Studies of fluorine auxochrome in C9-fluorenyl anthracenes on optoelectronic property for blue electroluminescent materials

Article abstract of DOI:10.1016/j.dyepig.2018.06.008

Two series of fluorine-containing fluorenyl anthracenes linked by a rigid sp³-hybridized carbon atom have been synthesized and characterized. The effect of a fluorine auxochrome at different positions in fluorene moiety on bulk properties, such as photophysical, electrochemical properties and thermal stabilities were investigated. The non-doped organic light-emitting diodes (OLEDs) utilizing C9-methylated fluorinated fluorenyl anthracene 4b as the emitter exhibits deep-blue emissions CIE x, y (0.159, 0.106) with efficiency of 1.31 cd/A and a maximum brightness of 2040 cd/m² at 14 V. It was found that the introduction of fluorine auxochrome to the C2 position of fluorene could afford the best performances of OLEDs among the fluorinated materials.

Full text of DOI:10.1016/j.dyepig.2018.06.008

Accepted Manuscript
Studies of fluorine auxochrome in C9-fluorenyl anthracenes on optoelectronic
property for blue electroluminescent materials
Guobin Ma, Huaxing Zhao, Jing Wang, Yuping Le, Haizhen Jiang, Hongmei Deng,
Jian Hao, Wen Wan
PII:
S0143-7208(18)30918-5
10.1016/j.dyepig.2018.06.008
DYPI 6813
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 April 2018
Revised Date: 5 June 2018
Accepted Date: 6 June 2018
Please cite this article as: Ma G, Zhao H, Wang J, Le Y, Jiang H, Deng H, Hao J, Wan W, Studies of
fluorine auxochrome in C9-fluorenyl anthracenes on optoelectronic property for blue electroluminescent
materials, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.06.008.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Studies of fluorine auxochrome in C9-fluorenyl anthracenes
on optoelectronic property for blue electroluminescent
materials
Guobin Ma a, Huaxing Zhao
Hao a, c *, Wen Wan a *
‡
a, Jing Wang a, Yuping Le a, Haizhen Jiang a, Hongmei Deng b, Jian
a Department of Chemistry, Shanghai University, Shanghai, 200444, China
b Laboratory of Microstructures, Shanghai University, China
c
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai, China
* Corresponding author. Tel.: +81 21 66132190; fax: +81 21 66132190.
E-mail address: wanwen@staff.shu.edu.cn, jhao@shu.edu.cn
‡
The author contributed equally to this work.
Abstract
Two series of fluorine-containing fluorenyl anthracenes linked by a rigid sp3-hybridized carbon
atom have been synthesized and characterized. The effect of a fluorine auxochrome at different
positions in fluorene moiety on bulk properties, such as photophysical, electrochemical properties
and thermal stabilities were investigated. The non-doped organic light-emitting diodes (OLEDs)
utilizing C9-methylated fluorinated fluorenyl anthracene 4b as the emitter exhibits deep-blue
emissions CIE x, y (0.159, 0.106) with efficiency of 1.31 cd/A and a maximum brightness of 2040
cd/m2 at 14 V. It was found that the introduction of fluorine auxochrome to the C2 position of
fluorene could afford the best performances of OLEDs among the fluorinated materials.
Keywords: Fluorenyl anthracenes, Twisted structure, Electroluminescence, Fluorine auxochrome,
Blue OLED.
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1. Introduction
In recent decades, great effort has been focused on the newly developed EL materials and the
fabrication of organic light-emitting diodes due to their promising applications in full-color
flat-panel displays and solid-state lighting [1-4]. Though a large number of EL materials have been
synthesized and investigated, exploitation of new blue-emitting materials with high performance
remains to be a great challenge. Compared with green and red light-emitting devices, the EL
properties of blue-emitting devices still needs to be improved, particularly in terms of efficiency,
color purity and lifetime for full-color applications [5-8]. Excellent blue-emitting materials are
important, not only as blue emitters, but also as hosts for dopant emitters to facilitate efficient
green and red emission.
Among the blue-emitting materials reported, fluorene and its derivatives are regarded as the
most promising candidates for blue OLEDs, due to its attractive building block with high
photoluminescence efficiency, high carrier mobility and easy modification [9-13]. In our previous
work, a series of C-9 fluorene functionalized anthracene derivatives [14, 15], in which the
anthracene and fluorene moieties were linked through a tetrahedral sp3-hybridized carbon atom,
were studied. Similar to the spiro-annulated rigid molecule, the tetrahedral sp3-hybridized atom
has presented an almost orthogonal linkage between the connected conjugated moieties. Due to its
structural twisted configuration and non-planarity, the C-9 linked fluorene functionalized
anthracene are expected to hinder close packing and intermolecular interactions, effectively
minimize its degree of intermolecular stacking in the solid state, so that the tendency for
molecules to crystallize should be reduced.
The fluorine atom is known as its strong electron-withdrawing ability and with isolated
electron couple and fluorinated organic compounds play a key role in the remarkable progress of
medicinal, agricultural, and material sciences [16-20]. Fluorination has been used in the past
decade as a route to improve the electron transport in small molecules and polymer because of its
strong electron-withdrawing ability [21-25]. Though fluorine containing materials used as
light-emitting materials have been intensively studied in phosphorescent luminescent iridium (III)
complexes [26-29], few examples of simple fluorine-containing organic compounds being directly
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used as fluorescent luminescence materials were presented in reported articles [30-32].
