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PFBP .Me β -CD.Si. PFBP .Bn. Electro-optical Studies of Threaded Molecular Wires. Sergio Brovelli, Gustaf Winroth and Franco Cacialli. London Centre for Nanotechnology, and Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom.

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Electro-optical Studies of Threaded Molecular Wires

Sergio Brovelli, Gustaf Winroth and Franco Cacialli

London Centre for Nanotechnology, and Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom

Optoelectronic properties of non polar polyrotaxane insulated molecular wires with high solubility in organic solvents

Enhanced electroluminescence from threaded molecular wires via fine tuning of their threading ratio

A new class of organic soluble polyrotaxanes was characterized by means of steady-state and time-resolved photoluminescence experiments. The experimental results confirm the molecular modeling that indicates a more insulated structure for the organic-functionalized materials. The organic-soluble polyrotaxanes were processed into polymer light-emitting diodes (PLEDs) by solution processing from non-polar organic solvents such as chloroform, thereby excluding ionic impurities from the active layer.

Effective control of intermolecular interactions and of their influence on the photophysics of conjugated polymers is needed for their best development and exploitation in low-cost, large-area, and even disposable consumer electronics items, such as light-emitting1 and photovoltaic diodes, and transistors.

Here, we combine spectroscopic, electrical and surface analysis techniques to elucidate how the threading ratio (TR) controls formation of interchain species and related physical properties.

We find that increasing the threading ratio enhances not only the solid-state photoluminescence (PL) and electroluminescence (EL) quantum efficiency.

  • Calculated structures of polymers. (a) PFBP.Bn, (b) PFBP.Hβ-CD,(c) PFBP.Bnβ-CD.Bn,(d) PFBP.Meβ-CD.Si. Solvent: CHCl3 for (a), (c) and (d); water for (b).

  • The decay kinetics of the PL arising from PFBP.Meβ-CD.COPr and PFBP.Meβ-CD.Si is single-exponential for about three decades, which indicates that the intermolecular interactions are weaker with respect to all the other compounds.

To unravel the effect of progressive encapsulation on the intrachain decay kinetics of the polymer backbone we also added an electron transfer quenching agent methyl viologen (MV), to the polymer solutions.

The time resolved data demonstrate that MV prevalently 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.

  • Two distinct emission regimes are clearly observable in the contour plots: a short lived decay at about 2.9 eV and a long lived emission at about 2.5 eV.

  • The slow component is almost completely removed in the polyrotaxane.

We also find that the spectroscopic properties of the aggregates are independent of the threading ratio.

This is very interesting, since upon the additional but reasonable assumption of a random distribution of the cyclodextrins along the backbone, indicates that the aggregates dimension is small compared to the length of the conjugated segments.

We can conclude that in PFBP.Meβ‑CD.Si, the polymer backbone is much more strongly shielded with respect to all the other organic-soluble polyrotaxane investigated.

  • Electroluminescent devices (PLEDs) were fabricated for each polymer, with unoptimised device structures (ITO/PEDOT:PSS/polymer/Ca/Al).

  • The current-voltage-luminance characteristics of the devices are consistent with the spectrosopic results with turn-on voltage higher for the devices fabricated with the insulated PFBP.Meb-CD.SiandPFBP.Bnβ-CD.Bn than for the naked PFBP.Bn

Time-resolved spectroscopy of Amylose complex submonolayer coverage on MICA

The morphology of submonolayer coverages of PDV.Li and PDV.LiAm on mica surface was investigated by AFM in ULP Node. The images reveal a different rearrangement of the polymer chains when wrapped with amylose. In particular, PDV.LiAm tends to form smaller aggregates (average diameter ~15 nm) when compared with its unwrapped analogue.

Enhanced electroluminescence efficiency from large cation rotaxinated polyelectrolytes blended with poly(ethylene oxide)

When polyrotaxanes are blended with poly(ethylene oxide), PEO, their electrolytic nature leads to complexation that reduces the tendency of the two materials to phase separate. This results in a suppression of the intermolecular interaction with effects similar to those induced by rotaxination.

Photophysics of thin films and optoelectronic properties of PLEDs of unthreaded and rotaxinated PDV.K are investigated as a function of the PEO fraction.

This submonolayer films are therefore an interesting model system for the investigation of the photophysical properties of amylose complex on surfaces.

  • The PL spectrum of the polyrotaxane is blue-shifted with respect to the unthreaded polymer.

  • As the PEO fraction is increased the PL spectra evolve with a systematic blue-shift and narrowing of the PL band.

  • The vibronic progression of the intrinsic excitonic PL band is clearly resolved for PEO fraction higher than 50 %.

The contour plots of the time decay of the PL intensity clarify the differences between the photophysics of unthreaded and rotaxinated PDV.Li submonolayer films.

Consistently with the morphological considerations, the long-lived contribution due to aggregates is clearly observable for PDV.Li and is partially suppressed for PDV.LiAm.

  • The decay profiles of pure compounds are multi-exponential with a faster initial component, followed by a long lived tail.

  • As the PEO content is raised up to 90% (full circles) a quasi-single-exponential behaviour is observed, resembling the molecular photophysics for about one and a half decades for PDV.K and two decades for the analogous polyrotaxane.

The combined effect of a threading ratio, a large counter cation and complexation with PEO increases the photoluminescence quantum efficiency due to suppressed intermolecular interactions.

By combing the steady-state and the time-resolved data we can obtain the PL profile due only to intrinsic exciton recombination.

PEO facilitates ion transport

The obtained Huang-Rhys factors, S, (S=0.81 for PDV.Li and S=0.95 for PDV.LiAm) suggest a larger delocalization of the excited state in unthreaded PDV.Li.

Higher electroluminescence external quantum efficiency for both materials (~10 fold increase for polyrotaxanes and ~20 fold increase for the unthreaded analogue)

We acknowledge the European Commission (THREADMILL - MRTN-CT-2006-036040) for financial support.