Functional single-wall carbon nanotube nanohybrids-associating SWNTs with water-soluble enzyme model systems

Article abstract of DOI:10.1021/ja050930o

We succeeded in integrating single-wall carbon nanotubes (SWNTs), several water-soluble pyrene derivatives (pyrene^-), which bear negatively charged ionic headgroups, and a series of water-soluble metalloporphyrins (MP⁸⁺) into functio

Full text of DOI:10.1021/ja050930o

Published on Web 06/15/2005
Functional Single-Wall Carbon Nanotube
NanohybridssAssociating SWNTs with Water-Soluble Enzyme
Model Systems
Dirk M. Guldi,* G. M. Aminur Rahman, Norbert Jux,* Domenico Balbinot,
Uwe Hartnagel, Nikos Tagmatarchis, and Maurizio Prato*
Contribution from the Institute for Physical and Theoretical Chemistry, Egerlandstrasse 3,
91058 Erlangen, Germany, Institut fu¨r Organische Chemie, Friedrich-Alexander-UniVersita¨t
Erlangen-Nu¨rnberg, Henkestrasse 42, 91054 Erlangen, Germany, and Dipartimento di Scienze
Farmaceutiche, UniVersita` di Trieste, Piazzale Europa 1, 34127 Trieste, Italy
Received February 14, 2005; E-mail: dirk.guldi@chemie.uni-erlangen.de
Abstract: We succeeded in integrating single-wall carbon nanotubes (SWNTs), several water-soluble pyrene
derivatives (pyrene-), which bear negatively charged ionic headgroups, and a series of water-soluble
metalloporphyrins (MP8+) into functional nanohybrids through a combination of associative van der Waals
and electrostatic interactions. The resulting SWNT/pyrene- and SWNT/pyrene-/MP8+ were characterized
by spectroscopic and microscopic means and were found to form stable nanohybrid structures in aqueous
media. A crucial feature of our SWNT/pyrene- and SWNT/pyrene-/MP8+ is that an efficient exfoliation of
the initial bundles brings about isolated nanohybrid structures. When the nanohybrid systems are
photoexcited with visible light, a rapid intrahybrid charge separation causes the reduction of the electron-
accepting SWNT and, simultaneously, the oxidation of the electron-donating MP8+. Transient absorption
measurements confirm that the radical ion pairs are long-lived, with lifetimes in the microsecond range.
Particularly beneficial are charge recombination dynamics that are located deep in the Marcus-inverted
region. We include, for the first time, work devoted to exploring and testing FeP8+ and CoP8+ in donor-
acceptor nanohybrids.
adsorptive properties as well as good chemical stability.5-7 Their
unique strength stems from their bonding structure; namely, they
Introduction
Nanoscale science, engineering, and technology are emerging
fields where scientists and engineers are beginning to manipulate
matter at the atomic and molecular scale levels, which leads to
dramatically improved materials and device properties.1
are composed entirely of sp2 bonds, which are appreciably
stronger than the sp3 bonds in diamond.
SWNT have structures similar to those of fullerenes, but
while a fullerene molecule is spherical, a nanotube is a one-
dimensional nanowire, with one end typically being capped with
half a fullerene molecule. In this light, it is possible to envisage
that SWNT accept electrons readily and transport them with
quasi-ballistic features along the tube axis.8 The high electron
Carbon nanotubes and, in particular, single-wall carbon
nanotubes (SWNT), are true examples of nanotechnology.2-4
SWNT are cylindrical carbon molecules (i.e., hollow cores and
large aspect ratios) with properties that make them potentially
useful in extremely small scale electronic and mechanical
applications. They exhibit unusual electronic, mechanical, and
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10.1021/ja050930o CCC: $30.25 © 2005 American Chemical Society

Functional SWNT Nanohybrids
A R T I C L E S
mobility s with values that are 25% higher than in any other
known semiconductor material s further strengthens this as-
sumption.9 Thus, SWNTs may find a prominent place in electro-
and photoactive nanocomposites with high surface areas s just
as other carbon modifications/allotropes have been tested
successfully as electron acceptors in recent research.10 However,
before SWNT may be integrated into functional arrays (i.e.,
donor-acceptor nanohybrids) or tested in practical applications
(i.e., photovoltaic devices), several key issues need to be
properly addressed. One of them entails the control over
modifying the SWNT surface with functional groups, such as
chromophores,11 electron donors,12 biomolecules, etc.13
High degrees of functionalization alter appreciably the
π-electronic structure and, subsequently, the properties of
SWNT.14 On one hand, the shape and size of functionalized
SWNT are largely retained. On the other hand, each cyclo-
addition step eliminates an equal number of π-electrons from
the curved surface,15 though the electronic structure of the
carbon framework is governed by the graphenic π-electrons.16
Thus, SWNT functionalization corresponds to doping, which
affects (i) the band gaps and (ii) the long-range conjugation.17
With an eye on keeping the electronic properties of SWNT
intact, alternative strategies are needed to develop donor-
acceptor SWNT nanohybrids. In particular, concepts should be
considered that assist in controlling contacts between electron
donors and electron acceptors while preserving the π-electronic
structure of SWNT. Supramolecular concepts fulfill such
requisites. Additional incentives that call for supramolecular
means (i.e., polymer wrapping, π-π interactions, etc.) are their
flexibility and tunability.18,19
We have shown earlier that cationic and anionic porphyrins
form ensembles with charged fullerene and SWNT deriva-
tives.10e,19b,20 Interestingly, electrostatic forces play crucial roles
in maintaining the tertiary and quarternary structure of enzymes
and also in their interactions with other biomolecules.21 An
excellent example is the hybridization of cytochrome c with
cytochrome c oxidase. As the terminal enzyme in the respiratory
chain, cytochrome c oxidase catalyzes the reduction of dioxygen
to water and pumps an additional proton across the membrane
for each proton consumed in the reaction.22 Although supramo-
lecular complexes of charged metalloporphyrins and SWNT are
certainly no models for heme-containing enzymes, it is reason-
able to assume that chemical processes happening in such
proteins can be performed in electrostatic SWNT/metallopor-
phyrin aggregates. The electron-accepting and/or charging
properties of SWNT hint at the possibility of an oxidation
catalysis of these aggregates, with the metalloporphyrin acting
as the reaction center and the SWNT ensuring the necessary
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A R T I C L E S
Guldi et al.
