Nonlinear optical properties of an ultrathin film containing porphyrin and poly (phenylenevinylene) units

An ultrathin nanoscopic multilayer film has been fabricated through the electrostatic layer-by-layer self-assembly of water-soluble porphyrin derivative 5,10,15,20-tetrakis-(3,4,5-trihydroxy-phenyl)-porphyrine (DHP) and oppositely charged poly{(2, 5-bis (3-bromotrimethylammoniopropoxy)-phenylene-1, 4-divinylene)-alt-1, 4-(2, 5-bis (2-(2-hydroxyethoxy) ethoxy))phenylenevinylene} (BH-PPV). UV-vis spectra showed a continuous and uniform deposition process of BH-PPV and DHP. The film structure was characterized by small-angle X-ray diffraction measurement. The nonlinear optical property of ultrathin film was characterized by Z-scan technique with laser duration of 8 ns at the wavelength of 532 nm. The BH-PPV/DHP film exhibited nonlinear saturated absorption and strong self-defocusing effect. The nonlinear absorption coefficient and refractive index of the film sample are − 1.9 × 10^− 5 m/W and − 5.57 × 10^− 12 m²/W, respectively.     

Electrical switching and memory phenomena observed in redox dendrimer thin films

This article reports on the fabrication of dendrimer molecular thin film sandwich devices with electrical switching and memory properties. The storage media consists of a redox-gradient N,N?-(1,4-phenylene)bis(N,N′N′N″,N″-pentakis(4-methoxyphenyl)-1,3,5-benzenetriamine) (4AAPD) dendrimer layer sandwiched between two N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) thin films. The TPD thin films are used as potential barriers. The 4AAPD layer acts as a potential well where redox-state changes and consequent electrical transitions of the embedded 4AAPD molecules can be effectively triggered. Experimental results indicate that electrical switching could be reproducibly obtained in the dendrimer sandwiches upon crossing a threshold bias voltage. After switching, the device conductivity can be increased more than three orders of magnitude, which is stable for several days in ambient conditions. This work demonstrates the possibility of using solid-state redox-gradient dendrimer layer as information storage media.     

Nanostructured carbon nanotubes/copper phthalocyanine hybrid multilayers prepared using layer-by-layer self-assembly approach

In this report, we demonstrate a convenient method of fabricating single-walled carbon nanotubes/organic semiconductor hybrid ultrathin multilayers using a layer-by-layer self-assembly approach. Single-walled carbon nanotubes were solubilized by water-soluble cationic alcian blue pyridine variant and anionic copper phthalocyanine-3,4′,4″,4′″-tetrasulfonic acid tetrasodium salt, which were then utilized for electrostatic layer-by-layer multilayer fabrication. The solubilization ability of single-walled carbon nanotubes was studied in water by UV-vis absorption spectroscopy. The composites were highly dispersed owing to the π-π interactions. In situ surface plasmon resonance spectroscopy during the layer-by-layer multilayer fabrication indicated a stepwise increase in reflectivity, indicating the successive formation of nanostructured hybrid ultrathin films. Cyclic voltammetry revealed that the electroactivity of the hybrid film was enhanced by the incorporation of single-walled nanotubes.     

Crystal structure,nonlinear optical and photophysical properties of a novel chromophore constructed with terpyridine,triphenylamine and ethyl cyanocaetate functional moieties

A novel chromophore (Z)-ethyl-3-(4-((4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl) (phenyl) amino)phenyl)-2-cyanoacrylate(3), constructed with triphenylamine moiety as the electron donor (D), 2, 2: 6, 2-terpyridine moiety as an electron acceptor (A), and ethyl cyanocaetate group as an auxiliary electron acceptor (A′), has been designed and synthesized. The crystal structures of 3 and its mediator 2 (4″-(4′-(4-(Diphenylamino) phenyl) aldehyde)-2,2′:6′,2″-Terpyridine), have been determined by single crystal X-ray diffraction analysis. The linear and nonlinear spectra of these chromophores were investigated on the basis of experimental and calculation methods. The two-photon absorption (TPA) cross-sections of 1–3 were determined by a femtosecond open-aperture Z-scan technique. The maximum value of the TPA cross-section (σ) for 3 is 7938.3 GM in DMF solution. However, much weaker two-photon absorption responses were observed at the same condition for chromophore 1 and 2, respectively. The results of the work indicate that, the photophysical properties of the D–A configuration group are influenced largely by the auxiliary moiety (A′) attached.     

