Functionalized dibenzo[g,p]chrysenes: Variable photophysical and electronic properties and liquid-crystal chemistry

Article abstract of DOI:10.1021/ol801029x

(Figure Presented) The photophysical and electronic properties of dibenzo[g,p]chrysenes bearing electron-rich and -deficient substituents vary markedly with these substituents. The chemistry of the first liquid-crystalline dibenzo[g,p]chrysene is also des

Full text of DOI:10.1021/ol801029x

ORGANIC
LETTERS
2008
Vol. 10, No. 14
3053-3056
Functionalized Dibenzo[g,p]chrysenes:
Variable Photophysical and Electronic
Properties and Liquid-Crystal
Chemistry
Rupsha Chaudhuri,† Ming-Yu Hsu,† Chia-Wen Li,† Cheng-I Wang,†
Chun-Jung Chen,‡ Chung K. Lai,‡ Li-Yin Chen,§ Su-Hao Liu,§ Chung-Chih Wu,§
and Rai-Shung Liu*,†
Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu, Taiwan, ROC,
Department of Chemistry, National Central UniVersity, Chungli, Taiwan, ROC,
and Department of Electric Engineering and Graduate Institute of Photonics and
Optoelectronics, National Taiwan UniVersity, Taipei, Taiwan, ROC
rsliu@mx.nthu.edu.tw
Received May 5, 2008
ABSTRACT
The photophysical and electronic properties of dibenzo[g,p]chrysenes bearing electron-rich and -deficient substituents vary markedly with
these substituents. The chemistry of the first liquid-crystalline dibenzo[g,p]chrysene is also described.
Discotic molecules of benzenoid polycyclic aromatic hy-
drocarbons (BPAHs) such as triphenylenes, dibenzopyrenes,
and hexabenzocorenenes represent organic materials of one
important class.1 These molecules tend to form columnar
(liquid)-crystalline because of outstanding hole-transport
mobility.2 Although substantial progress has been achieved
in preparing discotic molecules of new BPAHs, little work3
has targeted their functionalization to alter their photophysical
and electronic properties. Synthesis of these discotic mol-
ecules relies largely on Scholl oxidation,4–6 which generally
works for reacting benzenes tethered with alkoxy and alkyl
substituents rather than electron-deficient substituents.3
Dibenzo[g,p]chrysenes as discotic molecules of BPAHs
attract interest because of their notable fluorescent properties
such as good quantum yields, small Stoke shifts, and long-
(2) (a) van de Craats, A. M.; Warman, J. M.; Fechtenko¨tter, A.; Brand,
J. D.; Harbison, M. A.; Mu¨llen, K. AdV. Mater. 1999, 11, 1469. (b) van de
Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.;
Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2003,
15, 495.
† National Tsing-Hua University.
(3) To the best of our knowledge, electron-deficient BPAHs are only
known for hexafluorohexabenzocoronenen but remain unknown for small
discotic BPAHs including triphenylenes and dibenzopyrenes. For hexafluo-
rohexabenzocoronenen, see: Kikuzawa, Y.; Mori, T.; Takeuchi, H. Org.
Lett. 2007, 9, 4817.
‡ National Central University.
§ National Taiwan University.
(1) (a) Iyer, V. S.; Wehmeire, M.; Brand, J. D.; Keegstra, M. A.; Mu¨llen,
K. Angew. Chem., Int. Ed. 1997, 36, 1604. (b) Watson, M. D.; Fechtenko¨tter,
A.; Mu¨llen, K. Chem. ReV. 2001, 101, 1267. (c) Grimsdale, A. C.; Mu¨llen,
K. Angew. Chem., Int. Ed. 2005, 44, 5592. (d) Wu, J.; Pisula, W.; Mu¨llen,
K. Chem. ReV. 2007, 107, 718.
(4) For synthesis of triphenylenes via Scholl oxidation, see the following
reviews: (a) Kumar, S. Liq. Cryst. 2005, 32, 1089. (b) Kumar, S.; Manickam,
M. Chem. Commun. 1997, 17, 1615.
10.1021/ol801029x CCC: $40.75
Published on Web 06/19/2008
2008 American Chemical Society

