Substituent effects in twisted dibenzotetracene derivatives: Blue emitting materials for organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2014.07.007

A series of dibenzotetracene derivatives containing different substituents at the 11-,12-positions have been successfully synthesized and characterized. Single crystal X-ray analysis indicates that the new molecules have a twisted structure, which can effectively decrease the intermolecular aggregation in the solid-state. The effect of both substituent and packing on the corresponding physical properties was also investigated. In addition, organic light-emitting diodes doped with 11,12-difluoro-9,14-diphenyl-dibenzo[de,qr]tetracene into the emitter 9,9′-(1,3-phenylene)bis-9H-carbazole were fabricated and exhibit good performance.

Full text of DOI:10.1016/j.dyepig.2014.07.007

Dyes and Pigments 112 (2015) 176e182
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Substituent effects in twisted dibenzotetracene derivatives: Blue
emitting materials for organic light-emitting diodes
,
Jinchong Xiao a *, Zhenying Liu a, Xuemin Zhang a, Weichen Wu b, Tiejun Ren a, Bo Lv a,
,
*
Li Jiang c, Xuefei Wang d, Hua Chen a, Wenming Su b, Jianwen Zhao b
a College of Chemistry and Environmental Science, Key Laboratory of Chemical Biology of Hebei Province, Hebei University, Baoding 071002, PR China
b Printable Electronics Research Centre, Suzhou Institute of Nanotech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, PR China
c Beijing National Laboratory for Molecular Sciences, Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Beijing 100190, PR
China
d School of Chemistry and Chemical Engineering of University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of dibenzotetracene derivatives containing different substituents at the 11-,12-positions have
been successfully synthesized and characterized. Single crystal X-ray analysis indicates that the new
molecules have a twisted structure, which can effectively decrease the intermolecular aggregation in the
solid-state. The effect of both substituent and packing on the corresponding physical properties was also
investigated. In addition, organic light-emitting diodes doped with 11,12-difluoro-9,14-diphenyl-dibenzo
[de,qr]tetracene into the emitter 9,90-(1,3-phenylene)bis-9H-carbazole were fabricated and exhibit good
performance.
Received 18 April 2014
Received in revised form
1 July 2014
Accepted 4 July 2014
Available online 15 July 2014
Keywords:
Synthesis
© 2014 Elsevier Ltd. All rights reserved.
Single crystal
Twisted structure
Optoelectronic property
Acene
Organic light-emitting diodes
1. Introduction
diphenylphenoxy substituted perylene bisimide derivative which
exhibited strong solid-state fluorescence [21]. Shimizu reported a
During the past decade, a growing number of researchers have
paid intense attention tothe preparation of small, conjugated organic
molecules with favorable properties including high fluorescence
quantum yield in solution and/or solid-state, photochemical, chem-
ical and thermal stability, good packing model, processability and
tunability of color because these derivatives can be potentially
employed in many aspects such as artificial light harvesting systems,
organic light-emitting diodes (OLEDs), organicfield-effect transistors
(OFETs), solar cells, biosensors and lasers [1e19]. In general, most of
the synthesized molecules with high fluorescent quantum yield in
solution exhibit weak or no fluorescence in high concentration so-
lution and solid-state due to molecular aggregation [20]. Two ap-
proaches were used to alleviate the high-concentration/solid-state
quenching: decorating with bulky and nonplanar molecular struc-
tures, for example, Würthner and coworkers synthesized 2,6-
series of 1,4-bis-(alkenyl)-2,5-dipiperidinobenzenes and their color
could be tuned using different substituents at the alkenyl moieties
[22]. Moreover, some other bulky groups were introduced into the
conjugated frameworks, resulting in suppression of the aggregation
in the solid-state [23e26].
Currently, acene and its derivatives are still the major subjects of
extensive investigations owing to their high charge-carrier mobil-
ities, interesting opto-electrical properties and potential applica-
tions [27e34]. However, some detracting features such as easy
oxidation under ambient conditions, high photosensitivity, ease of
dimerization and extensive purification hamper their application in
organic electronics. Therefore much effort has been directed to-
wards overcome those limitations including random synthesis
[35e37] and step-by-step synthesis [27,38,39]. For example, phenyl,
triisopropylsilylethynyl (TIPS) and arylthio substituents are utilized
to produce persistent acene derivatives [40,41]; phenanthrene and
pyrene units can be grafted into acene building blocks, which not
only increases their stability and solubility in common organic sol-
vents but also makes them twist to effectively decrease the
* Corresponding authors.
