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Intermolecular interactions play a fundamental role on the performance of conjugated materials in organic electronic devices, as they heavily influence their optoelectronic properties. Synthetic control over the solid state properties of organic optoelectronic materials is crucial to access real life applications. Perylene diimides (PDIs) are one of the most highly studied classes of organic fluorescent dyes. In the solid state, π-π stacking suppresses their emission, limiting their use in a variety of applications. Here, we report the synthesis of a novel PDI dye that is encapsulated by four alkylene straps. X-ray crystallography indicates that intermolecular π-π stacking is completely suppressed in the crystalline state. This is further validated by the photophysical properties of the dye in both solution and solid state and supported by theoretical calculations. However, we find that the introduction of the encapsulating "arms" results in the creation of charge-transfer states which modify the excited state properties. This article demonstrates that molecular encapsulation can be used as a powerful tool to tune intermolecular interactions and thereby gain an extra level of control over the solid state properties of organic optoelectronic materials.

Porous silicon (PSi) is a promising material for future integrated nanophotonics when coupled with guest emitters, still facing challenges in terms of homogenous distribution and nanometric thickness of the emitter coating within the silicon nanostructure. Herein, it is shown that the nanopore surface of a porous silicon oxide (PSiO2) microcavity (MC) can be conformally coated with a uniform nm-thick layer of a cationic light-emitting polyelectrolyte, e.g., poly(allylamine hydrochloride) labeled with Rhodamine B (PAH-RhoB), leveraging the self-tuned electrostatic interaction of the positively-charged PAH-RhoB polymer and negatively-charged PSiO2 surface. It is found that the emission of PAH-RhoB in the PSiO2 MC is enhanced (≈2.5×) and narrowed (≈30×) at the resonant wavelength, compared with that of PAH-RhoB in a non-resonant PSiO2 reference structure. The time-resolved photoluminescence analysis highlights a shortening (≈20%) of the PAH-RhoB emission lifetime in the PSiO2 MC at the resonance versus off-resonance wavelengths, and with respect to the reference structure, thereby proving a significant variation of the radiative decay rate. Remarkably, an experimental Purcell factor Fp= 2.82 is achieved. This is further confirmed by the enhancement of the photoluminescence quantum yield of the PAH-RhoB in the PSiO2 MC with respect to the reference structure. Application of the electrostatic nanoassembling approach to other emitting dyes, nanomaterials, and nanophotonic systems is envisaged.

The aggregation-induced emission (AIE) phenomenon provides a new direction for the development of organic light-emitting devices. Here, we present a new class of emitters based on 4,4-difluoro-4-bora-3 a ,4 a -diaza- s -indacene (BODIPY), functionalized at different positions with tetraphenylethylene (TPE), which is one of the most famous AIE luminogens. Thanks to this modification, we were able to tune the photoluminescence of the BODIPY moiety from the green to the near-infrared (NIR) spectral range and achieve PL efficiencies of ~50% in the solid state. Remarkably, we observed an enhancement of the AIE and up to ~100% photoluminescence efficiencies by blending the TPE-substituted BODIPY fluorophores with a poly[(9,9-di- n -octylfluorene-2,7-diyl)- alt -(benzo[2,1,3]thiadiazol-4,7-diyl)] (F8BT) matrix. By incorporating these blends in organic light-emitting diodes (OLEDs), we obtained electroluminescence peaked in the range 650–700 nm with up to 1.8% external quantum efficiency and ~2 mW/cm2 radiance, a remarkable result for red/NIR emitting and solution-processed OLEDs.

Graphene transparent conductive films have been proposed as indium tin oxide (ITO) modifiers for application in optoelectronic devices. Here thermal and chemical treatments of graphene-based transparent films are systematically investigated with a view to increasing their conductivity and tuning their work function. We look at two different types of graphene, obtained either via spin-coating of graphene oxide (GO) aqueous dispersions, or vacuum-filtration of liquid-phase exfoliated (LPE) graphene in highly volatile, non-toxic solvents such as isopropanol and ethanol. As-deposited films are relatively resistive, but we are able to lower their sheet resistance by up to three orders of magnitude for both LPE and GO, so as to reach values of similar to 10(5) Omega/square via thermal treatments at temperatures of similar to 350/400 degrees C at pressures of similar to 10(-6)/10(-4) mbar. Most importantly, the physisorption of the molecule (CF3SO2)(2)NH (trifluoromethanesulfonimide) results in an increase of the films work function by up to 0.5 eV, to yield a value of similar to 5.3 eV. This is comparable or slightly better than what can be achieved with poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), depending on PSS concentration, thereby confirming the potentially beneficial role of chemical doping of liquid dispersions of graphene-derivatives for application to organic electronics. (C) 2015 Elsevier B.V. All rights reserved.

A series of polyfluorene-based polymers with a range of weight percentages (w/w) of a platinum(II)-containing porphyrin, 5,15-dimesityl-10,20-diphenylporphyrinato platinum(II) (MPP(Pt)), were synthesised and incorporated into organic light-emitting diodes. All polymers showed emission predominantly in the red/NIR region with only those polymers with porphyrin w/w of less than 2% showing residual tails at wavelengths lower than 600 nm, indicating increased emission from the porphyrin as w/w increases. The 2% loading of MPP(Pt) gave the highest efficiency LED (0.48%) and light output (2630 mW/m2).

