Arylamine-substituted hexa-peri-hexabenzocoronenes: Facile synthesis and their potential applications as "coaxial" hole-transport materials

Article abstract of DOI:10.1002/anie.200460174

The hole story: using a new synthetic concept the title hexa-peri-hexabenzocoronenes (HBCs) were synthesized with high atom economy. The coaxial arrangement of the HBCs and arylamines allowed a "double- cable" hole transport (see picture), that is, transp

Full text of DOI:10.1002/anie.200460174

Angewandte
Chemie
Electron Transfer
Arylamine-Substituted Hexa-peri-
hexabenzocoronenes: Facile Synthesis and Their
Potential Applications as “Coaxial” Hole-
Transport Materials**
Jishan Wu, Martin Baumgarten, Michael G. Debije,
John M. Warman, and Klaus Müllen*
Organic semiconductors for hole injection, hole transport,
and photoconduction are needed in thin-film electronics, such
as organic light-emitting diodes (OLEDs),[1] solar cells,[2]
field-effect transistors (FET),[3] and photorefractive sys-
tems.[4] Triarylamines are particularly useful because of their
ability to transport positive charge via their radical cations.[5]
Hexa-peri-hexabenzocoronenes (HBC) have recently been
introduced as discotic liquid-crystalline materials which
display high charge-carrier mobility along their one-dimen-
sional p-stacks.[6] Herein, a series of novel hole transport
materials 1–5 is described in which HBC is peripherally
substituted with arylamine moieties. They are shown to adopt
a columnar stacking owing to the strong p–p interactions
between the HBC cores, and thus allow charge-carrier
transport by both the HBC and arylamine moieties in a
coaxial supramolecular array (Scheme 1). The oxidative
formation of radical cations and higher charged cations
raises questions as to the intramolecular spin–spin interac-
tions and suggests a comparison between the HBC (super-
benzene) and the corresponding, much smaller benzene
species.[7,8]
Our synthesis of HBC materials by oxidative cyclodehy-
drogenation of appropriately substituted hexaphenylbenzene
precursors is not applicable owing to the radical cation
formation at the nitrogen centers.[8] An alternative approach
is by palladium-catalyzed Buchwald–Hartwig coupling[9]
starting from the planarized HBC building blocks carrying
bromo or iodo substituents. Thus, the mono- and bis- di(4-
methoxyphenyl)amino-substituted HBCs (1 and 2) were
synthesized by aryl amination between bis(4-methoxy-
phenyl)amine and the related mono- and dibromo HBC[10]
in 65% and 76% yield, respectively (see Supporting Infor-
[*] Dr. J. Wu, Dr. M. Baumgarten, Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
Dr. M. G. Debije, Prof. Dr. J. M. Warman
IRI, Delft Universityof Technology
Mekelweg 15, 2629 JB Delft (The Netherlands)
[**] This work was financiallysupported bythe Zentrum für Multi-
funktionelle Werkstoffe und Miniaturisierte Funktionseinheiten
(BMBF03N6500), the Deutsche Forschungsgemeinschaft
(Schwerpunkt Organische Feldeffekttransistoren, as well as the EU
project DISCELs (G5RD-CT-2000-00321) and MAC-MES (Grd2-
2000-30242).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 5331 –5335
DOI: 10.1002/anie.200460174
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5331