E-mail addresses: jcxiaoiccas@gmail.com (J. Xiao), jwzhao2011@sinano.ac.cn
(J. Zhao).
http://dx.doi.org/10.1016/j.dyepig.2014.07.007
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

J. Xiao et al. / Dyes and Pigments 112 (2015) 176e182
177
molecular packing in the solid-state. In addition the twisted de-
rivatives can be employed as emitters in organic electronics
[42e52].
Until now it has generally been accepted that organic blue
emitting material lagged behind the green and red emitters, and
thus pursuing high electroluminacent performance blue materials
is still a hot topic of the emitting material research.
In this manuscript, a series of dibenzotetracene derivatives
(3ae3f) were synthesized and fully characterized (Scheme 1). X-ray
diffraction analysis showed these compounds have twisted struc-
tures. Here we are more interested in the substituent effects in the
conjugated skeletons on the physical properties. Therefore, the
electro-optical and thermal properties of these compounds were
investigated in detail. In addition a good performing light-emitting
device was fabricated, based on compound 3c, through a vacuum
deposition method.
7.34e7.30 (m, 3H), 2.48 (s, 3H). 13C NMR (150 MHz, CDCl3, 298 K):
d
¼ 142.41, 136.43, 135.88, 135.53, 132.49, 132.39, 132.21, 130.73,
130.49, 130.47, 129.28, 129.08, 128.75, 128.55, 128.31, 128.20, 127.47,
127.45,126.80,125.92,125.57,125.53,124.68,124.63, 22.00. MS (EI):
Calc. for C37H24: [M]þ 468.19, found: [M]þ 468. HRMS (m/z): calcd
for C37H24, 468.1872, found 468.1872.
11,12-dimethoxyl-9,14-diphenyl-dibenzo[de,qr]tetracene (3b,
yield, 44%). Mp: 311.2e312.6 ꢀC. FT-IR (KBr): 3053, 2926, 1616, 1567,
1498, 1430, 1254, 1205, 1136, 1029, 833, 708 cmꢂ1 1H NMR
.
(600 MHz, CDCl3, 298 K):
7.84 (d, J ¼ 1.2 Hz,1H), 7.81 (d, J ¼ 1.2 Hz,1H), 7.80 (d, J ¼ 0.6 Hz,1H),
7.57e7.51 (m, 10H), 7.35e7.32 (m, 2H), 7.21 (s, 2H), 3.82 (s, 6H). 13
NMR (150 MHz, CDCl3, 298 K):
d
¼ 7.88 (s, 2H), 7.85 (d, J ¼ 0.6 Hz, 1H),
C
d
¼ 149.44, 142.76, 132.14, 130.71,
130.42, 129.20, 128.35, 128.25, 127.75, 127.52, 126.83, 125.61, 125.28,
124.57, 105.30, 55.65. MS (MALDI-TOF): Calc. for C38H26O2: [M]þ
514.19, found: [M]þ 514.2. HRMS (m/z): calcd for C38H26O2,
514.1927, found 514.1925. 11,12-difluoro-9,14-diphenyl-dibenzo
[de,qr]tetracene (3c, yield, 13%). Mp: 337.1e337.8 ꢀC. FT-IR (KBr):
3053, 1509, 1440, 1255, 883, 824, 736, 707 cmꢂ1. 1H NMR (600 MHz,
2. Experimental
2.1. Materials and instruments
CDCl3, 298 K):
d
¼ 7.89 (s, 2H), 7.88 (d, J ¼ 1.2 Hz, 1H), 7.87 (d,
J ¼ 0.6 Hz,1H), 7.82 (d, J ¼ 0.6 Hz,1H), 7.80 (d, J ¼ 1.2 Hz, 1H), 7.68 (t,
J ¼ 10.2 Hz, 2H), 7.58e7.54 (m, 6H), 7.52e7.50 (m, 4H), 7.33 (t,
All chemicals were purchased from J&K, TCI and Acros corpo-
rations and used without any purification. Column chromatography
was performed on silica gel (200e300 mesh). 1H NMR and 13C NMR
spectra were recorded on Bruker 600 MHz spectrometer. MALDI-
TOF mass spectra were performed on Bruker Biflex III MALDI-TOF.