In this work, the bottom-up template-assisted preparation of high-density lattices (up to 11 · 106 membranes/cm2) of suspended polymer membranes with micrometric size (in the order of few μm2) and sub-micrometric thickness (in the order of hundreds of nm) is demonstrated for both photoluminescent and non-photoluminescent polymers by capillarity-driven solvent evaporation. Solvent evaporation of low concentration polymer solutions drop-cast on an array of open-ended micropipes is shown to lead to polymer membrane formation at the inlet of the micropipes thanks to capillarity. The method is proven to be robust with high-yield (>98%) over large areas (1 cm2) and of general validity for both conjugated and non-conjugated polymers, e.g. poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT), poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), polystyrene (PS), thus breaking a new ground on the controlled preparation of polymer micro and nanostructures. Angle dependence and thermal stability of photoluminescence emission arising from F8BT membrane lattices was thorough investigated, highlighting a non-Lambertian photoluminescence emission of membrane lattices with respect to F8BT films. The method is eventually successfully applied to the preparation of both photoluminescent and non-photoluminescent micro Quick Response (μQR) codes using different polymers, i.e. F8BT, MDMO-PPV, PS, thus providing micrometric-sized taggants suitable for anti-counterfeiting applications.

We take advantage of a recent breakthrough in the synthesis of α,β-unfunctionalised 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties, which we symmetrically conjugate with oligothienyls in an unexpectedly stable form, and produce a “metal-free” A-D-A (acceptor-donor-acceptor) oligomer emitting in the near-infrared (NIR) thanks to delocalisation of the BODIPY low-lying lowest unoccupied molecular orbital (LUMO) over the oligothienyl moieties, as confirmed by density functional theory (DFT). We are able to retain a PL efficiency of 20% in the solid state (vs. 30% in dilute solutions) by incorporating such a dye in a wider gap polyfluorene matrix and demonstrate organic light-emitting diodes (OLEDs) emitting at 720 nm. We achieve external quantum efficiencies (EQEs) up to 1.1%, the highest value achieved so far by a “metal-free” NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.

Stimuli-responsive photonic crystals (PCs) represent an intriguing class of smart materials very promising for sensing applications. Here, selective ionic strength responsive polymeric PCs are reported. They are easily fabricated by partial sulfonation of polystyrene opals, without using toxic or expensive monomers and etching steps. The color of the resulting hydrogel-like ordered structures can be continuously shifted over the entire visible range (405-760 nm) by changing the content of ions over an extremely wide range of concentration (from about 70 μM to 4 M). The optical response is completely independent from pH and temperature, and the initial color can be fully recovered by washing the sulfonated opals with pure water. These new smart photonic materials could find important applications as ionic strength sensors for environmental monitoring as well as for healthcare screening.

Conjugated polyrotaxanes jacketed with hole-transport groups have been synthesized from water-soluble polyrotaxanes consisting of a polyfluorene-alt-biphenylene (PFBP) conjugated polymer threaded through β-cyclodextrin macrocycles. The hydroxyl groups of the oligosaccharides were efficiently functionalized with triphenylamine (TPA) so that every polyrotaxane molecule carries a coat of about 200 TPA units, forming a supramolecular coaxial structure. This architecture was characterized using a range of techniques, including small-angle X-ray scattering. Absorption of light by the TPA units results in excitation energy transfer (EET) and photoinduced electron transfer (ET) to the inner conjugated polymer core. These energy- and charge-transfer processes were explored by steady-state and time-resolved fluorescence spectroscopy, femtosecond transient absorption spectroscopy, and molecular modeling. The time-resolved measurements yielded insights into the heterogeneity of the TPA coat: those TPA units which are close to the central polymer core tend to undergo ET, whereas those on the outer surface of the polyrotaxane, far from the core, undergo EET. Sections of the backbone that are excited indirectly via EET tend to be more remote from the TPA units and thus are less susceptible to electron-transfer quenching. The rate of EET from the TPA units to the PFBP core was effectively modeled by taking account of the heterogeneity in the TPA-PFBP distance, using a distributed monopole approach. This work represents a new strategy for building and studying well-defined arrays of >100 covalently linked chromophores. © 2014 American Chemical Society.

Electronic supplementary information (ESI) available: Synthetic details and materials characterization. See DOI: 10.1039/c4tc00342j A family of phthalimide–thiophene copolymers with linear and branched alkyl chains attached to the imide nitrogen have been synthesized. Their optical and electronic properties were investigated along with their applications in OFETs and LEFETs. The phthalimide–thiophene copolymer having a C₁₆ straight alkyl chain on the phthalimide yielded the highest mobilities and PLQE with mobilities of 1 × 10̄̄̄̄⁻³ cm² V⁻¹ s⁻¹ for holes and 1 x 10⁻² cm² V⁻¹-s⁻¹ for electrons with a PLQE of ~28% in the solid state. Since these polymers are ambipolar and emissive, they have proven to be useful for applications as a host material for NIR PLEDs. In this study a 1% loading of NIR emitting DAD segments based on bisthienyl(thiadiazoloquinoxaline) or bisthienyl(benzotriazolothiadiazole) were incorporated into the phthalimide–thiophene polymerization. Using the branched CH(C₈H₁₇)₂ alkyl chain on the host phthalimide–thiophene copolymer combined with the bisthienyl(benzotriazolothiadiazole) emitter resulted in the most efficient (emission maximum ≥ 850 nm) single layer NIR-emitting PLED to date with an EQE of 0.27% emitting at 885 nm. Refereed/Peer-reviewed

