Effect of thermal annealing in atmosphere on photoluminescence of BTEO–PPV films

The photoluminescence (PL) spectra of poly[2,5‐bis‐(tri‐ethoxy)‐1,4‐phenylene vinylene] (BTEO–PPV) films are blue‐shifted with increasing thermal annealing temperature. It is known from the UV–vis absorption spectra that thermal annealing decreases the conjugation length of the polymer. For BTEO–PPV films, unlike with MEH–PPV films, the symmetric triethoxy side groups further block aggregation of the polymer chains. The absorption Fourier transfer infrared spectra showed that thermal annealing in atmosphere destroyed the chain structure of BTEO–PPV film by thermal oxidation to form aldehyde groups, which resulted in low PL efficiency of the annealed films. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Optical properties of MEH‐PPV conjugated polymer covered by silica nanoshells

Poly [2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene] (MEH‐PPV) covered by nanostructured silica shells were synthesized via sol–gel process and investigated after freeze‐drying and heat‐drying in vacuum. The freeze‐dried sample consists of a light pink powder while the heat‐dried sample presents a redder coarse‐grained material. The freeze‐dried sample was analyzed using small angle X‐ray scattering (SAXS). Both samples were analyzed using photoluminescence (PL) and Raman spectroscopy at room temperature. The PL spectra presented relatively large red shifts compared with that of the MEH‐PPV in tetrahydrofuran solution, which was taken as a reference sample. The energy shifts observed in the PL and Raman spectra strongly support an explanation based on denser packing conditions inside the nanostructured silica shells, which can effectively lead the polymer molecules to a higher interchain interaction via aggregate sites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5620–5626, 2006     

Morphology and properties of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐para‐phenylene vinylene]/silica nanoparticle hybrid films

Poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐para‐phenylene vinylene] (MEH‐PPV)/silica nanoparticle hybrid films were prepared and characterised. Three kinds of materials were compared: parent MEH‐PPV, MEH‐PPV/silica (hybrid A films), and MEH‐PPV/coupling agent MSMA/silica (hybrid B films), in which MSMA is 3‐(trimethoxysilyl) propyl methacrylate. It was found that the hybrid B films could significantly prevent macrophase separation, as evidenced by scanning electron and fluorescence microscopy. Furthermore, the thermal characteristics of the hybrid films were largely improved in comparison with the parent MEH‐PPV. The UV‐visible absorption spectra suggested that the incorporation of MSMA‐modified silica into MEH‐PPV could confine the polymer chain between nanoparticles and thus increase the conjugation length. The photoluminescence (PL) studies also indicated enhancement of the PL intensity and quantum efficiency by incorporating just 2 wt% of MSMA‐modified silica into MEH‐PPV. However, hybrid A films did not show such enhancement of optoelectronic properties as the hybrid B films. The present study suggests the importance of the interface between the luminescent organic polymers and the inorganic silica on morphology and optoelectronic properties. Copyright © 2004 Society of Chemical Industry     

Self‐organized MEH‐PPV domains in a TPU matrix and the consequences to the luminescence spectra

Self‐sustained cast films formed from heterogeneous blends of poly(2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV) conjugated polymer and thermoplastic polyurethane (TPU) were investigated by photoluminescence (PL) and scanning electron microscopy (SEM). A blue shift is observed for the pure electronic transition PL peak (E₀₀) with decreasing MEH‐PPV concentration. The two clear shoulders in the PL spectra at higher energy than the E₀₀ peak appear due to the formation of small conjugation segments of the MEH‐PPV molecules at the interface of the spherical MEH‐PPV domains. This assumption and the origin of the blue shift were confirmed by correlating the average size of the MEH‐PPV domains, observed from SEM images, and the analysis of the PL spectra at low temperatures. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and structure study of copolymers from thiadiazole fused indolocarbazole and dithienosilole

