Chiral Assembly of AIE-Active Achiral Molecules: An Odd Effect in Self-Assembly

Article abstract of DOI:10.1002/chem.201605458

The induction of chirality in supramolecular structures through hierarchical self-assembly of achiral compounds and the control of their handedness are closely related to the evolution of life and the chiral amplification found in nature. Herein, it is sh

Full text of DOI:10.1002/chem.201605458

A Journal of
Accepted Article
Title: Chiral assembly of AIE active achiral molecules: An odd effect in
self-assembly
Authors: Sheshanath V. Bhosale, Anuradha ., Duong Duc La,
Mohammad Al Kobaisi, and Akhil Gupta
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FULL PAPER
DOI: 10.1002/chem.200((will be filled in by the editorial staff))
Chiral assembly of AIE active achiral molecules: An odd effect in self-assembly
Anuradha., Duong Duc La, Mohammad Al Kobaisi, Akhil Gupta, Sheshanath V. Bhosale*
induction and control of supramolecular chirality within the assembly. These
Abstract: The induction of chirality
chirality into well-defined helical controlled assemblies of an AIE-active
superstructures by controlling the molecule are of potential practical
solvent composition and polarity. In value, such as templates for helical
particular, right handed supramolecular crystallisation, catalysis or chiral
in supramolecular structures through
hierarchical self-assembly of achiral
compounds, and the control of their
handedness, are closely related to the
evolution of life and the chiral
amplification found in nature. Here we
show that the combination of achiral
tetraphenylethylene, an AIE-active
luminophore bearing four alkyl chains
with either an odd or even number of
carbon atoms via an amide linkage in
the molecular structure, allows the
structures were produced from an even mechanochromic
number of carbon atoms in the alkyl superstructure formation.
chains, whereas an odd number led to
luminescent
the
assembly
of
The
left-handed
twisted
Keywords: AIE-effect;
tetraphenylethene; solvophobic-
control; self-assembly, helical
nanostructures
superstructures.
superstructure was visualised by
scanning electron microscopy, and
circular dichroism was used to observe
take, with a preferential direction according the number of carbons
in the substituted n-alkyl moieties [2]. This preferential orientation
further propagates into the self-assembly process via the alignment
of the n-alkyl chains [3], where the induced chirality evidenced by
CD signal can determine the direction of the twist in the self-
assembled helical structure [4]. This means that the position of the
chiral centre governs the chirality of the aggregates.
Introduction
Controlling the three dimensional (3D) chirality of supra-molecular
self-assembly is of significant importance in studying life processes
and designing structures with selective handedness. This is
especially important when the building block of chiral self-assembly
is a prochiral molecule. A prochiral molecule can become chiral
when confined in a self-assembled structure e.g. string (1D), surface
(2D) or various 3D structures. Self-assembly can occur in mirror
domains, with the overall monolayer consisting of an equal number
of left- and right-handed molecules, therefore remaining globally
achiral. In this work we use the odd/even effect of alkyl
substitutions to guide the self-assembly of series of achiral
tetraphenylethylene (TPE) molecules into chiral supramolecular
structures. The absorption of either left or right handed light by
circular dichroism (CD) in the series oscillated, according to the
number of methylene carbons. This CD phenomenon has also been
There are a few examples that demonstrate the formation of
twisted fibres from homo-chiral molecules, or a mixture of chiral
and achiral molecules, where the molecular chirality is transferred to
the handedness of the self-assembled helical structure [5]. However,
the induction and control (non-racemic) of supramolecular chiral
assembly from achiral functional organic molecules is a great
challenge. Previous reports describe the generation of twisted
structures from achiral molecules such as amphiphilic cyanine dyes,
cationic surfactants, asymmetric porphyrins and quinacridone. Some
reports describe inducing chirality through chiral control of
interfacial tension, however these chiral self-assemblies actually
produce a racemic mixture,[6] and aggregation-caused quenching
observed in helical superstructures [1]
.
(ACQ) effects in the case of fluorescent molecules[7]
.
