H. J. Geise

CHEMIRESISTIVE SENSORS OF ELECTRICALLY CONDUCTING POLY(2,5-THIENYLENE VINYLENE) AND COPOLYMERS: THEIR RESPONSES TO NINE ORGANIC VAPOURS

Abstract An apparatus and a method are described to test the sensing capacities, in the context of an electronic nose, of conductimetric sensors for organic vapours. Such a sensor was constructed using iodine doped poly(2,5-thienylene vinylene) (PTV) and tested in detecting nine ‘representative’ vapours. Responses after 1 min exposure are high and mostly linear with respect to concentrations. Recoveries, however, using 4 min of clean air, are only partial. Yet, responses and recoveries are reproducible and characteristic for a particular vapour. Partial methanol elimination from the methoxy-precursor polymer of PTV leads to new sensing materials with different selectivities. Also discussed are: (i) influence of the humidity of the carrier gas (air), (ii) possible causes of observed baseline-drift and (iii) possibilities of regeneration of the materials after prolonged use. It is concluded that PTV and derivatives show promise as active materials in sensor arrays.

Preparation and electrical conductivity of blends consisting of modified Wittig poly(para-phenylene vinylene), iodine and polystyrene, polymethyl methacrylate or polycarbonate

Eight poly(2,5-dialkoxyphenylene vinylene p-phenylene vinylene) and two poly(2,5-dialkoxyphenylene vinylene-2,5-thienylene vinylene) copolymers were synthesized via the Wittig reaction. Their solubility is more than 15 g/100 ml CHCl3, provided that the copolymer contains some cis-ethylene links and has alkoxy groups larger than OCH3. From solutions consisting of a conjugated copolymer, iodine and a host polymer (e.g., polystyrene, polymethyl methacrylate or polycarbonate), black electrically conducting films can be cast. The conductivity (0.1–2 Ω−1 cm−1) is very stable at room temperature and depends largely on the ratio of the components, somewhat on the nature of the copolymer, but not on the nature of the host. The stability is high, because the conducting species are encapsulated by the matrix of the host polymer.

Single-layer organic light-emitting diode with 2.0% external quantum efficiency prepared by spin-coating

We present a highly efficient organic light-emitting diode with a single-layer structure and air-stable Al cathode. The device consists of a spin-coated thin film of polystyrene in which rubrene and Alq3 molecules are dispersed. By optimizing the concentration of Alq3 and rubrene, a maximum external quantum efficiency of 2.0% photons/electron and a current efficiency of 8.5 cd/A at 22 V could be achieved in the device. The high electroluminescence efficiency is attributed to a combination of energy transfer from Alq3 to rubrene, and direct carrier trapping and recombination on rubrene, followed by an efficient fluorescent emission from rubrene.

The molecular structure of gaseous allyl alcohol determined from electron diffraction, microwave, infrared and geometry-relaxed ab-initio data

The structure of allyl alcohol was determined in the gas phase from electron diffraction, microwave and infrared data in combination with constraints obtained from ab-initio calculations (4-21G basis set). At 300 K the rotamer composition consists of 57(6)% (sp, sc) and 43% (ac,—sc) forms. The molecular orbital constrained joint analysis yields the following geometries (rg-distances, rα-angles): (sp, sc) conformer r(CC) = 1.334(5) A, r(CC) = 1.500(8) A, r(CO) = 1.425(5) A, ∠(CCC) = 124.7 (1.5)°, /o<(CCO) = 113.6(1.1)° and torsion (CCCO) = 22(9). (ac,—sc) conformer r(CC) = 1.335(5) A, r(CC) = 1.496(8) A, r(CO) = 1.431(5) A, ∠(CCC) = 125.3(1.5)°, ∠(CCO) = 112.2(1.1)° and torsion (CCCO) = 1.222(1.5)°. The average r(〈CH〉) = 1.096(6) A and r(OH) = 1.028(27) A in both rotamers.

Electrical conductivity of iodine-doped poly(para-phenylene vinylene) model compounds blended with polystyrene

Abstract We synthesized 15 trimeric poly(para-phenylene vinylene) (PPV) model compounds containing various substituents in the phenyl rings and characterized the structure by 1H and 13C NMR. The trans,trans-isomers are almost planar, while those containing methoxy groups in the central ring exhibit the ortho-methoxy-anti conformation. For cis,cis-isomers a non-planar form is proposed in which the three phenyl rings are perpendicular to the plane of the olefinic links. The solubilities were found to increase by (i) CH3 and OCH3 groups in the central ring, (ii) heavy OCH3 substitution in the terminal rings and (iii) cis-configurations of the ethylenic bonds. The more soluble compounds (> 2 g/100 ml CHCl3) were blended with polystyrene and iodine to give black electrically conducting films. Their conductivity ( 0.1-1 Ω −1 cm −1 ), which varies with the ratio polystyrene:model compound:iodine, is very stable at room temperature.