lived excited states.7 The synthesis of dibenzo[g,p]chrysenes
was generally hampered by long procedures before Swager’s
SbCl5/MeOH oxidation of bis(biaryl)acetylenes,7 but the
method is limited strictly to electron-rich benzenes. We
reported8a preliminary success in the synthesis of dibenzo-
[g,p]chrysenes 1 and 2a-c containing unsubstituted and
electron-rich substituents via sequential ICl-induced cycliza-
tion and Mizoroki-Heck coupling,9 as depicted in Scheme
1. We sought to vary the photophysical and electronic
Scheme 2
ties. As shown in Figure 1, the UV absorptions of fluoro-
and trifluoromethyl derivatives 3a-d have onsets at 392-402
Scheme 1
Figure 1. UV absorption spectra of dibenzo[g,p]chrysenes in
CH2Cl2.
properties of dibenzo[g,p]chrysenes bearing both electron-
rich and -deficient substituents. We here report a notable
liquid-crystalline (LC) behavior of one representative diben-
zo[g,p]chrysene 4.
nm, near 396 nm for parent species 1; these compounds have
similar energy gaps. For methoxy derivatives 2a-c, we
observed a significant shift in the UV absorption, ca. 25-30
nm red-shifted from that of compound 1. Figure 2 shows
We applied this synthesis to dibenzo[g,p]chrysenes 3a and
3b bearing bis(fluoro) and bis(trifluoromethyl) groups re-
spectively, which were obtained in 50% and 69% overall
yields in such a reaction sequence. As shown in Scheme 2,
this two-step synthesis works also for dibenzo[g,p]chrysenes
3c and 3d bearing four and eight fluoro groups, respectively;
these two products resulted from activation of the ortho
C(2)-H bond of the 3,5-difluorophenyl group. Minor side
products formed with 3c and 3d were removed through their
crystallization in hot THF.10
With dibenzo[g,p]chrysenes 1, 2a-c, and 3a-d bearing
electron-rich and -deficient substituents in hand, we examined
their UV absorption spectra to study their electronic proper-
(5) For synthesis of BPAHs via Scholl oxidation, see the following
ˇ
selected examples: (a) Tomovicˇ, Z.; Watson, M. D.; Mu¨llen, K. Angew.
Chem., Int. Ed. 2004, 43, 755. (b) Iyer, V. S.; Yoshimura, K.; Enkelmann,
V.; Epsch, R.; Rabe, J. P.; Mu¨llen, K. Angew. Chem., Int. Ed. 1998, 37,
2696. (c) Artal, M. C.; Toyne, K. J.; Goodby, J. W.; Barbera´, J.; Photinos,
D. J. J. Mater. Chem. 2001, 11, 2801. (d) Liu, W.-J.; Zhou, Y.; Ma, Y.;
Cao, Y.; Wang, J.; Pei, J. Org. Lett. 2007, 9, 4187. (e) Kumar, S.; Varshney,
S. Mol. Cryst. Liq. Cryst. 2002, 378, 59.
Figure 2
in CH2Cl2.
. Fluorescent emission spectra of dibenzo[g,p]chrysenes
(6) For a leading reference for Scholl oxidation, see: King, B. T.; Kroulik,
J.; Robertson, C. R.; Rempala, P.; Hilton, C. L.; Korinek, J. D.; Gortari,
L. M. J. Org. Chem. 2007, 72, 2279.
emission spectra of 1, 2a-c, and 3a-d, which exhibit small
Stoke shifts relative to their UV absorption spectra (∆ν <
20 nm). Compound 1 emits two maxima at 396 and 410
nm, whereas its fluoro derivatives 3a-d have the emissions
(7) Yamaguchi, S.; Swager, T. J. Am. Chem. Soc. 2001, 123, 12087.
(8) (a) Li, C.-W.; Wang, C.-I.; Liao, H.-Y.; Chaudhuri, R.; Liu, R.-S.
J. Org. Chem. 2007, 72, 9203. (b) Shen, H.-C.; Tang, J.-M.; Chang, H.-K.;
Yang, C. W.; Liu, R.-S. J. Org. Chem. 2005, 70, 10113.
3054
Org. Lett., Vol. 10, No. 14, 2008

