Screening interactions of zinc phthalocyanine-PPV oligomers with single wall carbon nanotubes - A comparative study

Article abstract of DOI:10.1039/c1jm10572h

In this paper, the ability to disperse single wall carbon nanotubes (SWNT) of several different-nature poly(p-phenylene vinylene) (PPV) oligomers having pendant zinc phthalocyanines (ZnPc) has been investigated. Based on the quenching of the ZnPc and SWNT

Full text of DOI:10.1039/c1jm10572h

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 8014
www.rsc.org/materials
PAPER
Screening interactions of zinc phthalocyanine–PPV oligomers with single wall
carbon nanotubes—a comparative study†
a
Juergen Bartelmess, Christian Ehli, Juan-Jose Cid, Miguel Garcıa-Iglesias, Purificacion Vazquez,
a
b
b
b
ꢀ
ꢀ
ꢀ
ꢀ
bc
a
*
*
and Dirk M. Guldi
ꢀ
Tomas Torres
Received 7th February 2011, Accepted 22nd March 2011
DOI: 10.1039/c1jm10572h
In this paper, the ability to disperse single wall carbon nanotubes (SWNT) of several different-nature
poly(p-phenylene vinylene) (PPV) oligomers having pendant zinc phthalocyanines (ZnPc) has been
investigated. Based on the quenching of the ZnPc and SWNT fluorescence in the supramolecular
assemblies, it has been shown that parameters such as p/n-type character of the oligomer, size and the
distance between the ZnPc moiety and the conjugated backbone play an important role in the strength
of the interactions. Important results suggest that n-type oligomers as well as certain flexibility in the
phthalocyanine arrangement are breakthroughs for immobilizing SWNT in THF, affording stable and
homogeneous suspensions. Transient absorption measurements confirm that upon photoexcitation the
photoexcited ZnPc triggers an intraensemble charge transfer to yield oxidized ZnPc and reduced
SWNT.
overcome this fact, another approach consisting in the supra-
molecular functionalization of SWNT by means of hydrophobic,
p-stacking or van der Waals interactions has been widely
investigated.5 In this way, the attachment of, for example,
polymers bearing photo- and electro-active molecules has proved
to be a suitable procedure to preserve the tube properties,
increase its solubility and control the organization between the
donor and acceptor units.6
Introduction
Single wall carbon nanotubes (SWNT) have become one of the
most important one-dimension nanomaterials for optoelec-
tronics applications due to their singular chemical, mechanical,
thermal and electronic properties.1 In addition, well ordered
SWNT forming a homogeneous active layer has revealed to be
indispensable in most of the devices.2 However, the lack of
solubility of the unfunctionalized SWNT makes them difficult to
handle, arrange or organise in a controlled manner. Particularly,
this fact becomes even more important in electronics and
photovoltaics (PV) when combined with other types of donor or
acceptor active units.3
On the other hand, among the numerous industrial, chemical
and technical applications of phthalocyanines (Pcs),7 a particular
interest as electron-donor molecules in the field of PV8 is
currently increasing. Factors such as (i) high light harvest at the
maximum of the solar photon flux (around 700 nm), (ii) excellent
photoconductivities, and (iii) reversible redox potentials are
responsible for these conjugated macrocycles to hold such
a prominent position. These properties turn the combinations of
Pcs with SWNT into the focus of research. In this regard, the
covalent functionalization of SWNT with phthalocyanines has
been exploited using different approaches such as cycloaddition
reactions.9 Because of the aforementioned reasons, the supra-
molecular functionalization of SWNT using either pyrene10 or
conjugated poly(p-phenylene vinylene) oligomers (oPPV),
bearing Pc’s as pendant arms,11,12 has already been investigated.
The latter has been shown that the ability to immobilize on the
sidewalls of SWNT—affording stable and homogeneous Pc–
oPPV/SWNT suspensions—depends on the n-type character of
the oligomer. The oligomers having an opposite p-type character
failed to be efficient. As a proof of concept, when a metastable
radical ion pair was formed upon photoexcitation of the Pc–
oPPV (n-type)/SWNT assembly the isolation of the Pc with
Unfortunately, the covalent approach to functionalize SWNT
walls and make them more soluble is responsible for partial
saturation of the extended p-system, and therefore, for impor-
tant modifications in both the electronic and spectroscopic
characteristics that make them so particular.4 In order to
aDepartment of Chemistry and Pharmacy & Interdisciplinary Center for
Molecular
Erlangen-Nu€rnberg, 91058 Erlangen, Germany. E-mail: dirk.guldi@
chemie.uni-erlangen.de; Fax: +49 9131 85 28307; Tel: +49 9131 85 27340
bDepartamento de Quꢀımica Orgaꢀnica, Universidad Autoꢀnoma de Madrid,
Cantoblanco, 28049 Madrid, Spain. E-mail: tomas.torres@uam.es; Fax:
+34 91 4973966; Tel: +34 91 4975097
Materials,
Friedrich-Alexander-Universita€t
cIMDEA-Nanociencia. Facultad de Ciencias, Cantoblanco, 28049 Madrid,
Spain
† Electronic supplementary information (ESI) available: Experimental
and the synthetic details of the new compounds and transient
absorption spectra of SWNT/2 and 2 in the visible region. See DOI:
10.1039/c1jm10572h
8014 | J. Mater. Chem., 2011, 21, 8014–8020
This journal is ª The Royal Society of Chemistry 2011

View Article Online
and used like that for the alkylation of 1011 affording in this way
the derivative 9. The subsequent substitution of the morpholine
rings by acetoxy groups led to bis(acetoxy)phthalonitrile 8. A
further step consisting of an oxidation of the bis(hydroxymethyl)
phthalocyanine 7 by means of periodinane IBX15 provided the
diformylphthalocyanine 6 in an 18% overall yield (Scheme 2).
