Asymmetrically 4,7-disubstituted benzothiadiazoles as efficient non-doped solution-Processable green fluorescent emitters

Article abstract of DOI:10.1021/ol9022563

Asymmetrically 4,7-disubstituted benzothiadiazole derivatives involving a carbazolyl moiety at one end and a solubilizing dendron at the opposite end have been synthesized and characterized. A two-layer electroluminescent device based on one of these solu

Full text of DOI:10.1021/ol9022563

ORGANIC
LETTERS
2009
Vol. 11, No. 22
5318-5321
Asymmetrically 4,7-Disubstituted
Benzothiadiazoles as Efficient
Non-doped Solution-Processable Green
Fluorescent Emitters
Yuan Li,† Ai-Yuan Li,† Bi-Xin Li,‡ Ju Huang,† Li Zhao,† Bao-Zheng Wang,†
Jun-Wen Li,† Xu-Hui Zhu,*,† Junbiao Peng,† Yong Cao,† Dong-Ge Ma,‡ and
Jean Roncali*,§
Institute of Polymer Optoelectronic Materials and DeVices and Key Laboratory of Special
Functional Materials of Ministry of Education, South China UniVersity of Technology,
Guangzhou 510640, China, State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China, and Linear
Conjugated Systems Group, CNRS, CIMA, UniVersity of Angers, 2 Bd LaVoisier,
Angers F-49045, France
xuhuizhu@scut.edu.cn; jean.roncali@uniV-angers.fr
Received September 30, 2009
ABSTRACT
Asymmetrically 4,7-disubstituted benzothiadiazole derivatives involving a carbazolyl moiety at one end and a solubilizing dendron at the
opposite end have been synthesized and characterized. A two-layer electroluminescent device based on one of these solution-processed
molecular emitters revealed a maximal luminous efficiency of ∼10.6 cd A-1 and green light emission with CIE coordinates (0.34, 0.58).
Solution-processable molecular semiconductors have re-
ceived increasing attention as active materials in organic
optoelectronic devices such as organic light-emitting diodes
(OLEDs),1,2 solid-state lasers,3 field effect transistors
(OFETs),4 and organic solar cells (OSCs).5 With respect to
their polymer counterparts, small molecular active com-
pounds present the advantages of a monodisperse unequivo-
cal chemical structure and thus of a potentially better
reproducibility of synthesis and purification and more
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P. A.; Huang, W.; Bradley, D. D. C. AdV. Funct. Mater. 2009, 19, 2844.
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† South China University of Techonology.
‡ Changchun Institute of Applied Chemistry.
§ University of Angers.
(1) Lo, S. C.; Burn, P. L. Chem. ReV. 2007, 107, 1097
.
10.1021/ol9022563 CCC: $40.75
Published on Web 10/23/2009
 2009 American Chemical Society

straightforward analysis of structure-properties relationships.
Furthermore, when adequately solubilized by design these
solution-processable molecular materials afford the possibil-
ity of low-cost large area electronics.
Scheme 1. Synthetic Routes to Compounds 1a/1b
The current development of solution-processed small
molecular RGB emitters in OLEDs, particularly for display
purpose, requires highly efficient light-emitting materials with
enhanced electron injection and transport properties.6,7
Although a number of emitters with predominant hole
transporting properties are available, their wide applicability
as a bulk emitter or dopant in a hole-transporting host might
present a potential challenge as it implies the use of hole
blockers combining sufficient device durability and conve-
nient synthetic accessibility.8
We have recently reported soluble red fluorescent molec-
ular glasses based on dithienylbenzothiadiazole combining
high solubility and excellent solution processability with
intrinsic morphological stability.3c,9 Notably, one of these
pure red emitters has demonstrated hole injection and electron
transporting properties,9b unlike many other red-emitting
materials that are largely dominated by hole transport.10,11
In this context, we present here new efficient non-doped
solution-processable green electroluminescent molecular
emitters based on asymmetrically 4,7-disubstituted ben-
zothiadiazoles 1a and 1b (Scheme 1).
A solubilizing dendron was attached at one side of 2,1,3-
benzothiadiazole of compound 1a to provide the desired
solubility. A 3,6-di(1-naphthyl)carbazolyl moiety12 was
introduced at the opposite side of the molecule through a
fluorenyl spacer in order to facilitate hole injection and glass
formation. Overall the branched structures are expected to
afford green emission with a potentially high photolumines-
cent (PL) efficiency and high color purity in the solid state
by suppressing strong intermolecular interactions generally
associated with donor-acceptor molecules. For compound
1b the introduction of a trifluoromethyl group onto the
carbazolyl ring is expected to further modulate charge
injection/transport properties.
The synthesis of compounds 1a/1b is outlined in Scheme
1. Compound 3a was prepared as already reported12 and
compound 3b was synthesized by Suzuki coupling of
6-bromo-2-(trifluoromethyl)carbazole (2) with 1-naphtyl bo-
ronic acid in 89% yield. Compound 2 was obtained by
treating 2-trifluoromethylcarbazole13 with NBS in the pres-
ence of LiClO4 and silica gel in ice-cooled CH2Cl2. Some
multibromination was observed when the reaction was carried
out at room temperature.
Ullmann coupling of compounds 3a/3b with the respective
2,7-dibromo-9,9-dialkylfluorenes (1.5 equiv) led to bromo
compounds 4a/4b in ca. 65% and 53% yields. Successive
treatment of 4a/4b with n-butyllithium and 2-isoproxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane in anhydrous THF
at -78 °C afforded the corresponding Suzuki reagents 5a/
5b.
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Org. Lett., Vol. 11, No. 22, 2009
5319

