New 4,7-dithienebenzothiadiazole derivatives with cyano-vinylene bonds: Synthesis,photophysics and photovoltaics

Two conjugated compounds P and T which contained 4,7-dithienbenzothiadiazole as central unit and terminal phenyl or thiophene rings, respectively, connected via a cyano vinylene bond, were synthesized. The stable intramolecular charge-transfer status makes possible the compounds to increase the optical absorption band. These compounds exhibited excellent film-forming properties which allowed the preparation of homogenous thin films by the spin-coating technique. The compounds were stable up to about 300 °C and showed glass transition temperatures of 64 and 50 °C. The absorption spectra were broad and extended from about 300 to 700 nm. They emitted yellow-orange light in solution with photoluminescence (PL) maximum at 574, 578 nm and reddish light in solid state with PL maximum at 663, 702 nm. Organic solar cells based on T (or P) as donor and C60 derivative as acceptor exhibit power conversion efficiencies (PCEs) of 0.11% (or 0.09%) under one sun of AM1.5 illumination.     

Luminescent monomer of substituted tetrastyrylpyrene and poly(p-phenylenevinylene) derivative with pyrene segments: Synthesis and photophysics

1,3,6,8-Tetrabromopyrene reacted with 1-tert-butyl-4-vinylbenzene to afford the luminescent monomer M. In addition, the reaction of 1,6-dibromopyrene with 1,4-didodecyloxy-2,5-divinylbenzene yielded a poly(p-phenylenevinylene) (PPV) derivative P. M dissolved partially in chloroform and completely in THF. P showed an excellent solubility being readily soluble in common organic solvents. M emitted intense blue–green light in THF solution with photoluminescence (PL) maximum at 507 nm and green–yellow light in thin film with PL maximum at 570 nm. Polymer P displayed in THF a PL maximum at 464 nm and a shoulder at 513 nm. Thin film of this polymer exhibited PL maximum at 572 nm. Both M and P emitted from excimers in solid state. The PL emission quantum yields in THF were 0.35 for M and 0.32 for P. The emission maximum of M thin film was red shifted by 9 nm after annealing at 150 °C for 24 h as a result of conformational changes of the molecules with increase in temperature.     

Alternating phenylenevinylene and thienylenevinylene copolymers with cyano groups: Synthesis,photophysics and photovoltaics

Two alternating conjugated copolymers TTP and TTT were synthesized by Heck coupling of 2,3-bis-(5-bromothiophene-2-yl)acrylonitrile with 1,4-dihexyloxy-2,5-divinylbenzene and 3-hexyl-2,5-divinylthiophene, respectively. The absorption spectra of the copolymers in THF solution showed three maxima around 270, 370 and 460 nm with optical band gaps of 2.30–2.34 eV. The electrochemically estimated band gaps of copolymers were 2.04–2.10 eV. The thin film absorption spectra were broad and extended about from 250 to 600 nm with a long wavelength maximum near 470 nm. The copolymers emitted yellow-orange light with maximum at 528–551 nm in THF solution and 567–610 nm in thin film. TTP showed the most red shifted emission maximum between the copolymers. The performance of the photovoltaic cells which were fabricated using blends of the copolymers with 6,6-phenyl C61-butyric acid methyl ester (PCBM) (1:1 and 1:4, w/w) as well as pure copolymers was investigated.     

Synthesis of selenopheno[3,2-c]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers

Various selenopheno[3,2-c]thiophene (STh) derivatives functionalized by 4-n-butylphenyl, 4-n-pentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and visible-near infrared (Vis-NIR) spectrophotometry. When compared with polymers of thieno[3,4-b]thiophene, poly(selenopheno[3,2-c]thiophene) (PSTh) was more easily oxidized by ca. 0.2 V (higher HOMO). On the other hand, the bandgap of electrochemically prepared PSTh varied with substituents, but showed similar values to those of poly(thieno[3,4-b]thiophene). While the resonance effect of phenyl substituents slightly lowered HOMO, the combination of resonance and inductive effects decreased LUMO more effectively, resulting in the lowest bandgap of 0.91 eV for 4-tert-butylphenyl functionalized PSTh.     