In this work, we report the synthesis and luminescent properties of a series of fluorine
containing fluorenyl-anthracene derivatives, in which the fluorine auxochrome was incorporated
onto different positions of the fluorene moiety. The influences of fluorine auxochrome at the
different position in the sterically twisted C9-fluorenyl anthracenes on optoelectronic property
were elucidated for blue electroluminescent materials.
2. Experimental
2.1. Materials and measurements
All the reagents and solvents used for the syntheses and measurements were purchased from
commercial suppliers and were used without further purification unless otherwise noted. The IR
spectra were determined on a PE-1700 IR spectrophotometer by dispersing samples in KBr disks.
1H, 13C, 19F NMR spectra were recorded on Bruker AV-500 spectrometer with CDCl3 as deuterium
solvent. High resolution mass spectra (HRMS) and Mass spectra (MS) were recorded using an
Electron impact (EI) or Electrospray ionization (ESI) techniques. Elemental analyses were
performed with Elemental Vario EL III instrument. UV-vis absorption spectra were recorded on
CRAY-100 spectrophotometer. Fluorescence measurements were carried out with Perkin-Elmer
LS55 in a solution of 10-6 mol/L and solid state, respectively. Oxidation-Reduction Potentials were
measured using a Solartron SI 1287 electrochemical workstation (vs SCE). The melting points
were measured by DSC on Netzstch STA409PC using a scanning rate of 10 oC /min with nitrogen
flushing. The redox potentials of the compounds were determined with cyclic voltammetry (CV)
using a Base 2000 CV system. HOMO values were recalculated from reported literature values as
HOMO = [Eonset]red/ox (vs SCE) + 4.4, Eg = 1241/UV(onset). The luminance-Current-voltage
characteristics were measured simultaneously by the programmable Keithley model 2400 and PR
650 spectrometer. All measurements were performed under an ambient atmosphere at room
temperature.
2.2. Synthesis of 9-(1-fluoro-9H-fluoren-9-yl)anthracene (3a)
Anthracene (1.78 g, 10 mmol), zinc powder (0.65 g, 10 mmol), and 9-bromo-1-fluoro-
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9H-fluorene (2.62 g, 10 mmol) were stirred in CS2 (50 ml) and refluxed for 24 hours. After the
reaction was cooled to room temperature and the solvent was removed by distillation, the residue
was extracted with hot toluene and filtered to remove excess zinc. On cooling, a light yellow
crystalline product was formed from the toluene solution, which was recrystallized from toluene
again, and dried in a vacuum oven to obtain pale yellow crystal 2.3 g (64 %). M.p. 198-199 oC, 1H
NMR (500 MHz, CDCl3): δ/ppm 8.68 (1H, d, J=9.0 Hz), 8.43 (1H, s), 8.08 (1H, d, J=8.0 Hz),
7.94 (1H, d, J=8.0 Hz), 7.88 (1H, d, J=8.5 Hz), 7.75(1H, d, J=7.5Hz), 7.61-7.58 (1H, m),
7.53-7.50 (1H, m), 7.41-7.37 (2H, m), 7.20-7.13 (2H, m), 7.03 (1H, d, J=7.5 Hz), 6.85-6.80 (2H,
m), 6.74 (1H, d, J=9.0 Hz), 6.62(1H, s). 13C NMR (125 MHz, CDCl3): δ/ppm 159.8 (1JC-F =247.5
Hz), 148.8, 143.8, 143.6, 139.6, 134.1, 131.9, 131.7, 130.4, 129.7 (3JC-F =17.5 Hz), 129.3 (3JC-F
=17.5Hz), 129.1, 128.2 (2JC-F =20.0 Hz), 127.4, 126.4, 125.3, 124.8, 120.8, 116.5 (4JC-F =3.7 Hz),
114.4 (2JC-F =20.0 Hz), 46.7 ( 3JC-F =17.5 Hz). 19F NMR (470 MHz, CDCl3): δ/ppm -118.00 (m).
IR (KBr) υmax/(cm-1) 3042, 1934, 1621, 1473, 1445, 1342, 1286, 1159, 1018, 877, 783, 740, 727.
HRMS (ESI-TOF): Found: [M+H]+ 361.1382; C27H18F requires [M+H]+ 361.1387. Elemental
Anal. Calcd for C27H17F: C, 89.97; H, 4.75 %; Found: C, 89.93; H, 4.78 %.
Compounds 3b and 3c were synthesized according to the procedure described for 3a.
2.3. Synthesis 9-(2-fluoro-9H-fluoren-9-yl)anthracene (3b)
Yield 53%. M.p 185-186 oC. 1H NMR (500 MHz, CDCl3): δ/ppm 8.64 (1H, d, J=4.5 Hz), 8.49
(1H, s), 8.14 (1H, dd, J=2.5 Hz), 7.95-7.91 (3H, m), 7.66-7.63(1H, m), 7.59-7.56 (1H, m), 7.43
(1H, t, J=7.5 Hz), 7.25-7.23 (1H, m), 7.17-7.14 (2H, m), 7.06 (1H, dd, J=7.5 Hz, J=2.5 Hz),
6.88-6.74 (1H, m), 6.69 (1H, dd, J=7.5 Hz, J=1.0 Hz), 6.64 (1H, d, J=1.0Hz), 6.49 (1H, s)
13C-NMR (125 MHz, CDCl3): δ/ppm 162.9 (1JC-F =252.5 Hz), 151.2, 148.9, 139.4, 136.3
(4JC-F=2.5Hz), 132.2, 131.8, 129.9, 129.0, 128.2, 127.3 (3JC-F=12.5Hz), 126.7, 125.5, 124.9, 124.5,
123.7, 121.5 (3JC-F=12.5 Hz), 120.2, 114.4 (2JC-F=22.5 Hz), 112.0 (2JC-F=22.5 Hz), 49.0. 19F-NMR
(470 MHz, CDCl3): δ/ppm -114.25 (m). IR(KBr) υmax/(cm-1) 3036, 1615, 1557, 1462, 1324, 1159,
1026, 973, 892, 730. HRMS (ESI-TOF): Found: [M+H]+ 361.1385; C27H18F requires [M+H]+
