A mesogenic triphenylene-perylene-triphenylene triad

Article abstract of DOI:10.1021/ol103018v

A straightforward synthesis of triphenylene-perylene-triphenylene triad structures has been achieved by using versatile triphenylene intermediates bearing a single oxyalkyl amine side chain. Among these, PBITP₁₀ showed a stable columnar mesophase implying favorably matched core-core separations in the structure. Importantly, the triad can be used as a vehicle for doping columnar triphenylene matrices with functional but incompatible perylene units and a mixture of hexahexyloxytriphenylene matrix doped with 0.1% PBITP₁₀ is homogeneous and liquid crystalline.

Full text of DOI:10.1021/ol103018v

ORGANIC
LETTERS
2011
Vol. 13, No. 4
764–767
A Mesogenic Triphenylene-Perylene-
Triphenylene Triad
Xiangfei Kong,† Zhiqun He,*,† Yinning Zhang,† Linping Mu,† Chunjun Liang,†
Bo Chen,‡ Xiping Jing,‡ and Andrew N Cammidge*,§
Key Laboratory of Luminescence and Optical Information, Ministry of Education,
Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044,
People’s Republic of China, College of Chemistry and Molecular Engineering, Peking
University, Beijing 100871, People’s Republic of China, and School of Chemistry,
University of East Anglia, Norwich, NR4 7TJ, United Kingdom
zhqhe@bjtu.edu.cn; a.cammidge@uea.ac.uk
Received December 13, 2010
ABSTRACT
A straightforward synthesis of triphenylene-perylene-triphenylene triad structures has been achieved by using versatile triphenylene
intermediates bearing a single oxyalkyl amine side chain. Among these, PBITP10 showed a stable columnar mesophase implying favorably
matched core-core separations in the structure. Importantly, the triad can be used as a vehicle for doping columnar triphenylene matrices with
functional but incompatible perylene units and a mixture of hexahexyloxytriphenylene matrix doped with 0.1% PBITP10 is homogeneous and liquid
crystalline.
Discotic liquid crystals (DLCs) are promising candi-
dates for use in organic electronic and optoelectronic
devices because they readily self-assemble into ordered
columnar structures that can in turn lead to efficient,
directional charge migration.1 For this reason they have
potential applications for use in optoelectronic devices,
such as thin film transistors, organic light emitting diodes
(OLEDs), or photovoltaic devices.2
Triphenylene derivatives are among the most widely
investigated discotic liquid crystals because they are robust
and show a strong tendency for columnar mesophase
formation. Synthetic advances have allowed a large array
of symmetrical and unsymmetrically substituted deriva-
tives to be prepared and evaluated, and the structural
factors controlling mesophase formation in simple mono-
meric systems are now largely understood.3 Furthermore,
† Beijing Jiaotong University.
‡ Peking University.
§ University of East Anglia.
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L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature
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10.1021/ol103018v
Published on Web 01/19/2011
2011 American Chemical Society