We report data from X-ray and neutron scattering on the structure and dynamics of P3HT–PCBM solid blends cast from three different solvents, characterized by different boiling points, namely chloroform (CF, boiling point Tb = 61.2 °C), chlorobenzene (CB, Tb = 131 °C) and ortho-dichlorobenzene (ODCB, Tb = 180.5 °C). Whereas the blend cast from CB shows already a significant degree of order in its “pristine” (as cast) state, blends prepared from CF and ODCB develop their order only upon annealing at 160 °C. In addition, blending seems to frustrate the polymer dynamics. We propose that such change in dynamics might be related to the polymer confinement within PCBM domains. However, no appreciable difference in the degree of dynamics (and/or of its frustration) is observed as a function of the solvent choice.

Synthesis, isolation and characterization of diketopyrrolopyrrole (DPP) dimers are described. These derivatives were prepared starting from symmetric or asymmetric DPP monomers through Pd-catalyzed homocoupling, Yamamoto coupling or Suzuki coupling. The spectroscopic properties of these newly synthesized DPP dimers were investigated and were supported by quantum-chemical calculations.

Aviation and space applications can benefit significantly from lightweight organic electronics, now spanning from displays to logics, because of the vital importance of minimising payload (size and mass). It is thus crucial to assess the damage caused to such materials by cosmic rays and neutrons, which pose a variety of hazards through atomic displacements following neutron-nucleus collisions. Here we report the first study of the neutron radiation tolerance of two poly(thiophene)s-based organic semiconductors: poly(3-hexylthiophene-2,5-diyl), P3HT, and the liquid-crystalline poly(2,5-bis (3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT. We combine spectroscopic investigations with characterisation of intrinsic charge mobility to show that PBTTT exhibits significantly higher tolerance than P3HT. We explain this in terms of a superior chemical, structural and conformational stability of PBTTT, which can be ascribed to its higher crystallinity, in turn induced by a combination of molecular design features. Our approach can be used to develop design strategies for better neutron radiation-tolerant materials, thus paving the way for organic semiconductors to enter avionics and space applications.

Control of intermolecular interactions is crucial to the exploitation of molecular semiconductors for both organic electronics and the viable manipulation and incorporation of single molecules into nano-engineered devices. Here we explore the properties of a class of materials that are engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4'-diphenylene vinylene) or polyfluorene through alpha- or beta-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects that red-shift and partially quench the luminescence. Our approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport. We used the polymers to prepare single-layer light-emitting diodes with Ca and Al cathodes, and observed blue and green emission. The reduced tendency for polymer chains to aggregate allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning force microscopy.

Hydrophilic polyanionic conjugated polyrotaxanes are readily synthesized in water by Suzuki coupling, but their high polarity and ionic nature limit the potential applications of these materials. Here, we demonstrate three methods for transforming these polar polyelectrolytes into nonpolar lipophilic insulated molecular wires. A water-soluble polyfluorene-alt-biphenylene β-cyclodextrin (CD) polyrotaxane was converted into nonpolar derivatives by methylation of the carboxylic acid groups with diazomethane and conversion of the hydroxyl groups of the CDs to benzyl ethers, trihexylsilyl ethers, benzoyl esters, and butanoate esters to yield polyrotaxanes that are soluble in organic solvents such as chloroform and cyclohexane. Elemental analysis, NMR spectroscopy, and gel permeation chromatography (GPC) data support the proposed structures of the organic-soluble polyrotaxanes. The extents of reaction of the polyrotaxane CD hydroxyl groups were 55% for trihexylsilyl chloride/ imidazole; 81% for benzyl chloride/sodium hydride; 72% for benzoyl chloride/pyridine/4- dimethylaminopyridine; and 98% butanoic anhydride/pyridine/4- dimethylaminopyridine. Alkylation, silylation, and esterification increase the bulk of the encapsulating sheath, preventing interstrand aggregation, increasing the photoluminescence efficiency in the solid state and simplifying the time-resolved fluorescence decay. The organic-soluble polyrotaxanes were processed into polymer light-emitting diodes (PLEDs) from solution in nonpolar organic solvents, thereby excluding ionic impurities from the active layer. © 2008 WILEY-VCH Verlag GmbH and Co. KGaA.

The operation of a supramolecular interaction between conjugated macromolecular polyelectrolytes and the ion-transporting polymer PEO at the single molecule level and in bulk solid films was investigated. The polyelectrolytic properties of insulated molecular wires (IMW) along with lithium carboxylate and sulfonate substituents were used for the formation of supramolecular complexes with polymers featuring ion-coordination properties. It was observed that the interaction increases the photoluminescence(PL) and electroluminescence(EL) efficiencies of light emitting polyelectrolytes. The results show that the external quantum efficiency increases by a factor of 200, reaching 0.40%.

