Columnar liquid crystalline bis-N-annulated quaterrylenes

Article abstract of DOI:10.1039/c1cc13398e

Columnar liquid crystalline bis-N-annulated quaterrylenes have been prepared from readily available N-annulated perylenes by DDQ/Sc(OTf)₃ oxidative coupling and ring-fusion. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc13398e

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COMMUNICATION
Columnar liquid crystalline bis-N-annulated quaterrylenesw
Yan Li,ab Linxiao Hao,ab Hongbing Fu,a Wojciech Pisula,*c Xinliang Fengc and
Zhaohui Wang*a
Received 8th June 2011, Accepted 20th July 2011
DOI: 10.1039/c1cc13398e
Columnar liquid crystalline bis-N-annulated quaterrylenes have
been prepared from readily available N-annulated perylenes by
DDQ/Sc(OTf)3 oxidative coupling and ring-fusion.
extensively studied,6 some important intermediates with large
rylene units have also been reported.7 To date, the higher homo-
logues have been still rarely explored due to the lack of selective
synthesis of these compounds as well as their improcessability
caused by their poor solubility and high melting points.
Liquid crystalline p-conjugated materials,1 which possess both
order and mobility at molecular, supramolecular and macroscopic
levels, are typically designed in such a way that flexible chains
are attached to a rigid p-delocalised core. These materials offer
the possibility for high degree of order and extensive p-orbital
overlap leading to high charge-carrier mobilities, and potentially
being more easily processed from solution or from an isotropic
melt than p-stacked crystalline materials. Molecular shape,
micro-segregation of incompatible parts, and specific molecular
interactions are important factors that drive the formation of
various LC phases. Rod-shaped molecules prefer an arrangement
in layer structures whereas molecules with disc-like geometry
preferably organize into columnar mesophases. Among them,
one-dimensional (1D) functional materials constructed by thermo-
tropic and lyotropic columnar liquid crystals, e.g., triphenylene,2
hexabenzocoronene3 and perylene diimide derivatives,4 have
attracted special attention, whereby the control of their molecular
packing and long-range organization through the design of the
building block represents a prime concern for the performance.5
In search of high performance organic semiconductors for
molecular devices, oligo(peri-naphthalene) (oligo-PN, Fig. 1) is
recognized as one of the most promising molecular scaffolds
because of its intriguing structural and electro-optical properties.
Whereas the physical properties of perylene (1, n = 0),
the first member in the series of the oligo-PNs, have been
We are particularly interested in the bottom-up synthesis of
graphene nano-ribbons-type p-conjugated molecules based on
perylene bisimides and heteroatom-annulated perylenes. Recently
we reported the facile synthesis of bis-N-annulated quaterrylenes
and tri-N-annulated hexarylenes from easily available
N-annulated perylene derivatives (NPs), which is conducted by
DDQ/Sc(OTf)3 oxidative coupling and ring-fusion.8 Herein, we
present our initial studies which promise the establishment of a
new type of highly ordered columnar liquid crystalline materials
based on bis-N-annulated quaterrylenes. Accordingly, we eluci-
date their structures by fiber wide-angle X-ray scattering
(WAXS) experiments performed at different temperatures.
The synthesis of liquid crystalline bis-N-annulated quater-
rylenes is outlined in Scheme 1. The key starting material were
Fig. 1 Oligo-PNs and bis-N-annulated quaterrylenes.
a National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China.
E-mail: wangzhaohui@iccas.ac.cn
b Graduate University of the Chinese Academy of Sciences, Beijing
100190, China
Scheme 1 Synthesis of liquid crystalline 8: (i) NBS, DMF, 25 1C, 2 h,
82%; (ii) 4-(methoxycarbonyl)benzeneboronic acid, Pd(PPh3)4, THF,
K2CO3, 66 1C, 12 h, 87%; (iii) Sc(OTf)3, DDQ, dichloromethane,
25 1C, 10 min, 98%; (iv) KOH, THF, 66 1C, 24 h, 98%; (v) 2-decyl-
1-tetradecanol, EDAC, DMAP, CH2Cl2, 25 1C, 12 h, 63%;
(vi) Sc(OTf)3, DDQ, toluene, 110 1C, 8 h, 62% for 8a; 60% for 8b.
c Max Planck Institute for Polymer Research, Ackermannweg 10,
D-55128 Mainz, Germany. E-mail: pisula@mpip-mainz.mpg.de
w Electronic supplementary information (ESI) available: Experimental
details and characterization of compounds 3 to 8. See DOI: 10.1039/
c1cc13398e
c
10088 Chem. Commun., 2011, 47, 10088–10090
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NPs (3) with dove-tailed alkyl chain and dodecyl chain, which
were obtained in 96% yield by deprotonation of N-annulated
perylene with NaH and subsequently alkylation with branched
alkyl bromide and n-dodecyl bromide, respectively. Subsequent
regioselective NBS bromination provided the key building blocks,
named mono-brominated NPs (4). Suzuki–Miyaura coupling of
4 and arylboronic acid affords mono-end-taped NPs (5). The
controllable oxidative homo-coupling of 5 allowed the prepara-
tion of the key precursor 6 due to the block of one active
peri-position by less reactive phenyl ester groups. When 7 was
treated with five equivalents of 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ) and scandium trifluoromethanesulfonate
[Sc(OTf)3]9 in toluene at 110 1C for 8 hours, bis-N-annulated
quaterrylenes with dove-tailed and linear alkyl chains were
successfully achieved in 60% yield. It is remarkable that under
this simple condition, the expansion of p-system with flexible
chains was successfully and highly effectively achieved along
