Photo-rechargeable battery effect in first generation cationic-cyanine dendrimers

Article abstract of DOI:10.1002/adma.200904464

A photobattery based on a cationic cyanine dendrimer is demonstrated. The battery can be recharged simply by exposure to light. The mechanism for charge storage and discharge is proposed.

Full text of DOI:10.1002/adma.200904464

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Photo-Rechargeable Battery Effect in First Generation
Cationic-Cyanine Dendrimers
By Ajay K. Pandey, Peter C. Deakin, Ross D. Jansen-Van Vuuren, Paul L. Burn,*
and Ifor D. W. Samuel*
Batteries are currently the most suitable and reliable form of
electricity for powering portable electronic devices. In their dif-
ferent variants rechargeable batteries mostly rely on combina-
tions of liquid or solid electrolytes and alkali salts for charge
storage.[1,2] Disposal of such batteries has environmental con-
sequences and so the development of new battery technologies
that are environmentally benign, light-weight and inexpensive
We studied the effect of varying the anion on the charge and
discharge cycles of the battery. Figure 1 shows the chemical
structures of the parent cyanine dye 1,[7] and dendronised ver-
sions 2 (with an iodide anion), and 3 (with a hexafluorophos-
phate anion). The first generation dendrimer with the iodide
anion, 2, was prepared in two steps from 5-bromo-1,2,3,3-
tetramethyl-3H-indolium iodide 4[8] (Scheme 1). In the first
step 4 was reacted with triethyl orthoformate in pyridine heated
at reflux to give cyanine dye 5 in a 79% yield. A simple Suzuki
reaction coupled the first generation dendron, ‘3,5-di(4-t-butyl-
phenyl) phenylboronic acid’[9] to either end of the cyanine core
5 in a 27% yield. The iodide anion was exchanged to the hex-
afluorophosphate anion by treating a hot acetonitrile solution
of 2 with a hot aqueous solution of potassium hexafluorophos-
phate. The hexafluorophosphate salt, 3, crystallised from the
solution and was collected at the filter in a 96% yield. Evidence
that the anion exchange had occurred came from the elemental
has been a topic of great interest over the last decade.[3–5]
A
photo-rechargeable battery is a device which converts incident
optical energy into electrochemical energy and delivers the
stored electrical power upon discharge to an appropriate load.
Hence ‘light powered’ batteries require two key properties;
the ability to form a charge separated state upon photoexcita-
tion, and the ability to store and release charge. Donor-acceptor
blends of organic semiconductors have shown considerable
charge generation capabilities in organic solar cells where,
under photo-excitation, a rapid charge transfer takes place from
the donor to the acceptor leading to generation of free charge
carriers.[6] However, these devices are specifically designed
for the separated charges to be extracted and if they are not
extracted from the layer in which they are generated the charges
recombine on a microsecond timescale.
In the present work, we report the charge creation and storage
abilities of cyanine-based materials, which can be recharged
simply through exposure to white light. The dendrimers con-
sisted of first generation biphenyl dendrons, t-butyl surface
groups, and a phenyl-1,1’,3,3,3’,3’-hexamethylindocarbocyanine
core with an iodide or hexafluorophosphate anion. The battery
effect is shown to arise from charge trapping sites within the
bulk of the film, which are created through internal charge
transfer from the balancing anion to the cationic cyanine.
1
analysis and NMR. The H NMR spectra of 2 and 3 showed
significantly different chemical shifts for the indole protons
nearest to the nitrogen and the methyl groups attached to the
nitrogens, while the fluorine NMR showed the large coupling
(≈712 Hz) normally associated with the hexafluorophosphate
anion.[10]
The first part of the study was to determine the optical
absorption profile of the materials and the solid-state spectra
are shown in Figure 1. 1 has absorption peaks at 532 and
578 nm, and a pronounced shoulder at 500 nm giving a much
broader absorption profile than 2 and 3. The presence of bulky
biphenyl groups on 2 and 3 prevents aggregation and hence
reduces the width of their absorption spectra. A slight spec-
tral shift (<20 nm) towards the red is attributed to extended
conjugation.