We use steady-state and time-resolved photoluminescence (PL) spectroscopy to investigate the luminescent properties of a sulfonated poly(diphenylenevinylene) lithium salt (PDV.Li) in water/propanol solutions at different concentrations, with a view to assessing its aggregation behavior. In particular, we compare results from uninsulated PDV.Li and cyclodextrin-threaded PDV.Li polyrotaxane (PDV.Li⊂β-CD). We find that addition of 1-propanol (≥20 weight%) leads to a significant blue-shift (of ∼0.20 eV) of the PL spectra, that we assign to suppressed interchain aggregation in PDV.Li solutions, with a concomitant fourfold increase in the fluorescence quantum efficiency (i.e. from 14 to 60%). Surprisingly, a moderate concentration of propanol increases further the luminescence efficiency even for PDV.Li⊂β-CD, whose supramolecular encapsulation already provides a shield against aggregation. Indeed, addition of propanol reduces the solvent polarity, and therefore helps solubilizing these materials that are still largely aromatic in nature. Interestingly, however, both uninsulated PDV.Li and polyrotaxane solutions exhibit signs of aggregation at high propanol fraction (>70%) with a distinctively weaker coupling than that of interchain states in PDV.Li at high water concentration and in pure water in particular. While we ascribe such behavior to a poor solvation of the polar moieties, we also report a different strength of aggregation for PDV.Li and PDV.Li⊂β-CD that can be attributed to the presence of the cyclodextrin rings. In PDV.Li⊂β-CD hydrogen bonding between the cyclodextrin rings may lead to closer packing between the polymer chains. We therefore suggest that a content of propanol between 30 and 70% provides a good balance of hydrophobic and hydrophilic interactions both for PDV.Li and PDV.Li⊂β-CD.

We present the synthesis of a novel diphenylanthracene (DPA) based semiconducting polymer. The polymer is solubilised by alkoxy groups attached directly to a DPA monomer, meaning the choice of co-monomer is not limited to exclusively highly solubilising moieties. Interestingly, the polymer shows a red-shifted elecroluminescence maximum (510 nm) when compared to its photoluminescence maximum (450 nm) which we attribute to excimer formation. The novel polymer was utilised as a host for a covalently-linked platinum(ii) complexed porphyrin dopant. Emission from these polymers was observed in the NIR and again showed almost a 100 nm red shift from photoluminescence to electroluminescence. This work demonstrates that utilising highly aggregating host materials is an effective tool for inducing red-shifted emission in OLEDs.

Highly oriented luminescent films are produced by stretching a 30-μm-thick polyvinyl alcohol matrix doped with water-soluble polyrotaxanes and their unthreaded analogues. Photoluminescence experiments reveal that over 95% of the emitted light is polarized along the orientation direction. A hybrid organic-inorganic light-emitting diode is built to investigate the possibility of using these films as polarizing filters for solid-state lighting and display technology. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

The fabrication of high-resolution nanostructures in both poly(p-phenylene vinylene), PPV, and a crosslinkable derivative of poly(9,9'-dioctylfluorene), F8, using scanning near-field optical lithography, is reported. The ability to draw complex, reproducible structures with 65000 pixels and lateral resolution below 60 nm (< lambda/5) is demonstrated over areas up to 20 mu m x 20 mu m. Patterning on length-scales of this order is desirable for realizing applications both in organic nanoelectronics and nanophotonics. The technique is based on the site-selective insolubilization of a precursor polymer under exposure to the confined optical field present at the tip of an apertured near-field optical fiber probe. In the case of PPV, a leaving-group reaction is utilized to achieve insolubilization, whereas the polyfluorene is insolubilized using a photoacid initiator to create a crosslinked network in situ. For PPV, resolubilization of the features is observed at high exposure energies. This is not seen for the crosslinked F8 derivative, r-F8Ox, allowing us to pattern structures up to 200 nm in height.

Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and cross-linked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures.

A systematic investigation is reported into the influence of the counter cations on the optical, electrical and electroluminescent properties of polyelectrolytic conjugated polymers and of their cyclodextrin-threaded rotaxanes. We compare conjugated polyelectrolytes with sulfonated side groups where the anionic charge is balanced by Li+, K+, Cs +, tetramethylammonium (Me4N+) and cryptate-encapsulated potassium (K+ at [2.2.2]). Narrowing (for the unthreaded analogues) and a slight red-shift of the absorption spectra (for the rotaxanes) are found upon exchange of Li+ for larger cations, together with a blue-shift and an efficiency enhancement of the luminescence. These effects are similar in nature to those induced by rotaxination, and are therefore assigned to a marked reduction of intermolecular interactions between the conjugated cores. Exchange of Li+ for K+, Cs +, or Me4N+ results in a higher electroluminescence external quantum efficiency (EQE) for both polyrotaxanes and unthreaded polymers. For polyrotaxane-based devices the EQE increased approximately 7 times upon substitution of Li+ with Cs+ or Me4N+, thereby demonstrating the importance of the selection of the counter-cations for optimizing the performance of polyelectrolytic conjugated polymers in light-emitting devices. © 2008 WILEY-VCH Verlag GmbH and Co. KGaA.