We report synthesis and characterization of two D-A polymers (PSDT-C12 and PSDT-EH) with different side chains. Both polymers are based on alternate 12,13-dioctyl-indolo[2,3-a][1,2,5]thiadiazolo[3,4-c]carbazole (TDZIC) and dithienosilole derivative units in polymer main chain. We used TDZIC to enlarge the 2D conjugated plane of acceptor monomers by fusing benzothiadiazole (BT) unit with an indole unit having alkyl groups. PSDT-C12 exhibited 10 nm redshift compared to PSDT-EH in solid films, while their absorption spectra were almost identical in solutions. Since the backbone and side chains on the indolocarbazole group are the same, the redshift on PSDT-C12 could be resulted from the dodecyl (C12) side chain on the dithienosilole unit and different molecular weight between these two polymers. PSDT-C12 has a larger molecular weight than PSDT-EH. Therefore possibly both side chains and molecular weight contributed to the difference in the absorption spectra in solid films. The straight C-12 side chain has less steric hindrance than the branched EH side chain in solid films. PSDT-C12 has a longer main chain (larger molecule weight) than PSDT-EH, which can favour a more extended main chain interaction. The vibronic peak at 519 nm and shoulder at 563 nm in the PSDT-C12 film further confirmed stronger main chain interaction. Geometry optimization showed that head–tail (HT)-PSDT had a more twisting conjugated backbone with larger dihedral angle between dithienosilole unit and thiadiazole-fused ring compared to head–head/tail–tail (HH/TT)-PSDT.     

Synthesis,photo‐, and electroluminescent properties of the soluble poly[(2,5‐diphenylene‐1,3,4‐oxadiazole)‐ 4,4′‐vinylene]

A novel soluble conjugated polymer, poly[(2,5‐diphenylene‐1,3,4‐oxadiazole)‐4,4′‐vinylene] (O‐PPV), containing an electron‐transporting group on the main chain of PPV, was synthesized according to HORNER mechanism. The oligo‐polymer with M_w = 1000 and T_d = 270°C is soluble in chloroform and tetrahydrofuran. The photoluminescent (PL) properties were investigated using different concentrations of solid‐state O‐PPV/PEO blends absorption and selective excitation measurements. The results show that PL arises from interchain charge‐transfer states in solid‐state O‐PPV. Compared with the analogous single‐layer device constructed with PPV (ITO/PPV/Al), which emits two peaks at λ = 520 nm and 550 nm (shoulder), the electroluminescence (EL) spectrum of the device [ITO/O‐PPV (80 nm)/Al] is a broad peak at λ_max = 509 nm. The quantum efficiency (0.13%) of the device ITO/O‐PPV/Al is much higher than that of the device ITO/PPV/Al, due to the introduction of the electron‐transporting group–oxadiazole units in the main chain of PPV. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3535–3540, 1999     

Novel 1,4‐diketo‐3,6‐diphenyl pyrrolo[3,4‐c]pyrrole (DPP)‐based copolymers with large Stokes shift

Two novel alternating π‐conjugated copolymers, named PDPPDOPV and PDPPDOPE, constituted of 1,4‐diketo‐3,6‐diphenyl pyrrolo[3,4‐c]pyrrole (DPP) with 2,5‐dioctyloxy‐1,4‐phenylenevinylene (DOPV) or 2,5‐dioctyloxy‐1,4‐phenyleneethynylene (DOPE), respectively, were synthesized and characterized by UV‐vis, FT‐IR, and photoluminescence spectroscopy. They are dark red solid readily soluble in various common organic solvents including THF and chloroform. The UV‐vis absorption spectra of the polymers show strong absorption bands, which correspond to the π‐π* transition of π‐conjugated segments. Photoluminescence (PL) spectra show that both polymer films and solution have large Stokes shifts. From their fluorescence behavior, Stokes shifts of 173 nm and 199 nm are derived for the films of PDPPDOPV and PDPPDOPE, respectively, which are the largest two values ever reported for DPP‐containing polymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Description and importance of interchain excited states in conjugated polymer photophysics

We compare the photoluminescence (PL) of poly-p-phenyl-enevinylene (PPV) to that of one of its soluble derivatives, poly (2-methoxy, 5-(2¹-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). We show that excimers due to π-orbital overlap of adjacent polymer backbones are formed in MEH-PPV in high quantum yield. They do not emit light at room temperature, potentially explaining the reduction of PL yield in MEH-PPV films relative to solution. The herringbone packing of polymer chains in PPV prevents π-π stacking, but low PL yields are observed anyway. We conclude that polymer design to inhibit interchain registration is a questionable strategy to increase luminescence yields. The low PL yield in PPV is ascribed to bound polaron pair formation and preliminary data suggest that these may also form upon photoexcitation of MEH-PPV films.     

