Martin Baumgarten

Polyphenylene-Based Materials for Organic Photovoltaics

Energy will be one of the most important factors to influence human society in the 21st century.1,2 Cost, availability, and sustainability of energy have a significant impact on the quality of our lives, development of global economies, relationships between nations, and the stability of our environment. Scientists are now focusing on the development of renewable energies3 generated from natural resources such as sunlight, wind, rain, tides, and geothermal heat. Among these, the sun has the potential to make the largest energy contribution: only one hour of sunshine (3.8 × 1023 kW) is more than enough to satisfy the highest human demand for energy for an entire year (1.6 × 1010 kW in 2005).4-7 Solar cells, also called photovoltaics,8 are devices based on solar technology which convert sunlight directly into electricity under the photovoltaic effect. Becquerel was the first to recognize this effect in 1839, when he shined light * To whom correspondence should be addressed. E-mail: muellen@ mpip-mainz.mpg.de. † Max Planck Institute for Polymer Research. ‡ BASF SE. Chen Li studied chemistry and completed his Master’s thesis under the supervision of Professor He Tian at the East China University of Science and Technology in 2003. In 2004, he joined the group of Professor Klaus Müllen at the Max Planck Institute for Polymer Research to work on functional rylene dyes for dye-sensitized solar cells. He received his Ph.D. in 2008 from the Johannes Gutenberg University of Mainz. Presently he works in the same group as a project leader, and his research interests are functional dyes and pigments as well as their applications.

High Performance Field‐Effect Ammonia Sensors Based on a Structured Ultrathin Organic Semiconductor Film

High performance organic field-effect transistor (OFET)-based ammonia sensors are demonstrated with ultrathin (4-6 molecular layers) dendritic microstripes of an organic semiconductor prepared via dip-coating. These sensors exhibit high sensitivity, fast response/recovery rate, good selectivity, low concentration detection ability, and reliable reversibility, as well as stability. Such a performance represents great progress in the field of OFET-based sensors.

Triangular Trinuclear Metal-N4 Complexes with High Electrocatalytic Activity for Oxygen Reduction

A new class of macrocyclic metal-N(4) complexes [MN(4)](n) (M = Co and Fe) were designed and synthesized based on a triangular ligand. Their unique triangular trinuclear structure provides a high density of active sites and facilitates the reduction of dioxygen via a four-electron pathway. Among them, a [CoN(4)](3)/C catalyst (20 wt %) exhibits high catalytic activity and long-time stability for the oxygen reduction reaction (ORR) in alkaline conditions, superior to the commercial Pt/C catalyst. Such structurally well-defined [MN(4)](n) complexes provide a platform for a new generation of nonprecious metal catalysts (NPMCs) for fuel cell applications.

Core, shell, and surface-optimized dendrimers for blue light-emitting diodes.

We present a novel core-shell-surface multifunctional structure for dendrimers using a blue fluorescent pyrene core with triphenylene dendrons and triphenylamine surface groups. We find efficient excitation energy transfer from the triphenylene shell to the pyrene core, substantially enhancing the quantum yield in solution and the solid state (4-fold) compared to dendrimers without a core emitter, while TPA groups facilitate the hole capturing and injection ability in the device applications. With a luminance of up to 1400 cd/m(2), a saturated blue emission CIE(xy) = (0.15, 0.17) and high operational stability, these dendrimers belong to the best reported fluorescence-based blue-emitting organic molecules.

Solution processable bulk-heterojunction solar cells using a small molecule acceptor

We report a small-molecule electron-acceptor based on 2-vinyl-4,5-dicyanoimidazole [Vinazene™] for use in solution processed organic solar cells. The material has a favourably located LUMO level of −3.6 eV and absorbs strongly in the visible spectrum up to 520 nm—attractive properties compared to the widely used acceptor (6,6)-phenyl-C60-butyric acid methyl ester (PCBM). The Vinazene derivative was blended with a poly(2,7-carbazole) donor—chosen for its complementary absorption range and comparatively high-lying HOMO level of −5.6 eV—and incorporated into bulk heterojunction devices. The influence of the donor/acceptor composition and annealing temperature on device performance were then investigated. The best performing devices exhibited reasonable power conversion efficiencies of 0.75% and open-circuit voltages of more than 1.3 V, substantially higher than previously reported devices using small molecule acceptors.

Franck–Condon spectra and electron-libration coupling in para-polyphenyls

Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-phenylene oligomers (OPP and OLPP) with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interaction within a standard molecular all-valence-electron semiempirical Hamiltonian. Adiabatic excitation energies, interstate distortions and normal modes are used to compute Franck–Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and rationalize the striking disparities in the linear optical response of OPP and OLPP. Whereas electron–phonon coupling in OLPP is essentially restricted to the carbon–carbon bond-stretching modes, photoexcitation, and emission processes in OPP are followed by significant relaxations in ring-torsional degrees of freedom. The broadening of spectra of OPP, especially pronounced in absorption, and the large Stokes shift be...

Rigorous Franck-Condon Absorption and Emission Spectra of Conjugated Oligomers from Quantum Chemistry

A harmonic Condon approach is used to calculate excitation and emission band shapes for the lowest dipole-allowed electronic transitions in conjugated oligomers: polyenes, oligorylenes, and para-phenylenevinylenes. Ground- and excited-state adiabatic energies, equilibrium structures, and vibrational modes are obtained within standard all-valence-electron molecular Hamiltonian incorporating extended configuration interaction. The interstate distortion is cast in normal coordinates and used to calculate transition probabilities from the zero-phonon initial state to the vibrational manifold of the final state. Spectral profiles are obtained as a superposition of Lorentzian line shapes. Theoretical band shapes reproduce prominent features in the absorption and fluorescence spectra of the oligomers in question. The strength of the bond-stretching vibronic progression increases with oligomeric length in polyenes, but decreases in para-phenylenevinylenes. In line with experiment, absorption and emission band sha...

Hexathienocoronenes: Synthesis and Self-Organization

Here we report hexathienocoronenes (HTCs), fully thiophene-annelated coronenes in which six double bonds in the periphery are thieno-fused. The derivatives tetrasubstituted with hexyl and dodecyl chains show a phase formation that strongly depends on the chain length. HTCs are remarkably stronger donors than the known thiophene-annelated coronenes but do not readily assemble into well-ordered films when deposited from the vapor phase. Thus, thin-film transistors fabricated by vacuum deposition have only modest field-effect mobilities of 0.002 cm(2) V(-1) s(-1).

Thiadiazoloquinoxaline–Acetylene Containing Polymers as Semiconductors in Ambipolar Field Effect Transistors

Two conjugated copolymers, PPTQT and PTTQT, were developed based on thiadiazoloquinoxalines connected via ethynylene π-spacer to thiophene units. PPTQT showed maximum hole and electron mobility of 0.028 and 0.042 cm(2)/V s, respectively, being the first example of an ambipolar semiconducting material bearing triple bonds in the polymer backbone.

RADICAL IONS - WHERE ORGANIC-CHEMISTRY MEETS MATERIALS SCIENCES

Radical ions generated by electron transfer reactions are known as important intermediates in organic chemistry. On the other hand, their formation, recombination and transport in organic materials is responsible for a series of attractive physical properties. Radical ion formation is often accompanied by structural changes being well understood in small organic molecules, which also constitute repeating units of intensively studied macromolecules. Therefore, an approach is described herein to compare and combine the structural and energetic description of monomeric and oligomeric radical ions with that of partially oxidized or reduced polymeric materials.