In situ reversible conversion of porphyrin aggregate morphology

Article abstract of DOI:10.1039/c2cc33438k

A porphyrin derivative possessing orthogonal self-assembly units displays in situ reversible transformation of aggregate morphology between nano-rods and hollow spheres upon exposure to different solvents.

Full text of DOI:10.1039/c2cc33438k

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In-situ reversible conversion of porphyrin aggregate morphologies
Ming-Cheng Kuoa, Hsiao-Fan Chena, Jing-Jong Shyueb, Dario M. Bassanic, Ken-Tsung Wonga*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
A porphyrin derivative possessing orthogonal self-assembly
units displays in-situ reversible transformation of aggregate
morphology between nano-rods and hollow spheres upon
exposure to different solvents.
50 biuret boronic ester 4 in 45 and 41% yield, respectively (two
steps).14 Compared to 1b possessing hydrophobic octyl side
chains which promote self-assembly via van der Waals
interactions, the biuret motifs are expected to induce self-
assembly through hydrogen-bonding (H-B) interactions in aprotic
55 media. In the case of 3, possessing both octyl chains and biuret
units, we may expect strong solvent-dependence of the self-
assembly process.
The design of well-defined self-assembled nanostructures
10 based on specific supramolecular interactions between
electroactive molecules is of interest for diverse applications such
as field-effect transistors,1 photovoltaics,2 and sensors,3 where the
morphology of the active material plays a key role in determining
the electrical properties of the final device. Organic materials
15 with tailored functional groups capable of inducing the formation
of nanostructures prior to or during deposition represent a route to
supramolecular devices via a “bottom-up” approach, in which an
ordered structural arrangement with nanometer precision over a
large scale is constructed.4 However, precise control of long-
20 range order using non-covalent molecular interactions is still a
challenge. Numerous factors, such as the nature and linking
topology of the self-assembly motifs and the chemical structure
of the functional core, as well as exterior factors such as solvent
composition, aging time and deposition conditions, combine to
25 form a complex ensemble from which the formation of specific
nanostructures emerges. In some cases, it is possible to control
the aggregation through an exterior stimulus. For example, a
vesicle to nanofiber transformation could be reversibly induced
by sonication and heating.5 More generally, the addition of co-
30 reagents,6 pH modulation,7 photochemical reaction,8 or variation
of solvent composition are used to induce transformation of
aggregate structures.9 Sometimes, reversible switching between
dissolved and aggregated states can be promoted through solvent
vapour annealing.10 Despite the fact that the real-time
35 interconversion between selected nanostructures is highly desired
for potential application of soft materials,11 the in-situ
transformation of an aggregate’s nano-morphology directly on a
substrate is rare.12
R
R
O
O
NH2
N
N
N
N
N
N
N
N
NH
e
NBS DCM M
OH
, ,
1
)
n
Z
HN
O
n
Z
NH
HN
O
H2N
2
Pd PPh
K
2CO3
,
a
4
(
THF
,
q
)
)
(
)
3
O
O
NH2
O
NH
B
NH
4
R
R
O
a:
=
= n
1
R
R
C
H
C8
:
2
:
3
=
C
H
C8
3H17
R
R
45
%
( )
3
:
1b
= n
H17 41
%
( )
Scheme 1 Synthesis of porphyrin derivatives 2, and 3.
60
The aggregation behavior of 1b, 2, and 3 was studied by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). SEM samples were prepared by evaporating
a solution of molecules 1b, 2, or 3 (10 µL, 0.1 mM) onto a
SiO2/Si substrate (1 × 1 cm2). Environmental SEM images (taken
65 under 0.45 Torr H2O or THF vapour, see SI) of the aggregates
formed by 1b and 2 are shown in Figure 1. For 1b, deposition
from THF solution leads to the formation of nanorods with ca.
50-nm width and ca. 700-nm length (Figure 1a),9 whereas
deposition from THF/MeOH (1:1) solution leads to the formation
70 of a non-uniform sheet topology (Figure 1b). In contrast, the
deposition of 2 from THF/MeOH solution (10:1, v/v) leads to the
formation of spherical nanoparticles with a uniform diameter of
ca. 200-nm (Figure 1c). The particles are hollow, as shown by
TEM images (Figure 1c, inset, also Fig. S1 in ESI). The
75 formation of self-assembled hollow spheres of
2 upon
evaporation of the solvent is analogous to previously reported
bis-biuret derivatives with a rigid conjugated core and is directly
attributed to the presence of H-B interactions. 14, 15 In contrast to
1b, increasing the proportion of methanol (e.g. THF/MeOH =
80 1:100 v/v) did not lead to changes in the aggregates' morphology,
but had the effect of dispersing the aggregates which retained
their hollow sphere morphology (Figure 1d). The increased
dispersion is not related to the Marangoni effect16 as there is only
a slight difference in surface tension between THF and methanol
85 (26.4 vs. 22.7 mN.m-1, respectively).
