Matthijs P. de Haas

Highly mobile electrons and holes on isolated chains of the semiconducting polymer poly(phenylene vinylene)

The nature of the charge carriers in ‘conducting’ polymers is of considerable interest at present,, largely on the basis of the technological potential of these materials for use as the semiconducting layer in field-effect transistors (FETs) and the emissive layer in light-emitting diodes (LEDs). One of the main outstanding questions concerns the relative importance of intra- versus inter-chain charge transfer in determining the overall rate of charge transport. Here we apply the pulse-radiolysis time-resolved microwave conductivity technique to dilute solutions of a soluble dialkoxy derivative of the semiconducting polymer poly(phenylene vinylene), PPV, by which means we determine the one-dimensional intra-chain mobilities of electrons and holes on isolated polymer chains free from inter-chain interactions. The values so obtained—0.5 and 0.2 cm2 V−1 s−1 respectively—are considerably larger than the mobilities measured previously for bulk PPV-based materials. This suggests that considerable improvement in the performance characteristics (in particular switching time and maximum current) of organic FET and LED devices should be possible if material purity and structural order can be better controlled.

Photon-induced molecular charge separation studied by nanosecond time-resolved microwave conductivity

Abstract The application of the time-resolved microwave conductivity (TRMC) technique to the quantitative measurement of charge separation in flash-photolysed molecular systems is desecribed. The apparatus required, background theory and problems involved in deriving absolute values related to the charge-separation process are fully discussed. The use of the technique is illustrated by experiments carried out on the flash photolysis of 4-dimethylamino-4′-nitrostilbene (DMANS) in several solvents. Values of the dipole moments of the T 1 and S 1 states have been estimated and for S 1 are in good agreement with literature values derived in other ways. Other parameters important to the understanding of the photophysics of DMANS are also derived.

Efficient Intermolecular Charge Transport in Self‐Assembled Fibers of Mono‐ and Bithiophene Bisurea Compounds

Hydrogen bonds between urea units allow self-organization of π systems in mono- and bithiophenes into fibers as shown schematically. In these fibers there is a surprisingly high mobility of charge carriers as determined by pulse-radiolysis time-resolved microwave conductivity measurements.

The study of the transient conductivity of pulse irradiated dielectric liquids on a nanosecond timescale using microwaves

Abstract The method of measuring conductivity changes in pulse irradiated liquids, on a ns timescale, by microwave absorption is described. Quantitative relationships between the measured absorption signal and the change in conductivity are derived for several cell designs. The method is compared with optical absorption and DC conductivity techniques for the detection of ion formation. The value of the product of the free ion yield and the electron mobility in 2.2.4 trimethylpentane (iso-octane) is found to be 1.62 × 10 -4 m 2 V -1 s -1 (100 eV) -1 . Electron-positive ion recombination in iso-octane is found to obey the Debye relation.

The direct observation of a highly mobile positive ion in nanosecond pulse irradiated liquid cyclohexane

Abstract The electrical conductivity induced by pulse irradiation of liquid cyclohexane has been studied by means of microwave absorption. The conductivity in pure cyclohexane, due principally to the excess electron, is reduced to less than 10% of the initial value on addition of 5 × 10 −4 M of the electron scavenger SF 6 . The conductivity remaining after addition of SF 6 is however more than an order of magnitude larger than expected for massive ions in cyclohexane and, since it is almost completely removed by the addition of 4 × 10 −3 M of the positive ion scavenger NH 3 , is attributed mainly to the high mobility of the positive hole in this liquid. The ratio of the electron to hole mobility is determined to be 15. The mean lifetime of the hole under the present conditions is 86 ns. The rate constant for reaction of the hole with NH 3 is determined to be 1.8 × 10 11 M −1 s −1 . From the conductivity remaining after removal of both the electron and the hole the sum of the mobilities of the resulting molecular ions is determined to be 8.4 × 10 −4 cm 2 V −1 s −1 .

