Molecular assembly of amino acid interlinked, topologically symmetric, π-complementary donor-acceptor-donor triads

Article abstract of DOI:10.3762/bjoc.9.178

Amino acid interlinked pyrene and naphthalenediimide (NDI) based novel donor-acceptor-donor (D-A-D) triads are designed to exploit their topological symmetry and complementary π-character for facile charge-transfer complexation. Consequently, free-floatin

Full text of DOI:10.3762/bjoc.9.178

Molecular assembly of amino acid interlinked,
topologically symmetric, π-complementary
donor–acceptor–donor triads
M. B. Avinash, K. V. Sandeepa and T. Govindaraju*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1565–1571.
Bioorganic Chemistry Laboratory, New Chemistry Unit, Jawaharlal
Nehru Centre for Advanced Scientific Research, Jakkur,
Bangalore-560064, India. Fax: +91 80 2208 2627; Tel: +91 80 2208
2969
doi:10.3762/bjoc.9.178
Received: 23 April 2013
Accepted: 08 July 2013
Published: 01 August 2013
Email:
T. Govindaraju* - tgraju@jncasr.ac.in
Associate Editor: T. P. Yoon
* Corresponding author
© 2013 Avinash et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
amino acids; charge transfer; designer functional molecules;
molecular assembly; molecular recognition
Abstract
Amino acid interlinked pyrene and naphthalenediimide (NDI) based novel donor–acceptor–donor (D-A-D) triads are designed to
exploit their topological symmetry and complementary π-character for facile charge-transfer complexation. Consequently, free-
floating high-aspect-ratio supercoiled nanofibres and hierarchical helical bundles of triads are realized by modulating the chemical
functionality of interlinking amino acids.
Findings
Geometric shapes, size and patterns have attracted the inquisi- essentially dependant on their molecular assembly [11-16], it is
tive human mind since time immemorial [1,2]. The signifi- extremely important to identify suitable donor–acceptor
cance of geometric complementarity and chemical functionality sequences for efficient energy or charge-transfer processes. It
can be exemplified by the well-known “lock and key” mech- is with this conception that we have designed pyrene based
anism of enzymes as well as the prototypal molecular recogni- D-A-D triads due to the topological symmetry of pyrene with
tion manifested in biochemical processes [1-4]. It is this that of NDI (Figure 1).
elementary yet elegant design strategy involving topological
symmetry of molecular structure that is explored in this current A variety of D-A systems [17-33] comprising pyrene and/or
communication. Herein, we have employed NDI as the func- NDI have been reported in the literature, viz., supramolecular
tional molecule due to its potential applications in field-effect n/p heterojunctions [17], adhesive π-clamping [18], catenane
transistors, photovoltaics and flexible displays [5,6]. Since the [19], rotaxane [20], self-healable supramolecular polymer [21],
performance of electro-active molecular materials [7-10] is aromatic stacking within a coordination cage [22], super-
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Figure 1: Molecular structures of donor–acceptor–donor traids. The red colouring highlights the topological symmetry of NDI and pyrene.
amphiphile [23] and thermochromic [24] materials. Notably, region of 270–400 nm due to characteristic π–π* transitions of
Wilson et al. have reported preferential π-stacking of pyrene pyrene and NDI (Figure 2a). With the successive addition of
and NDI amongst a pool of π-electron D-A combinations, when water to the solution of 1 in DMSO, a decrease in absorption
tethered to a polymer or oligomer [2]. This preferential aromatic intensity as well as broadening of the bands was observed.
interaction was proposed to be a consequence of frontier orbital Interestingly, at 40% aqueous DMSO, a red shift of ~13 nm was
congruence for the HOMO of pyrene with the LUMO of NDI. observed for the absorption band at 346 nm (λmax). However, at
Alternatively, NDI is one of the highly π-acidic molecules with 80% aqueous DMSO a blue shift of ~8 nm was observed with a
an estimated molecular quadrupole moment (Qzz) of +18.6 B net red shift of ~5 nm with respect to the absorption maxima in
(Buckinghams), which is in the range of the explosive TNT DMSO. This could be ascribed to the formation of offset type
[34]. This π-acidic behaviour of NDI is an interesting property, stacking of the triad due to solvatophobic interactions at 40%
because most of the aromatic compounds are π-basic, viz., aqueous DMSO, whereas the enhanced hydrophobic forces at
benzene has Qzz = −8.5 B, whereas pyrene has Qzz = −13.8 B. 80% aqueous DMSO reduce the offset by forming a stronger
Thus, pyrene and NDI represent a unique D-A system due to charge-transfer complex. Similar variations in the absorption
topological symmetry in their molecular structure and comple- spectra were observed for 2 in aqueous DMSO, while 3 exhibits
mentary π-character. Since NDI is one of the most promising only an incremental red shift of absorption maxima without any
organic n-type semiconductors, we anticipated that alternative indication of the existence of two different (described above)
stacking with a special donor such as pyrene would lead to kinds of aggregates as observed for 1 (see Supporting Informa-
facile charge-transfer interactions. Moreover, 1D organization tion File 1).
