Mixed-valence interactions in triarylamine-gold-nanoparticle conjugates

Article abstract of DOI:10.1039/b909715e

Optical and electrochemical investigations of triarylamine redox centres attached to gold nanoparticles via a π-conjugated bridge show intervalence charge-transfer bands which prove to be surprisingly strong interchromophore interactions.

Full text of DOI:10.1039/b909715e

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Mixed-valence interactions in triarylamine–gold–nanoparticle conjugatesw
Christian I. Mu
¨
ller,ab Christoph Lambert,*ab Markus Steeger,ab Frank Forster,c
c
c
cd
Michael Wiessner, Achim Scholl, Friedrich Reinert and Martin Kampe
¨
Received (in Cambridge, UK) 15th May 2009, Accepted 8th September 2009
First published as an Advance Article on the web 21st September 2009
DOI: 10.1039/b909715e
Optical and electrochemical investigations of triarylamine redox
centres attached to gold nanoparticles via a p-conjugated bridge
show intervalence charge-transfer bands which prove to be
surprisingly strong interchromophore interactions.
particles of different size (Au-PreS and Au-PreL, respectively).
The two fractions with different particle size and the terminal
alkyne 1 were then transformed via a Hagihara–Sonogashira
coupling reaction into the desired products Au-TaraS and
Au-TaraL.15 For comparison, the reference compound
Ref was synthesised via a palladium-catalysed reaction starting
from compound 1 (Scheme 1).
Gold nanoparticle–chromophore conjugates have a high potential
for biomedical,1,2 photoelectrochemical, catalytic and sensing
applications.3 In the advent of nanoparticle research gold
particles with aliphatic ligand shells were synthesised and
investigated. In recent years these aliphatic ligands were
functionalised with chromophores such as pyrenes4 or
porphyrins.5,6 While in most studies concerning nano-
particle–chromophore conjugates fluorescence properties are
in focus,1–3,5 in this study, we are interested in the optical
absorption properties of gold nanoparticles that are covered
by a p-conjugated bridging unit which is terminated by an
organic redox centre, i.e. a triarylamine. While in a recent study,
we were addressing the electron-transfer interactions between
triarylamine redox centres attached to solid gold electrodes,7 in
the present communication our goal is to elucidate how the
optical properties of triarylamine–gold–nanoparticle conjugates
depend on interactions between the triarylamine redox centre
via the conjugated bridge and the gold core, and on interactions
between two different triarylamines. That is, we use the gold
core as a template to bring many triarylamine units in close
contact. We use triarylamine redox centres as they can be used
as reversibly oxidisable redox catalysts8 and they are used as
hole transport components in many optoelectronic applications.9
Furthermore, they are used as redox centres in organic mixed-
valence compounds that serve as model systems for basic
electron-transfer studies.10–13
The gold core diameter distribution of the nanoparticles was
measured using dark-field scanning transmission electron
microscopy. Au-TaraS has an average diameter of 2.24
(Æ0.48) nm and Au-TaraL of 2.71 (Æ0.56) nm. Thus, in
Au-TaraS each gold core is covered by ca. 25 chromophores
(for details see ESIw). The composition and stability of the
gold core structure of the Au-Tara particles after Sonogashira
coupling was checked by UV/Vis/NIR- and X-ray photo-
electron-spectroscopy (XPS) (see Fig. S4 and S5w).
The surface plasmon band (SPB) is a characteristic feature
in the absorption spectrum of metallic nanoparticles. In the
case of spherical particles with the same organic layer,
the energy and the intensity of the SPB depend on the size
of the gold core.3 In agreement with this fact the larger particle
Au-TaraL shows an intense SPB at 19 000 cmÀ1 which
is practically absent in the smaller particle Au-TaraS
(Fig. 1 and Fig. S4w for the precursor particles).
The absorption spectrum of the reference compound Ref
displays two intense bands at 27 600 cmÀ1 (polarised along the
molecular axis) and 34 500 cmÀ1 (polarised perpendicularly,
see, Fig. S6w). The higher energy band is also seen with a slight
red shift in the spectrum of Au-TaraS but has a very weak
intensity in the case of Au-TaraL. Much in contrast, the
second characteristic band of Ref at 27 600 cmÀ1 is hardly
detectable in the spectra of Au-TaraL and Au-TaraS.
