Crosslinkable fluorene-based derivatives as materials for light emitting diodes

Article abstract of DOI:10.1080/15421400802462722

A series of cross-linkable fluorene-based derivatives have been synthesized by the multistep synthetic rout. Characterization of their structure by 1H NMR and mass spectrometry is presented. The synthesized materials were examined by various techniques including differential scanning calorimetry, thermogravimetry, UV, fluorescence and electron photoemission spectrometries. It is established that the new derivatives have large triplet energies and could be used as host materials in phosphorescent light emitting diodes. The device fabricated with the use of one of the new hosts reached a brightness of 1700cd/m2 and a current efficiency of 3.4cd/A.

Full text of DOI:10.1080/15421400802462722

This article was downloaded by: [McMaster University]
On: 14 October 2014, At: 10:04
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Crosslinkable Fluorene-Based
Derivatives as Materials for
Light Emitting Diodes
S. Lengvinaite a , J. V. Grazulevicius a , S.
Grigalevicius a , B. Zhang b & Z. Xie b
a Department of Organic Technology, Kaunas
University of Technology , Kaunas, Lithuania
b State Key Laboratory of Polymer Physics and
Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences ,
Changchun, China
Published online: 10 Jun 2010.
To cite this article: S. Lengvinaite , J. V. Grazulevicius , S. Grigalevicius , B. Zhang &
Z. Xie (2008) Crosslinkable Fluorene-Based Derivatives as Materials for Light Emitting
Diodes, Molecular Crystals and Liquid Crystals, 497:1, 202/[534]-210/[542], DOI:
10.1080/15421400802462722
To link to this article: http://dx.doi.org/10.1080/15421400802462722
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with

primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 497, pp. 202=[534]–210=[542], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802462722
Crosslinkable Fluorene-Based Derivatives as Materials
for Light Emitting Diodes
S. Lengvinaite1, J. V. Grazulevicius1, S. Grigalevicius1,
B. Zhang2, and Z. Xie2
1Department of Organic Technology, Kaunas University of Technology,
Kaunas, Lithuania
2State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, China
A series of cross-linkable fluorene-based derivatives have been synthesized by the
multistep synthetic rout. Characterization of their structure by 1H NMR and mass
spectrometry is presented. The synthesized materials were examined by various
techniques including differential scanning calorimetry, thermogravimetry, UV,
fluorescence and electron photoemission spectrometries. It is established that the
new derivatives have large triplet energies and could be used as host materials
in phosphorescent light emitting diodes. The device fabricated with the use of
one of the new hosts reached a brightness of 1700 cd=m2 and a current efficiency
of 3.4 cd=A.
Keywords: aromatic amine; cross-linkable material; ionization potential; light emitting
diode; triplet energy
1. INTRODUCTION
Improvements in the performance of organic light emitting diodes
(OLEDs) over the past decade have resulted in commercially available
products. Organometallic small molecule phosphor-doped devices have
demonstrated by far the highest efficiency [1,2]. In phosphorescent
devices, to reduce the quenching associated with relatively
long excited-state lifetimes of triplet emitters and triplet-triplet
Financial support of this research by the Lithuanian Science and Studies Foundation
and by the Alexander von Humboldt Foundation (Germany) is gratefully acknowledged.
Address correspondence to J. V. Grazulevicius, Department of Organic Technology,
Kaunas University of Technology, 19, Radvilenu Plentas, Kaunas LT50254, Lithuania.
E-mail: juozas.grazulevicius@ktu.It
202=[534]