Enhancement of green electroluminescence from 2,5-di-p-anisyl-isobenzofuran by double-layer doping strategy

A highly fluorescent compound, 2,5-di-p-anisyl-isobenzofuran (DABF), was synthesized and used as a dopant to fabricate efficient green electroluminescence (EL) devices. The highest occupied molecular orbital level of DABF suggests that it can be excited either by energy transfer or by direct charge trapping mechanism in the EL devices. Three kinds of devices were fabricated based on different emission mechanisms. A double-layer-doped device with DABF doped in both the hole-transporting layer and the electron-transporting layer of the ITO/NPB/TPBI/Mg:Ag device, where ITO is indium-tin-oxide, NPB is (4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl), and TPBI is (2,2′,2″-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole]), exhibited greatly enhanced brightness and efficiency comparing to the single-layer-doped devices. The brightness and efficiency enhancements are attributed to a combined contribution of energy transfer and direct charge trapping mechanisms in the double-layer-doped device.     

Langmuir-Blodgett films of ethylenedithiotetrathiafulvalene derivative containing hydroxyl groups

Langmuir-Blodgett (LB) films of an ethylenedithiotetrathiafulvalene (EDT-TTF) derivative containing hydroxyl groups, 4, 5-bis(11-hydroxyundecylthio)-4′, 5′-ethylenedithiotetrathiafulvalene were deposited without using stabilizer molecules. Doping of the film with iodine leads to oxidation of EDT-TTF moiety as evidenced by UV-vis and IR spectroscopy. X-ray diffraction studies reveal the presence of layered arrangement of the EDT-TTF derivative molecules in the LB film. Cyclic voltammetry studies indicate that the electrochemical oxidation of the EDT-TTF derivative in its LB film is irreversible. Due to the insulating nature of the alkyl chains, only the first layer of the LB film was found to be redox-active. The electrical conductivity of the 25 layer LB film was found to increase by two orders of magnitude on doping with iodine. On equilibrating the doped film in air, the conductivity gradually reduced with time and finally reached the conductivity of undoped film. This reversibility could be because of the steric hindrance induced by the two long alkyl groups. The alkyl chains help in rendering the EDT-TTF molecule amiable to LB film formation but are found to reduce the electrical conductivity of the films and also cause instability of the doped state. The hydroxyl groups at the end of the alky chain impart amphiphilic nature to the molecules and help in stabilizing the film at the air-water interface.     

Layer-by-layer deposition films of copper phthalocyanine derivative; their photoelectrochemical properties and application to solution-processed thin-film organic solar cells

Ultrathin films serving as a light-harvesting and hole-transporting material were fabricated by layer-by-layer deposition of a water-soluble phthalocyanine derivative, copper(II) phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid tetrasodium salt (CuPcTS), and poly(diallyldimethylammonium chloride). The blue-shift of absorption peak and the absorption dichroism of the Q band indicated that CuPcTS molecules in the layer-by-layer films form cofacial dimers or oligomers and that their molecular planes take a three- or two-dimensional orientation in a direction parallel to the substrate depending on a drying process of the film during the deposition. The diffusion constant of hole carriers among CuPcTS molecules in the film was evaluated to be 6.5 × 10^− 11 cm² s^− 1 in an acetonitrile solution from potential step chronoamperometry measurement. Solution-processed thin-film organic solar cells with a triple-layered structure were developed by combining a hole-transporting layer made of poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), a light-harvesting layer of CuPcTS, and an electron-transporting layer of fullerene, in this sequence. Photovoltaic properties of the cells strongly depended on the thickness of CuPcTS films and can be maximized by controlling the thickness at ca. 10 nm.     