J ¼ 7.8 Hz, 2H). 13C NMR (150 MHz, CDCl3, 298 K):
¼ 141.80,135.95,
d
132.03, 130.76,129.78, 129.60,129.44,128.62, 127.96, 126.88, 126.13,
125.84, 124.77, 112.89, 112.86, 112.80, 112.76. MS (MALDI-TOF): Calc.
for C36H20F2: [M] 490.15, found: [M] 490.1. HRMS (m/z): calcd for
FT-IR spectra were carried out on
a
HE-3100 spectrometer.
C36H20F2, 490.1528, found 490.1526.
Melting points were recorded on a TX-4 hot-stage apparatus.
UVeVis absorption and fluorescence spectra were carried out on
Shimadzu UV-2550 and RF5300PC spectrometer, respectively. Cy-
clic voltammograms were measured on CHI 630 A electrochemical
analyzer with Pt wire as the counter electrode, Ag/Agþ electrode as
the reference electrode and glassy carbon as the working electrode.
Tetrabutylammonium hexafluorophosphate dissolved in anhy-
drous methylene chloride (0.1 M) was used as the supporting
electrolyte. The scan rate is 0.1 V/s. X-ray crystallographic data were
11,12-dibromo-9,14-diphenyl-dibenzo[de,qr]tetracene
(3d,
yield, 28%). Mp: 329.2e330.4 ꢀC. FT-IR (KBr): 3053, 1606, 1576,
1488, 1440, 1312, 1110, 970, 882, 824, 706 cmꢂ1. 1H NMR (600 MHz,
CDCl3, 298 K):
d
¼ 8.18 (s, 2H), 7.88 (s, 3H), 7.87 (s, 1H), 7.77 (d,
J ¼ 6.0 Hz, 2H), 7.56e7.55 (m, 6H), 7.51e7.50 (m, 4H), 7.35 (t,
J ¼ 7.8 Hz, 2H). 13C NMR (150 MHz, CDCl3, 298 K): 141.10, 135.47,
132.18, 131.77, 131.26, 130.76, 130.50, 129.71, 129.44, 128.80, 128.08,
126.89, 126.35, 125.92, 124.82, 122.31. MS (MALDI-TOF): Calc. for
C
C
36H20Br2: [M]þ 612.35, found: [M]þ 612.0. HRMS (m/z): calcd for
36H20Br2, 609.9926, found 609.9933.
collected at 298 K with a graphite-monochromatic Mo K
a radiation
(l
¼ 0.71073 Å) on a Bruker APEX II CCD diffractometer.
Synthesis of 11,12-dinonylthio-9,14-diphenyl-dibenzo[de,qr]
tetracene (3e). Triflic acid (25 mL) was added to a mixture of 11,12-
dimethoxyl-9,14-dipheny-dibenzo[de,qr]tetracene (3b, 200 mg,
0.39 mmol) and 1-nonanethiol (144 mg, 0.90 mmol) in toluene
which was preheated at 110 ꢀC under a nitrogen atmosphere. After
stirring for 12 h, the reaction solution was cooled to room tem-
perature. Methylene chloride (30 mL) was added. The solution was
then washed with 5% aqueous solution of NaOH (50 mL ꢁ 3) and
brine (50 mL ꢁ 3). After the solvents were removed under reduced
pressure, the yellow residue was further purified by column chro-
matography on silica gel with methylene chloride/petroleum ether
(3:40, v/v) to afford 3e as a yellow solid (132 mg, 46%). Mp:
129.1e129.6 ꢀC. FT-IR (KBr): 3053, 2926, 2858, 1597, 1577, 1440,
1118,1000, 834, 727 cmꢂ1. 1H NMR (600 MHz, CDCl3, 298 K): 7.88 (s,
2H), 7.86e7.83 (m, 4H), 7.67 (s, 2H), 7.54e7.53 (m, 10H), 7.35 (t,
J ¼ 7.8 Hz, 2H), 2.81 (t, J ¼ 7.8 Hz, 4H), 1.63e1.60 (m, 4H), 1.31e1.26
(m, 24H), 0.90 (t, J ¼ 6.0 Hz, 6H). 13C NMR (150 MHz, CDCl3, 298 K):
142.20, 135.66, 135.27, 132.16, 130.77, 130.49, 130.26, 129.14, 128.95,
128.54, 127.59, 126.82, 125.84, 125.73, 124.66, 124.33, 33.12, 31.87,
29.49, 29.27, 29.21, 29.01, 28.85, 22.64, 14.04. MS (MALDI-TOF):
Calc. for C54H58S2: [M]þ 770.40, found: [M]þ 770.3. HRMS (m/z):
calcd for C54H58S2, 770.3974, found 770.3971.
