Modulating supramolecular helicity and electrical conductivity of perylene dyes through an achiral alkyl chain

Article abstract of DOI:10.1039/c4cc03563a

Here we have shown that it is possible to modulate hydrophobic and hydrophilic interactions to further adjust the supramolecular helicity by changing the length of an alkyl chain. These modulated supramolecular interactions have been translated to and observed in electrical conductivity, and provide a new pathway to construct a high-performance nanodevice. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03563a

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Modulating supramolecular helicity and electrical
conductivity of perylene dyes through an achiral
alkyl chain†
Cite this: Chem. Commun., 2014,
50, 8343
Received 12th May 2014,
Accepted 12th June 2014
Yongwei Huang,*a Junchao Wangab and Zhixiang Wei*b
DOI: 10.1039/c4cc03563a
www.rsc.org/chemcomm
Here we have shown that it is possible to modulate hydrophobic modulation of aggregate chirality of chiral amphiphiles are scarce,4
and hydrophilic interactions to further adjust the supramolecular although it has been well-known that a small modification of the
helicity by changing the length of an alkyl chain. These modulated achiral group may cause significant variations in the generation and
supramolecular interactions have been translated to and observed morphologies of chiral assemblies.5
in electrical conductivity, and provide a new pathway to construct a
high-performance nanodevice.
In the present study, we designed a series of sugar-based
perylenediimide derivatives (PTCDIs) bearing various achiral alkyl
chains (Scheme 1). The aggregation behaviour in THF–H2O solvent
The helix is a basic structural motif in natural biomacromolecules. systems and the electrical conductivity of the corresponding aggre-
Such distinct helical structures of biomacromolecules motivated gates were investigated. The combination here with a hydrophilic
chemists to construct various types of artificial helical supramolecular carbohydrate residue6 and a hydrophobic alkyl chain provides, at the
nanostructures from small molecules with controlled helicity that same time, a source of supramolecular chirality and driving forces for
mimic the structure and function of biological helices.1 Many studies the self-assembly of PTCDI molecules. Moreover, Density Functional
on helical aggregations have unveiled various fascinating phenomena, Theory (DFT) calculations revealed that the length of the alkyl chain is
among them, a reversible helix inversion between right-handed and also affected by the rotation angle of a stacked dimer that ultimately
left-handed has been discovered to be one of the most interesting and leads to a difference in changing electrical conductivity.
unique features of dynamic helical aggregation.2 Generally for synthetic
To discuss the structural dependence of the aggregate chirality
organic molecules, an external stimulus causes reversible helix clearly, it is helpful to first consider the solution-state characteristics.
inversion.2 However, chiroptical inversion is a particularly intriguing The absorption spectra of OBAG, DBAG and HBAG in dilute THF
phenomenon, and the relationship between the higher order struc- (a good solvent) show three pronounced peaks in the range 450–
ture and the chiroptical characteristics needs to be fully understood. 525 nm (Fig. S1 in ESI†). This is a typical spectrum of individual
Alkyl chains are the most widely used solubilizing groups in PTCDI molecules in solution (i.e., in a non-aggregated form), and
organic materials.3 On the one hand, alkyl chains can improve corresponds to the electronic p–p* transition superimposed with
the solubility of organic molecules in some organic solvent. On the vibrational transitions.7 Accordingly, no obvious Cotton effect in
other hand, especially for p-conjugated organic semiconductors, circular dichroism (CD) spectra was observed in THF solution
alkyl chains possess strong hydrophobic interactions in polar (Fig. S1 in ESI†), which fits well with absorption spectra.
solvent that can tune solute–solvent interactions to determine the
However, THF solution of OBAG, DBAG and HBAG containing
self-assembled process and aggregation morphology. Hence, it is H2O (a poor solvent) shows remarkable bisignate CD signals. Fig. 1
expected that chiral amphiphiles with various lengths of alkyl chains shows the CD and UV-vis spectra at various THF/H2O volume
may adopt right-handed and/or left-handed aggregation mode by the ratios. When H2O was gradually added into the THF solution,
modulation of the hydrophilic and hydrophobic interactions. How-
ever, reports concerning the effect of an achiral alkyl chain on the
a Medical College, Henan University, Kaifeng 475004, China.