Poly(fluorene)s and poly(p-phenylene)s with pyrenyltriazine segments: synthesis and photophysics

A Friedel-Crafts reaction between cyanuric chloride and pyrene afforded 2,4-dichloro-6-(pynen-1-yl)-1,3,5-triazine (1). This reacted with 4-bromophenol to yield 2,4-bis(4-bromophenoxy)-6-(pyren-1-yl)-1,3,5-triazine (2). A series of random copolymers PF-Pyr with various compositions were prepared by Suzuki polycondensation from 2,7-dibromo-9,9-di-2-ethylhexylfluorene (3) and 2. In addition, a series of random copolymers PP-Pyr were similarly prepared from 1,4-dibromo-2,5-didodecyloxybenzene (5) and 2. Solutions of copolymers PF-Pyr emitted blue light with photoluminescence (PL) maximum at 414–444 nm. Thin films of these copolymers emitted intense green light with PL maximum near 520 nm. An efficient energy transfer took place in thin films from the fluorene to the pyrenyltriazine segment even the content of the latter in copolymer was 0.5 mol%. Copolymers PP-Pyr behaved as blue light-emitting materials both in solution and solid state. Their PL maximum was red shifted with increasing the pyrenyltriazine content in copolymer. The PL quantum yields in solution were 0.42–0.56 for PF-Pyr and 0.27–0.35 for PP-Pyr.     

Electroluminescence in conjugated polymers: excited states in cyano-derivatives of poly(p-phenylenevinylene)

Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics both as electroluminescent diodes and as photoconductive diodes. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and, thus, bring about electron-hole capture to generate excitons at this interface. We present results on the photophysical properties of the cyanosubstituted poly(phenylenevinylene)s which provide electron transport layers if these heterostructure devices; we find evidence for strong interchain interactions (excimer formation) for the neutral excited states in these polymers, with strong red shifts of the luminescence, though preserving high photoluminescence efficiencies.     

Synthesis, photophysics and photovoltaics of alternating vinylene-copolymer and model compound containing triphenylamine moieties along the backbone

A soluble alternating vinylene-copolymer P and the corresponding model compound M were synthesized by Heck coupling. They contained triphenylamine moieties and cyano-substituted olefinic bonds along the backbone. Both samples were stable up to about 300 °C and afforded char yields of 76–89% at 800 °C. Their glass transition temperature (T_g) was 72–78 °C and the absorption maximum was located at 307–366 nm with optical band gaps of 2.19–2.35 eV. They emitted yellow-orange light with photoluminescence (PL) maximum at 560–576 nm. Bulk heterojunction photovoltaic cells with the configuration of ITO/PEDOT:PSS/Sample:PCBM(1:1, w/w)/LiF/Al were fabricated. The PCE of devices based on P and M reached 0.09% and 0.07%, respectively.     

Poly(p-phenylenevinylene) by chemical vapor deposition: synthesis,structural evaluation,glass transition,electroluminescence, and photoluminescence

Poly(p-phenylenevinylene) (PPV) was prepared via chemical vapor deposition (CVD) by vapor phase pyrolysis of α,α′-dihalogenated p-xylenes or α,α-dihalogenated p-xylenes. Differences in the product composition obtained are discussed. The advantages for the film formation as well as the limitations are reported with respect to the fabrication of light-emitting diode (LED) cells. PPV films were characterized by various techniques. They show a glass transition temperature at 220 °C, a hypsochromic shift of the S₁ → S₀ transition and higher degree of disorder as compared to ordinary PPV. The current-voltage characteristics of LED cells with PPV prepared via CVD show an unusually high rectification ratio. In contrast to PPV annealed at lower temperatures the formation of an unidentified defect structure was detected by ¹³C CP-MAS/TOSS spectroscopy with PPV films annealed at higher temperatures.     