2.2. Synthesis route
The typical synthetic procedures of 3ae3d are shown as follows.
Substituted 2-aminobenzoic acid (2) dispersed in degassed 1,2-
dichloroethane (DCE, 10 mL) solution and isoamyl nitrile (2 mL)
were added alternately into a solution of 1,3-diphenylcyclopenta[e]
pyrene-2-one (1) preheated at 90 ꢀC in (DCE, 10 mL). The mixture
was stirred overnight and then cooled to room temperature. Brine
(60 mL) was added to the reaction mixture. The aqueous phase was
further extracted with methylene chloride (30 mL ꢁ 3). The
collected organic layers were dried with Na2SO4 and the solvent
was removed under reduced pressure. The crude product was pu-
rified through silica gel column chromatography to afford target
compound (3) as solids.
11-methyl-9,14-diphenyldibenzo[de,qr]tetracene (3a, yield,
25%). Mp: 259.4e260.3 ꢀC. FT-IR (KBr): 3048, 2925, 1601, 1494,
1445, 829, 731, 702 cmꢂ1. 1H NMR (600 MHz, CDCl3, 298 k):
d
¼ 7.87
(s, 2H), 7.85e7.80 (m, 5H), 7.69 (s, 1H), 7.54e7.52 (m, 10H),
Synthesis of 11,12-dicyano-9,14-diphenyl-dibenzo[de,qr]tetra-
cene (3f). Cuprous cyanide (90 mg, 1 mmol) was added to a solution
of
11,12-dibromo-9,14-diphenyl-dibenzo[de,qr]tetracene
(3d,
150 mg, 0.25 mmol) in dry NMP. The mixture was stirred under
argon at 180 ꢀC for 3 days. After the mixture was cooled to 60 ꢀC, an
aqueous solution of [(NH4)2Fe(SO4)2] was added and stirred for 2 h.
Scheme 1. Chemical structures of compounds 3ae3f.

178
J. Xiao et al. / Dyes and Pigments 112 (2015) 176e182
After cooling to room temperature, the reaction mixture was
extracted with methylene chloride (30 mL ꢁ 3) and the combined
solvents were removed under reduced pressure. The brown residue
was further purified by column chromatography on silica gel with
methylene chloride/petroleum ether (2:7, v/v) to afford 3f as a
yellow solid (80 mg, 64%). Mp: 368.8e369.3 ꢀC. FT-IR (KBr): 3053,
2232, 1607, 1548, 1489, 1450, 1313, 912, 834, 707 cmꢂ1 1H NMR
.
(600 MHz, CDCl3, 298 K): 8.41 (s, 2H), 7.97 (d, J ¼ 7.2 Hz, 2H), 7.93 (s,
2H), 7.85 (d, J ¼ 7.2 Hz, 2H), 7.61e7.60 (m, 6H), 7.49e7.47 (m, 4H),
7.40 (t, J ¼ 7.8 Hz, 2H). 13C NMR (150 MHz, CDCl3, 298 K): 139.92,
139.88, 137.15, 135.43, 135.38, 133.72, 132.00, 131.32, 130.91, 130.89,
129.84, 129.81, 129.21, 128.82, 128.80, 127.54, 127.52, 127.07, 127.05,
126.17, 126.15, 125.11, 125.05, 116.49, 116.41, 108.90, 108.82. MS
(MALDI-TOF): Calc. for C38H20N2: [M]þ 504.16, found: [M]þ 504.0,
[MþNa]þ 527.0. HRMS (m/z): calcd for C38H20N2, 504.1621, found
504.1621.
3. Results and discussion
3.1. Synthesis
As outlined in Scheme 2, compounds 3ae3d were synthesized
in one-pot by a [4þ2] cycloaddition reaction of dienophile 2 and
diene 1 in the presence of isoamyl nitrile in anhydrous 1,2-
dichloroethane under nitrogen atmosphere. The intermediate 1
was obtained according to the literature [43]. Subsequently, com-
pound 3b was converted to the sulfide 3e via ipso-substitution
between aryl methyl ether 3b and 1-nonanethiol catalyzed by triflic
acid in toluene solution [53]. When compound 3d was treated with
10 equiv of CuCN in N-methyl pyrrolidone (NMP) at 180 ꢀC for three
days, the substitution reaction was finished to give 3f in 64% yield
[54]. The new molecules are light green solids and are readily sol-
uble in common solvents such as methylene chloride, chloroform,
toluene, 1,2-dichlorobene (ODCB) and THF. In addition, they are all
unambiguously characterized by 1H NMR, 13C NMR spectroscopy
and MALDI-TOF mass spectrometry.