E-mail: hywei79@126.com
b National Center for Nanoscience and Technology, Beijing 100190, China.
E-mail: weizx@nanoctr.cn
† Electronic supplementary information (ESI) available: Synthesis of OBAG,
DBAG and HBAG and additional UV-vis, CD, TEM, XRD and electrical conductivity.
Scheme 1 Chemical structures of OBAG, DBAG and HBAG.
See DOI: 10.1039/c4cc03563a
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the H2O content to 80 vol%, the absorption peak at 520 nm
disappeared and a new shoulder peak at 580 nm was observed for
OBAG and DBAG, indicating strong p–p stacking between PTCDI
molecules in 20/80 THF/H2O solution. Strong p–p stacking was also
found in X-ray diffraction (XRD) patterns of OBAG, DBAG and HBAG
nanowires/fibers obtained from 20/80 THF/H2O (v/v). As shown in
Fig. S3 (ESI†), a diffraction peak was observed for all PTCDI molecules
in the wide angle region at 2y = B25.41 with a d-spacing of 3.5 Å,
which is characteristic of the p–p stacking between p-conjugated
molecules.8 Extended p–p stacking along the long-axis of the nano-
wire/fiber has been proven to be effective for mediating charge
transfer through p-electron delocalization.9 Indeed, a dramatic
increase in conductivity was found for OBAG, DBAG and HBAG
nanowires/fibers in hydrazine vapour (see the discussion below).
To gain more insight into the origin and mechanism of the helix
inversion, it is crucial to examine the effect of alkyl chains on the
induction of helical sense. DFT calculations were performed to
investigate the thermodynamic process to form such chiral archi-
tectures. Due to the steric hindrance of the galactosyl groups and
alkyl chains, the adjacent molecules should rotate a certain angle to
reach the conformation with the minimal system energy. With most
energetically preferable molecules (Fig. S4 in ESI†) coaxially
arranged, the energy dependence on the rotation angle was calcu-
lated and is presented in Fig. 2.
Fig. 1 (a, c, e) CD and (b, d, f) UV-vis spectra for OBAG, DBAG and HBAG,
respectively, at different volume ratios of THF/H2O (0.05 mg mLÀ1).
As shown in Fig. 2, the binding energy of the system changes
from negative to positive with the rotation angle changing from 01 to
a well-resolved bisignate CD signal with a positive Cotton effect at 901and from 01 to À901. The positive binding energy means that the
430 nm and negative at 460 nm was observed at a THF/H2O volume system is unstable, whereas the negative binding energy means that
ratio of 40/60 (Fig. 1a). The bisignate positive/negative Cotton effect the system is stable. Detailed analysis reveals that the molecule of
with increasing wavelength indicates that the OBAG molecules OBAG, DBAG and HBAG with the right-handed rotation angle of 321,
adopt a left-handed helical arrangement during aggregation.7 With 281 and 301, respectively, has a lower binding energy (partially
a further increase of the H2O content to 80%, OBAG aggregation enlarged image in Fig. S5 in ESI†), indicating that such a structure
exhibited the opposite behaviour in terms of chirality, which is most stable. It is interesting to note that a rotation angle of À321,
indicated a right-handed helical arrangement in 20/80 THF/H2O.7 À301 and À311 (left-handed) for OBAG, DBAG and HBAG, respec-
Similar experimental results were also observed in the CD spectra of tively, was also revealed, and the system has a less stable local
HBAG in THF–H2O solutions in spite of a longer length of an alkyl minimum with a positive binding energy. This result suggests that
chain substituted at the imide nitrogen (Fig. 1e). In contrast, no although it is not the most thermodynamically preferred state, it still
chiroptical inversion effect was found for DBAG in THF–H2O can occur kinetically. Compared to DBAG, the energy difference in the
solutions, and a bisignate negative/positive Cotton effect suggests right-handed and left-handed state is smaller for OBAG and HBAG,
that HBAG molecules adopted a right-handed helical arrangement implying that left-handed aggregation was more easily achieved.
(Fig. 1c). The results were further supported by TEM examination,
and nanowires or nanofibers with corresponding helicity are shown
in Fig. S2 (ESI†). Comparing CD data and molecular structures
among these PTCDIs showed that only the length of the alkyl
chain changes, which indicates that a unique alkyl chain length-
dependent chiroptical inversion effect was achieved.
