Directional hydrogen bonding controlled 2D self-organization of phenyleneethynylenes: From linear assembly to rectangular network

Article abstract of DOI:10.1039/b927281j

The effect of the carboxylic acid substituent position of phenyleneethynylene systems on the 2D self-organization is revealed by scanning tunneling microscopy.

Full text of DOI:10.1039/b927281j

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Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 20 | 28 May 2010 | Pages 3405–3616
ISSN 1359-7345
COMMUNICATION
Ramesh and Thomas
FEATURE ARTICLE
Directional hydrogen bonding
controlled 2D self-organization of
phenyleneethynylenes: from linear
assembly to rectangular network
Hak-Fun Chow et al.
Conformational and supramolecular
properties of main chain and cyclic
click oligotriazoles and polytriazoles

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COMMUNICATION
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Directional hydrogen bonding controlled 2D self-organization
of phenyleneethynylenes: from linear assembly to rectangular networkw
A. R. Ramesh and K. George Thomas*
Received 24th December 2009, Accepted 23rd March 2010
First published as an Advance Article on the web 20th April 2010
DOI: 10.1039/b927281j
The effect of the carboxylic acid substituent position of
phenyleneethynylene systems on the 2D self-organization is
revealed by scanning tunneling microscopy.
(i) varying the alkyl chain length and (ii) by introducing
hydroxyl and aldehyde groups at the terminal positions.10
Intermolecular hydrogen bonding between carboxylic acids
is highly selective and directional.12 This type of hydrogen
bonding is effectively utilized in various molecular systems for
self-organization.13 Two phenyleneethynylene derivatives
possessing carboxylic acid groups at the para and meta
positions (p-acid and m-acid) were synthesized using a
Heck–Cassar–Sonagashira–Hagihara cross coupling reaction
(ESIw).
STM investigations of p-acid and m-acid were carried out by
drop casting 0.1 mM solution in a 9 : 1 mixture of 1,2,4-
trichlorobenzene and THF on to a freshly cleaved HOPG
surface, followed by drying in air. A well organized linear wire
like arrangement was observed for p-acid at various locations
with clear domain boundaries (Fig. 1A). High resolution STM
current images recorded at two different domains, at a scan
size of 6 ꢁ 6 nm2 are presented in Fig. 1B,C. Due to the high
electron density, phenyleneethynylene core units are observed
as bright rod like structures, with an average length of
1.8 ꢂ 0.1 nm. The alkyl chain regions of phenyleneethynylene
derivatives appeared as dark due to the large energy difference
between the electronic states of the aliphatic chain and the
Fermi level of the substrate.10 In contrast to the skewed type
arrangement observed for unsubstituted phenyleneethynylenes,
p-acid showed a linear arrangement which arises from two
types of basic arrangement of molecules: (i) a parallel strip-like
arrangement of phenyleneethynylenes along the ‘a’ axis
(a-strip) and (ii) a linear arrangement of phenyleneethynylenes
resulting from an end to end interaction along the ‘b’ axis
(b-strip).
Nature has utilized various noncovalent interactions for the
fabrication of nanostructures which can perform specific
biofunctions that maintain life on earth.1 Inspired by the
remarkable functions of natural nanostructured systems,
efforts have been focused in recent years on the construction
of molecular materials having potential applications as
components in optoelectronic devices.2 However, the design
of molecular materials with predictable composition,
structure, and function still remains a major challenge. An in
depth understanding of the arrangements of molecules, with
atomic resolution can provide valuable information on various
noncovalent interactions3 that control and direct their self-
organization to materials.4 Herein we report the role of
directionality of hydrogen bonding in the 2D self-organization
of phenyleneethynylenes possessing carboxylic acid groups at
the terminal positions (Scheme 1) by using scanning tunneling
microscopy (STM).
Phenyleneethynylenes are an interesting class of rigid rod
molecular systems which maintains p-electron conjugation at
any degree of rotation of phenyl rings and is proposed as
linker for electronic communication in molecular electronic
