Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV-violet electrofluorescence, blue and green phosphorescent OLEDs

Article abstract of DOI:10.1016/j.orgel.2014.03.035

A high efficient UV-violet emission type material bis[4-(9,9′- spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV-violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV-violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A^-1, respectively.

Full text of DOI:10.1016/j.orgel.2014.03.035

Organic Electronics xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Spirobifluorene/sulfone hybrid: Highly efficient
solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs
⇑
⇑
Xinxin Ban, Huange Xu, Guolong Yuan, Wei Jiang , Bin Huang, Yueming Sun
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, PR China
a r t i c l e i n f o
a b s t r a c t
A high efficient UV–violet emission type material bis[4-(9,90-spirobifluorene-2-yl)phenyl]
sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and
morphologically stable spirobifluorene into one molecule. The steric and bulky compound
SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (UPL = 92%),
high glass transition temperature (Tg = 211 °C) and high triplet energy (ET = 2.85 eV). In
addition, the uniform amorphous thin film could be formed by spin-coating from its
solution. These promising physical properties of the material made it suitable for using
as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs.
With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient
UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution
processed blue and green phosphorescent organic light emitting diodes showed a high
luminous efficiency of 13.7 and 30.2 cd Aꢀ1, respectively.
Article history:
Received 5 January 2014
Received in revised form 4 March 2014
Accepted 22 March 2014
Available online xxxx
Keywords:
OLEDs
Solution process
UV–violet emission
Phosphorescence
Sulfone
Host
Ó 2014 Published by Elsevier B.V.
1. Introduction
emitters with wide band gap can also be used as efficient
host materials for triplet dopant phosphorescent OLEDs
Organic light-emitting diodes (OLEDs) have attracted
much attention as it has been recognized as the next-gen-
eration high quality full color displays [1,2]. In recent
years, the rapid advancement of OLEDs has extended the
emission wavelengths over the whole visible range [3–5].
OLEDs with shorter wavelength emission, from the blue,
through the violet, to the ultra violet (UV) spectral regions,
have not progressed as well [6], although efficient UV-
emitting OLEDs hold unique promise for applications in
generating full-color OLED displays by irradiation of
luminescent dyes [7–9]; biological/fluorescent sensors
[10,11] and potential light source for high density informa-
tion storage devices [10–13]. In addition, these UV–violet
[12], which can harvest both singlet and triplet excitons,
yielding an internal quantum efficiency as high as 100%
[14–16].
As we known, it is difficult to obtain promising materi-
als for fabricating OLEDs with efficient UV–violet emission
because these materials usually possess a large band gap
[11]. To achieve a wide band gap, the extent of conjugation
in the molecule must be confined [9,10], which in turn
would impose constraints in molecular size and such short
conjugation length will directly reduce the conductivity
and photoluminescence efficiency of the material, which
is an essential requirement for producing high perfor-
mance OLEDs. Furthermore, the intramolecular charge
transfer (ICT) trend in such molecules indeed could also
lead to a remarkable bathochromic effect, which would
extremely impair the short wavelength emission and the
color purity [17]. Thereby, the key point for designing
⇑
Corresponding authors. Tel.: +86 02552090622.
E-mail addresses: 101011462@seu.edu.cn (W. Jiang), sun@seu.edu.cn
(Y. Sun).
http://dx.doi.org/10.1016/j.orgel.2014.03.035
1566-1199/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

2
X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
high-efficiency UV–violet electroluminescence materials is
to suitably control the -conjugation enlargement and the
emitting organic light-emitting device and a host material
for solution-processed phosphorescence OLEDs have been
demonstrated.
p
ICT trend inside the molecules [18]. Another challenge for
OLEDs is to reduce the fabrication cost. While many high
efficiency devices have been reported, most of them still
restricted by the low fabrication yield down to 50% arising
from the complicated multilayer thermal evaporation
techniques [19,20]. Solution processed organic light-emit-
ting diodes, such as spin-coating, inkjet printing and roll-
to-roll fabrication, offer an attractive alternative approach
in terms of their simple processing, low cost fabrication
and large size color patterning compared with vacuum
deposited OLEDs [5,21]. To date, solution processed OLEDs
using small organic molecules and conjugated polymers
have been reported [15,22]. In contrast to polymer coun-
terparts, small-molecule materials are advantageous, as
they can easily be synthesized and purified [14,23–25].
