Induction and control of columnar mesophase by charge transfer interaction and side chain structures of tetrasubstituted pyrenes

Article abstract of DOI:10.1080/154214090960081

The induction of the columnar phase is described in which intrinsically crystalline 1,4,6,9-tetraacyloxypyrenes have been mixed with tetranitrofluorenone, TeNF, as an electron acceptor. The pyrene derivative with linear nonyl chains, PC 9, has not shown any mesophases using trinitrofluorenone, suggesting that stronger charge transfer interaction is favored by TeNF. The shape of the substituents works to control the mesophase type: the mixture of PC 9 and TeNF has formed a rectangular columnar phase, and a hexagonal columnar phase is attained by rounder derivatives bearing branched side chains.

Full text of DOI:10.1080/154214090960081

Mol. Cryst. Liq. Cryst., Vol. 451, pp. 65–74, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/154214090960081
Induction and Control of Columnar Mesophase
by Charge Transfer Interaction and Side Chain
Structures of Tetrasubstituted Pyrenes
Takuji Hirose
Osamu Kawakami
Mikio Yasutake
Department of Applied Chemistry, Saitama University,
Sakura, Saitama, Japan
The induction of the columnar phase is described in which intrinsically crystalline
1
,4,6,9-tetraacyloxypyrenes have been mixed with tetranitrofluorenone, TeNF, as
an electron acceptor. The pyrene derivative with linear nonyl chains, PC , has
9
not shown any mesophases using trinitrofluorenone, suggesting that stronger
charge transfer interaction is favored by TeNF. The shape of the substituents
9
works to control the mesophase type: the mixture of PC and TeNF has formed a
rectangular columnar phase, and a hexagonal columnar phase is attained by
rounder derivatives bearing branched side chains.
Keywords: charge transfer interaction; columnar mesophase; side chain structures;
tetranitrofluorenone; tetrasubstituted pyrenes
INTRODUCTION
The organization of molecular materials is attracting a lot of interest.
In recent years, self-assembly has been recognized as a powerful
method of molecular organization in various fields [1]. Highly orga-
nized molecular assembly is important in nanotechnology as well as
in biological systems [2]. Various interactions are applied for self-
assembly and, among them, p–p interaction is a well known but
powerful tool to build up one-dimensional aromatic architecture [3].
In spite of the fact that one-dimensional stacking is a simple architec-
ture, the liquid crystalline (LC) one, an ordered columnar phase, is
Address correspondence to Takuji Hirose, Department of Applied Chemistry,
Saitama University, 255 Shimo-ohkubo, Sakura, Saitama 338-8570, Japan. E-mail:
hirose@apc.saitama-u.ac.jp
65

6
6
T. Hirose et al.
important for various applications [4] due to its anisotropic and
thermotropic properties, especially for electronic applications [3b,
5
–9]. However, the methodology to intentionally manifest the LC
phase has not been well established. Here we report on the induction
and control of the columnar mesophase from crystalline pyrene deriva-
tives by taking advantage of the electronic and structural characteris-
tics of 2,4,5,7-tetranitro-9-fluorenone, TeNF [10].
Most discotic compounds showing a columnar phase have large
and highly symmetric aromatic cores resulting from high melting tem-
peratures [4–6] and less solubility in common organic solvents [11].
In this study, therefore, we chose pyrene, which has an aromatic core
of C symmetry and designed some derivatives to achieve columnar
2
LC phases of lower melting temperatures. As far as we know, only a
few simple pyrene derivatives are known to have low melting
temperatures [7,8].
RESULTS AND DISCUSSION
The typical synthetic procedure for pyrene derivatives is shown in
Scheme 1 [7,12]. Simple crystalline esters PC (n ¼ 8, 9, 11) [7] were
n
prepared with long linear alkyl halides while branched side chains
were introduced as PCm.n (m.n ¼ 3.3, 2.4, 5.5) to reduce the p–p inter-
actions of the central cores. The DSC measurements showed, however,
that they were all crystalline and I-Cr transition temperatures were
higher for PC
than PC except PC (Table 1). The transition of
n 3.3
m.n
ꢀ
PC3.3 at 71 C was expected to be a crystal–crystal one from polarizing
optical microscopic (POM) observation. Although the effect of
SCHEME 1 Synthesis of tetrasubstituted pyrenes.