The development of semiconducting polymers with high solid-state luminescence efficiencies has enabled the fabrication of efficient polymer light-emitting diodes. Luminescence is often quenched in well-ordered molecular solids, as a result of inter-molecular dipolar coupling, and the general observation of efficient luminescence in semiconducting polymers is unexpected. We report here the synthesis and characterisation of a series of model 'glassy' poly(arylenevinylene)s where we control the cis to trans ratio about the vinylene linkage and also the phenylene linkage geometry. Photoluminescence efficiency is enhanced for more disordered materials, with highest values for 50 : 50 cis : trans ratios, when it exceeds 50%. We also find that the free volume associated with these glassy disordered polymers allows conformational relaxation of the excitonic state, via ring rotation at the vinylene linkage, causing a large Stokes' shift of the emission. We propose that high luminescence efficiency in these glassy polymers is due to emission from the more disordered regions, and that two effects due to disorder are simultaneously required: firstly that these regions are luminescent (prevention of aggregation by disorder), and, secondly, that the electronic excited state (exciton) can lower its energy below its value in more ordered regions by means of the ring-rotational coupling.

We report the optical characterization of near-infrared (NIR) emitters for polymer light-emitting diode (PLEDs) applications based on the copolymerization of a phthalimide-thiophene host polymer with a low-gap emitter containing the bisthienyl(benzotriazolothiadiazole) unit. We investigate different loadings of the low-gap emitter (in the range 1-3% by weight) and the substitution of a sulphur atom with a selenium atom to further extend the emission in the NIR up to 1000 nm. PLEDs based on copolymers with 1% loading give the best efficiency (0.09%) and show an almost pure NIR EL (95% in the NIR) peaking at 895 nm. Refereed/Peer-reviewed

We report our investigations of the photophysical properties of a so-called “donor-acceptor” (D-A) electroluminescent polymer incorporating N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI) as the electron acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) acceptor units. Polymer light-emitting diodes (PLEDs) based on this copolymer give the best efficiency (external quantum efficiency EQE∼0.04%) and show a pure near-infrared (NIR) electroluminescence (EL) with a peak at 870 nm.

Two alternating polyfluorene polyrotaxanes (3·TM-βCD and 3·TM-γCD) have been synthesized by the coupling of 2,7-dibromofluorene encapsulated into 2,3,6-tri-O-methyl-β- or γ-cyclodextrin (TM-βCD, TM-γCD) cavities with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester. Their optical, electrochemical and morphological properties have been evaluated and compared to those of the non-rotaxane counterpart 3. The influence of TM-βCD or TM-γCD encapsulation on the thermal stability, solubility in common organic solvents, film forming ability was also investigated. Polyrotaxane 3·TM-βCD exhibits a hypsochromic shift, while 3·TM-γCD displays a bathochromic with respect to the non-rotaxane 3 counterpart. For the diluted CHCl3 solutions the fluorescence lifetimes of all compounds follow a mono-exponential decay with a time constant of ≈0.6 ns. At higher concentration the fluorescence decay remains mono-exponential for 3·TM-βCD and polymers 3, with a lifetime τ = 0.7 ns and 0.8 ns, whereas the 3·TM-γCD polyrotaxane shows a bi-exponential decay consisting of a main component (with a weight of 98% of the total luminescence) with a relatively short decay constant of τ1 = 0.7 ns and a minor component with a longer lifetime of τ2 = 5.4 ns (2%). The electrochemical band gap (ΔEg) of 3·TM-βCD polyrotaxane is smaller than that of 3·TM-γCD and 3, respectively. The lower ΔEg value for 3·TM-βCD suggests that the encapsulation has a greater effect on the reduction process, which affects the LUMO energy level value. Based on AFM analysis, 3·TM-βCD and 3·TM-γCD polyrotaxane compounds exhibit a granular morphology with lower dispersity and smaller roughness exponent of the film surfaces in comparison with those of the neat copolymer 3.

We present the electro-optical properties of light-emitting diodes (LEDs) based on two blue-emitting polyfluorenes, with aluminum cathodes and having indium-tin oxide (ITO) anodes modified by self-assembled monolayers (SAM) of benzenesulphenyl chloride derivatives. The modification of ITO surface was confirmed by contact angle measurements. Electroabsorption measurements were used to assess the modification of ITO work function upon SAM formation. We find that the use of a benzenesulphenyl chloride derivative containing a nitro group is as effective at improving LEDs characteristics as the use of a hole-injection layer of well known PEDOT:PSS.

The color purity of polyfluorene-based blue-emitting polymers is often compromized by “long-wavelength” green emission bands, attributed to polymer interchain species first and more recently to formation of emissive fluorenone defects. Here, we study the nature and the suppression of such bands via characterization of a new class of polyfluorene derivatives modified by insertion of functional groups at the bridging points (position C9), so as to increase inter-ring torsion angles. We find that the solid-state photoluminescence spectra of random copolymers of the modified polyfluorenes and the homopolymer display a progressive decrease of the long-wavelength emission. Electroluminescence spectra also show efficient suppression of such bands in the copolymers with a concentration of ‘twisted' comonomer units of 40 % or greater. Quantum-chemical calculations on model oligomers address the influence of the bridging unit on the torsion angles, and the resulting excited-state properties; the impact on molecular packing is also explored with force-field calculations. We conclude that increase of intra-biphenyl torsion angles is a viable strategy for suppression of long-wavelength emission bands in polyfluorenes.