of D-A stacks could bring about significant improvement in the
conductivity through directional charge transfer. Herein, we On the contrary, in aqueous N-methyl-2-pyrrolidone (NMP)
have chosen three D-A-D triads (1–3) of pyrene and NDI inter- solvent system, 1 undergoes a decrease in absorption intensity
linked by different amino acids, namely alanine, phenylalanine until 60% aqueous NMP followed by a net red shift of ~5 nm at
and isoleucine (Figure 1). The choice of amino acids as linkers 80% aqueous NMP (see Supporting Information File 1). How-
is due to their compactness, variable side chain functionality ever, 2 and 3 in aqueous NMP exhibited only an incremental
and sequence-specific self-assembling properties and the desire red shift of absorption maxima similar to that observed in the
to exploit their inherent chiral information for helical assembly case of 3, in aqueous DMSO (see Supporting Information
of electro-active molecular materials [35-48].
File 1). In addition, the fluorescence emission for 1, 2 and 3 was
found to be quenched in DMSO and NMP as well as in their
A detailed procedure for the synthesis of the triads 1, 2 and 3 is aqueous medium (Figure 2b). Interestingly, in the case of 2,
given in Supporting Information File 1. Briefly, 1,4,5,8-naph- (both in aqueous DMSO as well as in aqueous NMP) a new
thalenetetracarboxylic acid dianhydride was condensed with weak emission band at ~460 nm was observed, which was
amino acids (alanine/phenylalanine/isoleucine) to give the attributed to the excimer of pyrene (see Supporting Information
respective symmetrically substituted NDIs, which were then File 1). In addition, UV–vis spectra clearly show a new broad
subjected to amide coupling with pyrenemethylamine to obtain absorption band in the range of 450 nm to 700 nm confirming
the triads (1–3) in good yield. UV–vis absorption spectra of 1 in the charge-transfer complexation of pyrene and NDI (Figure 2c)
dimethyl sulfoxide (DMSO) exhibit absorption bands in the [2]. Consequently, aqueous solutions of 1, 2 and 3 exhibited the
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Figure 2: (a) UV–vis and (b) fluorescence spectra of 1 (200 μM) in aqueous DMSO for various percentages of water in DMSO. Excitation at 345 nm.
For comparison fluorescence emission spectra of pyrenemethylamine hydrochloride is shown (black trace, pyrene) in (b). (c) UV–vis spectra of 1 mM
solutions of 1, 2 and 3 in 40% aqueous DMSO/NMP show the charge-transfer bands. The photograph shows the solid-state and free-floating aggre-
gates of 1, 2 and 3 with their characteristic colours due to charge-transfer complexation. A: 40% aqueous DMSO. B: 40% aqueous NMP.
effect of interlinking (amino acid) chemical functionality by phores. However, CD studies for 2 (Figure 3d) and 3 (Figure 3f)
their characteristic dark red, light pink and brownish red colour- in NMP show positive excitonic cotton effects indicating P-type
ation, respectively (Figure 2).
helicity. In aqueous DMSO as well as in aqueous NMP both 2
and 3 tend to approach towards CD silencing. The relatively
In order to gain further insights into the nature of molecular bulkier side chain functionality of the interlinking amino acids
interactions, 1, 2 and 3 were subjected to circular dichroism in 2 and 3 is believed to inhibit the effective excitonic coupling
(CD) spectroscopic studies. The CD spectra of 1 in DMSO of the chromophores in lower percentages (≤ 60%) of the
show positive excitonic cotton effects, indicating a P-type aqueous solution. However, at higher percentages of the
helical assembly (Figure 3a). However, with increase in the aqueous solution the enhanced hydrophobic forces render tran-
percentage of water in DMSO a bisignated cotton effect was sition of the angles between transition dipole moments of the
observed in the range of 300–400 nm, besides the appearance of stacked chromophores towards zero, resulting in CD silencing.
two new bands at ~420 nm and ~550 nm. The band at ~420 nm Moreover, the relative differences between the CD features of 2
could be attributed to the aggregation band of NDI, while the and 3 in aqueous DMSO and aqueous NMP could be ascribed
band at ~550 nm is assigned to the charge-transfer complexa- to solvatophobic interactions.