Induction and Control of Columnar Mesophase of Tetrasubstituted Pyrenes
67
Cr
TABLE 1 Thermal Properties of PC and PCm.n
n
Pyrene derivatives
I
X
PC
PC
8
9
.
.
.
.
.
.
76 (36.8)
67 (49.1)
72 (46.5)
172 (48.1)
130 (23.5)
151 (35.8)
–
–
–
.
–
–
.
.
.
.
.
.
PC11
PC3.3
PC2.4
PC5.5
71 (35.2)
ꢀ
ꢁ1
ꢀ
ꢁ1
PC
I: Isotropic, X: Undetermined, Cr: Crystalline.
n
: On cooling at 5 C min , PCm.n: On cooling at 10 C min .
branched side chains is ambiguous, Table 1 still shows that they
ꢁ1
reduce the transition enthalpy about 10J g .
It is known that thermotropic LCs can be formed when 2,4,7-
trinitro-9-fluorenone, TrNF [13], is mixed with discotic compounds,
which do not show mesomorphism themselves [14–16]. Electron-rich
discotic compounds are expected to form charge transfer (CT) com-
plexes with electron-deficient TrNF. In this study, PC and TrNF were
9
mixed in 2:1, 3:2, and 1:1 ratios to obtain a red solid. Although the
color change suggested the formation of CT complexes, no mesophases
were observed for any mixtures. The interaction between PC and
9
TrNF may not be strong enough to form stable cores for LC phases.
Accordingly, TeNF was prepared [10] to increase the interaction
with the pyrene derivatives. The mixture of PC and TeNF gave a dark
9
brown solid in various ratios, suggesting stronger CT interaction than
that between PC and TrNF. Their thermal properties were studied
9
with DSC and POM, and the results are summarized in Table 2. Table
2
suggests the LC phase for 2:1 ꢂ 1:1 PC –TeNF mixtures, and they
9
were all Col phases as demonstrated by the mosaic and sea grass
r
textures for the 1:1 PC –TeNF mixture (Fig. 1). The 4:3 mixture
9
ꢀ
was found to have the most stable LC phase between 67–112 C.
To compare the difference between PC -TrNF and PC -TeNF, the
9
9
UV-vis spectra of each 1:1 mixture were measured in the solid state,
and they are shown in Fig. 2. Both spectra reach to the visible region
(
above 450 nm), which proves CT, considering that none of PC , TrNF,
9
and TeNF have absorption in the same region. The kmax at longer
wavelength (555nm) for PC -TeNF indicated a smaller energy gap
9
between the HOMO of PC and the LUMO of TeNF than that for
9
PC -TrNF (k
¼ 501 nm). Higher electron acceptability of TeNF
9
max
seems to affect the CT from PC , making a stable core and leading
9
to the LC phase emergence. The HOMO-LUMO energy gaps were

