Engineering self-assembled fluorescent organic nanotapes and submicrotubes from π-conjugated molecules

Article abstract of DOI:10.1039/b925459e

Fluorescent elongated nanotapes and nearly monodispersed short submicrotubes were successfully prepared in a controlled manner from two tailor-made π-conjugated organic building blocks 1 and 2, respectively in dichloromethane solvent via a supramolecular

Full text of DOI:10.1039/b925459e

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Engineering self-assembled fluorescent organic nanotapes
and submicrotubes from p-conjugated moleculeswz
Naisa Chandrasekhar and Rajadurai Chandrasekar*
Received (in Cambridge, UK) 2nd December 2009, Accepted 12th February 2010
First published as an Advance Article on the web 9th March 2010
DOI: 10.1039/b925459e
Fluorescent elongated nanotapes and nearly monodispersed
short submicrotubes were successfully prepared in a controlled
manner from two tailor-made p-conjugated organic building
blocks 1 and 2, respectively in dichloromethane solvent via a
supramolecular self-assembly approach.
The design and preparation of 1-D functional organic
supramolecular nano and submicrostructures (tubes, fibres,
tapes/ribbons, and wires) by the hierarchical self-assembly of
organic building blocks with specific electronic properties
(magnetic, optical and electrical) are at the centre stage of
nanoscience and technology.1,2 Driving forces for self-assembly
are based on van der Waals interaction, hydrogen bonds,
coordination bonds, and p–p stacking.1,2 Recently, spontaneous
but controlled generation of well-defined 1-D fluorescent
nanotapes and nano/submicrotubes from organic building
blocks via electrostatic interaction mediated ‘‘bottom-up’’
self-assembly in certain solvents is of particular interest for
the construction of functional opto-electronic devices.3
Until now, most of the organic nano/submicrotubes were
Scheme 1 Illustration of the possible mechanism involved in the
fabricated from the self-assembly of disc-like molecules,4
dendrimers,5 amphiphiles6 and polymers.7 In contrary to big
molecules, small and low molecular weight organic molecules
offer good solubility, easy structural tunability and multi-
functionality. To the best of our knowledge only a few articles
report the synthesis of ‘‘fluorescent’’ organic nanotapes8 and
submicrotubes3a by the self-assembly of simple (low molecular
weight, p-conjugated, and non-amphiphilic) fluorescent
molecules adopting a simple slow solvent evaporation technique.
Herein, we report a controlled straightforward chemical
approach for the preparation of violet emitting elongated
nanotapes and monodispersed submicrotubes from two tailor-
made p-conjugated, non-amphiphilic, fluorescent molecules 1
and 2, respectively, via supramolecular H-bond mediated
self-assembly (Scheme 1). We also present the electron
(SEM/TEM) and confocal fluorescence microscopy images
of the nanostructures and also their optical properties
both in solution and solid state. Compound 1 was synthesized
as per our previously reported procedure.9 Molecule 2
was prepared by reacting 4,40-biphenyldiboronic acid with
formation of elongated nanotapes and submicrotubes from 1 and 2,
respectively.
2,6-di-pyrazol-1-yl-pyridin-4-yl-iodide under Suzuki reaction
conditions in a modest 50% yield. Molecules 1 and 2 have
unique structural features for hierarchical supramolecular self-
assembly. Both molecules have (i) a rigid rod shaped series of
aromatic rings (for 1 and 2: Npyꢀ ꢀ ꢀNpy distance = 1.14 nm and
1.57 nm, respectively) as central segments and (ii) four
pyrazole groups connected to the four corners of each
segment. Examination of the single X-ray structure of 1
showed that intermolecular hydrogen bonding plays a vital
role in the supramolecular ordering and self-assembly.9a Each
pyrazole ring nitrogen participates in two types of inter-
molecular hydrogen bonding interactions (i) with segmental
phenyl ring protons (Nꢀ ꢀ ꢀHPh=2.529 A) and (ii) with pyrazole
ring protons (Nꢀ ꢀ ꢀHPz = 2.593 A) (ESIw, Fig. S2 and S3).