However, the fabrication of highly efficient solution-pro-
cessed OLEDs derived from small molecules is limited by
the relatively low molecular weight, which makes it diffi-
cult to acquire the morphological stability that is necessary
for forming stable and uniform amorphous thin films
[26,27]. Hence, an exquisite molecular design is highly
desired to fully compromise the conflicting demands of
morphological stability and molecular size. Up to now,
the existing wide band gap materials used for UV–violet
emitting OLEDs or hosts of phosphorescent OLEDs are
mainly limited to a few functionalized structural features
[28], such as materials containing purine [29], carbazole
[18], fluorene [12] and silane [13] moieties. Recently, Ada-
2. Experiments
2.1. General information
All reactants and solvents, unless otherwise stated,
were purchased from commercial sources and used as re-
ceived. 1H NMR and 13C HMR spectra were measured on
a BRUKER AMX 300- or 500-MHz instrument with tetra-
methylsilane as the internal standard. Elemental analysis
was performed on an Elementar Vario EL CHN elemlental
analyzer. Molecular masses were determined by a FINNI-
GAN LCQ Electro-Spraying Ionization–Mass Spectrometry
(ESI-MS), or a BRUKER DALTONICS Matrix-assisted laser
desorption-ionization time-of-flight mass spectrometry
(MALDI-TOF-MS), with
a-cyano-hydroxycinnamic acid as
a matrix. Absorption and photoluminescence emission
spectra of the target compound were measured using a
SHIMADZU UV-2450 spectrophotometer and a HORIBA
FLUOROMAX-4 spectrophotometer, respectively. The
solution PL quantum efficiency (excited at 330 nm) was
measured by a relative method using quinine sulfate
(UF = 0.577 in 0.1 M H2SO4) as the standard in CH2Cl2 solu-
tions. The film PL quantum efficiency was measured with
an integrating sphere under an excitation wavelength of
330 nm. Phosphorescence spectra at 77 K were measured
in a dichloromethane solvent. Thermogravimetric analysis
(TGA) was recorded with a Netzsch simultaneous thermal
analyzer (STA) system (STA 409PC) under a dry nitrogen
gas flow at a heating rate of 10 °C minꢀ1. Glass-transition
temperature was recorded by (DSC) at a heating rate of
10 °C minꢀ1 with a thermal analysis instrument (DSC
2910 modulated calorimeter). The film surface morphol-
ogy was measured with AFM (Seiko Instruments,
SPA-400). Cyclic voltammetry (CV) was performed on a
CHI750C voltammetric analyzer in CH2Cl2 solutions at a
scan rate of 100 mV sꢀ1 with a platinum plate as the
working electrode, a silver wire as the pseudo-reference
electrode, and a platinum wire as the counter electrode.
The supporting electrolyte was tetrabutylammonium
hexafluorophosphate (0.1 M) and ferrocene was selected
to determine the potential of the silver wire electrode.
The solutions were bubbled with a constant nitrogen flow
for 15 min before measurements.
chi and co-workers demonstrated
a carbazole/sulfone
derivative can give efficient thermally activated delayed
fluorescence (TADF) through up-conversion of triplet to
singlet states and therefore lead to potentially 100%
internal singlet yields [30]. On the other hand, Kido and
co-workers reported an m-terphenyl-modifed sulfone
derivative as a host material for high-efficiency blue and
green phosphorescent OLEDs [31]. Furthermore, Zhang
et al. utilized a carbazole/sulfone hybrid design strategy
to obtain two novel efficient blue–violet fluorophores with
relatively weak ICT trends, which is beneficial for generat-
ing short-wavelength [18]. Thus, sulfone-containing deriv-
atives as primary emitters have been intensively studied
due to its high fluorescence efficiency [32] and appropriate
electron affinity energy [33–35].
In this work, we report the synthesis and characteriza-
tion of
a high luminescene UV–violet emittion type
material based on sulfone and spirobifluorene, bis[4-(9
,90-spirobifluorene-2-yl) phenyl] sulfone (SF-DPSO), in
which the spirobifluorene units are connected to the
sulfone moiety via two freely rotatable benzene rings. As
a result, the structural features endowed this compound
with UV–violet emission in the solid state, together with
high triplet energy (ET = 2.85 eV), which was sufficiently
high to serve as appropriate hosts for blue and green-emit-
ting phosphors. The introduction of spiro-configuration
gives the molecule a steric and bulky structure that further
enhances thermal and morphological stabilities and
retains high photoluminescent quantum yield (0.92 in
solid state). The application of this new compound as a
large band gap emitter for solution-processed UV–violet
2.2. Quantum chemical calculations
The geometrical and electronic properties of SF-DPSO
were performed with the Gaussian 03 program package.
