Low-threshold organic laser based on an oligofluorene truxene with low optical losses

A blue-emitting distributed feedback laser based on a star-shaped oligofluorene truxene molecule is presented. The gain, loss, refractive index, and (lack of) anisotropy are measured by amplified spontaneous emission and variable-angle ellipsometry. The waveguide losses are very low for an organic semiconductor gain medium, particularly for a neat film. The results suggest that truxenes are promising for reducing loss, a key parameter in the operation of organic semiconductor lasers. Distributed feedback lasers fabricated from solution by spin-coating show a low lasing threshold of 270 W/cm(2) and broad tunability across 25 nm in the blue part of the spectrum

Threshold-like Complexation of Conjugated Polymers with Small Molecule Acceptors in Solution within the Neighbor-Effect Model

In some donor-acceptor blends based on conjugated polymers, a pronounced charge-transfer complex (CTC) forms in the electronic ground state. In contrast to small-molecule donor-acceptor blends, the CTC concentration in polymer: acceptor solution can increase with the acceptor content in a threshold-like way. This threshold-like behavior was earlier attributed to the neighbor effect (NE) in the polymer complexation, i.e., next CTCs are preferentially formed near the existing ones; however, the NE origin is unknown. To address the factors affecting the NE, we record the optical absorption data for blends of the most studied conjugated polymers, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene) (P3HT), with electron acceptors of fluorene series, 1,8-dinitro-9,10-antraquinone (DNAQ), and 7,7,8,8-tetracyanoquinodimethane (TCNQ) in different solvents, and then analyze the data within the NE model. We have found that the NE depends on the polymer and acceptor molecular skeletons and solvent, while it does not depend on the acceptor electron affinity and polymer concentration. We conclude that the NE operates within a single macromolecule and stems from planarization of the polymer chain involved in the CTC with an acceptor molecule; as a result, the probability of further complexation with the next acceptor molecules at the adjacent repeat units increases. The steric and electronic microscopic mechanisms of NE are discussed

Dibenzothiophene-S,S-dioxide-fluorene co-oligomers. Stable, highly-efficient blue emitters with improved electron affinity

Incorporation of dibenzothiophene- S, S-dioxide units into conjugated fluorene oligomers changes the frontier orbital energy levels and presents an effective way to increase the electron affinity of these materials, which are highly fluorescent with bright blue emission in both solution and the solid state

Advances and Challenges in the Synthesis of Poly(p-phenylene vinylene)-Based Polymers

We provide an overview of the current progress in the synthesis of poly(p-phenylene vinylene) (PPV) polymers. The described synthetic approaches can be divided into four main categories: polymerization of quinodimethane intermediates, metathesis polymerization, nucleophilic polycondensation, and palladium-catalyzed cross-coupling. While such variety is remarkable and very high-molecular-weight PPV can be prepared by some methods, the synthetic chemistry of PPV still limits the structural variety and purity of arylene vinylene polymers. In particular, palladium-catalyzed cross-coupling reactions, the method of choice in the contemporary conjugated polymer chemistry, often shows limited success in PPV series. On the other hand, the possibility of using metathesis polymerization creates new opportunities not available for other classes of conjugated polymers

Supramolecular architecture of two charge-transfer complexes based on 2,7-(X,X)-4,5-dinitro-9-dicyanomethylenefluorenes (X=NO2 or CN) and tetrathiafulvalene

The crystal packings of two charge-transfer complexes based on tetrathiafulvalene and substituted fluorenes-2,4,5,7-tetranitro-9-dicyanomethylenefluorene (in complex 1) or 2,7-dicyano-4,5-dinitro-9-dicyanomethylenefluorene (in complex II)-are analyzed. Crystals of complex 11 involve a third component, namely, C6H5Cl solvate molecules. Crystals of both complexes are characterized by the formation of stacks composed of alternating donor and acceptor molecules and sheets in which the molecules are linked through different-type weak interactions. In structure 11, chlorobenzene molecules occupy cavities that are formed in stacks in the vicinity of the tetrathiafulvalene molecules due to the larger difference in size of the donor and acceptor molecules in complex 11 as compared to that in complex I. The chlorobenzene molecules provide a close packing. These molecules are involved in the system of weak interactions to form the Cl...N and C-H...N secondary bonds with the CN groups of the acceptor molecules in the sheets. (C) 2002 MAIK "Nauka/Interperiodica"