A significant difference was also observed in UV-vis spectra.
As shown in Fig. 1b, the intensity ratio of the absorption peak at
450 nm to that at 520 nm faintly increased in 40/60 THF/H2O solution
compared to that in THF (Fig. S1 is ESI†), indicating the weak
aggregation occurring between OBAG molecules.7 But, the intensity
ratio was dramatically increased compared to OBAG for DBAG and
HBAG in 40/60 THF/H2O solution (Fig. 1d and f), implying the onset
of strong aggregation. Moreover, a new peak was observed at
Fig. 2 Simulations to investigate the energy dependence on rotation of
the OBAG, DBAG and HBAG in a stacked dimer. The upper molecule
rotates with the rotation angle changing from 01 to 901 (right-handed) and
approximately 580 nm for HBAG, suggesting effective p–p stacking
from 01 to À901 (left-handed). The binding energy equals the total energy
of the dimer minus the energy of the two separate molecules.
in the cofacial PTCDI molecules (Fig. 1f). With a further increase of
8344 | Chem. Commun., 2014, 50, 8343--8345
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hydrazine through electron donor–acceptor complexation with
PTCDIs, followed by efficient long range electron transport through
the p–p delocalization (see Fig. S7 in ESI†).10 It is should be noted
that p–p overlap of vicinal PTCDI molecules also affects the electron
transport mobility, and evidence suggests that charge carrier
mobility is usually maximized along the direction of cofacial p–p
stacking of PTCDI molecules (i.e., the rotation angle between
vicinal perylene molecules is equal to zero).11 As revealed by DFT
calculations, DBAG molecules possess a smaller rotation angle
between vicinal perylene molecules compared to OBAG and HBAG.
The smaller rotation angle for DBAG (281 or À301) implies that a
larger p–p overlap of the vicinal perylene core is expected to
enhance electron mobility and lead to the most dramatic increase
Fig. 3 Current modulation (I/I0)–time (t) curves of OBAG, DBAG and
HBAG devices obtained from THF/H2O (20/80, V/V) in hydrazine vapor.
This provides theoretical support for our experimental observation in conductivity (about 5 orders). While a larger rotation angle for
of the left-handed aggregation for OBAG and HBAG in 40/60 THF/ OBAG (321 or À321) and HBAG (301 or À311) will reduce the p–p
H2O solution. Keeping the presented data in mind, we can now overlap of the vicinal perylene core so as to weaken electron
proceed to detailed analyses of remarkable differences in CD spectra transport mobility and ultimately lead to approximately 3 and
by the following arguments. As shown in Scheme 1, the amphiphiles 1.5 times smaller mobility for OBAG and HBAG with increasing
in our study are unique in that their amphiphilic structure consists magnitude of conductivity than that of DBAG.
of a perylene scaffold with a hydrophobic alkyl chain and a hydro-
In summary, we have shown that it is possible to modulate
philic galactosyl residue on either side. When dissolved in THF, the hydrophobic and hydrophilic interactions to further adjust the
alkyl chain, with addition of H2O, will aggregate due to its hydro- supramolecular helicity by simply changing the length of the
phobic effect in the latter solvent. In contrast, the hydrophilic alkyl chain. These modulated supramolecular interactions have
interactions that root in multiple O–HÁÁÁO hydrogen bonds between been translated to and observed in electrical conductivity, and
the OH groups from the galactosyl residue and water molecules will provide new pathways to construct a high-performance nano-
be enhanced. Concerning the OBAG in 40/60 THF/H2O solution, the device based on designed supramolecular interactions.
hydrophilic interaction will be dominant due to a shorter alkyl chain
We gratefully acknowledge the National Natural Science Founda-
that cannot lead a strong hydrophobic effect. This is supported by tion of China (No. 21202036 and 21125420), China Postdoctoral
the UV-vis spectrum and it suggests that weak aggregation was Science Foundation (No. 2013M530335) and Henan University for
achieved in 40/60 THF/H2O solution. In contrast, the hydrophobic financial support.
effect may direct the aggregation of HBAG in 40/60 THF/H2O
solution because of the longer alkyl chain that was also consistent
with the corresponding UV-vis spectrum. Hence, it is inferred that a
single dominated driving force, namely the hydrophilic or hydro-
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