The calculation was optimized at the B3LYP/6-31G(d) level
of theory. The molecular orbitals were visualized using
Gaussview.
2.3. Device fabrication and performance measurements
In
a general procedure, indium-tin oxide (ITO)-
coated glass substrates were pre-cleaned sequentially in
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
3
ultrasonic baths of a 2-propanol/deionized water (1:1 vol-
ume) mixture, toluene, deionized water, and acetone before
it was dried and treated with UV ozone. A 40 nm poly(eth-
ylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
aqueous solution was pin-coated onto the ITO substrate
and baked at 200 °C for 10 min under vacuum. The
substrates were then taken into a nitrogen glove box, where
a 70 nm thick layer of SF-DPSO doped with 10% FIrpic or 8%
Ir(mppy)3 was spin-coated onto the PEDOT:PSS layer from
1,2-dichloroethane solution and annealed 80 °C for
30 min. The substrate was then transferred into an evapora-
tion chamber, where the 2,20,200-(1,3,5-benzenetriyl)tris[1-
phenyl-1H-benzimidazole] (TPBI) was evaporated at an
involving air-sensitive reagents were performed under a
dry nitrogen atmosphere.
2.4.1. 2-Bromo-9,90-spirobifluorene
A 250 mL three-necked flask equipped with a tempera-
ture probe, an overhead stirrer was charged with THF
(80 mL) and 2-bromobiphenyl (9.2 g, 40.0 mmol) and put
under a nitrogen atmosphere. The flask was cooled to
ꢀ78 °C and n-BuLi (2.4 M in hexane, 20.0 mL) was added
dropwise via a syringe pump over 1 h, and the mixture
was stirred for an additional 1.5 h before addition of a solu-
tion of 2-bromo-9-fluorenone (10.8 g, 45.0 mmol) in THF
(100 mL). The ice bath was removed and the resulting
mixture was gradually warmed to ambient temperature
and quenched by adding saturated, aqueous NaHCO3
(200 mL). The mixture was extracted with dichlorometh-
ane and dried over anhydrous Na2SO4 before the solvent
was evaporated. A yellow powdery product was obtained.
The crude residue was placed in another three-necked
flask (100 mL) and dissolved in acetic acid (40 mL). A cata-
lytic amount of aqueous HCl (5 mol%, 12 N) was then
added and the whole solution was refluxed for 12 h. After
cooling to ambient temperature, purification by silicagel
chromatography using dichloromethane/n-hexane gave a
white powder (11.3 g, 28.8 mmol, 70%) [36,37]. 1H NMR
(CDCl3, 300 MHz): 7.83–7.77 (m, 3H), 7.67 (d, J = 8.1 Hz,
1H), 7.45 (dd, J = 8.1, 1.2 Hz, 1H), 7.37–7.30 (m, 3H),
evaporation rate of 1–2 Å sꢀ1 under
a pressure of
1 ꢁ 10ꢀ4 Pa and the Cs2CO3/Al bilayer cathode was evapo-
rated at evaporation rates of 0.2 and 10 Å sꢀ1 for Cs2CO3
and Al, respectively, under a pressure of 1 ꢁ 10ꢀ3 Pa. The
current–voltage–brightness characteristics of the devices
were characterized with Keithley 4200 semiconductor
characterization system. The electroluminescent spectra
were collected with a Photo Research PR750 Spectropho-
tometer. All measurements of the device were carried out
in ambient atmosphere without further encapsulations.
2.4. Materials
SF-DPSO was synthesized according to the procedure in
Scheme 1. Unless stated otherwise, all manipulations
Scheme 1. Synthetic routes toward SF-DPSO.
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

4
X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
7.11–7.06(m, 3H), 6.84 (s, 1H), 6.72–6.69 (d, J = 7.5 Hz, 3H).
MS (ESI): m/z 434.0 [M+K]+.
Besides, spirobifluorene was chosen as a bulky building
block for its spiro-structure, which enhances its morpho-
logical and thermal stability. More importantly, by linking
the two moieties via two freely rotatable benzene rings, p-
2.4.2. 2-(4,40,5,50-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9,
90-spirobifluorene
conjugated lengths of the molecular was effectively con-
fined, that makes the compound have high triplet energy.
