The hierarchical self-assembly of charge nanocarriers: A highly cooperative process promoted by visible light

Article abstract of DOI:10.1002/anie.201001833

Let there be more light: Triarylamine-based building blocks respond to visible-light exposure by the formation of cationic radicals that hierarchically self-assemble into molecular wires, which in turn combine within larger fibers (see picture). The stimu

Full text of DOI:10.1002/anie.201001833

Communications
DOI: 10.1002/anie.201001833
Self-Assembly
The Hierarchical Self-Assembly of Charge Nanocarriers:
A Highly Cooperative Process Promoted by Visible
Light**
Emilie Moulin, Frꢀderic Niess, Mounir Maaloum, Eric Buhler, Irina Nyrkova, and
Nicolas Giuseppone*
Angewandte
Chemie
6974
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One of the most important goals for chemistry at the interface
with materials and life science is to find new ways of building
responsive functional systems at the mesoscale.[1–3] In this
context, supramolecular chemistry has already proved to be a
powerful approach to achieve precise control over self-
assembled structures.[4–6] Indeed, the intrinsic reversibility of
the interactions between the well-defined molecular units
engaged in the formation of large supramolecular assemblies
provides relatively defect-free and self-healing characteristics
to these systems[7] and also makes them potentially responsive
to a number of external stimuli.[8] For instance, organic
photonic and optoelectronic materials have been shown to
benefit from supramolecular self-assembly within highly
organized aggregates such as vesicles,[9] nanotubes,[10]
layers,[11] and other architectures, including the artificial
photosynthetic systems developed by the research group of
Wasielewski[12] and the supramolecular n/p-heterojunctions
(SHJs) developed by Matile and co-workers.[13]
Triarylamine-type molecules are considered as very
efficient functional units that are widely used as photo-
conductors and charge carriers.[14] In particular, these com-
pounds display a high hole-transport mobility and are thus
incorporated in optoelectronic devices[15] such as organic
light-emitting diodes (OLEDs),[16] organic solar cells
(OSCs),[17] organic field-effect transistors (OFETs),[18] and
nonlinear materials,[19] and are even used as photoconductors
for the Xerox process in laser printers and photocopiers.[20]
However, despite the amount of studies performed on
triarylamine derivatives, this molecular core has never been
used as a structuring precursor that would be able to self-
assemble into ordered supramolecular architectures, possibly
with enhanced physical properties.
Figure 1. a) Structures and key synthetic step to access triarylamine
derivatives 1–8. b) Typical 1H NMR spectra (CDCl3) of 1 obtained
immediately after purification (A); after 10 min exposure to visible
light, (B); and after subsequent heating overnight at 608C (C);
[1]=10 mm.
Immediately after chromatographic purification, the
NMR spectrum of 1 indicated the presence of the pure
product (Figure 1b; spectrum A). Surprisingly, when the
solution was exposed to natural light for a couple of minutes,
a completely different spectrum was obtained (Figure 1b;
spectrum B). In this spectrum, the complete disappearance of
the aromatic protons (a–d), the aliphatic ether protons (e),
and the acetamide protons (g,h), seemed to contradict the
remaining resonance signals of the main aliphatic chains (f).
Two observations were made at this stage: 1) the initial
spectrum (A) was unchanged after one week in the dark, and
2) spectrum B, which was recorded after exposure of the
sample to visible light, was also unchanged after one week in
the dark. In addition, the color of the solution turned from
light yellow immediately after purification into light green
when exposed to light. The process was shown to be reversible
by simply heating the solution of 1 in chloroform overnight at
608C. This observation was illustrated by the reappearance of
protons a–e, g, and h (Figure 1b; spectrum C), and was
accompanied by the loss of the solutionꢀs green color. No
degradation of the product was observed by standard
spectroscopic techniques after at least three light exposure/
heating processes.
Recently, we observed the unique light-induced behavior
1
of precursor 1 by H NMR spectroscopy (Figure 1) in the
course of investigations into the design of SHJs. Compound 1
was synthesized by using a modified Ullmann[21] one-pot
coupling as a key process (see Figure 1a and the Supporting
Information).
