P-phenylenevinylene Units Research Articles

Switching of emissive and NLO properties in push–pull chromophores with crescent PPV-like structures

We report on a series of novel homologous push-pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, bearing, respectively, 1, 2 and 3 p-phenylenevinylene units in direct linear conjugation (compounds 7-9). The three compounds exhibit striking differences in their linear and nonlinear optical properties. The shorter compound 7 exhibits aggregation-induced emission with a strong luminescence in the solid state (blue emission, photoluminescence quantum yield 38%), and it is nonemissive in solution; the more extended conjugated systems 8 and 9 show classical aggregation-caused quenching in the solid state, while high quantum yield photoluminescence (21 and 93% in toluene) is restored in diluted solutions, through mechanisms involving intramolecular charge transfer in the excited states. EFISH measurements in solutions demonstrate a strong solvent and concentration dependence. As rationalized with the aid of molecular modelling, compounds 8 and, more markedly, 9 aggregate in stable centrosymmetric dimers in solution.

From red to blue shift: switching the binding affinity from the acceptor to the donor end by increasing the π-bridge in push–pull chromophores with coordinative ends

A series of homologous push–pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, exhibit striking differences in their supramolecular recognition of cations acting as Lewis acids. The shorter conjugated compound (one phenyl ring) coordinates a wide variety of lanthanide cations (Eu3+, Yb3+ and Er3+) in MeCN solutions to the 1,3-dicarbonyl acceptor end, resulting in an overall supramolecular polarization of the system (red shift of the intramolecular charge-transfer ICT band). With the “hard” cation Sc3+, recognition switches to the tertiary amine donor end, turning the conjugated system from D–π–A to A–π–A, and resulting in a blue shift of the ICT band upon complexation. Interestingly, increasing the conjugation by means of the insertion of sequential p-phenylenevinylene units into the ligand results in coordination to the donor end regardless of cation “hardness” (Sc3+, Eu3+ and Er3+), suggesting a relative change in the nucleophilicity of the two coordinating ends when increasing the length of the conjugated π-bridge. Such a hypothesis is supported by quantum chemical calculations on the ligands and subsequent atomic charges determination using two independent approaches (QTAIM and CHelpG). The characterization of the thermodynamic stabilities and the dimensionalities of the ligand–cation complexes in solution reveals striking differences from case to case, yet increasing affinities (from log Kav = 2.5 to log Kav = 4.9) are recorded with the increase of the π-conjugated bridge.

Structure and properties of p-phenylenevinylene-p-xylylene copolymers prepared by vapor-deposition polymerization

Films based on the p-phenylenevinylene-p-xylylene precursor are prepared via vapor-deposition polymerization during the pyrolysis of α, α′-dichloro-p-xylene on a copper grid in vacuum at substrate temperatures −196, 25, and 50°C. Subsequent annealing of the precursor at 250°C yields the final material: the copolymer of p-phenylenevinylene and p-xylylene. The structure, surface morphology, and optical properties of the copolymer are studied at different substrate temperatures and copper amounts in the pyrolysis zone. It is found that p-phenylenevinylene units mostly occur in trans configurations. The thermal treatment of the precursor is accompanied by an increase in the mean-square surface roughness and a decrease in roughness coefficient α from 0.84 ± 0.05 to 0.79 ± 0.05. As the content of copper in the pyrolysis zone increases, the concentration of p-xylylene fragments in the copolymer tends to increase; the band gap increases from −2.5 to 3.1 eV. Depending on synthesis conditions, the copolymer is characterized by a shift of the fluorescence spectrum maximum that achieves 150 nm in the visible spectral region.

Preparation of nano-thin films from conducting polymer by chemical vapor deposition method and its application for light emitting diodes (LED) and organic solar cells

It was demonstrated that a series of copolymers consisting of p-phenylenevinylene (PV) and p-phenyleneethylene (PE) units could be prepared from a single monomer, p-(methoxymethyl)benzyl chloride, via chemical vapor deposition polymerization (CVDP) method. The composition of the copolymers can be varied by altering the monomer activation temperature; the higher the temperature, the lower the content of the PV units becomes. The sufficient incorporation of the PE units into the poly(p-phenylenevinylene) backbone caused a significant blue shift in both photoluminescence and electroluminescence. The luminescence properties of the copolymer strongly depend on the amount of the PE units incorporated. The external quantum efficiency of the electroluminescence devices having the configuration of ITO/PEDOT-PSS/copolymer/Al-Li increased as the content of the PV units increased in the copolymer, which can be ascribed to the improved confinement of excitons. For the solar cell based on PPV derivative, the effect of the PCBM concentration on the performance of the solar cell is investigated. The results show that there is no interaction between the MEH-PPV and PCBM phase and the photoluminescence of MEH-PPV is quenched by the incorporation of PCBM. With increasing PCBM concentration, the short circuit current density and the overall energy conversion efficiency of the solar cell have been strongly improved.