electron transport.8,16 In the current work, we introduce a set of
new cationic metalloporphyrins, FeP8+ and CoP8+, and their
systematic integration into novel SWNT ensembles.
Crucial for our binding assays s in which two reference
systems (i.e., H2P8+ and ZnP8+) and two novel model systems
(i.e., FeP8+ and CoP8+) were tested s are SWNT templates
that bear carboxylate or sulfonate functionalities. These nega-
tively charged groups function as promoters for suspending
SWNT samples in aqueous media and likewise as anchors to
interact with one or several pyridinium headgroups present in
the different metalloporphyrins.19
Results and Discussion
FeP8+ and CoP8+. The syntheses of FeP8+ and CoP8+ were
accomplished via metalating the octabromomethylated porphyrin
H2Br8 under standard conditions. FeBr8 and CoBr8 were both
obtained in moderate to good yields. For CoBr8, both 1H NMR
and UV/vis spectra suggest a paramagnetic low-spin character
(i.e., cobalt(II) porphyrin). On the other hand, FeBr8 is a high-
spin iron(III) complex. The axial ligands of FeBr8 were, in
Figure 1. Absorption spectra of dilute solutions of 3 and SWNT/3,
displaying the UV, visible, and NIR regions. While for the 200-450 nm
range H2O solutions were prepared, the 450-1500 nm range was monitored
from a D2O solution.
solution (1 mM) with 1 mg of purified SWNT. To minimize
the amount of free pyrene-, which is expected to interfere in
the association assays, we isolated the SWNT/pyrene- from
the centrifuged solid. This approach bears the advantage to
eliminate free pyrene- that is not immobilized onto the SWNT
surface and leaves mostly the target compounds, SWNT/
pyrene-.
1
accordance with H NMR and microanalysis data, assigned as
hydroxide ligands s most likely stemming from a bromide/
hydroxide exchange that takes place during the column chro-
matography.
Next, FeBr8 and CoBr8 were reacted with 4-tert-butylpyridine
to give the desired cationic compounds FeP8+ and CoP8+ in
The absorption spectra of the typically black SWNT/pyrene-
solutions not only feature characteristics of both building blocks
but also testify to their mutual interaction. Figure 1 demonstrates
this. Here, a typical spectrum of 3 is compared with that of the
corresponding SWNT/3 dispersion, after matching the absorp-
tion at 350 nm. Several key features are discernible. First, the
450-1500 nm region reveals the SWNT van Hove singularities,
which fall into groups of metallic transitions (i.e., E11) in the
500-600 nm range and of semiconductor transitions in the
600-900 nm (i.e., E22 semiconductor transitions)/1000-1500
nm regions (i.e., E11 semiconductor transitions).24 Second, the
characteristic pyrene transitions in 3 are seen in the 200-400
nm region. Third, insight into van der Waals interactions
between SWNT and 3 came from bathochromic shifts (i.e., ∼2
nm) in SWNT/3, which suggests mutually interacting π-systems.
Similar data are gathered for the analogous SWNT association
with 1, 2, and 4. Figure S1 (Supporting Information) reviews
the NIR sections of all the different SWNT/pyrene- (i.e., 1, 2,
3, and 4).
A fundamental advantage for our work is that the pyrene
compounds are also strong fluorophores.25 This feature renders
them convenient and sensitive markers for intrahybrid interac-
tions, which when compared in different samples provides
valuable insight into excited-state deactivations with SWNT.