The UV/Vis absorbance and emission properties of 1 and 2
in dichloromethane (DCM) solvent is presented in Fig. 1 and
Table 1. Compound 1 showed two absorbance maxima at
250 nm and 283 nm. While 2 revealed two absorbance bands
one centred at 250 nm and the other slightly 31 nm red shifted
to give a maximum at 314 nm in contrast to 1 due to its longer
p-conjugation length. Excitation of 1 at lex = 283 nm showed
an emission maximum at 380 nm, whereas 2 displayed an
emission maximum at 378 nm with a shoulder at 364 nm for
the excitation performed at lex = 314 nm. Both solution and
solid (crystal and powder) samples of 1 and 2 exhibited strong
violet fluorescence upon excitation with UV light (365 nm).
School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road,
Gachi Bowli, Hyderabad – 500 046, India.
E-mail: rcsc@uohyd.ernet.in, chandrasekar100@yahoo.com;
Fax: +91(40)23134824; Tel: +91(40)23134824
w Electronic supplementary information (ESI) available: General
experimental details, crystal packing diagrams, and SEM images of
2. See DOI: 10.1039/b925459e
z Dedicated to Prof. M. Baumgarten on the occasion of his 52nd
birthday.
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2915–2917 | 2915

View Article Online
Fig. 1 Absorbance and fluorescence spectra of 1 and 2 in dichloro-
methane (c = 1 ꢂ 10ꢃ5 M). lex is 283 nm and 313 nm for 1 and 2,
respectively.
The solution state quantum yield of 1 (Ff = 0.39) is weaker
than 2 (Ff = 0.57).
The generation of self-assembled nanostrucutures of both 1
and 2 is a solvent mediated process. The variation in their
solubilty in DCM plays a crucial role in the final nanostructure
morphology as evident from the electron microscopy exam-
inations. At room temperature 1 readily dissolves in DCM,
but 2 is sparingly soluble. Especially for 2, in addition to
solubility the concentration is also one of the crucial factors
that determined the formation of sheet-like morphology,
completely grown tubular structures, and also their relatively
high monodispersity. In fact, we employed different concen-
trations to unravel the structural evolution of submicrotubes
and found that a concentration of ca. 1 ꢂ 10ꢃ3 M is ideal for
the tubular growth and its near monodispersity. We have
adopted two different techniques to reproduce the controlled
preparation of nanotapes and tubes from 1 and 2, respectively.
Firstly, slow evaporation of a clear sample solution (ca. 10ꢃ4 M)
at room temperature to obtain cloudy turbid solution.
Secondly, casting two drops of ca. 10ꢃ3 M sample solution
on the glass slide followed by slow evaporation at room
temperature.
Fig. 2 SEM micrographs of 1 (a) elongated nanotapes network (scale
bar is 2 mm), (b) stacked nanotapes are forming multi-layer micro-
bundles (scale bar is 2 mm) (c) individual nanotapes. White dotted
circle: helically twisted nanotape (scale bar is 1 mm) and Bright-field
TEM micrographs of 1. (d) Solid black arrow: multilayered nanotapes.
Black dotted circle: thin nanotapes with a helical twist (scale bar is
200 nm), (e) and (f) close-ended feature of a single tape (scale bars are
50 nm and 20 nm, respectively).