Fluorene Derivatives as Materials for Multilayer Light Emitting Diodes 203=[535]
annihilation, etc., triplet emitters of heavy-metal complexes are
normally used as emitting guests in a host material, and thus suitable
host materials are of equal importance for phosphorescent OLEDs. For
the electrophosphorescence from triplet guests, it is essential that the
triplet level of a host be larger than that of the triplet emitter to
prevent the reverse energy transfer from the guest back to the host
and to effectively confine triplet excitons on guest molecules [3,4].
We have observed earlier that carbazole- and diphenylamine-based
derivatives demonstrate good hole transport properties and large trip-
let energies [5,6]. Here, we report the molecular design of materials
containing the non-conjugated linking of fluorene with carbazol-9-yl
or diphenylamino fragments. The substituted fluorenes demonstrate
rather large triplet energies and are potential host materials for green
or red electro-phosphorescent devices. On the other hand, the deriva-
tives contain cross-linkable oxetanyl groups and could be used for the
preparation of a insoluble host-guest system which is suitable for the
preparation of multilayer OLEDs by spin-coating [7].
2. EXPERIMENTAL
2.1. Instrumentation
1H NMR spectra were recorded using a Varian Unity Inova (300 MHz)
apparatus. Mass spectra were obtained on a Waters ZQ 2000 spec-
trometer. FTIR spectra were recorded using a Perkin Elmer FT-IR
System. UV spectra were measured with a Spectronic GenesysTM
8
spectrometer. Fluorescence (FL) spectra were recorded with a MPF-
4 spectrometer. Differential scanning calorimetry (DSC) measure-
ments were carried out using a Bruker Reflex II thermosystem.
Thermogravimetric analysis (TGA) was performed on a Netzsch STA
409. The TGA and DSC curves were recorded in a nitrogen atmos-
phere at a heating rate of 10ꢀC=min.
The ionization potentials of the layers of the compounds synthesized
were measured by the electron photoemission method in air which was
described earlier [8]. The measurement method was, in principle, simi-
lar to that described by Miyamoto et al. [9].
The electro-phosphorescent devices were fabricated on glass sub-
strates and had the typical structure with the organic layers sand-
wiched between a bottom ITO anode and a top metal cathode
(Ca=Al). Before use, the ITO-coated glass substrates were carefully
cleaned and treated with UV=ozone right before the deposition of
organic layers. PEDOT layers were deposited by spin-coating and
heated at 120ꢀC for 30 min. The electro-phosphorescent layers were

204=[536]
S. Lengvinaite et al.
made by spin-coating: e.g., a ꢁ100-nm layer of the host (mixture-7:3
of 3 or 6 with PBD) containing 2 wt.% of guest {Ir(ppy)3} onto the
substrates with PEDOT. Evaporation of the cathode was done in a
vacuum evaporation equipment.
The current–voltage and luminance–voltage characteristics were
recorded under a forward bias using a computer-controlled Keithley
2400 source meter and a PR650 spectrometer.
2.2. Materials
2-Bromofluorene (1), 2,7-dibromofluorene, 9H-carbazole, diphenyl-
amine, 3-methyldiphenylamine, benzyl triethyl ammonium chloride,
potassium carbonate, copper powder, 18-crown-6, Pd2(dba)3, (t-Bu)3P,
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), tris(2-
phenylpyridine) iridium (III) {Ir(ppy)3} and sodium tert-butoxide were
purchased from Aldrich. 3-Bromomethyl-3-methyl-oxetane was bought
from Chemada Fine Chemicals (Israel). All the chemicals were used
without further purification.
2-Bromo-9,9-di(3-methyloxetan-3-ylmethyl)fluorene (2) was pre-
pared by the reaction of 2-bromofluorene (1) with 3-bromomethyl-3-
methyl-oxetane in a two-phase system [10,11]. 2-Bromofluorene
(5.0 g, 20 mmole) and benzyl triethyl ammonium chloride (0.2 g,
0.92 mmole) were dissolved in DMSO (20 ml), and 20 ml of a sodium
hydroxide aqueous solution (50%) were added. In 5 min, 3-bromo-
methyl-3-methyl-oxetane (8.42 g, 50 mmole) was added dropwise into
the reaction mixture. The two-phase system was stirred at 80ꢀC for
6 h under argon. After the TLC control, the reaction mixture was
treated with 2 N HCl, and the product was extracted with chloroform.
The organic layer was dried over MgSO4. After filtration and evapo-
ration of the solvent, the crude product was purified by silica gel
column chromatography using ethylacetate=hexane (vol. ratio 1:3) as
an eluent. Yield: 5.6 g (68%) of yellow powder.
2,7-Dibromo-9,9-di(3-methyloxetan-3-ylmethyl)fluorene was pre-
pared by a similar procedure as monobromo derivative 2. 2,7-Dibromo-
fluorene (5 g, 15 mmole) and benzyl triethyl ammonium chloride
(0.15 g, 0.69 mmole ) were dissolved in DMSO (20 ml), and 20 ml of a
sodium hydroxide aqueous solution (50%) were added. In 5 min, 3-bro-
momethyl-3-methyl-oxetane (6.36 g, 37.5 mmole) was added dropwise
into the reaction mixture. The two-phase system was stirred at 80ꢀC
for 5 h under argon. After the TLC control, the reaction mixture was
treated with 2 N HCl, and the product was extracted with chloroform.
The organic layer was dried over MgSO4. After filtration and evapor-
ation of the solvent, the crude product was purified by silica gel

