Elisabeth Klemm

Synthesis and Properties of Novel Well-Defined Alternating PPE/PPV Copolymers

The polycondensation reactions of luminophoric dialdehydes 5 and bisphosphonates 3 provide a new type of π-conjugated polymers 7 with well-defined structure (—Ar—C≡C—Ar—C≡C—Ar—CH=CH—Ar— CH=CH—)n , which was confirmed by NMR, infrared and UV-Vis spectroscopy. High molecular weight (Mw up to 500 000 g/mol), thermostable, soluble and transparent film-forming materials were obtained. The grafting of large alkoxy side chains enhances the solubility and processability of the new compounds. The incorporation of triple bonds into the PPV backbone increases the oxidation and reduction potentials, thus making these polymers potentially good electron-transporting materials if used in light-emissive-diode devices. The polymers are photoconductive and show very good photoluminescent properties in solution as well as in the solid state. Identical absorption (λmax,abs = 445 nm) and emission (λmax,em = 490 nm) behaviors were observed for all polymers in solution (CHCl3), resulting in a fluorescence quantum yield of 70%, but the photophysical behavior in solid state was side group dependent. Polymer 7ac shows a very large Stokes shift (137 nm) and lower fluorescence quantum yield (19%), whereas 7aa, 7bb and 7ab, consisting of side groups equal to or longer than O(CH2)11CH3 are characterized by smaller Stokes shifts (around 30 nm) and comparatively higher fluorescence quantum yields (34 to 44%).

Conjugated polymers with 2,2′-bipyridine and diethinylenebenzene units: absorption and luminescence properties

Alternating oligomers and polymers consisting of 2,2′-bipyridine and diethinylenebenzene units and corresponding model compounds were synthesized and investigated in dilute solutions by absorption spectroscopy and by stationary and time-resolved emission spectroscopy. The strictly linear (rod-like) π-chain oligomers/polymers were compared with the angularly linked oligomers/polymers and with related model compounds. The model compounds which already show the essential spectroscopic properties of the oligomers/polymers consist of three (hetero)aromatics linearly connected by two diethenylene groups. These models exhibit fluorescence quantum yields close to unity and short fluorescence decay times around 1 ns. Fluorescence anisotropy and rotational relaxation times are consistent with the Stokes–Einstein equation and the Perrin equation. The absorption and emission spectra of the polymers and their radiative rate constants determined by fluorescence quantum yield and lifetime and according to the Strickler/Berg equation show a conjugation length of one to two repetition units. The conjugation along the chain is stronger in linear than in angular polymers and stronger in alkoxy-substituted than in unsubstituted polymers. In angular polymers at least two different emitting segments were found. The shortened mean lifetimes and the reduced fluorescence quantum yields and anisotropies of the oligomers/polymers indicate an additional radiationless deactivation channel which is opened by energy migration along the chain. Rates of energy transfer calculated for linear and angular polymers correlate with rates of radiationless deactivation. Copyright

Optical and ESR studies on poly(3-alkylthiophene)/fullerene composites for solar cells

Optical measurements (absorption coefficients, photoluminescence) and light-induced electron spin resonance ( LESR ) technique were used to study photoinduced charge generation and charge transfer in conjugated polymer / fullerene composites. The quenching of fluorescence by C 60 gives a first indication of an effective charge transfer from P3DDT to C 60 . At 77 K two kinds of LESR signals were identified, one of polaron (P +. ) on the polymer chain and one of radical anion (C 60 -.) of fullerene. The relaxation rates of generated paramagnetic centres were estimated by microwave power saturation studies. Moreover for the first centre (P +. ) two contributions to the LESR signal were observed: a permanent one and a photoinduced one. The light intensity dependence of the photoinduced P +. and C 60 -. ESR signals is of bimolecular type (I 0.5 ). Solar cells based on P3DDT/ C 60 and P3DDT/ PCBM are prepared and characterised.

Investigation of quantitative SiOH determination by the silane treatment of disperse silica

Abstract The concentration of free silanol groups after treatment of disperse silica with 3-(trimethoxysilyl) propyl methacrylate (A-174) was studied in substance pellets by IR spectroscopy. There are significant structural differences between the silica which was treated with gaseous silane and that which was treated in solution. By gas-phase silanation a monolayer of methacrylate was formed, while in solution a polylayer was formed. This conclusion results from a determination of carbon content by means of elemental analysis in addition to an IR spectroscopic determination of the SiOH concentration remaining in the silane-treated silica surface. Silylation by means of hexamethyldisilazane was found to be a convenient method to determine the concentration of these residual silanol groups.

Characterization of potential donor acceptor pairs for polymer solar cells by ESR, optical, and electrochemical investigations

In this report new PPV-PPE copolymers DE 69, DE 11 were compared with the state of the art materials MDMO-PPV and poly(3-alkylthiophenes) (P3DDT, P3OT). The optical band gap energy of the two copolymers DE 69, DE11 is somewhat higher than that one of MDMO-PPV. The electrochemical band gap was found to be lower for DE 69, DE11 related to for MDMO-PPV. The absorption coefficient of the new PPV-PPE copolymers is higher than for MDMO-PPV but in the same order of magnitude. Films from composites of MDMO-PPV/PCBM and DE69 or DE11 with PCBM show a clear photoluminescence quenching effect. At 77 K two kinds of LESR signals were identified, one of polarons (P+.) and one of radical anions of fullerenes. The LESR results show strong differences in kinetics between the separation and recombination processes of photoexcited charge carriers. The relaxation rates of paramagnetic centers were estimated by microwave power saturation experiments. Photovoltaic devices were prepared under ambient conditions on flexible PET-ITO foils with MDMO-PPV/ PCBM (1:3 wt. %) with ηAM1.5 = 2.4% and DE 69/ PCBM (1:3 wt. %) with ηAM1.5 = 1.75% (A=0.25 cm2, Pin = 100 mW/cm2).

