Reinhold Wannemacher

Stimulated Emission Properties of Sterically Modified Distyrylbenzene-Based H-Aggregate Single Crystals.

J-aggregation has been shown to be beneficial for light amplification in single crystals of π-conjugated organic molecules. In the case of H-aggregation, the criteria for such processes are still under debate. It has also been shown that H-aggregate arrangements with considerable π-π overlap are detrimental for light amplification. We show here that a proper alignment of the molecules in the crystal lattice, which minimizes π-π overlap between adjacent molecules, gives rise to (random) stimulated emission from cofacial arrangements similar to that of the herringbone aggregates.

Stimulated Resonance Raman Scattering and Laser Oscillation in Highly Emissive Distyrylbenzene-Based Molecular Crystals

Three-in-one: A novel distyrylbenzene-based material forms J-type aggregates in single crystals with highly polarized and bright red emission, giving rise to optical gain narrowing, for which different mechanisms (amplified spontaneous emission, laser emission and stimulated resonance Raman scattering) are observed. These are correlated with the favorable intrinsic and macroscopic properties of the crystal, in particular to the orientation of the molecules to the crystal surface.

The effect of oxygen induced degradation on charge carrier dynamics in P3HT:PCBM and Si-PCPDTBT:PCBM thin films and solar cells

Due to their light weight, transparency and flexibility, organic photovoltaic (OPV) devices are ideal for building integration. As this application requires solar cell life times of more than twenty years and oxygen ingress cannot be avoided at competitive cost on this time scale, OPV modules must be intrinsically stabilized against photo-oxidation. To this end, the mechanism of rapid performance loss of OSCs due to oxygen-induced degradation must be understood. Here, we combine transient absorption experiments with electrical studies in P3HT:PCBM and Si-PCPDTBT:PCBM thin films and solar cells after controlled photo-oxidation, studying charge carrier dynamics on the femtosecond to millisecond time scale. We find that oxygen-induced degradation does not significantly influence charge generation, while its influence on charge recombination is strong in both materials. A dramatic retardation of charge recombination already at low levels of oxygen-induced degradation is attributed to a substantial reduction of charge mobilities. We also observe a significant increase of the background concentration of charge carriers with the level of degradation, which leads to a crossover from second order towards pseudo-first order recombination behaviour. Extraction is shown to be retarded even more strongly than recombination, possibly by a reduction of the extraction field by the background carriers. Overall, the recombination yield is increased with degradation, explaining the strong performance loss already at low degradation levels.

Excited State Features and Dynamics in a Distyrylbenzene-Based Mixed Stack Donor-Acceptor Cocrystal with Luminescent Charge Transfer Characteristics.

Combined structural, photophysical, and quantum-chemical studies at the quantum mechanics/molecular mechanics (QM/MM) level precisely reveal the structure-property relationships in a mixed-stack donor-acceptor cocrystal, which displays vibronically structured fluorescence, strongly red-shifted against the spectra of the parent donor and acceptor, with high quantum yield despite the pronounced CT character of the emitting state. The study elucidates the reasons for this unusual combination, quantifies the ordering and nature of the collective excited singlet and triplet state manifold, and details the deactivation pathways of the initially created Franck-Condon state.

Flexible all-polymer waveguide for low threshold amplified spontaneous emission

The fabrication of all polymer optical waveguides, based on a highly fluorescent conjugated polymer (CP) poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and a mechanically flexible and biodegradable polymer, cellulose acetate (CA), is reported. The replication by hot embossing of patterned surfaces in CA substrates, onto which high quality F8BT films can be easily processed by spin coating, is exploited to produce an entirely plastic device that exhibits low optical loss and low threshold for amplified spontaneous emission (ASE). As a result, highly transparent and flexible waveguides are obtained, with excellent optical properties that remain unaltered after bending, allowing them to be adapted in various flexible photonic devices.

Controlled Suppression of Wear on the Nanoscale by Ultrasonic Vibrations.

Wear on the nanoscale, as evidenced by the formation of periodic ripples on a model polystyrene thin film while a sharp tip is sliding on it with a normal force in the μN range, is shown to be suppressed by the application of ultrasonic vibrations of amplitude Aexc. An accurate calibration of the transducer excitation amplitude is achieved by a home-built setup based on a laser Doppler vibrometer. The corrugation of the typical ripple pattern that is formed in the absence of vibrations is reduced when the excitation frequency matches the contact resonance of the system and Aexc progressively increases. Above a critical value of Aexc, the ripples completely disappear, while the friction levels off at a finite value determined by the normal force and the vibration amplitude. This value can be significantly smaller than the value of the macroscopic friction coefficient. In addition to the control of wear in general, this opens up the possibility of controlled nanolithography with improved accuracy.

Highly efficient organic photocatalysts discovered via a computer-aided-design strategy for visible-light-driven atom transfer radical polymerization

AbstractOrganocatalysed photoredox-mediated atom transfer radical polymerization (O-ATRP) is a very promising polymerization method as it eliminates concerns associated with transition-metal contamination of polymer products. However, reducing the amount of catalyst and expanding the monomer scope remain major challenges in O-ATRP. Herein, we report a systematic computer-aided-design strategy to identify powerful visible-light photoredox catalysts for O-ATRP. One of our discovered organic photoredox catalysts controls the polymerization of methyl methacrylate at sub-ppm catalyst loadings (0.5 ppm—a very meaningful amount enabling the direct use of polymers without a catalyst removal process); that is, 100–1,000 times lower loadings than other organic photoredox catalysts reported so far. Another organic photoredox catalyst with supra-reducing power in an excited state and high redox stability facilitates the challenging polymerization of the non-acrylic monomer styrene, which is not successful using existing photoredox catalysts. This work provides access to diverse challenging organic/polymer syntheses and makes O-ATRP viable for many industrial and biomedical applications.Organocatalysed photoredox-mediated atom transfer radical polymerization is a very promising method, although many challenges still lie ahead. Now, Kwon, Gierschner, Kim and co-workers present a computer-aided-design strategy to identify organic photoredox catalysts for this process. The success of the design strategy is demonstrated by polymerizations of methyl methacrylate and styrene.