Synthesis and electroluminescent properties of new amorphous cyclohexyl- and hexyl-substituted ter(9,9-dialkylfluorene)s

Article abstract of DOI:10.1246/cl.2008.686

A series of ter(9,9-dialkylfluorene)s (TFs) containing cyclohexyl and hexyl side groups has been synthesized via the Suzuki coupling reaction. Their chemical structures were characterized by ¹HNMR and ¹³CNMR. All TFs were soluble in common organic solvents and showed good thermal stability up to 400 °C. In particular, T(CHC)F composed of two 9,9-bis(cyclohexylmethyl)fluorenes and one dihexylfluorene showed improved thermal stability over T(HHH)F and was found to be amorphous, unlikely with T(CCC)F. A light-emitting diode device fabricated with an ITO/2-TNATA/NPB/TFs/ Alq₃/Liq/ Al configuration exhibited blue light emission with EL maxima at 450 nm and EL efficiency of 1.1 cd/A. Copyright

Full text of DOI:10.1246/cl.2008.686

686
Chemistry Letters Vol.37, No.7 (2008)
Synthesis and Electroluminescent Properties of New Amorphous
Cyclohexyl- and Hexyl-substituted Ter(9,9-dialkylfluorene)s
Eunhee Lim,1;3 Byung-Jun Jung,1 Sung Min Kim,2 Bong Ok Kim,2 and Hong-Ku Shimꢀ1
1Department of Chemistry and School of Molecular Science (BK21),
Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea
2GRACEL Corp. 374-2, 284-25 Samyang Technotown, Seungsu2-Ga, Seungdong-Gu, Seoul 133-833, Korea
3Melville Laboratory for Polymer Synthesis, Department of Chemistry,
University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
(Received March 24, 2008; CL-080316; E-mail: hkshim@kaist.ac.kr)
A series of ter(9,9-dialkylfluorene)s (TFs) containing cyclo-
hexyl and hexyl side groups has been synthesized via the Suzuki
coupling reaction. Their chemical structures were characterized
by 1H NMR and 13C NMR. All TFs were soluble in common
organic solvents and showed good thermal stability up to
400 ꢁC. In particular, T(CHC)F composed of two 9,9-bis(cyclo-
hexylmethyl)fluorenes and one dihexylfluorene showed im-
proved thermal stability over T(HHH)F and was found to be
amorphous, unlikely with T(CCC)F. A light-emitting diode de-
vice fabricated with an ITO/2-TNATA/NPB/TFs/Alq3/Liq/
Al configuration exhibited blue light emission with EL maxima
at 450 nm and EL efficiency of 1.1 cd/A.
Fluorene-based oligomers and polymers are generally re-
garded as the most promising candidates for blue organic
light-emitting diodes (OLEDs). Oligomers have advantages over
polymers in terms of purity and structure uniformity.1 Fluorene
trimers are known to be long enough to emit blue light. Howev-
er, oligomers are limited by a tendency toward crystallization
and a low glass-transition temperature, in comparison to poly-
mers. Since crystallization tends to create grain boundaries that
limit charge transport and cause poor contact, it is desirable to
create thermally and morphologically stable amorphous molecu-
lar materials that do not tend to be crystallized on heating above
glass-transition temperature (Tg) for practical applications. In
this manner, glass-forming conjugated oligomers with improved
thermal stability have been synthesized, which includes starburst
molecules and spiro-linked oligo(p-phenylene)s and oligo(tri-
phenylamine)s.2–5 In particular, spiro-linked terfluorenes have
attracted much attention for use as blue-emitting materials
for OLEDs, which form stable nonpolymeric organic glasses
with high glass-transition temperatures.6–9 Here, we report on
the synthesis and thermal and optical properties of cyclohexyl-
and/or hexyl-substituted terfluorenes as new amorphous materi-
als emitting blue light. We outline our strategy for improving the
material properties of terfluorenes. The thermal stability and the
phase-transition behavior were varied with the incorporated side
groups of terfluorenes.
Scheme 1. Synthetic procedure of TFs: (i) 1-bromooctane,
tetra(n-butyl)ammonium bromide, NaOH (aq, 50 wt %), toluene,
reflux; (ii) n-BuLi, THF, ꢂ78 ꢁC for 2 h; (iii) 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane, ꢂ78 ꢁC for 1 h, then
room temperature overnight; (iv) Pd(PPh3)4, toluene, ethanol,
Na2CO3 (aq, 2 M), reflux.
Table 1. Thermal properties of terfluorenes
a
a
Tg /ꢁC
Tk /ꢁC
Tma/ꢁC
T5%b/ꢁC
c
c
T(HHH)F
T(CHC)F
T(CCC)F
30
70
100
—
—
—
—
380
390
400
c
c
210
255
aTg: glass-transition temperature, Tk: crystallization tempera-
