Highly Deformed o-Carborane Functionalised Non-linear Polycyclic Aromatics with Exceptionally Long C-C Bonds.

The effect of substituting o-carborane into the most sterically hindered positions of phenanthrene and benzo(k)tetraphene is reported. Synthesised via a Bull-Hutchings-Quayle benzannulation, the crystal structures of these non-linear acenes exhibited the highest aromatic deformation parameters observed for any reported carborane compound to date, and among the largest carboranyl C-C bond length of all organo-substituted o-carboranes. Photoluminescence studies of these compounds demonstrated efficient intramolecular charge-transfer, leading to aggregation induced emission properties. Additionally, an unusual low-energy excimer was observed for the phenanthryl compound. These are two new members of the family of carborane-functionalised non-linear acenes, notable for their peculiar structures and multi-luminescent properties.

Carborane-Induced Excimer Emission of Severely Twisted Bis-o-Carboranyl Chrysene.

The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o-carboranyl groups is reported. The molecule exhibits a complex, excitation-dependent photoluminescence, including aggregation-induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o-carborane. This is the first time that o-carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene-based small molecule in solution. Bis-o-carboranyl chrysene is thus an initial member of a new family of o-carboranyl phenacenes exhibiting a novel architecture for highly-efficient multi-luminescent fluorophores.

Heteroatomic Conjugated Polymers and the Spectral Tuning of Electroluminescence via a Supramolecular Coordination Strategy.

The unique electronic structures of heteroatomic conjugated polymers (HCPs) offer an attractive platform to tune optoelectronic properties via a supramolecular coordination strategy. This study reports on an sp2 nitrogen heteroatom containing fluorene-based copolymer namely poly(9,9-dioctylfluorene-co-9,9-dioctyldiazafluoren-2,7-yl) (PF8-co-DAF8), with ≈20% DAF8 units. Tuning the optoelectronic properties of PF8-co-DAF8 via supramolecular coordination with a Lewis acid (B(C6 F5 )3 or AlCl3 ) is explored. Formation of either the PF8-co-DAF8-B(C6 F5 )3 or PF8-co-DAF8-AlCl3 adducts reduces the optical gap and causes an attendant redshift of the photoluminescence spectra. Controlling the degree and strength of the coordination allows the emission color to be tuned from blue through to green and yellow. This strategy is successfully implemented for polymer light-emitting diodes, confirming the large degree of spectral tuning whilst maintaining good device performance. Maximum luminous efficiencies, η ≈ 1.55 cd A-1 @ 2120 cd m-2 , 1.32 cd A-1 @ 1424 cd m-2 , and 2.56 cd A-1 @ 910 cd m-2 are, respectively, recorded for the blue-emitting diodes with Commission Internationale de L'Eclairage (CIE) (x, y) coordinates = (0.16, 0.16), the white-emitting diodes with CIE (x, y) = (0.28, 0.38) and the green-emitting diodes with CIE (x, y) = (0.33, 0.52). The results highlight the versatility of the supramolecular coordination strategy in modifying the electronic structure of HCPs.

Wavefront kinetics of plasma oxidation of polydimethylsiloxane: limits for sub-µm wrinkling.

We investigate the surface plasma oxidation of polydimethylsiloxane (PDMS) elastomers and its implication for the morphologies attainable by wrinkling of glassy-elastomer 'bilayers'. The kinetics of glassy skin formation is found to follow a logarithmic dependence with plasma exposure time t and, for various plasma intensities I, the relevant control variable is shown to be dose (≡I × t). We model the mechanism and kinetics of glassy film formation by plasma oxidation with a frontal propagation coarse-grained model, describing the spatio-temporal evolution of a conversion order parameter (ϕ) orthogonal to the film surface. The model is validated by X-ray reflectivity experiments, which confirm the logarithmic growth and quantify the initial growth of a transient, incomplete, skin layer during the early stage of plasma exposure. Three regimes are identified as (I) induction, (II) formation and (III) propagation with a combination of X-ray and wrinkling experiments. The simultaneous increase in thickness and skin mechanical modulus is found to be responsible for an unexpected minimum wavelength λmin attainable, which depends on critical strain εc and is ultimately limited by mechanical failure of the elastomer (λmin ≃ 140 nm is demonstrated at ε = 200%). We conclude by establishing a 1D surface morphology diagram, in terms of wavelength λ and amplitude A, limitations and capabilities for producing highly ordered (sub-)micropatterns over macroscopic areas using plasma oxidised PDMS under uniaxial strain.

Confined surface plasmon-polariton amplifiers.

