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.
In 1962, Mark and Helfrich demonstrated that the current in a semiconductor containing traps is reduced by N/Nt(r), with N the amount of transport sites, Nt the amount of traps and r a number that depends on the trap energy distribution. For r > 1, the possibility opens that trapping effects can be nearly eliminated when N and Nt are simultaneously reduced. Solution-processed conjugated polymers are an excellent model system to test this hypothesis, because they can be easily diluted by blending them with a high-bandgap semiconductor. We demonstrate that in conjugated polymer blends with 10% active semiconductor and 90% high-bandgap host, the typical strong electron trapping can be effectively eliminated. As a result we were able to fabricate polymer light-emitting diodes with balanced electron and hole transport and reduced non-radiative trap-assisted recombination, leading to a doubling of their efficiency at nearly ten times lower material costs.
A series of conjugated polyrotaxane insulated molecular wires are synthesised by aqueous Suzuki polymerisation, using hydrophobic binding to promote threading of the cyclodextrin units. These polyrotaxanes have conjugated polymer cores based on poly(para-phenylene), polyfluorene, and poly(diphenylene-vinylene), threaded through 0.9-1.6 cyclodextrins per repeat unit. Bulky naphthalene-3,6-disulfonate endgroups prevent the macrocycles from slipping off the conjugated polymer chains. Dialysis experiments show that the cyclodextrins become unthreaded only if smaller stoppers are used. MALDI TOF mass spectra detect oligomers with up to ten threaded cyclodextrins, and reveal the presence of some defects that result for oxidative homo-coupling of boronic acids. Weight-average molecular weights were determined by analytical ultracentrifugation, demonstrating that step-growth polymerisation is efficient enough to achieve degrees of polymerisation up to approximately 20 repeat units (84 para-phenylenes). The fluorescence spectra of these polyrotaxanes indicate that the presence of the threaded cyclodextrin macrocycles reduces the flexibility of the conjugated polymer pi-systems. Both the solution and the solid-state photoluminescence quantum yields are enhanced upon threading of the conjugated polyaromatic cores through alpha- or beta-cyclodextrins, and the emission spectra of the polyrotaxanes are blue-shifted compared to the corresponding unthreaded polymers. The greater weight of the 0-0 transition in the emission spectra, as well as the smaller Stokes shift, indicate that the polyrotaxanes are more rigid than the unthreaded polymers.
We have investigated the ferroelectric phase diagram of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA). The binary nonequilibrium temperature composition diagram was determined and melting of αand β-phase PVDF was identified. Ferroelectric β-PVDF:PMMA blend films were made by melting, ice quenching, and subsequent annealing above the glass transition temperature of PMMA, close to the melting temperature of PVDF. Addition of PMMA suppresses the crystallization of PVDF and, as a consequence, the roughness of blend films was found to decrease with increasing PMMA content. Using timedependent 2D numerical simulations based on a phase−field model, we qualitatively reproduced the effect of PMMA on the crystallization rate and the spherulite shape of PVDF. The remnant polarization scaled with the degree of crystallinity of PVDF. The thermal stability of the polarization is directly related to the Curie temperature. We show that, at high temperature, the commodity ferroelectric PVDF:PMMA blends outperform the commonly employed specialty copolymer poly(vinylidene fluoride−trifluoroethylene) (P(VDF−TrFE)).
Solution processability endows polymer semiconductors with several intriguing prospects, from low-cost processing, such as inkjet printing, to the possibility of creating new materials by simply mixing solutions. Polymer blends have already been exploited in light-emitting diodes (LEDs) [1] and photovoltaic diodes, [2,3] as well as light-emitting electrochemical cells (LECs), [4] although the factors controlling their supramolecular structures [5] and properties are not fully understood. Most polymer blends undergo phase segregation. [6] This has been used to generate large-area heterojunctions, [2] but it can be detrimental where solid solutions are sought to increase photoluminescence (PL) efficiency, and where rough surface morphology is a problem. Here we use three supramolecular strategies to prepare a complex material that has none of these drawbacks and benefits from enhanced electroluminescence properties: firstly, ionic interactions favor mixing of a conjugated polyelectrolyte with poly(ethylene oxide), PEO, preventing phase segregation and boosting the PL efficiency; secondly, the PEO facilitates ion transport and allows fabrication of LEC-like devices which display a two orders-of-magnitude increase in the electroluminescence (EL) efficiency; thirdly, threading the conjugated polymer through cyclodextrins gives higher PL efficiencies at small PEO loadings, and increases the EL efficiency over the full range of PEO concentrations. Insulated molecular wires, IMWs, consisting of conjugated polymers threaded through cyclodextrin rings (b-CD-poly(paraphenylene) (b-CD-PPP), b-CD-poly(fluorene) (b-CD-PF), a-CD-poly(4,4′-diphenylene vinylene) (a-CD-PDV), and b-CD-poly(4,4′-diphenylene vinylene) (b-CD-PDV); Fig. 1) are versatile supramolecular architectures [7] that display a reduced degree of interchain interactions reflected in higher electroluminescence efficiency, blue-shifted absorption/emission, and reduced luminescence quenching and packing density, when compared to their uninsulated analogues (PPP, PDV, and PF). [8] In this paper we exploit their polyelectrolytic nature, and use the presence of lithium carboxylate and sulfonate substituents to drive the formation of supramolecular complexes with polymers featuring ion-coordination properties. This supramolecular assembly enables us to reduce the tendency of the different components to phase separate, to promote smooth surface morphologies, and to boost the PL and EL efficiency. The interaction of PEO with polyrotaxanes in aqueous solution was tested by fluorescence titration, using b-CD-PDV and PDV. This experiment revealed that both conjugated polymers bind PEO strongly even under extremely dilute conditions (1 ppm PEO by weight, ca. 1 × 10 -8 mol dm -3 of both components). The fluorescence spectra of PDV at a range of PEO concentrations are shown in Figure 2a, and the corresponding titration curve is plotted in Figure 2b (see Fig. S1 in the Supporting Information for analogous data for b-CD-PDV). The titration curves for b-CD-PDV and PDV fit remark...
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