For charge storage studies, devices consisting of a layer of
1, 2 or 3 sandwiched between ITO and Al electrodes were first
illuminated at 100 mW cm−2 for 5 minutes and then the stored
charge was measured by discharging the device through a
10 MΩ load. Figure 2 shows the discharge currents obtained for
1, 2 and 3. Dendrimer 2 gives a higher discharge current than its
non dendronised version 1 and dendrimer 3. It is important to
recall here that 2 and 3 have almost identical absorption profiles
and chemical structures, and they differ only in the nature of
the counter anion associated with the cyanine core. Dendrimer
2 with the iodide anion shows a higher discharge current and
slower decay, while 3 with hexafluorophosphate anion produces
much less current and decays faster. This shows that the nature
of the anion has a strong effect on the photo-generated charge
storage. As dendrimer 2 showed the best storage properties we
selected it to study the effect of film thickness on the charge
∗
[ ] Dr. A. K. Pandey, Prof. I. D. W. Samuel
Organic Semiconductor Centre. SUPA
School of Physics and Astronomy, North Haugh
University of St Andrews, KY16 9SS (UK)
E-mail: idws@st-andrews.ac.uk
Dr. P. C. Deakin
Department of Chemistry
University of Oxford
Chemistry Research Laboratory
Mansfield Rd, OX1 3TA (UK)
R. D. Jansen-Van Vuuren, Prof. P. L. Burn
Centre for Organic Photonics & Electronics
University of Queensland
Chemistry, Building, QLD, 4072 (Australia)
E-mail: p.burn2@uq.edu.au
DOI: 10.1002/adma.200904464
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Figure 1. Chemical structures and optical absorption spectra of 1, 2 and 3. The film thicknesses for the absorption spectra were the same ∼110 nm.
Also shown is the photobattery device structure used in this study.
storage ability. Discharge curves for devices with three different
thicknesses of dendrimer 2, namely 70, 110 and 150 nm, are com-
pared in Figure 3. An increase in the amount of charge stored
with increasing device thickness is observed with the 150 nm
film having ∼10 μC/cm2 stored.
Cationic cyanine dyes have been traditionally used for sensi-
tizing silver halide photographic films to enhance their spectral
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
1
2
3
no irradiation
0
100
200
300
400
500
600
Time (s)
Figure 2. Discharge current obtained for 1, 2 and 3. The amount of photo-
generated charge (8.22 μC/ cm2 for 1) is obtained from the area between
the discharge current first measured in dark and then after 100 mW/cm2
illumination for 5 min. Data shown for ‘no irradiation’ is representative
of dark current from devices made with 1.
Scheme 1. i) CH(OMe)3, pyridine, reflux, dark; ii) 3,5-di(4-t- butylphenyl)
phenylboronic acid, Pd(PPh3)4, PhMe, EtOH, CsF, 110 °C, dark; iii) KPF6,
MeCN/H2O, 60 °C→3 °C.
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0.05
0.04
0.03
0.02
0.01
70 nm
110 nm
150 nm
0.00
0
100
200
300
400
Time (s)
Figure 3. Discharge current for 2 with different layer thicknesses showing
charge storage increases with active layer thickness.
Figure 4. Proposed mechanism of photocharging and discharging cycles.
Photoexcitation of cyanine (Step 1) is followed by an electron transfer
from the iodide anion to the excited cyanine (Step 2) leading to forma-
tion of a radical. Application of a load initiates the charge flow out of the
device and discharging occurs (Step 3).
sensitivities. Their high absorption co-efficient has also been
utilized for energy harvesting in organic solar cells.[11,12] At
high concentration such dyes are known to form dimeric or
polymeric aggregates causing spectral broadening and forma-
tion of charge trapping sites. The counterions associated with
a cationic cyanine are also associated with the formation and
stabilization of such aggregates giving rise to deep traps within
the bulk.[13] It has also been reported that the presence of iodide
anions can potentially work as hole trapping sites in cationic
cyanine dyes.[14] Such trapping of photo-generated charges can
give rise to current storage in the dark.
the first oxidative process is therefore due to the iodide anion.
That is, the iodide anion has the potential to reduce the excited
core chromophore. This assignment is further supported by
cyclic voltammetry of tetra ( n-butyl)ammonium iodide under
the same conditions, which was found to undergo an oxida-
tion with Ep at 0.42 V. The relatively long-lived nature of the
charge storage state may be due to trans-trans-trans to cis-trans-
trans isomerisation of the cyanine chromophore as a result of
decreased bond order of the double bonds upon excitation and
reduction.[15–17]
The cyanines studied here are cationic dyes and are sur-
rounded by either iodide or hexafluorophosphate anions.