On the molecular level, this handedness in the self-assembly
behaviour can be assigned to a dynamic propeller conformation that
derivatives of molecules such as TPE and triphenylamine (TPA) can
To overcome the ACQ effect, the unusual fluorescence
properties of tetraphenylethylene (TPE) and its derivatives have
recently attracted significant interest. These chromophores are weak
or non-emissive in nature as un-associated monomers, but they
become strongly fluorescent upon aggregation i.e. they display the
aggregation-induced emission (AIE) effect [8]. AIE properties are of
interest in diverse areas including electroluminescent organic
Mrs. Anuradha ., Mr. D. D. La, Dr. M. Al. Kobaisi, Dr. A. Gupta, Dr. S.
V. Bhosale
materials[9], pH sensors[10], supramolecular assembly[11], interaction
[13]
with DNA[12], proteins
or other biostructures[14]. Recently, we
School of Science
RMIT University, GPO Box 2476
Melbourne, VIC-3001, Australia
Fax: (+) 61 3 9925 2680
have reported the control and induction of supramolecular chirality
from an achiral TPE. AIE-active luminophore analogues bearing
four long alkyl chains via amide linkages (alkyl-TPE) form well-
defined, uniform, right-handed helical structures through
solvophobic interactions[15]. The odd/even effect in molecules with
n-alkane moieties have been also utilized in the field of materials
E-mail: sheshanath.bhosale@rmit.edu.au
Supporting information for this article is available on the WWW under
http://www.chemeurj.org/ or from the author.
science in light-emitting diodes (LED)[16]
,
self-assembled
1
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10.1002/chem.201605458
Chemistry - A European Journal
[19]
monolayers[17], liquid crystals[18], and drag-reducing surfactants
,
Ultraviolet-visible (UV–Vis) absorption and fluorescence
spectroscopy
and to examine the origin of VOCs[20]
.
The UV–Vis absorption spectra of alkyl-TPE (1-4) derivatives
in tetrahydrofuran (THF) give two typical absorption bands near 273
and 343 nm, which are characteristic of the S0→S1 transition. Fig.
3A shows the absorption spectra of 1–4 in an acetonitrile/THF
(AA/T) solvent mixture of 9:1 (v:v). Typically, the absorption
Results and Discussion
In this manuscript we report for the first time the self-assembly of
TPE derivatives (1–4), each bearing alkyl chains of either an odd (1:
C7 and 3: C9) or even (2: C8 and 4: C10) number of methylene units
(See Fig. 1, Synthesis shown in ESI Scheme S1). With solvophobic
control, these achiral molecules formed into twisted ribbons, where
an odd number of methylene units give a left handed twist and an
(B)
(A)
evens number gave
respectively. This “odd/even” effect is
a
right handed twist to the structures
widely observed
a
phenomenon in physics, chemistry, materials and biology [21]. The
effect is described as an alteration in the structure and properties of
an object through the number of “repeat” units employed, with
examples observed in self-assembled in monolayers (SAMs)[22]
,
O
H
N
chiral recognition systems[23], molecular tunnelling junctions, field-
O
H
N
N
H
O
N
H
O
O
H
O
O
H
H
N
N
H
O
effect transistors (FETs), molecular diodes and many others[24]
.
N
H
N
O
O
H
H
N
N
H
N
O
N
H
O
O
O
H
N
H
N
O
N
H
N
O
O
H
N
N
H
O
O
H
N
O
R
H
O
N
H
O
O
H
H
N
O
H
N
O
H
N
N
H
H
R = n-Heptyl
n-Octyl
(1)
(2)
(3)
(4)
N
N
N
H
O
O
H
O
O
H
N
H
N
O
N
R
O
H
N
H
N
N
H
N
O
O
O
n-Nonyl
n-Decyl
O
Figure 2. Schematic representation of the self-assembly of C9 (3) and C10
(4) substituted alkyl-TPE, where the prochiral molecules arrange in
opposite directions; C9 (3) left handed and C10 (4) right handed self-
assembly.