361.1387. Elemental Anal. Calcd for C27H17F: C, 89.97; H 4.75 %; Found: C, 89.99; H, 4.77 %.
2.4. Synthesis of 9-(3-fluoro-9H-fluoren-9-yl)anthracene (3c)
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Yield 62%. M.p. 177-178 C. H-NMR (500 MHz, CDCl3): δ/ppm 8.53 (1H, d, J=8.5 Hz),
8.37 (1H, s), 8.02 (1H, d, J=7.5 Hz), 7.85 (1H, d, J=8.0 Hz), 7.80 (1H, d, J=8.0 Hz), 7.59 (1H, dd,
J=9.0 Hz, J=2.5 Hz), 7.51-7.44 (2H, m), 7.35 (1H, t, J=7.5 Hz), 7.14-7.07 (2H, m), 6.99 (1H, dd,
J=7.5 Hz, J=1.5 Hz) , 6.89 (1H, dd, J=8.5 Hz, J=1.5 Hz), 6.80-6.74 (2H, m), 6.63 (1H, dd, J=9.0
Hz, J=2.5 Hz), 6.37(1H, s). 13C-NMR (125 MHz, CDCl3) δ/ppm = 162.8 (1JC-F =242.5 Hz), 149.9,
144.4 (4JC-F =2.5 Hz), 142.1 (3JC-F =8.7 Hz), 139.4 (4JC-F =3.7 Hz), 132.2, 132.0, 131.1, 129.7,
129.5, 128.9, 128.0, 127.3, 126.6, 125.5 (3JC-F=8.7 Hz), 124.8, 123.7, 120.8, 114.3 (2JC-F =23.7
Hz), 107.6 (2JC-F =23.7 Hz), 48.4. 19F-NMR (470 MHz,CDCl3) δ/ppm = -115.34 (m). IR(KBr)
υmax/(cm-1) = 3034, 1613, 1552, 1463, 1324, 1155, 1029, 978, 897, 736. HRMS (ESI-TOF): Found:
[M+H]+ 361.1383; C27H18F requires [M+H]+ 361.1387. Elemental Anal. Calcd for C27H17F: C,
89.97; H, 4.75 %; Found: C, 89.92; H, 4.80 %.
2.5. Synthesis of 9-(1-fluoro-9-methyl-9H-fluoren-9-yl)anthracene (4a)
A solution of 9-(1-fluoro-9H-fluoren-9-yl) anthracene (720 mg, 2 mmol) in anhydrous THF
(20 ml) was added with 1 ml of n-Butyl lithium (2.5 mmol 1.6 M) at 0 oC. After stirring for 3 hrs,
0.25 ml (4 mmol) of iodomethane was added to the reaction mixture and stirred for 2 hrs. Then,
the reaction mixture was added to saturated ammonium chloride solution. The mixture was
separated and extracted twice by 50 ml of ethyl acetate. The combined organic layer was washed
with water, dried over magnesium sulfate and filtered. The solvent was removed and the residue
was purified by recrystallization from dichloromethane and petroleum ether to afford white crystal
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620 mg (83%). M.p. 194-195 C. H-NMR (500 MHz, CDCl3): δ/ppm 8.28 (1H, s), 7.96 (1H, d,
J=7.0 Hz), 7.90 (2H, t, J=8.0 Hz), 7.72 (1H, d, J=8.5 Hz), 7.63 (1H, dd, J=7.5 Hz, J=2.5 Hz), 7.50
(1H, dd, J=8.0 Hz, J=2.5 Hz), 7.32 (1H, t, J=8.0 Hz,), 7.21 (3H, m), 7.12 (1H, dd, J=8.5 Hz, J=1.5
Hz), 7.07 (1H, t, J=8.0 Hz), 6.98 (1H, d, J=7.5 Hz), 6.90 (1H, dd, J=8.5 Hz, J=2.5 Hz), 6.73 (1H,
dt, J=7.5 Hz, J=1.5 Hz), 1.84 (3H, s). 13C-NMR (125 MHz, CDCl3): δ/ppm 162.8 (1JC-F =241.2
Hz), 155.7, 150.3 (4JC-F =2.5 Hz), 141.3 (3JC-F =8.7 Hz), 138.5, 134.1, 132.8, 130.9, 130.7, 129.2
(4JC-F =3.7 Hz), 128.8, 128.6, 127.4 (3JC-F =10.0 Hz), 127.2, 126.7, 125.4, 125.3 (3JC-F =8.7 Hz),
124.9, 124.6, 124.5, 124.4, 124.3,123.5, 120.9, 114.6 (2JC-F =23.7 Hz), 107.6 (2JC-F =23.7 Hz),
55.0, 30.9. 19F-NMR (470 MHz, CDCl3): δ/ppm -115.39(m). IR(KBr) υmax/(cm-1) = 3032, 1616,
1560, 1460, 1328, 1159, 1028, 976, 898, 739. HRMS (ESI-TOF): Found: [M+H]+ 375.1540;
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C28H20F: requires [M+H]+ 375.1544. Elemental Anal. Calcd for C28H19F: C, 89.81; H, 5.11 %;
Found: C, 89.85; H, 5.14 %.
