Frustrated 2D molecular crystallization

Article abstract of DOI:10.1021/ja0742535

Grafting isophthalic acid groups to linear connectors produces tetracarboxylic acids 2-4, which are extended analogues of trimesic acid (1). Normal pairwise association of -COOH groups induces trimesic acid to form a hexagonal network held together by six

Full text of DOI:10.1021/ja0742535

Published on Web 10/19/2007
Frustrated 2D Molecular Crystallization
Hui Zhou,† Hung Dang,† Ji-Hyun Yi,‡ Antonio Nanci,‡ Alain Rochefort,§ and James D. Wuest*,†
De´partement de Chimie and Faculte´ de Me´decine Dentaire, UniVersite´ de Montre´al, Montre´al, Que´bec H3C 3J7,
Canada, De´partement de Ge´nie Physique, EÄ cole Polytechnique de Montre´al, Montre´al, Que´bec H3A 3A7, Canada
Received June 11, 2007; E-mail: james.d.wuest@umontreal.ca
Molecules with well-defined shapes and multiple sites that engage
in strong directional interactions are now widely used to build new
ordered materials by design.1 Such molecules, which have been
called tectons,2 associate spontaneously to form networks with
architectures that can often be predicted in detail. Tectons are a
prolific source of engineered three-dimensional (3D) and two-
dimensional (2D) crystals with predefined structures and properties.3
Figure 1. Portions of networks built from trimesic acid (1) and tetraacids
2-4, with 1,3,5-trisubstituted phenyl groups represented by triangles and
hydrogen bonds shown as broken lines.
Trimesic acid (1) and its salts are prototypic tectons.4 3D and
2D crystallization of the acid is typically directed by association
Figure 2. STM images of the 2D crystallization of tectons 2 (Figure 2a)
of the three -COOH groups as cyclic hydrogen-bonded pairs, which
generates the planar hexagonal network represented by structure I
(Figure 1).4,5 Expanded versions of the network can be built from
molecules derived from triacid 1 by inserting spacers between the
-COOH groups and the 1,3,5-trisubstituted phenyl core.6 However,
the utility of tecton 1 and its extended analogues is limited by (1)
the inability of normal pairwise association of the -COOH groups
to program the construction of networks other than polymorph I7
and (2) the small number of hydrogen bonds (6) in which each
molecule can participate.
Related tetraacids 2-4, as well as other derivatives with multiple
isophthalic acid groups grafted to suitable cores, can provide planar
networks that are both richer in variety and more robust. In
particular, we have found that 2D crystallization of tectons 2-4
can be controlled to produce two polymorphs, parallel network II
or Kagome´ network III.8 Moreover, the special connectivity of these
networks allows their growth to be interrupted by a smooth
transition to the alternative polymorph, without necessarily intro-
ducing defects in which tectons are missing, improperly oriented,
or unable to form the optimal number of hydrogen bonds with
neighbors. We show that this can frustrate crystallization, particu-
larly when tectons 2-4 are mixed.
and 4 (Figure 2b) on HOPG (deposition from heptanoic acid, with Vbias
)
-1.5 V and Iset ) 50 pA). The unit cells are highlighted in light blue, the
Kagome´ network formed by tecton 4 is shown in dark blue, and molecular
models are superimposed as an aid to visualization.
boxylate and 1,4-diethynylbenzene into extended tecton 4 in 68%
overall yield.
In a typical 2D crystallization, a droplet of a saturated solution
of tecton 2 in heptanoic acid was placed on freshly cleaved
highly oriented pyrolytic graphite (HOPG) at 25 °C, and the
resulting physisorbed assembly was imaged by scanning tunneling
microscopy (STM) in the constant-height mode (Figure 2). Figure
2a reveals the formation of an open parallel network in which the
tectons associate by hydrogen bonding according to motif II (Figure
1), with unit cell parameters a ) b ) ∼1.36 nm and γ ) ∼90°.
This interpretation is reinforced by the observation of homologous
sheets in the 3D crystal structure of tecton 2.9 In the 3D structure,
tecton 2 resembles other biphenyls by adopting a nonplanar
conformation in which the torsional angle between the aryl rings
is approximately 40°. Prominent differences of contrast within each
molecule in Figure 2a may indicate that tecton 2 also adopts a
twisted conformation in its 2D crystals.12
In contrast, adsorption of extended tecton 4 on HOPG was found
to produce a Kagome´ network (Figure 2b and motif III in Figure
1), with unit cell parameters a ) b ) ∼3.43 nm and γ ) ∼60°.
This surprising selectivity cannot be explained by differences in
the density of packing or in the number of hydrogen bonds formed
per tecton (8), which are identical for both motifs II and III. Instead,