Compound 2 is soluble in polar solvents such as THF or
DMF, which makes easier the purification and characterization
1
in solution by H-NMR, UV-Vis, FT-IR, and GPC (gel perme-
ation chromatography) techniques. 1H-NMR spectroscopy
showed broad signals due to the presence of isomers from both
Pc and PPV moieties. The signals (in THF-d8) at 9.7–8.0 ppm are
ascribed to the two types of hydrogen atoms surrounding the Pc
core and are split into two groups. The phenyl rings of the
conjugated polymer are observed at 7.9–7.3 ppm. At 7.2–6.4
ppm, the vinylene linkers within the PPV backbone resonance
showing the usual shift for trans-isomer vinylenes. Finally, the
signals between 4.6 and 4.2 ppm and 4.2 and 3.6 ppm were
assigned to the methylene groups linked to the oxygen. The trans-
configuration of the vinylene linkers was also confirmed by FT-
IR with a doop(C]C–H) at 979 cmÀ1. Absorption spectroscopy
of 1 reveals the known features of ZnPc. In particular, these are
strong Soret bands around 350 nm, and Q bands at around
675 nm. In addition, a broad band is seen around 480 nm, which
is ascribed to the PPV backbone (Fig. 2).
Fig. 1 ZnPc oligomers 1–5.
regard to the carbon tube caused a ratio of charge separation rate
to charge recombination rate of nearly 3 orders of magnitude.
As a consequence of the aforementioned results, we have
focused on the study of a longer PPV oligomer with the Pc
separated from the conjugated backbone and comparing it with
the previous ones having the Pc unit directly attached to the
oligomer chain in order to shed light onto the interactions taking
place among the different components. Specifically, the oligo-
mers subject of this study are, on one hand, p-type PPV with
varying number of repeat units 3–511,12 or with a hexyl spacer that
links ZnPc to the PPV backbone 2, and, on the other hand,
n-type (CN-functionalized) short PPV 111,12 as depicted in Fig. 1.
Strong interactions between n-type PPV and SWNT have
already been documented, but the fact that also p-type PPVs
interact with SWNT is surprising and warrants further investi-
gations.12 To this end, inserting alkane spacers between ZnPc and
the PPV backbone brings about structural flexibility, which
emerges as a decisive factor in the interactions with SWNT.
The GPC results in THF using polystyrene standards are
summarized in Table 1 for all the series. These data allow esti-
mating the length of the polymeric chains with regard to their
weight-average molecular weight in mass (Mw). Moreover, the
number-average molecular weight, taking the size distributions
of the chains (Mn) into account, was determined. The ratio Mw/
Mn provided insight into the regularity in size or dispersion
(polydispersity, PDI). It is important to remark that in all the
compounds each repeat unit contains two PPV units and one Pc.
Synthesis
Compounds 1 and 3 as well as 4–6 were prepared as described
previously by us, elsewhere.11,12 Oligomer 2 was synthesized
following a related procedure, that is, by a Wadsworth–Horner–
Emmons reaction of dialdehyde 6 and 2,5-di-n-octyloxy-1,4-
xylene-bis(diethylphosphonate)ester13 with tBuOK, in refluxing
THF (Scheme 1). Subsequent separation on a SEC (size exclu-
sion chromatography) column afforded the corresponding main
fraction named as compound 2, in a moderate yield (19%).
The synthesis of the key phthalocyanine 7 was accomplished
by cross-condensation of 8 with 4-tert-butylphthalonitrile in the
presence of Zn(AcO)2 followed by methanolysis. Phthalonitrile 8
was prepared from phthalonitrile precursor 1214 after tosylation,
Scheme 1 Synthesis of oligomer 2.
Scheme 2 Synthetic pathway of diformylphthalocyanine 2.
J. Mater. Chem., 2011, 21, 8014–8020 | 8015
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Fig. 2 Room temperature absorption spectra of 1–5 in THF, normal-
ized for identical Q band absorption maxima at 675 nm. 1: black; 2: grey;
3: brown; 4: red; and 5: magenta. Please note that due to the different
number of repeat units of oligomers 1–5, no concentrations are given.
Fig. 3 Room temperature fluorescence spectrum of 2 in THF upon 450
nm excitation of PPV with optical density at the excitation wavelength of
0.01 (1.7 Â 10À8 M).
further confirm this notion. The ZnPc fluorescence quantum
yield—upon exclusive PPV excitation at 480 nm—is 0.021 for 1
and between 0.076 and 0.045 for 3–5, while a lower value of 0.028
has been determined for 2. Comparing the fluorescence quantum
yields upon ZnPc excitation at 610 nm, the following trend is
observed. The fluorescence quantum yield for the ZnPc reference
is in the order of 0.316 and decreases steadily with the length of
the oligomer chain—see Table 2. A similar trend is seen when
inspecting the fluorescence lifetimes. Longer chain lengths result
in shorter fluorescence lifetimes. The explanation for these find-
ings implies interactions between neighboring ZnPc as
commonly observed in aggregates of ZnPc.17 As the chain lengths
increase, the ZnPc/ZnPc interactions intensify.