Synthesis,spectroscopy and electrochemistry study on a novel di-silyl substituted poly(p-phenylenevinylene)

A novel di-silyl substituted poly(p-phenylenevinylene) (PPV) derivative is synthesized through the Gilch route. The polymer is solution processable and shows high thermal stability in air. UV–Vis spectra for the film and solution samples illustrate some different features, in which the absorption peak corresponding to the π–π* transition of the delocalized electrons along the polymer back bone is red shifted and broadened in the film state compared to the solution one. The fluorescent emission spectra are quite similar for the polymer in solution and in the solid states. Cyclic voltammetric investigation reveals a partially reversible n-doping process and an irreversible p-doping process. The cathodic current is much greater than that of the anodic one, which means the polymer has the tendency to be charged through n-doping rather than p-doping process. The improved affinity of the polymer to electrons may be owing to the low interface barrier between the polymer films and the electrode surface. The HOMO and LUMO energy levels have also been estimated from the data of oxidation and reduction onset potentials. All of the results indicate that the synthesized polymer is a promising green emissive material for the light-emitting device application.     

X-shaped tetra-substituted pyrenes: Synthesis,photophysics, and electroluminescence

Two X-shaped compounds (7 and 9) based on pyrene have been synthesized and well characterized. These compounds exhibit good thermal stabilities and high fluorescence yields. In addition, an interesting phenomenon is discovered that the fluorescence of compound 9 can be reversibly converted from red to orange in the solid state. Organic light-emitting diodes have been fabricated using the X-shaped compounds as the host emitter with low turn-on voltage up to 2.35 V. The device based on compound 7 exhibits orange-red electroluminescence (CIE: 0.58, 0.42) with a maximum luminance of 4798.4 cd/m², a maximum current efficiency 1.26 cd/A, and a power efficiency of 1.06 l m/W.     

Poly (p-pyridine) - and poly (p-pyridyl vinylene) -based polymers: their photophysics and application to SCALE devices

Photophysics and light-emitting device applications of poly (p-pyridine) - and poly(p-pyridyl vinylene)-based polymers are presented. Extensive time-resolved (ps to ms) photoluminescence, stimulated emission and photoinduced absorption studies of solutions, powders and films demonstrate that the primary photoexcitation of these polymers is an intrachain singlet exciton. The presence of (n,π^*) states leads to enhanced intersystem crossing to triplet excitons for the powder form, while aggregate formation plays a key role in the films. Polarons are important at longer times. These polymers were used to fabricate ‘conventional’ polymer light-emitting diodes. In addition, these polymers were used to demonstrate a novel light-emitting structure, the symmetrically configured a.c. light-emitting (SCALE) device. These new devices have potential advantages in their use with high workfunction electrodes, such as gold, and also in their a.c. operation.     

Synthesis of two benzo[1,2-b:3,4-b′]dithiophene-based conjugated polymers with different side chains and their applications in photovoltaic devices

Two D–A conjugated polymers (named PBDTC₆DBT and PBDTC₁₀DBT) with benzo[1,2-b:3,4-b′]dithiophene (BDT) as donor and 4,7-dithiophen-2,1,3-benzothiadiazole (DBT) as acceptor, were designed and synthesized. PBDTC₆DBT and PBDTC₁₀DBT have hexyl and decyl as side chains in the DBT units, respectively. The polymers were characterized by spectroscopic (NMR and UV–vis) methods, GPC, DSC and cyclic voltammetry. In order to study the photovoltaic properties of the two conjugated polymers, photovoltaic devices with the configuration of ITO/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS)/polymer:[6,6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM)/LiF/Al, were fabricated, in which PBDTC₆DBT or PBDTC₁₀DBT acted as electron donor in active layers. According to the experimental data, when the blend ratio of PBDTC₆DBT and PC₆₁BM reached 1:2, the best photovoltaic properties among the PBDTC₆DBT-based devices were obtained, and the open-circuit voltage, the short-circuit current density and the power conversion efficiency were measured to be 0.63 V, 6.18 mA/cm² and 1.86%, respectively. As to PBDTC₁₀DBT-based devices, the maximum of power conversion efficiency reached 0.99% with the blend ratio of PBDTC₁₀DBT and PC₆₁BM being 1:2. PBDTC₁₀DBT and PBDTC₆DBT were both promising donor candidates in the application of polymer solar cells.     