It is important that we and others have recently demonstrated
that in covalently linked SWNT/pyrene nanohybrids intra-
molecular fluorescence quenching prevails.11 The quenching is
attributed to intramolecular transduction of singlet excited
energy s from the high-energy pyrene singlet excited state to
1
71% and 65% yields, respectively. The H NMR spectrum
confirms that FeP8+ remains as a high-spin iron(III) complex
with a partially intermediate spin. The high symmetry of the
1H NMR spectrum indicates that the iron cation carries two
identical axial ligands, whereas the pyrrole proton resonance at
3
63.9 ppm indicates an intermediate spin (S ) /2) at the iron
center. Both results indicate that two water molecules are bound
to the central metal. Surprisingly, the 1H NMR spectra specify
that CoP8+ transformed to a diamagnetic species, although the
resonances are somewhat broadened. In the UV/vis spectrum,
significant shifts of the Soret and Q-band absorptions of
CoP8+ s compared with those of CoBr8 s further support the
notion that a change in redox state took place. Which axial
ligands are bound to the cobalt center, bromide or 4-tert-
butylpyridine, both present in the reaction mixture, remains,
however, an open question. Since (i) no signals appear for the
R- and â-pyridine proton resonances and (ii) the total number
1
of signals in the H NMR spectrum shows that CoP8+ still
possesses D4h symmetry on the NMR time scale, it is likely
that a fast exchange of ligands takes place.23
SWNT/Pyrene-. Our approach to form dispersable SWNT
ensembles involves the immobilization of water-soluble pyrene-
derivatives (i.e., 1-pyreneacetic acid (1), 1-pyrenecarboxylic acid
(2), 1-pyrenebutyric acid (3), and 8-hydroxy-1,3,6-pyrenetrisul-
fonic acid (4)) with pristine SWNT (see Scheme 1). In
particular, strong van der Waals forces are operative between
the two π-systems. Stable dispersions of SWNT/pyrene- were
prepared by stirring, sonicating, and centrifuging 10 mL of a
sodium tetraborate decahydrate buffer (pH ) 9.2) pyrene-
(24) (a) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano,
M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.;
Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002,
297, 593. (b) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.;
Smalley R. E.; Weisman, R. B. Science 2002, 298, 2361.
(25) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Dekker: New York, 1993.
(23) The open question regarding the axial ligands is of only minor concern,
since aqueous buffer solutions are used in the association and electron-
transfer studies. Hereby, water or hydroxide ligands will replace any other
donor molecule complexed by the central metal. Detailed investigations
on the behavior of FeP8+ and CoP8+ in aqueous solutions are currently
being performed.
9
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A R T I C L E S
Scheme 1. Structures of Compounds Used in This Work
the low-energy manifold of SWNT. In line with this conclusion,
we see strong fluorescence quenching in all the new SWNT/
pyrene- ensembles. The quenching is as strong as 80%, for
example, in SWNT/3, as shown in Figure 2. A reasonable
assumption for the residual fluorescence is free, not immobilized
pyrene- in solution.
tion of photoexcited pyrene-, k . 2.0 × 1010 s-1. However,
based on the π-π stacking in SWNT/pyrene-, we have to
In parallel with the steady-state experiments, we tested sets
of pyrene- and SWNT/pyrene- in time-resolved fluorescence
decay measurements. We noted in all four different sets only
the long-lived pyrene fluorescence. For example, following the
375 nm fluorescence in oxygen-free samples of 3 and SWNT/3
led to lifetimes of 93 ( 2 ns. This confirms our earlier
hypothesis that the residual fluorescence, noted in the steady-
state experiments, is largely due to non-immobilized pyrene-.
As far as SWNT/pyrene- interactions are concerned, we have
to assume that they happen in a time domain that is essentially
covered by our instrumental resolution of around 100 ps. Thus,
we can only estimate a lower limit for the intrahybrid deactiva-
Figure 2. Fluorescence spectra of dilute solutions of 3 and SWNT/3 that
display equal absorption at the 335 nm excitation wavelength.
9
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A R T I C L E S
Guldi et al.
Figure 3. Differential absorption spectrum (visible) obtained upon fem-
tosecond flash photolysis (387 nm) of SWNT/pyrene- in nitrogen-saturated
aqueous solutions with several time delays between 0 and 4 ps at room
temperature.
consider rates somewhere in the neighborhood of 1011 s-1. These
values were independently confirmed in femtosecond resolved
transient absorption spectroscopic measurements (vide supra).
Exact information on the dynamic excited-state interactions
in SWNT/pyrene- was obtained by utilizing transient absorp-
tion changes starting with a fast 150 fs excitation at 387 nm.
For pyrene- alone, characteristic long-lived singlet-singlet
features develop (no significant decay on the 1500 ps time scale).
An ultrafast deactivation, on the other hand, is seen for the same
pyrene--centered features in SWNT/pyrene- as summarized
in Figure 3 for 3. From the corresponding time-absorption
profiles (see Figure S2, Supporting Information), a rate constant
of (1.8 ( 0.3) × 1012 s-1 is determined, which is in good
agreement with our estimate that is based purely on the steady-
state experiments.
Figure 4. Transmission electron micrographs of purified pristine SWNT
at low (a) and high (b) magnifications and water-soluble SWNT/3 at low
(c) and high (d) magnifications. The absence of metal nanoparticle impurities
and the presence of thinner bundles in SWNT/3 are evident due to the
purification process followed and the solubilization/dispersion upon im-
mobilization of water-soluble pyrene- onto SWNT, respectively.