maximum at the same region (407-412 nm); methoxy
derivatives 2a, 2b, and 2c exhibited fluorescence spectra at
longer wavelengths in blue regions, respectively, with
maxima at 426, 452, and 442 nm, respectively. For their
fluorescence spectra, as shown in Table 1, compound 1 and
its fluoro derivatives 3a-d have the quantum yields ca.
0.12-0.19, but methoxy derivatives are more fluorescent
with Φ ) 0.26-0.49,11 which is presumably enhanced by
the steric effects of methoxy groups to reduce intermolecular
energy transfer.11 We estimated the HOMO energy levels
optical microscopy, and powder X-ray diffraction, is sum-
marized in Figure 3. The DSC analysis showed typical
12
ox
from their oxidation potentials (E1/2
)
and obtained the
LUMO energy levels from the onset of their UV absorptions;
Table 1. Photophysical Properties of Functionalized
Dibenzo[g,p]chrysenes
ox
compd Φ (%) E1/2 (V) HOMO (V) band gap (V) LUMO (V)
1
19.3
35.2
48.7
26.0
12.4
16.8
12.4
13.2
0.85
0.55
0.49
0.61
1.12
1.39
1.00
1.18
5.65
5.35
5.29
5.41
5.92
6.19
5.80
5.98
3.16
2.88
2.84
2.91
3.18
3.10
3.18
3.18
2.49
2.47
2.45
2.50
2.74
3.09
2.62
2.80
2a
2b
2c
3a
3b
3c
3d
Figure 3. (a) Temperature-dependent XRD diffraction scan from
210 to 30 °C. (b) Structure of compound 4. (c) Optical texture
observed at 210 °C. (d) DSC thermographs.
columnar phase transitions, crystal-to-columnar-to-isotropic
(Cr f Col fI). Compound 4 melted into columnar phase
at 117.8 °C and cleared at 230.3 °C into an isotropic state.
This species is thermally stable above that clearing temper-
ature, but it decomposes above 376 °C. The temperature
range of its columnar phase is wide, ca. ∆T ) 112.5 °C on
heating curve. Furthermore, a large enthalpy (∆H ) 9.55
KJ/mol on heating cycle) was observed for the Colh f I
transition, indicating a large molecular order in the columnar
phase. Under an optical microscopy, upon heating, species
4 melted to a fluid phase with columnar superstructures, and
typical pseudo-focal-conic textures with linear birefrigent
defects were observed. The dark area implies a homeotropic
domain, in which the molecules are all perpendicularly
aligned within the two glass slides. The diffraction pattern
performed at 150.0 °C showing a strong peak with d ≈ 30.10
Å and two weaker peaks with d ≈ 17.42 Å and 15.09 Å
was observed. These patterns are characteristic of two-
dimensional hexagonal columnar arrangement (Colh) with
a d-spacing ratio of 1:(1/3)1/2:(1/4)1/2, which were assigned
to Miller indices 100, 110 and 200.15 These data imply a
lattice constant of a ) 34.76 Å. A liquid-like correlation, a
so-called halo peak, between the rigid core was observed at
a wide-angle region for 4.30-4.44 Å. These diffraction data
indicate that the hexagonal columnar structural arrangement
persisted at 30 °C.16
the results appear in Table 1. Although fluoro derivatives
3a-d have the same energy gaps (3.10-3.18 eV) as their
parent species 1 (3.16 eV), both the HOMO and LUMO
energy levels of 3a-d were lowered equally (ca. 0.13-0.6
eV) by fluoro and trifluoromethyl substituents. This phe-
nomenon was observed also for hexafluoro-substituted hexa-
benzocoronenes.3 In contrast, we observed a decrease in the
energy gaps (2.84-2.91 eV) for compounds 2a-c because
their methoxy groups raise their HOMO orbitals more than
their LUMO orbitals.
A columnar liquid crystal structure is an important
characteristic for discotic molecules, and such a mesophase
is virtually unknown13 for pure dibenzo[g,p]chrysens that
have a twisted nonplanarity.14 We prepared dibenzo[g-
,p]chrysene 4 tethered with four benzoate chains using
tetra(methoxy)-substituted species 2a as an initial reagent
(see the Supporting Information). The phase behavior of
compound 4, characterized by thermal analysis, polarized
(9) For the synthesis of new BPAHs via Mizoroki-Heck coupling, see:
Wegner, H. A.; Reisch, H.; Rauch, K.; Demeter, A.; Zachariasse, K. A.; de
Meijere, A.; Scott, L. T. J. Org. Chem. 2006, 71, 9080.
(10) Compounds 1, 2a-c, and 3a,b have good solubility in THF,
CH2Cl2, benzene, and toluene, whereas species 3c and 3d are less soluble
in these solvents.
(11) Swager and co-workers reported7 Φ ) 0.24-0.35 for some alkoxy-
substituted dibenzo[g,p]chrysenes, near values of compounds 2a-c.
(12) The cyclic voltammograms of compounds 1, 2a-c, and 3a-d are
provided in the Supporting Information.
We measured charge-transport properties of compound 4
using the time-of-flight (TOF) transient photocurrent tech-
nique with the device configuration: glass/ITO (120 nm)/
(13) Octaalkoxy-substituted dibenzo[g,p]chrysenes showed a columnar
phase in the presence of charge transfer (C-T) complex trinitrofluorenone
in a 2:1 molar ratio. In the absence of that C-T additive, these alkoxy-
substituted species only showed melting points at 139-165 °C, respectively.
Compound 4 is the first reported dibenzo[g,p]chrysene having a columnar
liquid-crystalline structure. See ref 5e.
(15) Wang, J. Y.; Song, J. H.; Lin, Y. S.; Lin, C.; Sheu, H. S.; Lee,
G. H.; Lai, C. K. Chem. Commun. 2006, 4912.
(16) For fluorescent polycyclic aromatic compounds, see: Boydston,
A. J.; Pecinovsky, C. S.; Chao, S. T.; Bielawski, C. W. J. Am. Chem. Soc.
2007, 129, 14550.
(14) The twisted structure of dibenzo[g,p]chrysene 1 has been reported:
Herbstein, F. H. Acta Crystallogr. 1979, B35, 1661.
Org. Lett., Vol. 10, No. 14, 2008
3055

