M.P. de Haas

Exciplex-type emission from folded and extended conformations of a donor-acceptor molecule with limited flexibility

Abstract The fluorescent behaviour of a donor-bridge-acceptor compound was studied in apolar solvents as a function of temperature. In the ground-state conformation donor and acceptor are held far apart by the interconnecting saturated hydrocarbon bridge. Photoexcitation at room temperature in an apolar solvent nevertheless leads to exciplex-like emission at 454 nm from a “folded” conformation. Emission spectra at low temperatures reveal a totally different fluorescence band at 380 nm that is ascribed to a CT state with a stretched conformation. With time-resolved fluorescence measurements it was shown that this state is the precursor to the folded exciplex-state, indicating that charge separation takes place in an extended conformation. At room temperature charge separation is followed by rapid Coulomb-induced interconversion to a folded conformation, thereby efficiently quenching the 380 nm fluorescence. Further confirmation of our interpretation was obtained by time-resolved microwave conductivity experiments.

The lifetime of the fluorescent excited state of liquid cyclohexane

Abstract The average lifetime of the fluorescence of liquid cyclohexane was determined by means of pulse radiolysis with a time resolution of ca. 100 ps and found to be 0.95 ns.

Charge carrier mobilities in mesomorphic α, ω-dihexylquaterthiophene : A comparative Microwave Conductivity and Thin Film Transistor study

Abstract The charge carrier mobility in α,ω-dihexylquaterthiophene has been determined as a function of temperature using the Pulse-Radiolysis Time-Resolved Microwave Conductivity (PR-TRMC) and Thin Film Transistor (TFT) techniques. At room temperature a minimum value of 0.015 cm 2 /Vs was obtained with PR-TRMC for the amorphous powder, close to the mobility obtained in TFT experiments on a well-ordered film. The PR-TRMC mobility decreases by 30% at 81 °C and drops to less than 10 −4 cm 2 /Vs above 180°C. These temperatures correspond to two reversible phase transitions discernable in DSC scans. With TFT experiments only the first phase transition could be studied, showing a decrease in mobility very similar to that observed in the PR-TRMC study.

Charge migration in pulse-irradiated undoped C60 powder studied with the Time-Resolved Microwave conductivity technique

Abstract The conductivity of C60 buckminsterfullerene was studied using the pulse-radiolysis time resolved microwave conductivity (PR-TRMC) technique. Conductivity transients with disperse decay kinetics and a first half-life of 2 ns were obtained in the temperature range from 210 to 450 K. The charge carrier mobility at high temperatures (T>300 K) is thermally activated (E a =46 meV). A minimum for the sum of the charge carrier mobilities of holes and electrons was found to be 1×10 −5 m 2 /Vs.

High-energy radiation monitoring based on radio-fluorogenic co-polymerization II: fixed fluorescent images of collimated x-ray beams using an RFCP gel

We have produced an optically clear, close to water-equivalent gel that is radio-fluorogenic, i.e. fluoresces in UV light after exposure to high-energy radiation. Its potential as a 2D and 3D dosimetric medium is demonstrated by fixed fluorescent images of the cross-section, track and intersection of collimated (10 × 10 or 5 × 5 mm(2)) 205 kVp x-ray beams. The images, produced by doses on the order of 10 Gy, are formed instantaneously and can be digitally recorded and scanned with a spatial resolution on the order of 0.1 mm. No loss of spatial resolution occurs on standing under ambient conditions for at least 3 days.

Development of a microwave transmission setup for time-resolved measurements of the transient complex conductivity in bulk samples

We describe and characterize a microwave transmission setup for the measurement of radiation-induced transient conductivities in the frequency range between 26 and 38GHz (Q band). This technique combines the virtues of two already existing techniques. On one hand, the microwave transmission technique is well established for the determination of (quasi)static conductivities, but requires adaptations to be suitable to the determination of transient conductivities with 1ns temporal resolution. On the other hand, the transient conductivity technique is well established, too, but in its present form (using a reflection configuration) it suffers from a poor signal to noise ratio due to unwanted interferences. These interferences are due to the circulator, which diverts part of the incoming microwave flux directly to the detector. We characterized the transmission setup by measuring the real and imaginary components of the conductivity of pulse irradiated CO2 gas at different pressures, and compared these result...

Mobile charge carriers in pulse-irradiated poly(3-n-alkyl)thiopenes

Abstract Upon pulsed irradiation with high energy electrons from a Van de Graaff accelerator long lived conductivity transients are found in polythiopenes with n -hexyl, n -octyl and n -decyl sidechains, using the time resolved microwave conductivity (TRMC) technique. The sum of the charge carrier mobilities is found to at least 7×10 −7 m 2 /Vs, independent of the sidechain length. The radiation-induced conductivity transients show disperse decay kinetics. The timescale on which the conductivity decays increases with increasing sidechain length. The conductivity transients observed in the compounds synthesized with a MgI-Grignard reagent show no effect of aging up to a period of 4 months. Conductivity transients of poly( n -hexyl)thiopene synthesized with FeCl 3 showed a strong dependence on residual Fe content.

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.

Frequency dependence of the charge carrier mobility in DH4T

The charge carrier mobility in bulk samples of α,ω-dihexylquaterthiophene has been determined as a function of temperature using the Pulse-Radiolysis Time-Resolved-Microwave-Conductivity and Thin Film Transistor (TFT) techniques. The mobility values obtained at room temperature were 0.01 cm 2 /Vs for both methods However upon cooling to 150 K the FET mobility decreases by an order of magnitude whereas the TRMC mobility remains almost constant. This behaviour is interpreted as an effect of the frequency of the electric field that is used in the experiments and could be described theoretically using an over-the-barrier hopping model for the mobility.

Electrodeless measurement of photoconductivity in thin polymer films

A recent development in the time-resolved microwave conductivity technique is the ability to study photogenerated charge carriers in thin polymer films (of only a few tens of nanometres thickness). This brings the experimental conditions much closer to those in device applications, while circumventing the problems associated with applying electrode contacts. A variety of samples (including polythiophene, poly(phenylene-vinylene) and polyfluorene derivatives) has been investigated.