THREADMILL

THREADed Molecular wIres as supramoLecularly engineered muLtifunctional materials

Poly(9,9'-dioctyl fluorene) (F8) and poly(4,4'-diphenylene diphenylvinylene) (PDPV) are conjugated polymers with optical transitions that are nonresonant thanks to the particular structural features of PDPV that yield a very large Stokes’ shift (1.14 eV) between absorption and emission spectra. We present steady-state and time-resolved photoluminescence (PL) experiments showing that F8:PDPV blends are “optically disconnected” systems for which the emission spectra and PL quantum yields are the linear combination of the contributions of the individual constituents with weights given by the respective absorption coefficients and concentration in the films. Single-layer light-emitting diodes incorporating F8:PDPV blends show white electroluminescence resulting from the simultaneous exploitation of the spectral features of both blend constituents (Commission Internationale de l’Eclairage, CIE, coordinates: x=0.27 and y=0.36, in the case of 8:2 F8:PDPV molar ratio).

(a) EL EQE and (c) EL spectra for (ITO/PEDOT:PSS/polymer/Ca/Al) LEDs incorporating F8, PDPV and the relative binary blends. (b) Position of the HOMO and LUMO levels for the isolated materials with no applied field. (d) CIE chromaticity diagram of EL and PLspectra.

Enhanced Luminescence Properties of Highly Threaded Conjugated Polyelectrolytes with Potassium Counter-Ions upon Blending with Poly(ethylene oxide)
Gianluca Latini, Gustaf Winroth, Sergio Brovelli, Shane O. McDonnell, Harry L. Anderson, Jeffrey M. Mativetsky, Paolo Samorì and Franco Cacialli
Published Online: 22 June 2010
DOI: 10.1063/1.3372616

The photophysics and electroluminescence (EL) of thin films of unthreaded and cyclodextrin-encapsulated poly(4,4'-diphenylenevinylene) (PDV) with potassium countercations, blended with poly(ethylene oxide) (PEO) are investigated as a function of the PEO concentration.
We show that three main factors contribute to increasing the photoluminescence (PL) quantum efficiency as a result of suppressed intermolecular interactions, namely: the high degree of encapsulation of the polyrotaxanes, the relatively large countercation (e.g., compared to lithium), and the complexation of the rotaxanes with PEO. By facilitating cationic transport to the negative electrodes, PEO also leads to devices with enhanced electron injection and improved charge balance, whose operation therefore resembles that of “virtually unipolar” light-emitting electrochemical cells. This effect, together with the enhanced PL efficiency, leads to higher EL efficiency for both polyrotaxanes and unthreaded polymers, upon addition of the PEO.We show that the concurrent exploitation of the various strategies above lead to an overall EL efficiency that is approximately twice the value previously reported for Li-based PDV. A blueshift of the EL spectrum during the devices turn-on is also reported and analyzed in terms of interference and doping effects.

A schematic band diagram for an LED device incorporating polyelectrolytes
as active layer. At zero applied voltage (a), the mobile countercations migrate at the interface with ITO thus partially screening the bulk
from the built-in electric field. Under forward bias (b), the positive ions accumulate at the cathode, reducing the width of the electron injection barrier. (c) External quantum efficiency . The average of the maximum external quantum effciency of the devices tested (more than 10 devices per type) as a function of the PEO concentration for the unthreaded (dashed line) and threaded polymers (solid line). Error bars are the standard deviations.

Electrochemical synthesis of PEDOT derivatives bearing imidazolium-ionic liquid moieties (p 3010-3021)
Markus Döbbelin, Cristina Pozo-Gonzalo, Rebeca Marcilla, Raúl Blanco, José L. Segura, Jose A. Pomposo, David Mecerreyes
Published Online: May 7 2009 10:55AM
DOI: 10.1002/pola.23384

Novel poly(3,4-ethylenedioxythiophene) (PEDOT) polymers bearing imidazolium-ionic liquid moieties were synthesized by electrochemical polymerization. The polymers could be dissolved in a range of polar organic solvents such as dimethylformamide, propylene carbonate, and dimethyl sulfoxide making them interesting candidates for wet processing methods. Interestingly, the hydrophobic character of electropolymerized films could be modified depending on the anion type. The hydrophobicity followed the trend PF > (CF3SO2)2N- > BF > pure PEDOT as determined by water contact angle measurements.