Fluorene Derivatives as Materials for Multilayer Light Emitting Diodes 205=[537]
column chromatography using ethylacetate=hexane (vol. ratio 1:3) as
an eluent. Yield: 4 g (52.5%) of yellow powder.
2-(9-Carbazolyl)-9,9-di(3-methyloxetan-3-ylmethyl)fluorene (3). 0.5 g
(1.2 mmole) of 2-bromo-9,9-di(3-methyloxetan-3-ylmethyl)fluorene (2),
0.6 g (3.6 mmole) of 9H-carbazole, powdered potassium carbonate
(0.66 g, 4.8 mmole), copper powder (0.3 g, 4.8 mmole), and 18-crown-6
(0.07 g, 0.24 mmole) were stirred in o-dichlorobenzene (15 ml) at 180ꢀC
under nitrogen for 18 h. Copper and inorganic salts were filtered off,
and the solvent was removed by distillation. After precipitation into
hexane, the crude product was purified by silica gel column chromato-
graphy using ethyl acetate=hexane (vol. ratio 2:1) as an eluent. Yield:
0.5 g (83%).
2-Diphenylamino-9,9-di(3-methyloxetan-3-ylmethyl)fluorene
(4).
Pd2(dba)3 (11 mg, 0.012 mmole) and (t-Bu)3P (6 mg, 0.03 mmole) were
stirred in 25 ml of toluene under nitrogen at room temperature for
10 min. 1 g (2 mmole) of 2-bromo-9,9-di(3-methyloxetan-3-ylmethyl)-
fluorene (2), 0.4 g (2.4 mmole) of diphenylamine and 0.58 g (6 mmole)
of sodium tert-butoxide were added into the reaction mixture, and it
was stirred for 8 h at 80ꢀC. The reaction mixture was cooled and
quenched by the addition of ice water. The product was extracted into
ethylacetate, and the extract was dried over anhydrous MgSO4. The
crude product was purified by silica gel column chromatography using
ethyl acetate=hexane (vol. ratio 1:3) as an eluent. Yield: 0.48 g (39%).
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 7.69–6.9 (m, 16H, Ar),
4.23 (d, 2H, CH2 of oxetane ring, J ¼ 5.5 Hz), 4.18 (d, 2H, CH2 of oxe-
tane ring, J ¼ 5.7 Hz), 3.65 (d, 2H, CH2 of oxetane ring, J ¼ 5.4 Hz),
3.51 (d, 2H, CH2 of oxetane ring, J ¼ 5.6 Hz), 2.47–2.29 (m, 4H,
2 ꢂ CH2), 0.50 (s, 6H, 2 ꢂ CH3).
MS(APCIþ, 20 V), m=z (%): 502.3 ([M þ H]þ, 100).
2,7-Di(9-carbazolyl)-9,9-di(3-methyloxetan-3-ylmethyl)fluorene (5).
0.5 g (1 mmole) of 2,7-dibromo-9,9-di(3-methyloxetan-3-ylmethyl)fluor-
ene, 1 g (6 mmole) of 9H-carbazole, powdered potassium carbonate
(0.96 g, 7 mmole), copper powder (0.25 g, 4 mmole), and 18-crown-6
(0.05 g, 0.2 mmole) were stirred in o-dichlorobenzene (15 ml) at 180ꢀC
under nitrogen for 12 h. Copper and inorganic salts were filtered off,
and the solvent was removed by distillation. After precipitation into
hexane, the crude product was purified by silica gel column chromato-
graphy using ethyl acetate=hexane (vol. ratio 1:3) as an eluent. Yield:
0.2 g (30%).
2,7-Di(3-methyldiphenylamino)-9,9-di(3-methyloxetan-3-ylmethyl)
fluorene (6). Pd2(dba)3 (11 mg, 0.012 mmole) and (t-Bu)3P (6 mg,
0.03 mmole) were stirred in 25 ml of toluene under nitrogen at
room temperature for 10 min. 1 g (2 mmole) of 2,7-dibromo-9,9-di