3.2. Single crystal structure
Fig. 1. Single crystal structure and packing model of (a) (b) 3a, (c) (d) 3b and (e) (f) 3c.
Carbon, oxygen and fluoride atoms are colored in gray, red, green, respectively and
hydrogen atoms are omitted for clarity. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
To obtain more information about their spatial arrangement,
single crystal X-ray measurement were performed. The suitable
single crystals for determination (3a, 3b and 3c) were obtained by
slow evaporation of CH2Cl2/CH3OH solutions. The crystal of 3c is
observed as thin needles, while the crystals of 3a and 3b are light
greenprisms. The crystal structures and packing models (3ae3c) are
presented in Fig.1 and the detailed crystallographic data are given in
Table 1. Compounds 3a (CCDC number: 927068) and 3b (CCDC
number: 991580) are observed to be triclinic crystal systems, space
group P-1, but 3c (CCDC number: 991581) has monoclinic lattice and
space group P21. The unit cell parameters of a ¼ 10.6832(17) Å,
b ¼ 12.7735 (19) Å, c ¼ 19.528 (3) Å,
b
¼ 76.156(3)ꢀ for 3a,
a ¼ 15.657(2) Å, b ¼ 16.770 (3) Å, c ¼ 23.094 (3) Å,
b
¼ 107.491 (10)ꢀ
for 3b, a ¼ 12.5285(16) Å, b ¼ 25.530 (3) Å, c ¼ 7.5269 (10) Å,
b
¼ 98.477(3)ꢀ for 3c are also observed. As depicted 3ae3c unam-
biguously exhibit twisted structures owing to the strong interaction
between benzo units in the pyrene unit and the proximal phenyl
groups. The twisted angles between the pyrene moiety and naph-
thalene unit of 3ae3c are 28.56ꢀ, 29.74ꢀ and 36.26ꢀ, respectively.
These results clearly suggest that the different terminal substituents
could influence the aggregation in the solid-state. In addition, the
intermolecular distances are larger than the sum of van der Waals
radii (3.83 Å), and the mismatch arrangements in the single crystals
leads to significant decrease of the pep intermolecular interactions.
Thus weak intermolecular interaction should be taken to account for
Scheme 2. Synthetic procedure of compounds 3ae3f.
the high fluorescent yield in the solid-state.

J. Xiao et al. / Dyes and Pigments 112 (2015) 176e182
179
Table 1
10 nm compared to those in diluted solution and broader profiles. It
should be noted that new shoulder absorption peaks for these
compounds are observed in the lowest energy absorption bands.
Crystal data of compounds 3a, 3b and 3c.
3a
3b
3c
Such red-shift might be assigned to the
pep stacking interaction
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
a
C37H24
468.56
Triclinic
P-1
C38H26O2
514.59
Triclinic
C36H20F2
490.52
Monoclinic
P21
12.5285(16)
25.530(3)
7.5269(10)
90ꢀ
98.477(3)ꢀ
90ꢀ
among individual molecules in the packing films [55]. However,
there are almost no variation of the maximum absorption wave-
lengths for compounds 3d and 3e.