ture, Tm: melting temperature. bTemperature resulting in 5%
weight loss based on the initial weight. Not detected.
c
showed good thermal stability, as weight loss was less than
5% on heating to about 400 ꢁC. The phase transition tempera-
tures (i.e., Tg), investigated with differential scanning calorime-
try (DSC) in nitrogen, were dramatically increased by increasing
the number of cyclohexyl-substituted fluorene units (See Table 1;
T(HHH)F < T(CHC)F < T(CCC)F). Upon heating beyond Tg,
T(CCC)F composed of three-cyclohexyl-substituted fluorenes
was found to crystallize at 210 ꢁC, referred to as the crystalliza-
tion temperature, Tk. This is an indication of morphological in-
stability, as commonly encountered with nonpolymeric organic
materials. Further heating revealed the crystalline melting point,
New ter(9,9-dialkylfluorene)s (TFs), T(HHH)F, T(CHC)F,
and T(CCC)F, were synthesized by using the Suzuki coupling
reaction.10 The synthetic procedures of TFs are outlined in
1
Scheme 1. Their chemical structures were verified by H NMR
and 13C NMR. The resulting oligomers were well soluble in
common organic solvents due to the introduced side groups.
The thermal stabilities of the TFs were evaluated under nitrogen
atmosphere by thermogravimetric analysis (TGA). All TFs
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.7 (2008)
687
electron affinity Ea of the terfluorenes are ꢂ5:85 and ꢂ2:65 eV
respectively, as determined by cyclic voltammetry (CV).
In order to investigate the electroluminescence properties
and the current–voltage–luminance characteristics of the synthe-
sized oligomers, devices with the configuration ITO/2-TNATA
(60 nm)/NPB (40 nm)/TFs (35 nm)/Alq3 (20 nm)/Liq (3 nm)/
Al were fabricated. 4,4,4-Tris[N-(2-naphthyl)phenylamino]tri-
phenylamine (2-TNATA), N,N0-di(l-naphthyl)-N,N0-diphenyl-
1,10-biphenyl-4,40-diamine (NPB) and tris(8-hydroxyquino-
linato)aluminum (Alq3) were used as hole-injecting, hole-trans-
port, and electron-transport materials, respectively.11 The 8-
hydroxyquinolinatolithium (Liq) was used as an electron-
injection layer, which showed better performance tolerance to
the injection layer thickness than lithium fluoride (LiF).12,13
TF compounds were used as blue emitters. Three TFs showed
similar EL characteristics. As an example, the EL spectra and
the current–voltage–luminance (I–V–L) characteristics of the
device using T(CHC)F are shown in Figure 2. All TFs emitted
light of a blue color with a maximum emission at 450 nm and
a full width at half maximum (fwhm) of only 60 nm. This
emission has the CIE coordinates (0.15 and 0.09), which are
very close to the standard for blue used by the National Televi-
sion System Committee (NTSC) (0.15 and 0.08) and are superior
to the previously reported oligofluorene derivatives. The
maximum EL efficiency was found to be 1.1 cd/A and up to
about 200 mA/cm2 the efficiency is maintained near 1.0 cd/A.
In summary, a series of cyclohexyl- and hexyl-substituted
ter(9,9-dialkylfluorene)s have been synthesized. Among them,
T(CHC)F is found to be amorphous and shows improved thermal
stability. New TFs can be used as emitting materials for produc-
ing stable blue emission for full-color organic EL devices.
Figure 1. DSC thermograms of TFs and optical microscopy
image of T(CCC)F at 200 ꢁC. Scale bar represents 200 mm.
Tm, at 255 ꢁC. An optical microscopy image of T(CCC)F film
measured at 200 ꢁC is also shown in Figure 1. In contrast,
T(HHH)F and T(CHC)F were morphologically stable upon
heating beyond their Tg’s.
Three TFs showed similar optical properties. The UV–vis
absorption maxima of TFs appeared at 350 nm. The PL emission
spectra of the TFs in the chloroform solution showed typical vi-
bronically structured bands comprising a maximum, a shoulder
and a tail, which appeared at 393, 414, and 440 nm respectively.
As films, the emission maxima of TFs were red-shifted (399 nm)
compared to those of the solution state. The normalized UV–vis
absorption and PL emission spectra of the T(CHC)F film are
shown in Figure 2a. In addition, the ionization potential Ip and
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Figure 2. (a) UV–vis, PL, and EL spectra of T(CHC)F films.
Inset: An energy diagram of the device (in eV). (b) Luminance
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Published on the web (Advance View) May 31, 2008; doi:10.1246/cl.2008.686