Compounds 4b and 4c was synthesized according to the procedure described for 4a
2.6. Synthesis of 9-(2-fluoro-9-methyl-9H-fluoren-9-yl)anthracene (4b)
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Yield 67%. M.p. 194-195 C. H-NMR (500 MHz, CDCl3): δ/ppm 8.31 (1H, s), 8.05 (1H, d,
J=7.5 Hz), 7.99 (1H, d J=8.5 Hz), 7.96-7.92 (2H, m), 7.81 (1H, d, J=8.5 Hz), 7.59-7.55 (1H, dd,
J=7.5 Hz, J=1.0 Hz), 7.40-7.36 (1H, m), 7.31-7.28 (1H, m), 7.24 (1H, s), 7.19-7.18 (2H, m),
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7.13-7.09 (2H, m), 7.07-7.04 (1H, m), 6.74 (1H, dd, J=8.0 Hz, J=2.5 Hz), 1.92(3H, s). C-NMR
(125 MHz, CDCl3): δ/ppm 158.5 (1JC-F =252.5 Hz), 154.7, 138.5, 138.1, 135.3 (4JC-F =2.5 Hz),
132.8, 131.0, 130.6, 129.2 (3JC-F =8.75 Hz), 129.1, 128.7, 127.6, 127.4, 127.3, 126.7, 125.6, 125.4,
125.3, 124.8, 124.5 (3JC-F =8.7 Hz), 123.3, 121.7 (3JC-F =8.7 Hz), 121.6, 120.3, 114.5 (2JC-F =22.5
Hz), 110.9 (2JC-F =23.7 Hz), 55.5 (4JC-F =1.2 Hz), 30.8. 19F-NMR (470 MHz, CDCl3): δ/ppm
-113.42 (m). IR(KBr) υmax/(cm-1) 3034, 1620, 1562, 1460, 1324, 1159, 1029, 975, 897, 737.
HRMS (ESI-TOF): Found: [M+H]+ 375.1546; C28H20F: requires [M+H]+ 375.1544. Elemental
Anal. Calcd for C28H19F: C, 89.81; H, 5.11 %; Found: C, 89.77; H, 5.09 %.
2.7. Synthesis of 9-(3-fluoro-9-methyl-9H-fluoren-9-yl)anthracene (4c)
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Yield 78%. M.p. 165-166 C. H-NMR (500 MHz, CDCl3): δ/ppm 8.28 (1H, s), 7.96 (1H, d,
J=7.0 Hz), 7.90 (2H, t, J=8.0 Hz), 7.72 (1H, d, J=8.5 Hz), 7.63 (1H, dd, J=7.5 Hz, J=2.5 Hz), 7.50
(1H, dd, J=8.0 Hz, J=2.5 Hz), 7.32 (1H, t, J=8.0 Hz,), 7.21 (3H, m), 7.12 (1H, dd, J=8.5 Hz, J=1.5
Hz), 7.07 (1H, t, J=8.0 Hz), 6.98 (1H, d, J=7.5 Hz), 6.90 (1H, dd, J=8.5 Hz, J=2.5 Hz), 6.73 (1H,
dt, J=7.5 Hz, J=1.5 Hz), 1.84 (3H, s). 13C-NMR (125 MHz, CDCl3): δ/ppm 162.8 (1JC-F =241.2
Hz), 155.7, 150.3 (4JC-F =2.5 Hz), 141.3 (3JC-F =8.7 Hz), 138.5, 134.1, 132.8, 130.9, 130.7, 129.2
(4JC-F =3.7 Hz), 128.8, 128.6, 127.4 (3JC-F =10.0 Hz), 127.2, 126.7, 125.4, 125.3 (3JC-F =8.7 Hz),
124.9, 124.6, 124.5, 124.4, 124.3,123.5, 120.9, 114.6 (2JC-F =23.7 Hz), 107.6 (2JC-F =23.7 Hz),
55.0, 30.9. 19F-NMR (470 MHz, CDCl3) δ/ppm = -115.39(m). IR(KBr) υmax/(cm-1) 3032, 1616,
1560, 1460, 1328, 1159, 1028, 976, 898, 739. HRMS (ESI-TOF): Found: [M+H]+ 375.1542;
C28H20F: requires [M+H]+ 375.1544. Elemental Anal. Calcd for C28H19F: C, 89.81; H, 5.11 %;
Found: C, 89.82; H, 5.13 %.
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3. Results and Discussion
3.1. Synthesis of Fluorine-Containing Fluorenyl-Anthrancene Derivatives
The main synthetic routes of the fluorine-containing blue light-emitting materials are shown in
Scheme 1. Three Fluorine-containing 9-fluorenones were synthesized from different starting
materials through rationally designed ways with good yields and the experiment details were
summarized in the supporting information for this article. Reduction of 9-fluorenones with sodium
borohydride in ethanol provided the fluorine-containing 9-hydroxyl fluorene derivatives 1a-1c,
which were further treated with phosphorus tribomide in methylene dichloride to give the
corresponding 9-bromo fluorene derivatives 2a-2c. Anthracene was refluxed with compounds
2a-2c in the presence of zinc powder for 24 hours to generate 3a-3c in modest yields (53%-64%).
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Finally, lithiation of compounds 3a-3c with n-butyl lithium (n-BuLi) at 0 C, followed by the
treatment with iodomethane, gave the C-9 methylated compounds 4a-4c in good yields (67-83%).
