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.

The information that can be obtained on the mobility and relaxation kinetics of electronic charge carriers in bulk molecular materials using the pulse-radiolysis time-resolved microwave conductivity technique is illustrated by results on several dialkoxy-substituted phenylene-vinylene polymers. The results demonstrate the sensitivity of the electronic properties of such conjugated polymers to their morphology. Thus, despite having the same conjugated backbone, the mobility and the relaxation kinetics (due to trapping and/or charge recombination) depend strongly on the nature of the alkyl-chain substituents, with in particular a marked difference between symmetrically and unsymmetrically dialkoxy-substituted compounds. For the latter, high-temperature annealing has a substantial positive effect on the mobility and lifetime of mobile carriers. The mobilities found for annealed materials range from a low of 0.0025 cm2 V-1 s-1 for the methoxy, ethyl-hexoxy derivative, MEH-PPV, to a high of 0.036 cm2 V-1 s-1 for the di-octadecoxy derivative, (OD)2-PPV. The latter compound becomes a free-flowing liquid above 190ºC but still displays a high charge-carrier mobility of 0.017 cm2 V-1 s-1. For all compounds the temperature dependence of the mobility after annealing is only slight over the range from −50 to +150ºC with an energy of activation ≤0.1 eV. Saturation of vinylene residues (breaking the conjugation) results in a marked decrease in the mobility. For very high accumulated doses of radiation the mobility on a nanosecond timescale remains unaffected but the decay of the mobile carriers at longer times becomes faster. This effect is completely reversed on annealing at 150ºC.

Discotic molecules have planar, disklike polyaromatic cores that can self-assemble into "molecular wires". Highly anisotropic charge transfer along the wires arises when there is sufficient intermolecular overlap of the pi-orbitals of the molecular cores. Discotic materials can be applied in molecular electronics, field-effect transistors, and-recently with record quantum efficiencies-photovoltaics (Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Frien, R. H.; MacKenzie, J. D. Science 2001, 293, 1119). A combination of quasielastic neutron scattering (QENS) measurements with molecular dynamics simulations on the discotic molecule hexakis(n-hexyloxy)triphenylene (HAT6) shows that the dynamics of the cores and tails of discotic molecules are strongly correlated. Core and tail dynamics are not separated, the system being characterized by overall in-plane motion, on a time scale of 0.2 ps, and softer out-of-plane motions at 7 ps. Because charge transfer between the molecules is on similar time scales, these motions are relevant for the conducting properties of the materials. Both types of motion are dominated by van der Waals interactions. Small-amplitude in-plane motions in which the disks move over each other are almost entirely determined by tail/tail interactions, these also playing an important role in the out-of-plane motion. The QENS measurements reveal that these motions are little changed by passing from the columnar phase to the isotropic liquid phase, just above the clearing temperature. The model of four HAT6 molecules in a column reproduces the measured QENS spectrum of the liquid phase, suggesting that correlations persist within the liquid phase over about this number of disks.

ExperimentalMaterials: 1,2-Bis(2-methyl-1-benzothiophen-3-yl)hexafluorocyclopentene (1) and 1,2-bis(2-methyl-6-phenyl-1-benzothiophen-3-yl)hexafluorocyclopentene (2) were prepared according to the literature [8,9]. Poly(cyclopentene) polymer (Zeonex 480) was purchased from the ZEON Corporation. Polymer films were prepared by solution casting. Diarylethene 2 (10.3 mg) and Zeonex 480 (258 mg) were dissolved in chloroform (5 mL). The solution was cast on a glass plate (ca. 100 cm 2 ). Removal of the solvent formed photochromic films doped with diarylethene derivatives. The polymer film was peeled off before the measurement. Film thickness was measured by micrometer (Mitutoyo Corporation, M110-25) as 35 lm.Measurements: The IR spectra were measured using a HORIBA FT-710 FTIR interferometer and IR images were monitored on a Perkin Elmer Spectrum Spotlight 300. UV light was produced using a TOPCON PU-2 black light lamp. Visible light irradiation was carried out using an USHIO 500 W highpressure mercury lamp with a Toshiba Y-50 cutoff filter.Computational Details: The molecular structures were optimized and vibrational analysis was performed by the Hartree±Fock method with 6-31G basis in Gaussian 98 [10]. The calculated frequencies were scaled by 0.88.

The fullerene derivative, phenyl-C 61 -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) [1][2][3][4][5][6][7][8][9][10] and as the electron-transport layer in organic field-effect transistors (FETs). [11][12][13][14][15][16] 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 [7][8][9]17,18] and pure PCBM films.

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CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. 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" cm?Vs) were found for a series of cyclohexyl-endcapped thiophenes, while significantly higher values of 0.01-0.02 cm'/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.

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