Diverse dimerization of molecular tweezers with a 2,4,6-triphenyl-1,3,5- triazine spacer in the solid state

Article abstract of DOI:10.1039/c0cc00454e

Molecular tweezers with a 2,4,6-triphenyl-1,3,5-triazine spacer exhibit diverse dimerization through π-π stacking interactions in the solid state, and these dimeric species form highly organized supramolecular networks.

Full text of DOI:10.1039/c0cc00454e

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Diverse dimerization of molecular tweezers with a
2,4,6-triphenyl-1,3,5-triazine spacer in the solid statew
Yosuke Hisamatsu and Hidenori Aihara*
Received 18th March 2010, Accepted 24th May 2010
First published as an Advance Article on the web 9th June 2010
DOI: 10.1039/c0cc00454e
Molecular tweezers with a 2,4,6-triphenyl-1,3,5-triazine spacer
exhibit diverse dimerization through p–p stacking interactions in
the solid state, and these dimeric species form highly organized
supramolecular networks.
Noncovalent interactions of aromatic units such as p–p stacking
interactions play significant roles in the stabilization and
regulatory functions of both natural (e.g. base stacking of
DNA and higher order structures of proteins) and synthetic
systems.1 In supramolecular chemistry1 and crystal engineering,2
controllable self-assembly process based on p-conjugated
molecules in the solid state has been a challenging research
topic and should provide an efficient bottom-up strategy for
Scheme 1 Synthesis of 1–4.
constructing supramolecular architectures, which should be
useful as supramolecular electronic materials.3 In comparison
with other conventional interactions, such as hydrogen-bonding
and metal coordination, the predictable use of weaker p–p
stacking, CH/p and van der Waals interactions for self-assembly
systems has remained a great challenge.
nitrogen atmosphere, and their high decomposition temperatures
(Td5, corresponding to 5% weight loss) indicated good thermal
stabilities (Td5 of 1: 474 1C, 2: 479 1C, 3: 517 1C and 4: 542 1C).
These results suggest that compounds 1–4 would be stable
under thermal conditions such as in sublimation for purification
and vacuum deposition for the fabrication of electronic
devices. Indeed, single crystals of 1–4 suitable for X-ray
diffraction analysis were obtained by a temperature-gradient
vacuum sublimation method.8
Molecular tweezers are host molecules with aromatic
pincers separated by spacers at a distance of approximately
7 A that can efficiently bind planar aromatic guests through
the formation of p-sandwich complexes.4 Most previous
studies on molecular tweezers have focused on the formation
of heterocomplexes between the tweezers and planar guest
molecules.4
The X-ray crystal structure of 1 shows a syn orientation of
the anthracene pincers (intercentroid distance between the two
anthracene rings = 7.08 A). As shown in Fig. 2, compound 1
forms a unique self-complementary dimeric structure in which
a TPT spacer (p-acceptor) of 1 is sandwiched between the
In this communication, we report that new molecular
tweezers 1–4 exhibit diverse dimerization through p–p stacking
interactions in the solid state by combining a 1,3,5-triphenyl-
2,4,6-triazine (TPT)5 spacer and anthracene or acridine
moieties. Furthermore, these dimeric species form highly
organized supramolecular networks.6
Molecular tweezers 1 and 3 were readily prepared by a
Pd(0)-catalyzed Suzuki–Miyaura cross-coupling reaction7 of
2,4-bis(3-bromophenyl)-6-phenyl-1,3,5-triazine (5) or 2,4-bis-
(3-bromophenyl)-6-(4-bromophenyl)-1,3,5-triazine (6) with
9-anthryl boronic ester. For the introduction of acridine
moieties, compounds 5 and 6 were converted to diboronic
ester (7) and triboronic ester (8), followed by Suzuki–Miyaura
coupling reaction with 9-bromoacridine to give 2 and 4,
respectively (Scheme 1). The thermal stabilities of 1–4 were
investigated by thermogravimetric analysis (TGA) under a
Fig. 1 Molecular structures of 1–4.
Sagami Chemical Research Center, Hayakawa, Ayase 252-1193,
Japan. E-mail: aihara@sagami.or.jp; Fax: +81 467-77-4113;
Tel: +81 467-77-4112
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis and NMR studies of 1–4. CCDC
770283–770286. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc00454e
Fig. 2 X-Ray structure of dimer 1Á1.
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anthracene pincers (p-donors) of a neighboring molecule.9,10
The distance between the anthracene mean plane and the
centroid of the triazine ring is 3.47 A, which indicates p–p
stacking interactions between the anthracene pincers and TPT
moiety. The dimer 1Á1 (Fig. 3a) forms an infinite rhombic grid
2-D network sheet via slipped p–p stacking interactions
between the anthracene moieties of dimeric species 1Á1
(Fig. 3b). Simultaneously, phenyl groups attached to the
6-position of the triazine ring of dimer 1Á1 act as supra-
molecular tenons (Fig. 3a), which are slightly twisted (dihedral
angle between the phenyl plane and triazine plane = 141,
Fig. 2), and attach to supramolecular mortises (Fig. 3b) of the
rhombic grid sheet by CH/p and van der Waals interactions.
As a result, dimeric species 1Á1 form a highly organized 3-D
network (Fig. 3c) by the infinite mortise and tenon stacking
(Fig. 3d) of rhombic grid sheets.
The X-ray crystal structure of 2 with acridine pincers, which
are more electron-deficient than anthracene,4c,j also shows a
syn orientation of the two acridine rings (intercentroid
distance between the acridine pincers = 7.81 A). As shown
in Fig. 4a, compound 2 forms a self-complementary dimeric
structure11 in which one acridine ring of 2 is sandwiched face-
to-face between the acridine pincers of a neighboring molecule
with an antiparallel orientation so as to maximize the dipole–