Under photo-excitation excitons are generated on the core
chromophore but unlike organic solar cells there is no high
affinity material to act as an electron acceptor in our devices.
This means that the excited electron is not removed from the
exciton and a majority of the photo-generated excitons simply
decay back to the ground state. However, given that stored
charge is created on photoexcitation means that not all excitons
simply decay. We propose that the charging process occurs
as shown in Figure 4. First, on irradiation a singlet exciton is
formed on the cationic core chromophore. Instead of decaying
back to the groundstate the exciton is reduced by transfer of
an electron from the iodide anion thus forming a radical on
the chromophore. During the discharge process the radical
on the cyanine migrates to the electrode interface before com-
pleting the circuit and reducing the iodine radical back to the
iodide, with the cyanine chromophore reverting to its most
thermodynamically stable structure. Evidence for this mecha-
nism comes from the electrochemical properties of dendrimers
2 and 3 (Figure 5). In Figure 5 it can be seen that dendrimer
2 (iodide anion) has two oxidation processes (a shoulder and
a peak) while dendrimer 3 with hexafluorophosphate anion
has one (a peak) and at more positive potential. Given that
the only difference between the two materials is the anion,
Figure 3 is consistent with this model as the thicker devices
have more trapping sites, leading to more stored charge. In
30
2
3
20
10
0
-10
-20
-30
-40
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Potential (V)
Figure 5. Oxidation cyclic voltammagrams for 2 and 3. The shoulder in
the region of 0.4 V shows that 2 has an extra oxidation process, which is
ascribed to oxidation of the iodide anion.
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distilled from potassium hydroxide before use. Light Petroleum refers to
the fraction with boiling point of 40–60 °C. Thin layer chromatography
was performed on Merck aluminum plates coated with silica gel F254
and visualized using 254 nm UV irradiation or by iodine staining.
Column chromatography was performed using either gravity feed
chromatography or the flash chromatography technique, with silica
gel (flash) C60, 40–60 mesh. Where solvent mixtures are used their
proportions are given by volume.
Synthesis of 5,5’-Dibromo-1,1’,3,3,3’,3’-hexamethylindocarbocyanine
Iodide (5) : A mixture of 5-bromo-1,2,3,3-tetramethyl-3H-indolium iodide
4 (10 g, 26.4 mmol) and triethyl orthoformate (2.2 mL, 13.2 mmol)
in pyridine (80 mL) was deoxygenated by placing under vacuum and
backfilling with nitrogen four times. The resultant mixture was heated
at reflux for 18 h in the dark and after being allowed to cool to room
temperature the resultant crystals were collected by filtration. The solid
was washed with light petroleum (2 × 100 mL), and dried in vacuo
to give 5 as deep purple crystals (6.7 g, 79%); dec. 269 °C; (Found:
C, 46.7; H, 4.3; N, 4.3. C25H27Br2N2I requires C, 46.8; H, 4.2; N, 4.4%);
the absence of a load a self-discharge does take place but it is
slow and can take as long as 30–40 minutes. Unlike the iodide
anion the hexaflurophosphate anion gives a much weaker
response, as it is not easily oxidized due to its electronegativity
being higher. Charge and discharge cycles were performed for
each device at least 10 times, and the observed loss of charge
storage capacity at the end of this test was less than 5% of the
initial value. The total storage capacity of the 150 nm thick
cyanine layer from the experimental results was calculated to
be 0.20 mA.h/g, which for the output voltage of 0.35 V corre-
sponds to an energy density of 0.07 W.h/Kg. The overall charge
storage capacity is sufficient to demonstrate the principle of
the photobattery, but needs to be increased. This could poten-
tially be achieved by increasing the rate of reduction of the
exciton on the cyanine chromophore and will require further
investigation.
λ
max(CH2Cl2)/nm (log10ε) 527 (4.90), 562 (5.14); δH(400 MHz; CDCl3)
To summarize, we have demonstrated a novel organic semi-
conductor device, which acts as a battery that can be recharged
by light. The nature of the counteranion associated with the cat-
ionic cyanine chromophore is shown to strongly influence the
battery effect. A mechanism has been proposed explaining the
mechanism of the photo-battery effect observed in the single
layer-single material thin film devices. Such ultrathin photo-
rechargeable batteries could be useful for integrated power gen-
eration and storage in flexible electronic devices.