[*] Dr. E. Moulin, F. Niess, Prof. Dr. N. Giuseppone
SAMS research group—icFRC, Institut Charles Sadron—CNRS
University of Strasbourg
23 rue du Loess, BP 84087, 67034 Strasbourg Cedex 2 (France)
Fax: (+33)3-8841-4099
E-mail: giuseppone@unistra.fr
Homepage: http://www-ics.u-strasbg.fr/spip.php?rubrique49
Prof. Dr. M. Maaloum, Dr. I. Nyrkova
Institut Charles Sadron, CNRS—University of Strasbourg
23 rue du Loess, BP 84087, 67034 Strasbourg Cedex 2 (France)
Prof. Dr. E. Buhler
Matiꢀre et Systꢀmes Complexes (MSC) Laboratory
University of Paris Diderot—Paris VII, UMR 7057
Bꢁtiment Condorcet, 75205 Paris Cedex 13 (France)
[**] We wish to thank the CNRS, the icFRC, and the University of
Strasbourg for financial support. This work was supported by a
doctoral fellowship of the French Ministry of Research (F.N.). We
acknowledge the ESF-COST action on Systems Chemistry. We are
also grateful to E. Aweke, S. Coulibaly, M. Bernard, and J. Raya for
help at various stages.
We subsequently studied the photophysical properties of a
solution of 1 (1 mm) in chloroform by UV/Vis–NIR as a
function of the irradiation time (the sample was irradiated
with a 20 W white light between measurements). The spectra
(Figure S1 in the Supporting Information) revealed the
appearance of a new absorption band at 786 nm that was
indicative of the production of a small quantity of the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001833.
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triarylammonium radical.[22] Similar results were obtained by
and could be detected by averaging over the dipolar
interactions. The observed reappearance of the resonance
signals that disappeared after light exposure supports this
claim, although an in-depth comparison between the two
tensors of dipolar coupling and electronic paramagnetism did
not allow for a definitive conclusion to assess the predominant
phenomena (see Figure S3 in the Supporting Information).
Importantly, chlorinated solvents appeared to be necessary to
initiate the oxidation of the triarylamine. For instance, both
the green color of the solutions and the disappearance of
irradiating solutions that were strictly deoxygenated by
freeze–thaw cycles. To obtain quantitative data, EPR spec-
troscopy was performed on a nonirradiated solution, the
1H NMR spectrum of which showed a full set of resonance
signals (Figure 2a). As expected, no trace of radicals was
detected in the dark (t = 0; point A). However, after irradiat-
1
signals in the H NMR spectrum also occurred when deu-
terated dichloromethane and 1,2-tetrachloroethane were
used as solvents, but not when deuterated methanol, toluene,
acetonitrile, or dimethylsulfoxide were used. These data are
consistent with reported studies in which the excited state of
the triarylamine can reversibly transfer one free electron to
chloroform and liberate chloride counterions.[24–26]
We then carried out a structure–property study by
synthesizing derivatives 2–8 and investigating the sensitivity
of their corresponding solutions in chloroform to light (see
chart in Figure 1a). Here, the criterion of the response to light
excitation was the disappearance (yes) or the persistance (no)
of the NMR signals of the aromatic protons when the samples
were submitted to irradiation for one hour. The results reveal
Figure 2. a) Quantitative EPR data, as a function of time, showing the
evolution of the ratio of the triarylammonium radical 1C+ over neutral
1: without visible light excitation at RT (point A); upon visible light
excitation (A–B); in the absence of light at RT (B–C); and after
subsequent heating (608C) in the dark (from C; [1init.]=10 mm).
Dotted line: y-axis value of 6ꢂ10ꢀ3. b) Time autocorrelation function
of the scattered electric field vector for an irradiated solution of 1
([1init.]=7.5 mm in chloroform at T=208C) and for a scattering angle
q=908. Solid line: exponential fit.
