Drahomír Výprachtický

Enhanced electroluminescence from light-emitting devices based on poly(9,9-dihexadecylfluorene-2,7-diyl) and polysilane blends

Photoluminescence and electroluminescence (EL) of poly(9,9-dihexadecylfluorene-2,7-diyl) (PFC16) and its blends with hole-transporting poly[methyl(phenyl)silanediyl] modified with pyrene were studied. Efficient blue light-emitting devices were fabricated. Blending of PFC16 with modified polysilane led to a significant improvement in the EL efficiency and stability compared with the devices fabricated from neat PFC16. An increase in the EL efficiency of up to two orders of magnitude was achieved. The increase in the EL efficiency was attributed to the modification of the charge transport and recombination in the blend.

Synthesis and Photophysical and Electroluminescent Properties of Poly(1,4-phenylene−ethynylene)-alt-poly(1,4-phenylene−vinylene)s with Various Dissymmetric Substitution of Alkoxy Side Chains

The synthesis and characterization of a set of conjugated polymers, poly(1,4-phenylene–ethynylene)-alt-poly(1,4-phenylene–vinylene)s (PPE–PPVs), with a dissymmetrical configuration (partial or total) of alkoxy side chains is reported. Five new polymers bearing octyloxy and/or octadecyloxy side chains at the phenylene–ethynylene and phenylene–vinylene segments, respectively, were obtained. Two symmetrical substituted polymers were used for comparison. Polymers with weight-average molecular weight, Mw, up to 430 000 g/mol and degree of polymerization between 17 and 322 were obtained by a Horner–Wadsworth–Emmons olefination polycondensation reaction of the respective luminophoric dialdehydes and bisphosphonates. As expected, identical conjugated backbones in all polymers results in very similar photophysical response in dilute solution, with high fluorescence quantum yields between 50% and 80%. In contrast, the thin film properties are dependent on the combinatorial effects of side chain configuration, molecular weight, and film thickness parameters, which are the basis of the resulting comparison and discussion.

Aggregation of Poly(γ-benzyl L-glutamate)s Followed by Time-Resolved Emission Anisotropy

Syntheses of poly(7-benzyl L-glutamate)s (PBLGs) labeled with various fluorophores (tryptophan, dansyl, and anthracene) having different molecular weights are reported. Association of PBLG chains was studied by time-resolved emission anisotropy in the solvents supporting the aggregation process (1,4-dioxane and tetrahydrofuran) and in N,N-dimethylformamide, where the aggregates were not formed. The influence of molecular weight and polymer concentration on PBLG association was studied as well. The limiting emission anisotropy (r∞) and rotational correlation times (o) were determined, The chain relaxation dynamics were compared with the fluorescence lifetimes of the fluorophores and spectroscopically suitable labels were selected. Tryptophan was found to be an inconvenient fluonophore for the association study of PBLGs because of its short excited-state life time. Dansyl and anthracene fluorophores, however, proved to be suitable labels for the chain dynamics study of PBLGs in solution. The mobilities of PBLG chains in 1,4-dioxane were slower than those in tetrahydrofuran and N,N-dimethylformamide because of PBLG association in this solvent.

Semiconducting Conjugated Copolymer Series for Organic Photonics and Electronics

Donor–acceptor copolymer series containing 4,6-di (thien-2′-yl) thieno [3,4-c][1,2,5] thiadiazole or its derivatives serving as electron-acceptor units and various electron-donor units such as 9,9-bis (alkyl) fluorene, benzene, bithiophene or carbazole derivatives is reported. These copolymers possess narrow optical band gap in the range of 1.0 - 1.5 eV depending on the character of the donor units. They exhibit relatively high electron affinity. Absorption of copolymer thin films covers the whole visible spectral region extended up to NIR for some copolymers. The influence of side chain nature and molecular weight on their photophysical properties is shown. Selected copolymers are used in the blends with fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) as active layers in bulk heterojunction photovoltaic devices. The results are discussed in relation to the copolymer structure, side chain nature and molecular weight.