We report the fabrication of a two-dimensional periodic structure made from the conjugated, fluorescent polymer poly(p-phenylene vinylene), PPV, by direct-write near-field lithography of a photosensitive precursor of the conjugated polymer. The precursor film is illuminated with ultraviolet radiation using the apertured fibre of a home-built scanning near-field optical microscope (SNOM), and unexposed areas are subsequently dissolved in methanol. The fully conjugated polymer is obtained by thermal conversion under vacuum. The structure is approximately 10 mu m x 10 mu m, and consists of 30 nm tall pillars on a 330 nm periodicity lattice. We present a calculation of the photonic properties and propose that structures made with the method investigated here are promising candidates for photonic crystal lasers. We also report the results of preliminary investigations aimed at increasing the pillar height.

We report on the synthesis and luminescence properties of a new random fluorene-based copolymer, BTBOX, combining 9,9-dioctylfluorene, benzothiadiazole and phenylene units substituted with pendant oxetane groups, which can undergo a ring-opening polymerisation. BTBOX exhibits efficient green-yellow emission. Upon UV irradiation of solid films of BTBOX containing a photoacid the oxetane groups polymerise, leading to the formation of an insoluble crosslinked network, r-BTBOX.

We report on the use of indium-tin oxide surface modification, by grafting of highly polar p-disubstituted benzenes, in the fabrication of light-emitting diodes. The polar compounds possess COCl or SO2Cl grafting groups and CF3 or NO2 as highly electronegative groups, leading to the formation of a dipolar monolayer, which brings about an increase in ITO work function, thereby reducing the barrier for hole injection into luminescent polymers. We observe that the effect of this self-assembled monolayer, in terms of light-onset voltage, efficiency and luminance, is at least comparable to the use of a hole injection layer of doped poly[(3,4-ethylenedioxy)thiophene] for LEDs using poly({2-[(2-ethylhexyl)oxy]-5-methoxy-1,4-phenylene}vinylene) (MEH-PPV) and polyfluorene blends as active layers.

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor device physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the noninvasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky–Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g., Bardeen model), except for electrodes that extensively react with the polymer, and introduce deep gap states. In addition, we show results that validate the approximation of rigid tilting of polymer energy levels with bias (for biases for which no significant injection of carriers occurs). To investigate further the consequences of the electronic line-up on device operation, we complemented the electroabsorption measurements with characterization of the emissive and transport properties of the light-emitting diodes, and confirmed that the cathodic barrier lowering in CsF/Ca/Al and LiF/Ca/Al electrodes leads to the best improvements in electron injection. We found that luminance and overall current are greatly affected by the barrier-reducing cathodes, indicating a truly bipolar transport, with comparable electron and hole currents. We also found significant indications of CsF/Ca/Al cathodes strongly reacting with the polymer, which is suggestive of CsF dissociation and diffusion in the bulk of the polymer.

We report on the synthesis and characterization (including structural, optical, electrochemical and electroluminescence properties) of three alternating F-alt-X copolymers, where F is 9,9-bis(2 0 -ethylhexyl)fluorene unit and the X comonomer varies from a phenylene, to a thiophene and to a thiophene-S,S-dioxide unit. Among these X comonomers, the phenylene group is at the origin of a blue-emitting copolymer with unitary luminescence efficiency in solution, while thiophene-S,S-dioxide promotes the highest electron affinity. These copolymers are also used in the fabrication of light-emitting diodes. q 2003 Elsevier Science Ltd. All rights reserved.

We study the effect of indium-tin oxide surface modification by self assembling of highly polar molecules on the performance of single-layer light-emitting diodes (LEDs) fabricated with polyfluorene blends and aluminium cathodes. We find that the efficiency and light-output of such LEDs is comparable to, and sometimes better than, the values obtained for LEDs incorporating a hole injection layer of poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid. This effect is attributed to the dipole-induced work function modification of indium-tin oxide.

A series of alternating copolymers of the type A- alt -B, where A is 9,9-bis(2′-ethylhexyl)fluorene and B is a thiophene-based moiety, was synthesised via the palladium-catalysed Suzuki coupling. These copolymers were characterised by optical and electrochemical methods and used in the fabrication of light-emitting diodes. We find that upon increasing the conjugation length of the B moiety, from a thiophene unit to 2,5-bis(2′-vinyl thienyl)thiophene, the fluorescence quantum yields are reduced both in solution (from 51 to 11%) and in solid state. Both absorption and emission are red-shifted, with the solid state emission maximum increasing from 476 nm (green) to 600 nm (orange). When the B unit is changed from thiophene to thiophene- S , S -dioxide, there is a significant increase in both the ionisation potential and electron affinity, and a concurrent reduction of the solution fluorescence efficiency (down to 16%). These polymers appear promising materials for optoelectronic applications.

We present picosecond time-resolved studies of thin films of two polyfluorene blends, combining a relatively high energy-gap poly(9,9-dioctylfluorene), PFO, and a polyfluorene derivative with a smaller gap. These studies show both efficient energy transfer from the donor to the acceptor, as well as spatial confinement of the excitons in the acceptor polymers. The optoelectronic characteristics of light-emitting diodes fabricated with such blends reflect the contribution of these two processes. The electroluminescence is almost exclusively from the acceptor polyfluorenes and we observe a significant improvement of the electroluminescence efficiency of the light-emitting diodes fabricated with such blends in comparison to those based on neat films of the polyfluorenes.