6
8
T. Hirose et al.
TABLE 2 Thermal Properties of the Mixtures of PC , PC3.3, PC2.4, and
9
PC5.5-TeNF
Pyrene derivatives PC
PC
n
–TeNF ratio
I
Col
r
Col
h
Cr
9
4:1
.
.
.
.
.
.
.
.
.
61 (45.9)
83 (9.0)
112 (19.4)
75 (5.3)
75 (6.6)
56 (35.6)
–
.
.
.
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
5:3
4:3
6:5
5:5
5:6
66 (41.5)
67 (27.6)
66 (46.3)
67 (35.4)
a
b
.
–
.
.
.
PC3.3
PC2.4
PC5.5
1:1
4:3
1:1
138 (5.6)
102 (5.2)
156 (5.1)
88 (24.9)
40 (16.7)
84 (19.1)
ꢀ
ꢁ1
.
On cooling at 5 C min
r h
I: Isotropic, Col : Discotic rectangular columnar, Col : Discotic hexagonal columnar,
Cr: Crystalline.
a
Phase separation of I and Col
h
was observed.
b
Phase separation of Col
h
and Cr was observed.
estimated for tetraacetyloxy pyrene, PC , TrNF, and TeNF by PM3
1
molecular orbital calculations [17]. The computations were performed
with MM2 and MOPAC 97 programs. The HOMO energy of PC was
1
ꢁ
7.98eV and the LUMO energy levels were ꢁ2.94 and ꢁ3.34eV for
TrNF and TeNF, respectively.
Although all mixtures were crystalline at rt, no single crystals
were obtained for the present PCs in Table 1. To obtain structural
FIGURE 1 Mosaic (a) and sea grass (b) textures of Col
at 122 C.
r
of PC
9
-TeNF (1:1)
ꢀ

Induction and Control of Columnar Mesophase of Tetrasubstituted Pyrenes
69
FIGURE 2 UV-vis spectra of (a) PC
9 9
-TeNF (1:1), (b) PC -TrNF (1:1), (c) TeNF,
(
d) TrNF, and (e) PC in their crystalline phases.
9
information, therefore, PC1 was prepared, and the mixtures with
TrNF and TeNF have colors similar to PC -TrNF and PC -TeNF.
9
9
Their single crystals were analyzed by X-ray crystallography and
the Ortep diagrams for PC -TrNF and PC -TeNF are shown in
1
1
Figs. 3 and 4, respectively.
For PC -TrNF, the columnar structure of their rings was formed in a
1
1
:1 ratio with PC and TrNF aromatic rings parallel to each other. The
1
˚
distance was 3.34A but TrNF was disordered (Fig. 3). In addition, two
characteristics were found: (1) the long axes of PC1 and TrNF
are almost crossed with each other and (2) the center of PC deviated
1
from that of TrNF. These should be due to the asymmetrical charge
density of the LUMO. On the other hand, the PC -TeNF crystal struc-
1
ture was constructed in a 1:2 ratio, even prepared from the 1:1 mixture.
The units were arranged in a zigzag way along the b axis direction, and
the unit was perpendicular to the neighboring units, as shown in Fig. 4.
In addition, three interesting features were observed: (1) the long
axes of PC and TeNF are almost eclipsed, but (2) the PC and TeNF
1
1
rings are not parallel to each other with the center-to-center distance
˚
of 3.84A, whereas (3) the two TeNF rings are parallel. The symmetrical
structure and the four bulky nitro groups of TeNF should be their
cause but the reason for the 1:2 composition ratio is unexplained at
present.

7
0
T. Hirose et al.
9
FIGURE 3 Ortep diagram of PC -TrNF (a) and view showing the frame
format of Ortep (b) and crystal packing diagram (c) of the PC -TrNF (1:2)
9
complex.
Based on the result of PC , PC5.5 with branched side chains was also
9
blended with TeNF. The PC5.5:TeNF mixture in a 1:1 ratio was observed
to have a hexagonal columnar phase Col by POM (Fig. 5). The phase
h
transition behaviors are summarized in Table 2. In the PCm.n
–
TeNF composition range studied, that is, between 2:1 and 2:3, all
mixtures of PC2.4, PC3.3, and PC5.5 formed Col phases (Table 2).
h
Table 2 shows a couple of interesting features of PC-TeNF mixtures:
(
1) the rectangular PC derivative with linear side chains like PC9
formed a Colr phase (Fig. 6a); (2) rounder PC3.3 and PC5.5 with
branched side chains turned to the Col phase (Fig. 6b) with higher
h
transition temperatures; and (3) PC2.4, having asymmetrical branches
with the same carbon number as PC , showed the lowest Col -Cr
3
.3
h
ꢀ
transition temperature at 40 C. It is noteworthy that the LC-Cr
transition temperatures were higher for all PCm.n with smaller