Poly(4,4-diphenylenevinylene) rotaxanes and [2]rotaxanes with -, -, -cyclodextrin macrocycles were synthesised and their sensitivities to fluorescence quenching by methyl viologen in aqueous solution were determined, relative to uninsulated analogues. Stern–Volmer analysis revealed that the fluorescence quenching response of polyrotaxanes is strongly dependent on the diameter of the cyclodextrins. Polyrotaxanes, composed of the smaller diameter - or -cyclodextrins, are the least easily quenched, with Stern–Volmer constants about two orders of magnitude smaller than from the wider -cyclodextrin polyrotaxane and the uninsulated polymer. Time-resolved photoluminescence results demonstrate the crucial role of interchain aggregation on the sensitivity to fluorescence quenchers. The materials with the highest Stern–Volmer constants exhibit the most biexponential photoluminescence decay, which is indicative of aggregation, and the emission spectra of solutions containing methyl viologen resemble the early-time emission spectra (0–3 ns after excitation) of the unquenched samples. The results show that the threaded -cyclodextrin is effective in preventing aggregation, and in hindering fluorescence quenching, even when only a small fraction of the conjugated polymer is encapsulated. This conclusion is relevant to the application of these materials in optoelectonic devices, such as light-emitting diodes, where it is essential to prevent luminescence quenching without hindering charge transport.

(J. Mater. Chem., 2009, 19, 2846 - 2852, DOI: 10.1039/b821950h)

Tuning Intrachain versus Interchain Photophysics via Control of the Threading Ratio of Conjugated Polyrotaxanes
Sergio Brovelli, Gianluca Latini, Michael J. Frampton, Shane O. McDonnell, Francine E. Oddy, Oliver Fenwick, Harry L. Anderson and Franco Cacialli

Effective nanoscale control of intermolecular interactions in conjugated polymers is needed for the optimal development and exploitation of the latter in low-cost, large-area consumer electronics items, such as light-emitting and photovoltaic diodes, or transistors. Here we report our investigations on insulated molecular wires constituted by conjugated polymers threaded into cyclodextrin rings. Until now, there has been no detailed quantitative understanding of the role of progressive cyclodextrin encapsulation (quantifiable by the so-called “threading ratio”, TR, or number of cyclodextrins per repeat unit) in tailoring the photophysics of the conjugated polymeric wires. We combine spectroscopic, electrical and surface analysis techniques to elucidate how the TR of cyclodextrin-threaded molecular wires controls formation of interchain species and related physical properties (0 < TR = 2.3; the maximum theoretical TR for close-packed CDs is 2.8). Increasing TR enhances the solid-state photoluminescence (PL) and electroluminescence quantum efficiency. To unravel the effect of progressive encapsulation on the intrachain decay kinetics of the polymer backbone, we added an electron-accepting quenching agent, methyl viologen (MV), to the polymer solutions. MV predominantly quenches the aggregate PL, thus enabling measurement of the decay kinetics of the intrinsic exciton even for low-TR polyrotaxanes, for which the different contributions are otherwise difficult to disentangle.
(Nano Lett., 2008, 8 (12), pp 4546–4551)

Control of Rapid Formation of Interchain Excited States in Sugar-Threaded Supramolecular Wires 
Annamaria Petrozza, Sergio Brovelli, Jasper J. Michels, Harry L. Anderson, Richard H. Friend, Carlos Silva, Franco Cacialli

The time-resolved photoluminescence of cyclodextrin-threaded conjugated molecular wires and their unthreaded analogues is studied to probe suppression of intermolecular interactions by supramolecular encapsulation. The polyrotaxane dynamics are exponential and independent of concentration, different from those of unthreaded semiconductors, which display instead optical signatures of interchain species. This is fundamental insight into the photophysics of semiconducting polymers, and crucial for the development of organic electronics.
(Advanced Materials, 2008, 20, 17, 3218-3223)