The strong damping of the band at 27 600 cmÀ1 and the red
shift of the band at 34 500 cmÀ1 are caused by an interaction
between the gold core and the chromophore.4,6,16 The different
degree of damping between the 27 600 cmÀ1 band and the
The first step to synthesise triarylamine-functionalised gold
nanoparticles (Au-Tara) was to generate the precursor Au-Pre
by reduction of HAuCl4 with LiBH4 in the presence of
4-bromothiophenol.14 Extraction of the crude product with
dichloromethane (dcm) only or with both dcm and thf yields
a Institute of Organic Chemistry, University of Wurzburg,
¨
Am Hubland, D-97074, Wurzburg, Germany.
¨
E-mail: lambert@chemie.uni-wuerzburg.de
b Conrad Wilhelm Rontgen Center for Complex Material Systems,
¨
Wurzburg, Germany. Fax: +49 931 888 4606;
¨
Tel: +49 931 888 5318
c Experimentelle Physik II, University of Wurzburg, Am Hubland,
¨
D-97074, Wurzburg, Germany
¨
d Forschungszentrum Karlsruhe, Gemeinschaftslabor fur Nanoanalytik,
D-75021, Karlsruhe, Germany
¨
e Technische Physik, University of Wurzburg, Am Hubland, D-97074,
Wurzburg, Germany
¨
¨
w Electronic supplementary information (ESI) available: Synthesis,
absorption spectra, fluorescence anisotropy spectra, XPS spectra and
OSWV. See DOI: 10.1039/b909715e
Scheme
1 Synthesis of the nanoparticles Au-Pre and Au-Tara
(S small, L large nanoparticle) and the chromophore Ref.
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Table 1 Redox potentials of Ref, Au-TaraS and Au-TaraL vs.
ferrocene (Fc/Fc+) in dcm–TBAH
Redox potential/mV
E(Ox1)
E(Ox2)
Ref
Au-TaraS
Au-TaraL
280a
950a
980a
980a
290b, 420b
280b, 420b
a
b
Measured by OSWV. Determined by
functions.
a
fit with two Voigt-
band at 27 600 cmÀ1 decreases with increasing potential while
at the same time the characteristic band for the monoradical
cation at 13 000 cmÀ1 and another band at 23 000 cmÀ1
increase (see Fig. S7w). The band at 13 000 cmÀ1 is associated
with a p–p* transition of the dianisylphenylamine radical
cation.10–13
Fig. 1 Absorption spectra of the triarylamine-functionalised particles
Au-TaraS (black line) and Au-TaraL (red line) and the reference
compound Ref (blue line) in dcm.
34 500 cmÀ1 band might be due to the different relative
orientation of the chromophore transition dipole to the
surface normal of the gold particle.17 In fact, it is likely that
the orientation of the arylthiol units deviates strongly from the
gold surface normal.18
For a better overview we divide the SEC of Au-TaraS into
three sequential processes which are grouped according to
isosbestic points (Fig. 3). The characteristics of the three
processes are the increase of the triarylamine radical cation
band at 13 100 cmÀ1 accompanied by a decrease of the
band at 33 300 cmÀ1. During this first process the weak bands
at 24900 cmÀ1 and 27 900 cmÀ1 decrease. Even more interesting
is the observation of a weak band at about 10 000 cmÀ1
(e ca. 2000 MÀ1 cmÀ1), which increases during the second
process and then decreases during the third process (see inset
in Fig. 3). This behaviour is indicative of an intervalence
charge-transfer band (IVCT) which is associated with an
optically induced hole transfer between an oxidised and a
neutral neighbouring triarylamine. The IVCT band should be
maximal at 50% oxidation of all triarylamine centres as indeed
observed. IVCT bands of similar intensity are seen in many
bis(triarylamine) radical cation systems10–13,19 and are also
reported for ruthenium particles with attached ferrocenes.20
In contrast to the mixed-valence bis(triarylamine) radical
cations where hole transfer occurs over the conjugated
bridges that connect the two triarylamine moieties, the hole
in the partially oxidised triarylamine–gold–nanoparticle
conjugates is transferred through space between the
triarylamines.