Communications
virtually insoluble HBC building block
hexakis(4-iodophenyl)-peri-hexabenzo-
coronene (6) appeared highly reactive
during the palladium catalyzed Hagi-
hara–Sonogashira coupling reactions[12]
to afford a series of soluble HBC materials
which are highly ordered liquid crystals or
carry electroactive substituents, such as
the HBC derivative 5. Herein, two new
insoluble building blocks 7 and 8, are
introduced and utilized for the synthesis
of the corresponding tris (3)- and hexa-
amines (4).
On route to a C3 symmetric tris(bi-
phenylyl)benzene
precursor
14
(Scheme 2) the selective Suzuki cou-
pling[13] between 1-bromo-2-iodobenzene
(9) and 3-(trimethylsily)phenyl boronic
acid (10)[14] afforded 2-bromo-3’-(trime-
thylsilyl)biphenyl (11) in 93% yield. The
bromo functionality in 11 was then con-
verted into boronic acid 12 in 90% yield,
and the threefold Suzuki coupling
between 12 and 1,3,5-tribromobenzene
gave
1,3,5-tris[3’’-(trimethylsilyl)-2’-
biphenyl]benzene (13) in 58% yield.
After replacement of the trimethylsilyl
groups by iodo units (91% yield), the resulting precursor 14
was submitted to FeCl3 oxidative cyclodehydrogenation
conditions to give the fused HBC building block 7 as an
insoluble yellow powder in 92% yield. The extremely poor
solubility of compound 7 only allows solid-state MALDI-
Scheme 1. Schematic presentation of a coaxial “double-cable”
approach for hole transport.
mation). The synthesis of di(4-octylphenyl)amino-substituted
HBC-arylamine materials (3–5) was based on a novel
synthetic concept recently developed in our group.[11]
A
Scheme 2. Synthesis of molecule 3: a) [Pd(PPh3)4], K2CO3(aq.), tolu-
ene, 958C, 92%; b) 1) nBuLi, ꢀ788C; 2) B(OCH3)3, ꢀ788C!RT;
3) HCl (aq.), 90%; c) [Pd(PPh3)4], K2CO3(aq.), toluene, 958C, 58%;
d) ICl, chloroform, 91%; e) FeCl3 (24 equiv), CH3NO2/CH2Cl2, 92%;
f) [Pd2(dba)3(PtBu3)], toluene, 808C, 24%. dba=dibenzylideneacetone
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TOF mass spectroscopic characterization. The subsequent
functionalization of 7 with bis(4-octylphenyl)amine by Buch-
wald–Hartwig coupling reactions[9] afforded soluble HBC-
arylamine materials 3 in 24% yield, allowing full structural
characterization (see Supporting Information).
Another D6h symmetric HBC, namely hexakis(4-iodo)-
peri-hexabenzocoronene (8), was prepared by similar oxida-
tive cyclodehydrogenation of the precursor hexakis(4-iodo-
phenyl)benzene (15)[15] in nearly quantitative yield (see
Supporting Information). Although virtually insoluble, 8
underwent sixfold Hagihara–Sonogashira coupling[12] with
measurements in Supporting Information). At room temper-
ature there is a tilted columnar stacking of the HBC discs
(Figure 1b) with an orthorhombic 2D unit cell (a = 2.28, b =
1.78 nm). A series of reflections at the wide-angle area in the
equatorial and meridianal directions can be correlated to the
positional or rotational order between the arylamine arms in
the columnar stacking. While HBC 3 only displayed a
crystalline phase bellow the melting point of 3878C, the
hexaamine substituted HBCs 4 and 5 did not show any phase
transition between ꢀ1008C and 4008C. The 2DWAXD
measurements on the extruded fibers of 4 and 5 clearly
revealed a hexagonal columnar stacking in a wide temper-
ature range (Figure 1c). Thus, except molecule 3, all the
arylamine substituted HBCs clearly display ordered coaxial
columnar stacking in the liquid-crystalline phase or micro-
crystalline phase; such coaxial stacking affords the opportu-
nity of a “double-cable” hole transport (see scheme 1). At the
same time, smooth thin films with thickness of tens to one
hundred nanometers and roughness around 1 nm can be
easily obtained for compounds 3–5 by spin-coating the
solutions onto different substrates, such as quartz, silicon
wafer, and highly oriented pyrolytic graphite (HOPG) as
studied by atomic force microscope (AFM). The smooth UV/
Vis absorption spectra can be obtained for these thin films.
On the other hand, the thin films of compounds 1 and 2
showed typical crystalline domains on quartz mainly because
of their crystalline properties at room temperature (see
Supporting Information).
N,N’-bis(4-octylphenyl)-N’’-(4-ethynylphenyl)amine
(16)
smoothly affording the soluble HBC 4 in 61% yield
(Scheme 3).
Scheme 3. Synthesis of molecule 4: a) FeCl3 (24 equiv), CH3NO2/
CH2Cl2, quantitative; b) [Pd(PPh3)4]/CuI, piperidine, 508C, 61%.
Compounds 1–5 can be easily oxidized to stable radical
cations or higher cationic charged species, as indicated by the
cyclic voltammetric and differential-pulse voltammetric
measurements (Table 1 and Supporting Information). The
The self-assembly properties of the arylamine substituted
HBCs 1–5 in the bulk were investigated by differential
scanning calorimetry (DSC), polarized optical microscopy
(POM), and two-dimensional wide-angle X-ray diffraction
(2DWAXD) techniques.[16] Upon heating, 1 entered a hexag-
onally ordered columnar liquid-crystalline phase at 1628C, as
indicated by the typical fan-type texture in POM and
2DWAXD diagrams (see Supporting Information). Upon
cooling from the isotropic melt above 3758C, the compound
passed through the columnar liquid-crystalline phase and
gave rise to a columnar microcrystalline phase at 1278C. The
typical 2D X-ray diffractogram at room temperature (Fig-
ure 1a) shows the “X” shaped reflections beyond the
equatorial and meridianal directions, which are correlated