H
R
R
N
H
N
O
Figure 1. TPE structures (1-4) used in this study.
maximum in AA/T (90%, v/v) shows a red-shift with an increase in
the number of methylene groups in the side chains. For example,
derivative 1 shifts 6 nm, 2 shifts 12 nm, 3 shifts 17 nm and 4 shifts
22 nm in comparison to the peak for 4 in pure THF (AT), which is
Here we report studies on the synthesis and self-assembly of
achiral TPE homologues bearing odd and even units, namely amide
linked alkyl chains of varied lengths, to elucidate the effect on the
supramolecular structures. In this contribution we report the
typical for J-type aggregates [26]
.
controlled
hierarchal
organized
chiral
supramolecular
microarchitecture of TPE derivatives bearing an alkyl chain, varying
in length, linked via an amide bond. This molecular structure has
three important features that result in the formation of chiral, AIE
active, assemblies; (i) --interactions between the aromatic TPE
cores, (ii) hydrogen bonding via the amide functionality, and (iii)
long alkyl chains on the p-position of the aromatic units, with an
odd or even number of carbons which are designed to adjust the
dispersive van der Waals interactions[15]. The alignment of these n-
alkyl moieties in self-assembly is the cause of preferential chirality
of the structure.
A
B
As shown by Tahara et al.[3a], the geometric difference in
alignment caused by the n-alkyl chain with either an odd or even
chain length is critical in the formation of a preferred direction in the
self-assembled monolayer. Based upon the known TPE compound
arrangement in the crystal structure [25], Fig. 2 shows a schematic
representation of the opposing handedness of C9 (3) and C10 (4)
substituted alkyl-TPE self-assembled structures. The structures
allow for terminal methyl units to position themselves clear of the
polar amide linking groups, and thus maximise van der Waals
interactions. This results in a surface pattern where molecules are
oriented into a 2D chiral configuration. This in-plane chiral self-
assembly further extends into 3 Dimensions by stacking, to produce
ribbons with an opposite twist direction from the odd or even alkyl
substituted TPEs respectively.
C
D
Figure 3. Spectroscopic measurements of 1–4, to study the solvent effect
on alkyl-TPE self-assembly and chirality; (A) UV–Vis spectroscopy of 1–
4 in acetonitrile in THF 9:1 (v/v) and 4 in neat THF. (B) Fluorescence
spectra of 1–4 in 90% (v/v) acetonitrile in THF and neat THF (ex = 343).
(C) FL quantum yield (ΦF) of 1–4 in various ratios of acetonitrile in THF
solvent mixtures. (D) Photographic image of 1–4 in THF and 90%
acetonitrile in THF taken under ex = 365 nm irradiation.
2
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10.1002/chem.201605458
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Furthermore, the fluorescence (FL) emission spectroscopy of the
alkyl-TPE (1-4) series shows a signal at 520 nm in neat THF upon
excitation at 343 nm (Fig. 3B) with a FL quantum yield (ΦF) of
approximately 2.3 and 3.1% for 1 and 2, respectively. 3 and 4
showed no detectable signal and the FL quantum yield (ΦF) was
only 0.13 and 0.07%. However, a 20 fold enhancement of emission
A
B
with ΦF 18.3% and 19.7% was observed for 3 and 4 in an
of
acetonitrile/THF (fA/T= 90%) solvent mixture, respectively. Similar
enhancement of emission was observed for compound 1 (ΦF
=
14.7%) and 2 (ΦF = 17.4%) with an increasing fraction of ACN in
THF (Fig. 3C). The fluorescence quantum efficiency (ΦF) of the
samples with absorption (intensity ~0.05) was estimated using
fluorescein in ethanol (ΦF = 70%) as a standard solution, and ΦF of
the solid films was measured using an integrating-sphere photometer.