Figure 2. Molecular structure of compound 1c.
The UV-vis and PL spectra of 1a/1b in dilute solution
and as thin solid films are shown in Figure 1, and the relevant
data are summarized in Table 1. Thin films of 1a and 1b
spun-cast from p-xylene solutions show emission maxima
at 518 and 521 nm, respectively. These highly green emissive
solids show absolute solid-state PL efficiencies of ca. 0.72
for 1a and 0.51 for 1b, measured in an integrating sphere
under 325 nm He-Cd laser excitation. Furthermore, com-
parison of the solid state PL spectra of 1a/1b to those of
compound 1c (λem ) 568 nm, see Supporting Information)
and 4,7-bis(9,9-diarylfluoren-2-yl)-2,1,3-benzothiadiazole
(λem ) 544 nm)14 shows that the new compounds present a
significantly improved green color purity, thus confirming
the effectiveness of the 3,5-di(p-sec-butoxylphenyl)phenyl
end-capping group to reduce intermolecular interactions.
Figure 1. UV-vis absorption and PL spectra of 1a/1b in p-xylene
solution and as-cast films on quartz (PL excitation ) 380 nm).
dioxaborolane resulted in compound 6 in 86% yield, which
underwent further lithiation and boronation reactions to yield
Sukuki reagent 7. Precursor compound 8 was thus prepared
in 78% yield by a mono-Suzuki coupling of 7 and 4,7-bromo-
2,1,3-benzothiadiazole.
Cyclic voltammetry (CV) was performed in methylene
chloride/acetonitrile in the presence of Bu4NPF6 as supporting
electrolyte. Compounds 1a/1b are reversibly reduced at ca.
-1.40 V vs Ag/AgCl (Figure 3). With respect to 1a, the
introduction of CF3 substituent on the carbazole ring in 1b
results in a substantial increase of the oxidation potential
but leaves the reduction potential pratically unaffected. The
LUMO energy levels estimated from the CV data of -2.93
eV for 1a and -2.94 eV for 1b seem appropriate for efficient
electron injection (Table 1). The anodic peak potentials lead
to estimated HOMO levels of ca. -5.61 and -5.84 eV for
1a and 1b, respectively, in good proximity to the values of
-5.85 and -6.12 eV obtained by UPS measurements.15
Finally, the target compounds 1a and 1b were obtained
in ca. 93% yields by coupling a slight excess of precursor 8
with Suzuki reagents 5a and 5b, respectively. This high
coupling efficiency greatly facilitated purification by routine
column chromatography. Furthermore, the high solubility of
compound 8 in ethanol and petroleum ether allows easy
removal of the excess of reagent. The identity and purity of
compounds 1a/1b was confirmed by 1H NMR, MALDI-TOF
mass spectrometry, and microanalysis (see Supporting In-
formation). These new compounds show a high solubility
in common organic solvents such as CH2Cl2, THF, toluene,
and p-xylene. For instance, 25 mg of 1a/1b is rapidly
dissolved in 1 mL of p-xylene at room temperature without
saturation.
Thermal gravimetry analysis shows that compounds 1a/
1b are stable well above 350 °C. Differential scanning
calorimetry (DSC) measurements on as-prepared samples
reveal a distinct glass transition with Tg of ∼95 °C for 1a
Table 1. Summary of Optical and Electrochemical Data
UV-vis (λabsmax, nm)a
PL (λemmax, nm, (quantum yield))
solution
solid
solutionb
solidc
reductiond(V)
oxidatione(V)
LUMOf (eV)
HOMO (eV)
1a
1b
302, 408
300, 400
285, 413
301, 413
504 (0.67)
500 (0.58)
518 (0.72)
521 (0.51)
-1.42
-1.41
1.26
1.49
-2.93
-2.94
-5.61g (-5.85)h
-5.84g (-6.12)h
a In p-xylene. b ∼10-6 mol L-1 in p-xylene using quinine bisulfate in 0.1 mol L-1 H2SO4 as the reference (φr ) 0.54). c Measured in an integrating sphere
under 325 nm laser excitation. d Derived from E0 values. e Derived from anodic peak potentials vs Ag/AgCl reference electrode. f Derived from E0 values.
g Derived from anodic peak potentials with reference to the energy level of ferrocene of -4.8 eV vs vacuum level. h Measured by UPS.
5320
Org. Lett., Vol. 11, No. 22, 2009