Scheme 1. Synthetic route towards fluorine-containing C-9 fluorene-functionalized anthracene derivatives
Thus, novel fluorine-containing fluorene-functionalized anthracene derivatives 3a-3c, in which
a rigid linkage of a tetrahedral sp3-hybridized carbon atom between fluorinated fluorene and
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anthracene moieties (at the C-9 position of the fluorene) were prepared. In order to elevate rigidity,
as well as electrochemical stabilties of these fluorine-containing fluorene-functionalized
anthracenes, further methylation of compounds 3a-3c at the active C9-H resulted the formation of
C9-methylated anthracene derivative 4a-4c, which were expected to exhibit stronger rigidity due
to the existance of steric effect of methyl group. The chemical structures of 3a-3c and 4a-4c were
confirmed by 1H, 13C, 19F NMR and FT-IR spectroscopy. The results of the mass spectrometry and
elemental analysis also supported the formation of the six blue emitting materials and matched
well with the calculated data. Properties such as UV-vis absorption, photoluminescene, and
electroluminescence including EL efficiceny and color purity were evaluated. Compounds 3a-3c
and 4a-4c have good solubility in common organic solvents so that they can be easily purified by
column chromatography and recrystallization to high purity for spectroscopy characterization and
OLEDs application.
3.3. Thermal Properties
Thermal properties of the six fluorine-containing C-9 fluorenyl-anthrancene compounds 3a~3c
and 4a~4c were investigated by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) and summarized in Table 1. The Td is defined by a 5%-weight-loss
temperature under a nitrogen atmosphere. It is found that the six compounds exhibit thermal
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stabilities with decomposition temperatures above 300 C, as shown in the TGA measurements.
Compounds 3a-3c show a little higher thermal stability than those of the corresponding 4a-4c,
which indicate that the introduction of the methyl substituent at the C9-position of fluorene is
unbeneficial for improving the morphological stability and resistance to the thermal
decomposition. The tendency of both Td and Tm for 3a＞3b＞3c and 4a＞4b＞4c show that the
closer position of fluorine atom to anthracene moiety would afford higher thermal stability.
3.4. Optical and Photoluminescent Characteristics
UV–vis absorption of each compound was tested in hexane, dichloromethane, chloroform,
ethylacetate, tetrahydrofuran and ethanol, respectively, to investigate the solvent effects. All
compounds exhibit the characteristic vibrational patterns of isolated anthracene group with three
strong absorption bands in the region of 340-420 nm, indicating the existence of a π-π* transition
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derived from the substituted anthracene backbone. Only 1~4 nm shift based on different solvents
are observed (seen Fig.S7-Fig.S12 in SI). A comparison of the absorptions in dichloromethane
solution in Fig. 1 demonstrate that compounds 3a-3c give nearly the same vibronic patterns at 350
nm, 369 nm and 389 nm, while C9-methylated compounds 4a-4c provide the peak wavelength
around 343 nm, 363 nm and 381 nm. These results indicate that the fluorine auxochrome at
different position on the fluorene moiety in compounds 3a-3c exhibit much less influence on
Uv-vis absorption than those of C9- methylated analogues. A blue shift of less than 10 nm is
presented in the absorption spectra of C9- methylated 4a-4c. Compounds 4a-4c show bigger UV
band edge than 3a-3c, suggesting that the C9-methylated compounds would have larger energy
gap.
3a
3b
3c
4a
4b
4c
300
320
340
360
380
400
420
Wavelength (nm)
Figure 1. UV–vis absorption of compounds 3a-3c and 4a-4c in CH2Cl2 solution (1×10-6 M)
The photoluminescence of each compound was also measured in hexane, dichloromethane,
chloroform, ethyl acetate, tetrahydrofuran and ethanol, respectively, to investigate the influence of
solvents (Seen Fig.S13-Fig.S18 in SI). Shifts of less than 4 nm are observed for three main
emission around 390 nm, 415 nm and 445 nm, which are similar to the emission peaks of
anthracene. Fig. 2 (a) and Fig. 2 (b) show the photoluminescence of all compounds in
dichloromethane solution and solid state at room temperature, respectively. The PL spectra of
compounds 3a-3c in solution from Fig. 2(a) present almost identical fluorescent emission pattern
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with three emission peaks at 394 nm, 418 nm and 444 nm, only 1 nm shift difference for each
compound 3a-3c with fluorine-containing group at different substituted position. Compared with
the analogues of compounds 3a-3c, a blue shift of around 10 nm is detected on PL spectra of
emission peaks located around 384 nm, 408 nm and 431 nm for compounds 4a-4c.
The PL spectra in solid states shown in Fig. 2(b) for all synthesized compounds exhibit a
maxima emission peak in the blue region of 420-450 nm, which have red shift about 10 nm in
comparison with those in dilute solution. The bathochromic shift on the fluorescent emissions is
probably due to the difference in dielectric constant of the environment. The PL spectra for 4a and
4b showed 2-3 nm red shift, compared with that of 4c. The PL spectra in solid states for
compounds 4a-4c show almost the same spectra around 420 nm and 440 nm, suggesting that the
intermolecular interactions are restrained effectively by the twisted bulky fluorenyl-anthracene
units after methylation of C9-H in compounds 3a-3c. These phenomena of PL in solid state are
also observed by careful studies between compounds 3a, 3b, which reveal that the fluorine
auxochrome at the 1- or 2-position of fluorene moiety cause little effect on the emission process.
However, the PL spectra around 430 nm and 451 nm for 3c, which has a fluorine substituent at the
3-position of fluorene, presents a stronger bathochromic effect with the respect of 3a and 3b with
fluorine atom at 1- or 2-position. The fluorescence quantum yield was measured by calibrating
against quinine sulfate (φf ≈ 0.55) [33]. The emission quantum yields are high in the region of
0.79-0.92, indicating that these fluorine containing compounds would have good efficient
electroluminescent properties in OLED devices.