MEH-PPV and dialkoxy phenylene vinylene copolymers. Synthesis and lasing characterization

We report the use of the HORNER-type polycondensation for the synthesis of MEH-PPV and some strictly alternating copolymers (MEH-DOO-PPV) and regular terpolymers (TPD-M3EH-PPV) all of which having MEH-PV subunits. High molecular weight, solution processable waveguiding materials of increased Tg (80 - 160 °C) were achieved. For comparison also statistical dialkoxy-PPV copolymers and blends of MEH-PPV and DPOP-PPV have been studied. All these luminescent polymers exhibit travelling-wave lasing (amplified spontaneous emission-ASE) in neat films. Emission occurs in the red spectral region. ASE is achieved in waveguiding neat films by picosecond pulse excitation. The lasing is identified by spectral narrowing, temporal shortening, and threshold behaviour of light emission.

Anisotropic optical properties of conjugated polymer and polymer/fullerene films

Abstract We have studied the optical properties of thin films of substituted polyphenylene ethynylene/polyphenylene vinylene copolymers. Two novel materials of this type were investigated: poly[1,4-phenyleneethylene-1,4-(2,5-didodecyloxylphenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-didodecyloxyphenylene)ethene-1,2-diyl] (DE15) and poly[1,4-phenyleneethylene-1,4-(2,5-di(2-ethylhexyloxy)phenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-di(2-ethylhexyloxy)phenylene)ethene-1,2-diyl] (DE42). Both pure and fullerene-doped films were prepared by spin coating onto silicon substrates. Their optical properties were obtained by determination of the dielectric function by means of spectroscopic ellipsometry. A large anisotropy of the dielectric function was found both for the pure materials and for their composites with 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM) with the optical axis normal to the film surface. The origin of this phenomenon and the influence of the anisotropy on the performance of plastic solar cells are discussed. The optical properties of such DE15/PCBM and DE42/PCBM composites are successfully described in terms of the Bruggemann effective medium approximation.

Investigation on radical polymerization of 2-methylene-1,3-dioxolanes and 2-methylene-1,3-oxazolidines

The reaction of 2-methylene-1,3-dioxolanes and 2-methylene-1,3-oxazolidines with benzoyl peroxide (acceptor radical) and with N-ethylmaleimide (acceptor) was investigated. It was shown that benzoyl peroxide adds to monomers 1a and 1b, giving the corresponding linear diester amides 1a and 1b respectively. The oxazolidine 1c adds benzoyl peroxide, without ring opening, by addition to the exomethylene group. Together with N-ethylmaleimide the oxazolidines 1a or 1b produce deep-colored charge transfer complexes, resulting in high molecular poly-N-ethylmaleimides probably via a radical mechanism. The 1,3-dioxolanes 2a and 2b were radically polymerized to produce polyacetals by vinyl polymerization. 2c was polymerized to produce randomly containing acetal units and ester units. The mechanism of polymerization of 2e is complex, affording polymers of nonuniform character. 2-Methylene-4-phenyl-1,3-dioxolane polymerization leads to polyester as the main structure, and to a lesser degree polyacetal structures. The chemical structures of the polymers were confirmed by NMR spectra and elemental analysis.

Unusual addition by the thiol-ene photopolymerization

SummaryThe photoaddition of aromatic dithiols (1) onto aliphatic alkenes (2a-2c) yielding polysulfides is studied. Polysulfides having anti-Markownikow and Markownikow moieties in the main chain are obtained by the reaction of dithiols (1) with ethyleneglycoldivinylether (2a). The unusual Markownikow orientation is explained on the basis of a radical cation intermediate.

Travelling-wave lasing of triphenylamine-based poly(phenylene vinylene)

Travelling-wave lasing (amplified spontaneous emission) is reported for a series of triphenylamine-based poly(phenylene vinylene)-copolymers (TPA-PPVs) that have different substituents R at the vinylene double bond (R=H, CN, methoxyphenyl) and slight modifications in the alkoxy side chains (dioctyl, MEH). Wave-guiding neat films on glass substrates are transversally pumped with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). The laser emission occurs in the wavelength region between 515 and 560 nm. Optical parameters (refractive index spectra and absorption coefficient spectra) and spectroscopic parameters (absorption cross-section and stimulated emission cross-section spectra, fluorescence quantum distributions, internal fluorescence quantum yields, and fluorescence lifetimes) of the polymers are determined. The lumophore size is extracted from fluorescence lifetime and fluorescence quantum yield measurement and found to be approximately one repeat-unit. The lasing is characterised by measuring the spectral narrowing, the temporal shortening, the laser output energy versus input pump pulse energy, and the effective length of amplification. The obtained lasing parameters compare favourably well with those of previously studied MEH-PPV and TPD-PPVs. The laser threshold pump pulse energies are <650 nJ. The effective lengths of amplification are about 1 mm. The spectral widths of emission are <9 nm.