Figure 3
. Cyclic voltammograms of 1a/1b in 5:1 CH2Cl2/CH3CN,
Figure 4. L-V-J characteristics of devices ITO/PEDOT:PSS/PVK/
1a(1b)/Ba/Al.
supporting electrolyte 0.10 M n-Bu4NPF6, scan rate 50 mV s-1
.
and ∼130 °C for 1b in the first heating run. Neither
crystallization nor melting was observed upon heating up to
300 °C.
green fluorescent molecular emitters (see Supporting Infor-
mation).17 These preliminary characterization of the single-
and two-layer light-emitting devices also suggest that electron
transport is facilitated in these new emitting materials.
The new compounds exhibit excellent film-forming prop-
erties. Atomic force microscopy images of spin-cast films
of 1a and 1b from p-xylene solution show smooth, continu-
ous, and featureless morphology with a surface root-mean-
square roughness value of 0.35 and 0.42 nm, respectively
(see Supporting Information). This stable homogeneous
amorphous morphology appears favorable for light-emitting
diodes with reduced leak currents and improved thermal
stability during device operation.
Compounds 1a/1b were first investigated as non-doped
solution-processed emitters in single-layer diode structures
ITO/PEDOT:PSS/1a(1b)/Ba/Al. The devices showed nearly
pure green emission (λem ≈ 520 nm) with CIE coordinates
(0.305-0.318, 0.60), very close to the European Broadcast
Union color gamut green point CIE (x ) 0.29, y ) 0.60).
Onset voltages of ∼2.8 and 3.3 V (at a luminance of 1 cd
m-2) were determined for 1a and 1b, respectively. These
low values suggest efficient charge injection into the emitting
layer. Furthermore, this result also suggests that transport
of at least one of the two types of charge carriers could be
efficient. However the low device efficiencies (LEmax, ∼1
cd A-1) suggest unbalanced charge injection or poor charge
confinement in the active layer.
Consequently, double-layer devices consisting of an
electron blocking PVK layer ITO/PEDOT:PSS/PVK/1a (1b)/
Ba/Al were fabricated. The device performance was greatly
improved with a maximal luminous efficiency of 10.6 cd
A-1 (corresponding to ηextmax of ∼3.7%16) for 1a and 7.5 cd
A-1 (ηext ∼2.6%) for 1b, respectively. At a current density
of ca. 20 mA cm-2, V ) 8.7 V, L ) 1228 cd m-2, LE )
6.70 cd A-1, λmax ) 528 nm with CIE coordinates (0.344,
0.580) for 1a and V ) 9.0 V, L ) 1308 cd m-2, LE ) 6.18
cd A-1, λmax ) 528 nm with CIE coordinates (0.336, 0.577)
for 1b. It is important to note that the device characteristics
achieved here, especially without a hole-blocker, rank among
the best reported so far for non-doped solution-processed
In summary, soluble green fluorescent molecular glasses
based on 2,1,3-benzothiadiazole derivatives have been
synthesized and characterized. These truly solution-process-
able molecular green emitters have been used as active
materials in highly efficient green light-emitting devices. We
have shown that the 3,5-di(p-sec-butoxylphenyl)phenyl
moiety is an efficient end-cap block to alleviate intermo-
lecular interactions, leading to green emission with improved
color purity and PL efficiency. An in-depth analysis of the
device operation, in particular, charge transport, is underway
which should warrant further optimization of the device
architecture and molecular structure.
Acknowledgment. X.H.Z. is deeply grateful for the
financial support of NSFC, MOE and MOST (Grant Nos.
50603006, 50911130174, 2006NCET, U0634003, 2009-
CB930601, and 2009CB623604). Ms. Chan Sun at Fudan
University and Dr. Hai-Tao Xu and Prof. Xiang Zhou at Sun
Yat-sen University are acknowledged with gratitude for the
assistance in MALDI-TOF and UPS measurement.
Supporting Information Available: Experimental and
characterization detail of all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL9022563
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