Induction and Control of Columnar Mesophase of Tetrasubstituted Pyrenes
71
9
FIGURE 4 Ortep diagram of PC -TeNF (a) and view showing the frame format
of Ortep (b) and crystal packing diagram (c) of the PC -TeNF (1:2) complex.
9
transition enthalpies than PC . It indicates that the side chain struc-
n
ture can control the morphology for the LC phase effectively as
presented for porphyrine derivatives by Ohta et al. [18].

7
2
T. Hirose et al.
ꢀ
FIGURE 5 Snow-like texture of Colh of PC5.5-TeNF (1:1) at 122 C.
DATA
X-ray crystallographic data of PC -TrNF: empirical formula
1
C18.50H11.50N1.50 O7.50; formula weight 374.79; black prisms; crystal
3
dimensions 0.20ꢃ 0.20 ꢃ 0.30mm ; monoclinic; space group P2 =n
1
ꢀ
˚
(
#14); a ¼ 11.855(5), b ¼ 7.770(5), c ¼ 18.721(5) A, b ¼ 102.907(5) ,
FIGURE 6 Schematic view showing the frame format of rectangular (a) and
hexagonal (b) structures of PC derivatives.

Induction and Control of Columnar Mesophase of Tetrasubstituted Pyrenes
73
3
3
˚
V ¼ 1680.9(14) A , Z ¼ 4; Dcalcd ¼ 1.481 g=cm ; F(000) ¼ 772.00;
ꢁ1
ꢀ
l(MoKa) ¼ 0.117 cm ; 2h
¼ 55.0 ; no. of reflections measured
max
ꢀ
3
847; no. of observations (I > 2.00r (I), 2h < 55.0 ) 3847; no. of
variables 338; reflection=parameter ratio was 11.38; R ¼ 0.0792;
R ¼ 0.1538; GOF ¼ 0.841.
w
X-ray crystallographic data of PC -TeNF: empirical formula
1
C H N O ; formula weight 577.39; black prisms; crystal dimen-
2
5
13
4
13
3
sions 0.2ꢃ 0.3 ꢃ 0.2mm ; monoclinic; space group P2 =a (#14);
1
ꢀ
˚
a ¼ 10.7720(4), b ¼ 18.7790(9), c ¼ 12.4590(5) A, b ¼ 90.958(3) ,
3
3
˚
V ¼ 2519.95(18) A , Z ¼ 4; D
¼ 1.522g=cm ; F(000) ¼ 1180.00;
calcd
ꢁ1
ꢀ
l(MoKa) ¼ 0.126 cm ; 2h
¼ 55.0 ; no. of reflections measured
max
ꢀ
1
7314; no. of observations (I > 2.00r (I), 2h < 55.0 ) 4922; no. of vari-
ables 420; reflection=parameter ratio was 11.72; R ¼ 0.0690; R ¼
w
0
.2167; GOF ¼ 1.562.
Crystallographic data (excluding structure factors) for the struc-
tures reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications no.
CCDC-244511 (PC -TrNF) and 244510 (PC -TeNF). Copies of the data
1
1
can be obtained free of charge upon request to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [Fax: þ44(1223)336033; E-mail: deposit@
ccdc.cam.ac.uk].
SUMMARY
Some pyrene derivatives with four acyloxy chains were prepared.
Although all of them were crystalline, the addition of TeNF allowed
for LC phase emergence caused by CT complexation. It was shown
that the LC phase type can be controlled by the structure of the side
chain: linear side chains formed Col whereas branched ones led to
r
Col . A more comprehensive study on the LC emergence of PC deriva-
h
tives is under way, and the relationships with the side chain structure
and the LC properties will be reported in the near future.
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