We report the incorporation of graphene-oxide quantum dots (GOQDs) into films, diluted solutions, and light-emitting diodes (LEDs) as part of a water-soluble derivative of poly(p-phenylene vinylene), or PDV.Li, to investigate their impact on the light-emission properties of this model conjugated polymer. Despite the well-known ability of graphene and graphene oxide to quench the photoluminescence of nearby emitters, we find that the addition of GOQDs to diluted solutions of PDV.Li does not significantly affect the photoluminescence (PL) dynamics of PDV.Li, bringing about only a modest quenching of the PL. However, loading the polymer with GOQDs led to a substantial decrease in the turn-on voltage of LEDs based on GOQD–PDV.Li composites. This effect can be attributed to either the improved morphology of the host polymer, resulting in an increase in the charge mobility, or the enhanced injection through GOQDs near the electrodes.

The photophysical properties of two polyrotaxanes (PFBTh•PSβCD and PFBTh•PMeβCD) composed of fluorene and bithiophene encapsulated into permodified β-cyclodextrin cavities have been investigated and compared with those of the reference PFBTh. Rotaxane formation results in improvements of the thermal stability, solubility in common organic solvents, as well as better film forming ability combined with a high transparency. As expected PFBTh and its encapsulated forms absorb at wavelengths beyond 510 nm, and time-resolved photoluminescence (PL) in solution shows a well-define vibronic structures with a predominance of the 0-0 transitions and an energy difference of 0.16 eV. The fluorescence lifetimes follow a monoexponential decay with a value τ = 630 ± 30 ps. Atomic force microscopy, AFM, indicated a tendency of polyrotaxanes to organize into fibers. The advancing contact angles indicated higher surface hydrophobicity and lower surface free-energy values for polyrotaxanes compared with their unthreaded analogues. The device based on PFBTh•PSβCD:PCBM in a 1/1 w/w ratio under simulated AM 1.5G illumination at 100 mW cm-2 exhibited improved photovoltaic parameters of cells, resulted in high Voc (0.68 V), Jsc (1.65 mA cm-2), FF (31.6%), and PCE (0.35) values, compared with PFBTh or PFBTh•PMeβCD, respectively. © 2013 Wiley Periodicals, Inc.

We report investigations of organic light-emitting diodes with an active layer consisting of poly(9,9-dioctylfluorene), PFO, blended with a green emitting derivative (F8BT). We used indium tin oxide (ITO) anodes with and without a hole transporting layer, consisting of doped poly(3,4-ethylene dioxythiophene). We also studied the incorporation of an oxadiazole based hole-blocking/electron transporting layer, HBL, namely 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole, PBD, between the emissive layer and the Ca/Al cathodes. The presence of this layer reduces the range of radiative recombination in a region closer to the HBL interface, and therefore, allows investigation of depth-dependent structural non-uniformities of the emissive layer. We find marginal alterations of the electroluminescence spectra depending on the weight concentration of the F8BT in the range 5–25%. Efficient energy transfer from the host (PFO) to the guest (F8BT) ensures that the emission spectra are dominated by the green component. However, we find that blue emission from the PFO host is much more intense for the structures incorporating the HBL than for those without it. We propose that phase separation phenomena of the blend induced by the spin-coating of the PBD-based layer, are responsible for the higher weight of the blue component.

De-mixing of Polyfluorene Based Blends by Contact with Acetone: Electro-and Photoluminescence Probes

We present the photo- and electroluminescence properties of the statistical copolymer, poly[2,5-bis(dimethyloctylsilyl)-1,4-phenylenevinylene)]-co-[2,5-bis(triethoxymethoxy)-1,4-phenylenevinylene)], BDMOS-co-BTEM-PPV, designed to combine the high efficiency of BDMOS-PPV, with the ion-coordinating ability of BTEM-PPV. We find that the photoluminescence efficiency is increased to about 17%, from 8.8% for BTEM-PPV. Single-layer light-emitting diodes with calcium (aluminum) cathodes exhibit luminances up to 1300 (10) cd/m2 with an efficiency of ≈0.5 (0.01) cd/A. ITO/BDMOS-co-BTEM-PPV:LiTf (8 wt %)/Al devices display behavior typical of light-emitting electrochemical cells, LECs, with efficiencies up to 0.03 cd/A. These results show that this statistical copolymer approach is successful at improving the photoluminescence efficiency, while also granting sufficient ion coordination for LEC applications.

The influence of carrier-injecting layers on the built-in potential of polymer light-emitting diodes (PLEDs) is analysed by means of electroabsorption spectroscopy. Two promising materials on which research has recently focused are: first, poly(ethylene-dioxythiophene)/poly(styrene sulphonic acid) (PEDOT:PSS), a doped conducting polymer employed as hole injector; and secondly, lithium fluoride (LiF), which has been shown to enhance electron injection of Al-based cathodes. In the first samples LEDs with an indium tin oxide (ITO) anode were compared to those incorporating a PEDOT:PSS layer (in an ITO/PEDOT:PSS bilayer anode). In the second set of samples the comparison was between LEDs with an aluminium cathode and those incorporating a LiF layer (in an Al/LiF bilayer cathode) as a function of LiF thickness. Electroabsorption spectroscopy results supported by work function measurements clearly prove that both PEDOT:PSS and LiF introduce a large reduction in the barrier height at the anode and cathode interfaces, respectively. The ensuing enhancement in carrier injection is considered to be responsible for the marked improvement in device performance (lower operating voltages, higher luminous efficiencies and longer lifetimes).