DNA: a molecular wire?

Abstract Radiation-induced conductivity measurements on hydrated DNA provide evidence for highly mobile charge carriers within the B-DNA superstructure. The lack of anisotropy in the conductivity for aligned fibre samples and the second-order nature of the decay argue against one-dimensional conduction via a ‘π-way’ type mechanism involving electron transport confined to the base-pair core.

The delayed absorption of microwaves due to electron thermalization in nanosecond pulse irradiated N2, He, and Ar at atmospheric pressure

The absorption of microwaves in pulse irradiated C2H6 (ethane), N2, He, and Ar at atmospheric pressure has been investigated on a nanosecond timescale. In the case of C2H6, a time of less than 1 nsec following the pulse is required for the absorption signal to reach a time independent plateau value. For N2, He, and Ar, a considerable delay in the development of the absorption signal is observed. This increase in absorption coefficient over 20, 50, and 300 nsec in the case of N2, He, and Ar, respectively, is attributed to a time dependence of the electron collision frequency resulting from post‐pulse thermalization of the electron energy. Using momentum transfer collision frequency data from swarm studies, the time dependence of the electron energy has been derived and is discussed in terms of the energy exchange rate coefficient Ku, defined by −d?/dt=Ku [?−(3/2) kT]N. For N2, Ku is found to be almost independent of electron energy over the range kTe=0.03–0.7 eV and has a mean value of 1.55×10−11 cm3 sec−1...

Electron migration in hydrated DNA and collagen at low temperatures. Part 1.—Effect of water concentration

The migration of charges formed in hydrated DNA and collagen at low temperatures by nanosecond pulses of 3 MeV electrons has been studied by measuring the microwave conductivity on nanosecond and microsecond timescales. The radiation-induced conductivity is found to be critically dependent on the water concentration of the samples. No radiation induced conductivity could be detected below water concentrations of 0.41 and 0.79 gram water per gram dry collagen and DNA, respectively. Above the critical water concentration the conductivity increases approximately linearly with water concentration. It is suggested that the observed conductivity is due to a highly mobile ‘dry’ electron migrating in the ice-like water layer around the biopolymer, with a mobility similar to that of the excess electron in pure ice (2.5 × 10–3 m2 V–1 s–1) but with a considerably longer lifetime.

Rate constants for the reaction of positive ions with solutes in irradiated liquid cyclohexane

Abstract The effect of added solutes on the decay kinetics of primary positive ions in irradiated cyclohexane has been studied using the microwave absorption method. Absolute rate constants, in the range of 1 to 3.5 × 10 11 M −1 s −1 , have been determined for the reaction of positive ioins with ethanol, ammonia, cyclopropane, benzene, dimethylaniline and biphenyl. For all of the above solutes the activation energy for reaction is found to be less than 1 kcal/mole. The rate constants obtained are compared with reactivity parameters available from steady-state radiolysis experiments. Taking for the yield of geminate ions a value of 5.0 (100eV) −1 , the characteristic geminate ion lifetime, λ −1 , in pure cyclohexane is estimated to be 2.2 ± 0.5 ps.

Measurement of ionic mobilities in dielectric liquids by means of concentric cylindrical electrodes

When a liquid, placed in an electric field between coaxial cylindrical electrodes, is ionized uniformly by a short pulse of x rays, the current resulting from the subsequent motion of the inward‐moving ions decays with time differently from that due to the outward‐moving ions. This effect provides a convenient method, here described in detail, for assigning mobilities to the positive and negative ions present. Mobilities were determined as a function of temperature for the ions formed in dilute solutions of the electron‐trapping molecules SF6, CH3I, CH3Br, CH3Cl, or O2 in cyclohexane or 2,2,4‐trimethylpentane. In all these cases the negative ions were more mobile than the positive. Only one major negative ion mobility and one major positive ion mobility was to be seen in the solutions, with the exception of O2 in 2,2,4‐trimethylpentane, which gave two negative ions of different mobilities.