Table 1 GPC results of oligomers 1–5 in THF and size estimated based
on Mn data
Size based on
Mn (repeating units)
Entry
Mw
8495
37 896
7211
Mn
5730
12 883
5132
6365
PDI
1
2
3
4
5
1.482
2.941
1.405
3.429
2.696
ca. 4
ca. 27
ca. 3
ca. 9
ca. 16
21 823
32 038
11 882
Results and discussion
To test the interactions between ZnPc–PPV oligomers 1–5 and
SWNT, in THF suspended SWNT were titrated with small
amounts of the dissolved oligomers. The SWNT suspensions
were prepared following our recently published procedure.10,12
For all of the oligomers, interactions with SWNT are
discernible. For example, in the visible region, the Q band
absorptions of ZnPc red-shift from 675 nm to 695 nm–see Fig. 4.
In addition, the ZnPc fluorescence is quenched following the
addition to the SWNT suspensions. As a complement to the
aforementioned, in the near-infrared SWNT absorptions red-
shift as well upon addition of ZnPc–PPV oligomers 1–5. In line
with this observation, the near-infrared fluorescence of SWNT is
susceptible to parallel red-shifts, when photoexcited with a Nd:
YAG laser at 532 nm. However, upon further addition of the
ZnPc–PPV oligomers, the fluorescence features of ZnPc reap-
pear, a finding, which is due to free, non-SWNT bound ZnPc–
PPV oligomer in solution. This is accompanied by a shoulder in
the absorption spectra at around 675 nm, which matches the Q
band absorption seen in experiments lacking SWNT—see Fig. 4.
Notable differences between the different ZnPc–PPV oligomers
do not emerge.
The absorption features of the different oligomers (1–5) are
a combination of those of ZnPc and PPVs. In particular, ZnPc
reveals two major absorption bands in THF, namely the Soret
band with a maximum at 350 nm and the Q band with the
maximum at 675 nm. PPVs show broad absorptions in the region
between 400 and 500 nm—see Fig. 2. Oligomer 1, for example,
shows just a weak PPV absorption, whilst in the remaining
oligomers the intensity of the PPV centered absorption increases
with the number of repeat units. Interestingly, 2, in which the
ZnPc/PPV distance has been enlarged, the PPV absorption is red-
shifted.
In steady-state fluorescence experiments with ZnPc and 1–5,
very similar fluorescence patterns emerged that exhibit
a pronounced maximum at around 680 nm. Excitation of 1 and
3–5 into the PPV absorption at 480 nm did not lead to any
appreciable PPV fluorescence. Instead, intense phthalocyanine
fluorescence is seen. To unravel the mechanism of producing the
ZnPc fluorescence, excitation spectra were taken. The excitation
spectra were exact matches of the ground state absorption of the
PPV and ZnPc with maxima at 400–500 and 675 nm, respec-
tively. An excited state deactivation via an energy transfer from
the donating PPV to the accepting ZnPc is taking place.
Notably, 2, which features a somewhat larger PPV/ZnPc
distance, reveals a different trend—see Fig. 3. Here, we note dual
fluorescence, that is, a PPV and a ZnPc centered fluorescence
with maxima at 490 and 680 nm, respectively. The reason
assigned to this behaviour is an incomplete energy transfer
between the two constituents. Fluorescence quantum yields
Further studies on interactions between the ZnPc–PPV
oligomers and SWNT necessitated the preparation of more
stable and more concentrated suspensions of SWNT.10,12,18
Importantly, the simple use of the procedure developed for 112
fails for 2–5, but nevertheless we managed to modify our
procedure to prepare a stable suspension of SWNT with all of the
ZnPc–PPV oligomers. To ensure that comparable amounts of
SWNT are added, a pre-suspension of SWNT in THF was
8016 | J. Mater. Chem., 2011, 21, 8014–8020
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Table 2 Overview of the number of repeat units of the compounds. Fluorescence quantum yields (FPPV 480 nm excitation; Rhodamine 6G reference;
FZnPc 610 nm excitation; ZnPc reference; THF) and fluorescence lifetimes (647 nm excitation, 684 nm detection, THF)
Compound
Pc units per molecule
FPPV
FZnPc
sZnPc/ns
Relative weight
ZnPc
1
4
27
3
9
16
—
0.30
0.23
0.07
0.18
0.13
0.04
3.49
3.21
2.85
3.42
3.13
3.03
1.00
0.90
0.96
0.99
0.99
0.98
1
2
3
4
5
0.021
0.028
0.076
0.045
0.032
backbone is not decisive, only the shortest oligomer 3 seems to be
slightly favored to yield stable suspensions. The overall limiting
factor is the weak nature of the p–p-interactions to drive the
PPV immobilization onto SWNT. Repulsive forces as they result
between p-type PPV19 and p-type SWNT20 further contribute to
the overall limitations. 1, on the hand, has been shown to be
efficient SWNT suspender due to the strong n-type character of
the PPV of SWNT.12,19 A closer inspection at 2–5 reveals an
interesting trend—see Fig. 5. SWNT suspensions with 2 show
a superior stability when compared to those prepared with 3–5,
although never reaching the performance of 1. A likely rationale
involves the structural flexibility of 2, which enables the PPV
backbone to interact better with the curved SWNT surface and,
in turn, secures an enhanced stability of the resulting suspen-
sions. The stability of the SWNT suspensions was further tested
by centrifugation to remove the non-debundled SWNT. 1 and 2
give rise to acceptable stabilities upon centrifugation for 15
minutes at 9.6 kG, whilst 3–5 failed these tests. Here, only
residual amounts of SWNT remain in suspensions.