ꢀ
that both the free N H bond of the amide moiety (H-bonding
interactions) and the long alkyl (or benzyl) R1 chains must be
present, thus indicating the requirement for additional van
der Waals (or supplementary p-stacking) stabilizations of the
hypothesized self-assembled structures.
ing the sample with a white light (20 W) for one hour (A to
B), the formation of a radical was clearly observed in a
proportion of up to 11% compared to the total number of
molecules (see the Supporting Information). The observed
paramagnetic hyperfine pattern was consistent with that
expected for a radical localized on the nitrogen atom of a
triarylammonium derivative (see Figure S2 in the Supporting
Information).[23] After one hour of irradiation, the amount of
radicals was recorded as a function of time in the dark
(Figure 2a; point B). A very smooth decay was observed over
a period of 16 hours; the decay asymptotically reached a
plateau at a concentration of 6 radicals per 1000 triarylamine
molecules (points B to C). This concentration then remained
constant for at least one week as verified by an EPR control
experiment, thus reinforcing the idea of a radical-stabilization
mechanism in the solution. Finally, the subsequent heating of
the solution (point C) led to fast destruction of the remaining
stabilized radicals over a period of 2 hours. This quantification
of the radical confirmed its very low concentration at
equilibrium and its unusual stability. As a consequence of
We further investigated the solutions by dynamic light
scattering (DLS) experiments (Figure 2b).[27,28] Prior to
irradiation with visible light and consistent with previous
observations, no evidence for the presence of assemblies or
large objects could be deduced from DLS measurements on a
solution of 1 in chloroform. However, after one hour of
irradiation with visible light, the normalized time autocorre-
lation function of the concentration fluctuations, g(1)(q,t),
could be characterized by a simple exponential relaxation
(Figure 2b). The angular dependance revealed a diffusive
relaxation with a characteristic time inversely proportional to
q2, where q is the scattered wave vector. In the case of a
diffusive process, g(1)(q,t), is given by Equation (1):
À
Á
hdcðq; 0Þdcðq; tÞi
gð1Þðq; tÞ ¼
¼ exp ꢀDq2t
ꢀ
ꢁ
ð1Þ
2
dcðq; 0Þ
where dc(q,t) and dc(q,0) represent fluctuations of the
concentration at time t and zero, respectively. The formation
of self-assemblies in solution was thus demonstrated by the
presence of this diffusive relaxation in the correlation
function. In highly diluted solutions, the fit of g(1)(q,t) allows
to determine the diffusive coefficient, D = 2 ꢁ 10ꢀ12 m2 sꢀ1,
and, by using Equation (2), the average hydrodynamic radius
can be determined, where hw is the solvent viscosity.
1
the stability, the H NMR spectra observed at equilibrium
after light exposure (spectrum B in Figure 1b) might indicate
that the total disappearance of the aromatic signals is not due
to the presence of 6 paramagnetic species per 1000 molecules,
but more likely caused by a self-association/delocalization
process by charge transfer of these radicals when self-
assembled with their neutral precursors. To test this assump-
tion, we performed a high-resolution magic angle spinning
(HRMAS) 1H NMR experiment in solution; the possibly
highly p-stacked aromatic parts of the molecules would
behave as a very anisotropic domain in such an experiment
kBT
q!0 6phwD
ð2Þ
RH ¼ lim
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In CDCl3 (hw = 0.57 cP at T= 208C), the apparent average
hydrodynamic radius of the main species in dilute solution
was found to be (180 ꢁ 20) nm for [1] = 7.5 mm. The DLS
analysis and complementary neutron and X-ray scattering
measurements showed that the assemblies are dense and
polydisperse (see the Supporting Information).
units. To correlate these structures with what was previously
observed in solution, we then studied a liquid film deposited
onto mica by spin coating of a 1 mm irradiated solution of 1 in
chloroform (Figure 3d). While the resolution in the liquid
state was lower than in the solid state, we observed the
presence of a gel-like structure formed from dense entangled
filaments, which can be precursors for larger and stiffer fibers
at higher concentrations.