1.71 (12 H, s, CH3), 3.81 (6 H, s, NCH3), 7.03 (2 H, d, J = 8.5, core
phenylH), 7.45–7.48 (4 H, m, core phenylH and vinylH), 7.53 (2 H, dd,
J = 2, J = 8.5, core phenylH), 8.38 (1 H, dd, J = 13.5, J =13.5, vinylH);
δC(125 MHz; CDCl3) 28.1, 33.0, 48.8, 105.9, 112.2, 118.5, 125.5, 131.9,
141.7, 142.3, 150.8, 173.7; m/z [LDI] Found: 512.5 (52%), 514.4 (100%),
516.2 (51%); C25H27Br2N2 requires 513.0, 515.0, 517.0.
Synthesis
of
5,5’-[3,5-Di(4-t-butylphenyl)phenyl]-1,1’,3,3,3’,3’-
hexamethylindocarbocyanine Iodide (2) : A mixture of 5 (1.00 g, 1.56 mmol),
3,5-di(4-t-butylphenyl)phenylboronic acid (1.5 g, 3.9 mmol), and cesium
fluoride (2.37 g, 15.6 mmol) in toluene (30 mL) and ethanol (30 mL)
was deoxygenated by placing under vacuum and backfilling with
nitrogen four times. Tetrakis(triphenylphosphino)palladium(0) (180 mg,
156 μmol) was added and the mixture deoxygenated a further three
times. The reaction was then heated at 110 °C in the dark for 19 h.
After being allowed to cool to room temperature, water (15 mL) and
dichloromethane (50 mL) were added. The organic layer was separated
and the aqueous layer was extracted with dichloromethane (2 × 100 mL).
The combined organic extracts were washed with water (2 × 100 mL),
brine (200 mL), dried over magnesium sulfate, filtered, and the
solvent removed in vacuo. The crude material was purified by column
chromatography over silica using a methanol/chloroform mixture (1:9)
as eluent to give 2 as a metallic gold-purple coloured solid (518 mg,
27%). mp 248 °C; (Found: C, 79.3; H, 7.2; N, 2.55. C77H85N2I requires C,
79.4; H, 7.35; N, 2.4%); λmax(CH2Cl2)/nm (log10ε) 548 (5.03), 582 (5.17);
δH(500 MHz; CDCl3) 1.40 (36 H, s, t-Bu), 1.80 (12 H, s, CH3), 3.89 (6
H, s, 2 × NCH3), 7.24 (2 H, d, J = 9, core phenylH), 7.39 (2 H, d, J =
13, vinylH), 7.54 (8 H, 1/2AA’BB’, spH), 7.63-7.66 (10 H, m, spH, core
phenylH), 7.72-7.74 (6 H, m, core phenylH and G1-bpH), 7.81 (2 H, s,
G1-bpH), 8.46 (1 H, dd, J = 13, J = 13, vinylH); δC (125 MHz; CDCl3)
28.2, 31.5, 32.9, 34.5, 48.9, 105.8, 111.1, 121.0, 124.7, 125.4, 126.0, 127.0,
128.0, 138.1, 139.0, 141.2, 141.25, 142.27, 142.5, 150.4, 150.8, 173.9; m/z
[ESI+] 1038.9 (33%); C77H85N2 requires 1038.5.
Experimental Section
General Experimental: 1H nmr spectra were recorded on Bruker
DPX-400 (400 MHz) or Bruker AV-500 (500 MHz with cryoprobe)
spectrometers. 13C nmr were recorded on Bruker DPX-400 (100 MHz)
or AV-500 (125 MHz with cryoprobe) spectrometers and were fully
decoupled. Chemical shifts (δ) are reported in parts per million (p.p.m.)
and are referenced to the residual solvent peak. Coupling constants are
reported to the nearest 0.5 Hz. G1-bpH = first generation branching
phenyl H, spH = surface phenyl H, core phenyl H = cyanine phenyl
H. Electrospray mass spectra were recorded on a Micromass Q-TOF
Micro or Fisons Platform. Linear Desorption Ionisation Time-Of-Flight
mass spectra were recorded on a Micromass TofSpec spectrometer.