Electrostatic interactions between negatively charged SWNT/
pyrene- and positively charged MP8+ (i.e., M ) Zn, H2, Fe,
and Co) were tested in a series of investigations. These include
microscopy (i.e., transmission electron microscopy (TEM) and
atomic force microscopy (AFM)) and spectroscopy (i.e., absorp-
tion and fluorescence spectroscopy). Figure 4 shows TEM
photographs of SWNT/pyrene- samples, where we found linear
objects tens of nanomenters in diameter, while their lengths
reached several micrometers. Metal nanoparticles are absent due
to the thermal and acid treatment of the nanotubes prior to the
immobilization of the water-soluble pyrene- derivatives. In
addition, the presence of small-diameter bundles proves that
our SWNT approach toward water-soluble SWNT/pyrene-
ensembles exfoliates individual SWNT from the original carbon
nanotube bundles. Visualization of the negatively charged
SWNT/pyrene- and positively charged MP8+ ensembles
through AFM reveals the presence of two to three isolated
nanotubes interacting in the buffer solution, as is nicely
demonstrated in Figure 5.
Figure 5. Atomic force microscopy images of SWNT/pyrene-/H2P8+
showing the presence of isolated nanotubes. The height measure corresponds
to 1.2 nm.
,
SWNT/Pyrene-/ZnP8+. In the next step, we employed
absorption spectroscopy to test the interplay between SWNT/
pyrene- and ZnP8+. First, we titrated ZnP8+ solutions with
several concentrations of SWNT/pyrene-. A representative
example is given in Figure 6 for SWNT/3, from which we
observe the following changes. First, we note red-shifts of the
Soret- and Q-band features of ZnP8+. The bands at 435, 565,
and 605 nm shift to 439, 569, and 609 nm, respectively. Second,
Figure 6. Absorption spectra of a dilute aqueous solution of ZnP8+ upon
addition of several concentrations of SWNT/3. The 420-460 nm range is
amplified in the inset.
we see the presence of two isosbestic points at 426 and 437
nm. Finally, we find that the Soret transition at the endpoint of
the titration has a slightly larger half-width and a slightly lower
9
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A R T I C L E S
exponential fluorescence decay, in the absence of SWNT/
pyrene-, is substituted, in the presence of SWNT/pyrene-, by
a decay function that is best represented by a biexponential rate
law. We see a long-lived (i.e., 2.6 ( 0.1 ns) component and a
short-lived (i.e., 0.22 ( 0.05 ns) component, which are related
to the unquenched fluorescence in free ZnP8+ and the quenched
fluorescence in immobilized ZnP8+ (i.e., SWNT/pyrene-/
ZnP8+), respectively. Overall, the two lifetimes are maintained
throughout the titration assay. Important is that with increasing
SWNT/pyrene- concentration, the pre-exponential factor of the
short-lived part increases, while that of the long-lived part falls
steadily. Ultimately, the plateau region is reached, where only
the short-lived component is monitored. Addition of acid restores
the original ZnP8+ fluorescence intensity, concomitant with the
disappearance of the short-lived component and the reappearance
of the long-lived component.
Figure 7. Fluorescence spectra of a dilute aqueous solution of ZnP8+ upon
addition of SWNT/3 (complementary to Figure 6). Excitation wavelength
is 437 nm.
When ZnP8+ is brought together with just pyrene-, the
picture deviates substantially from what has been concluded
from the absorption spectra. As Figure S4 (Supporting Informa-
tion) indicates, only red-shifts of the fluorescence peaks s
although much weaker s occur. The 613 and 668 nm peaks
shift to 617 and 671 nm, respectively. However, no quenching
of the steady-state fluorescence or changes in the fluorescence
lifetime is seen. A tentative hypothesis implies electrostatic
pyrene-/ZnP8+ binding without meaningful photophysical
extinction. All these data indicate a successful SWNT/pyrene-/
ZnP8+ complex formation, driven by electrostatic interactions
between pyrene- and ZnP8+. The exact same changes were
noted when ZnP8+ was titrated with 1, 2, and 4. Important is
the fact that the typical van Hove singularities (i.e., 660, 745,
830, and 888 nm) of the SWNT remain unaffected by the ZnP8+
association.
The absorption features of ZnP8+ containing variable con-
centrations of just pyrene- s no SWNT, not shown s represent
an important reference experiment. Essentially, the shifts are
superimposable with those observed upon SWNT/pyrene-
addition (i.e., 435 nm f 439 nm; 565 nm f 570 nm; 605 nm
f 609 nm). The same isosbestic points developed at 426 and
437 nm. All these experiments lead to the conclusion that
electrostatic forces are selective tools for associating SWNTs
with ZnP. Long-range interactions, such as π-π stacking, could
not be confirmed for SWNT and ZnP solely on the basis of
absorption spectroscopy.
consequences for photoexcited ZnP8+
.
In the last step, we tested the fluorescence behavior of ZnP8-
in the absence and in the presence of SWNT/pyrene-. At the
beginning of the titration experiment, the characteristic ZnP8-
fluorescence is seen, which manifests itself through a set of
maxima at 612 and 665 nm. The fluorescence decay, as
measured at both maxima, is monoexponential, from which a
lifetime of 2.8 ( 0.1 ns is derived.20 When variable amounts
of SWNT/pyrene- were added, no overall changes were
detected, except a 15% fluorescence quenching, which is
attributed to competitive light absorption at the excitation
wavelength.
Control tests were carried out by following the spectral
changes for SWNT/pyrene- using a negatively charged ZnP8-
.
On the basis of the aforementioned results we reach the
following conclusion. The decrease of fluorescence intensity
in ZnP8+, the shift of the ZnP8+ fluorescence, and evidence
for a short-lived emissive component, which is invariant
throughout the titration assay s all realized upon addition of
SWNT/pyrene- s suggest a static quenching event inside a
well-defined supramolecular SWNT/pyrene-/ZnP8+ complex.
