Nanoscale organization of a phthalocyanine-fullerene system: Remarkable stabilization of charges in photoactive 1-D nanotubules

Article abstract of DOI:10.1021/ja043283u

Induction of self-organization between zinc phthalocyanine (ZnPc) and C₆₀ moieties in a novel amphiphilic ZnPc-C₆₀ salt results in uniformly nanostructured 1-D nanotubules. Their photoreactivity, in terms of ultrafast charge separation (i.e., ～1012 s^-1) and ultraslow charge recombination (i.e., ～103 s^-1), is remarkable. In addition, the observed ZnPc^?+-C₆₀^?- lifetime of 1.4 ms implies, relative to that of the monomeric ZnPc-C₆₀ (～3 ns), an impressive stabilization of 6 orders of magnitude. Copyright

Full text of DOI:10.1021/ja043283u

Published on Web 04/05/2005
Nanoscale Organization of a Phthalocyanine-Fullerene System: Remarkable
Stabilization of Charges in Photoactive 1-D Nanotubules
Dirk M. Guldi,*,† Andreas Gouloumis,‡ Purificacio´n Va´zquez,‡ Toma´s Torres,*,‡
Vasilios Georgakilas,§ and Maurizio Prato*,§
UniVersita¨t Erlangen, Institute for Physical and Theoretical Chemistry, 91058 Erlangen, Germany, UniVersity of
Notre Dame, Radiation Laboratory, Notre Dame, Indiana 46556, Departamento de Qu´ımica Orga´nica (C-I),
UniVersidad Auto´noma de Madrid, Cantoblanco 28049 Madrid, Spain, and Dipartimento di Scienze Farmaceutiche,
UniVersita` di Trieste, Piazzale Europa 1, Trieste 34127, Italy
Received November 8, 2004; E-mail: dirk.guldi@chemie.uni-erlangen.de; tomas.torres@uam.es; prato@units.it
The construction of precisely defined molecular materials,1,2
designed to perform specific functions such as electrical conductiv-
ity, requires the incorporation of functional building blocks and a
control of the supramolecular assembly process.
Scheme 1. Synthesis of Pc-C60 System 8
Phthalocyanines (Pcs),3 one of the best known synthetic por-
phyrin analogues, are highly versatile and stable chromophores with
unique physicochemical properties that make them, alone or in
combination with many other electro- and photoactive moieties,4
ideal building blocks for the construction of molecular materials
having special electronic and optical properties. In general, the
design of Pcs linked to strong acceptor moieties appears particularly
promising. We have recently described the preparation of strongly
coupled ZnPc-C60 dyads5 in which long-lived and highly emissive
charge-separated states are formed.6
Additionally, owing to their extended flat hydrophobic aromatic
surface, phthalocyanine molecules can interact with each other by
attractive π-π stacking interactions,3a,7 leading to aggregation both
in solution and in the solid state. However, the formation of long-
range ordered Pc aggregates based on the mutual noncovalent
recognition8 of Pcs is a challenging task and requires the introduc-
tion of additional structural features within the Pc ring.
Relatively few recognition motifs have been employed for the
construction of nanosized supramolecular architectures based on
Pcs. Among them are the introduction of adequate peripheral sub-
stituents to promote a mesomorphic behavior,9 the cation complex-
ation of crowned-phthalocyanines,10 and the use of hydrogen-
bonding11 and metal-ligand12 interactions to self-assemble phthalo-
cyanine molecules forming either discrete supramolecular structures
or infinite ill-defined aggregates.
In this connection, we have described recently the first Pc
supramolecular assembly, driven by strong donor-acceptor interac-
tions between Zn(II)- and Ni(II)-phthalocyanine rings that possess
alkoxy and alkylsulfonyl substituents at the periphery, respectively,
which has allowed the construction of donor-acceptor Pc-
nanoaggregates.13
In this communication, we describe the synthesis, nanoscale
organization, and photophysical properties of a novel Pc-C60
system 8. Interestingly, unique electron-transfer benefits stem from
the tertiary organization of 8 into perfectly ordered one-dimensional
(1-D) ZnPc-C60 nanotubules (NTs). When dissolving 8 or its
precursor 7 homogeneously, for example, in nonpolar solvents, fast
charge recombination occurs (i.e., ns), whereas in polar solvents,
utilizing the amphiphilic structure of 8, aggregates form which lead
to very long charge-separated states (i.e., ms).
The synthesis of formylphthalocyanine 1 was carried out starting
from 4-formylphthalonitrile (2)5 in a three-step sequence (Scheme
1).
The reduction of the formyl group in 2 was performed with an
excess of NaBH4, thus affording 4-hydroxymethylphthalonitrile (3)
in almost quantitative yield. The statistical crossover condensation
of 314 with 4-tert-butyl-phthalonitrile (4) in the presence of
Zn(OAc)2 in refluxing N,N-dimethylaminoethanol (DMAE) afforded
tri-tert-butyl-hydroxymethylphthalocyanine (5) in 22% yield, after
isolation from the statistical mixture of phthalocyanines by column
chromatography on silica gel. Further oxidation of 5 to the Pc-
aldehyde 15 was carried out by treatment with an excess of the
SO3-pyridine complex and triethylamine in dry DMSO at 50 °C
to afford 1 in 84% yield. Compound 1 was much easier to obtain
† Universita¨t Erlangen and University of Notre Dame.
‡ Universidad Auto´noma de Madrid.
§ Universita` di Trieste.
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10.1021/ja043283u CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 5811-5813