We report on the luminescence of a tetraphenylporphyrin, TPP-d, blended into poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV. We find significant energy transfer from MEH-PPV to the porphyrin, in spite of the low absorption of the porphyrin at the emission wavelength of MEH-PPV, reflected in a Forster transfer radius (2.5 nm) smaller than for materials with larger spectral overlap. The overall photoluminescence, PL, efficiency decreases monotonically with increasing porphyrin content, whereas the porphyrin contribution to the total efficiency, referred as an “apparent” PL efficiency, exhibits a maximum at 1.4% porphyrin content (by weight). We attribute this non-monotonic behaviour to the interplay of the exciton transfer probability and PL quenching, both of which increase with concentration. We also observed the energy transfer under electrical excitation, but noticed that, at low concentrations, the porphyrin contribution to the electroluminescence is higher than that observed in PL. This indicates significant emission from excitons formed directly at the porphyrin sites, which are likely to act as charge trapping sites. We also compare the luminescence properties of the blends with those of copolymers based on the same host–guest pair.

We have investigated the influence of the addition of indium chloride (InCl 3 ) on the luminescence properties of poly( p -phenylene vinylene) (PPV). We find that the photoluminescence (PL) efficiency of PPV converted on spectrosil, about 27%, is reduced to 21% upon addition of 0.5% molar of InCl 3 , but remains constant for further increase of the InCl 3 concentration up to 10% molar. The PL efficiency of PPV converted on ITO also decreases from 19% to 11% by addition of InCl 3 , as observed for the PPV converted on spectrosil, with similar independence on the InCl 3 concentration. Photothermal deflection spectroscopy (PDS) studies of the PPV–InCl 3 blends show no evidence for subgap absorption bands, as expected for charge transfer doping. We consider that the decrease of the PL efficiency of PPV by addition of InCl 3 is due to ion-induced exciton quenching.

We present the optical properties of four poly(p-phenylenevinylene) (PPV)-based statistical copolymers, which combine monomers of highly luminescent polymers and monomers carrying crown ether side groups. The photoluminescence efficiency for films on spectrosil ranges from 28% to 52%, whereas the electroluminescence (EL) efficiency of single-layer diodes with Al cathodes reaches 0.6 cd/A (internal quantum efficiency of 1.4%). Surprisingly, the efficiency for Ca cathodes diodes is lower than for Al ones. Slight variation of the EL spectra from pixel to pixel affects all the copolymers of the family, and we consider that it is indicative of chain aggregation induced by the crown ether moieties. Studies of light-emitting electrochemical cells, LECs, based on one of these polymers show that the ion mobility induced by the presence of the crown ethers is relatively small. Typical LEC behaviour was observed upon addition of poly(ethylene oxide), with EL internal quantum efficiency reaching 2.3%.

We report photo and electroluminescence efficiency of a thiophene oligomer functionalized to enhance its solid-state efficiency. We find that the absolute PL quantum yield is up to 37% for spin-coated thin films of the compound. The material has been used as active material in organic light-emitting diodes (LEDs). EL efficiencies up to 0.9 cd/A are demonstrated in LEDs prepared with a blend indium tin oxide and Ca–Al electrodes.

Achieving balanced electron-hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (

A new series of soluble porphyrin-containing copolymers have been synthesized via the Gilch methodology [1], for use in light-emitting devices (LEDs). These materials consist of random copolymers of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and a porphyrin-substituted phenylene vinylene. Low concentrations of porphyrin monomers were used in order to improve solubility, and help prevent aggregation (and consequent fluorescence quenching) of the porphyrin units. Emission is red-shifted from that expected in dialkoxy-PPVs such as MEH-PPV owing to an efficient exciton transfer to the fluorescent porphyrin unit.

Angle-resolved X-ray photoelectron spectroscopy and photothermal deflection spectroscopy are used to study the oxygen-plasma or aquaregia treated indium-tin oxide (ITO) anodes for organic light-emitting diodes. Detailed analysis of the Ols core-level spectra and their dependence on photoemission angle was performed. The results indicate the presence of different chemical forms of oxygen atoms (two types of O2-, OH-, organic oxygens and H2O) which evolve with surface treatment. We find that the treatments lead to a modification of the surface chemical states and therefore of the physico-chemical properties of ITO, which in turn control the performance of organic light-emitting diodes. (C) 1999 Elsevier Science B.V. All rights reserved.

We report the phase-separation and the optoelectronic properties of an alternating poly(p-phenylene vinylene)-based copolymer, to which poly(ethylene oxide), PEO, and or lithium triflate are added. The alternating units of this copolymer (DB-BTEM-PPV) are 2,3-dibutoxy-1,4-phenylene vinylene, and 2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene, a moiety containing side groups allowing ion solvation and transport. Upon addition of the ion-transporting polymer PEO to DB-BTEM-PPV blended with lithium triflate, we have found a sizeable efficiency increase (from 0.9 to 1.5 cd/A), with a concomitant increase of the response time. We propose that this is due to solvation and complexation of lithium triflate by the PEO, which simultaneously reduces the quenching of photoluminescence (and electroluminescence) efficiency by the ionic charge, and the effectiveness of formation of highly doped, low-barrier, polymer/electrode interfaces. We discuss charge transport and injection in the copolymer and in the blend with reference to the diodes characteristics, and to the phase separation of PEO, which we investigated with atomic force microscopy.