Porphyrin derivatives decorated with self-assembly motifs
40 have been widely used as molecular scaffolds for the construction
of well-defined organic nanostructures such as fibres, sheets, rods,
tubes, and cubes.13 Herein, we report a porphyrin-based system
that exhibits reversible solvent-induced transition between rod-
like aggregates and hollow spherical structures directly on a
45 substrate upon exposure to different solvents. The structure and
synthesis of porphyrin derivatives 1 – 3 are shown in Scheme 1.
Tetraphenylporphyrins
2 and 3 bearing bis-urea (biuret)
substituents were obtained via NBS bromination of
diphenylporphyrins 1a and 1b followed by Suzuki coupling with
Some understanding of how the intermolecular forces control
aggregation can be gleaned from the solid-state structures of 1b
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chains on the spherical surface would also account for the more
40 dispersed nature of the aggregates in THF and suggests that they
should be destabilized by the presence of a polar solvent. To
verify this, the substrates containing the spherical aggregates
were treated with (THF/MeOH = 2:5 v/v, 10 μL) to weaken the
H-B interactions and enhance van der Waals forces between the
45 octyl chains. In line with our expectations, rod-like aggregates
once again re-emerged (Figure 3c). The process can be repeated
(Figure 3d), successfully demonstrating the in-situ reversible
conversion between self-assembled structures of different
morphology.
(b)
(d)
(a)
(c)
5 μm
0.5 μm
0.5 μm
5 μm
50
Fig. 3 Evolutionary SEM images (0.45 Torr H2O or THF vapour, see SI)
for morphology transition of 3 at room temperature. (a) As cast film from
THF/MeOH (2:5, v/v). (b) The same substrate after treating with THF (10
µL). (c) After THF/MeOH (2:5, v/, 10 µL) treatment from (b). (d) After
55 THF (10 µL) treatment from (c). All solvent treatments were directly
done on the same substrate.
Fig. 1 SEM of 1b (0.1 mM) prepared in (a) THF and (b) THF/MeOH (1:1,
v/v) and 2 (0.1 mM) prepared in (c) THF/MeOH (10:1, v/v) and (d)
THF/MeOH (1:100, v/v) at room temperature. The inset of (c): TEM of 2
5 prepared in THF/MeOH (10:1, v/v), showing hollow sphere structures. (e)
Schematic representation of the proposed self-assembly of hollow
spherical aggregates of 2.
The electronic absorption spectra of 3 in THF and THF/MeOH
(v/v = 2/5) and their corresponding aggregates (state I: sphere, II:
rod) cast from solution (0.1 mM) are shown in Figure 4.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
solution (THF)
solution (THF/MeOH = 2/5)
state I (THF)
and 4. Examination of the crystal structure of 1b (Fig. S2) reveals
that the porphirin cores in 1b are not co-planar and that they are
10 separated by a distance of 6.54 Å, whereas the octyl chains are in
the all-trans configuration and interdigitated. This indicates that
van der Waals interactions between octyl chains control
intermolecular self-assembly in the crystal rather than co-facial
π−π stacking as is often observed for planar π-cores with
15 peripheral long chains.17 Based on the crystal packing of 4 (Fig.
S3), the biuret unit induces the formation of H-B ribbons. In the
case of 2 which possess two biuret units at 180°, such ribbons
would be interconnected and lead to the formation of sheets
whose folding would result in hollow aggregates being formed
20 (Figure 1e).
The deposition of compound 3 from solution of (THF/MeOH
= 2:5 v/v) also led to the formation of rod-like aggregates (Figure
3a) similar to those obtained from 1b. However, the addition of
methanol did not result in a transformation of the aggregates as
25 had been observed for 1b in THF/MeOH = 1:1 (v/v) and distinct
rod-like aggregates were still observed for 3 even at high
methanol concentrations. Moreover, the average length of the
rod-like aggregates (ca. 15 µm) was significantly longer than
those of 1b derived from THF. Treatment of the rod-like
30 aggregates deposited onto the substrate with THF (10 µL)18 led to
their transformation into homogeneous spherical aggregates with
an average diameter of ca. 200 nm (Figure 3b). The self-assembly
behavior observed for 3 after THF treatment is the same as that of
compound 2 deposited from THF solution, suggesting a similar
35 process for the formation of the spherical aggregates in both cases.
