Controlled self-assembly of squaraines to 1D supramolecular architectures with high molar absorptivity

Article abstract of DOI:10.1039/b718054c

A tailor made squaraine dye 1 upon binding with Ca²⁺ self-assembles to form a spherical micellar assembly that reorganises to thermodynamically stable 1D cylindrical rods with high molar absorptivity. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718054c

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Controlled self-assembly of squaraines to 1D supramolecular
architectures with high molar absorptivitywz
Ayyappanpillai Ajayaghosh,* Parayalil Chithra, Reji Varghese and
Kizhumuri P. Divya
Received (in Cambridge, UK) 22nd November 2007, Accepted 18th December 2007
First published as an Advance Article on the web 7th January 2008
DOI: 10.1039/b718054c
A tailor made squaraine dye 1 upon binding with Ca2+ self-
assembles to form a spherical micellar assembly that reorganises
to thermodynamically stable 1D cylindrical rods with high molar
absorptivity.
absorption maximum at 650 nm with two additional bands at
619 and 580 nm. The emission of the dye occurs at around
670 nm with a quantum yield of 0.004. On the other hand, the
dye 2 with a flexible podand chain showed a relatively broad
absorption with a better fluorescence quantum yield (0.014).
These observations indicate a strong excitonic interaction
between the dye moieties in 1 due to the positional confine-
ment on an aromatic platform.
Titration of Ca(ClO4)2ꢂ4H2O to a solution of 1 (1.8 ꢀ 10ꢁ6
M) in acetonitrile exhibited a decrease in the intensity of the
absorption maximum at 651 nm with the formation of a new
band at 547 nm (Fig. 1a). The Job plot revealed a 1 : 1 binding
of the cation with the dye. Addition of Mg2+ caused a similar
effect, however Sr2+ and Ba2+ showed relatively weak
changes to the absorption and emission spectra.11 The dye 2
exhibited a relatively weak response towards Ca2+. The
absorption maximum of 2ꢂCa2+ occurred at 560 nm which is
ca. 13 nm red-shifted with relatively weak molar absorptivity
Self-assembly of functional dyes is a topic of current interest in
the ‘bottom-up’ creation of nanoarchitectures due to their
importance in the mimicry of natural processes and to the
design of organic electronic devices.1,2 Of particular impor-
tance is the self-assembly of organic dyes to 1D structures with
high aspect ratio.3 A variety of organic dyes, such as porphy-
rins,4 phthalocyanines,5 merocyanines,6 perylenes7 and squa-
raines,8 have been reported as versatile synthons for the
creation of functional supramolecular assemblies. Though
extensive reports are available on the aggregation of squar-
aines, only a few examples are known of extended self-
assembly leading to supramolecular architectures.8,9 Herein
we report an unprecedented effect of a cation binding to the
self-assembly pathway and optical properties of a catechol-
linked bis-squaraine dye 1. Such an effect leading to a 0–1D
structural change with strongly enhanced visible absorption
upon cation complexation has not been reported previously.
In this work we explore the Ca2+ and Mg2+ binding ability
of flexible chain-linked bis-squaraines.10 The bis-squaraines 1
and 2 were synthesized and characterized by NMR and mass
spectral analyses.11 1 in acetonitrile (1 ꢀ 10ꢁ6 M) exhibited an
Photosciences and Photonics Group, Chemical Sciences and
Technology Division, National Institute for Interdisciplinary Science
and Technology (NIIST), CSIR, Trivandrum 695019, India.
E-mail: ajayaghosh62@gmail.com; Fax: +91-471-2490186;
Tel: +91-4712515306
w Electronic supplementary information (ESI) available: Synthetic
procedures, emission change of 1 with Ca2+, absorption changes of
1 with Mg2+, Sr2+, Ba2+, mass spectra of 1 before and after
complexation with Ca2+, AFM images of 1ꢂCa2+ complex after 9 h,
TEM image of 2. See DOI: 10.1039/b718054c
Fig. 1 Absorption spectral change of (a) 1 and (b) 2 (1.8 ꢀ 10ꢁ6 M) in
acetonitrile upon addition of Ca(ClO4)2ꢂ4H2O (0–2 ꢀ 10ꢁ6 M) in
acetonitrile. Insets show the corresponding Job plots.
z This is contribution No. NIIST-PPG-257.
ꢃc
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Chem. Commun., 2008, 969–971 | 969