to a p–p stacking distance of 4.5 Š along the stacking axis and
indicate that the discs are tilted about 308 with respect to the
columnar axis.[17] Similarly, upon heating, 2 entered a
columnar liquid-crystalline phase at 2178C from a columnar
microcrystalline phase (see POM textures and 2DWAXD
Table 1: Cyclic voltammetryand differential pulse voltammetrydata of
compound 1–5 in 1,2-dichlorobenzene.[a]
Event
1
2
3
4
5
1
E1/2 (V)
0.47(1e)
0.94(me) 0.50(1e)
–
0.34(1e)
0.51(1e)
0.72(2e)
0.77(me) 0.71(me)
1.10(me) 0.92(me)
1.24(me) 1.07(me)
2
E1/2 (V)
3
E1/2 (V)
0.97(me) 1.23(me)
[a] For full details see the Supporting Information. The half-wave
potential E1/2 was refer to an AgNO3/Ag reference electrode and was
calibrated with an internal standard, ferrocene/ferrocenium redox
system (E1/2(Fc)=0.232 V). 1e=one electron transfer; 2e=two electron
transfer, and me=multi-electron transfer.
splitting of the oxidation of the amines in 2 and 3 into two
steps (before the muli-electron event) suggests effective
charge delocalization through the HBC core in the mixed-
valence radical monocation species (2+C and 3+C). We thus
monitored the Vis/NIR and electron spin resonance (ESR)
spectra upon oxidative titration. The UV/Vis/NIR spectra of
2+C in CH2Cl2 during stepwise oxidation by thianthrenium
perchlorate (THClO4) revealed the absorption band of a
radical monocation centered 601 nm and 709 nm, together
with a unique long-wavelength absorption band above
1200 nm, where the maximum peak is beyond 2200 nm
(< 0.56 eV; Figure 2). The five-line ESR spectrum observed
Figure 1. Representative 2D WAXD diagrams of extruded fibers of
compound 1 (a), 2 (b), and 5 (c) at room temperature.
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order of magnitude, with room temperature values of 0.11,
0.21, 0.01, 0.03 and 0.04 cm2 (Vs)ꢀ1 for 1–5, respectively. The
order of the mobilities for compounds 1, 2, and 5, that is, 2 >
1 > 5, are seen to be in qualitative agreement with the
sharpness of the WAXD images shown in Figure 1. This
supports the expected dependence of charge transport on the
degree of columnar order. These largest mobility values
which approach those found for the crystalline and liquid-
crystalline phases of a hexadecyl-substituted HBC[6b] would
be surprising if a large portion of the positive charges resided
on the peripheral amine moieties: either the amines are
sufficiently well-organized within the intercolumnar space to
allow rapid hole transport between them, or the electrons,
which remain localized on the HBC cores, have an intra-
columnar mobility which is comparable with that of holes.
Unfortunately the TRMC technique cannot differentiate
between these two possibilities since it is insensitive to the
sign of the charge of the major carrier; but the former
explanation suggests that the hole mobilities in the present
discotic materials are considerably larger than those found in
disordered triarylamine solids.[21]
In conclusion, the new synthetic concept towards electro-
active arylamine substituted HBC materials can be used to
broaden the HBC family with high atom economy. Combi-
nation of the columnar superstructure formation of HBC and
the hole-transporting ability of both the HBC and arylamines
led to new kind of hole-transporting materials with high
carrier mobility, good film formation capability (expect
compound 1 and 2), and low ionization potential, which are
promising properties for organic devices.[1,2] The mixed-
valence compounds of oxidized 1–5, can be regarded as
ideal models for studying the intramolecular charge transfer
as a function of the molecular symmetry and distance between
the nitrogen centers, and the intermolecular association of
charged p systems.[22]
Figure 2. UV/Vis/NIR spectroscopic titration profile of compound 2 in
CH2Cl2 with THClO4 solution. Inset: temperature-dependent ESR spec-
tra of the oxidized mono radical cation of 2.
at 310 K for 2+C reflects an intramolecular spin exchange
between two nitrogen centers (Figure 2, inset). Upon going to
250 K, only a three-line signal is seen which resembles the
situation found for 1+C.[18] Upon lowering the temperature the
intensity of the ESR signals of 1+C and 2+C decreases,
suggesting p–p aggregate formation in solution (Figure 2,
inset). The long-wavelength absorption band of 2+C, thus can
be explained by the molecular cation absorption, which is
predicted to be around 1400 nm (AM1-CI calculation), and
an additional intramolecular charge-transfer process. Intra-
molecular charge-transfer reactions in mixed-valence triaryl-
amine systems have often been studied.[7,8,19] Herein, we
provide evidence for aggregation of the radical cations of 1–3
even at low concentration (about 10ꢀ5 m) and additional
intramolecular charge-transfer interactions between aryla-
mine moieties but no clear indication could be found for the
intermolecular charge transfer other than their p–p aggrega-
tion.
Received: March 31, 2004
Revised: June 17, 2004
The intracolumnar charge-carrier mobilities for com-
pounds 1 through 5, determined using the pulse-radiolysis
time-resolved microwave conductivity technique (PR-
TRMC)[6b,20] are shown as a function of temperature in
Figure 3. The mobilities are seen to differ by more than one
Keywords: arylamines · electron transfer · hole transport ·
.
liquid crystals · pi interactions
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