Different columnar hexagonal phases (Colh, Colhp, Colobl and Colr) have been identified using optical polarization microscopy, differential scanning calorimetry and X-ray diffraction. A chiral 1,3,5-benzenetrisamide derivative forms columnar stacks with a single helical sense, both in an apolar solvent and in a film, as observed by circular dichroism studies. Pulse radiolysis time-resolved microwave conductivity (PR-TRMC) studies show that the high ordering in a Colhp phase results in high charge carrier mobilities. On the other hand, the highest mobility was observed for the chiral compound, which has a Colr ordering.

Supramolecular Control of Charge Transport in Molecular Wires

The transport of positive charges along porphyrin-based molecular wires has been probed in dilute solution using pulse radiolysis time-resolved microwave conductivity. The mobility of positive charges along isolated single-strand porphyrin wires was found to be 0.084 cm2/(V s). Addition of 4,4‘-bipyridyl leads to the formation of a double-strand ladder complex, locking each polymer strand into a more planar structure and thus enhancing the porphyrin−porphyrin π-conjugation, leading to an increase in the charge carrier mobility by an order of magnitude to 0.91 cm2/(V s).

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.

Organic Field-Effect Transistors Utilizing Solution-Deposited Oligothiophene-Based Swivel Cruciforms

Two types of highly soluble oligothiophene-based swivel cruciforms are presented as semiconducting materials in OFETs. Incorporation of these penta-aryl oligomers into a swivel-cruciform type architecture greatly enhances the solubility of the oligomers, which lends itself to the fluidic preparation of thin films of the materials. XRD and AFM analysis of the resulting thin films reveals that they are highly crystalline. OFETs with pentathiophene-based swivel cruciforms exhibit a maximum field-effect mobility of 0.012 cm 2 /Vs, which is among the highest values reported to date for wet-processed OFETs utilizing oligothiophenes. In contrast, transistors made from bithiophene-phenyl-based cruciforms give overall lower mobilities, although the layers appear more crystalline in the AFM images. Pulse-radiolysis timeresolved microwave conductivity (PR-TRMC) experiments of the swivel cruciforms suggest that charge transport between crystalline domains may ultimately be the determining factor.

Photoluminescence and Conductivity of Self‐Assembled π–π Stacks of Perylene Bisimide Dyes

The self-assembly of a new, highly fluorescent perylene bisimide dye 2 into pi stacks, both in solution and condensed phase, has been studied in detail by NMR spectroscopy, vapor pressure osmometry (VPO), UV/Vis and fluorescence spectroscopy, differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and X-ray diffraction. The NMR and VPO measurements revealed the formation of extended pi-pi stacks of the dye molecules in solution. The aggregate size determined from VPO and DOSY NMR measurements agree well with that obtained from the concentration and temperature-dependent UV/Vis spectral data by employing the isodesmic model (equal K model). In the condensed state, dye 2 possesses a hexagonal columnar liquid crystalline (LC) phase as confirmed by X-ray diffraction analysis. The columnar stacking of this dye has been further explored by atomic force microscopy (AFM). Well-resolved columnar nanostructures of the compound are observed on graphite surface. A color-tunable luminescence from green to red has been observed upon aggregation which is accompanied by an increase of the fluorescence lifetime and depolarization. The observed absorption properties can be explained in terms of molecular exciton theory. The charge transport properties of dye 2 have been investigated by pulse radiolysis-time resolved microwave conductivity measurements and a 1D charge carrier mobility up to 0.42 cm(2) V(-1) s(-1) is obtained. Considering the promising self-assembly, semiconducting, and luminescence properties of this dye, it might serve as a useful functional material for nano(opto)electronics.

The Negative Effect of High‐Temperature Annealing on Charge‐Carrier Lifetimes in Microcrystalline PCBM

The fullerene derivative, phenyl-C61-butyric acid methyl ester (PCBM) has been the subject of intense recent study as the electron-acceptor component in heterojunction blends for all-organic photovoltaic devices (OPVs) and as the electron-transport layer in organic field-effect transistors (FETs). In most of the studies referenced, the effect of high-temperature annealing has been investigated and has usually been found to have a positive influence on device characteristics, including the external quantum efficiency of OPVs and the electron mobility in FETs. This has been attributed to reorganization and/or aggregation within the initially amorphous or nanocrystalline spin-cast films resulting in the formation of larger well-organized domains. The occurrence of PCBM aggregation to form microcrystallites on annealing of spin-cast films has been well-documented by atomic force and electron beam microscopy studies of polymer blends with high PCBM content and pure PCBM films. In a previous paper we reported results on the mobility and decay kinetics of the mobile charge carriers formed on pulsed ionization of microcrystalline PCBM powder. An unexpected finding was that on returning to room temperature after a period of only a few minutes at approximately 120 °C, the lifetime of the mobile carriers was substantially shorter than found before heat treatment. The effect was even more pronounced on heating temporarily to about 170 °C. We concluded that a structural or chemical change of the material was occurring at elevated temperatures, which resulted in the irreversible formation of trapping sites for electrons and holes. Since the previous sample was not deaerated, it was thought that the effects observed could have been due to a thermally induced chemical change involving atmospheric gases such as oxygen or water vapor. It was therefore decided to carry out a further study on a sample which could be subjected to rigorous evacuation. The results, which are reported here, show that this negative influence of high-temperature annealing on the lifetime of charge carriers occurs even for this in vacuo sample. This could have important ramifications in view of the often routine thermal-annealing treatment applied to OPV and FET devices based on PCBM. In the present pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) experiments, the microcrystalline powder sample of PCBM was contained in a cell that allowed the sample to be continuously evacuated in situ to a residual pressure of less than 10 Torr (1 Torr ≈ 133 Pa). All of the conductivity transients reported were obtained using singleshot 2 ns pulses of ionizing radiation (3 MeV electrons) with, at most, four pulses being used for signal averaging. Prior to presenting results on thermal annealing, we consider the effects of evacuation and radiation dose on the radiation-induced conductivity of the pristine sample at room temperature. In Figure 1, transients are shown for an air-equilibrated sample at room temperature and for the same sample after exposure to an accumulated radiation dose equivalent to about 150 pulses of 2 ns duration each. The end-of-pulse conductivity is seen to be unchanged and the decay to be only C O M M U N IC A TI O N S

Efficient Charge Transport along Phenylene−Vinylene Molecular Wires

We have studied the motion of charge carriers along isolated phenylene-vinylene (PV) chains using a combination of experimental and theoretical methods. The conductive properties of positive charges along PV chains in dilute solution were studied by using the pulse-radiolysis time-resolved microwave conductivity (TRMC) technique. This technique enables the measurement of high-frequency (tens of GHz) charge carrier mobilities along isolated PV chains without the use of electrodes. The charge carrier mobility along PV chains with finite and infinite length was studied theoretically by charge transport simulations with parameters from density functional theory (DFT) calculations. The high-frequency charge carrier mobility is found to depend strongly on the conjugation length of the PV chains and is found to increase both with increasing length of the PV chain and with increasing conjugation fraction. The experimental results are in good agreement with the calculated results. On the basis of this combined experimental and theoretical study an intrachain charge carrier mobility of a few tens of cm2/Vs is expected for an infinitely long PV chain without conjugation breaks.

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.