the preferences must arise from small variations in the strengths of
the hydrogen bonds between the adsorbates or from other subtle
effects related to solvation or interactions of the adsorbates with
each other and with the underlying surface.
Tetraacid 2 was prepared by the reported method,9 and elongated
analogues 3 and 4 were synthesized by standard procedures.10
Specifically, Sonogashira coupling of diethyl 5-iodo-1,3-benzene-
dicarboxylate with diethyl 5-ethynyl-1,3-benzenedicarboxylate,11
followed by hydrolysis, provided tecton 3 in 71% overall yield.
An analogous route converted diethyl 5-iodo-1,3-benzenedicar-
† De´partement de Chimie, Universite´ de Montre´al.
‡ Faculte´ de Me´decine Dentaire, Universite´ de Montre´al.
§ EÄ cole Polytechnique de Montre´al.
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10.1021/ja0742535 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
smoothly from motif II to motif III without introducing defects,
and (2) the structural similarity of tectons 3 and 4, which allows
them to be interchanged without completely interrupting the growth
of an extensively hydrogen-bonded network. Further experiments
revealed that 2D crystallization of tecton 4 can be thwarted by the
addition of less than 15% of compound 3.
Our work demonstrates that a deep understanding of 2D and 3D
molecular assembly can be attained by an integrated approach using
the tools of X-ray diffraction, STM, computation, and molecular
synthesis. In particular, our results provide new guidelines for
designing 2D and 3D molecular glasses,16 as well as detailed models
for their structures and the phenomenon of polyamorphism.
Furthermore, our work confirms that molecules with well-defined
shapes and multiple sites that engage in strong directional interac-
tions are a consistently productive source of new materials with
properties not previously observed.
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council of Canada, the Canadian Institutes
of Health Research, the Ministe`re de l’EÄ ducation du Que´bec, the
Canada Foundation for Innovation, and the Canada Research Chairs
Program for financial support.
Figure 3. (a) STM image of the assembly produced by the adsorption of
tecton 3 on HOPG (deposition from heptanoic acid, with Vbias ) -1.5 V
and Iset ) 50 pA). (b and c) Higher-resolution images of the areas in Figure
3a highlighted in green and blue, respectively, with superimposed models.
These images show regions of local order according to motifs II (parallel
network) and III (Kagome´ network), as well as the smooth transition
between them.
According to a recent study of the 2D crystallization of tectons
on HOPG, directional interactions between adsorbates can be more
important than diffuse interactions with the surface in determining
the ultimate structure and its orientation.13 Extensive hydrogen
bonding of tectons 2-4 should make interadsorbate interactions
particularly important. Indeed, the tetraethyl esters of tectons 3-4
produced distinctly different nanopatterns on HOPG. Moreover,
networks derived from tectons 2 and 4 did not show a preferred
orientation with respect to the underlying surface.
Supporting Information Available: Syntheses of compounds 3-4,
detailed description of the calculations, and additional STM images.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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the Kagome´ alternative III; in contrast, elongated analogue 4 is
estimated to favor the Kagome´ structure by 0.42 kcal/mol.
For intermediate compound 3, however, the two possible
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tion of tecton 3 on HOPG generated a nanopattern strikingly unlike
those produced by analogues 2 and 4 (Figure 3a). In the new pattern,
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because they are held in place by multiple hydrogen bonds. Motifs
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local order (Figure 3b,c); however, large-scale periodicity apparently
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assembly shows a high degree of order, and most molecules have
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The frustrated crystallization of tecton 3 suggested that the 2D
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HOPG under the standard conditions failed to yield large periodic
domains, as determined by STM.10 Again, the images showed
clearly resolved molecules oriented locally according to motifs II
and III; however, sustained growth of single crystalline phases was
not observed. We attribute the frustrated crystallization to a
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