3a
a
3a
3b
3c
4a
4b
4c
b
3b
3c
4a
4b
4c
360
380
400
420
440
460
480
360
390
420
450
480
510
540
570
600
Wavelength (nm)
Wavelength (nm)
Figure 2. Photoluminescene in dichloromethane solvent (a) (1×10-6 M) and in solid states (b)
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3.5. Electrochemical Behavior
To explore the influence of chemical structure on the electrochemical properties of these
materials, the redox potentials of the compounds were determined with cyclic voltammetry (CV)
measurements, which were carried out in a three-electrode cell setup with 0.1 M
tetrabutyl-ammonium hexafluorophosphate (Bu4NPF6) as a supporting electrolyte in anhydrous
CH2Cl2 to probe the electrochemical behavior of the materials. The molecular orbital energy levels
of compounds 3a-3c and 4a-4c have been derived from cyclic voltammetry (CV) (Fig. 3) in
combination with the optical band gaps (Eg) determined by the onset of the absorption in solution
and are presented in Table 1.
HOMO values were calculated from HOMO= [Eonset]red/ox (vs SCE) + 4.4, Eg=1240/ UV(onset)
[34]. All compounds show a reversible oxidation process, indicating their electrochemical
stabilities. As seen from Fig. 3 and Table 1, all HOMO energies of the fluorine-containing
fluorenyl-anthracene derivertives are at about 5.62 - 5.70 eV. Such a low HOMO energy level
greatly reduces the energy barrier for hole injection from indium tin oxide (ITO) to the emissive
fluorine-containing anthracene-fluorene derivatives. The energy band gaps (Eg), which are
estimated from the onset of the absorption spectra, are located around 3.10 eV for compounds
3a-3c and 3.15 eV for compounds 4a-4c, respectively, suggesting that these newly prepared
compounds are potentially suitable as blue emitting materials. A lager energy gaps (Eg) for 4a-4c
than those of 3a-3c would be resulted from the distorted π-conjugation on the fluorenyl anthracene
molecule due to the replacement of C9-H with the bulky methyl group. The LUMO energies are
calculated around 2.5 eV-2.6 eV by combining the HOMO energy levels together with the
bandgaps obtained from the absorption edge. Compared with the analogues of compound 3a-3c,
compounds 4a-4c show relatively lower HOMOs and LUMOs energies with larger bandgaps (Eg).
The substitution position of the fluorine atom on fluorene moiety doesn’t show considerable
difference on molecular orbital energy on the series 3 and 4. The energy gap for these compounds
are found to show little relationship with the substituted position of fluorine atom on fluorine
moiety.
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3a
3b
3c
4a
4b
4c
6
4
2
0
-2
-4
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Voltage / V
Figure 3. The curves of cyclic voltammetry for the fluorine-containing compounds 3a-3c and 4a-4c
Table 1. Optical, thermal and electrical properties of compounds
E1/2
HOMO LUMO
Eg
Td
Tm
λmax, Abs
λmax, PL
λmax, PL
QE
(φ) c
0.79
0.82
0.86
0.89
0.92
0.84
Compd.
(nm) a
(nm) a
(nm) b
(eV)
(eV)
(eV)
(eV)
(oC)
(oC)d
3a
3b
3c
4a
4b
4c
350, 369, 389
351, 370, 390
352, 369, 389
343, 362, 381
344, 363, 382
343, 362, 381
394, 417, 442
395, 418, 443
394, 418, 442
384, 407, 430
385, 408, 431
439
1.29
1.31
1.31
1.23
1.22
1.23
5.69
5.70
5.70
5.62
5.63
5.63
2.61
2.60
2.60
2.47
2.49
2.49
3.08
3.10
3.10
3.15
3.14
3.14
385
355
332
339
312
308
198
185
177
194
183
165
440
430, 451
420, 442
420, 443
419, 440
384, 408, 430
a
b
Measured in CH2Cl2 solution. Film samples. c Fluorescence quantum efficiency, relative to quinine sulfate in
cyclohexane (φ ≈ 0.55). d Melting point
f
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Three-dimensional geometries and the frontier molecular orbital energy levels of compounds
3a-3c and 4a-4c were accomplished using density functional theory (DFT) calculations at the
B3LYP/6-31G* level on Gaussian 03 programs [35-37]. Their ground geometries and the orbital
plots of the HOMO-LOMOs were illustrated in Figure 4. Obviously, they exhibit large torsion
angles in the optimized geometries. The calculated geometries and that comparison with X-ray
diffraction analysis of the known fluorine-free analogue [14, 15] demonstrate that the
fluorine-containing fluorene and the anthracene moieties are significantly twisted against each
other through the linkage of the sp3-hybridized carbon atom, resulting in a non-coplanar twisted
conformation in each molecule. Due to the introduction of the bulky methyl group, compound
4a-4c exhibit an almost orthogonal conformation. These dramatically twisted conformations can
effectively prevent intermolecular interactions between π-systems of aromatic rings, which are
proposed to suppress the crystallization of the materials and finally improve the morphological
stability of these molecules. Such a unique twisted structural feature of fluorenyl-anthracene is
beneficial for their suppressing on fluorescent quenching induced by the aggregation in the solid
states and for the formation of uniform amorphous morphology in the film state of OLED device.
The calculated electron densities of the HOMOs and LUMOs are mainly localized on the
anthracene moiety, indicating that the absorption and emission process are only attributed to the
π-π* transition centered on the anthracene moiety. Thus, the excellent luminescence luminescene
efficiency of anthracene moiety in the synthesized compounds can be maintained.
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Compds.