可溶性聚对苯乙炔共聚物的合成及性质

采用脱氯化氢法,从１,４－双氯甲基－２,５－二甲基苯（ＢＣＭＤＭＢ）和１,４－双氯甲基－２－甲氧基－５－己氧基苯（ＢＭＭＯＨＯＢ）分别合成出了聚（２,５－二甲基）对苯乙炔和聚（２,５－乙氧基）对苯乙炔;并对ＢＣＭＤＭＢ和ＢＣＭＭＯＨＯＢ的共聚进行了研究。结果表明,随着单体摩尔比率Ｒ（Ｒ＝ＢＣＭＤＭＢ／ＢＭＣＭＯＨＯＢ）的减小,共聚物的溶解性能增大,当Ｒ＝２／９时,共聚物在室温下可溶于氯仿中,成膜性     

Enhancement of the photoelectrochemical response of poly(terthiophenes) by CdS(ZnS) core-shell nanoparticles

We have investigated the photoelectrochemical properties of hybrid films of polythiophenes poly(4,4″dimethoxy-3′-methyl-2,2′:5′,2″ terthiophene) (PDM), poly(4,4″dipentoxy-3′-methyl-2,2′:5′,2″ terthiophene) (PDP), and cadmium sulfide/zinc sulfide (CdS(ZnS)) core-shell nanoparticles. Although CdS(ZnS) nanoparticles present enhanced exciton trapping, light harvesting by hybrid films was enhanced when compared to those of pure PDM and PDP films. This enhancement is explained in terms of electron and hole transfer mechanisms at different excitation wavelengths. The more efficient light harvesting of PDM/CdS(ZnS) when compared to that of PDP/CdS(ZnS) was attributed to its broader absorption spectrum and more efficient electron hopping.     

On the optical limiting and Z-scan of hexamethylin-dotricarbocyanine perchlorate dye

Measurement of Reverse Saturable Absorption (RSA) and optical limiting response in hexamethylin-dotricarbocyanine perchlorate (HITC perchlorate) is reported for the first time and results are compared with C₆₀ and various other dyes under the nanosecond pulse excitation. HITC perchlorate shows better optical limiting than well known C₆₀ and results reveal that photo-excited triplet state absorption cross-section is higher than the ground state absorption cross-section, suggesting the presence of Reverse Saturable Absorption (RSA) in these materials. Limiting threshold for HITC perchlorate is 0.45 J/cm² (at 70% linear transmission), which is lower than C₆₀ (1.2 J/cm², at 70% linear transmission). Z-scan shows negative nonlinearity (self-defocusing) for HITC perchlorate. Ethanol soluble and low cost HITC perchlorate displaying better optical limiting as compared to other conventional materials. Absence of photochemical or photothermal destruction processes in the studied materials suggests their perspective application as a stable optical limiter in photonic devices.     

Influence of electron beam irradiation on nonlinear optical properties of Al doped ZnO thin films for optoelectronic device applications in the cw laser regime

We present the studies on third-order nonlinear optical properties of Al doped ZnO thin films irradiated with electron beam at different dose rate. Al doped ZnO thin films were deposited on a glass substrate by spray pyrolysis deposition technique. The thin films were irradiated using the 8 MeV electron beam from microtron ranging from 1  kG y to 5  kG y. Nonlinear optical studies were carried out by employing the single beam Z-scan technique to determine the sign and magnitude of absorptive and refractive nonlinearities of the irradiated thin films. Continuous wave He–Ne laser operating at 633 nm was used as source of excitation. The open aperture Z-scan measurements indicated the sample displays reverse saturable absorption (RSA) process. The negative sign of the nonlinear refractive index n₂ was noted from the closed aperture Z-scan measurements indicates, the films exhibit self-defocusing property due to thermal nonlinearity. The third-order nonlinear optical susceptibility χ(3) varies from 8.17 × 10⁻⁵ esu to 1.39 × 10⁻³ esu with increase in electron beam irradiation. The present study reveals that the irradiation of electron beam leads to significant changes in the third-order optical nonlinearity. Al doped ZnO displays good optical power handling capability with optical clamping of about ∼5 mW. The irradiation study endorses that the Al doped ZnO under investigation is a promising candidate photonic device applications such as all-optical power limiting.     