It is important to mention that enhancement of FL ocurred with an
increasing number of methylene units in the alkyl chains of the
alkyl-TPE molecules, i.e. 4>3>2>1 (Fig. 3D). The emission of
alkyl-TPE (1) in its crystalline form exhibited strong luminescence
with an emission peak at 545 nm (ΦF = 18.3%), which represents a
25 nm red shift. The FL results clearly demonstrate that the alkyl-
TPE (1-4) series is AIE-active. These results strongly suggest the
formation of large J-type aggregates, which are typical of AIE-
active species, and observed in all four of theTPE derivatives
Figure 4. CD spectra of the alkyl-TPE series. (A) CD spectra of 2 in THF
and 1–4 in 90% acetonitrile in THF. (B) Temperature dependent CD
spectroscopy of 3 in 90% acetonitrile in THF between 20 and 85 °C.
In addition, chirality induction during the self-assembly of alkyl-
TPE and the formation of supramolecular aggregates was
investigated using temperature dependent CD spectroscopy (Fig. 4A
and Fig. S1). As expected, with an increase in the temperature above
40 °C, the intensity of the CD peaks decreased, due to the increased
solubility of the aggregates on the single molecule level, rendering
the solution chirally inactive at ~80 °C [35]. This finding indicates
that at high temperature the molecular self-assembly of alkyl-TPE 4
diminishes, while at low temperature they are assembled into helical
aggregates as demonstrated by the UV-vis absorption spectroscopy
(Figure S2a).[1d] Interestingly, the linear dichroism (LD) spectrum
shows a small response, which increases after stirring (Figure S2b).
The UV/Vis, CD, and LD spectra clearly indicate that upon
lowering the temperature, the helical fiber length increases and the
increase in the LD is most likely associated with the alignment of
the alkyl-TPE (4) helical fibruous aggregates with a increasing
lenght upon lowing the temeratyre, and upon stirring which is due to
participitating the long helical fibers in solution. This clearly suggest
that the certain fiber length is required to induce the LD and CD
effects. LD spectra in solution shows that the alkyl-TPE (4) fibers
in solution orient preferentially in the vertical direction, consistent
with interpretation of the CD effect for right-handed helical strcture.
Fig. 5 shows a graphical representation of the process of
chirality induction in the self-assembled structures of the odd and
even methylene alkyl-TPE In the absence of a rotational degree of
freedom a racemic mixture forms at higher temperatures, whereas
chiral structures and handed self-assemblies are produced when the
rotation freezes at lower temperature, andpromote self-assembled
molecules.
studied here [15, 27]
.
Circular dichroism
[3a,
STM
c], sum frequency generation (SFG) [28], low-frequency
[29]
Raman
,
solid-state 13C NMR, circularly polarized
photoluminescence (CPPL) electroluminescence (CPEL) [30], and
[31]
vibrational circular dichroism spectroscopy
, are used in
characterizing and studying the properties resulting from the
odd/even effect in self-assembly, but CD spectroscopy in the UV–
Vis region is the common theme in these types of studies.
Here CD spectroscopy is utilized to gain an insight on the
influence of the odd/even effect on the chirality of self-assembly in
the series 1-4 [32]. The CD spectrum of 2 (C8 chain) was measured in
THF, and was shown to be CD inactive, with the molecules well
solvated in a non-aggregated state as expected for achiral molecules
[33]. The CD spectrum of 2 in ACN/THF (9:1, v/v) shows significant
Cotton effects, through an isodichroic point at the zero-crossing at
325 nm, with strong positive (364 nm) and negative (286 nm) CD
signals. Similar behaviour were observed for alkyl-TPE (4) bearing
alkyl chains with an even number of methylene units (C10) (Fig. 4A).
The CD spectra of alkyl-TPE 2 and 4, bearing even numbers of
methylene units clearly supports formation of a right handed chiral
aggregation [34]. An interesting result is that completely opposite
bisignated CD signals, with negative (362 nm) and positive (289
nm) maxima were observed for alkyl-TPE derivatives (1 and 3)
bearing an odd number of methylene units on their alkyl (C7 and C9)
chains (Fig. 4A). This is characteristic of excitonically-coupled left
handed helical organization of the TPE chromophores [33]. The
perfect mirror image Cotton effects of alkyl-TPE bearing an odd
number of carbon atoms in the chains (1 and 3), and even number of
carbon atoms (2 and 4) in the chains, clearly suggest chirality with
opposite handedness as a result of the self-assembly [34]
.
