Alexander Kraus

Charge Separation in Localized and Delocalized Electronic States in Polymeric Semiconductors

Conjugated polymers such as poly(p-phenylene vinylene)s (PPVs) allow low-cost fabrication of thin semiconducting films by solution processing onto substrates. Several polymeric optoelectronic devices have been developed in recent years, including field-effect transistors, light-emitting diodes, photocells, and lasers. It is still not clear, however, whether the description of electronic excitations in these materials is most appropriately formulated within a molecular or semiconductor (band-theory) picture. In the former case, excited states are localized and are described as excitons; in the latter they are delocalized and described as free electron–hole pairs. Here we report studies of the electronic states associated with optical excitations in the visible and ultraviolet range for the conjugated polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), by means of photocurrent measurements and quantum-chemical calculations. We find several photocurrent spectral features between 3 and 5 eV which are coupled with bands in the absorption spectrum. On modelling the excited states in this energy range, we have discovered an important feature that is likely to be general for materials composed of coupled molecular units: that mixing of delocalized conduction- and valence-band states with states localized on the molecular units produces a sequence of excited states in which positive and negative charges can be separated further at higher energies. In other words, these excited states facilitate charge separation, and provide a conceptual bridge between the molecular (localized) and semiconductor (delocalized) pictures.

Covalent attachment of various substituents in closest proximity to the C60-core: A broad synthetic approach to stable fullerene derivatives

Abstract α-Substttulcd o-quinodimethanes facihtate the covlcnt attachment of many attractive molecules in closest proximity to the C 60 -core Thermal isomcrizalion of various (±)-hydroxyben/ocyclobutenc esters 4. 8. 10 and 13 in the presence of C 60 provided the corresponding fullerene adducts 5(n). 7(n). 9(n). 11(n) and 12(n) in high yields Using a dumbbell shaped bis-o-quinodimethane precursor 19 . accessible from (±)-7-hydroxybenzocyclobutcne and dodccancdicarboxyhc acid dichloride. a soluble C 60 -contaimng polymer 20 was formed

The repetitive Diels-Alder reaction: A new approach to [60]fullerene main-chain polymers

Abstract Maindashchain polymers with incorporated [60]fullerene units are accessible by reaction of buckminsterfullerene with functionalized bis -o- quinodimethanes . Cross linking can be effectively suppressed by carefully directed admixing of a mono -o- quinodimethane . The polymer 11(n) of high molecular weight and excellent solubility, consisting of 80 [60]fullerene units on the average, is affordable. This polymer can be processed to thin amorphous films of high quality by spin coating.

Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars

Magnetars are young neutron stars with very strong magnetic fields of the order of 1014–1015 G. They are detected in our Galaxy either as soft γ-ray repeaters or anomalous X-ray pulsars. Soft γ-ray repeaters are a rare type of γ-ray transient sources that are occasionally detected as bursters in the high-energy sky. No optical counterpart to the γ-ray flares or the quiescent source has yet been identified. Here we report multi-wavelength observations of a puzzling source, SWIFT J195509+261406. We detected more than 40 flaring episodes in the optical band over a time span of three days, and a faint infrared flare 11 days later, after which the source returned to quiescence. Our radio observations confirm a Galactic nature and establish a lower distance limit of ∼3.7 kpc. We suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths, and for which the long-term X-ray emission is short-lived. In this case, a new manifestation of magnetar activity has been recorded and we can consider SWIFT J195509+261406 to be a link between the ‘persistent’ soft γ-ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.A. J. Castro-Tirado, A. de Ugarte Postigo, J. Gorosabel, M. Jeĺınek, T. A. Fatkhullin, V. V. Sokolov, P. Ferrero, D. A. Kann, S. Klose, D. Sluse, M. Bremer, J. M. Winters, D. Nuernberger, D. Pérez-Ramı́rez, M. A. Guerrero, J. French, G. Melady, L. Hanlon, B. McBreen, F. J. Aceituno, R. Cunniffe, P. Kubánek, S. Vitek, S. Schulze, A. C. Wilson, R. Hudec, J. M. González-Pérez, T. Shahbaz, S. Guziy, S. B. Pandey L. Pavlenko, E. Sonbas, S. A. Trushkin, N. N. Bursov, N. A. Nizhelskij and L. Sabau-Graziati

Covalent incorporation of an alkoxysilyl-substituted [60]fullerene derivative in sol-gel matrices

A [60]fullerene adduct bearing a reactive triethoxysilyl functional group has been synthesized. The high solubility of this adduct in polar solvents enables its covalent incorporation into a sol-gel silica matrix. In comparison to the conventional admixing of ‘naked’ fullerene to silica sols, this approach has the following distinct advantages: (i) even with an adduct content of up to 2 wt% (related to SiO 2 ), the reaction mixture remains homogeneous during the entire sol-gel process, and (ii) the covalently attached fullerene adduct is fixed in the silica matrix and therefore cannot migrate.

Efficient charge carrier transfer from m-LPPP to C60 derivatives

Abstract The improvement of dissociation of singlet excitons by electron or hole capturing species like C 60 is essential for efficient photovoltaic cells based on conjugated systems. We investigate the charge transfer in blends of methyl-substituted ladder type poly-(paraphenylene) (m-LPPP) with new well soluble C 60 derivatives, which are characterized by sidegroups of different electronic properties (donor, acceptor nature). It is shown, that the electronic properties of the C 60 sidegroups strongly influence the efficiency of the charge transfer in these blends and hence significant different behaviours in the photocurrent and photoinduced absorption is observed.

Synthesis and Chemical Transformations of the Diels-Alder Adduct of [60] Fullerene and 4-Amino-O-quinodimethane

Abstract Diels-Alder reaction of 4-amino-o-quinodimethane (6) with [60]fullerene (1) affords the thermally stable and chemically reactive adduct 7(n). The nucleophilic amino group of 7(n) is well suited for the covalent attachment of different molecules to the [60]fullerene core by further chemical reactions.