Enhanced performance in organic light-emitting diodes by sputtering TiO2 ultra-thin film as the hole buffer layer

Organic light-emitting diodes were prepared using titanium oxide (TiO₂) ultra-thin film by RF magnetron sputtering as the hole buffer layer. The device configuration is ITO/TiO₂/N-N′-diphenyl-N-N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine/tris(8-quinolinolato)-aluminum/LiF/Al. The maximum luminous efficiency for the 1.2 nm TiO₂ device is increased by approximately 46% (6.0 cd/A), in comparison with that of the control device (4.1 cd/A). The atomic force microscopy shows that with the insertion of TiO₂ buffer layer, the roughness of ITO surface decreases, which is favorable to improve the device luminance and increase the device lifetime. The mechanism behind the enhanced performance is that the TiO₂ layer enhances most of the holes injected from the anode and improves the balance of the hole and electron injections.     

Preparation of active layers in polymer solar cells by aerosol jet printing

Active layers of polymer solar cells were prepared by aerosol jet printing of organic inks. Various solvents and additives with high boiling points were screened for the preparation of high-quality polymer films. The effects on device performance of treating the films by thermal and solvent vapor annealing were also investigated. The components of the solvent were important for controlling the drying rate of the liquid films, reducing the number of particle-like protrusions on the film surface, and realizing high molecular ordering in the polymer phases. The optimized solar cell device with poly(3-hexylthiophene) and a C(60) derivative showed a high fill factor of 67% and power conversion efficiency of 2.53% without thermal annealing. The combination of poly[N-9-heptadecanyl-2,7-carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-diethylhexyl-2,5-dihydropyrrolo-[3,4-]pyrrole-1,4-dione] and a C(70) derivative led to power conversion efficiency of 3.92 and 3.14% for device areas of 0.03 and 1 cm(2), respectively.     

Crystal structure of 1,4?-Dimethoxy-4,1′:4′,1″:4″,1?-quaterphenylene

1,4?-Dimethoxy-4,1′:4′,1″:4″,1?-quaterphenylene crystallizes in space group Pbca with a = 7.4989(5) Å, b = 6.1544(3) Å, c = 40.359(2) Å, V = 1862.6(2) ų, and Z = 4. In the crystal structure the molecules are arranged in layers, which are stacked in the direction of the crystallographic c-axis. The torsion between the molecules in a layer amounts to 69.06° resulting in a herringbone structure. The phenyl rings of the molecule are approximately planar with a torsion angle of 4.64° between the outer and the center rings. UV/Vis transmission measurements of the colorless crystal reveal a band gap of ~ 3.2 eV.     

Regiospecifity-driven self-assembly of methyl substituted quaterthiophenes at surface

A study on the self-assembly of regiochemically differentiated methyl substituted quaterthiophenes is presented. Hierarchical self-assembly of 5,4′,3″,5?-tetramethyl-2,2′:5′,2″:5″,2?-quaterthiophene molecule, 7 leads to unprecedented fluorescent superstructures on glass and mica surfaces made by reiteration of a peculiar V-shaped recurring motif that extended at least for three generations.     

Thermally stable triphenylene-based hole-transporting materials for organic light-emitting devices

The thermally stable hole transport layer (HTL) materials, 1,4-bis[(N,N′-di(2-naphthyl)-N,N′-diphenyl)aminophenyl]triphenylene (NPAPT) and 1,4-bis[(N,N′-di(2-naphthyl)-N,N′-diphenyl) aminophenyl]-2,3-diphenyl triphenylene (NPAPPT), were synthesized and the device performances of the organic light-emitting diodes (OLEDs) with NPAPT and NPAPPT as a hole transport layer were investigated. The glass transition temperatures of NPAPT and NPAPPT could be enhanced to 153 °C and 157 °C by the introduction of a rigid triphenylene backbone in the main chain. The use of NPAPT and NPAPPT as a HTL for OLEDs lowered the driving voltage and enhanced the light-emitting efficiency. The power efficiencies of triphenylene-based devices with tris(8-hydroxyquinoline)aluminum as an emitting material could be improved by 20% compared with that of N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine based devices.     