Solution U.V.-visible spectra were recorded on a Perkin-Elmer UV Lambda
15 spectrometer in spectroscopic grade dichloromethane with the peak
maxima in the visible region reported. Solid-state spectra were measured
using a Cary 300 spectrometer. Melting points were determined on a
Gallenkamp melting point apparatus and are uncorrected. Microanalyses
were carried out by Mr. Stephen Boyer, London Metropolitan University,
London. Electrochemistry was performed using
a BAS Epsilon
Synthesis
of
5,5’-[3,5-Di(4-t-butylphenyl)phenyl]-1,1’,3,3,3’,3’-
electrochemistry station using a glassy carbon working, Ag/AgNO3 in
acetonitrile reference, and platinum counter electrodes, and a 100 mV/s
scan rate. All measurements were made at room temperature on samples
at a 1 mM concentration in acetonitrile. The acetonitrile was dried over
anhydrous potassium carbonate, further dried over 3Å molecular sieves
before being distilled and stored over 3Å molecular sieves. 0.1 M Tetra-
n-butylammonium perchlorate was used as the electrolyte. The solutions
were deoxygenated with argon and the ferricenium/ferrocene couple
was used as standard.[18] In all cases several scans were carried out to
confirm the chemical reversibility of the redox-processes.
Reactions that were deoxygenated by vacuum-nitrogen purging were
carried out in a sealed tube fitted with a Young’s valve unless otherwise
stated. Materials which were purified by column chromatography eluted
with mixtures containing more than 5% methanol in chloroform were
further dissolved in dichloromethane and filtered through Celite, eluted
with dichloromethane, to remove any dissolved silica. Pyridine was
hexamethylindocarbocyanine Hexafluorophosphate (3) : 2 (250 mg,
0.215 mmol) was dissolved in the minimum amount of hot acetonitrile
(∼70 mL) and hot filtered into a hot (∼60 °C) solution of potassium
hexafluorophosphate (200 mg, 1.1 mmol) in water (20 mL). Water was
added until crystals began to form. This mixture was allowed to cool to
room temperature and then cooled to 3 °C. The solid product was isolated
by filtration and dried in vacuo to give 3 as purple-gold crystals (244 mg,
96%); mp 256 °C; (Found: C, 78.2; H, 7.3; N, 2.3. C77H85F6N2P requires
C, 78.1; H, 7.2; N, 2.4%); λmax(CH2Cl2)/nm (log10ε) 548 (5.21), 582
(5.36); δH(500 MHz; CDCl3) 1.40 (36 H, s, t-Bu), 1.80 (12 H, s, CH3), 3.74
(6 H, s, NCH3), 6.70 (2 H, d, J = 13.5, vinylH), 7.25 (2 H, d, J = 8.5, core
phenylH), 7.54 (8 H, 1/2AA’BB’, spH), 7.63-7.66 (10 H, m, core phenylH
and spH), 7.72-7.74 (6 H, m, core phenylH and G1-bpH), 7.81 (2 H, s,
G1-bpH), 8.42 (1 H, dd, J = 13.5, J = 13.5, vinylH); δC(125 MHz; CDCl3)
28.2, 31.4, 31.5, 34.6, 49.0, 104.7, 111.1, 121.0, 124.7, 125.4, 125.9, 127.0,
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128.1, 138.1, 139.1, 141.1, 142.2, 142.5, 150.2, 150.8, 174.2; m/z [ESI+]
1038.0 (100%); C77H85N2 requires 1038.5; δF(235 MHz; CDCl3) −73.0
(d, JP,F = 713, PF6−).
Samuel holds an EPSRC Senior Research Fellowship (grant number EP/
C542398).
Received: December 29, 2009
Revised: March 11, 2010
Published online: July 30, 2010
DeviceFabricationandCharacterization: Tostudytheirchargegeneration
and storage properties several devices of the structure shown in Figure 1
were made on ITO coated glass substrates (Merck EL Grade-Series 800
with sheet resistance < 1 5 Ω/ꢀ). The substrates were sequentially cleaned
in ethanol, acetone and de-ionized water and then oxygen-plasma
cleaned at a power of 100 W for 5 min to increase the work-function and
improve the adhesion of the subsequent layer. A 40 nm PEDOT:PSS layer
(H.C. Stark, VP AI 4083) was then spin-coated in air on the treated ITO
and samples were then transferred to dry at 120 °C inside a nitrogen
filled glove box for 20 min. Thin layers of the different cyanines were then
spin-coated from a chloroform solution at a concentration of 20 mg/ml
[1] G.-A. Nazri, G. Pistoia, Lithium Batteries: Science and Technology,
Kluwer Academic, Plenum, Boston, 2004.