P-1
10.6832(17)
12.7735(19)
19.528(3)
74.376(3)ꢀ
76.156(3)ꢀ
87.626(3)ꢀ
2491.1(7)
4
15.657(2)
16.770(3)
23.094(3)
109.099(10)ꢀ
107.491(10)ꢀ
96.075(2)ꢀ
5322.9(15)
8
The corresponding fluorescence spectra of these compounds in
diluted solution and solid-state were recorded at room tempera-
ture. The normalized spectral shapes are shown in Fig. 2b. Com-
pounds 3ae3e in methylene chloride emit blue light with the
emission maxima bands at 437 nm, 432 nm, 434 nm 444 nm and
451 nm, respectively. In contrast, the fluorescence spectrum of 3f
exhibits emission at 474 nm under the excitation at 349 nm. Note
that compounds 3ae3e have well-defined spectral shapes except
for 3f having a broad band. In addition, the Stokes shifts of all these
compounds are over 100 nm, which might be ascribed to the for-
mation of twisted structure that makes the ground and excited
b
g
Volume [Å3]
2381.2(5)
4
Z
Density (calculated)
1.249 Mg/m3
1.284 Mg/m3
1.368 Mg/m3
3.3. Optical, electrochemical and thermal properties
states distinct. The quantum yields (Фf) of 3ae3f were measured in
The spectroscopic properties of all six dibenzotetracene de-
rivatives were measured at room temperature both in methylene
chloride and in solid-state. Clearly, the introduction of different
substituents at the 11- and 12-positions brings about an obvious
impact on the absorption spectra. Fig. 2a shows that compound 3a,
3b and 3c have nearly identical absorption profile and maxima
diluted methylene chloride using 9,10-diphenylanthracene
(ethanol solution, Фf ¼ 0.95) as the reference [56,57]. Compounds
3ae3c and 3f display strong fluorescence and the quantum yields
are 0.25, 0.43, 0.47 and 0.33, respectively. However, 3d (Фf ¼ 0.03)
and 3e (Фf ¼ 0.07) present weak fluorescence, which might be
caused by the heavy atom effect, which results into the intersystem
crossing to the nonradiative triplet state [58]. The fluorescence
spectra of 3ae3f in thin films are displayed in Fig. 2d. The solid-
state emission spectra of compounds 3a and 3f with broad
featureless profile are distinctly bathochromically shifted with
respect to the case of solution. The emission maxima bands of
3be3e correlate well with those in methylene chloride, and the
intensity order of 3d and 3e is reversed.
(lmax) at 333 nm, 332 nm and 330 nm, respectively, which is
ascribed to the * transition of the conjugated building block. In
pep
comparison, bromo- and alkyl sulfanyl-substituted compounds 3d
and 3e display the red-shifted absorption peaks (3d: 330 nm,
342 nm and shoulder peak at 367 nm, 3e: 332 nm, 345 nm). In the
case of 3f, the largest bathochromic shift of absorption maximum to
349 nm and 366 nm is observed. These phenomena can be ascribed
to the increase of the conjugation length and the electron density
with variation of substituents. The solid-state absorption spectra of
compounds 3ae3f are presented in Fig. 2c. The solid-state ab-
sorption of compounds 3ae3c and 3f show a red-shift of about
The electrochemical behavior of the new molecules were con-
ducted at room temperature in dry methylene chloride solution
containing 0.1
M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) as a supporting electrolyte. As shown in Fig. 3, the
Fig. 2. UVeVis (a: in methylene chloride, c: in thin film) and fluorescence spectra (b: in methylene chloride, d: in thin film) of compounds (i) 3a, (ii) 3b, (iii) 3c, (iv) 3d, (v) 3e, (vi) 3f.

180
J. Xiao et al. / Dyes and Pigments 112 (2015) 176e182
Fig. 3. Cyclic voltammogram of compounds in TBAPF6/CH2Cl2 solution. Scan
rate ¼ 0.1 V/s (i) 3a, (ii) 3b, (iii) 3c, (iv) 3d, (v) 3e, (vi) 3f.
oxidation peaks of 3ae3f are electrochemically irreversible. The
first oxidation waves for compound 3ae3f are observed at 0.92 V,
0.81 V, 1.07 V, 1.06 V, 0.80 V and 1.19 V, respectively. Dibenzote-
tracene derivatives 3b and 3e are about 0.11 V and 0.12 V, lower
than the methyl substituted 3a, indicating of a better electron-
donating ability, which is ascribed to the introduction of alkoxyl
and alkyl sulfanyl groups. However, the first half-wave oxidation
potentials of 3c, 3d and 3f are increased by about 0.15 V, 0.14 V and
0.27 V relative to 3a, which provides a support of their strong
electron-accepting ability. It should be pointed out that the elec-
trochemical oxidations of compounds 3a, 3b and 3e containing
electron-donating groups display two one-electron processes cor-
responding to the formation of dibenzotetracene monoanion and
dianion. While only one oxidative wave is observed for 3c, 3d and
3f. Taking the CV experimental results into consideration, one does
believe that the obvious oxidation potentials of these derivatives
(3a, 3b and 3e) containing electron-donating units arise from the
oxidation of substituents (methyl, oxygen and sulfur) and the sec-
ond redox processes belong to the oxidation of core building blocks.