We demonstrate the realization of confined surface plasmon polariton amplifiers using a thin layer of the organic gain medium 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dispersed in a tris(8-hydroxy-quinolinato)aluminum matrix. Complete loss compensation, which occurs at a pump fluence of approximately 200 µJ/cm(2), is directly observed in the time domain and studied for a range of waveguide lengths. The power dependence is also reported, and a significant net gain of 93 dB/mm is observed at the highest fluence.

Simple and Versatile Platforms for Manipulating Light with Matter: Strong Light-Matter Coupling in Fully Solution-Processed Optical Microcavities.

Planar microcavities with strong light-matter coupling, monolithically processed fully from solution, consisting of two polymer-based distributed Bragg reflectors (DBRs) comprising alternating layers of a high-refractive-index titanium oxide hydrate/poly(vinyl alcohol) hybrid material and a low-refractive-index fluorinated polymer are presented. The DBRs enclose a perylene diimide derivative (b-PDI-1) film positioned at the antinode of the optical mode. Strong light-matter coupling is achieved in these structures at the target excitation of the b-PDI-1. Indeed, the energy-dispersion relation (energy vs in-plane wavevector or output angle) in reflectance and the group delay of transmitted light in the microcavities show a clear anti-crossing-an energy gap between two distinct exciton-polariton dispersion branches. The agreement between classical electrodynamic simulations of the microcavity response and the experimental data demonstrates that the entire microcavity stack can be controllably produced as designed. Promisingly, the refractive index of the inorganic/organic hybrid layers used in the microcavity DBRs can be precisely manipulated between values of 1.50 to 2.10. Hence, microcavities with a wide spectral range of optical modes might be designed and produced with straightforward coating methodologies, enabling fine-tuning of the energy and lifetime of the microcavities' optical modes to harness strong light-matter coupling in a wide variety of solution processable active materials.

Planar refractive index patterning through microcontact photo-thermal annealing of a printable organic/inorganic hybrid material.

We demonstrate proof-of-concept refractive-index structures with large refractive-index-gradient profiles, using a micro-contact photothermal annealing (µCPA) process to pattern organic/inorganic hybrid materials comprising titanium oxide hydrate within a poly(vinyl alcohol) binder. A significant refractive index modulation of up to Δn ≈ +0.05 can be achieved with µCPA within less than a second of pulsed lamp exposure, which promises the potential for a high throughput fabrication process of photonic structures with a polymer-based system.

Understanding hierarchical spheres-in-grating assembly for bio-inspired colouration.

The vivid iridescent response from particular butterflies is as an excellent example of how micro-engineered hierarchical architectures that combine physical structures and pigmentary inclusions create unique colouration. To date, however, detailed knowledge is missing to replicate such sophisticated structures in a robust, reliable manner. Here, we deliver spheres-in-grating assemblies with colouration effects as found in nature, exploiting embossed polymer gratings and self-assembled light-absorbing micro-spheres.

Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films.

The combination of efficient light emission and high charge-carrier mobility has thus far proved elusive for polymer semiconductors, with high mobility typically achieved by cofacial pi-electron system to pi-electron system interactions that quench exciton luminescence. We report a new strategy, comprising the introduction of a limited number of more effective hopping sites between otherwise relatively isolated, and thus highly luminescent, polyfluorene chains. Our approach results in polymer films with large mobility (mu approximately 3-6 x 10(-2) cm2 V-1 s-1) and simultaneously excellent light-emission characteristics. These materials are expected to be of interest for light-emitting transistors, light-emitting diode sources for optical communications and may offer renewed hope for electrically pumped laser action. In the last context, optically pumped distributed feedback lasers comprising one-dimensional etched silica grating structures coated with polymer have state-of-the-art excitation thresholds (as low as 30 W cm(-2) (0.1 nJ per pulse or 0.3 microJ cm-2) for 10 Hz, 12 ns, 390 nm excitation) and slope efficiencies (up to 11%).

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends.

Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4-5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.

Ultrastable Supramolecular Self-Encapsulated Wide-Bandgap Conjugated Polymers for Large-Area and Flexible Electroluminescent Devices.