columnar triphenylenes can spontaneously form highly
aligned mesophases as thin films on solid substrates for
device fabrication.1,4,5
bridged by a phenylene carbamate linkage and flexible
spacers for application as a charge transporting layer in
an organic electroluminescent device and found that
the flexible spacer and the hydrogen bonds contributed
to generate organized thin films.10 In our design of fused
triphenylene-perylene systems we therefore chose simple
hydrocarbon linkers of varying length. The final triad
design is shown in Figure 1.
Perylene is another important core for organic materials
and several columnar mesophases have been observed
in suitably functionalized structures.6,7 Perylenes have
received particular interest because of their absorption
and emission properties in both solution and solid state.
For example, Langhals6a and Jancy6b reported that pery-
lene bisimide (PBI) has almost 100% fluorescence quan-
tum yield in solution. Of particular interest is the use of
combinations of aromatic/columnar structures in specific
device architectures. For example, a perylene bisimide
derivative has demonstrated an interesting photovoltaic
performance of 2% in conjunction with hexabenzocoro-
nene discotic materials.2c In thin film applications, such as
OLEDS or PV devices, phase separation and immiscibility
between organic components can be a desirable or indeed
essential property. Other applications rely on doping the
host columnar matrix with a complementary or active
component. The columnar mesophases formed by discotic
materials havea strong tendency toexclude other organics,
even other mesogenic discogens, so it is normally impos-
sible to introduce a low concentration dopant simply by
mixing or formulation.
We reasoned that a possible general approach to addres-
sing this issue would be to fuse the discotic (triphenylene)
and functional aromatic component (perylene) in a cova-
lent manner. Phase separation is then impossible in such a
homogeneous system and a doped matrix can be produced
solongasmesophaseformationisretained. Inthisletterwe
report the fusion of discotic triphenylenes onto a central
perylene bisimide unit to give triphenylene-perylene-
triphenylene triads.
Figure 1. Target triphenylene-perylene-triphenylene triads
PBITPn.
Perylene bisimide derivatives are most conveniently
prepared by reaction between commercially available 3,4:
9,10-perylene-tetra-carboxylic acid dianhydride (19) and 2
equiv of a primary amine. Simple, short-chain deriva-
tives are known to be poorly soluble, however, so we
selecteda routethatwould introduce the highlysubstituted
triphenylenes onto the perylene in the final step so that
any solubility and purification issues were avoided.
The synthesis therefore required a discotic triphenylene
bearing a single amine on the end of one side chain. The
synthesis of the key precursors is shown in Scheme 1. The
synthesis requires monohydroxypentahexyloxy tripheny-
lene 3 and we recently reported a convenient protocol that
allows such intermediates to be prepared easily in a single
step via a mixed cyclization between hexyloxphenol and
1,2-bishexyloxybenzene using iron chloride.11
Potassium phthalimide was alkylated12 with dibromo-
alkane in a statistical synthesis to give convenient access to
masked bromoalkylamines that were employed to alkylate
monohydroxytriphenylene 3. The triphenylene amine pre-
cursors were then prepared by deprotection using Ing-
Manske reaction conditions13 This sequence worked
well for all medium and long chain lengths but failed in
the synthesis of the ethylamine intermediate during the
triphenylene alkylation step. For this amine an alterna-
tive synthesis was employed, shown in Scheme 2. Mono-
hydroxy triphenylene 3 was alkylated with 1,2-dibromo-
ethane under phase transfer conditions and the resulting
bromide 16 treated with potassium phthalimide. Depro-
tection with hydrazine proceeded smoothly to yield the
Hexasubstituted triphenylenes generally have a strong
tendency to form columnar mesophases so long as the link
atom can conjugate to the core (alkoxides are the most
widely employed).3c In twinned and oligomeric assemblies
of triphenylene discotics, columnar mesophase forma-
tion is only retained if the link groups are flexible and
compatible with the intercolumn separation.8 Rigid linkers
lead to nematic mesophases in some cases.9 For example,
Freudenmann et. al synthesized a conjugated-bridged
triphenylene dimer having a large spectral red-shift2a used
in OLEDs but liquid crystalline properties were lost.
On the other hand, Mao reported a triphenylene dimer
(4) (a) Vauchier, C.; Zann, A.; Le Barney, P.; Dubois, J. C.; Billard, J.
Mol. Cryst. Liq. Cryst. 1981, 66, 103. (b) Kawata, K. Chem. Rec. 2002, 2,
59. (c) Wang, J.; He, Z.; Zhang, Y.; Zhao, H.; Zhang, C.; Kong, X.; Mu,
L.; Liang, C. Thin Solid Films 2010, 518, 1973.
(5) Li, J.; He, Z.; Zhao, H.; Kong, X.; Gopee, H.; Xu, M.; An, X.;
Jing, X.; Cammidge, A. N. Pure Appl. Chem. 2010, 82, 1993.
(6) (a) Langhals, H. Helv. Chim. Acta 2005, 88, 1309. (b) Jancy, B.;
Asha, S. K. J. Phys. Chem. B 2006, 110, 20937.
(10) Mao, H.; He, Z.; Wang, J.; Zhang, C.; Xie, P.; Zhang, R.
J. Lumin. 2007, 122-123, 942.
(11) Kong, X.; He, Z.; Gopee, H.; Jing, X.; Cammidge, A. N.
Tetrahedon Lett. 2011, 52, 77.
(12) Salzberg, P. L.; Supniewski, J. V.; Whitmore, F. C.; Benedict,
(7) Wicklein, A.; Lang, A.; Much, M.; Thelakkat, M. J. Am. Chem.
Soc. 2009, 131, 14442.
(8) (a) Kumar, S. Liq. Cryst. 2005, 32, 1089. (b) Cammidge, A. N.;
Gopee, H. Liq. Cryst. 2009, 36, 809. (c) Li, J.; He, Z.; Gopee, H.;
Cammidge, A. N. Org. Lett. 2010, 12, 472.
H. C. Org. Synth. 1941, 1, 119.
(13) (a) Ueda, T.; Ishizaki, K. Chem. Pharm. Bull. 1967, 15, 228. (b)
Khan, M. N. J. Org. Chem. 1995, 60, 4536.
(9) (a) Kumar, S.; Varshney, S. K. Org. Lett. 2002, 4, 157. (b) Zhang,
L.; Gopee, H.; Hughes, D. L.; Cammidge, A. N. Chem. Commun. 2010,
46, 4255.
Org. Lett., Vol. 13, No. 4, 2011
765

