Solvophobic control of core-substituted naphthalene diimide nanostructures

Article abstract of DOI:10.1039/b920015k

The requirements for the self-assembly of 2,3-annulated core-substituted NDIs into discrete worm-like nanostructures are explored in MeOH-CHCl ₃ solutions. AFM was used to visualize and characterize the structures formed at precise proportions

Full text of DOI:10.1039/b920015k

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Solvophobic control of core-substituted naphthalene diimide
nanostructuresw
Sheshanath V. Bhosale,* Chintan Jani, Cecilia H. Lalander and Steven J. Langford*
Received (in Cambridge, UK) 30th September 2009, Accepted 27th November 2009
First published as an Advance Article on the web 14th December 2009
DOI: 10.1039/b920015k
The requirements for the self-assembly of 2,3-annulated
core-substituted NDIs into discrete worm-like nanostructures
are explored in MeOH–CHCl3 solutions. AFM was used to
visualize and characterize the structures formed at precise
proportions of solvent mixes.
Self-assembly and self-organization play important roles in the
construction of thermodynamically-stable structures at
both the cellular and sub-cellular levels within nanometre to
Fig. 1 Schematic structure of 1 and the proposed self-assembled layer
millimetre dimensions. These paradigms have been translated
successfully to the laboratory, where complex, unnatural
systems for molecular materials applications have been
developed via the so-called ‘bottom-up’ strategy.1 In this
way, molecular curiosities, such as catenanes and rotaxanes,
to highly structured liquid crystals, self-assembled monolayer
and network structures have been produced. An area still
under strong development is the controlled growth of
assemblies within a pseudo one dimension to form filled wires
or hollow tubules in a replicable and easy fashion. One
approach relevant to applications in optoelectronic devices is
the use of p–p-stacking and solvophobicity as a means
of producing supramolecular polymers.2 This has proved
especially effective with the larger macrocyclic polyaromatics
such as hexabensocoronene and perylenediimides (PDI)
to produce elaborate and well-defined systems.3,4 Recent
developments in the synthesis of core-substituted naphthalene
diimides (NDIs) have allowed several groups to probe
the function of this interesting class of dye molecules in a
molecular5 and supramolecular sense.6 Core-substitution can
be seen as an opportunity to extend the planar, rigid naphthalene
core of NDIs which could be used to prepare novel structures
which promote spontaneous self-assembly.7 Here we report on
the development and use of small annulated core-substituted
NDIs for the controlled assembly of nanostructures through
solvent effects (Fig. 1).
formed from 40% v/v methanol in chloroform.
Scheme 1 Synthesis of core-substituted NDIs used in this study.
across the entire core), which adopts the central focus for
recognition leading to the reported structures.
Compounds 1–3 were prepared by reacting the tetrabromo-
NDI 48 with ortho amines 5–7 (Scheme 1) in hot, dry DMF
solution. NDI 1 was obtained in 74% yield from 4,5-bis-
(dodecyloxy)benzene-1,2-diamine as a deep green crystalline
solid after purification by column chromatography on silica.
Compounds 2 and 3 were used to probe the structural features
that dictate the self-assembly process. The fact that mono-
annulation results in high yield can be attributed to two
factors: aniline amines such as 5 are less nucleophilic than
the aliphatic amines used in examples where tetrasubstitution
takes place,8 and, once the first addition is complete the
deactivating effect of the two amino groups precludes further
addition. This result is important as it allows variation to the
structure through the two remaining aryl bromide groups.
The absorption of 1 in CHCl3 showed absorption bands at
582 and 643 nm (Fig. 2) that are red-shifted, yet characteristic
for the S0–S1 transition of the isolated NDI chromophore.8,9
The results demonstrate that the electron-donating 2,3-diamino
substituent introduced to the naphthalene core affects the
electronic structure of NDI in a similar fashion to 2,6- or
2,3,6,7-core-substitution. Furthermore, Fig. 2 shows the
absorption spectra of NDI 1 in various ratios of methanol
and chloroform. Aggregation effects in which a reduction in
the peak intensity along with a significant blue-shift of the
The main naphthalene diimide used in this study, 1,
bears two important features consistent with other specific
methodologies for studying self-assembly—suggesting