Controlling chain behavior through smart molecular design provides the potential to develop ultrastable and efficient deep-blue light-emitting conjugated polymers (LCPs). Herein, a novel supramolecular self-encapsulation strategy is proposed to construct a robust ultrastable conjugated polydiarylfluorene (PHDPF-Cz) via precisely preventing excitons from interchain cross-transfer/coupling and contamination from external trace H2 O/O2 . PHDPF-Cz consists of a mainchain backbone where the diphenyl groups localize at the 9-position as steric bulk moieties, and carbazole (Cz) units localize at the 4-position as supramolecular π-stacked synthon with the dual functionalities of self-assembly capability and hole-transport facility. The synergistic effect of the steric bulk groups and π-stacked carbazoles affords PHDPF-Cz as an ultrastable property, including spectral, morphological stability, and storage stability. In addition, PHDPF-Cz spin-coated gelation films also show thickness-insensitive deep-blue emission with respect to the reference polymers, which are suitable to construct solution-processed large-scale optoelectronic devices with higher reproducibility. High-quality and uniform deep-blue emission is observed in large-area solution-processed films. The electroluminescence shows high-quality deep-blue intrachain emission with a CIE (0.16, 0.12) and a very narrow full width at half-maximum of 32 nm. Finally, large-area and flexible polymer light-emitting devices with a single-molecular excitonic behavior are also fabricated. The supramolecular self-encapsulation design provides an effective strategy to construct ultrastable LCPs for optoelectronic applications.

Identifying triplet pathways in dilute pentacene films.

Building efficient triplet-harvesting layers for photovoltaic applications requires a deep understanding of the microscopic properties of the components involved and their dynamics. Singlet fission is a particularly appealing mechanism as it generates two excitons from a single photon. However, the pathways of the coupled triplets into free species, and their dependence on the intermolecular geometry, has not been fully explored. In this work, we produce highly ordered dilute pentacene films with distinct parallel and herringbone dimers and aggregates. Using electron paramagnetic resonance spectroscopy, we provide compelling evidence for the formation of distinct quintet excitons in ambient conditions, with intrinsically distinctive electronic and kinetic properties. We find that the ability of quintets to separate into free triplets is promoted in the parallel dimers and this provides molecular design rules to control the triplets, favouring either enhanced photovoltaic efficiency (parallel) or strongly bound pairs that could be exploited for logic applications (herringbone).

Controlling Molecular Conformation for Highly Efficient and Stable Deep-Blue Copolymer Light-Emitting Diodes.

We report a novel approach to achieve deep-blue, high-efficiency, and long-lived solution-processed polymer light-emitting diodes (PLEDs) via a simple molecular level conformation change of an emissive conjugated polymer. We introduce rigid ß-phase segments into a 95% fluorene-5% arylamine copolymer emissive layer. The arylamine moieties at low density act as efficient exciton formation sites in PLEDs, whereas the conformational change alters the nature of the dominant luminescence from a broad, charge transfer like emission to a significantly blue-shifted and highly vibronically structured excitonic emission. As a consequence, we observe a significant improvement in the Commission International de L'Eclairage ( x, y) coordinates from (0.149, 0.175) to (0.145, 0.123) while maintaining high efficiency and improved stability. We achieve a peak luminous efficiency, η = 3.60 cd/A, and a luminous power efficiency, ηw = 2.44 lm/W, values that represent state-of-the-art performance for single copolymer deep-blue PLEDs. These values are 5-fold better than for otherwise-equivalent, ß-phase poly(9,9-dioctylfluorene) PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a simple but effective method to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

Pentafluorobenzene end-group as a versatile handle for para fluoro "click" functionalization of polythiophenes.

A convenient method of introducing pentafluorobenzene (PFB) as a single end-group in polythiophene derivatives is reported via in situ quenching of the polymerization. We demonstrate that the PFB-group is a particularly useful end-group due to its ability to undergo fast nucleophilic aromatic substitutions. Using this molecular handle, we are able to quantitatively tether a variety of common nucleophiles to the polythiophene backbone. The mild conditions required for the reaction allows sensitive functional moieties, such as biotin or a cross-linkable trimethoxysilane, to be introduced as end-groups. The high yield enabled the formation of a diblock rod-coil polymer from equimolar reactants under transition metal-free conditions at room temperature. We further demonstrate that water soluble polythiophenes end-capped with PFB can be prepared via the hydrolysis of an ester precursor, and that such polymers are amenable to functionalization under aqueous conditions.

Supramolecular Polymer-Molecule Complexes as Gain Media for Ultraviolet Lasers.

A novel supramolecular system comprising a complex of 9,9'-diphenyl-9H,9'H-2,2'-bifluorene-9,9'-diol (DPFOH) with poly(methyl methacrylate) (PMMA) is presented as an attractive system for optical gain in the ultraviolet. The analogue compound 9,9'-diphenyl-9H,9'H-2,2'-bifluorene (DPFO8) without an -OH substituent was synthesized alongside DPFOH to confirm the importance of its chemical structure to the thin-film microstructure. A hydrogen-bonding interaction allows the molecule such as DPFOH and a combination of DPFOH and PMMA to have an excellent solution-processed high quality coating film. In stark contrast to the DPFO8 system, we find that the addition of 1 wt % DPFOH to PMMA leads to spontaneous formation of a supramolecular complex via hydrogen bonding interactions, giving rise to a homogeneous film with relatively high photoluminescence quantum efficiency ∼38 (±5)%. The demonstration of ultraviolet laser action with peak wavelength emission at 385 nm provided further evidence of the high optical quality of the DPFOH/PMMA supramolecular complex films. The DPFOH/PMMA supramolecular complex has great potential for use in low-cost solution-processed optoelectronic devices.