devices.5,6 Phenyleneethynylenes possess tunable optical
properties both in solution and in the solid state.5 They are
proposed as components in optoelectronic devices; for
example, efficient blue electroluminescence was achieved in a
single layer OLED using
a phenyleneethynylene based
polymer system.7 For solid state device applications, it is
extremely important to understand the self-organization of
phenyleneethynylenes,8 since the interchromophoric inter-
actions significantly influence their optoelectronic properties.9
Recent studies on the self-organization of phenyleneethynylenes,
by our group10 and others,11 have revealed that these systems
can be well organized on surfaces. Based on our studies, it is
concluded that the organization of phenyleneethynylenes on
2D surfaces is mainly driven by alkyl CHꢀ ꢀ ꢀacetylenic p
interactions (CHꢀ ꢀ ꢀp), which can be further modulated by
Based on our earlier studies it is well understood that the
parallel arrangement of phenyleneethynylenes along the
a-strip arises through the alkyl CHꢀ ꢀ ꢀacetylenic p interactions
(CHꢀ ꢀ ꢀp; CH from the terminal –CH3 of the hexyloxy moiety)
and interdigitation of the hexyloxy chain.10 The average
distance between identical points on the ‘a’ axis is 1.3 ꢂ 0.1 nm
which suggests that there is enough room for accommodating
hexyloxy chains (B0.89 nm in the extended conformation).
With respect to the alkyloxy groups, two modes of CHꢀ ꢀ ꢀp
interactions are possible: either from the ortho or meta positions
with an angle of orientation of 65 ꢂ 51 and 85 ꢂ 51,
respectively.10 Images of p-acid presented in Fig. 1B and 1C
Photosciences and Photonics, Chemical Sciences and Technology
Division, National Institute for Interdisciplinary Science and
Technology (NIIST), CSIR, Trivandrum 695 019, India.
E-mail: kgt@vsnl.com; Fax: +91 471 2491712;
Tel: +91 471 2515364
w Electronic supplementary information (ESI) available: Details of
synthesis, and crystal data for p-acid. CCDC 758463. For ESI and
crystallographic data in CIF or other electronic format see
DOI: 10.1039/b927281j
Scheme
1
Chemical structure of p-acid and m-acid under
Chem. Commun., 2010, 46, 3457–3459 | 3457
investigation.
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Fig. 2 (A) ORTEP view of the single crystal structure of the p-acid
obtained from DMSO solvent. (B) Sliced packing diagram of the
crystal structure of the p-acid along a plane showing a skewed
organization due to the capping of DMSO molecules on to the
carboxylic acid.
Fig. 1 (A) STM current image of p-acid on HOPG showing linear 1D
organization with clear domain boundary, scan size 40 ꢁ 40 nm2;
V
bias = ꢄ500 mV; It = 653 pA. High resolution STM current image of
(B) ortho arrangement: scan size 6 ꢁ 6 nm2; Vbias = ꢄ570 mV;
It = 405 pA. and (C) meta arrangement: scan size 6 ꢁ 6 nm2;
Vbias = ꢄ500 mV; It = 653 pA.
intermolecular hydrogen bonding between the carboxylic acid
groups. The terminal carboxylic acid groups of p-acid are
solvated with the DMSO molecules through strong hydrogen
bonding between the O(2)Hꢀ ꢀ ꢀO0(4) in which the distance
between the H and O(4) atoms is 1.77 A. Interaction between
a carboxylic acid group and DMSO was reported earlier for a
dehydrobenzo[12]annulene based molecular system.14 In the
present case, DMSO acts like a cap for the p-acid at both ends
and the p-acid.2DMSO assemblies were further interlocked to
a skewed arrangement as presented in Fig. 2B. The capped
DMSO molecules further act as the glue for the construction
of the 3D structure through two weak hydrogen bonding
interactions: (i) C00(19)Hꢀ ꢀ ꢀO(3) and (ii) C00 0(14)Hꢀ ꢀ ꢀO0(4).
Trapping of the solvent molecules is difficult in a monolayer
of p-acid on the HOPG surface and the linear arrangement of
molecules was observed in an STM image resulting from the
dimerization of carboxylic acid groups.
show an orientation angle of 641 and 871 along the ‘a’ axis
indicating that the interactions do occur through the ortho and
meta positions, respectively. The strong intermolecular hydrogen
bonding interactions between the carboxylic acids along the ‘b’
axis result in the formation of a linear 2D organization of
p-acid (Scheme 2).
Attempts were made to crystallize p-acid from various
solvents to obtain a correlation between their organizations
on surface and crystal. Single crystals of p-acid were obtained