206=[538]
S. Lengvinaite et al.
(3-methyloxetan-3-ylmethyl)fluorene, 0.96 g (6 mmole) of 3-methyldi-
phenylamine, and 1.15 g (6 mmole) sodium tert-butoxide were added
into the reaction mixture, and it was heated for 8 h at 80ꢀC. The reac-
tion mixture was cooled and quenched by the addition of ice water.
The product was extracted into ethylacetate, and the extract was
dried over anhydrous MgSO4. The crude product was purified by sil-
ica gel column chromatography using ethyl acetate=hexane (vol. ratio
1:2) as an eluent. Yield: 0.55 g (42%).
3. RESULTS AND DISCUSSION
The synthetic rout towards the condensed aromatic amines containing
the fluorene core with two (photo)polymerizable oxetanyl groups (3–4)
is shown in Scheme 1. 2-Bromo-9,9-di(3-methyloxetan-3-ylmethyl)-
fluorene (2) as a key material was synthesized from commercially
available 2-bromofluorene (1) and 3-bromomethyl-3-methyl-oxetane
in a two-phase system by a similar procedure as alkylation of fluo-
rene [10,11]. The obtained bromo compound 2 was then treated with
9H-carbazole by the Ullmann reaction [12], or diphenylamine by
Buchwald-Hartwig [13,14] palladium-catalyzed amination, respect-
ively, to afford arylamino mono substituted derivatives 3 and 4. Dicar-
bazol-9-yl and di(3-methyldiphenylamino) substituted derivatives 5
and 6 were synthesized by the same procedures from commercially
available 2,7-dibromofluorene.
All the newly synthesized compounds were identified by mass spec-
1
trometry and by H NMR spectroscopy. The data were found to be in
SCHEME 1 Synthesis of fluorene-based compounds 3 and 4; structures of
compounds 3–6.

Fluorene Derivatives as Materials for Multilayer Light Emitting Diodes 207=[539]
TABLE 1 Thermal Characteristics of Compounds 3–6
Compound
Tg (ꢀC)
Tm (ꢀC)
TID (ꢀC)
3
4
5
6
94
60
60
62
209
185
193
157
370
350
300
275
good agreement with the proposed structures. Materials (3–6) were
soluble in common organic solvents and could form transparent thin
films by spin-coating from their solutions.
The behavior under heating of compounds 3–6 was studied by DSC
and TGA. The values of glass transition temperatures (Tg), melting
points (Tm), and the temperatures at which 5% loss of mass
was observed (TID) are summarized in Table 1. All materials (3–6)
demonstrated a high thermal stability. The mass loss occured at
temperatures in the range from 275 to 370ꢀC as confirmed by TGA.
All compounds 3 were crystallized after purification and obtained as
crystalline materials. When their samples were heated, Tm were
observed in the range 157–209ꢀC (Table 1), however the materials
readily formed glasses, when their melt samples were cooled on stand-
ing in air or with liquid nitrogen. As an example, the DSC thermo-
grams of 5 are shown in Figure 1. When the crystalline sample was
heated, the endothermic peak due to melting was observed at 193ꢀC.
When the melt sample was cooled down and heated again, the glass-
transition phenomenon was observed at 60ꢀC, and no peaks due to
crystallization and melting appeared on the further heating.
FIGURE 1 DSC curves of the material 5. Heating rate – 10ꢀC=min.