Solution-Crystallization and Related Phenomena in 9,9-Dialkyl-Fluorene Polymers. I. Crystalline Polymer-Solvent Compound Formation for Poly(9,9-dioctylfluorene).

Polymer-solvent compound formation, occurring via co-crystallization of polymer chains and selected small-molecular species, is demonstrated for the conjugated polymer poly(9,9-dioctylfluorene) (PFO) and a range of organic solvents. The resulting crystallization and gelation processes in PFO solutions are studied by differential scanning calorimetry, with X-ray diffraction providing additional information on the resulting microstructure. It is shown that PFO-solvent compounds comprise an ultra-regular molecular-level arrangement of the semiconducting polymer host and small-molecular solvent guest. Crystals form following adoption of the planar-zigzag ß-phase chain conformation, which, due to its geometry, creates periodic cavities that accommodate the ordered inclusion of solvent molecules of matching volume. The findings are formalized in terms of nonequilibrium temperature-composition phase diagrams. The potential applications of these compounds and the new functionalities that they might enable are also discussed. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1481-1491.

Solution-crystallization and related phenomena in 9,9-dialkyl-fluorene polymers. II. Influence of side-chain structure.

Solution-crystallization is studied for two polyfluorene polymers possessing different side-chain structures. Thermal analysis and temperature-dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X-ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar-zigzag chain conformation termed the ß-phase, which is observed for certain linear-side-chain polyfluorenes, is necessary for the formation of so-called polymer-solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side-chains prevents formation of the ß-phase conformation and results in non-solvated, i.e. melt-crystallization-type, polymer crystals. Unlike non-solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the ß-phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter-chain interactions. The presented results clarify the fundamental differences between the ß-phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1492-1506.

High-efficiency, solution-processed, multilayer phosphorescent organic light-emitting diodes with a copper thiocyanate hole-injection/hole-transport layer.

Copper thiocyanate (CuSCN) is introduced as a hole-injection/hole-transport layer (HIL/HTL) for solution-processed organic light-emitting diodes (OLEDs). The OLED devices reported here with CuSCN as HIL/HTL perform significantly better than equivalent devices fabricated with a PEDOT:PSS HIL/HTL, and solution-processed, phosphorescent, small-molecule, green OLEDs with maximum luminance ≥10 000 cd m(-2) , maximum luminous efficiency ≤50 cd A(-1) , and maximum luminous power efficiency ≤55 lm W(-1) are demonstrated.

Dip-pen patterning of poly(9,9-dioctylfluorene) chain-conformation-based nano-photonic elements.

Metamaterials are a promising new class of materials, in which sub-wavelength physical structures, rather than variations in chemical composition, can be used to modify the nature of their interaction with electromagnetic radiation. Here we show that a metamaterials approach, using a discrete physical geometry (conformation) of the segments of a polymer chain as the vector for a substantial refractive index change, can be used to enable visible wavelength, conjugated polymer photonic elements. In particular, we demonstrate that a novel form of dip-pen nanolithography provides an effective means to pattern the so-called ß-phase conformation in poly(9,9-dioctylfluorene) thin films. This can be done on length scales ≤500 nm, as required to fabricate a variety of such elements, two of which are theoretically modelled using complex photonic dispersion calculations.

Pulse delay and propagation through subwavelength metallic slits.

The transmission properties of a 2D metallic grating are investigated at optical wavelengths for an incident Gaussian pulse having pulse widths from 100 fs to 10 ps. The slits in the grating are subwavelength which can nevertheless allow significant transmission in the narrow wavelength regions where the so-called surface plasmon polariton (SPP) and waveguide mode resonances occur. The solution is obtained for each spectral component of the pulse by using the rigorous coupled wave approach and then the temporally varying output pulse is reconstructed by the standard method of taking an inverse Fourier transform. The delay of the pulse and the output pulse widths are determined by taking the first and second order moments of the Poynting vector with respect to time. It is shown that the time delay may be significant, as much as 256 fs for a pulse width of 200 fs for the SPP resonance but quite small (32 fs) for the waveguide mode resonance. The focus of the work is on demonstrating how the pulse delay evolves as the pulse propagates in the half-space beyond the grating. It is shown that the distance over which the time delay develops is much larger than the actual longitudinal dimension of the grating structure and it is approximately the same distance over which the stored energy and the vortices of the Poynting vector extend.