[2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature
2000, 407, 496.
[3] Y. Oumellal, A. Rougier, G. A. Nazri, J-M. Tarascon, L. Aymard,
Nature Mater. 2008, 7, 916.
[4] K. Oyaizu, H. Nishide, Adv. Mater. 2009, 21, 1.
[5] A. Züttel, Mater. Today 2003, 24, 33.
[6] a) N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 1992,
258, 1474; b) J.-L. Bredas, J. E. Norton, J. Cornil, V. Coropceanu, Acc.
Chem. Res 2009, 17, 1691.
[7] Z.-H. Peng, L. Qun, X.-F. Zhou, S. Carroll, H. J. Geise, B.-X. Peng,
R. Dommisse, R. Carleer, J. Mater. Chem. 1996, 6, 559.
[8] S. Yagi, K. Maeda, H. Nakazumi, J. Mater. Chem. 1999, 9,
2991.
[9] M. Kimura, T. Shiba, M. Yamazaki, K. Hanabusa, H. Shirai, N.
Kobayashi, J. Am. Chem. Soc. 2001, 123, 5636.
[10] L. B. Ebert, D. R. Mills, J. C. Scanlon, H. Selig, Mat. Res. Bull. 1981,
16, 831.
[11] A. Mishra, R. K. Behera, P. K. Behera, B. K. Mishra, G. B. Behera,
Chem. Rev. 2000, 100, 1973.
°
and spin speed of 1000 rpm. Samples were pre-heated at 60 C on a
hotplate, in inert atmosphere, before loading in a vacuum evaporator for
deposition of a metallic aluminum electrode at 0.05 nm/s. The device
area was defined by the aluminium and was 6 mm2. In order to evaluate
the photo-charging and discharging characteristics of the devices, the
photo-recharge ability measurements were performed in ambient
conditions. Devices were first irradiated for 5 minutes at 100 mW/cm2
from a metal halide lamp solar simulator source (SolarCell Test 575,
KHS Steuernagel GmbH) simulating the solar spectrum and then left
in the dark for 10 seconds. The discharge current was then measured
by connecting the ITO and Al electrodes with a load of 10 MΩ for up
to 10 minutes; data were taken every 10 seconds. The load resistor is
very much smaller than the 10 GΩ input impedance of the Keithley 2000
multimeter used for the measurement. For reference, the discharge
current without photo-irradiation, that is, in dark, was also measured.
After each discharge a complete recharge takes place within 100 sec of
exposure to light. The charge density of the cyanine dendrimer 2 was
calculated from the dimensions of the organic layer in the device (4 mm
× 1.5 mm, area with a thickness of 150 nm) and the measured charge
stored assuming a density of 1 g/cm3.
[12] F. S. Meng, K. C. Chen, H. Tian, L. Zuppiroli, F. Nuesch, Appl. Phys.
Lett. 2003, 82, 3788.
[13] K. Nakatsu, H. Yoshioka, H. Morishita, Acta Cryst. 1977, B33,
2181.
[14] R. C. Nelson, J. Chem. Physics 1963, 39, 859.
[15] S. Yagi, K. Maeda, H. Nakazumi, J. Mater. Chem. 1999, 9, 2991.
[16] D.-L. Jiang, T. Aida, Nature 1997, 388, 454.
[17] Y. Wakabayashi, M. Tokeshi, D.-L. Jiang, T. Aida, T. Kitamori,
J. Luminescence, 1999, 83-84, 313.
Acknowledgements
Professor Paul L. Burn is receipient of an Australian Research Council
Federation Fellowship (project number FF0668728). Professor Ifor
[18] G. Gritzner, J. Kuta, Electrochim. Acta 1984, 29, 869.
©
3958 wileyonlinelibrary.com
2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2010, 22, 3954–3958