Columnar mesomorphism of fluorescent board-shaped quinoxalinophenanthrophenazine derivatives with donor-acceptor structure

Article abstract of DOI:10.1021/ol303375x

Quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (QPP) dyes have been studied as electron acceptor materials, fluorophores, and building blocks for self-organizing organic semiconductors. Condensation of tetraketopyrene with electron-rich diamino-terphenylene and -triphenylene derivatives generates new donor-acceptor QPP derivatives that display columnar mesomorphism over wide ranges of temperature; are fluorescent in solution, liquid crystal, and solid phases; and have electron acceptor properties. Also reported are the synthesis and properties of the first diamino-(tetraalkoxy) triphenylene as a valuable new synthon.

Full text of DOI:10.1021/ol303375x

ORGANIC
LETTERS
2013
Vol. 15, No. 3
558–561
Columnar Mesomorphism
of Fluorescent Board-Shaped
Quinoxalinophenanthrophenazine
Derivatives with DonorꢀAcceptor
Structure
Shuai Chen,† Farah S. Raad,†,‡ Mohamed Ahmida,† Bilal R. Kaafarani,*,‡ and
S. Holger Eichhorn*,†
Department of Chemistry and Biochemistry, University of Windsor, Windsor,
Ontario N9B 3P4, Canada, and Department of Chemistry, American University
of Beirut, Beirut 1107-2020, Lebanon
bilal.kaafarani@aub.edu.lb; eichhorn@uwindsor.ca
Received December 10, 2012
ABSTRACT
Quinoxalino[20,30:9,10]phenanthro[4,5-abc]phenazine (QPP) dyes have been studied as electron acceptor materials, fluorophores, and building
blocks for self-organizing organic semiconductors. Condensation of tetraketopyrene with electron-rich diamino-terphenylene and -triphenylene
derivatives generates new donorꢀacceptor QPP derivatives that display columnar mesomorphism over wide ranges of temperature; are
fluorescent in solution, liquid crystal, and solid phases; and have electron acceptor properties. Also reported are the synthesis and properties of
the first diamino-(tetraalkoxy)triphenylene as a valuable new synthon.
The quinoxalino[20,30:9,10]phenanthro[4,5-abc]phenazine
(QPP) core (1) has been used in the design of organic
semiconductors,1ꢀ3 sensors,4,5 nanofibers,6,7 and EPR spin
labels8 because of its high rigidity, thermal stability, elec-
tronic, and optical properties (Figure 1). A common goal of
several reports has been the conversion of 1 into columnar
liquid crystalline derivatives via the attachment of flexible
side chains (2).2,9ꢀ12 However, a large number of the
reported derivatives display (lamellar) soft crystal rather
than columnar liquid crystal phases because of their large
† University of Windsor.
‡ American University of Beirut.
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r
10.1021/ol303375x
Published on Web 01/23/2013
2013 American Chemical Society

aspect ratios and the attachment of only two side chains to
each end of the molecule (tetracatenar motive). An ad-
vantage of columnar arrangements is the possible accom-
modation of πꢀπ stacking with a nonparallel orientation
of the QPP cores that should enhance their emission
properties and affects transport of charge carriers (e.g.,
may bias electron over hole transport).
Scheme 1. Synthesis of QPP 3
To promote columnar mesomorphism over lamellar
arrangements and to lower transition temperatures we
decided to attach four side chains on each side by ex-
changing R with dialkoxy phenyl (3) and tetraalkoxyphe-
nanthrene groups (4). Both the larger number of side
chains and their orientation at an angle of about 60°
relative to the long axis of the core increase the lateral
packing volume of the side chains, which has been shown
to disfavor lamellar and promote columnar mesomorph-
ism in polycatenar-type liquid crystals.13
Our recent results on similar compounds showed that
the oxidative coupling is successful when the 2 and 11
positions of QPP 3 are blocked by alkyl groups. This
suggests that oxidative polymerization occurs via the 2
and 11 positions of QPP 3,17,18 which is astonishing con-
sidering that the didodecyloxyphenyl groups should be
more easily oxidized than the pyrene core (see topology
of HOMO in Table S2, Supporting Information (SI)).
However, we did not attempt protection of the 2 and 11
positions for the preparation of QPP 4 but, instead, decided
to prepare 6,7,10,11-tetrakis(dodecyloxy)triphenylene-2,3-
diamine 9 (Scheme 2) for a subsequent coupling to tetra-
ketopyrene 8. Tetrakis(alkoxy)triphenylene-2,3-diamine
derivatives have not been previously reported but are of
broader interest as valuable building blocks for the synthesis
of discotic liquid crystals and azaarenes.
Figure 1. QPP core 1 and previously described tetraalkoxy and
tetraalkylthio-substituted QPP derivatives 2. QPP derivatives 3
and 4 are new liquid crystalline derivatives reported here.
Scheme 2. Synthesis of Diaminotetradodecyloxytriphenylene 9
In our first synthetic attempt toward compounds 3 and 4
we followed a procedure recently reported by Lixiang Wang
(Scheme 1).2 Suzuki coupling between 4,5-dibromobenzene-
1,2-diamine (5) and boronic acid 6 gave ortho-terphenyl 7,
which was reacted with tetraketopyrene 85,14ꢀ16 to afford
tetraphenyl QPP 3 in an overall yield of 54%. Unexpectedly,
oxidative coupling of the dialkoxyphenyl rings to generate
the planar QPP 4 was not successful. Instead a yellow
insoluble compound was obtained that is likely a polymer
of compound 3 according to its IR and UVꢀvis spectra.
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5743.
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Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.;
Wiley-VCH: Weinheim, 1998; Vol. 2B, p 865.
Diaminoterphenyl 7 is stable in air for months, but
expectedly, the electron-rich diaminobenzene group causes
many side reactions under oxidative coupling conditions.
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Org. Lett., Vol. 15, No. 3, 2013
559