In conclusion, we have investigated responsive self-
assembled suprastructures that are based on a triarylamine
unit as a building block and react to light exposure. The self-
assembly process occurs as a result of highly intriguing
synergistic phenomena that start by the formation of tri-
arylammonium cationic radicals upon stimulation with light
(Figure 4). This radical can in turn be transferred to neutral
We found AFM[29] (Figure 3) to be a key technique for the
investigation of the self-assembled suprastructures. The
irradiated green solution of 1 was quickly evaporated to
dryness; the green residue obtained revealed the presence of
Figure 4. Hierarchical self-assembly processes that occur upon irradi-
ation of the neutral triarylamine 1 (a). The electron transfer that occurs
between light-excited triarylamines and the solvent produces a small
amount of triarylammonium radical (b). In this electronic configura-
tion, the charge transfer and various supramolecular interactions
initiate the self-assembly of 1D supramolecular polymers (c) which
combine to produce larger 3D fibers (d).
Figure 3. a) AFM height and b) phase images (surface scales
500ꢂ500 nm2) of the self-assembled structures obtained by concen-
tration to dryness of an irradiated solution of 1 in chloroform. c) AFM
high-resolution image (surface scale 50ꢂ50 nm2) of a single self-
assembled structure obtained by concentration to dryness of an
irradiated solution of 1 in chloroform. d) AFM image (surface scale
500ꢂ500 nm2) of the self-assembled structures obtained by probing a
liquid film made from an irradiated solution of 1 ([1init.]=1 mm in
chloroform).[30]
triarylamines by charge hopping, thus allowing the supra-
molecular polymerization of units that contain a delocalized
cationic radical. We also assume that, as indicated by EPR
spectroscopy, one radical is sufficient to stabilize the stacking
of about 160 triarylamine units along the long axis of the self-
assembled wires. In addition to the charge-transfer phenom-
ena and the p–p stacking interactions, the structure–property
study revealed that at least one hydrogen bonding and van der
Waals interaction of lateral chains should be combined to
produce stable structures. Moreover, we have demonstrated
that these molecular wires, which protect holes from quench-
ing by the solvent, can combine into very strongly packed
bundles of larger fibers.
From a self-assembly point of view, this phenomenon
represents a sort of “supramolecular living radical polymer-
ization” process that can be triggered by charge transfer and
be reversibly broken up by heating, because the radicals are
spontaneously reduced by the solvent when they are not
stabilized within the self-assembled structures. These results
provide new insights into charge transfer self-assemblies and
open a number of interesting possibilities by using these
triarylamine synthons as new stimuli-responsive supramolec-
ular scaffolds. From a functional point of view, such highly
organized triarylamine-based architectures with enhanced
physical properties (strong stabilization and hole conduction)
highly organized structures.[30] Large fibrillar 3D aggregates
(10–50 nm in width and 50–1000 nm in length) were observed
in both height mode and phase mode (Figure 3a, and b,
respectively). High-resolution scanning of these objects
revealed their “corn-like” surface, which is composed of
bundles of individual 1D fibers strongly packed in a concen-
tric-layer organization (Figure 3c). For each individual 1D
fiber, the separation between the bright dots was measured to
be approximately (1.0 ꢁ 0.3) nm in length. This distance is too
large to represent the distance between individual stacked
molecules; the dots might indicate a periodic pattern of
triarylamines for which the lateral p-substituents would
alternatively point into a particular direction (see Fig-
ure S6d,e in the Supporting Information for a suggested
arrangement of the molecules). The lateral separation
between two individual 1D fibers in the 3D aggregates was
measured to be approximately (1.3 ꢁ 0.4) nm, which is con-
sistent with the dimensions of individual molecular wires
made from the polymerization of a single row of triarylamine
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Received: March 28, 2010
Revised: June 9, 2010
Published online: July 19, 2010
Keywords: materials science · nanostructures · self-assembly ·
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