Efficient blue polymer light-emitting diodes from a novel biphenyl derivative

Abstract A novel blue-emitting soluble biphenyl derivative 4,4-bis[2-(3,4,5-trimethoxyphenyl)vinyl]biphenyl (TMPVB) was synthesized. It was dispersed into poly[2-(6′-cyano-6′-methylheptyloxy)-1,4-phenylene] (CNPPP), to form a blend film as the emissive layer in organic light-emitting diodes (OLEDs). Efficient Forster energy transfer from CNPPP to TMPVB molecules is proved in this film. We find that the emission is dominantly from the TMPVB molecules during the device operation. The best performing device has a peak external quantum efficiency of 1.2%, which is comparable to the best results obtained for devices based on blue-emitting polymers. Forster energy transfer and carrier trapping are considered to be the main mechanisms for exciton formation on TMPVB molecules under electrical excitation.

Spectroscopy and conductivity of the separate cis and trans-isomers of a PPV oligomer

2,5-Dimethoxy-1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene was synthesized using the Wittig reaction and the mixture of the three possible cis,trans-isomers was separated by preparative HPLC. The spectroscopy, conductivity and solubility of the isomers are presented. NMR measurements provided information about the molecular structure and dynamics of the separate isomers.

Conductivity and electron density of undoped model compounds of poly(phenylene vinylene)

Fourteen derivatives of trans,trans-1,4-bis[2-phenylethenyl]benzene were synthesized by Wittig reactions as model compounds of poly(paraphenylene vinylene). Structure, configurational homogeneity and absence of ionic impurities were controlled by mass spectrometry, infrared and neutron activation analysis, respectively. Crystallographic unit cell parameters were obtained from X-ray powder patterns and measurements of electrical conductivities were performed on undoped samples. The data of four more compounds containing one or more thiophene rings instead of phenyl rings were added from the 3iterature. If NO2 and Cl groups are excluded from the electron count a good linear correlation is found between the logarithm of the conductivity and the non-σ electron density (e A -3). The position of the substituents, on the central or on the terminal ring, also plays a role in as much as it affects the molecular volume of the compound but not the non-σ electron density. The correlation between the logarithm of the conductivity and the absorption coefficient of the longest wavelength of UV absorption identifies the π electrons in the chromophore as the principal charge carriers.

Solid state cross polarization/magic angle spinning 13C NMR investigation of alkoxy-substituted poly(p-phenylene vinylene) homo- and copolymers

The comparison of solid state and solution 13C NMR spectra of the model compound trans, trans-2,5-dimethyl-1,4-bis(2-(3,4,5-trimethoxyphenyl)ethenyl) benzene showed that essentially the same conformation exists in both states. The molecule is almost planar exhibiting an ortho-methyl-syn form. For the cis, cis-isomer a non-planar form is proposed in which the three phenyl rings are perpendicular to the plane containing the double bonds. The information from the model compounds was used to analyse the cross polarization/magic angle spinning (CP-MAS) 13C NMR spectra of a series of 2,5-di-alkoxy-substituted homo- and copolymers of poly(para-phenylene vinylene) (PPV). For the all-trans polymers we propose the ortho-alkoxy-anti conformation and for the cis-containing polymers we propose the conformation in which the phenyl rings are perpendicular to the olefinic plane. The first atom of the alkoxy chain is always in the phenyl plane. Estimates of the degree of polymerization gave 9<n<18 for these compounds synthesized via Wittig reactions. The value agrees well with results of other methods. Measurements of relaxation times (T1p(H), T2(H), TCH, T2(C)) were performed on a selection of the compounds. The T1p(H) values vary linearly with the length of alkoxy chain. The molecular dynamics of alkoxy-substituted PPVs is dominated by a segmental mobility of these chains, with minimal interference from other molecular motions. Methyl rotation is more rapid in long and unbranched chains than in short, or branched ones. A rationalization of specific densities observed in these polymers can be given in terms of the observed molecular mobility.

Characterization and d.c. conductivity of poly(p-phenylene vinylene) films doped with FeCl3

Films of poly(p-phenylene vinylene) (PPV) were synthesized via pyrolysis of a water-soluble sulphonium polyelectrolyte, derived from p-xylylene bis(tetrahydrothiophenium chloride). The thermal elimination reaction of the sulfide group was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure of the completely converted, unoriented PPV films was characterized using IR spectroscopy and X-ray diffraction. Doping of PPV films with FeCl3 in nitromethane was studied. Films of different dopant concentrations y = (FeClx/C8H6) were obtained by varying the molarity of the doping solutions and by changing the immersion time. The electrical conductivity of FeCl3-doped PPV films was measured as a function of concentration and doping time. The d.c. conductivity reached a maximum value of σdc = 35 ohm−1 cm−1.