Synthesis and characterization of a novel side-chain liquid crystalline poly(p-phenylenevinylene)

We describe the synthesis of soluble liquid crystalline poly(p-phenylenevinylene)s with cyanobiphenyl as the mesogenic group linked to the vinylene group via a flexible alkyl chain. The polymers were well characterized by spectroscopic methods (NMR, IR, UV—Vis, photoluminescence (PL)) and showed PL emission of green light with a maximum around 540 nm. Polarized UV—Vis spectroscopy of polymer films prepared on oriented substrates indicated a high orientation of the backbone and mesogenic units.     

Synthesis,theoretical characterisation and spectra of thiophene–fluorene π-conjugated derivatives

The syntheses, spectral measurements and the theoretical study of 2-((9H-fluoren-9-ylidene)methyl)thiophene (A), ((9H-fluoren-9-ylidene)methyl)-5-(thiophen-2-yl)thiophene (B), 1-(5-((9H-fluoren-9-ylidene)methyl)thiophen-2-yl)propan-1-one (C), 1- (5-((9H-fluoren-9-ylidene)methyl)thiophen-2-yl)pentan-1-one (D), 1,2-bis(1-(5-((9H-fluoren-9-ylidene) methyl)thiophen-2-yl)propylidene)hydrazine (E) and 1,2-bis(1-(5-((9H-fluoren-9-ylidene)methyl)thiophen-2-yl)pentylidene)hydrazine (F) are presented. The electron absorption and fluorescence spectra of prepared molecules are measured in polymer matrix and solution. The influence of the conformation (e.g. cis- and trans-forms in the case of E and F molecules) on electronic spectra are interpreted by ZINDO/S method. Our results indicate that the thiophene rings connected with terminal fluorene chromophores through the double bounded carbon atom seem to be suitable candidates for the subsequent preparation of the electro-optical materials.     

Synthesis and characterization of polyimides from triphenylamine-based diamine monomers with thiophene or trifluoromethyl side group

Triphenylamine-based novel diamine monomers with side group, such as 4-(2,2′-bithiophenyl)-4′,4″-diaminotriphenylamine (2TTPA) and 4-(3,5-bis(trifluoromehyl)phenyl)-4′,4″-diaminotriphenylamine (6FTPA) were prepared and used for polyimide synthesis. 4-Bromo-dinitrotriphenylamine (BrTPA), prepared from 4-bromoaniline and 1-fluoro-4-nitrobenzene, was reacted with 2,2′-bithiophenene-5-boronic acid (2TBB) or 3,5-bis(trifluoromethyl)benzeneboronic acid (6FBB), followed by hydrogenation to afford 2TTPA and 6FTPA, respectively. After characterization by FT-IR, ¹H-NMR, EA and melting point analyzer, the triphenylamine-based dimaines were utilized to prepare polyimides with 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) or 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The polymers were characterized by FT-IR and ¹H-NMR. Thermal, optical and electrical properties were also evaluated by DSC/TGA, UV–vis/photoluminescence spectrometery and cyclic voltammetry (CV), respectively. The 6FDA–6FTPA polyimide exhibited high glass transition temperatures (291 °C), high thermal stability (>488 °C) and light blue emission (480 nm) with very good solubility even after imidization.     

Synthesis and properties of chemically coupled poly(thiophene)

We describe the synthesis and physical properties of poly(thiophene) with a molecular weight of approximately 4000 consisting of 46 – 47 thiophene rings (184 – 188 carbons along the backbone). The pristine polymer exhibits an ESR line similar to that of cis-(CH)_x; the small number of unpaired spins is consistent with relatively high-quality material. The electrical conductivity can be increased by nearly ten orders of magnitude to values in excess of $\text{10 Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ upon partial oxidation by AsF₅. The magnitude and temperature dependence of the thermopower of the doped polymer (24 mol%) are consistent with metallic behavior. The decrease in conductivity as the temperature is lowered and the magnitude of π at room temperature apparently result from the macroscopic transport being limited by interparticle contacts.     