Molecular Engineering of Organic Sensitizers Containing p-Phenylene Vinylene Unit for Dye-Sensitized Solar Cells

Three organic sensitizers containing bis-dimethylfluorenyl amino donor and a cyanoacrylic acid acceptor bridged by p-phenylene vinylene unit were synthesized. The power conversion efficiency was quite sensitive to the length of bridged phenylene vinylene groups. A nanocrystalline TiO2 dye-sensitized solar cell was fabricated using three sensitizers. The maximum power conversion efficiency of JK-59 reached 7.02%.

Multifunctional Molecular Carbon Materials — From Fullerenes to Carbon Nanotubes

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Multifunctional molecular carbon materials—from fullerenes to carbon nanotubes

This critical review covers the timely topic of carbon nanostructures-fullerenes and carbon nanotubes-in combination with metalloporphyrins as integrative components for electron-donor-acceptor ensembles. These ensembles are typically probed in condensed media and at semi-transparent electrode surfaces. In particular, we will present a comprehensive survey of a variety of covalent (i.e., nanoconjugates) and non-covalent linkages (i.e., nanohybrids) to demonstrate how to govern/fine-tune the electronic interactions in the resulting electron-donor-acceptor ensembles. In the context of covalent bridges, different spacers will be discussed, which range from pure "insulators" (i.e., amide bonds, etc.) to sophisticated "molecular wires" (i.e., p-phenylenevinylene units, etc.). Furthermore, we will elucidate the fundamental impact that these vastly different spacers may exert on the rate, efficiency, and mechanism of short- and long-range electron transfer reactions. Additionally, a series of non-covalent motifs will be described: hydrogen bonding, complementary electrostatics, pi-pi stacking and metal coordination-to name a few. These motifs have been successfully employed by us and our collaborators en route towards novel architectures (i.e., linear structures, tubular structures, rotaxanes, catenanes, etc.) that exhibit unique and remarkable charge transfer features.

Synthesis and optical characterization of new highly luminescent poly(m,p-phenylenevinylene) derivatives

Poly(phenylenevinylene) (PPV) derivatives consisting of alternating m-phenylenevinylene and p-phenylenevinylene units were synthesized using the Wittig and the Wittig-Horner reactions. P1 which was prepared by the Wittig method contained both cis and trans double bonds in significant amounts. P2 was prepared by the Wittig-Horner reaction. Its vinylene configuration was nearly exclusively trans (>95%). The m-linkages reduced the π-conjugation length in an effective way leading to blueshifted UV-Vis absorption and PL emission spectra compared to analogous previously described PPV derivatives without m-phenylenevinylene units. A dependence of solubility, film forming properties, molecular weight and optical spectra on the amount of substituents per repeating unit was observed. A LED fabricated from P2 exhibited low threshold voltages for the electroluminescence (4,3V) and high luminance (1000 cd/m 2 at 10V).

Metathetic incorporation of preformed defined p-phenylenevinylene sequences into commercially available neodymium butadiene rubber

The metathetic incorporation of preformed oligomers of 1,4-divinylbenzene into neodymium butadiene rubber (Nd-BR) was achieved using the stable tungsten carbene W(CHAr′)(NAr)[OCMe(CF 3) 2] 2(THF) (Ar=2,6-Me 2C 6H 3, Ar′= o-MeOC 6H 4), leading to butadiene copolymer containing p-phenylenevinylene units with sequence lengths depending on the length of the oligomer employed. UV—Vis spectroscopic characterization of the copolymers proved that at suitable reaction temperatures the length of p-phenylenevinylene sequences remains intact.

Ring-opening copolymerization of [2.2] paracyclophane-1,9-diene (PCPDE) and cyclooctene (COE); a route to soluble products with p-phenylenevinylene sequences

Cyclooctene (COE) and [2.2] paracyclophane-1,9-diene (PCPDE) can be copolymerized via ring-opening metathesis polymerization (ROMP) with well-defined tungsten carbene. Distribution of p-phenylenevinylene units occurs statistically in manner of Bernoulli. Copolymers with higher content of PCPDE (> 5%) show physical properties influenced by p-phenylenevinylene sequences.

Ring-opening metathesis copolymerization of cyclopentene and [2.2] paracyclophane-1,9-diene: UV-Vis-spectroscopic determination of sequence-length distribution of p-phenylenevinylene units

Ring-opening metathesis copolymerization of cyclopentene (CPE) with [2.2]paracyclophane-1,9-diene (PCPDE) was achieved using the stable tungsten carbene W(CHAr′)(NAr)[OCMe(CF 3) 2] 2(THF) (Ar = 2,6-Me 2C 6H 3, Ar′ = o-MeOC 6H 4) (I).UV-Vis-spectroscopic characterization of the soluble copolymers containing O.1–1% PCPDE proved that PCPDE rings are not only opened to yield [=CH-Phenylene-CHCH-Phenylene-CH=] units, but as the reaction continues these are largely split to give [=CH-Phenylene-CH=] units which are distributed statistically in the polymer chain.