Repulsive forces between the two negatively charged species
are reflected by the lack of appreciable changes. Figure S3
(Supporting Information) summarizes a representative titration
experiment.
Regarding the electron donor (i.e., ZnP)-acceptor (i.e.,
SWNT) interactions, fluorescence spectroscopy proved to be a
very sensitive tool for their study. In the corresponding assays
we recorded the ZnP8+ fluorescence (i.e., λmax ) 613 and 668
nm; Φ ) 0.04; τ ) 2.6 ns) in the absence and in the presence
of variable SWNT/pyrene- concentrations, generated by excit-
ing at one of the isosbestic points s for example at 437 nm.
As can be seen in Figure 7, when SWNT/pyrene- is present
the ZnP8+ fluorescence decreases with an exponential relation-
ship relative to the SWNT/pyrene- concentration, until a
plateau value is reached, where the complexation of ZnP8+ is
assumed to be complete and the effective concentration of
SWNT/pyrene-/ZnP8+ is on the order of >90%. Please note
that a purely diffusion driven process can be ruled out on the
basis of the applied SWNT/pyrene- concentrations and the
short lifetime of the ZnP8+ singlet excited state (vide supra).
Also, the fluorescence shifts progressively to the red (i.e., 613
nm f 619 nm; 668 nm f 674 nm). This trend parallels the
changes seen in the absorption spectra. Finally, the mono-
In SWNT/pyrene-/ZnP8+, the fluorescent state of ZnP8+
,
which is a good electron donor, is quenched by electron transfer
to the electron-accepting SWNT and, accordingly, substituted
by a long-lived charge-separated state.
To shed light on the nature of the product evolving from this
electron-transfer deactivation, complementary transient absorp-
tion measurements (i.e., 8 ns laser pulses at 532 nm) were
necessary. Excitation at 532 nm, which guarantees irradiation
into the Q-band absorption of ZnP8+ while leaving pyrene-
untouched, brings about the following triplet characteristics (i.e.,
Figure S5, Supporting Information): transient minimum and
maximum at 540 and 835 nm, respectively. This triplet excited
state decays in the absence of molecular oxygen to the singlet
ground state with a time constant of ∼105 s-1 26
.
The picture associated with the nanosecond absorption
(26) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111, 6500.
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A R T I C L E S
Guldi et al.
is complemented by clean isosbestic points at 413 and 432 nm.
In contrast to the aforementioned ZnP8+ case, none of the
Q-bands (i.e., 518, 548, 587, and 647 nm) seem to shift at all.
Apparently, H2P8+ is much less susceptible to electronic
perturbations.
Our findings are still in agreement with an electrostatically
driven association of SWNT/pyrene-/H2P8+. Tests on the
electrostatic attractions in pyrene-/H2P8+ and electrostatic
repulsion between SWNT/pyrene- and H2P8- further proved
this hypothesis. For example, an experiment with pyrene-/
H2P8+ gave rise to the same 1 nm red-shift, while no appreciable
changes were seen when titrating H2P8- with SWNT/pyrene-.
Interestingly, one of the SWNT transitions, namely, the one
typically seen around 745 nm (i.e., E22 semiconductor transi-
tions), is perturbed by additions of H2P8+. For 1, 2, and 3 the
band shifts to 736, 738, and 734 nm, respectively. The remaining
Figure 8. Differential absorption spectrum (visible and NIR) obtained upon
nanosecond flash photolysis (532 nm) of SWNT/3/ZnP8+ in nitrogen-
saturated solutions with a time delay of 100 ns.
E
22 semiconductor transitions at 660, 830, and 890 nm, on the
other hand, were not affected. It is, however, unclear at present
why this trend evolves exclusively for H2P8+, while ZnP8+ does
not give any analogous shifts.
In steady-state experiments, the H2P8+ fluorescence is
characterized by maxima at 655 and 715 nm (see Figure S6,
Supporting Information). Evidence for the electron-transfer
deactivation came from titration experiments with variable
SWNT/pyrene- concentrations. Specifically, upon excitation
at 432 nm, which corresponds to the isosbestic point of the
ground-state absorption, a SWNT/pyrene- concentration-de-
pendent decrease in fluorescence intensity is seen. A closer
inspection reveals, however, that the quenched H2P8+ fluores-
cence (i.e., at 655 and 715 nm) is not accompanied by any
appreciable red- or blue-shifts. This tracks the trend seen in the
ground-state spectrum upon SWNT/pyrene-/H2P8+ association.
Prior to the addition of SWNT/pyrene-, the fluorescent signal
of H2P8+ is well fitted by a monoexponential decay, for which
a lifetime of 10.5 ( 0.5 ns was determined in deoxygenated
aqueous solutions. Similar to the case with SWNT/pyrene-/
ZnP8+, after addition of SWNT/pyrene-, the fluorescent signal
of H2P8+ exhibits a double-exponential decay with lifetimes of
10.5 ( 0.5 and 0.32 ( 0.05 ns. An interpretation implies that
the fast decaying component corresponds to intrahybrid electron-
transfer quenching within SWNT/pyrene-/H2P8+, while the
slow decay represents deactivation in free H2P8+, not im-
mobilized by SWNT/pyrene-. The pre-exponential factors vary
throughout the titration s the one associated with the 0.32 (
0.05 ns lifetime increases with increasing SWNT/pyrene-,
while that of the 10.5 ( 0.5 ns lifetime decreases steadily.