2 formed short tubes of lengths ca. 10 mm with varying widths
ranging from ca. 340 nm–1 mm. Fully grown tubes exhibited
relatively high monodispersity and smooth surfaces (Fig. 3a
and b). The open-ended features of the nanotubes are clearly
revealed in the tubular formation. The generation of sub-
microtubes possibly involved three major steps: (i) growth of
individual sheets to form precursor sheets (ESIw, Fig. S4
(a and b)), (ii) rolling up of precursor sheets to form a curled
sheets (ESIw, Fig. S4 (c–i), and (iii) seaming of the two
opposing edges of each curled sheet to form an open-ended
hollow tube (Fig. 3). In general the tendency of the above
process (curling of sheet, seaming and tube formation) is
energetically favoured since this route ideally lowers the excess
surface energy through reducing surface areas. Importantly,
each fully grown tube showed edges along the tube axis
(in Fig. 3(b) see black arrows) confirming the folding process
involved in the tubes growth. Additionally, a close look at the
open-ended features (Fig. 3 (d–m)) of different tubes showed
dissimilar features. The origin of diverse defects at the open-
ended features of each tube is probably due to the seaming of
the curled sheet having variation in lengths along the long
sheet axis (see, black arrows in Fig. 3 (d, f, and l) and ESIw,
Fig. S4 (b, d, and f)) and similar width. It is identified that
diverse dominant intermolecular interactions are responsible
Electron microscopy images shown in Fig. 2 and 3
exemplifies the remarkable difference in growth of nanotapes
and submicrotubes from two custom-made molecules such as
1 and 2, respectively. Building block 1 prefers to form long
nanotapes of a length extending up to 10 mm with widths
varying from ca. 75 nm to 130 nm. Each nanotape aggregates
in a stacked fashion forming a network of nanotape bundles of
ca. 2 mm width (Fig. 2b). Interestingly, both SEM and TEM
micrographs exhibited helical twists in the nanotapes (Fig. 1c
and d). TEM image of 1 clearly showed the difference between
thin (twisted) and multi-layered nanotapes (Fig. 2d). Some
thin nanotapes showed multiple helical twists. In contrast,
Table 1 Photophysical properties of 1 and 2
lmax in nm
Solution
Solida
Cmpd
labs in nm (e in Mꢃ1cmꢃ1
)
Ffb
1
2
250 (39884)
250 (51043)
283 (44014)
314 (69872)
380
364 and 378
452
452
0.39
0.57
a
b
Confocal microscopic excitation (lex = 250) data. Quinine sulfate
as reference (Qref = 0.577) and excitation at 331 nm for all samples.
ꢁc
This journal is The Royal Society of Chemistry 2010
2916 | Chem. Commun., 2010, 46, 2915–2917

View Article Online
In summary, we have demonstrated the controlled generation
of violet emitting elongated nanotapes and nearly mono-
dispersed submicrotubes from two tailor-made p-conjugated
molecules 1 and 2, respectively, differing only in their
conjugation lengths in an organic solvent during the slow
evaporation process via self-assembly. The driving force for
the nanotape and tube formation is intermolecular hydrogen
bonded interactions between pyrazole nitrogens and phenyl/
pyrazole protons. The evolution mechanism of the tubular
structure possibly involved a sequence of steps: (i) sheet-like
structure development forming a precursor sheet, (ii) curling
of a precursor sheet and (iii) seaming of the curled sheet
opposite edges to form a tube. These nanotapes and tubes
may find application in the field of nanohybrid materials,
optoelectronics and biological transport systems.
We are grateful to DST (Fast Track Scheme for Young
Scientist Grant No. SR/FTP/CS-115/2007), New Delhi for
financial support. Our special thanks to the Centre for
Nanotechnology (CNT), UoH for providing the TEM facility
and also CIL, UoH for SEM and Confocal Fluorescence
Microscopy facility. We extend our thanks to Prof. T. P.
Radhakrishnan and Prof. A. Samanta for providing the
instrument facility. NC thanks CSIR New Delhi for JRF.
We express our gratitude to the referees for their suggestions.
Notes and references
Fig. 3 SEM micrographs of 2 (a) submicrotubes (scale bar is 10 mm),
(b) front, back and side view of tubes. White dotted circle: front view
of an open ended feature. Black arrows: edges along the length of the
tubes (scale bar is 10 mm), (c) Left white dotted rectangle: an uncurled
precursor sheet-like structure (scale bar is 10 mm). (d) White arrows:
Defects in the open-ended feature. (e–m) Selected tubes showing their
different types of open-ended features (scale bars are 2 mm). (h) Side
view of a tube.
1 (a) J.-M. Lehn, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4763;
(b) G. W. Whitesides and B. Grzybowski, Science, 2002, 295, 2418;
(c) L. Brunsveld, B. J. B. Folmer, E. W. Meijer and R. P. Sijbesma,
Chem. Rev., 2001, 101, 4071; (d) D. T. Bong, T. D. Clark,
J. R. Granja and M. R. Ghadiri, Angew. Chem., Int. Ed., 2001,
40, 988; (e) A. P. H. J. Schenning and E. W. Meijer, Chem.