To get deeper insights into the SWNT/ZnPc–PPV oligomer
hybrids, the suspensions prepared by the aforementioned
procedure were subjected to fluorescence measurements, in
which the near-infrared fluorescence of SWNT is recorded at
various excitation wavelengths, the so-called 3D fluorescence
plot. These experiments are decisive, since they generate
comparable fluorescence landscapes of SWNT upon variable
Fig. 4 Upper part – titration of SWNT suspended in THF with 2.
Black—absorption spectrum of pure SWNT, grey—absorption spectrum
of 2.9 Â 10À9 M of 2 with SWNT, brown—absorption spectrum of 5.8 Â
10À9 M of 2 with SWNT, red—absorption spectrum of 1.1 Â 10À8 M of 2
with SWNT, and purple—absorption spectrum 1.7 Â 10À8 M of 2 with
SWNT. Lower part: corresponding fluorescence spectra after 610 nm
excitation.
excitation, namely between 550 and 800 nm, by a Xe-lamp.21
A
SWNT/SDBS suspension in D2O, in which the SWNT are
capped by SDBS and, in turn, lack the electronic communication
with the environment (i.e., ZnPc–PPV), was used as a reference.
The fact that different solvents were used in these assays should
prepared. We started with 0.2 mg of solid SWNT and 3 mL of
THF, which are sonicated for 10 minutes to disperse the SWNT.
Although these SWNT suspensions are largely heterogeneous, it
guarantees the use of equal amounts of SWNT. Next, 0.2 mL of
this pre-suspension were added to 8 mL of THF solutions of
ZnPc–PPV oligomers 1–5 that exhibit identical Q band absorp-
tions of ZnPc. This was followed by 10 minutes of ultra-
sonication and recording absorption spectra. Going through
several repetitions of these steps we could realize stable SWNT
suspensions, which contain SWNT immobilized ZnPc–PPV
oligomers. To this end, stability tests were carried out for up to
a week without showing notable SWNT aggregation. A further
increase of the SWNT concentration initiated aggregation,
precipitation, etc. The number of repeat units in the PPV
Fig. 5 Absorption spectra of stable suspensions of SWNT/1–5 in THF
with the highest possible amount of dispersed SWNT prior to their
precipitation. Oligomer solutions exhibited similar Q band absorption of
1–5 at the starting point of the preparation procedure—see Fig. 2.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8014–8020 | 8017

View Article Online
not invalidate the interpretation. All of the suspensions were of
comparable SWNT absorptions all throughout the excitation
area. Resembling the absorption features, all SWNT character-
istic fluorescence maxima are red-shifted in SWNT/1–5 relative
to SWNT/SDBS. Although 1–5 evoke no particular differences
in terms of red-shifts, significant changes are seen in terms of
fluorescence intensities.
(Fig. 7). Initially after photoexcitation, a broad maximum in the
500 to 620 nm range and a minimum located at around 700 nm,
reflecting the ZnPc singlet excited state absorption and the
ground state bleaching, are discernible—see Fig. S1†. The
ground state bleach deactivates rapidly within 6.1 Â 1012 sÀ1
,
while in Fig. S2† showing non-SWNT bound ZnPc–PPV
oligomer 2, a slow intersystem crossing to the ZnPc triplet excited
state is observable. Inspecting the near-infrared region sheds
light on SWNT based features. In particular, bleaching of the
SWNT’s van Hove singularities evolves as a minimum at around
1350 nm and as shoulders at around 1160 and 1280 nm.
However, taking the fact of the diluted SWNT suspension into
account, no further features have been identified. All of the
SWNT centered features decay very fast—1.6 Â 1012 sÀ1—a value
that matches quite well with the singlet excited state decay of
ZnPc. Together with the decay of the ZnPc and SWNT charac-
teristics, the SWNT exNIR bands shift towards shorter wave-
lengths and an 840 nm maximum evolves. The earlier feature,
namely a blue shift of about 10 to 20 nm of the SWNT absorp-
tion features, is indicative for the formation of the charge
transfer product, that is, reduced SWNT—see Fig. 7, while the
latter reflects the well-known absorption of the ZnPc radical
cation,12,23 for which we have determined a lifetime of around
100 ps. Such a short lifetime relates to the short distance between
the electron donating ZnPc and the SWNT sidewalls. Interesting
is the comparison with SWNT/1—similar lifetimes corroborate
our assumption that the nature of the PPV oligomer plays no
In fact, the fluorescence intensities at an excitation wavelength
of 725 nm—see Fig. 6—show the weakest quenching of SWNT
fluorescence for the longest ZnPc–PPV oligomer, that is, 5, fol-
lowed by 4 and 3. Phthalocyanines that are in contact with the
surface of SWNT by either covalent or non-covalent9b,10,12,22
binding reveal quenched fluorescence due to charge transfer
interactions. Due to the fact that an excess of oligomer was
omnipresent during these assays, we postulate that the immobi-
lization of ZnPc–PPV oligomers 1–5 onto the SWNT surface is at
its maximum. Taking this into consideration we rationalize the
weaker SWNT fluorescence quenching, especially for the longer
ZnPc–PPV oligomers to fewer ZnPc immobilized onto SWNT.