a
generality in design. The first is the use of long (C8–C10) alkyl
chains at the periphery through both the imide nitrogens and
alkoxy linkage to the core-substituted NDI. The second
feature is a near planar core (despite the lack of aromaticity
School of Chemistry, Monash University, Clayton, Victoria, Australia.
E-mail: steven.langford@sci.monash.edu.au,
sheshanath.bhosale@sci.monash.edu.au; Fax: +613 9905 4597;
Tel: +613 9905 4597
w Electronic supplementary information (ESI) available: Synthesis of
1, and details of UV/Vis and AFM data for all assemblies and
aggregates. See DOI: 10.1039/b920015k
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Samples of 1 ([1] = 1 Â 10À4 M) in 10–40% v/v methanol in
CHCl3 were spin coated onto silica (111) wafers and analyzed
by AFM in the tapping mode. Little supramolecular structure
is seen by this technique at 10–30% v/v MeOH, mainly
irregular aggregates (see ESIw), that in hindsight could be
argued as the progeny to the events that occur at higher v/v
levels of MeOH solvent. Images of the aggregates formed
at 40% v/v MeOH show well-formed nanostructures that
extend to micrometres in length (Fig. 3). There appear to be
variations in the observed lengths (ca. 1–2 mm), widths and
heights of assemblies of 1 (Fig. 3a). Fig. 3b profiles three of the
nanostructures as identified in Fig. 3a. The heights of profiles
1–3 are 27 nm, 54 nm and 82 nm, respectively—consistent with
a doubling then trebling of the thinnest of the nanostructures.
Fig. 3c and d illustrate that further aggregation of the nano-
structures into bundles is possible then into higher aggregates
again, leading to the observed height differences. As a result,
the thickness of the nanostructures also varies up to several
hundred nanometres (Fig. 3d). The ratio of the diameter and
height of the nanostructures formed by 1 also suggests a
flattening upon being transferred from solution to silicon
surface.11 Increasing the amount of methanol to beyond
Fig. 2 UV/Vis spectra of NDI 1 in CHCl3 ([1] = 1 Â 10À4 M) and
with increasing concentration of methanol at 20 1C.
absorption maximum and a loss of the fine structure are seen
at higher %v/v MeOH.
These features suggest the formation of face-to-face p-stacks
of NDI chromophores in a similar effect observed in the case
of J- and H-aggregates in perylenediimides,10 and prompted us
to further investigate the aggregates of 1 at or around the point
of aggregation using atomic force microscopy (AFM).
Fig. 3 AFM images of well-defined nanostructures formed by 1 from 1 Â 10À4 M solutions in MeOH–CHCl3 (v/v 4 : 6) (a) topography image,
(b) cross-section analysis of three nanostructures indicated in (a), (c) and (d) phase images at varying magnifications.
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40% causes the compound to precipitate out of solution and
no detailed micro or nanostructure can be observed anymore
indicating the delicate balance of solvophobicity with non-
covalent interactions allowing the nanostructures to form.
Importantly, and from a design issue, compounds 2 and 3 in
which the alkoxy groups have been removed or replaced with
more rigid sulfonamide groups failed to give any well-defined
nanostructures at any proportions of MeOH indicating the
subtleties of the self-assembly process. So although the self-
assembly is predominantly driven by p–p stacking interactions
between the large aromatic cores of 1, in exacting proportions
of MeOH–CHCl3, solvophobicity of the peripheral alkyl
chains plays an important role in the formation of each stack
(Fig. 1) also resulting in the merging of a number of nano-
structures into larger structures by similar forces.
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In conclusion, we report for the self-assembly behaviour
of a 2,3-annulated, core-substituted naphthalene diimide 1,
which forms regular wire, or worm-like, nanostructures in
MeOH–CHCl3. The core design is unique in that total
aromaticity of the core is a tolerant feature that could be
exploited to an array of novel systems. We are currently
investigating analogues of 1 to further understand this
behaviour and its scope,12 to look at covalent-modification
strategies13 and other self-assembly behaviour such as liquid
crystallinity as well as in single molecule spectroscopy. These
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SVB and SJL gratefully acknowledge the Australian
Research Council for support under the Discovery program
(DP0878220).
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Chem. Commun., 2010, 46, 973–975 | 975