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C O M M U N I C A T I O N S
Figure 3. Steady-state fluorescence spectra of ZnPc-C60 NTs (lower line)
Figure 1. Ground-state absorption spectra of ZnPc-C60 7 (dashed
spectrum) in THF and ZnPc-C60 NT (salt 8) (solid spectrum) in H2O.
in H2O and ZnPc reference (upper line) in THF at room temperature with
matching absorption at the 375 nm excitation wavelength (i.e., OD375nm
0.2).
)
namely, other nanorods. These long shapes are reminiscent of those
observed for a similar donor-acceptor array, where a smaller
porphyrin macrocycle was attached to C60 instead of to the
phthalocyanine macrocycle.15 Obviously, phthalocyanines and
fullerenes are strong elements of order in this system, and self-
organization occurs through intermolecular forces.
Despite the strong fluorescence that the phthalocyanine chro-
mophore exhibits in the ZnPc reference (Φ ) 0.3) and in the ZnPc-
C60 7 (Φ ≈ 0.005), we determined a fluorescence quantum yield
for a ZnPc-C60 NT dispersion of <10-5 at 375 or 710 nm
excitation. Figure 3 illustrates the comparison of ZnPc fluorescence
in the ZnPc reference and in the ZnPc-C60 NTs. Implicit in the
near quantitative fluorescence quenching is an ultrafast electron-
transfer activationsonce the phthalocyanine chromophore is
photoexcitedsto the electron-accepting C60 core.
Figure 2. Nanotubules formed in water solution deposited on a TEM grid.
by this method than by the alternative ozonolysis procedure of the
corresponding vinyl Pc described previously by us.5 The structures
of all compounds were fully supported by their analytical and
spectroscopic data. The UV-vis spectrum of 5 presents a sharp
Q-band at 678 nm, whereas the MS-MALDI-TOF spectrum shows
the molecular ion peak at m/z 776-778 as multiprotonated isotopic
pattern without relevant fragmentations.
Formylphthalocyanine 1 was condensed in toluene at reflux with
N-functionalized glycine 6 to afford the expected fulleropyrrolidine
ZnPc-C60 7, which was characterized by NMR and IR and whose
purity was confirmed by direct-phase HPLC (see Supporting
Information). The amino group in 7 was deprotected by reaction
with excess of trifluoroacetic acid (TFA) to afford the ammonium
salt 8. Due to the presence of the positively charged end groups,
this latter compound could be dispersed in aqueous solutions. Figure
1 compares the ground-state spectrum of an aqueous ZnPc-C60 8
with that of monomeric ZnPc-C60 7 in THF. In line with the tight
π-stacking of the ZnPc and C60 moieties, the absorption spectrum
of such dispersions is characterized by substantial broadening, which
is further augmented by light scattering of the aggregated species.
Important also are the red-shifts of the 350 and 690 nm maxima to
375 and 710 nm, respectively.
Next, we complemented the steady-state fluorescence experi-
ments with time-resolved measurements. In particular, the long-
lived ZnPc fluorescence, for which we determined a lifetime of
3.1 ( 0.2 ns in the reference, emerges also as a sensitive probe for
such studies. However, we could not detect any appreciable
fluorescence signal for the NTs made from amphiphilic 8, because
the electron-transfer deactivation of the ZnPc singlet excited-state
occurs on a time scale much faster than our instrumental time
resolution. In fact, with the help of the fluorescence quantum yields
(i.e., ZnPc reference and ZnPc-C60 NT) and the fluorescence
lifetime (i.e., ZnPc reference) we extrapolated a rate constant of
electron transfer of about 1012 s-1
.
Transient absorption measurements confirmed that the product
of the ultrafast excited-state deactivation is indeed the radical ion
pair, ZnPc•+-C60•-. Spectroscopic evidence for the electron-transfer
product came from the featuresssee Figure 4sdeveloping im-
mediately with the conclusion of the laser pulse (i.e., 18 ps or 6
ns). In the far-visible region, the observed maximum at 850 nm
corresponds to the one-electron oxidized π-radical cation of ZnPc
(i.e., ZnPc•+), while in the near-infrared region the 1010 nm
maximum resembles the signature of the one-electron reduced form
of C60 (i.e., C60•-).6c
To study the aggregation properties, amphiphilic ZnPc-C60 salt
8 was dispersed in water, and the resulting mixture was sonicated
in an ultrasonic bath. The insoluble material was removed by
centrifugation at 3000 rpm, and one drop of the solution was
transferred to a TEM copper grid, coated with Formvar film. Long,
extremely well-organized nanorods were consistently observed
throughout the gridsFigure 2 (see also Figure S1 in Supporting
Information). Although atomic resolution was not possible, the
nanotubules reported here are formed by many other substructures,
An additional feature of the new transient is bleaching in a region
which is dominated by the broadened, ground-state absorptions
see Figure 1. It is important that both spectral attributes, namely
ZnPc•+ and C60•-, are persistent on the pico-, nano-, and micro-
second time scale. It is only in the millisecond regime that ZnPc•+
and C60•- start to decay slowly. Time-absorption profilessdepicted
•-
as an insert to Figure 4sillustrate that ZnPc•+-C60 decay via a
single step. The charge recombination dynamics within the ZnPc-
C60 NTs were determined accurately by fitting the decays
9
5812 J. AM. CHEM. SOC. VOL. 127, NO. 16, 2005