3
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10.1002/chem.201605458
Chemistry - A European Journal
an odd-number of carbon chains. However, solid state CD shows
broad peaks, which may be due to bundle type helical structure
formations. This CD effect implies that during aggregation the
alkyl-TPE scaffolds are helically arranged with a preferred screw
sense, which is induced by the odd- or even number of carbons in
the chains periphery to the TPE-core. This confirms that opposite
helically arranged fibers are due to the interaction of hydrophobic
alkyl chains as illustrated in Figure 2. It is important point to note
that helical structures are kinetically favored, and their assembly
depends on the conditions, such as temperature. Also, the addition
of 1-4 from THF solution injected into MeCN causes the assembly
to be irreversible.
Figure 5. Graphical illustration of the supramolecular self-assembly resulting
from alkyl-TPE with an even (left) and odd (right) number of methylene
units. Oppositely oriented chiral aggregates are seen on the molecular scale,
and oppositely twisted ribbons on teh microstructure level.
Field Emission Scanning Electron Microscopy (FE-SEM)
Figure 6. Visualisation of twisted superstructures by SEM analysis. SEM
images of alkyl-TPE (1–4) microstructures deposited on silicon wafer by
solvent evaporation THF/ACN (1:9, v/v): (A) alkyl C7-TPE (anti-clockwise
twist), (B) alkyl C8-TPE (clockwise twist), (C) alkyl C9-TPE (anti-clockwise
twist), and (D) alkyl C10-TPE (clockwise twist).
To examine the morphology of the hierarchical twisted
superstructures resulting from the self-assembly of the achiral alkyl-
TPE (1–4; 10-5 M) molecules in THF/ACN (1:9, v/v) solvent
mixtures, SEM images were obtained after deposition and solvent
evaporation of the solutions on a silicon wafer substrate (Fig. 6, Fig.
S3). The alkyl-TPE bearing alkyl chains with an odd number of
methylene units (C7 and C9) produced well-defined left handed
twisted ribbons as illustrated in Fig. 6A and 6C. In particular, the
image of alkyl C9-TPE (anti-clockwise twist) revealed the long
twisted ribbons at the macroscopic level, which are approximately
145 nm wide and ~65 nm in diameter, with a helical pitch of
approximately 500 nm (Fig. 6C). SEM images of the
microstructures of C7-TPE also show the dimensions of the anti-
clockwise twisted ribbons, which are approximately ~210 nm in
diameter with a helical pitch of approximately 1.2 µM (Fig. S3).
However, as expected, the SEM of alkyl-TPE bearing alkyl chains
with an even number of methylene units (C8 and C10) resulted in
well-defined right handed twisted ribbons, tens of micrometres in
length (Fig. 6B and 6D). The SEM images clearly show the long
twisted ribbons at the macroscopic level, which are approximately
0.5–1.0 µM wide and ~60 nm in diameter with a helical pitch of
approximately 1.6 and 3.6 µM, respectively (Fig. 6D, Fig. S3). To
confirm the twisted helical fibers in the solid state CD spectroscopy
was also performed. The CD spectra of alkyl-TPE (4) exhibited
strong Cotton effects at the wavelength 365, 308 (positive) and
negative at 280 nm, through an isodichroic point at the zero-crossing
Twisted ribbons are the main microstructures formed by alkyl-
TPE self-assembly (Fig. 6), but in several instances nanotubes (Fig.
7A) could be observed as preliminary structures, which upon growth
in diameter, or alignment parallel to each other collapse to form the
twisted ribbons (Fig. 7B). The majority of twisted ribbons are
composed of stacks of thinner ribbons, only a few tens of
nanometres in thickness, as seen in the SEM image in Fig. 7C.