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in a 2 : 1 complexation mode when compared to those of 1ꢂ
Ca2+
indicating weak complexation in the former
,
a
(Fig. 1b). This is clear from the comparison of A547/A651 for
1ꢂCa2+ and A560/A640 for 2ꢂCa2+, which are 1.31 and 0.74,
respectively. Binding of Ca2+ to the dye 1 is confirmed by
mass spectral (FAB) analyses which gave values of 944.13
(M + 2H) for 1 and 982.33 (M + Ca2+) for 1ꢂCa2+ 11
.
Surprisingly, after the addition of Ca(ClO4)2ꢂ4H2O to 1, a
three-fold increase in the intensity of the absorbance is ob-
served within a time span of 10 h (Fig. 2). Such a persistent
increase in the absorbance of a supramolecular dye–cation
complex with time (hyperchromism) has not been reported
previously. This phenomenon is attributed to the difference in
the dipole orientation of the chromophores with time.12 In
such a situation we speculated that the initially formed kine-
tically controlled aggregates may change to thermodynami-
cally favored structures in which the dipole orientation of the
dye may change significantly. In addition, a considerable
decrease in molecular shadowing may be possible when the
morphology changes to cylindrical rods from spherical mi-
celles.13 Insight into this hypothesis is obtained by the detailed
atomic force microscopic (AFM) and transmission electron
microscopic (TEM) analyses of 1 in the absence and presence
of Ca2+. Interestingly, the Ca2+ complex of the dye 2 did not
show any change in the absorption spectra even on standing
for several hours.
Fig. 3 Tapping mode AFM images of 1ꢂCa2+ recorded after different
intervals; (a) initial micellar assemblies, (b) histogram showing the
particle distribution (Gaussian fit, R2 = 0.9510) of the spherical
assemblies of 1ꢂCa2+, (c) after 6 h and (d) zoomed image of the same.
AFM samples were prepared by casting an acetonitrile solution (1.8 ꢀ
10ꢁ6 M) over a mica surface after adding an equivalent amount
of Ca(ClO4)2ꢂ4H2O, z-scales of AFM images: (a) 10 nm and (c),
(d) 30 nm.
after 6 h revealed the complete transformation of the spheres
into extended assemblies of 10–100 nm in width and micro-
metres in length with an average height of 15 nm (Fig. 3c). The
broad distribution of the width of the fibers indicates the
formation of elementary 1D fibers that tend to form large
bundles on the mica substrate. The morphology remained
almost unchanged when imaged after 6 h.11
AFM images of the dried samples of 1 prepared from
acetonitrile (1.8 ꢀ 10ꢁ6 M) in the presence of one equivalent
of Ca(ClO4)2ꢂ4H2O on freshly cleaved mica substrates showed
spherical particles of 5–200 nm in diameter after deducting the
tip broadening factor (Fig. 3a).14 The corresponding histo-
gram reveals an average particle size of 60 nm. AFM images
In order to confirm the effect of the substrate on the
morphology as well as the formation of a 1D self-assembly
of 1ꢂCa2+ complex, transmission electron microscopy (TEM)
studies were performed on carbon coated grids. TEM images
of 1 (1.8 ꢀ 10ꢁ6 M) in acetonitrile in the presence of
Ca(ClO4)2ꢂ4H2O exhibited spherical structures that tended to
agglomerate on the carbon grid (Fig. 4a). However, in the
absence of the cation, no specific morphology could be seen.
The diameter of the smallest particle observed is ca. 4 nm
which is twice the length of the molecule as determined from
the energy minimized structure. This observation indicates the
micellar nature of the self-assembly. The TEM images after 6 h
revealed the transformation of the spherical micellar
Fig. 2 (a) Time dependent absorption spectral change of 1 (1.8 ꢀ
10ꢁ6 M) in acetonitrile after the addition of one equivalent of
Ca(ClO4)2ꢂ4H2O. (b) Comparison of plots of the absorbance at
548 nm of 1ꢂCa2+ and at 560 nm of 2ꢂCa2+ against time showing
the hyperchromic effect of the former.
Fig. 4 TEM (unstained) images of 1ꢂCa2+ recorded after different
intervals; (a) initial micellar assemblies, and (b) after 6 h. Insets
represent zoomed areas, highlighting different morphologies.
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Fig. 5 Schematic representation of Ca2+ induced self-assembly processes in 1 leading to spherical and extended micellar structures.
aggregates to 1D nanostructures of ca. 4 nm in diameter and a
few micrometres in length (Fig. 4b). After 6 h, the morphology
remained more or less the same. Interestingly, the TEM
analysis of the dye 2 showed spherical dye aggregates;11
however, in the presence of Ca(ClO4)2ꢂ4H2O no stable mor-
phology could be seen, probably due to the weak complexa-
tion of 2 with Ca2+. Thus the unusual hyperchromic behavior
of 1ꢂCa2+ with time is in agreement with the morphology
changes as observed by the AFM and the TEM analyses.
On the basis of the above observations, the self-assembly
processes in dye 1 are rationalized as shown in Fig. 5. The
amphiphilic nature of 1ꢂCa2+ allows initial self-assembly into
the kinetically favored spherical micellar aggregates. Over a
period of time the 0D spherical micelles rearrange into a
thermodynamically stable 1D micellar assembly which is a
slow process.15 On the other hand, the dye 2 with a flexible
podand chain forms a weak complex with Ca2+ which neither
shows a hyperchromic effect with time nor forms a stable self-
assembly. These observations reveal that the confinement of
the dye moieties on the aromatic platform as in 1 has a
significant role in the hyperchromic effect and the self-assem-
bly processes. In the case of the dye 1, formation of the stable
1 : 1 complex may be partly assisted by the cation–arene
interaction of the aromatic moiety of the podand chain. The
initially formed stable complex facilitates the hierarchical
assembly to spherical micelles and extended micellar structures
through peripheral arene–arene interactions (Fig. 5). The
weak 2 : 1 complexation in dye 2 with the cations does not
allow these processes in the absence of the aromatic moiety of
the podand chain.
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In conclusion, we have illustrated that even a subtle varia-
tion in the structure of the dye has a significant impact on the
hierarchical self-assembly, initially to 0D spherical and finally
to 1D extended micellar structures, accompanied by a hyper-
chromic effect. The presence of the aromatic moiety of the
catechol linker in dye 1 plays the key role in the cation
controlled self-assembly process. This is clear from the fact
that the dye 2 without the catechol linker did not show stable
self-assembly or hyperchromism. The 1D structures of the dye
1 with high absorbance may find application in organic
electronic and photonic devices that require a high molar
absorption cross-section.
11 See supporting information.
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Chem. Commun., 2008, 969–971 | 971