Fig. 4. HOMO and LUMO molecular orbitals and energy levels (eV) of compounds
3ae3f calculated by DFT at B3LYP/6-31G.
the energy levels. The energy gap of these compounds were
calculated to be ꢂ3.72 eV for 3a, ꢂ3.75 eV for 3b, ꢂ3.67 eV for
3c, ꢂ3.62 eV for 3d, ꢂ3.59 eV for 3f and ꢂ3.40 eV for 3f, which is
close to the band gaps determined from the UVeVis absorption and
electrochemical experimental results.
In
addition,
according
to
the
empirical
formula
EHOMO ¼ ꢂ[4.8 ꢂ EFOC þ Eoxonset] eV, the HOMO energy levels of
compounds 3ae3f were estimated (onset potentials) to
be ꢂ5.32 eV, ꢂ5.21 eV, ꢂ5.47 eV, ꢂ5.46 eV, ꢂ5.20 eV and ꢂ5.59 eV,
respectively [59,60]. The thermal properties of the new molecules
were also investigated through thermogravimetric analysis (TGA)
in a nitrogen atmosphere. Their onset temperatures of the weight
loss (5%) are ca. 362 ꢀC (3a), 378 ꢀC (3b), 341 ꢀC (3c), 386 ꢀC (3d),
363 ꢀC (3e) and 403 ꢀC (3f), suggesting that they exhibit good
thermal stability (Fig. S1).
3.5. OLED
Taking the high quantum yield of 3c into consideration, a 3c-
based multilayer OLED was fabricated with the following device
structure: ITO/TAPC(20 nm)/mCP:3c(%1 or 3%)(30 nm)/TPBi
(50 nm)/Liq(2 nm)/Al(150 nm). ITO refers to indium-tin-oxide,
TAPC
is
1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane
(host material), mCP is 9,90-(1,3-phenylene)bis-9H-carbazole and
TPBi is 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (elec-
tron transporting material).
3.4. Theoretical calculations
Density functional theory (DFT) at the B3LYP/6-31G level was
used to optimize their geometry [61] and the plots of HOMOs and
LUMOs are presented in Fig. 4. The HOMOs are different between
compounds 3a, 3b, 3e containing electron-donating groups and 3c,
3d, 3f bearing electron-withdrawing units. The molecular orbitals
of 3a, 3b and 3e are mainly located on the benzo[f]tetraphene
moiety and extended up to the methyl carbon, methoxyl oxygen,
nonanethiol sulfur atoms, whereas those of 3b, 3c and 3f are
delocalized on benzo[e]pyrene fragments and the substituents (eF,
eBr, eCN) do not contribute to HOMOs. In contrast, the LUMOs of
all the compounds are mainly on benzo[f]tetraphene segments.
As shown in Fig. 5, the doping concentrations are 1% and 3% for
3c in mCP. The devices emit strong blue light with CIE coordinate
of (0.1582, 0.0868) for 1% doping and (0.164, 0.1116) for 3% dopant
concentration, corresponding to the deep blue region. The turn on
voltages with a brightness of 1.0 cd/m2 for 1% and 3% doping 3c
were 4.5 V and 4.3 V, respectively. The devices could reach the
same maximum brightness of 1850 cd/m2 at 14 V, clearly implies
that there is no obvious difference for the luminance with the
increase of dopant concentration. In addition, the current effi-
ciency of 3% doping is larger than that of 1% doping concentration
of 3c at the starting low bias (Fig. 5b), indicating a higher carrier
injection and transport [62e64]. Evidently, the current efficiency
is relatively low, and further optimization should be carried out in
the feature.
Obviously, both the electron-donating (eCH3, eOCH3, eSC9H19
)
and electron-withdrawing (eF, eBr, eCN) substituents participated
in the LUMO orbitals. The HOMO energy levels are in the range
of ꢂ5.13 to ꢂ5.81 eV, which matches well with the cyclic voltam-
metry data. These results also support the introduction of different
substituents in the twisted dibenzotetracenes can effectively tune
The device exhibits the electroluminescence spectra at 435 nm
and 458 nm, being consistent with the emission spectra in meth-
ylene chloride (434 nm and 457 nm) and solid-state (437 nm and

J. Xiao et al. / Dyes and Pigments 112 (2015) 176e182
181
Province (B2014201007), and the introduction of overseas students
foundation of Hebei Province.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.07.007.
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