208=[540]
S. Lengvinaite et al.
FIGURE 2 UV absorption (a) and FL emission (b) spectra of 3–6 dilute THF
solutions.
UV absorption and FL spectra of dilute solutions of the compounds
synthesized were recorded. The pertinent data are presented in
Figure 2. The electronic absorption energy of the derivatives is similar,
and the kmax values are in the range 290–360 nm. Their FL emission
maxima appear in the region of wavelengths from 350 to 395 nm.
The comparison of the UV absorption and FL spectra of monosubsti-
tuted derivative 3 and 4 with those of corresponding disubstituted
derivatives 5 and 6 revealed a small bathochromic shift of the spectra
of the disubstituted compounds. The bathochromic shift of the spectra
indicates different conjugations of derivatives 5 and 6, and layers of
the compounds are expected to show the lower ionization potentials
(Ip) than those of 3 and 4.
Triplet energies (i.e., the transition energy of T1 ! S0) of the synthe-
sized materials were estimated from their highest energy phosphor-
escence peaks at 77 K. Carbazolyl substituted fluorenes (3 and 5)
have large triplet energies of ꢁ2.73 eV. Diphenylamino substituted
derivatives (4 and 6) show a small red shift in phosphorescence, lead-
ing to a lower triplet energy of 2.57 eV. Such triplet energies are large
enough for the use of materials 3–6 as hosts for green and red electro-
phosphorescence, including their use for confining triplet excitons to
phosphorescent dopants and for preventing the reverse energy
transfer.
The electron photoemission spectra of layers of the compounds
synthesized are presented in Figure 3. The values of Ip of the materials
range from 5.62 to 5.8 eV. It is evident that the charge injection
properties of the synthesized materials depend on the nature of substi-
tuents connected to the fluorene core. Diphenylamino substituted deri-
vatives demonstrate lower Ip than those with carbazolyl fragments (cf.
Ip of 3 with that of 4 or Ip of 5 with that of 6). On the other hand,
disubstituted fluorenes have lower Ip than those of monosubstituted

Fluorene Derivatives as Materials for Multilayer Light Emitting Diodes 209=[541]
FIGURE 3 Electron photoemission spectra of layers prepared using 3–5.
derivatives (cf. Ip of 3 with that of 5 or Ip of 4 with that of 6) probably
due to different conjugations, as it was observed from UV absorption
and FL spectra.
Derivatives 3 and 6 were tested as host materials in the mixture
(7:3) with electron transporting derivative PBD. Commercially avail-
able tris(2-phenylpyridine) iridium {Ir(ppy)3} was used as a quest.
The electro-phosphorescent OLEDs were of the following configur-
ation: ITO=PEDOT=host with Ir(ppy)3 (2 wt.%)=(Ca=Al). The
emitting layers were made by spin-coating from the host mixture (3
or 6 with PBD) containing the guest. When a positive voltage was
applied to the device, bright green electro-phosphorescence of Ir(ppy)3
was observed with the emission maximum at around 510 nm, suggest-
ing the confinement of triplet excitons on triplet dopants.
Figure 4 shows the current–voltage and luminance–voltage charac-
teristics of the OLEDs containing host systems with 3 or 6. The turn-
on voltage of the device containing host of 6 was rather high, and the
maximum brightness reached only 240 cd=m2. For the device having
host of 3, the turn-on voltage decreased to 13 V. The EL efficiency of
the device reached 3.4 cd=A, and the maximum brightness exceeded
1700 cd=m2. It should be pointed out that these characteristics were
obtained in a non-optimized test device under ordinary laboratory
conditions. The device performance may be further improved by both
the optimization of the host system and the injection of holes and elec-
trons into the emitting layer. The OLEDs containing cross-linked
layers of the materials will be reported later.
In conclusion, cross-linkable carbazol-9-yl or diphenylamino substi-
tuted fluorenes have been synthesized and characterized. All the