In summary, steady-state and time-resolved fluorescence
measurements testify that a rapid electron-transfer decay of the
H2P8+ singlet excited-state prevails in the SWNT/pyrene-/
H2P8+ assemblies. Thus, to confirm the electron deactivation
in SWNT/pyrene-/H2P8+, transient absorption spectroscopy s
following 8 ns laser excitation at 532 nm s was performed with
H2P8+ in the absence and in the presence of SWNT/pyrene-.
In the absence of SWNT/pyrene- (i.e., 3), differential
absorption spectra, recorded ca. 100 ns after the short laser pulse
for a deoxygenated aqueous solution of H2P8+, are characterized
by strong bleaching of the porphyrin Soret band and Q-band
absorptions at 425 and 550 nm, respectively (not shown). Further
in the red, an instantaneous formation of broad absorptions at
wavelengths >600 nm, including a distinct maximum at 770
Figure 9. Absorption spectra of a dilute aqueous solution of H2P8+ upon
addition of several concentrations of SWNT/1. The 410-450 nm range is
amplified in the inset.
spectroscopy in the presence of SWNT/pyrene- (i.e., 3) is
notably different. Despite the unequivocal excitation of ZnP8+
at 532 nm, no porphyrin triplet-triplet absorption was found
at short delay times following the 8 ns laser pulse in oxygen-
free solutions. Instead, a new transient develops as a result of
the rapid intrahybrid deactivation of the ZnP8+ excited state.
A key feature of the new product is a broad transition between
600 and 900 nm (see Figure 8). This new transient resembles
the one-electron-oxidized π-radical cation of ZnP8+ 27
Due to
.
the large spatial separation between the oxidized ZnP8+ and
reduced SWNT, the radical ion pair features decay rather slowly,
with a lifetime of 1.6 µs. The charge recombination process is
governed by a monoexponential recovery of the singlet ground
state rather than population of a porphyrin triplet excited state.
Again, bringing just pyrene- and ZnP8+ together leads
exclusively to the formation of the above-described ZnP8+ triplet
characteristics, due to the lack of electron donor-acceptor
deactivation.
SWNT/Pyrene-/H2P8+. As far as the absorption spectra are
concerned, titration of H2P8+ with SWNT/pyrene- leads to
minute alterations. The inset to Figure 9 corroborates that the
Soret band is subject to only a moderate red-shift of 1 nm, which
(27) Guldi, D. M.; Neta, P.; Hambright, P. J. Chem. Soc., Faraday Trans. 1992,
88, 2013 and references therein.
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A R T I C L E S
Conclusions
We accomplished the integration of SWNTs, several water-
soluble pyrene-, and a series of water-soluble porphyrins into
functional nanohybrids through a combination of associative
van der Waals and electrostatic interactions. The SWNT
functions as electron acceptor, while MP8+ metalloporphyrins
are employed as excited-state electron donors. Crucial for the
binding studies were SWNT/pyrene- templates that bear
carboxylate or sulfonate appendages. These negatively charged
groups function (i) as promoters for suspending SWNT samples
in aqueous media and (ii) as anchors for interaction with one
or several pyridinium headgroups present in the different MP8+
.
Spectroscopic (i.e., absorption and fluorescence) and micro-
scopic (i.e., AFM and TEM) methods were used to characterize
Figure 10. Differential absorption spectrum (visible and NIR) obtained
upon nanosecond flash photolysis (532 nm) of SWNT/3/H2P8+ in nitrogen-
saturated solutions with a time delay of 100 ns.
the resulting SWNT/pyrene- and SWNT/pyrene-/MP8+
.
Important for our steady-state and time-resolved work was that
all the novel nanohybrids formed stable suspensions in aqueous
media. A key feature of our SWNT/pyrene- and SWNT/
pyrene-/MP8+ systems is that an efficient debundling was
achieved, which in some cases gave isolated nanohybrid
structures.
We could demonstrate with the help of transient absorption
and fluorescence measurements that SWNT serve as the electron
acceptor component in SWNT/pyrene-/MP8+ (i.e., H2P8+ and
ZnP8+). Intrahybrid electron transfer occurs on the 200-300
ps time scale and evolves from the photoexcited H2P8+ and
ZnP8+ to SWNT. This event leads to the reduction of the
electron-accepting SWNT and oxidation of the electron-donating
MP8+. The radical ion pairs are long-lived and benefit from
charge recombination dynamics that are located in the Marcus-
inverted region.
nm, is discernible. These spectral features are attributes of the
long-lived H2P8+ triplet excited state.26 In sharp contrast, the
transient absorption changes for SWNT/pyrene-/H2P8+ reveal
the characteristic broad band of one-electron-oxidized π-radical
cation H2P8+ that absorbs strongly in the visible region around
700 nm.27 Figure 10 depicts these characteristics for SWNT/
3/H2P8+ with a time delay of 100 ns. In the case of 3, the
lifetime of the transient radical pair is 2.4 µs; this is significantly
longer-lived relative to what was seen in the case of SWNT/
3/ZnP8+ (vide supra). Considering that among the two porphy-
rins ZnP8+ is the better electron donor, our finding is the first
indication that also in SWNT s similar to the electron-accepting
C609- s charge recombination dynamics are located in the
Marcus-inverted region,28 where kinetics slow down substan-
tially with increasing free energy change (i.e., -∆G).