dipole interactions (distance between acridine A (mean
plane)–B (centroid): 3.54 A, B (mean plane)–C (centroid):
3.53 A and C (centroid)–D (mean plane): 3.54 A).4c,d This
result indicates that the self-complementary dimerization of 1
and 2 depends on the p-electron property of the pincers. In
dimer 2Á2, a TPT spacer also forms p–p stacking with that of a
neighboring molecule with an antiparallel orientation. As a
result, dimeric species of 2 exhibit a tightly packed molecular
arrangement due to the good overlapping of p-orbitals
(Fig. 4b).
Fig. 4 X-Ray structure of (a) dimer 2Á2 and (b) molecular arrangement
of 2Á2.
Acenes are promising candidates in molecular electronics,
especially in organic field-effect transistors (OFETs).12
A
herringbone (edge-to-face) structure with the minimal over-
lapping of p-orbitals in the solid state is shown in most of the
crystal packing of the acenes.13 Therefore, to achieve greater
charge carrier transport capability, an approach to enforce
face-to-face p–p stacking interactions of acenes has been an
important topic in OFETs.14 We expected that the introduction
of an additional anthracene moiety might compete with the
self-complementary dimerization of 1 and facilitate the
formation of a p-sandwich complex between the anthracene
moieties. Thus, we designed compound 3 as shown in Fig. 1.
The X-ray crystal structure of 3 (intercentroid distance between
anthracene pincers = 7.49 A) shows an infinite 1-D network
structure based on a head-to-tail dimeric structure (Fig. 5),
Fig. 3 X-Ray structure of (a) monomer 1 and dimer 1Á1, (b) infinite rhombic grid network formed from 1Á1, (c) 3-D network structure formed
from 1Á1 and (d) tenon and mortise stacked structure of 1Á1.
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5 1,3,5-Triphenyl-2,4,6-triazine derivatives are expected to have
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crystalline materials,5d,e and nonlinear optical materials,5f see:
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Fig. 5 X-Ray structure of 3 forming an infinite 1-D network based on
head-to-tail dimerization.
rather than the self-complementary mode as in 1Á1 (Fig. 2).
The distances between the anthracene mean plane and the
centroid of each anthracene pincer are 3.51 and 3.59 A, which
indicates the formation of a p-sandwich complex between the
anthracene moieties (Fig. 5). This result suggests that the
formation of an infinite 1-D polymeric network promotes
face-to-face p–p stacking interactions. A similar result was
obtained in the case of 4, in which the anthracene moieties of 3
are replaced by acridine moieties (see ESIw). The X-ray crystal
structure of 4 (intercentroid distance between the acridine
pincers = 7.59 A) also reveals the formation of an infinite
1-D network structure by p-sandwich complexes between the
acridine moieties (distances between the acridine mean plane
and the centroid of each acridine pincer are 3.53 A and
3.61 A).
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9 This type of self-complementary dimerization has rarely been
encountered either in the solid state or in solution, see:
(a) T. Korenaga, Y. Kawauchi, T. Kosaki, T. Ema and T. Sakai,
Bull. Chem. Soc. Jpn., 2005, 78, 2175–2179; (b) T. Haino, T. Fujii,
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U. P. Spitz, L. Toledo, S. Kubik and J. J. Rebek, J. Am. Chem.
Soc., 1995, 117, 12733–12745.
In summary, we have demonstrated the diverse dimerization
of molecular tweezers 1–4 through combination with a 1,3,5-
triphenyl-2,4,6-triazine spacer and anthracene or acridine
moieties in the solid state.15,16 The dimeric species make it
possible to form well-organized supramolecular networks
through p–p stacking, CH/p and van der Waals interactions.
These results represent an important example for the construction
of higher order structures based on the diverse dimerization of
p-conjugated molecules. We are trying to develop supra-
molecular electronic materials5 that have a unique assembled
state, and the application of these tweezers, especially 3, to
OFETs is ongoing.
10 Head-to-tail 1-D structure4c,9c due to the formation of the
p-sandwich complex between an anthracene donor and TPT
acceptor might be prevented by the phenyl groups attached to
the 6-position of the triazine ring on 1.
11 (a) D. A. McMorran and P. J. Steel, Chem. Commun., 2002,
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15 When a CDCl3 solution of 1 at 25 1C was diluted from 2.0 mM to
0.4 mM, negligible changes (o0.01 ppm) were observed in the
chemical shift values. A similar result was obtained in the case of 3.
We were unable to perform a dilution study of 2 or 4 in CDCl3
because of their poor solubility. In addition, changes in the
chemical shifts of 1–4 were not observed at 50 1C. These results
indicate minimal contribution of self-assembly of 1–4 in CDCl3.
We thank Dr M. Yamazaki, Rigaku Corporation, for help
in X-ray analysis of compound 1.
Notes and references
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16 Compound
1 and 2,4,7-trinitrofluorenone (TNF) formed a
p-sandwich complex in CDCl3. The stoichiometry of the complex
1ÁTNF was confirmed to be 1 : 1 by a Job plot.16a Due to the
conformational flexibility of the TPT spacer, the Ks value of 1 : 1
complex 1ÁTNF (Ks = 80 MÀ1 at 25 1C)16b was 30-fold lower than
that of a molecular tweezer with a pyridine–pyrimidine–pyridine
spacer.4c(a) P. Job, Ann. Chim. Appl., 1928, 9, 113–203; (b) the Ks
value was determined by the non-linear least-squares curve-
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Interscience, New York, 1987.
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This journal is The Royal Society of Chemistry 2010
4904 | Chem. Commun., 2010, 46, 4902–4904