Synthesis and Optoelectronic Properties of Nonpolar Polyrotaxane Insulated Molecular Wires with High Solubility in Organic Solvents 
Michael J. Frampton, Giuseppe Sforazzini, Sergio Brovelli, Gianluca Latini, Emily Townsend, Charlotte C. Williams, Ana Charas, Leszek Zalewski, Naeem S. Kaka, Mallena Sirish, Lisa J. Parrott, Joanne S. Wilson, Franco Cacialli, Harry L. Anderson

Alkylation, silylation, and esterification are used to inflate the insulation around a conjugated polymer backbone, making the polyrotaxane soluble in nonpolar solvents, increasing the photoluminescence efficiency in the solid state, and simplifying fluorescence decay behavior.
(Advanced Functional Materials, 2008, 18, 3367–3376)

Ground- and Excited-State Pinched Cone Equilibria in Calix[4]arenes Bearing Two Perylene Bisimide Dyes
Catharina Hippius, Ivo H. M. van Stokkum, Ennio Zangrando, René M. Williams, Michael Wykes, David Beljonne and Frank Würthner

We report on a series of bis-chromophoric compounds o2c, g2c, and r2c, afforded by linking two identical orange, green, or red perylene bisimide (PBI) units, respectively, through a calix[4]arene spacer unit.
(J. Phys. Chem. C 2008, 112, 14626–14638 )

Radical Cation Stabilization in a Cucurbituril Oligoaniline Rotaxane
Rienk Eelkema, Kiminori Maeda, Barbara Odell, and Harry L. Anderson
A cucurbituril-encapsulated oligoaniline rotaxane is synthesized in high yield by reductive amination in aqueous acid. Spectroscopic and electrochemical measurements show that encapsulation by cucurbit[7]uril increases the thermodynamic and kinetic stability of the radical cation of the threaded oligoaniline. This radical cation has a stoichiometry similar to that of the conducting emeraldine salt form of polyaniline, which implies that cucurbituril insulation of polyaniline might drastically alter the oxidation potential of the threaded conjugated polymer chain.
(J. Am. Chem. Soc. 2007, 129, 12384–12385)

Cyclodextrin-Threaded Conjugated Polyrotaxanes for Organic Electronics: The Influence of the Counter Cations 

Gianluca Latini, Lisa-Jodie Parrott, Sergio Brovelli, Michael J. Frampton, Harry L. Anderson, Franco Cacialli
The influence of counter cations on the optical properties of conjugated polyelectrolytes and of their cyclodextrin-threaded rotaxanes is studied. Exchange of Li+ with K+, Cs+, or Me4N+ results in higher electroluminescence efficiency for both rotaxanes and unthreaded polymers, narrowing (unthreaded analogues) and a slight red-shift of the absorption spectra (rotaxanes) and a blue-shift and PL efficiency enhancement of the luminescence.
(Advanced Functional Materials, 18, 16,2419-2427,2008)

Amylose-wrapped luminescent conjugated polymers

Michael J. Frampton, Timothy D. W. Claridge, Gianluca Latini, Sergio Brovelli, Franco Cacialli and Harry L. Anderson

Highly luminescent inclusion complexes consisting of poly(para-phenylene) ( PPP) or poly(4,4-diphenylene-vinylene) ( PDV) in the helical cavity of amylose have been synthesised, structurally characterised by nuclear Overhauser spectroscopy and used to fabricate electroluminescent light-emitting diodes.
(Chemical Communications, 24, 2797-2799, 2008 )

Nucleation-Governed Reversible Self-Assembly of an Organic Semiconductor at Surfaces: Long-Range Mass Transport Forming Giant Functional Fibers 

G. De Luca, A. Liscio, P. Maccagnani, F. Nolde, V. Palermo, K. Müllen, P. Samorì

Solvent-vapor annealing has been successfully employed to increase the degree of order in the self-organization of a perylene-bis(dicarboximide) derivative cast on different substrates from solution (see figure). Upon exposure to tetrahydrofuran vapors, the starting needle-like structures rearrange into millimeter-long fibers, which have a sub-micrometer cross section, with remarkable mass transport at the surface.
(Adv. Funct. Mater., 2007, 17, 3791-3798 & 3687)