C O M M U N I C A T I O N S
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In conclusion, induction of self-organization between ZnPc and
C60 moieties in an amphiphilic ZnPc-C60 salt 8, which occurs
predominantly through intermolecular forces, results in uniformly
nanostructured 1-D nanotubules. Their photoreactivity, in terms of
ultrafast charge separation (i.e., ∼1012 s-1) and ultraslow charge
recombination (i.e., ∼103 s-1), is remarkable. In addition, the
observed ZnPc•+-C60•- lifetime of 1.4 ms implies, relative to that
of the monomeric ZnPc-C60 7 (τ ≈ 3 ns), an impressive
stabilization of 6 orders of magnitude. The charge separation
lifetime found in ZnPc-C60 NTs reaches into a time domain
typically found in thin solid films of donor-acceptor composites.
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Acknowledgment. This work was supported by CICYT, Co-
munidad de Madrid (Spain), and the European Union through
Grants BQU2002-04697, 07N/0030/2002, GR/MAT/0513/2004,
HPRN-CT-2000-0127, HPRN-CT-2000-0020, HPRN-CT-2002-
00177 and by Italian MIUR (PRIN 2004, prot. 2004035502 and
prot. 2003054199) and the Office of Basic Energy Sciences of the
U.S. Department of Energy. This is document NDRL-4599 from
the Notre Dame Radiation Laboratory.
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Supporting Information Available: Synthesis and spectroscopic
characterization of compounds 1, 7, and 8; preparation of the samples
of the amphiphilic ZnPc-C60 salt 8 for TEM microscopy; and
measurement conditions for photophysical study. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) Compound 3 was already described in the literature by reduction of
4-carboxymethylphthalonitrile without relevant characterization data:
Young, S. D.; Anthony, N. J.; Graham, S. L.; Tran, L. O.; Bell, I. M.;
Desolms, S. J.; Gomez, R. P.; Sparks, K. M.; Lumma, W. C., Jr.; Perlow,
D. S.; Shaw, A. W.; Wai, J. S. PCT Int. Appl. 1998, CAN 129:109089.
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Zerbetto, F. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5075-5080.
(16) To ensure a reliable interpretation of the data, fits were only admitted
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