New two-step synthesis of N-(2-ethylhexyl)-2,7-diiodocarbazole as a monomer for conjugated polymers

A reasonably efficient two-step synthesis of N-(2-ethylhexyl)-2,7-diiodocarbazole is presented. In the first step, the 4,4′-diiodobiphenyl was nitrated to a mixture of 4,4′-diiodo-2-nitrobiphenyl and 4-iodo-4′-nitrobiphenyl. In the second step, the mixture of these compounds was converted by simultaneous carbazole ring closure and N-alkylation to the N-(2-ethylhexyl)-2,7-diiodocarbazole by means of tris(2-ethylhexyl) phosphite. The synthesis represents a shorter alternative to the known more tedious procedures. The prepared compound is a suitable monomer for synthesis of conjugated polymers for optoelectronic applications.

Novel and Simple Synthesis of Brominated 1,10-Phenanthrolines

A novel, simple, and reasonably efficient synthesis of 3,8-dibromo-1,10-phenanthroline, 3,6-dibromo-1,10-phenanthroline, 3,5,8-tribromo-1,10-phenanthroline, and 3,5,6,8-tetrabromo-1,10-phenanthroline is presented herein. The crucial role of a new catalyst (sulfur dichloride – SCl2) for the bromination of 1,10-phenanthroline is reported. The bromination of 1,10-phenanthroline monohydrate in the presence of SCl2 and pyridine yielded the brominated compounds, previously only possible through the complicated multi-step and tedious Skraup synthesis method. The application of the bromination catalyst SCl2 as a medium-strength Lewis acid is demonstrated for the first time, and the results are compared with the behaviours of known weak (sulfur chloride – S2Cl2) and strong (thionyl chloride – SOCl2) bromination catalysts. A reaction mechanism was proposed.

Time‐Resolved Emission Anisotropy of Anthracene Fluorophore in the Backbone of Stereoregular Poly(methyl methacrylate)

Syndiotactic fluorescent poly(methyl methacrylate) labeled with anthracene in the middle of the polymer chain was prepared by living anionic polymerization (s-PMMA-A). Absorption and emission transition moments of the label are oriented in the direction of the polymer backbone, therefore the fluorophore really reflects the mobility of polymer segments. Stereocomplex formation between isotactic poly(methyl methacrylate) (i-PMMA) and s-PMMA-A was studied by time-resolved fluorescence depolarization method in solvents with different complexing ability and at different i-PMMA/s-PMMA-A ratios. The stoichiometry of the stereocomplex i-PMMA/s-PMMA-A was 1/2 and 1/1.5 in N,N-dimehtylformamide and 1,4 dioxane, respectively. The stereocomplex was not formed in chloroform. Determination of the rotational correlation times at different temperatures enabled us to evaluate the height of the potential barrier to rotational motion of the anthracene fluorophore (ΔU r ) or activation enthalpy and entropy (ΔH ¬= , ΔS ¬= ) of this process in the presence and absence of the stereocomplex. Time-resolved emission anisotropy data, r(t), were analyzed by intermediate zone formula of the torsion dynamics theory for stiff macromolecules. The torsional rigidity of the stereocomplex i-PMMA/s-PMMA-A, a = 1.9 w 10 -18 N.m in N,N-dimethylformamide at 30°C, is larger than that obtained for DNA (a = 3.8 x 10 -19 N.m). Self-aggregation of s-PMMA takes place probably in N,N-dimethylformamide.

Distance and orientation factor of fluorescence labels in complexed tactic and atactic poly(methyl methacrylate)s

Isotactic, syndiotactic and atactic poly(methyl methacrylate)s (i-, s- and a-PMMA) labeled with a donor (carbazole) or an acceptor (anthracene) were prepared. The distance between the donor and acceptor fluorophores in complexed tactic PMMA was measured using Forsters energy transfer method and compared with that found for atactic PMMA. The measured average distance of the fluorophores in the stereocomplex (˜3.7 nm) was found to be shorter than that in atactic systems where stereocomplexation does not take place. The upper and lower boundaries for the dipole-dipole orientation factor (K2) in Forsters theory were determined by time-resolved emission anisotropy. The orientational freedom of the fluorophores was found to be restricted for tactic PMMA in N,N-dimethylformamide as a result of stereocomplex formation but for tactic PMMA in 1,4-dioxane and atactic PMMA in 1,4-dioxane and N,N-dimethylformamide, the dynamic average value of 〈K2〉 = 2/3 was used.