When the twisted ribbons grow further, in some instances helical
ribbons were observed, as seen for alkyl-TPE (4) in Fig. 7D. In a
few instances, the twisted ribbon structure stability is easily
counterbalanced by adhesion forces on the substrate surface, where
the twist has completely unwound, resulting in a flat ribbon in
contact with the silicon wafer surface, which is a morphological
state of higher thermodynamic stability (Fig. S5). To gain further
insight about influence of the surface in helical assembly, we
performed SEM analysis of alkyl-TPE (4) using HOPG surface.
The SEM images of dried drop-cast solution of 4 self-assembled
systems on HOPG showed large bundles of long fibers with right-
handed structure along with with slight flattening of the structure,
which is due to inteaction of structures with HOPG surface (Fig. S6).
Figure S7 shows TEM micrographs of superstructures formed by C7
and C8-TPE confirming the multilayered twisted ribbon formations
seen in Figure 7C.
at 300 nm, after deposition onto
a quartz surface from
acetonitrile/THF (9:1, v/v), and room temperature evaporation
(Figure S4). Similar to the solution CD signal, completely
opposite Cotton effects werre observed with alkyl-TPE 3, bearing
4
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10.1002/chem.201605458
Chemistry - A European Journal
molecular level, but does not necessarily have the same intensity.
Shorter alkyl chains result in smaller deviations in the self-assembly,
and display a smaller twist in the microstructure. The twist of the
phenyl-imide-alkyl moiety off the molecule plane occurs at very
similar angles for all derivatives, with opposite directions for even
and odd alkyl chains, with the stacking of the molecules occuring at
smaller angles for smaller alkyl chains. The formation of the flat
ribbons is attributed to a balance between the intermolecular
interactions, including - stacking that requires a perfect alignment,
and H-bonding and van der Waals interactions that require a torsion
adjustment angle to relieve the hindrance in the self-assembly. This
results in a smaller intermolecular torsion angle in TPE derivatives
with a smaller alkyl moiety (Fig. 6A and 6C) [35b]. It is also clear
from Fig. 7 that the mechanical forces applied by the surrounding
objects have resulted in disturbing the regularity of the twisted
ribbon. This shows that the conformation in the supramolecular
arrangement is very sensitive to the mechanical interactions with
surrounding environment including other self-assembled structures
the substrate surface causing the twist in the ribbon to deform or
flatten and produce energetically more stable state. Nevertheless, it
is clear that, odd/even effect is predominant and controlled the
assembly through solvophobic effect. FTIR spectroscopy confirms
the formation of intermolecular hydrogen bonding within self-
assembly of alkyl-TPE (1–4; 10 M), i.e. shift of N-H stretch
vibrations from 3328 cm-1 to 3309–3302 cm-1 increasing shift occur
with the increasing number of carbon in the alkyl chain (Fig. S8).
Figure 7. SEM images of alkyl-TPE (1–4) microstructures showing the
size and shape evolution of the supramolecular structures of alkyl-TPE
aggregates after solvent evaporation. (A) Nanotubes, (B) twisted ribbons,
(C) multilayered twisted ribbons, and (D) helical ribbons. SEM images of
alkyl-TPE (1–4) microstructures showing the size and shape evolution of the
supramolecular structures of alkyl-TPE molecules aggregates after solvent
evaporation. (A) Nanotubes, (B) twisted ribbons, (C) multilayered twisted
ribbons, and (D) helical ribbons.
The mechanistic stepwise microstructures growth via self-
assembly of solvated molecules due to solvophobic effects is
presented schematically in Fig. 8.
Conclusion
In summary, we have demonstrated the controlled chiral self-
assembly of alkyl-substituted achiral TPE derivatives bearing alkyl
chains with odd and even number of methylene. The CD
spectroscopy confirms the formation of right-handed, twisted chiral
supramolecular structures from alkyl(even)-TPE and left-handed
structures from alkyl(odd)-TPE. SEM images clearly support
formation of chiral assemblies, and teh structure due to aggregation
varies, including chiral tubes, twisted ribbons, and helical ribbons.
The perfect, controlled morphological features of the assembled
structures of alkyl-TPE derivatives make them attractive for nano-
and bio-material research. We are currently working on use of these
controlled structures for chiral recognition and separation.