Consequently, the directconversion of7 to 9 is not feasible,
although 9 was chromatographically isolated from a com-
plex product mixture in yields of 5ꢀ8%. Protection of the
two amino groups as a 2,1,3-thiadiazole ring renders the
central phenyl group inert to the chosen oxidative coupling
conditions (1:1 mixture of VOF3 and BF3•Et2O), and
triphenylene 11 was generated in close to quantitative
yields. Only 2 equiv of VOF3 are sufficient for this cou-
pling, since product 11 is apparently not oxidized under
these conditions.19ꢀ21 Reduction of 11 with LiAlH4 gen-
erates the desired diaminotriphenylene 9 in an overall yield
of 65% starting from 7.
Pure compound 9 is a white solid, but a brown solid is
obtained when handled in air, which is attributed to the
presence of oxidation products. However, the brown
product gave satisfactory NMR spectra and was used
without further purification for the subsequent condensa-
tion reaction with 8. Samples of 9 that were stored for one
month at ꢀ30 °C under argon gave identical yields.
Condensation of tetraketopyrene 8 with 2.2 equiv of
diamine 9 in acetic acid did not generate QPP 4 but the
monocondensation product 12 in close to quantitative
yield based on 8 (Scheme 3). An excess of 9 was recovered
from the reaction mixture, which confirms that no side
reactions occurred. The formation of 12, a valuable inter-
mediate for the preparation of unsymmetrical QPP deri-
vatives, is solely a result of its low solubility in acetic acid
and does not require stoichiometric control.2,3,6 QPP 4 was
obtained in 80% yield when the reaction was conducted in
a 1:8 mixture of acetic acid and THF.
Figure 2. Phase behavior of QPPs 3 and 4 based on POM, DSC,
and pXRD. Transition temperatures (onset) and enthalpies are
given in °C (kJ/mol). a It is likely a Colr-type mesophase (see SI).
mesophase (Colh) and an isotropic liquid phase (Iso). The
larger planar core of 4 clearly increases the thermal stabi-
lity of its Cr and Col phases in comparison to 3 and
suppresses the formation of Colh and Iso phases because
thermal decomposition occurs first. Lower transition tem-
peratures and the additional Colh and Iso phases most
importantly benefit the processing of QPP 3 into aligned
films for device applications. For example, 3 self-organizes
into millimeter-sized domains of columnar stacks with
vertical (homeotropic) alignment to a substrate when
cooled from its isotropic liquid state between glass slides.
This vertical self-alignment is particularly remarkable for
a compound of a board shape rather than a disc shape and
is only slightly disturbed at the transition into the biaxial
Colr phase.
The electronic properties of QPPs 3 and 4 were probed
by absorption spectroscopy, cyclic voltammetry, and DFT
calculations (Table 1). Extension of the conjugated system
in 4 decreases Egap and ELUMO by 0.2 eV with regard to 3.
A comparison of the red/ox potentials of QPPs 3 and 4
with the potentials of tetraalkoxy and tetraalkylthio sub-
stituted QPP derivatives 29,15 reveals an increase of EHOMO
and a decrease of ELUMO by 0.2 and 0.1 eV, respectively,
for 3 and almost twice these values for 4. Consequently,
Egap values of QPPs 3 and 4 are between 0.2 and 0.8 eV
smaller than those for QPPs 2. These properties qualify
3 and 4 as organic dyes with good electron acceptor
properties.
Scheme 3. Condensation of 8 and 9 to 12 and QPP 4
Table 1. Red/Ox Potentials, EHOMO/LUMO, and Optical
HOMOꢀLUMO Gaps of QPPs 3 and 4
The thermal properties of compounds 3, 4, 7, 9, 10, 11,
and 12 were studied by polarized optical microscopy
(POM), differential scanning calorimetry (DSC), and vari-
able temperature powder X-ray diffraction (pXRD) mea-
surements. Mesomorphism was observed only for QPP
derivatives 3 and 4 that display hexagonal and rectangular
columnar mesophases over wide ranges of temperature
(Figure 2).
ox a
E1/2red/ E1/2
d
e
f
EHOMO/ELUMO /Egap EHOMO/ELUMO /Egap Egap
3 ꢀ1.24/1.17b
4 n.a.c
ꢀ5.49/ꢀ3.08/2.41
ꢀ5.13/ꢀ2.18/2.95
ꢀ5.03/ꢀ2.28/2.75
2.48
2.21
n.a.c
a First potentials in V vs Ag/Agþ (CH2Cl2, glassy carbon working
electrode. b Estimated from peak potential of irreversible oxidation at
1.20 V. c Insufficiently soluble for solution cyclic voltammetry. d Calculated
ox
based on E1/2 of ferrocene/ferrocenium as internal standard (0.48 V vs
Ag/Agþ = 4.80 eV). e Based on DFT calculations (B3LYP/6-31g(d,p)) in
vacuum. f Based on onset of longest wavelength absorption; Energy values
are given in eV vs vacuum.22
QPPs 3 and 4 both display crystalline phases (Cr) and
rectangular columnar mesophases (Colr) while only QPP
3 also displays a high temperature hexagonal columnar
The absorption and emission properties of QPPs 3 and
4 agree with the measured and calculated electronic pro-
perties and show the expected bathchromic shift of the
absorption/emission maxima from 445/557 nm for 3 to
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560
Org. Lett., Vol. 15, No. 3, 2013