the long molecular axis, which opens a new avenue towards
liquid crystalline graphene ribbon-type molecules.
yields (FF) (see ESIw) of 0.40 for 8a and 0.30 for 8b in THF
using cresyl violet as the standard. Time profiles of fluorescence
intensities of both compounds are shown in Fig. S1 (ESIw),
revealing a relatively long fluorescence lifetime (t1 = 2.77 ns),
which indicates that these two compounds are rare liquid crystal-
line materials with strong fluorescence in the near-infrared region.
The phase behaviour of compounds 8 was investigated with
differential scanning calorimetry (DSC, Fig. S3, ESIw) and
polarized optical microscopy (POM). The DSC thermograms
of 8a show two phase transitions in the second heating cycle.
The first appears at 126 1C (enthalpy 12.6 J gÀ1) and is attributed
to a crystal–liquid crystal phase change. The below discussed
structure study reveals the molecular packing for each solid
phase. The second peak at 276 1C (enthalpy 7.4 J gÀ1) was
verified by POM as the isotropization point. Two phase transi-
tions were also observed for 8b at 15 1C (15.3 J gÀ1) and 174 1C
(1.4 J gÀ1). The latter peak is related to the isotropic melting,
while despite the relatively high enthalpy the low temperature
transition does not induce any structural modification. This is in
accordance with the optical textures for 8b which display in
POM identical birefringent fan-shaped domains, as also 8a,
over the whole temperature range for the solid phases (Fig. 3).
The structural analysis was performed on two-dimensional
wide-angle X-ray scattering (2D WAXS) results obtained for
different temperatures. Both compounds 8a and 8b assemble in all
determined phases into quasi one-dimensional lamellate stacks.
The small-angle equatorial reflections in the pattern in Fig. 4a
indicate that the 1D stacks of 8a are arranged in an orthorhombic
lattice to each other with a = 3.90 nm and b = 2.76 nm as the
parameters (top view Fig. 5). The parameter a is in agreement
with the length of the long molecular axis, while the relatively
large b is assigned to a herringbone arrangement of the molecules
(side view Fig. 5). The meridional reflection in the wide-angle
region corresponds to an intracolumnar period of 0.47 nm of
tilted, p-stacked molecules (Fig. 4a). Such molecular tilting
towards the columnar axis is characteristic for crystalline phases
The absorption and fluorescence spectra of 8a and 8b in THF
are shown in Fig. 2. In comparison with tert-butyl substituted
quaterrylene, compounds 8 display a bathochromic shift (18 nm)
absorption band probably due to the electron-donating effect of
nitrogen bridges, with maxima at 678, 622, 576, 528 nm. It is
interesting to emphasize that although the absorption and
emission maxima of 8a and 8b are almost the same, the
introduction of a sterically bulky end group at the lateral
position of the core has a strong influence on the absorption
coefficient as a reflection of the strong aggregation of 8a.
It should be also noted that although the emission quantum
yields of perylene and terrylene are high, those of quaterrylene
and longer rylenes are very weak. However, compounds 8a and 8b
exhibit very strong fluorescence. Theoretical calculations reveal
that the optically active 1Bu state lies below the dark B3g or 2Ag
state for perylene and terrylene, which results in strong fluores-
cence, while a crossover to S1 = 2Ag occurs and leads to much
weaker fluorescence for quaterrylene. When heteroatom bridges
are introduced at the lateral position of the core, the charge
distribution at the frontier orbitals is significantly modified and
the lowest-lying state is stabilized from even-parity to odd-parity,
thus allowing light-emission.10 The fluorescence efficiencies of 8a
and 8b were estimated by measuring the fluorescence quantum
Fig. 3 POM images for (a) 8a and (b) 8b both recorded with cross-
polarisers at 30 1C. The cooling rate was 1 1C minÀ1. The scale bar
corresponds to 100 mm.
Fig. 4 Two-dimensional wide-angle X-ray scattering (2D WAXS)
patterns of 8a at (a) 30 1C (crystalline phase), (b) 140 1C (liquid
crystalline phase), (c) 8b at À60 1C.
Fig. 2 UV/vis absorption (solid line) and fluorescence emission spectra
(dotted line) of 8a (red line) and 8b (blue line) in THF (1 Â 10À5 M).
c
This journal is The Royal Society of Chemistry 2011
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we thank the National Natural Science Foundation of China
(Grant Nos. 50873106 and 21021091), the 973 Program,
NSFC-DFG joint project TRR61, and the Chinese Academy
of Sciences.
Notes and references
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Fig. 5 Schematic illustration of the arrangement of 8a at 30 1C.
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In conclusion, a new efficient synthetic method towards
liquid crystalline bis-N-annulated quaterrylenes from easily
available N-annulated perylene derivatives by DDQ/Sc(OTf)3
oxidative coupling and ring fusion is presented. Theoretical
calculation reveals that due to the introduction of the heteroatom
bridges at the lateral position of the core, the charge distribution
at the frontier orbitals is significantly modified, which leads to
the tremendous changes from weak fluorescence of quaterrylene
to strong fluorescence of bis-N-annulated quaterrylene. The
structural analysis in the solid-state demonstrated that the
combination of the intermolecular p–p interaction of bis-N-
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c
10090 Chem. Commun., 2011, 47, 10088–10090
This journal is The Royal Society of Chemistry 2011