tion of NDI and pyrene. With further increase in the percentage
of added water in DMSO, the amplitude of the CD signal 2D NOESY studies of 1, 2 and 3 in DMSO revealed the pres-
approaches zero, suggesting a probable CD silencing [37]. ence of weak aromatic interactions between pyrene and NDI
Similar changes in the CD spectra were observed for 1 in (see Supporting Information File 1). Moreover the amide NH
aqueous NMP (Figure 3b). In contrast, 2 and 3 show signifi- proton of 1, 2 and 3 showed spatial interactions with the proton
cantly distinct excitonic cotton effects (Figure 3c–f). In DMSO, of the α-carbon and the side chain functionality of the inter-
2 (Figure 3c) and 3 (Figure 3e) exhibit a weak CD signal above linking amino acid, which suggests a trans configuration for the
350 nm suggesting minimal excitonic coupling of NDI chromo- amide group. Powder X-ray diffraction (PXRD) studies for ag-
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Figure 3: CD spectra of (a) and (b) 1, (c) and (d) 2, and (e) and (f) 3 for various percentages of water in aqueous DMSO (a, c and e) and aqueous
NMP (b, d and f). Concentration of 1, 2 and 3 used was 400 μM.
gregates of 1, 2 and 3 showed a broad peak corresponding to
d-spacing of ~3.4 Å, indicating a strong aromatic stacking (see
Supporting Information File 1). Remarkably 1, 2 and 3 form
free floating aggregates within a couple of hours in aqueous
DMSO and aqueous NMP (Figure 2). These free floating aggre-
gates were subjected to morphological studies using field emis-
sion scanning electron microscopy (FESEM). FESEM images
of 1 in 40% aqueous NMP showed the presence of supercoiled
nanofibres of ~10 nm diameter and a helical pitch of ~30 nm
(Figure 4a). At 80% aqueous NMP, a dense network of
Figure 4: FESEM image of (a) 1 obtained from 40% aqueous NMP,
(b) 2 obtained from 80% aqueous NMP.
nanofibres of 1 was observed (see Supporting Information
File 1). Interestingly, at 80% aqueous NMP, 2 was found to
form hierarchical helical bundles of ~1 μm length and ~200 nm
diameter (Figure 4b). However, 3 showed only the presence of nature of assembly in general. In the case of alanine derivative
spherical or random aggregates both in aqueous DMSO as well 1, the amino acid side chain is a methyl group (surface area:
as in aqueous NMP (see Supporting Information File 1).
86 Å2), which renders minimal steric repulsion for the inter-
molecular D-A stacking due to its smaller size [49]. On the
In these triads, the primary molecular interaction for the other hand, the phenyl group (surface area: 194 Å2) in 2 renders
assembly is charge-transfer complexation. The amino acids, relatively stronger steric repulsion for intermolecular D-A
which are employed to link the donor (pyrene) and acceptor stacking, as evident from weaker CD signals (Figure 3c, 3d).
(NDI) modules can also influence the assembly due to its Similarly, the side chain of isoleucine (surface area: 155 Å2) in
hydrophobicity, steric bulkiness and helical propensity amongst 3 also facilitates weaker D-A stacking (Figure 3e, 3f). Unlike 1,
others. So when there is intermolecular stacking of donors and the much favoured orthogonal charge-transfer interactions of D
acceptors, the interlinking amino acid plays the crucial role in and A are hindered in the cases of 2 and 3, and hence would be
determining the extent of charge-transfer complexation and the expected to result in random architectures. However, the pres-
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ence of the phenyl functionality in 2 is believed to facilitate
some favourable aromatic interactions with NDI and pyrene
modules, and thereby results in hierarchical helical bundles.
Further, energy minimized molecular structures of the triads, 1
and 2 were obtained by using the Gaussian-09 software
(Hartree–Fock method with 3-21G basis set) to get additional
insights about their conformations (Figure 5a, 5b). After careful
evaluation of absorption, fluorescence, circular dichroism,
NMR as well as PXRD data, the plausible molecular stacking
model depicting the charge-transfer complexation under
enhanced hydrophobic forces thus obtained is shown in
Figure 5c and 5d.
Supporting Information
Supporting Information File 1
Experimental details, synthesis procedures, FESEM
images, absorption, photoluminescence, PXRD and
2D NOESY spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-178-S1.pdf]
Acknowledgements
Authors thank Prof. C. N. R. Rao, FRS for constant support and
encouragement; JNCASR, DST, IYBA-DBT and the govern-
ment of India for financial support; Prof. G. U. Kulkarni for
the FESEM facility; Satyanarayana Bonakala for computa-
tional studies; and Selvi and Mahesh for their help in various
measurements.
Conclusion
In conclusion, we have designed and elucidated the molecular
assembly of amino acid interlinked novel D-A-D triads. Herein,
a relatively weaker noncovalent interaction like charge-transfer
complexation has been achieved by employing a topologically
symmetric and π-complementary pyrene and NDI. Consequent-
ly, free-floating high-aspect-ratio supercoiled nanofibres and
hierarchical helical bundles of triads are realized by modulating
the chemical functionality of interlinking amino acids. Such
molecular-recognition processes embodied in pertinent designer
functional molecules are believed to pave the way for program-
mable molecular assemblies with advanced applications.
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