Theoretical structure
HOMO
LUMO
3a
3b
3c
4a
4b
4c
Figure 4. HOMO and LUMO electronic density distributions of 3a-3c and 4a-4c
3.6. OLED Fabrication
To investigate their potential applications in OLED, these fluorine-containing
fluorenyl anthrancene derivatives 3a-3c and 4a-4c were used as emitting layer by thermal
evaporation technique with following configuration to fabricate simple non-doped OLED
devices. The device structure is depicted in Figure 5 (a): ITO/MoOx (4 nm)/NPB (20 nm)
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/Emitting layer(30 nm) /Bphen (20 nm) /LiF (0.7 nm)/Al, where N, N’-diphenyl-N, N’-bis
(1-naphthyl)-(1,1-biphenyl)-4,4’-diamine (NPB), 4,7-diphenyl-1,10- phenanthroline (Bphen) were
used as hole-transporting and electron-transporting layer, respectively. The combination of a thin
LiF layer and Al were applied to form an efficient cathode and ITO (indium tin oxide) was used as
anode. The ITO glass substrate was cleaned with detergent and deionized water and dried in an
oven. It was then treated with UV-ozone for 30 minutes before loading into a deposition chamber.
The organic films of NPB, emitting material, Bphen and LiF/Al were sequentially deposited onto
the ITO substrate at a pressure of around 3×10-4 Pa. The EL spectra and luminance were recorded
on a PR650 spectrometer. Luminance-voltage and current-voltage characteristics were measured
at room temperature.
b
3a
3b
3c
4a
4b
4c
50 mA
300
400
500
600
700
800
Wavelength (nm)
Figure 5. Schematic energy level of different layer (a) and EL spectra of six fluorine- containing
fluorenyl-anthracene derivatives at 50 mA (b)
As shown in the schematic energy level diagram of OLEDs in Fig.5 (a), the hole-injection
barrier at NPB/emitter is about 0.3 V, while the electron-injection barrier at the emitter is about 0.4
V. These small injection barriers could enhance the efficient injection of both holes and electrons
into the emission layer. Fig. 5 (b) showes the typical EL spectra of OLEDs based on 3a-3c and
4a-4c as the emitting layers, suggesting that carriers recombination is effectively restricted in
these blue materials when compared with their PL ones. All devices exhibit blue emission with
peak maxima between 444-468 nm. A stable emission in EL spectra and CIE coordinates from all
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diodes obtained on different applied voltages demonstrate that these fluorine-containing
compounds reveal excellent operating stabilities for blue emission. In comparison with the
corresponding PL of solid states, the El spectra for compounds 3a-3c and 4a-4c are found to
afford a red shift of about 20 nm, which may be caused by the intermolecular interactions and the
electrical field polarization in the excited states. Compounds 3a shows the emission peak at 468
nm with a width at half maximum (FWHM) about 80 nm and compounds 3b, 3c afford the
emission peak at 464 nm with a width at half maximum (FWHM) about 70 nm, giving only 4 nm
blue shift difference according the study on the influence of different fluorine substituted position.
While, compound 4a provides the emission peak at 468 nm with a FWHM about 80 nm and
compound 4c gives the emission peak at 462 nm with a FWHM about 70 nm, respectively. It is
interesting to find that compound 4b, which has a fluorine substitutent at 2-position on fluorene
moiety of C9-methylated fluorene-anthracene derivative, exhibits the emission peak at 444 nm
with the narrowest FWHM around 60 nm among all the derivatives, giving nearly 20 nm blue shift
in comparsion with 3b or 4a. Those results indicate that the incorporation of a fluorine atom into
the C2-position of fluorene moiety in the methylated fluorene-anthracene would have significant
steric hindrance and greatly reduce the aggregation formation emitting at longer wavelength [38,
39].
The EL performance of the six devices is studied in Figure 6, and the key parameters are
collected in Table 2.
The devices based on compounds 3a, 3b and 4a show a sky-blue emission with CIE
coordinates for 3a (0.169, 0.313), 3b (0.168, 0.230), 4a (0.157, 0.272), while a deep-blue emission
for devices from 3c (0.160, 0.165), 4c (0.162, 0.122), especially that for compound 4b (0.159,
0.106), which is very close to the pure blue light (0.159, 0.080) presented. However, it is found
that compounds 3a, 3b and 4a exhibit CIE coordinates of y>0.23 with an obvious observation of
red shift of emission peak and wider FWHM. By analysis on the coordinates of compounds 3a, 3b
and 3c, it can be clearly observed that incorporation of fluorine atom further away from
anthracene moiety would provide deeper blue (smaller y coordinates) emitting. Interestingly,
compound 4b is endowed with significant deep-blue emission among 4a-4c. This might be
ascribed to the collaborative effect for an isolated electron couple, electron-withdrawing ability of
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fluorine atom and the steric effect of C9 methyl group.
As shown by the voltage-current density-luminance characteristics (Fig. 6 (a) and Fig. 6(b))
and the efficiency curves (Fig.6 (c)), the devices based on the blue emitters 3a-3c exhibit a
maximum current efficiency of 1.01 cd/A, 1.06 cd/A and 1.17 cd/A with a maximum brightness of
1219 cd/m2, 1808 cd/m2 and 1826 cd/m2 at 14 V, respectively. However, much-higher EL
efficiencies are obtained with a maximum current efficiency of 1.46 cd/A, 1.31 cd/A, 1.27 cd/A
and a maximum brightness of 2070 cd/m2, 2040 cd/m2, 1795 cd/m2 for 4a, 4b and 4c at 12V,
respectively. It is interesting to find that a better luminescence efficiency would be obtained when
the fluorine substituent is positioned away from anthracene moiety in compound 3a-3c. In contrast,
an opposite trend of luminescence efficiency would be observed in the case of C9-methylated
fluorenyl-anthrancene 4a-4c. It is obvious that the current density and maximum brightness for
4b-based device provided the best performance among these of the corresponding devices, despite
the identical device configuration applied. As it is known that the fluorine atom exhibits very
strong electron-withdrawing ability and the possible interaction of fluorine atom with the adjacent
aromatic-H,
compound
4b
would
probably
present
the
most
suitable
electric and steric effect among the prepared C9-methylated fluorinated fluorenyl anthracene, due
to the incorporation of the fluorine atom into the 2-position on fluorenyl moiety.