Experimental Section
SWNT/Pyrene-/CoP8+ and SWNT/Pyrene-/FeP8+. Spec-
tral transformations associated with CoP8+ and FeP8+ were
followed spectroscopically. A dilute aqueous solution of CoP8+
was titrated with SWNT/pyrene-. Figure S7 (Supporting
Information) shows that a clean conversion from CoP8+ to
SWNT/pyrene-/CoP8+ is observed, as evidenced by an iso-
sbestic point at 446 nm and through red-shifts of the Soret and
Q-bands. In the case of 1 and 3, the maxima at 438, 552, and
591 nm shift to 440, 557, and 596 nm, respectively. Even
stronger red-shifts were observed for 2 to 443, 560, and 600
nm. Interestingly, in 2 the carboxylic acid is directly linked to
the pyrene core. Perhaps a closer proximity between the two
π-systems contributes toward an enhanced electronic perturba-
tion of the porphyrin macrocycle.
Relative to SWNT/pyrene-/CoP8+, the absorption changes
accompanied with the formation of SWNT/pyrene-/FeP8+ were
different. In particular, prior to the addition of SWNT/2 the
following maxima were recorded: 422 nm (i.e., Soret band)
and 625 and 661 nm (i.e., Q-band). While for SWNT/pyrene-/
FeP8+ the Soret band reveals the usual red-shift to 436 nm, the
Q-band transitions shift surprisingly to the blue with new
maxima at 598 and 637 nm (Figure S8, Supporting Information,
illustrates these shifts). However, we cannot offer at this stage
of the investigation any meaningful rationalization for the
observed trend.
Preparation of SWNT/Pyrene-. Water-soluble SWNTs were
obtained in analogy to previous work, using 1-pyreneacetic acid (1),
1-pyrenecarboxylic acid (2), 1-pyrenebutyric acid (3), and 8-hydroxy-
1,3,6-pyrenetrisulfonic acid (4) (pyrene-).18b However, to reduce the
amount of free pyrene in solution, SWNT/pyrene- complex was
allowed to precipitate, and the centrifuged solid was resolubilized in
water.
Femtosecond transient absorption studies were performed with 387
nm laser pulses (1 kHz, 150 fs pulse width) from an amplified Ti:
sapphire laser system (Clark-MXR, Inc.). Nanosecond laser flash
photolysis experiments were performed with 532-nm laser pulses from
a Quanta-Ray CDR Nd:YAG system (6 ns pulse width) in a front face
excitation geometry. Fluorescence lifetimes were measured with a Laser
Strobe fluorescence lifetime spectrometer (Photon Technology Inter-
national) with 337 nm laser pulses from a nitrogen laser fiber-coupled
to a lens-based T-formal sample compartment equipped with a
stroboscopic detector. Details of the Laser Strobe systems are described
on the manufacture’s web site. Emission spectra were recorded with
an SLM 8100 spectrofluorometer. The experiments were performed at
room temperature. Each spectrum represents an average of at least five
individual scans, and appropriate corrections were applied whenever
necessary.
Synthesis. Chemicals and solvents were used as received unless
otherwise noted. Solvents were dried using standard procedures. Column
chromatography was performed on silica gel 32-63, 60 Å (MP
1
Biomedicals). H and 13C NMR spectra were recorded on an Avance
300 instrument (Bruker Analytische Messtechnik GmbH). FAB mass
spectrometry was performed with Micromass Zabspec and Varian MAT
311A machines. Standard UV/vis spectra were recorded on a Shimadzu
(28) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
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J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005 9837

A R T I C L E S
Guldi et al.
UV-3102 PC UV/vis NIR scanning spectrophotometer. IR spectra were
taken with a Bruker Vector 22 spectrometer.
mmol), and sodium acetate (5 mg, 0.062 mmol) were dissolved in THF
(20 mL). The reaction mixture was heated for 4 h under reflux. The
solvent was removed and the residue chromatographed on silica gel
with CH2Cl2 as eluent to remove unreacted starting material. The
product was finally eluted with ethyl acetate. FeBr8 was crystallized
from CHCl3/hexanes to give dark blue microcrystals. Yield 78 mg
CoBr8. 5,10,15,20-Tetrakis-(4’-tert-butyl-2’,6’-bis(bromomethyl)-
phenyl)porphyrin29 (150 mg, 0.094 mmol), CoBr2 (75 mg, 0.34 mmol),
and sodium acetate (28 mg, 0.34 mmol) were dissolved in THF (25
mL). The reaction mixture was heated in the dark for 24 h (bath
temperature 60 °C). The solvent was removed and the residue
chromatographed on silica gel with CH2Cl2/hexanes 30:70) as eluent.