Restraining the aggregation of poly(9,9‐dihexylfluorene) by crosslinked poly(methyl methacrylate) networks

A crosslinked network was used to restrain undesired interchain aggregation of fluorescent polymers in this study. Primarily, solutions of poly(9,9‐dihexylfluorene) (PF) in toluene and in curable liquid monomer mixtures of methyl methacrylate (MMA) and ditrimethylolproane tetracrylate (DTTPT) were compared to study the effect of concentration on absorption and emission behaviors. Results from ultraviolet–visible absorption, photoluminescence (PL) emission, and PL excitation reveal that the degree of aggregation depended strongly on the concentration and was correlated to the size of the polymer coil as determined from light scattering. Solutions of known PF content in MMA/DTTPT were then photoirradiated to yield PF/crosslinked poly(methyl methacrylate) (X‐PMMA) composite films. Comparison among the pure PF, PF/MMA/DTTPT mother liquid, and PF/X‐PMMA solid composites were then conducted to evaluate the effect of crosslinking on the degree of aggregation. With small amounts of imbedded PF (0.0015 wt %), the PF/X‐PMMA composite had a PL quantum yield (0.70) higher than the pure PF (0.65) itself. Most notably, the PF/X‐PMMA composites after annealing at 120 and 200°C for 5 h still retained a reasonable emission efficiency (with related PL quantum yields of 0.61 and 0.43), in contrast to the large PL quantum yield reductions to 0.32 and 0.19 for the annealed PFs after the same treatment. Suggestively, interchain aggregation during a high‐temperature annealing process can be largely inhibited by the surrounding crosslinked network matrix. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Synthesis and characterization of poly(terphenylenevinylene) derivatives containing alkoxy substituents and (or) phenyl pendant group

The blue electroluminescent polymers, poly(terphenylenevinylene) derivatives that have advantages of PPP and PPV, were prepared by Suzuki coupling reaction. The structure and properties of the polymers were analyzed by various spectroscopic methods. Poly(MHTPPV) and poly(TPPV) with phenyl pendant group in a vinyl bridge showed blue shifted absorption spectra, large band gap and enhanced thermal stability as compared with that of poly(MHTPV). The PL spectra of the films of the polymers showed maximum peaks at 450-460 nm, which are pure blue emissions. The blue electroluminescence was obtained with the turn on voltage of 8-10 V, when the simple light-emitting diodes of ITO/polymer/Al were fabricated. The excimer emission due to interchain interaction was reduced by the introduction of substituents into the vinyl bridge and (or) the main chain.     

Synthesis,characterization, and electro‐optical properties of a soluble conjugated polymer containing an oxadiazole unit in the main chain

A novel copolymer, poly{[2,5‐diphenylene‐1,3,4‐oxadiazole‐vinylene]‐alt‐[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene]}(MEH‐OPPV) containing a high‐electron‐affinity unit of aromatic oxadiazole in the main chain is synthesized through the Wittig condensation reaction. The obtained copolymer is easily soluble in conventional organic solvents. The structure of the copolymer was characterized by Fourier transform infrared, ¹H nuclear magnetic resonance, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and ultraviolet‐visible and photoluminescence spectroscopy. The TGA result indicates that the copolymer has very high thermal stability (stable up to 310°C in nitrogen), while DSC investigation demonstrates that the glass transition temperature (T_g) is 143°C, which might be a merit for the long‐life operation of light‐emitting devices. The absorption spectrum of film sample of the copolymer reveals two peaks at 310 and 370 nm, respectively, and the edge absorption corresponds to a band gap of 2.46 eV. A single‐layer light‐emitting diode device ITO/MEH‐OPPV/Al is successfully fabricated. The device emits visible yellowish‐green light above the bias voltage of 4.0 V under ambient condition. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2618–2623, 2003     