We thus conclude that the spherical aggregates derived from 3
present the octyl chains organized at the suprafacial and
antarafacial positions of the 2D surface. The location of the octyl
state II (THF/MeOH = 2/5)
400
500
600
700
Wavelength (nm)
60
Fig. 4 Absorption spectra of 3 in THF (10-6 M, solid line) and
THF/MeOH (v/v = 2/5) solution (4 × 10-6 M, dashed line) and their
respective aggregates (states I and II) cast from their corresponding
solutions (10-4 M).
65
In solution, 3 possesses sharp absorption bands that are
independent of solvent composition, suggesting isolated molecule
behavior. Compared to solution, both the Soret and Q bands of
the porphyrin cores are red-shifted and significantly broadened
when deposited onto a quartz substrate. These spectral changes
70 are commonly observed in porphyrin aggregates and assigned to
the formation of J-type aggregates with a slipped-stacked
geometry.19 The Soret band of 3 in the aggregates is split into two
peaks centred at 415 and 441 nm for state I and 415 and 459 nm
for state II, respectively. The two peaks have been previously
75 assigned to the optical transitions from the transition dipole
moments oriented parallel (longer wavelength) and perpendicular
(shorter wavelength) to the aggregation direction.20 The parallel
transition dipole is sensitive to the distance between the
porphyrin cores,21 suggesting that the inter-porphyrin distance is
80 increased in the aggregates formed in state II with respect to
those in state I.
To better understand the evolution of the aggregates of 3 upon
2
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50 aDepartment of Chemistry, National Taiwan University, Taipei 10617,
solvent treatment, the process was followed using TEM. The
Taiwan. Fax +886 2 33661667; Tel: +886 2 33661665; E-mail:
kenwong@ntu.edu.tw
transition from rod-like to spherical aggregates upon exposure to
THF is shown in Figure 5. By treatment of the spherical
aggregates (Figure 5b) twice with methanol (5 µL), most
bResearch Center for Applied Sciences, Academia Sinica, Taipei 115,
Taiwan.
5
transform back to rod-like aggregates (Figure 5c, d). Within the 55 cUniv. Bordeaux, ISM, CNRS UMR 5255, 351, Cours de la Libération,
33400 Talence, France.
same substrate, a small portion of architectures that may
correspond to the transition from the spherical to the rod-like
morphology are also observed. Their morphology suggests that
the transition occurs by fusion of the spherical aggregates into
† Electronic Supplementary Information (ESI) available: Synthesis,
1
characterization, H, 13C NMR spectra for 5b, 1b, 2, 3 , crystal structure
parameters for 1b (CCDC 881738) and
4 (CCDC 881739). See
60 DOI: 10.1039/b000000x/
10 needles (Figure 5e), which then grow in length. In addition, larger
and more complex assemblies involving hierarchical aggregation
of the rods onto thin sheets (Figures 5f~h) were also observed.
This phenomenon resembles the merging behavior exhibited by
1b aggregates in the presence of methanol. More importantly, the
15 interconversion between vesicle and rod-like aggregates is
reversible by switching between THF and methanol.
1
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Fig. 5 Evolutionary TEM images for the transition of 3 at room
temperature. (a) As cast film from THF/MeOH (2:5, v/v). (b) After THF
20 (2 × 5 µL) treatment from as cast film. (c)-(h) show evolutionary
progression of morphological transition from hollow spheres to sheet
aggregates after MeOH (2 × 5 µL) treatment from (b). All solvent
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7
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9
In summary, we have developed a porphyrin derivative
25 incorporating amphipathic groups whose self-assembly can be
reversibly controlled through exposure to different solvents. Polar
protic environments disrupt H-B interactions between the biuret
groups and favor the hydrophobic interactions between the octyl
chains (van der Waals forces), leading to the formation of rod-
30 like aggregates. The reverse occurs in less polar aprotic media,
which favor H-B over hydrophobic interactions and induce the
formation of spherical aggregates. These results are in agreement
with model systems that show that the rod-like and spherical
aggregates arise from hydrophobic and H-B interactions,
35 respectively. The switching between aggregate morphologies in 3
is thus understood on the basis of solvent favoritism for specific
functional groups. Of particular interest is the fact that the
process is entirely reversible and that it can be conducted directly
onto the substrate analogously to solvent annealing procedures.
40 Given the large difference in morphology between the spherical
and rod-like aggregates, we believe the transition occurs via
(partial) dissolution of the material in the liquid phase.
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a block co-polymer was shown to exhibit in-situ
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18 A small drop of the solvent (10 µL) was gently deposited onto the
substrate and allowed to evaporate under argon.
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The authors gratefully acknowledge the financial support from
National Science Council of Taiwan (NSC-99-2923-M-002-002-
45 MY3) and the Agence Nationale de la Recherche (ANR-09-
BLAN-0387) and technical assistant of Technology Commons,
College of Life Science and Precision Instrumentation Center,
NTU with TEM analysis.
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Notes and references
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