475/654 nm for 4 because of its enlarged conjugated system
(Figure 3). The observed large Stokes shifts of 112 and 179 nm
for QPPs 3 and 4, respectively, are probably generated by
intramolecular charge transfer (ICT) states, and their
formation is in agreement with the distinct spatial separa-
tion of HOMO and LUMO orbitals (donorꢀacceptor
structure)23 in calculated structures (see SI, Table S2).
below 0.3%,24,25 and we hypothesize that the ether groups
are involved in the more competitive nonradiative deacti-
vation processes. The ether groups in QPP 3 only weakly
electronically interact with the pyrene fluorophore since
the phenyl groups are tilted out of plane.
Both QPPs 3 and 4 also fluoresce in their crystalline and
columnar liquid crystal phases (see SI). This is rather
unusual for πꢀπ stacking materials and suggests a weak
interaction between the transition dipole moments, as it
has been observed in columnar J-aggregates and com-
pounds that stack orthogonally.26,27
In conclusion, we describe different synthetic ap-
proaches to new QPP derivatives 3 and 4 that display
several types of columnar mesophases over wide ranges of
temperature. Both compounds consist of donorꢀacceptor
structures and can be categorized as organic dyes, electron
acceptors, and fluorophores in liquid and solid phases.
Their properties as organic semiconductors are presently
under investigation.
Acknowledgment. S.H.E. acknowledges support by
NSERC, CFI, OIT, and Xerox, and B.R.K. is grateful
for support by the ACS-PRF (Grant No. 47343-B10) and
the University Research Board of the American University
of Beirut.
Figure 3. Absorption and fluorescence spectra of compounds 3
(1.0 ꢁ 10ꢀ5 M (absorption) and 1.0 ꢁ 10ꢀ6 M (fluorescence) in
CH2Cl2) and 4 (8.7 ꢁ 10ꢀ6 M in CHCl3 at 45 °C). Excitation
wavelengths for 3 and 4 were 445 and 475 nm, respectively.
Supporting Information Available. Synthesis and char-
acterization of all compounds; UVꢀvis and fluorescence
spectra, cyclic voltammetry, DFT calculations, POM,
TGA, DSC, and pXRD of QPPs 3 and 4. This material
is available free of charge via the Internet at http://pubs.
acs.org.
QPP 3 has a high fluorescence quantum yield of 31% in
solution whereas QPP 4 only reaches 0.72%. Similarly, the
fluorescence quantum yields of QPPs 2 are low with values
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B.; Guillon, D.; Eichhorn, S. H. J. Mater. Chem. 2010, 20, 1292.
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€
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The authors declare no competing financial interest.
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