Synthesis, photophysical properties and color tuning of highly fluorescent 9,10-disubstituted-2,3,6,7-tetraphenylanthracene

Article abstract of DOI:10.1039/b912289c

We report the synthesis of 9,10-disubstituted-2,3,6,7- tetraphenylanthracenes, of which the photophysical properties are altered significantly by the 9,10-functionalities; the emission wavelengths range from 410 to 610 nm with outstanding quantum yields f

Full text of DOI:10.1039/b912289c

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Synthesis, photophysical properties and color tuning of highly fluorescent
9,10-disubstituted-2,3,6,7-tetraphenylanthracenew
Sheng-Hsun Lin, Fang-Iy Wu and Rai-Shung Liu*
Received (in Cambridge, UK) 23rd June 2009, Accepted 1st October 2009
First published as an Advance Article on the web 15th October 2009
DOI: 10.1039/b912289c
We report the synthesis of 9,10-disubstituted-2,3,6,7-tetraphenyl-
anthracenes, of which the photophysical properties are altered
significantly by the 9,10-functionalities; the emission wavelengths
range from 410 to 610 nm with outstanding quantum yields for
most fluorophores.
4,6-bis(4-tert-butylphenylethynyl)isophthalaldehyde with
diphenylacetylene derivatives 1a–1c (10 equiv.) with Cu(OTf)2
(10 mol%) and difluoroacetic acid (5 equiv.) in hot dichloro-
ethane (100 1C, 5 h) produced the desired 2,3,6,7-tetraphenyl-
anthracenes 2a–2c in 5–55% yields, with species 2c giving the
best yield and high fluorescent quantum yield (j = 0.19);
4-tert-butylbenzoic acid was presumed to be the concurrent
product in this benzannulation.10 To our pleasure, species 2c
has a band gap ca. 2.92 eV, that is ca. 0.98 eV smaller than that
of parent anthracene. The molecular structure of species 2c
was identified by X-ray diffraction.12
There is considerable interest in using small polycyclic
aromatic hydrocarbons (PAH) in organic light-emitting diodes
(OLEDS) because of their high thermal stability and oxidation
resistance;1–4 these compounds include anthracenes,1 pyrenes,2
perylenes3 and chrysenes.4 One strategy to improve the
efficiencies of devices of these aromatic compounds is to add
aryl, amino or hetero groups on the parent framework, not
only to disrupt p–p stacking, but also to increase emission
wavelengths. Nevertheless, these aromatic fluorophores are
used exclusively as blue emitters.1–4 There appears to be no
precedents for the tuning of the emissions of small PAH to
wavelengths over a wide range with common functional
groups.5,6 We envisage that small PAH with variable energy
gaps should have many applications including OLED devices,
thin-film transistors, molecular sensors and liquid crystal
chemistry.7–8 Here we report the color tuning of 2,3,6,7-
tetraphenylanthracene frameworks, to emission wavelengths
over a wide range (419–611 nm) with enhanced quantum
yields. Related to this work is a report1 on a 9,10-disubstituted
anthracene which shows an emission in the deep blue region.
Anthracene has a large energy gap, ca. 3.90 eV,9 and the
tuning of this gap is insensitive to the inductive effect of the
substituent because every anthracenyl carbon has an appreciable
coefficient for both HOMO and LUMO orbitals. An increase
in the p-conjugation of anthracene with its tethered p-substituent
may be a viable route to decrease this energy gap. We envisage
that attachment of four phenyl groups at the 2,3,6,7-positions
would affect the HOMO–LUMO gap. To achieve the synthesis
of this target framework, we employed a Cu(OTf)2-catalyzed
benzanulation reported by Yamamoto et al.;10 this reaction
has not been applied to synthesis of PAH. The literature
method to synthesize 2,3,6,7-tetraphenylanthracene is troubled
with tedious long procedures from starting material 2,3,6,7-
tetraphenylanthraquinone, as well as an inseparable byproduct
in the final step.11 As shown in Scheme 1, treatment of
Encouraged by this success, we prepared 9,10-disubstituted
derivatives based on species 2c; the protocol is depicted in
Scheme 2. We obtained 9,10-dibromo derivatives 3 in 81%
yield through treatment of 2c with Br2 in CH2Cl2 (0 1C, 0.5 h).
We selected arene, alkyne and diarylamine as the substituents;
the corresponding substitution reactions were accomplished
through Suzuki,13 Sonogashira14 and Buchwald–Hartwig15
coupling reactions of dibromide 3, giving compounds 4a–4g
in 41–80% yields. Remarkably, compounds of these three
families show blue, green and yellow-red emissions, respec-
tively. The molecular structures of 9,10-alkynyl substituted
anthracene 4c and 4d were characterized by X-ray diffraction
studies.12
Table 1 presents key data for the photophysical and thermal
properties of compounds 2c and its 9,10-disubstituted deri-
vatives 4a–4g. Within our expectation, all these compounds
are thermally stable; the observed decomposition temperatures
exceeded 43 1C. Relative to unsubstituted species 2c, we
observed substantial alterations in both uv absorption and
emission spectra of 9,10-disubstituted species 4a–4g, as
depicted in Fig. 1a and b. The uv absorption maxima was
observed at 311 nm for 2c, which was gradually red-shifted to
318–322 nm for aryl-substituted species 4a–4d, and further to
338–355 nm for diphenylamino species 4e–4f.
The emission spectra clearly reflect the substituent effect
that produced emission over a wide range, 419–611 nm for 2c
and 4a–4g, covering the blue, green yellow and red regions.
Particularly notable are the high quantum yields of species
Department of Chemistry, National Tsing-Hua University, Hsinchu,