from DMSO. The ORTEP structure and packing diagram
along a plane are presented in Fig. 2. The shortest inter-
molecular contact distance C(1)Hꢀ ꢀ ꢀC0(11) of 3.23 A and the
intermolecular distance between identical points of 1.3 nm
indicate that the CHꢀ ꢀ ꢀp interactions assist the strip
formation. An earlier report on the crystal structure of
unsubstituted phenyleneethynylene (OPE) showed that there
are two major interactions: (i) CHꢀ ꢀ ꢀp interactions and
(ii) interdigitation of alkyl chains.5 In the present case, the
meta CHꢀ ꢀ ꢀp interactions with an orientation angle of 891
assist the strip formation of the p-acid. However, we have
observed that the DMSO solvent molecules are entrapped
during the crystallization of p-acid which prevents the
STM imaging of phenyleneethynylene possessing carboxylic
acids at the meta positions (m-acid) was carried out under
similar conditions in order to understand the role of the
directionality of hydrogen bonding. In contrast to the regular
linear arrangement for p-acid, a distinctly different organization
was observed for m-acid over a large area with a rectangular
network having periodic cavities (Fig. 3A). High resolution
imaging was carried out at lower scan size (13 ꢁ 13 nm2) to
understand the molecular arrangement in the network.
A periodic rectangular network having cavity dimensions
of B(2.3 ꢁ 2.5) nm2 was observed in the high resolution
images (Fig. 3B) which are aesthetically appealing. One of the
noticeable points is that the m-acid possesses the same
functionality as that of p-acid, however directionality of the
carboxylic acid group played a significant role in their 2D
organization. The shortest intermolecular distance between
two parallel molecules in the image is 2.3 nm, which rules
out the possibility of CHꢀ ꢀ ꢀp interactions and alkyl chain
interdigitation. In the absence of these interactions, hydrogen
bonding directed by the carboxylic acid at the meta position is
the main driving force. The phenyleneethynylene core
appeared as bright rods on the four sides of the rectangle and
multiple hydrogen bonding between the terminal carboxylic
acid units at the meta position holds the molecules as a
Scheme 2 Schematic representation of the 2D organization of p-acid
into linear wires through (A) ortho and (B) meta interaction.
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Fig. 3 STM current images of m-acid showing rectangular network type organization: (A) scan size: 45 ꢁ 45 nm2; Vbias = ꢄ663 mV; It = 672 pA;
(B) scan size: 13 ꢁ 13 nm2; Vbias = ꢄ730 mV; It = 544 pA. (C) Schematic representation of the 2D molecular arrangement for m-acid into a
rectangular network.
rectangular network having periodic cavities. Multiple hydrogen
bonding can occur at the corners of the rectangle in several
ways and a possible arrangement is presented in Fig. 3C. It is
interesting to note that the alkyl chains do not play a
significant role in the organization of m-acid as in the case of
OPE and p-acid. The crystal structure of m-acid can provide
more information on the 3D organizations, however our
attempt to crystallize the molecule was not successful. The
rectangular porous network on the surface is regular and the
cavities having dimensions of B(2.3 ꢁ 2.5) nm2 are an
excellent template for host molecules such as fullerenes.
In summary, we have demonstrated that the position of
carboxylic acid plays a crucial role in the self-organization of
phenyleneethynylenes on a surface. Two point hydrogen bonding
interactions between the carboxylic acid groups lead to a linear
wire type arrangement for p-acids and a periodic rectangular
network having cavity dimensions of B(2.3 ꢁ 2.5) nm2 for
m-acids. Multiple hydrogen bonding between the terminal
carboxylic acid units at the meta position holds phenylene-
ethynylenes as a rectangular network having periodic cavities.
Thus, the organization of the phenyleneethynylene building
block is greatly influenced by the position of the functional
group and an understanding of the optical and electronic
properties of these assemblies is essential for designing next
generation molecular devices.
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We acknowledge financial support from CSIR (NWP-23)
and the Indo-Italian Program of DST, Government of India.
SAIF, IITM, Chennai is gratefully acknowledged for single
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