Synthesis and photovoltaic properties of polythiophene derivatives with side chains containing C60 end group

A donor–acceptor double‐cable polythiophene derivative ( PT‐F1 ) with side chain containing C₆₀ end group was synthesized, and characterized by infrared, UV‐vis absorption and photoluminescence (PL) spectroscopy, and electrochemical cyclic voltammetry. Cyclic voltammogram of PT‐F1 shows the oxidation peak of the polymer main chains and the reduction peaks of the C₆₀ end groups, indicating that there is no interaction between the polymer main chains and side chain C₆₀ groups on the ground state. The UV‐vis absorption spectrum of PT‐F1 film is red‐shifted in comparison with that of its chloroform solution. The PL spectrum of the polymer main chain was quenched by the C₆₀ pendant on the side chain. Polymer solar cell with the structure of ITO/PEDOT:PSS/ PT‐F1 /Ca/Al was fabricated. The power conversion efficiency of the device based on PT‐F1 reached 0.274% under the illumination of AM 1.5, 100 mW/cm². © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Fluorescence upconversion properties of a chiral polybinaphthyl induced by two‐photon absorption

The fluorescence properties of a chiral polymer based on optically active polybinaphthyls were studied in tetrahydrofuran solution. One‐photon excited fluorescence of the polymer was located in the range of ～ 596 nm and the corresponding lifetime was ～ 4.38 ns. From the excitation spectra and emission spectra excited at 800 nm, the upconversion fluorescence emission was observed. When excited by using 800 nm fs laser pulses with different input irradiances, the peak fluorescence intensity of the solution provides square dependence with the input irradiance power, giving an evidence for two‐photon excited fluorescence. Furthermore, open aperture Z‐scan measurements were performed at different irradiation intensities to confirm two‐photon absorption property of the solution at 800 nm excitation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electrochemical synthesis of polypyrrole layers doped with glutamic ions

Electrochemically synthesized polypyrrole thin films doped with glutamic ions were investigated as interesting materials for potential use as molecularly selective surfaces. Pyrrole and glutamate interact in aqueous solution, resulting in the formation of a prominent band at 240 nm in the absorption spectra of the solution. This feature was assigned to the formation of a complex, which significantly influences the electrochemical deposition of polypyrrole films doped with glutamate. It was also detected that oxidation of these complexes leads to the appearance of a shoulder in the absorption spectrum at 270 nm, which corresponds to the yellowish color of pyrrole aged under influence of air. A mechanism is proposed for polymer film growth in aqueous solutions containing monosodium L ‐glutamate and pyrrole. It takes into account the observed influence of the pH of the solution, the concentration of reagents, the zwitterionic properties of glutamate on deposition, and results from spectroscopic measurements. It is also shown that glutamic ions can electrochemically be released from the synthesized polymer layers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Branched poly(p‐phenylenevinylene): Synthesis,optical and electrochemical properties

A series of poly(p‐phenylene vinylene) (PPV) derivatives with phenylene vinylene side chains (branched PPVs), PPV0, PPV1, PPV2, and PPV3, were synthesized by the Heck coupling reaction and characterized by TGA, absorption spectra, photoluminescence (PL) spectra, and electrochemical cyclic voltammetry. The branched PPVs showed two absorption peaks in the UV–vis region, corresponding to the conjugated side chains (UV absorption) and the main chains (the visible absorption). Especially the absorption spectrum of PPV3 covers a broad wavelength range from 300 to 500 nm. Introducing the electron‐donating alkoxy substituents on the PPV main chains and increasing the content of the alkoxy groups lead to bathochromic shift of both absorption and PL spectra from PPV1 to PPV2 to PPV3. The onset oxidation potential of the branched PPVs is lower by 0.1–0.2 V than that of PPV, indicating that the electron‐donating ability of the branched PPVs enhanced in comparison with that of PPV. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis of CdS nanoparticles dispersed within solutions and polymer films using amphiphilic urethane acrylate chains