Osteryoung square wave voltammetry (OSWV) of Ref in
tetrabutylammonium hexafluorophosphate (TBAH) dcm solution
shows two peaks (E(Ox1) = 280 mV, E(Ox2) = 950 mV) which can
be assigned to the first and second oxidation of the triarylamine
centres (Fig. 2). Only the first oxidation peak is reversible.
The OSWV of Au-TaraS and Au-TaraL reveals a splitting of
the first triarylamine oxidation into two reversible oxidation
processes. A fit of the voltammogram with two Voigt-functions
(see Fig. S8w) reveals a separation (seen as a shoulder) of about
130 mV for Au-TaraS and 140 mV for Au-TaraL (Table 1).
This splitting is caused by a very strong coupling between
neighbouring triarylamine centres (the statistical value for two
non-interacting redox centres is 35.6 mV). Such a splitting of the
first oxidation is often seen in bis(triarylamine) systems10–13,19
and is also seen for ferrocenes attached to a ruthenium particle
via a vinylene unit.20
Further information about the electronic coupling between
the triarylamine centres on the particles was obtained by
spectroelectrochemistry (SEC) at a transparent gold-minigrid
electrode. Only the oxidation to the triarylamine radical cation
is discussed because the oxidation to the dication is irreversible.
The SEC of Ref shows that the typical triarylamine absorption
Fig. 2 OSWV in dcm (TBAH) of Au-TaraS (black line), Au-TaraL
(red line) and Ref (blue line).
Fig. 3 SEC of Au-TaraS (black line) in dcm–TBAH. The SEC is
composed of three processes coloured yellow, red and blue.
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and the SEC measurements of the nanoparticles revealed a
strong coupling between neighbouring triarylamine units
which is due to through space intervalence interactions24 in
these metal–organic mixed-valence conjugates. The SEC also
showed for the larger particle that by charging the
chromophore–nanoparticle system, the gold core transitions
are damped. These multi-electron redox active nanoparticle
systems may be interesting building blocks in the field of
charge storage (electron sponge) as they can be loaded to a
high degree at constant potential.
We thank the Deutsche Forschungsgemeinschaft for
financing this project within the Research Training School
GRK1221.
Notes and references
Fig. 4 SEC of Au-TaraL (black line) in dcm–TBAH. The SEC is
composed of three processes coloured yellow, red and blue. The
coloured arrows symbolise the changes of the band intensities for
each process.
1 M. Hu, J. Y. Chen, Z. Y. Li, L. Au, G. V. Hartland, X. D. Li,
M. Marquez and Y. N. Xia, Chem. Soc. Rev., 2006, 35, 1084–1094.
2 C. J. Murphy, A. M. Gole, J. W. Stone, P. N. Sisco,
A. M. Alkilany, E. C. Goldsmith and S. C. Baxter, Acc. Chem.
Res., 2008, 41, 1721–1730.
The SEC of Au-TaraL can also be described by three
sequential processes. (Fig. 4). Only the first process is fully
reversible. Initial increasing of the potential (first process)
leads to a dramatic decrease of all characteristic gold core
band intensities. With further increase of the potential
the triarylamine radical cation band at 13 100 cmÀ1 rises
(second process) and the gold core bands gain more intensity.
This process is associated with an increase in a band at
ca. 10 000 cmÀ1. As the radical cation band continues to
increase, the gold core bands and the band at 10 000 cmÀ1
decreases (third process). Again, as in Au-TaraS the latter
band is interpreted to be an IVCT band.
3 M. C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.
4 B. I. Ipe and K. G. Thomas, J. Phys. Chem. B, 2004, 108,
13265–13272.
5 H. Imahori, Y. Kashiwagi, Y. Endo, T. Hanada, Y. Nishimura,
I. Yamazaki, Y. Araki, O. Ito and S. Fukuzumi, Langmuir, 2004,
20, 73–81.