Commun., 2005, 3245; (f) L. C. Palmer and S. I. Stupp, Acc. Chem.
Res., 2008, 41, 1674; (g) A. Ajayaghosh and V. K. Praveen, Acc.
Chem. Res., 2007, 40, 644.
2 (a) A. Langner, S. L. Tait, N. Lin, C. Rajadurai, M. Ruben and
K. Kern, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 17927;
(b) S. L. Tait, A. Langner, N. Lin, R. Chandrasekar, M. Ruben
and K. Kern, ChemPhysChem, 2008, 9, 2495.
3 (a) Y. S. Zhao, W. Yang, D. Xiao, X. Sheng, X. Yang, Z. Shuai,
Y. Luo and J. Yao, Chem. Mater., 2005, 17, 6430; (b) N. Diaz,
F.-X. Simon, M. Schmutz, M. Rawiso, G. Decher, J. Jestin and P. J.
M. Lsini, Angew. Chem., Int. Ed., 2005, 44, 3260; (c) P. Samori,
V. Francke, K. Muellen and J. P. Rabe, Chem.–Eur. J., 1999, 5,
2312; (d) S. Allard, M. Forster, B. Souharce, H. Thiem and
U. Scherf, Angew. Chem., Int. Ed., 2008, 47, 4070;
(e) A. Ajayaghosh, C. Vijayakumar, R. Varghese and
S. J. George, Angew. Chem., Int. Ed., 2006, 45, 456; (f) Y. Tian,
Q. He, C. Tao and J. Li, Langmuir, 2006, 22, 360; (g) P. Xue, R. Lu,
G. Chen, Y. Zhang, H. Nomoto, M. Takafuji and H. Ihara,
Chem.–Eur. J., 2007, 13, 8231.
Fig. 4 Confocal fluorescence microscopic images of nanotapes 1
(scale bar is 10 mm) and submicrotubes 2 (scale bar is 15 mm). Left:
after excitation. Right: before excitation. Extreme Right: solid state
emission data of 1 and 2 (lex = 250).
4 (a) M. R. Ghadiri, J. R. Granja, R. A. Milligan, D. E. McRee and
N. Khazanovich, Nature, 1993, 366, 324; (b) S. Iijima, Nature, 1991,
354, 56; (c) P. M. Ajayan and T. W. Ebbeson, Rep. Prog. Phys.,
1997, 60, 1025; (d) L. Shu, M. Muri, R. Krupke and M. Mayor, Org.
Biomol. Chem., 2009, 7, 1081.
5 T. Yamaguchi, N. Ishii, K. Tashiro and T. Aida, J. Am. Chem. Soc.,
2003, 125, 13934.
6 G. John, M. Masuda, Y. Okada, K. Yase and T. Shimizu, Adv.
Mater., 2001, 13, 715.
¨
for the formation of organic nano- and submicrostructures
with distinctive shapes. The formation of nanotapes and
submicrotubes from 1 and 2, respectively is clearly mediated
by H bonding interactions.
Furthermore, the confocal fluorescence microscopy images
(Fig. 4) of 1 and 2 clearly displayed that both nanotapes and
tubes are indeed emitting violet colours with varying intensity
upon excitation with laser light (lex = 250). The solid state
emission intensity of 1 is lower than 2 which is in accordance
with their respective fluorescence quantum yields in solution.
7 E. N. Savariar, K. Krishnamoorthy and S. Thayumanavan, Nat.
Nanotechnol., 2008, 3, 112.
8 H. Ozawa, H. Tanaka, M. Kawao, S. Unoa and Nakazato, Chem.
Commun., 2009, 7411.
9 (a) C. Rajadurai, O. Fuhr, R. Kruk, M. Ghafari, H. Hahn and
M. Ruben, Chem. Commun., 2007, 2636; (b) S. Basak, P. Hui and
R. Chandrasekar, Synthesis, 2009, 23, 4042.
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2915–2917 | 2917