The steric and electronic configuration of 4b would contribute to the best performance among
4a-4c. It is also found from Fig. 6 (c) that the efficiencies for these fluorine-containing derivatives
have only a mild decrease as the current density increase. The devices based on 4a–4c provide
better luminous efficiencies than those for the counterpart 3a–3c, indicating that the highly twisted
structure and strong rigidity of the fluorine-containing fluorenyl-anthracene molecules induced by
the steric effect of the methyl group would play an important role in the improvement of the
luminous efficiency. Additionally, all devices have turn-on voltages of 4.2-5.1 V at a brightness of
1 cd/m2. One important reason for such similar turn-on voltages is that the injection barriers in the
devices are very close to each other.
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1750
1500
1250
1000
750
500
250
0
(a)
3a
3b
3c
4a
4b
4c
0
2
4
6
8
10
12
Voltage (V)
(b)
180
150
120
90
3a
3b
3c
4a
4b
4c
60
30
0
0
2
4
6
8
10
12
Voltage (V)
(c)
1.5
1.2
0.9
0.6
0.3
0.0
-0.3
-0.6
3a
3b
3c
4a
4b
4c
-20
0
20 40 60 80 100 120 140 160 180 200
Current density (mA / cm2)
Figure. 6. Luminescence-Voltage (V) (a), Current density (I)-voltage (V) (b) and L-C (luminance efficiency vs.
current density) characteristics (c) for the six fluorine- containing fluorenyl- anthracene derivatives
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Table 2. Electroluminescene properties of compounds
ELmax
(nm)
Brightness
(cd/m2)
Efficiency
(cd /A)
CIE
Compd.
(x, y)
3a
3b
3c
4a
4b
4c
468
464
464
468
444
462
1219 a
1808 a
1826 a
2070 b
2040 b
1795 b
1.01
1.06
1.17
1.46
1.31
1.27
0.169, 0.313
0.168, 0.230
0.160, 0.165
0.157, 0.272
0.159, 0.106
0.162, 0.122
a the data were measured at 14 V, b: the data were obtained at 12 V
Conclusion
In summary, six fluorinated fluorenyl functionalized anthracenes, which contain a twisted
configuration due to the rigid linkage of a sp3-hybridized carbon atom were prepared and
described. The DFT theoretical calculation supports that the synthesized materials have
non-coplanar structures with large intramolecular torsion angles, which would avoid π-π stacking
in the solid state. The experimental results were well supported by DFT theoretical calculations.
The compounds 3a-3c and 4a-4c exhibit strong blue PL emissions, good thermal stability, and
good film forming property. The effects of the fluorine auxochrome at the substituted position of
the fluorene moiety on the photophysical, electrochemical properties and thermal stabilities, as
well as the performance of non-doped blue-light-emitting OLED have been discussed. The
introduction of fluorine atom into fluorenyl functionalized anthracene at C2-position (4b) could
give the excellent electroluminescent performance among the host emitter materials synthesized in
this work.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No.
21572128, 21672139) and the Science Foundation of Shanghai Municipal Commission of
Sciences and Technology (No. 15230724800). The authors thank the Laboratory of
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Micro-structures of Shanghai University for structure analysis.
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Scheme and Figure captions
Scheme 1. Synthetic route towards fluorine-containing C-9 fluorene-functionalized anthracene
derivatives
Fig. 1. UV–vis absorption of compounds 3a-3c and 4a-4c in CH2Cl2 solution (1×10-6 M)
Fig. 2. Photoluminescene in dichloromethane solvent (a) (1×10-6 M) and in solid states (b)
Fig. 3. The curves of cyclic voltammetry for the fluorine-containing compounds 3a-3c and 4a-4c
Fig. 4. HOMO and LUMO electronic density distributions of 3a-3c and 4a-4c
Fig. 5. Schematic energy level of different layer (a) and EL spectra of six fluorine- containing
fluorenyl-anthracene derivatives at 50 mA (b)
Fig. 6. Luminescence-Voltage (V) (a), Current density (I)-voltage (V) (b) and L-C (luminance
efficiency vs. current density) characteristics (c) for the six fluorine- containing fluorenyl-
anthracene derivatives
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►Six new fluorine-containing C9-fluorenyl anthracenes linked by a rigid sp3-hybridized carbon
atoms were synthesized and characterized. ► The effect of fluorine auxochrome at different
position in fluorene moiety on photophysical, electrochemical properties and thermal stabilities
were investigated. ► The non-doped organic light-emitting diodes (OLEDs) utilizing
C9-methylated fluorinated fluorenyl anthracene 4b as the emitter exhibits deep-blue emissions
CIE x, y (0.159, 0.106) with efficiency of 1.31 cd/A and a maximum brightness of 2040 cd/m2 at
14 V. ► The introduction of fluorine atom into fluorenyl functionalized anthracene at C2-position
(4b) could give the excellent electroluminescent performance among the host emitter materials
synthesized in this work.