6 J. Ohyama, Y. Hitomi, Y. Higuchi, M. Shinagawa, H. Mukai,
M. Kodera, K. Teramura, T. Shishido and T. Tanaka, Chem.
Commun., 2008, 6300–6302.
7 C. Lambert and V. Kriegisch, Langmuir, 2006, 22, 8807–8812.
8 E. Steckhan, Angew. Chem., Int. Ed. Engl., 1986, 25, 683–701.
9 M. Thelakkat, Macromol. Mater. Eng., 2002, 287, 442–461.
10 C. Lambert, S. Amthor and J. Schelter, J. Phys. Chem. A, 2004,
108, 6474–6486.
11 D. R. Kattnig, B. Mladenova, G. Grampp, C. Kaiser,
A. Heckmann and C. Lambert, J. Phys. Chem. C, 2009, 113,
2983–2995.
In the first process the gold core gets charged. Such a core-
charging alters the electronic structure of the gold core and
is known to affect some gold transitions within the gold
nanoparticle.21,22 It might also be possible that the electrolyte
(TBAH) mediates electrostatic interactions between the
charged particles which causes an aggregation of particles.
Chandrasekharan et al. found that aggregation of a gold
nanoparticle coated with Rhodamine 6G was responsible for
a damping and broadening of the SPB.23 Whatever the reasons
may be for this effect, oxidation of the triarylamine centres in
the second step disturbs it strongly. In contrast, the smaller
particle size of Au-TaraS and/or the superimposition with the
more intense triarylamine bands could be the reasons that a
damping of the gold core band intensities are not detectable in
the SEC of Au-TaraS.
12 C. Lambert and G. Noll, J. Am. Chem. Soc., 1999, 121, 8434.
¨
13 S. Amthor and C. Lambert, J. Phys. Chem. A, 2006, 110,
1177–1189.
14 M. P. Rowe, K. E. Plass, K. Kim, C. Kurdak, E. T. Zellers and
A. J. Matzger, Chem. Mater., 2004, 16, 3513–3517.
15 C. Xue, G. Arumugam, K. Palaniappan, S. A. Hackney, H. Liu
and J. Liu, Chem. Commun., 2005, 1055–1057.
16 D. Li, Y. Zhang, J. Jiang and J. Li, J. Colloid Interface Sci., 2003,
264, 109–113.
17 A. J. Haes, S. L. Zou, J. Zhao, G. C. Schatz and R. P. Van Duyne,
J. Am. Chem. Soc., 2006, 128, 10905–10914.
18 P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and
R. D. Kornberg, Science, 2007, 318, 430–433.
19 A. Heckmann, S. Amthor and C. Lambert, Chem. Commun., 2006,
2959–2961.
20 W. Chen, S. Chen, F. Ding, H. Wang, L. E. Brown and
J. P. Konopelski, J. Am. Chem. Soc., 2008, 130, 12156–12162.
21 G. C. Lica, B. S. Zelakiewicz, M. Constantinescu and Y. Y. Tong,
J. Phys. Chem. B, 2004, 108, 19896–19900.
22 V. L. Jimenez, D. G. Georganopoulou, R. J. White, A. S. Harper,
A. J. Mills, D. Lee and R. W. Murray, Langmuir, 2004, 20,
6864–6870.
In summary, we present the first synthesis of triarylamine-
functionalised gold nanoparticles of two different sizes. The
absorption properties of these two conjugates differ strongly
both in the neutral state and in the ligand oxidised state. In the
neutral state this is due to different electric fields emanating
from the gold particle which interacts with the transition
dipole of the ligands. In the ligand oxidised state, the OSWV
23 N. Chandrasekharan, P. V. Kamat, J. Hu and G. J. Il, J. Phys.
Chem. B, 2000, 104, 11103–11109.
24 D.-L. Sun, S. V. Rosokha, S. V. Lindeman and J. K. Kochi, J. Am.
Chem. Soc., 2003, 125, 15950–15963.
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Chem. Commun., 2009, 6213–6215 | 6215