Due to sterical reasons, smaller ZnPc–PPV oligomers have more
possibilities to attach to the SWNT surface. 1 and 2 show
a comparable quenching of the SWNT fluorescence. The latter is
nearly as strong as that seen for 3. For 1 and 2, high SWNT
surface coverage arises either from a favored n-type/p-type
interaction between 1 and SWNT or from an efficient interaction
of SWNT and 2 due to its structural flexibility. All of the
aforementioned results were independently corroborated by
fluorescence spectra recorded upon 532 nm Nd:YAG laser
excitation.
Lastly, we turned to transient absorption measurements. The
low concentration of SWNT suspensions containing oligomers
3–5 rendered their investigations difficult. Thus, we focused our
work exclusively on the characterization of SWNT/2 in
comparison to the results published recently for SWNT/1.12
During the preparation of SWNT suspensions in THF—by
following the aforementioned procedure—special care was taken
to avoid free ZnPc–PPV oligomer in solution. To this end, the
ZnPc Q band absorption at 695 nm served as a convenient
marker for SWNT immobilized ZnPc–PPV. The differential
absorption spectra of SWNT/2, obtained upon laser excitation at
387 nm, show great similarities with those seen for SWNT/1
Fig.
7 Upper part—differential absorption spectra (near-infrared)
obtained upon femtosecond laser flash photolysis (387 nm) of SWNT/2 in
THF at room temperature with several time delays between 0.95 and 4.0
ps representing the bleaching of the SWNT van Hove singularities and
formation of reduced SWNT. Lower part—the absorption time profile of
the above spectra at 1350 nm.
Fig. 6 SWNT NIR fluorescence spectra of SWNT/1–5 in THF upon Xe-
lamp 725 nm excitation of SWNT/1–5 and SWNT/SDBS/D2O reference.
All samples show similar absorption at 725 nm.
8018 | J. Mater. Chem., 2011, 21, 8014–8020
This journal is ª The Royal Society of Chemistry 2011

View Article Online
2007, 19, 1439; (i) P. Avouris, M. Freitag and V. Perebeinos, Nat.
Photonics, 2008, 2, 341; (j) T. Umeyama and H. Imahori, Energy
Environ. Sci., 2008, 1, 120; (k) V. Sgobba and D. M. Guldi, Chem.
Soc. Rev., 2009, 38, 165.
significant or only a minor role in impacting the charge transfer
properties.
4 (a) Y. Saito and S. Uemura, Carbon, 2000, 38, 169; (b) A. Kuznetsova,
I. Popova, J. T. Yates, Jr, M. J. Bronikowski, M. C. B. Huffman,
J. Liu, R. E. Smalley, H. H. Hwu and J. G. Chen, J. Am. Chem.
Soc., 2001, 123, 10699; (c) W. Huang, Y. Lin, S. Taylor, J. Gaillard,
A. M. Rao and Y.-P. Sun, Nano Lett., 2002, 2, 231; (d) J. Zhang,
H. Zou, Q. Qing, Y. Yang, Q. Li, Z. Liu, X. Guo and Z. Du, J.
Phys. Chem. B, 2003, 107, 3712; (e) S. Bellucci, Phys. Status Solidi
C, 2005, 2, 34; (f) S. Osswald, E. Flahaut and Y. Gogotsi, Chem.
Mater., 2006, 18, 1525; (g) A. Lucas, C. Zakri, M. Maugey,
M. Pasquali, P. van der Schoot and P. Puolin, J. Phys. Chem. C,
2009, 113, 20599.
5 (a) N. Nakashima, Y. Tomonari and H. Murakami, Chem. Lett.,
2002, 31, 638; (b) A. Star, Y. Liu, K. Grant, L. Ridvan,
J. F. Stoddart, D. W. Steuerman, M. R. Diehl, A. Boukai and
J. R. Heath, Macromolecules, 2003, 36, 553; (c) J. Zhang, J.-K. Lee,
Y. Wu and R. W. Murray, Nano Lett., 2003, 3, 403; (d)
H. Murakami, T. Nomura and N. Nakashima, Chem. Phys. Lett.,
2003, 378, 481; (e) D. M. Guldi, G. M. A. Rahman, V. Sgobba and
C. Ehli, Chem. Soc. Rev., 2006, 35, 471; (f) D. Tasis,
N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006, 106,
1105; (g) H. Murakami and N. Nakashima, J. Nanosci.