characteristic textures when viewed under a polarizing
optical microscope. Further cooling tends to lead to a
glassy structure that slowly crystallizes.
Scheme 1. Preparation of Triphenylene with Primary Amine
Groups (n = 4, 6, and 10)
The final triphenylene-perylene-triphenylene triads
(PBITPn) were prepared in a single efficient step from
triphenylene amines 13-15 and 18 by reaction with
3,4:9,10-perylene-tetra-carboxylic dianhydride 19 by using
a trace ofglacial acetic acid asa catalyst and a modification
of the procedure developed by Rohr14 in which the triphe-
nylene amines were employed as the limiting reactants
(Scheme 3).
Scheme 3. Synthesis of Triphenylene-Perylene-Triphenylene
Triads
Thermotropic mesophase behavior of the triads was in-
vestigated by using polarizing optical microscopy (POM)
and differential scanning calorimetry (DSC). PBITP2,
PBITP4, and PBITP6 showed only crystalline phases with
their melting temperatures reduced as the length of the
flexible spacer increased, and this behavior is typical for
systems that combine two different discotic units. PBITP10
is unique in the series and particularly interesting because it
does form a surprisingly stable liquid crystal phase below
around 165 °C and shows a texture typical of a columnar
hexagonal (Figure 2). There is clearly a subtle structural
balance required in such linked systems. In this case there
is a match beween the separation required in columns
formed solely by triphenylenes (the triphenylene core plus
OC6H13) and the triphenylene-perylene separation (the
broader perylene core is compensated by a -OC10H12O-
bridge). Figure 2 also depicts a cartoon of the triad
structure and a likely arrangement of the units within a
columnar mesophase. It could also be speculated that
charge transfer interactions could stabilize the columnar
arrangement but there is no evidence for any such interac-
tion from solution-state UV-visible spectroscopy. Indeed,
the triads show preserved fluorescence from the perylene
core providing a further indication for absence of charge-
transfer or other significant face-to-face interactions
that would be expected to cause complete quenching.
target primary amine 18. Triphenylene amines 13-15 and
18 are versatile and useful precursors for grafting discotic
mesogenic units onto further functional moieties. They
are interesting in their own right. When cooled from
the isotropic liquid they form columnar mesophases with
Scheme 2. Alternative Synthesis of Triphenylene Amine 18
€
(14) Rohr, U.; Kohl, C.; Mullen, K.; Van de Craats, A.; Warman, J.
J. Mater. Chem. 2001, 11, 1789.
766
Org. Lett., Vol. 13, No. 4, 2011

clearing temperature. A sample doped with 2% PBITP10
remained an isotropic liquid when cooled to 75 °C at which
point slow crystallization began. The same observation
resulted from the 0.5% doped sample but in both cases the
outcomes were interesting because there was no phase
separation between the triad and HAT6, with crystals
forming from the doped mixture. Significant transition
suppression was observed when an even lower doping
concentration was employed (0.1%) but importantly in
this case cooling to 76 °C led to formation of a columnar
mesophase, again with no separation of the triad from its
matrix. The mesophase structure was retained (as a glass)
down to room temperature.
In summary, a straightforward synthesis of triphenylene-
perylene-triphenylene triad structures has been achieved
by using versatile triphenylene intermediates bearing a
single oxyalkyl amine side chain. The final triad structures
with linking chain lengths of 2, 4, and 6 methylene units are
nonmesogenic, but the triad with decyl linkers forms
a stable columnar mesophase below 165 °C. As such it is
a rare example of a columnar material comprising signifi-
cantly different cores. There is no evidence for charge
transfer or other ground state interactions and the stability
of the columnar structure is likely to arise through favor-
ably matched core-core separations. The fluorescence
of the central perylene core is preserved, and the excita-
tion spectrum clearly demonstrates that excitation of the
triphenylene leads to emission from the perylene. Impor-
tantly, the triad can be used as a vehicle for doping
columnar triphenylene matrices with functional but in-
compatible perylene units. Doping hexahexyloxytripheny-
lene with 0.1% of mesogenic triad PBITP10 gives a
columnar phase below 76 °C with no separation of the
triad from its host matrix. These results suggest this
approach to incorporation of functional components in
columnar materials can be more widely applied so long as
careful design principles are used to match the core and
linker structures.
Figure 2. A cartoon representation of PBITP10 and its likely
arrangement in the columnar mesophase, and the mesophase
texture from PBITP10 at 160 °C observed by polarizing optical
microscopy (scale bar =200 μm). Thermal transitions from
DSC are inset.
The absorption spectrum of the triad shows essentially a
combination of the absortion spectra for the isolated
triphenylene7 and perylene units.9 Fluorescence spectro-
scopy, however, shows that there is communication be-
tween the different units in the excited state. Observing
fluorescence from the perylene, the profile of the excitation
spectrum for the triad essentially follows the absorption
spectrum, unambiguously confirming that excitation of the
triphenylene leads to emission from the central perylene.
Observation of a stable columnar phase for PBITP10
suggested the potential for using the triad as a vehicle for
introducing incompatible functional components into the
columnar matrices of the parent hexaalkoxytriphenylenes
without inducing phase separation. Hexahexyloxytriphe-
nylene (HAT6) was used as host material. In its pure state
it forms a columnar mesophase between 70 and 100 °C.
Doping with triad PBITP10 led to a suppression of the
Acknowledgment. X.K. thanks the China Scholarship
Council, Ministry of Education. This work is supported by
the funds from NSFC (20674004), ISTCP (2008DFA61420),
and the 111 project (B08002).
Supporting Information Available. Experimental pro-
cedures and compound characterization data and ab-
sorption and fluorescence spectra for PBITP10. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 4, 2011
767