1
(76%); mp > 200 °C (dec). H NMR (300 MHz, CDCl3, 25 °C): δ
82.1 (br s, pyrr. H), 16.8 (br s, m-Ar-H), 14.8 (br s, m-Ar-H), 9.0,
4.5, 2.8. MS (FAB, NBA): m/z 1636 (M+), 1557 [(M - Br+). UV/vis
(CH2Cl2): λmax (ꢀ, M-1 cm-1) 365 sh (50 000), 392 (62 200) 422
(90 700), 508 (12 700), 576 (6300), 652 (4500). IR (KBr): ν 2955,
2861, 1527, 1478, 1201, 1102, 1069, 997, 802, 752. Anal. Calcd for
C68H68Br8FeN4-OH‚CHCl3 (1762.74): C, 46.75; H, 3.98; N, 3.16.
Found: C, 46.61; H, 3.78; N, 2.70.
FeP8+. FeBr8 (50 mg, 0.031 mmol) was dissolved in a mixture of
4-tert-butylpyridine (0.18 mL, 168 mg, 1.240 mmol) and toluene (20
mL). The mixture was refluxed for 12 h. The solvent was removed
and the residue dissolved in dry methanol. Dry diethyl ether was added
to precipitate the desired cationic porphyrin. The dissolving/precipitation
procedure was repeated twice to give a dark greenish powder. Yield
60 mg (71%); mp > 200 °C (dec). 1H NMR (300 MHz, D2O, 25 °C):
δ 63.9 (br s, â-pyrr), 10.4, 6.1, 2.6, 1.2. UV/vis (H2O): λmax (relative
intensity) 334.5 (sh), 421.5 (1.00), 480.5 (0.14), 516.5 (0.09), 560 (0.05),
624.5 (0.04), 661 (0.05). IR (KBr): ν 3116, 3040, 2965, 2871, 1638,
1461, 1114, 1006, 852, 806.
1
Yield 76 mg (49%) of a dark orange solid; mp > 200 °C (dec). H
NMR (300 MHz, CDCl3, 25 °C): δ 14.94 (br s, 8H, â-pyrr), 9.74 (br
s, 8H, Ar-H), 5.80 (br s, 16H, CH2), 2.65 (br s, 36H, t-Bu). 13C NMR
(75 MHz, CDCl3, 25 °C): δ 154.60, 148.99, 144.50 (2C), 129.09 (2C),
35.39, 34.34, 31.43. MS (FAB, NBA): m/z 1639 (M+), 1560 (M+
-
Br), 1478 (M+ - 2Br). UV/vis (CH2Cl2): λmax (ꢀ, M-1 cm-1) 415
(290 000), 531 (15 500), 564 (6800). IR (KBr): ν 2963, 2867, 1635,
1364, 1349, 1261, 1213, 1074, 1001, 884, 755, 717. Anal. Calcd for
C68H68Br8CoN4 (1639.46): C, 49.82; H, 4.18; N, 3.42. Found: C, 49.92;
H, 4.21; N, 3.42.
CoP8+. CoBr8 (75 mg, 0.046 mmol) was dissolved in 4-tert-
butylpyridine (7.4 mL, 6.78 g, 50.2 mmol). The mixture was heated in
the dark for 5 days (bath temperature 140 °C). The excess of 4-tert-
butylpyridine was removed in vacuo, and the residue was suspended
in diethyl ether, filtered, and thoroughly washed with diethyl ether.
The solid was dissolved in a little CH2Cl2 and layered with diethyl
ether. The precipitate was collected and washed with diethyl ether. The
dissolving and precipitating procedure was repeated three times. The
residue was dried in high vacuo. Yield 35 mg (65%) of an orange solid;
Acknowledgment. This work was carried out with partial
support from the EU (RTN network “WONDERFULL”), MIUR
(PRIN 2004, prot. 2004035502), DFG (SFB 583), and the Office
of Basic Energy Sciences of the U.S. Department of Energy.
This is document NDRL-4606 from the Notre Dame Radiation
Laboratory.
1
mp > 220 °C (dec). H NMR (300 MHz, CDCl3, 25 °C): δ 9.51 (br
s, 16H, R-pyridine), 9.35 (br s, 8H, â-pyrr), 7.83 (br s, 16H, â-pyridine),
6.31 (br s, 8H, Ar-H), 5.53 (m, 16H, CH2), 1.29 (br.s, 72H, t-Bu-
pyridine), 1.13 (br s, 36H, t-Bu). 13C NMR (75 MHz, CDCl3, 25 °C):
δ 173.20, 155.49, 147.58, 143.65, 138.84, 135.12, 126.13, 121.46,
112.41, 62.88, 38.15, 36.46, 32.57, 31.44. UV/vis (CH2Cl2): λmax (ꢀ,
M-1 cm-1) 439 (205 000), 549 (15 000), 586 sh (2500). IR (KBr): ν
2964, 2924, 2865, 1638, 1562, 1461, 1372, 1274, 1163, 1113, 1061,
897, 723.
Supporting Information Available: NIR absorption spectra,
fluorescence spectra, and nanosecond spectra of reference
systems and those of CoP8+ and FeP8+. This material is
available free of charge via the Internet at http://pubs.acs.org.
FeBr8. 5,10,15,20-Tetrakis-(4’-tert-butyl-2’,6’-bis(bromomethyl)-
phenyl)porphyrin (100 mg, 0.063 mmol), iron(II) bromide (50 mg, 0.23
(29) Jux, N. Org. Lett. 2000, 2, 2129.
JA050930O
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9838 J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005