In this study, a new process for preparation of CdS/polymer nanocomposite films using a new amphiphilic oligomer chain (Urethane Acrylate Nonionomer: UAN) is presented. UAN chains have hydrophilic polyethylene oxide segments and polypropylene oxide-based hydrophobic segments as part of the same backbone. In addition, these chains also have reactive vinyl groups at their hydrophobic segments. For the preparation of CdS nano-colloid solutions, UAN chains acted as a stabilizer for CdS nanoparticles dispersed in a solvent. For the fabrication of CdS/polymer nanocomposite films, UAN chains in CdS nano-colloid solutions were polymerized through reactions between their vinyl groups, resulting in the formation of CdS nanoparticle dispersed films swollen by the solvent. The nature of the solvent used strongly influenced the size of the CdS nanoparticles, which was confirmed by UV absorption spectra, PL emission spectra and TEM images. Smaller sized CdS nanoparticles were formed in higher polar solvents such as DMAc and methanol, which can be explained by the higher solubility of UAN chains and the more effective dissociation and stabilization of cadmium salts and CdS nanoparticles by UAN chains in the polar solvent.     

Study of Photoluminescence Excitation Spectra of Tris(8-hydroxyquinoline)aluminum(III) (Alq3) in Solutions and Films

Absorption, photoluminescence, and photoluminescence excitation (PLE) spectra have been investigated for tris(8-hydroxyquinoline)aluminum(III) (Alq₃) in solution at various Alq₃ concentrations and in doped and neat films. The redshifted PLE spectra, with respect to the absorption spectra, are obtained, together with a dip located near the absorption band peak. The dip becomes deeper with increasing concentration. It is found that the unusual PLE spectra are caused by the penetration depth effect, not by previously suggested microcrystallites of Alq₃.     

Synthesis and characterization of new anthracene‐based semiconducting polyethers

New anthracene‐based polyethers, anthracene/bisphenol A (An–BPA) and anthracene/fluorinated bisphenol A (An–BPAF), were synthesized and characterized. An–BPA and An–BPAF were fully soluble in common organic solvents and had number‐average molecular weights of 2580 and 3240, respectively. The optical properties of the polymers were investigated with ultraviolet–visible absorption and photoluminescence spectroscopy. Blue photoluminescence was observed in dilute solutions. In solid thin films, π–π interactions influenced the optical properties, and redshifted photoluminescence spectra were obtained; a green emission (504 nm) for An–BPAF and a green‐yellow emission (563 nm) for An–BPA were found. By cyclic voltammetry, the electrochemical band gap was estimated to be 2.72 and 3.05 eV for An–BPA and An–BPAF, respectively. Single‐layer diode devices of an indium tin oxide/polyether/aluminum configuration were fabricated and showed relatively low turn‐on voltages (3.5 V for An–BPA and 3.7 V for An–BPAF). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The influence of tetrakis-ethylhexyloxy groups substituted in PPV derivative for PLEDs

New electroluminescent polymer with tetrakis-alkoxy group, poly[2,3,5,6-tetrakis(2-ethylhexyloxy)-1,4-phenylenevinylene] (TEH-PPV), has been synthesized by the Gilch polymerization. In solid film state, the TEH-PPV exhibits absorption spectra with maximum peaks at 451 nm, and PL spectrum at 505-545 nm. As compared to MEH-PPV, TEH-PPV with tetrakis-ethylhexyloxy groups in phenylene unit can get shorter conjugation length, and had more blue shifted absorption and emission peaks due to steric hindrance, in spite of increasing the number of alkoxy substituents which may increase the effective conjugation length caused by the electron-donating effect. The polymer LEDs (ITO/PEDOT/polymer/Al) of TEH-PPV showed emission with maximum peaks at around 505-590 nm. Tetrakis-ethylhexyloxy groups induced very typical vibronically structured band in solid film state, since the conjugated backbone is twisted by steric hindrance. And they can enhance the internal efficiency of the conjugated polymer as emissive layer in PLED because of the restraint of inter-chain interaction by the avoidance of close packing to give decent device performance.