Nanotechnol., 2006, 6, 16; (h) C. Ehli, C. Oelsner, D. M. Guldi,
A. Mateo-Alonso, M. Prato, C. Schmidt, C. Backes, F. Hauke and
A. Hirsch, Nat. Chem., 2009, 1, 243; (i) A. S. D. Sandanayaka,
E. Maligaspe, T. Hasobe, O. Ito and F. D’Souza, Chem. Commun.,
2010, 46, 8749.
Conclusions
In summary, we have developed a novel synthetic route of a key
diformylphthalocyanine 6 as a monomer for the preparation of
conjugated PPV oligomers having the Pc connected to the
conjugated backbone through a flexible spacer. With the
comparative study among oligomers of different lengths,
p/n-type character, and separation between the Pc and the
conjugated backbone, we have documented the interactions of
ZnPc–PPV oligomers with the surface of SWNT. We were able to
show that either electronic modifications (1) or implementation
of structural flexibility (2) in simple ZnPc–PPV oligomers is
indispensable to realize highly concentrated and stable SWNT
suspensions. On the contrary, oligomers of a varied number of
repeat units did not show significant differences regarding the
stability of the suspensions. When inspecting, however, the
quenching of the SWNT fluorescence in the near-infrared
the length of the ZnPc–PPV oligomer (3–5) matters. In parti-
cular, shorter oligomers cover more of the SWNT surface leading
to a stronger SWNT fluorescence quenching. The structurally
modified oligomers, such as 1 and 2, reveal the strongest SWNT
fluorescence quenching. This is primarily due to facilitated
immobilization onto SWNT, that is either based on n-type/p-type
interactions in the case of 1 or the structural flexibility in the case
of 2. Time-resolved transient absorption measurements verified
charge transfer that evolves from the photoexcited ZnPc to
SWNT yielding a 100 ps stable charge transfer product. In terms
of charge transfer, the nature of the PPV backbone has at best
a minor role.
6 (a) M. Shim, A. Javey, N. W. S. Kam and H. Dai, J. Am. Chem. Soc.,
2001, 123, 11512; (b) J. Chen, H. Liu, W. A. Weimer, M. D. Halls,
D. H. Waldeck and G. C. Walker, J. Am. Chem. Soc., 2002, 124,
9034; (c) Z. Li, V. Saini, E. Dervishi, V. P. Kunets, J. Zhang,
Y. Xu, A. R. Biris, G. J. Salamo and A. S. Biris, Appl. Phys. Lett.,
2010, 96, 033110.
7 (a) G. de la Torre, C. G. Claessens and T. Torres, Chem. Commun.,
2007, 2000; (b) C. G. Claessens, U. Hahn and T. Torres, Chem.
ꢀ
Rec., 2008, 8, 75; (c) Y. Rio, M. S. Rodrıguez-Morgade and
T. Torres, Org. Biomol. Chem., 2008, 6, 1877; (d) G. de la Torre,
G. Bottari, U. Hahn and T. Torres, Struct. Bonding, 2010, 135, 1;
(e) J. Mack and N. Kobayashi, Chem. Rev., 2011, 111, 281.
8 (a) J. Xue, S. Uchida, B. P. Rand and S. R. Forrest, Appl. Phys. Lett.,
2004, 84, 3013; (b) K. Suemori, T. Miyata, M. Yokoyama and
M. Hiramoto, Appl. Phys. Lett., 2005, 86, 063509; (c) J.-J. Cid, J.-
Acknowledgements
Financial support from the DFG (GU 517/4-2 and Cluster of
Excellence), the MICINN and MEC, Spain (CTQ2008-00418/
BQU, CONSOLIDER-INGENIO 2010 CDS 2007-00010
Nanociencia Molecular, PLE2009-0070), COST Action D35,
ꢀ
H. Yum, S.-R. Jang, M. K. Nazeeruddin, E. Martınez-Ferrero,
E. Palomares, J. Ko, M. Gratzel and T. Torres, Angew. Chem., Int.
€
ꢀ
Ed., 2007, 46, 8358; (d) J.-J. Cid, M. Garcıa-Iglesias, J.-H. Yum,
ꢀ
ꢀ
€
A. Forneli, J. Albero, E. Martınez-Ferrero, P. Vazquez, M. Gratzel,
Md. K. Nazeeruddin, E. Palomares and T. Torres, Chem.–Eur. J.,
2009, 15, 5130; (e) B. E. Hardin, E. T. Hoke, P. B. Armstrong, J.-
and
CAM
(MADRISOLAR-2,
S2009/PPQ/1533)
is
acknowledged.