210=[542]
S. Lengvinaite et al.
FIGURE 4 OLED characteristics of the device ITO=PEDOT=host with Ir(p-
py)3=(Ca=Al). The host system contains the materials 3 or 6, respectively.
materials are thermally stable and form amorphous films with glass
transition temperatures in the range 60–94ꢀC. The ionization poten-
tials of amorphous layers of the materials depend on their chemical
structure and range from 5.62 to 5.8 eV. All the synthesized materials
have large triplet energies of 2.57–2.73 eV and are suitable as hosts
for green and red phosphorescent light emitting diodes. The device
fabricated with the use of one of the new hosts reached a brightness
of 1700 cd=m2 and a current efficiency of 3.4 cd=A.
REFERENCES
[1] Adachi, C., Baldo, M. A., Thompson, M. E., & Forrest, S. R. J. (2001). Appl. Phys.,
90, 5048.
[2] The Special Issue on Organic Electronics. (2004). Chem. Mater., 16, 4381.
[3] Adachi, C., Kwong, R. C., Djurovich, P., Adamovich, V., Baldo, M. A., Thompson, M.
E., & Forrest, S. R. (2001). Appl. Phys. Lett., 79, 2082.
[4] Avilov, I., Marsal, P., Bredas, J.-L., & Beljonne, D. (2004). Adv. Mater., 16, 1624.
[5] Grigalevicius, S., Getautis, V., Grazulevicius, J. V., Gaidelis, V., Jankauskas, V., &
Montrimas, E. (2001). Materials Chemistry and Physics, 72, 395.
˙
[6] Tsai, M.-H., Hong, Y.-H., Chang, C.-H., Su, H.-C., Wu, C.-C., Matoliukstyte, A.,
˙
Simokaitiene, J., Grigalevicius, S., Grazulevicius, J. V., & Hsu, C.-P. (2007).
Advanced Materials, 19, 862.
[7] Mullen, K. & Scherf, U. (Eds.). (2005). Organic Light Emitting Devices – Synthesis,
Properties and Applications, Wiley-VCH: Weinheim.
[8] Grigalevicius, S., Blazys, G., Ostrauskaite, J., Grazulevicius, J. V., Gaidelis, V.,
Jankauskas, V., & Montrimas, E. (2002). Synth. Met., 128, 127.
[9] Miyamoto, E., Yamaguchi, Y., & Yokoyama, M. (1989). Electrophotography, 28, 364.
[10] Jiang, G., Yao, B., Geng, Y., Cheng, Y., Xie, Z., Wang Jin, L. X., & Wang, F. (2006).
Macromolecules, 39, 1403.
[11] Burn, P. L., Kraft, A., Baigent, D. R., Bradley, D. D. C., Brown, A. R., Friend, R. H.,
Gymer, R. W., Holmes, A. B., & Jackson, R. W. (1993). J. Am. Chem. Soc., 115, 10117.
[12] Gauthier, S. & Frechet, J. M. J. (1987). Synthesis, 1, 383.
[13] Hartwig, J. F. (1997). Synlett., 329.
[14] Hartwig, J. F., Kawatsura, M., Hauck, S. I., Shaughnessy, K. H., & Alcazar-Roman,
L. M. (1999). J. Org. Chem., 64, 5575.