Photo-induced molecular alignment of azo dye derivative

Photo-induced molecular alignment behavior of azo dye derivative (sodium 4,4′-bis (4-hydroxy-3-carboxy-phenylazo) benzidine-2,2′-disulphonate: SD1) was investigated using SD1 film prepared by spin-coating method. As-prepared SD1 film was composed of an amorphous layer with smooth surface. Upon linearly polarized UV light irradiation, the film surface was roughened slightly and X-ray diffraction measurement and transmittance electron microscope observation indicated that SD1 molecules crystallized with orientation. Molecular plane of SD1 aligned parallel to the substrate surface along the normal to the polarization direction of irradiated UV light through the trans-cis and cis-trans isomerizations of the azobenzene chromophore.     

Nonlinear optical investigations on Al doping ratio in ZnO thin film under pulsed Nd:YAG laser irradiation

In the present study, it has been reported on the effect of Al doping on linear and nonlinear optical properties of ZnO thin films synthesized by spray pyrolysis method. The structural properties of ZnO thin films with different Al doping levels (0–4 wt%) were analyzed using X-ray diffraction (XRD). The results obtained from XRD analysis indicated that the grain size decreased as the Al doping value increased. The UV–Vis diffused refraction spectroscopy was used for calculation of band gap. The optical band gap of Al-doped ZnO (AZO) thin films is increased from 3.26 to 3.31 eV with increasing the Al content from 0 to 4 wt%. The measurements of nonlinear optical properties of AZO thin films have been performed using a nanosecond Nd:YAG pulse laser at 532 nm by the Z-scan technique. The undoped ZnO thin film exhibits reverse saturation absorption (RSA) whereas the AZO thin films exhibit saturation absorption (SA) that shows RSA to SA process with adding Al to ZnO structure under laser irradiation. On the other hand, all the films showed a self-defocusing phenomenon because the photons of laser stay on below the absorption edge of the ZnO and AZO films. The third-order nonlinear optical susceptibility, χ⁽³⁾, of AZO thin films, was varied from of the order of 10^?5–10^?4 esu. The results suggest that AZO thin films may be promising candidates for nonlinear optical applications.     

White polymer light-emitting diodes based on poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) doped with a poly(p-phenylene vinylene) derivative

Efficient white polymer light-emitting diodes based on the polymer blend of poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) doped with poly{2-[3′,5′-bis(2?-ethylhexyloxy) benzyloxy]-1,4-phenylenevinylene}-co-poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) were fabricated. The electroluminescence (EL) spectrum is easily controlled by changing the dopant concentration. A white light emission was realized on the device with the dopant concentration of 0.194‰ and the emission light is less sensitive to the applied voltage in a wide voltage range. The maximum luminance and the maximum EL efficiency of the single-layer device were 2330 cd/m² and 0.29 cd/A, respectively. By introducing an Alq₃ layer as an electron transporting and hole blocking layer, the overall performance of the double layer device was dramatically improved, the maximum luminance and the maximum EL efficiency reached 3300 cd/m² and 2.37 cd/A, respectively.     

A new precursor to electroconducting conjugated polymers: synthesis and opto-electrical properties of luminescent devices based on these PPV derivatives

A variety of novel conjugated PPV-related polymers have been synthesised using a synthetic approach that allows the tailoring of the chemical structure of the polymer backbone. In this way we have been able to prepare not only poly(p-phenylene vinylene) (PPV) (1), but also some electron-rich PPV derivatives such as poly(2,5-dimethyl-1,4-phenylene vinylene) (PDMetPV) (2) and poly(2,5-dimethoxy-1,4-phenylene vinylene) (PDMeoPV) (3) and PPV derivatives with an enlarged aromatic system, poly(4,4′-bisphenylene vinylene) (PBPV) (4) and poly(2,6-naphtaln vinylene) (PNV) (5). These materials were characterised in terms of molecular weight (GPC), elimination temperature (TGA, FT-IR), optical properties (UV-VIS), photoluminescence (PL) and device characteristics of the LEDs.