Highly efficient synthesis of thieno[3,4-b]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers

Various thieno[3,4-b]thiophene derivatives functionalized by n-octyl, 4-tert-butylphenyl, 4-n-butylphenyl, and 4-n-pentylphenyl were synthesized in a concise and efficient way. Previously reported synthetic processes were modified to produce target molecules in relatively high yields. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and Vis–NIR spectrophotometry. The bandgap of electrochemically prepared polymers varied with substituents, ranging from 0.91 eV to 0.98 eV. While HOMO of conjugated polymers was raised by inductive effect of alkyl substituents, the bandgap was mainly determined by resonance stabilization of phenyl substituents.     

Electronic structure and third-order nonlinear optical response of conjugated polymer chains bearing periodically arranged side groups

The electronic structure of an infinite one-dimensional polyene chain bearing periodically arranged phenyl side groups is calculated using the Green function method within a tight-binding (Hückel) model. It is shown that the side groups have a striking effect on the energy band structure: they split the energy bands of the polyene chain into a set of minibands and minigaps, the number and width of which can be governed by changing the distance between the side groups and the degree of their conjugation with the polyene backbone. We calculated the third-order nonlinear optical susceptibility χ⁽³⁾ caused by non-parabolicity of the electron dispersion law in the lowest conduction miniband which is supposed to be partially filled with π-electrons. It is found that χ⁽³⁾ is directly proportional to the width of the miniband and shows a four-power dependence on the distance between the side groups. The numerical estimates indicate that in the transparency region χ⁽³⁾ can reach values comparable to those of inorganic materials with strong nonlinear optical properties.     

Synthesis,photophysics and electroluminescence of dendronized styryl-substituted fluorene with triphenylamine units

Compound 2, a dendronized styryl-substituted fluorene with bulky hole-transporting triphenylamine units, was synthesized by a simple two-step procedure. At the first step of synthesis two triphenylamine units were attached to the C-9 of fluorene while at the second step styryl segments were introduced both on fluorene and triphenylamine by Heck coupling. Compound 2 dissolved in common organic solvents like THF, chloroform, dichloromethane and toluene. This compound was stable up to ～320 °C and showed high glass transition temperature (T_g) of 175 °C. Compound 2 absorbed around 370 nm with optical band gap of 2.89 eV. It behaved as blue-green light emitter with photoluminescence (PL) emission maximum at 444–465 nm and PL quantum yield of 0.36 in THF solution. The HOMO and LUMO levels of compound 2, estimated from cyclic voltammograms, were ?5.20 and ?2.40 eV, respectively. The electroluminescence (EL) emission maxima (530–570 nm) were very different from PL maximum (465 nm), which could be explained by the direct cross recombination transition between electrons and holes trapped on triphenylamine subunits. The maximal brightness (current efficiency) of the EL devices (ITO/PEDOT:PSS/2/Ca/Al) was 1348 cd/m² (0.12 cd/A) and the color showed obvious blue-shift with an increase in operating voltage.     

Synthesis of a side chain conjugated polythiophene copolymer and its photovoltaic property

A new soluble polythiophene copolymer with phenylene–vinylene conjugated side chain (PEHOPVTh-Th) was synthesized. Polymer photovoltaic solar cells using PEHOPVTh-Th as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) as acceptor was demonstrated. The absorption spectrum of the PEHOPVTh-Th showed strong absorption in the visible region and a 55 nm red-shift in comparison with that of its homopolymer (PEHOPVTh). By adjusting the ratio of PEHOPVTh-Th to PC₆₁BM, the polymer solar cells based on PEHOPVTh-Th showed a power conversion efficiency of 0.37% under 100 mW/cm² AM 1.5G simulated sunlight when the weight ratio of the polymer to PC₆₁BM equals to 1:3.