Transcript UCL - THREADMILL

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
Enhanced electroluminescence from threaded molecular wires via
fine tuning of their threading ratio
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.
A new class of organic soluble polyrotaxanes was characterized by means of steady-state and timeresolved 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.
• Calculated structures of polymers. (a) PFBP.Bn, (b) PFBP.HβCD,(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βCD.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.
PFBP.Bn
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.
50 nm x 50 nm
PFBP.Meβ-CD.Si
•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 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.Si
andPFBP.Bnβ-CD.Bn than for the naked PFBP.Bn
Enhanced electroluminescence efficiency from large cation
rotaxinated polyelectrolytes blended with poly(ethylene oxide)
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.
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.
This submonolayer films are therefore an interesting model system for the investigation of the
photophysical properties of amylose complex on surfaces.
Photophysics of thin films and optoelectronic properties of PLEDs of unthreaded and rotaxinated
PDV.K are investigated as a function of the PEO fraction.
• 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 %.
all
isolated small
particles
PDV.LiAm
.LiAm
1 μm x 1 μm
1 μm x 1 μm
PDV.LiAm
particles
1 μm x 1 μm
PDV.Li
1 μm x 1 μm
m
isolated small
particles
1 μm x 1 μm
PDV.Li
PDV.Li
large aggregates
500 nm x 500 nm
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
AFM topography
AFM topography
AFM
observable for PDV.Li and is partially suppressed
fortopography
PDV.LiAm.
PDV.LiAm
large aggregates PDV.LiAm
PDV.LiAm
large aggregates
Round particles
 coiled?
isolated
small
particles
Round particles
 coiled?
50500
nmnm
x 50
nm nm
x 500
1 μm x 1 μm
50 nm x 50 nm
500 nm x 500 nm
Round particles
 coiled?
PDV.LiAm
1500
μmnm
x 1xμm
500 nm
By combing the steady-state and the
time-resolved data we can obtain the
PL profile due only to intrinsic
exciton recombination.
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.
50 nm x 50 nm
• The decay profiles of pure compounds are multiexponential 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.
PEO facilitates ion transport
Higher electroluminescence external quantum
efficiency for both materials (~10 fold increase for
polyrotaxanes and ~20 fold increase for the
unthreaded analogue)
External EL Quantum Efficiency (%)
500 nm x 500 nm
PDV.LiAm
Time-resolved spectroscopy of Amylose complex submonolayer
coverage on MICA
AFM50topography
nm x 50 nm
 coiled?
Round particles
 coiled?
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.
ates
m
Optoelectronic properties of non polar polyrotaxane insulated
molecular wires with high solubility in organic solvents
1.2
1.0
0.8
0.6
0.4
0.2
PDV.K
PDV.K-CD
0.0
0
10
20
30
40
50
PEO fraction (%w)
We acknowledge the European Commission (THREADMILL - MRTN-CT-2006-036040) for financial support.