Experimental Section
General Methods & Materials: TPE, chloroform (CHCl3), chloroform-d (CDCl3),
methanol (MeOH), dichloromethane (DCM), Tetrahydrofuran (THF), N,N’-
dimethylformamide (DMF) were purchased from Aldrich and used without purification,
Figure 8. Schematic illustration of the aggregation process of alkyl-TPE,
where (A) shows well solvated racemic molecules, (B) small chiral
aggregates of alkyl-TPE, (C, D) large single molecule sheets, (E) sheets are
folded to form chiral nanotubes, (F) when the diameter of the nanotubes is
too high to support the structure, the tube collapses and forms a twisted
ribbon, and (G) is the stacking of many twisted ribbons resulting in a helical
ribbon microstructure.
unless otherwise specified. Fluorescence measurements were performed on
a
FluoroMax-4, Horiba Jobin Yvon, equipped with an injector port, a stirrer and a
temperature controller (25°C). 1H NMR, 13C-NMR spectra were recorded on a Bruker
spectrometer using CDCl3 and MeOD as solvent and tetramethylsilane as an internal
standard. The solvents for spectroscopic studies were of spectroscopic grade and used as
received. Mass spectra (MS) were obtained by using Bruker AutoFlex Matrix Assisted
Laser Desorption/Ionisation (MALDI) Time of Flight (TOF)-Mass Spectrometer
(MALDI-TOF-MS). The X-ray diffraction (XRD) pattern spectra were performed on a
Bruker D8 FOCUS diffractometer using a Cu target radiation source (λ= 0.15418 nm).
These supramolecular structures varied in size and dimension,
where the smallest observed feature we can see is a wavy tubular
formation. The tubular structures hierarchically assemble to form
the larger ribbon like structures due to the solvophobic interactions
as graphically illustrated in Fig. 8. The observed widths of the
ribbons are significantly wider than the estimated diameter of alkyl-
TPE, suggesting that the ribbons are composed of bundles of stacks
(Fig. 6 and 7). The alkyl-TPE bearing alkyl chains with an odd
number of methylene units (C7 and C9) produced left-handed twisted
ribbons with a longer winding period, and in many instances rather
flat ribbon structures (Fig. 6A and 6C). This is evidence that the
optical activity of the aggregates occur on the molecular level, and
the handedness on the microstructure level is carried over from the
Spectroscopic measurements
UV–Vis measurements: UV–Vis absorption spectra were recorded in a Cary-50, and
UV–Vis spectrometer in 1cm path length cuvette. A 0.2 mL aliquot of the stock solution
of alkyl-TPE (conc. = 10-3 M) from THF was transferred to various ratios ACN/THF in
different volumetric flasks, and made up to 2 mL volume. The solutions were allowed
to equilibrate for 2 h prior to the spectroscopic measurements.
Fluorescence Measurements: Fluorescence emission spectra were recorded on a
Horiba Jobin Yvon FluoroMax®-4–Spectrofluorometer. Fluorescence measurements
and quenching experiments were performed on a FluoroMax-4 equipped with an
injector port and stirrer at 25°C. All experiments were performed in a quartz cell with a
1 cm path length with 365 nm excitation wavelength.
5
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10.1002/chem.201605458
Chemistry - A European Journal
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6
This article is protected by copyright. All rights reserved.

10.1002/chem.201605458
Chemistry - A European Journal
We describe the combination of
achiral tetraphenylethylene, an AIE-
active luminophore bearing alkyl
chains with either an odd or even
number of carbons via an amide
linkage. These molecules self
asemble into chiral supramolecular
structures and well-defined helical
superstructures due to a solvophobic
effect. Right handed supramolecular
structures were produced from an
even number of C in alkyl chains,
whereas an odd nuber of C led to left-
handed superstructures.
An odd effect in self-assembly
Anuradha., D. D. La , M. Al
Kobasis, A. Gupta, S. V. Bhosale*
………...… Page – Page
Chiral assembly of AIE active
achiral molecules: An odd effect
in self-assembly
7
This article is protected by copyright. All rights reserved.