compound 4 (5 µm)/ITO(120 nm)/glass. After capillary
injection at its melted state (250 °C), compound 4 was then
annealed around 230 °C (40 min) to induce liquid-crystalline
molecular arrangements. By rapidly cooling the aligned
sample to 30 °C (ca. 60 °C/min), the molecular alignment
mediated by the fluidic mesomorphism was preserved to form
a rather uniform solid film as monitored by polarized light
microscope.17,18 Conoscopy characterization of the molecular
alignment orientation on the ITO surface reveals the ho-
meotropic arrangement of molecules (i.e., the face-on con-
figuration).17 Compound 4 exhibits bipolar carrier transport;
the field-dependence of mobilities in Figure 4 follows the
often observed in noncrystalline organic systems and could
be attributed to effects of energetic and positional disorder
on the hopping conduction.16,17 Compound 4 shows a hole
mobility up to 9.3 × 10-6 cm2/V·s for fields from 2.4 × 105
to 4.0 × 105 V/cm and electron mobility up to 3.9 × 10-6
cm2/V·s for fields from 3.2 × 105 to 4.4 × 105 V/cm.
In summary, we report variations of the photophysical and
electronic properties of dibenzo[g,p]chrysenes by their
electron-rich and -deficient substituents, in particular their
HOMO-LUMO energy levels, UV absorptions, and fluo-
rescent emissions. We report also the first liquid-crystalline
dibenzo[g,p]chrysene 4,21 which exhibited an hexagonal
columnar structure supercooled at 30 °C; this structural
arrangement allows reasonable hole and electron mobility
in a thin film prepared on capillary injection.22,23
Acknowledgment. We thank the National Science Coun-
cil, Taiwan, for support of this work.
Supporting Information Available: Experimental pro-
cedures including synthesis, cyclic voltammograms, spectral
data of key compounds, measurement of charge mobilities,
DSC curve, MALDI-TOF mass spectrum, and TOF transients
of compound 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL801029X
Figure 4.
Mobilities vs E1/2 for compound 4 at 25 °C.
(20) Ba¨ssler, H. Philos. Mag. 1992, B 65, 795
.
(21) Triphenylenes represent small discotic BPAHs, which have been
studied much more extensively than other molecules. The liquid-crystal
temperature ranges of triphenylenes are highly dependent on their substit-
uents; only few exceptions showed a wide range (>100 °C)22 with a
columnar strcuture that can be supercooled at room temperatures.23a
Reported triphenylenes only conducted hole transport with outstanding
mobility.23
universal Poole-Frenkel relationship: µ ) exp(ꢀE1/2), where
ꢀ is the Poole-Frenkel factor.19,20 Such a relationship is
(17) Born, M.; Wolf, E. Principles of Optics, 4th ed.; Pergamon, Oxford,
1969
.
(22) Kumar, S. Liq. Cryst. 2004, 31, 1037.
(18) The details in the measurement of charge mobilities, structural
characterization of thin film on ITO, and TOF transients of compound 4
are provided in the Supporting Information.
(23) (a) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.;
Siemensmeyer, K.; Etzbach, K. H.; Ringdorf, H.; Haarer, D. Nature 1994,
371, 141. (b) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B. J. Mater.
Chem. 1999, 9, 2081. (c) Arikainen, E. O.; Boden, N.; Bushby, R. J.;
(19) Wu, C.-C.; Liu, T.-L.; Lin, T.-T.; Hung, W.-Y.; Ke, T.-H.; Wong,
K.-T.; Chao, T.-C. Appl. Phys. Lett. 2004, 85, 1172
.
Clements, J.; Movaghar, B.; Wood, A. J. Mater. Chem. 1995, 5, 2161.
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Org. Lett., Vol. 10, No. 14, 2008