30043, Taiwan (ROC). E-mail: rsliu@mx.nthu.edu.tw;
Fax: (+)886-3-5711082
w Electronic supplementary information (ESI) available: General
procedure, cyclic voltammograms, H and C NMR data and spectras,
EL spectra and OLED performance. CCDC 735891–735893. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b912289c
Scheme 1 Synthesis of compound 2.
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Fig. 2 The energy levels of HOMO–LUMO orbitals of 2c and 4a–4g.
2.92–2.12 eV; the values follow the order 4a–4d 4 4e–4g,
corresponding to aryl, alkynyl and diarylamino substituted
species.
Scheme 2 Synthesis of compound 3 and 4a–g.
Compounds 2c and 4a–4f show one-electron quasireversible
oxidation in CH2Cl2, whereas species 4g exhibits two-electron
qausireversible oxidations as revealed by cyclic voltammetry
measurements9 (see ESI).w The p-methoxyphenyl groups of
species 4g are capable of stablizing the dicationic centers, each
located at the two amino groups respectively, although the
1/2
corresponding two Eox values 4.95 V and 5.15 V are quite
distinct. We determined their HOMO-energy levels from the
1/2
Eox potentials, and subsequently calculated their LUMO
orbitals using the energy gaps determined by uv absorption.
Species 4a and 2c have similar LUMO orbitals, but the
HOMO orbital of 4a is ca. 0.19 eV higher than that of
anthracene 2c. The remaining species 4b–4g have HOMO-
orbitals lying at higher energy levels; the LUMO-orbitals have
energy lower than those of parent species 2c, resulting in a
decreased band gap (see Fig. 2). This phenomenon is indicative
of a pronounced p-delocalization between the anthracene and
alkynyl or diphenylamino substituents.
Compound 4a shows blue fluorescent emission; an OLED
device was fabricated from species 4a by thermal evaporation.
The multilayer configuration consists of ITO/TCTA (50 nm)/
4a (3%) in DMPPP (30 nm)/BCP (10 nm)/TPBI(30 nm)/
LiF(1 nm)/Al; the molecular structures of TCTA, DMPPP,
BCP and TPBI are shown in Scheme 3. The EL emission
spectra of 4a has a maximum at 456 nm that is unaffected by
the applied voltage (Fig. 3). The CIE(x,y) coordinates are 0.14,
0.14 with efficiencies 5.0 cd Aꢁ1 and 2.09 lm Wꢁ1. The
maximum brightness attains 11777 cd mꢁ2 with Zext
=
4.24% and a relatively high turn-on voltage (6.5 V). Such
an outstanding EL performance reflects the suitability of
compound 4a as a deep blue emitter after further structural
or device modification.
Fig. 1 Absorption and emission spectra of compounds 2c and 4a–4g.
In summary, this work serves as the first demonstration
that structural modification of small PAH produces a great
alteration of the electronic states, giving HOMO–LUMO
gaps over a wide range. According to our approach, the
anthracene framework has attached to it four phenyl groups
at the 2,3,6,7 positions, and two additional phenyl, alkynyl
and diphenylamino groups at the 9,10 positions. Such
substituent modification not only effectively tunes the
HOMO–LUMO energy gap, but also enhances fluorescent
quantum yields. Such PAH with diverse energy gaps should
have widespread applications in materials chemistry.
4a–4e with j = 0.56–0.83, which greatly exceed that (j = 0.19)
of unsubstituted species 2c. The quantum yields decreased to
j = 0.37 and 0.03, respectively, for orange and red fluoro-
phores 4f and 4g; their small energy gaps are typically
accompanied by nonradiative process.16,17
The enhanced fluorescent emission of species 4a–4f is
attributed to the steric effect of 9,10-substituents that minimize
intermolecular energy transfer. On the basis of uv absorption,
we obtained energy gaps of 4a–4g with a wide range of
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Table 1 Photophysical and thermal properties of compounds 2c and 4a–4g
Q.Y.a,c
(%)
HOMO/LUMOd
(eV)
Band gape
(eV)
Td
(1C)
a
a,b
f
Absmax
(nm)
PLmax
(nm)
Compound
2c
4a
4b
4c
4d
4e
4f
311
318
319
335
330
296, 338
302, 346
305, 351
419, 444
449, 474
454
481, 513
511, 454
543
19.1
78.2
56.3
80.3
82.5
60.4
36.7
3.4
5.55/2.63
5.36/2.62
5.44/2.71
5.50/2.97
5.37/2.99
5.20/2.83
5.20/2.96
4.95/2.83
2.92
2.74
2.73
2.53
2.38
2.37
2.24
2.12
524
439
485
472
510
448
447
451
573
611
4g
a
f
b
c
In CH2Cl2, 1 ꢂ 10ꢁ5 M. Excitation wavelengths are shown in ESI. Coumarin 1 as a standard. The energy levels of HOMO are determined
d
from CV. Band gaps were calculated from uv absorption. Determined from TGA.
e
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are only known for hexabenzocoronenen, triphenylenes and
dibenzochrysenes, but the change of energy gaps are rather small
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Scheme 3 Structure of TCTA, BCP, TPBI, DMPPP.
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charge from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif and are available in the
ESIw.
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Fig. 3 EL spectra of the device using compound 4a as dopant at
various voltage.
Notes and references
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17 The charge transfer is much more pronounced for species 4g than
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511, 534 and 556 nm, respectively. Compared to those in CH2Cl2,
we observed the magnitude of red-shifts, 32, 39 and 55 nm for 4e,
4f and 4g, respectively. We can not exclude the possibility of an
adverse effect of the charge transfer on the poor quantum yield
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ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6961–6963 | 6963