ꢀ
H. Yum, P. Comte, T. Torres, J. M. J. Frechet,
M. K. Nazeeruddin, M. Gratzel and M. D. McGehee, Nat.
€
ꢀ
Photonics, 2009, 3, 406; (f) M. V. Martınez-Dıaz, G. de la Torre
and T. Torres, Chem. Commun., 2010, 46, 7090; (g) B. E. Hardin,
ꢀ
References
ꢀ
J.-H. Yum, E. T. Hoke, Y. C. Jun, P. Pechy, T. Torres,
M. L. Brongersma, Md. K. Nazeeruddin, M. Gratzel and
M. McGehee, Nano Lett., 2010, 10, 3077; (h) K. Driscoll, J. Fang,
N. Humphry-Baker, T. Torres, W. T. S. Huck, H. J. Snaith and
1 (a) S. Iijima, Nature, 1991, 354, 56; (b) S. Niyogi, M. A. Hamon,
H. Hu, B. Zhao, P. Bhowmik, R. Sen, M. E. Itkis and
R. C. Haddon, Acc. Chem. Res., 2002, 35, 1105; (c) H. Dai, Acc.
Chem. Res., 2002, 35, 1035; (d) E. T. Thostenson, C. Li and T.-
W. Chou, Compos. Sci. Technol., 2005, 65, 491.
€
ꢀ
R. H. Friend, Nano Lett., 2010, 10, 4981; (i) M. Garcıa-Iglesias, J.-
J. Cid, J.-H. Yum, A. Forneli, P. Vazquez, Md. K. Nazeeruddin,
ꢀ
2 P. Diao and Z. Liu, Adv. Mater., 2010, 22, 1430.
€
E. Palomares, M. Gratzel and T. Torres, Energy Environ. Sci., 2011,
4, 189.
3 (a) D. M. Guldi, G. M. A. Rahman, F. Zerbetto and M. Prato, Acc.
Chem. Res., 2005, 38, 871; (b) D. M. Guldi, G. M. A. Rahman,
V. Sgobba, N. A. Kotov, D. Bonifazi and M. Prato, J. Am. Chem.
Soc., 2006, 128, 2315; (c) V. Sgobba, G. M. A. Rahman,
D. M. Guldi, N. Jux, S. Campidelli and M. Prato, Adv. Mater.,
2006, 18, 2264; (d) C. Ehli, G. M. A. Rahman, N. Jux, D. Balbinot,
D. M. Guldi, F. Paolucci, M. Marcaccio, D. Paolucci, M. Melle-
Franco, F. Zerbetto, S. Campidelli and M. Prato, J. Am. Chem.
Soc., 2006, 128, 11222; (e) R. Chitta, A. S. D. Sandanayaka,
A. L. Schumacher, L. D’Souza, Y. Araki, O. Ito and F. D’Souza, J.
Phys. Chem. C, 2007, 111, 6947; (f) A. Kongkanand,
9 (a) B. Ballesteros, S. Campidelli, G. de la Torre, C. Ehli, D. M. Guldi,
M. Prato and T. Torres, Chem. Commun., 2007, 2950; (b)
B. Ballesteros, G. de la Torre, C. Ehli, G. M. A. Rahman,
F. Agullo-Rueda, D. M. Guldi and T. Torres, J. Am. Chem. Soc.,
2007, 129, 5061; (c) S. Campidelli, B. Ballesteros, A. Filoramo,
ꢀ
ꢀ
D. Dıaz-Dıaz, G. de la Torre, T. Torres, G. M. A. Rahman,
C. Ehli, D. Kiessling, F. Werner, V. Sgobba, D. M. Guldi,
C. Cioffi, M. Prato and J.-P. Bourgoin, J. Am. Chem. Soc., 2008,
ꢀ
130, 11503; (d) K. H. Le Ho, L. Rivier, B. Jousselme, P. Jegou,
A. Filoramo and S. Campidelli, Chem. Commun., 2010, 46, 8731; (e)
G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev.,
2010, 110, 6768.
ꢀ
R. M. Domınguez and P. V. Kamat, Nano Lett., 2007, 7, 676; (g)
P. Avouris, Z. Chen and V. Perebeinos, Nat. Nanotechnol., 2007, 2,
605; (h) B. L. Allen, P. D. Kichambare and A. Star, Adv. Mater.,
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8014–8020 | 8019

View Article Online
10 J. Bartelmess, B. Ballesteros, G. de la Torre, D. Kiessling,
S. Campidelli, M. Prato, T. Torres and D. M. Guldi, J. Am. Chem.
Soc., 2010, 132, 16202.
Synth. Met., 2001, 122, 401; (g) D. A. M. Egbe, T. Kietzke,
€
€
B. Carbonnier, D. Muhlbacher, H.-H. Horhold, D. Neher and
T. Pakula, Macromolecules, 2004, 37, 8863; (h) S. W. Chasteen,
S. A. Carter and G. Rumbles, J. Chem. Phys., 2006, 124, 214704; (i)
V. Boucard, Macromolecules, 2001, 34, 4308; (j) M. Hanack,
11 J.-J. Cid, C. Ehli, C. Atienza-Castellanos, A. Gouloumis, E.-M. Maya,
ꢀ
P. Vazquez, T. Torres and D. M. Guldi, Dalton Trans., 2009, 3955.
ꢀ
ꢀ
12 J. Bartelmess, C. Ehli, J.-J. Cid, M. Garcıa-Iglesias, P. Vazquez,
T. Torres and D. M. Guldi, Chem. Sci., 2011, 2, 652.
€
B. Behnisch, H. Hackl, P. Martinez-Ruiz and K.-H. Schweikart,
Thin Solid Films, 2002, 417, 26; (k) J.-P. Morin, N. Drolet, Y. Tao
and M. Leclerc, Chem. Mater., 2004, 16, 4619; (l) P. Teranekar,
M. Abdulbaki, R. Krishnamoorti, S. Phanichphant, P. Waenkaew,
D. Patton, T. Fulghum and R. Advicula, Macromolecules, 2006, 39,
3848.
€
13 A. Drury, S. Meier, M. Ruther and W. J. Blau, J. Mater. Chem., 2003,
13, 485.
ꢀ ꢀ
14 D. Dıaz-Dıaz, J.-J. Cid, P. Vazquez and T. Torres, Chem.–Eur. J.,
ꢀ
2008, 14, 9261.
15 (a) M. Frigerio, M. Santagostino, S. Sputore and G. Palmisano, J.
Org. Chem., 1995, 60, 7272; (b) M. Frigerio, M. Santagostino,
S. Sputore and G. Palmisano, J. Org. Chem., 1999, 64, 4537; (c)
J. D. More and N. S. Finney, Org. Lett., 2002, 4, 3001.
16 P. G. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969, 31, 1.
17 (a) J. Vacus and J. Simon, Adv. Mater., 1995, 7, 7997; (b) S. Dhami
and D. Phillips, J. Photochem. Photobiol., A, 1996, 100, 77; (c) X.-
Y. Li, X. He, A. C. H. Ng, C. Wu and D. K. G. Ng,
Macromolecules, 2000, 33, 2119; (d) T. Ngai, G. Zhang, X.-Y. Li,
D. K. P. Ng and C. Wu, Langmuir, 2001, 17, 1381; (e) A. Ogunsipe,
J.-Y. Chen and T. Nyokong, New J. Chem., 2004, 28, 822.
20 (a) M. Shim, A. Javey, N. Wong Shi Kam and H. Dai, J. Am. Chem.
Soc., 2001, 123, 11512; (b) A. Javey, J. Guo, Q. Wang, M. Lundstrom
and H. Dai, Nature, 2002, 424, 654; (c) V. Derycke, R. Martel,
J. Appenzeller and P. Avouris, Appl. Phys. Lett., 2002, 80, 2773; (d)
Y. Zhou, A. Gaur, S.-J. Hur, C. Kocabas, M. A. Meitl, M. Shim
and J. A. Rogers, Nano Lett., 2004, 4, 2031.
21 (a) M. J. O’Connell, S. M. Bachilo, C. B. Huffman, V. C. Moore,
M. S. Strano, E. H. Haroz, K. L. Rialon, P. J. Boul, W. H. Noon,
C. Kittrell, J. Ma, R. H. Hauge, R. B. Weisman and R. E. Smalley,
Science, 2002, 297, 593; (b) R. B. Weisman and S. M. Bachilo,
Nano Lett., 2003, 3, 1235; (c) A. Hartschuh, H. N. Pedrosa,
L. Novotny and T. D. Krauss, Science, 2003, 301, 1354; (d)
R. B. Weisman, S. M. Bachilo and D. Tsyboulski, Appl. Phys. A:
Mater. Sci. Process., 2004, 78, 1111; (e) D. Yoon, S.-J. Kang, J.-
B. Choi, Y.-J. Kim and S. Baik, J. Nanosci. Nanotechnol., 2007, 7,
3727.
ꢀ
18 J. Bartelmess, A. R. M. Soares, M. V. Martınez-Dıaz,
M. G. P. M. S. Neves, A. C. Tome, J. A. S. Cavaleiro, T. Torres
ꢀ
ꢀ
and D. M. Guldi, Chem. Commun., 2011, 47, 3490.
19 (a) N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. C. Friend
and A. B. Holmes, Nature, 1993, 365, 628; (b) J. J. M. Halls,
C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend,
S. C. Moratti and A. B. Holmes, Nature, 1995, 376, 498; (c) G. Yu
and A. J. Heeger, J. Appl. Phys., 1995, 78, 4510; (d) S. C. Moratti,
R. Cervini, A. B. Holmes, D. R. Baigent, R. H. Friend,
22 (a) P. W. Barone, S. Baik, D. A. Heller and M. S. Strano, Nat. Mater.,
2005, 4, 86; (b) B. C. Satishkumar, L. O. Brown, Y. Gao, C.-C. Wang,
H.-L. Wang and S. K. Doorn, Nat. Nanotechnol., 2007, 2, 560.
23 (a) T. Nyokong, Z. Gasyna and M. J. Stillman, Inorg. Chem., 1987,
26, 548; (b) M. E. El-Khouly, O. Ito, P. M. Smith and F. D’Souza,
J. Photochem. Photobiol., C, 2004, 5, 79; (c) A. Kahnt,
€
N. C. Greenham, J. Gruner and P. J. Hamer, Synth. Met., 1995, 71,
2117; (e) M. M. de Souza, G. Rumbles, D. L. Russell,
I. D. W. Samuel, S. C. Moratti, A. B. Holmes and P. L. Burn,
Synth. Met., 2001, 119, 635; (f) Y. Liu, G. Yu, Q. Li and D. Zhu,
ꢀ
M. Quintiliani, P. Vazquez, D. M. Guldi and T. Torres,
ChemSusChem, 2008, 1, 97.
8020 | J. Mater. Chem., 2011, 21, 8014–8020
This journal is ª The Royal Society of Chemistry 2011