As shown in Scheme 1, 2-bromobiphenyl was reacted with
2-bromo-9-fluorenone to form 2-bromo-9,90-spirobifluo-
rene. Then, this bromo-compound was transformed into
boronic esters via lithiation with n-butyllithium followed
by reaction with 2-isopropoxy-4,40,5,50-tetramethyl-1,3,2-
dioxaborolane. Finally, the Suzuki cross-coupling reactions
of 4,40-dichlorodiphenyl sulfone with 2-(4,40,5,50-tetra-
methyl-1,3,2-dioxaborolan-2-yl)- 9,90-spirobifluorene led
to SF-DPSO in the presence of Pd2(dba)3 (yield: 60%). The
target compound was purified by silica column chroma-
tography and recrystallization. 1H NMR, 13C NMR, mass
spectrometry, and elemental analysis were employed to
confirm the chemical structures of above-mentioned
compounds as described in the experimental section.
To a solution of 2-dibromo-9,90-spirobifluorene (4.0 g,
10.5 mmol) in anhydrous THF (100 mL) at ꢀ78 °C, 22 mL
(5.0 ml,12 mmol) of n-butyllithium (2.4 M in hexane) was
slowly added. The mixture was stirred at ꢀ78 °C for 2 h.
Then, 3.0 mL (15.0 mmol) of 2-isopropoxy-4,40,5,50- tetra-
methyl-[1,3,2]-dioxaborolane was added rapidly to the
solution, and the resulting mixture was warmed to room
temperature and stirred overnight. The mixture was
poured into water and extracted with CH2Cl2. The organic
extracts were washed with brine and dried over MgSO4.
The solvent was removed by rotary evaporation, and repre-
cipitation with methanol and THF gave product (3.0 g,
6.8 mmol, 65%).1H NMR (CDCl3, 300 MHz) d[ppm]: 7.84
(d, J = 9.6 Hz, 5H), 7.35 (t, J = 7.5 Hz, 3H), 7.18 (s, 6H),
7.11–7.06 (m, 3H), 6.92 (t, J = 8.1 Hz, 3H), 1.24 (s, 12H).
MS (ESI): m/z 465.0 [M+Na]+.
3.2. Thermal properties
2.4.3. Bis[4-(9,90-spirobifluorene-2-yl)phenyl] sulfone (SF-
DPSO)
The thermal properties of SF-DPSO were investigated
using thermal gravimetric analysis (TGA) and different
scanning calorimetry (DSC) under a nitrogen atmosphere.
The introduction of spirobifluorene groups to the para-po-
sition of diphenylsulfone renders the molecule highly
bulky backbone structure and non-planar. Such a molecu-
lar configuration is strongly beneficial to the thermal
stability. Fig. 1 displays the DSC curves recorded over the
4,40-Dichlorodiphenyl sulfone (0.86 g, 3.0 mmol),
2-(4,40,5,50-tetramethyl-1,3,2-dioxaborolan-2- yl)-9,90-spi-
robi-fluorene (3.0 g, 6.8 mmol), and Tris(dibenzylideneace-
tone)dipalladium (0) (0.14 g, 0.15 mmol) were added to a
round-bottomed flask equipped with a reflux condenser
and dissolved in 50 mL of 1,4-dioxane. After adding
10 mL of aqueous 2 N potassium phosphate solution, the
reaction mixture was heated at 110 °C for 24 h. The cooled
crude mixture was poured into water and extracted with
CHCl3 and dried over MgSO4, filtered, and evaporated to
yield a crude product. Flash column chromatography using
CHCl3 followed by reprecipitation with methanol and THF
gave a product (3.7 g, 6.21 mmol, 60%) [38]. 1H NMR
(500 MHz, CDCl3) d[ppm]: 6.71 (d, J = 7.5 Hz, 6H), 6.80
(s, 2H), 7.07 (t, J = 7.5 Hz, 4H), 7.11 (t, J = 7.5 Hz, 2H),
7.34–7.39 (m, 10H), 7.51 (d, J = 8 Hz, 2H), 7.74
(d, J = 8 Hz, 4H), 7.83–7.88 (m, 8H); 13C NMR (300 MHz,
CDCl3): d[ppm] 152.40, 151.83, 150.94, 148.38, 144.91,
144.37, 143.48, 142.58, 141.33, 130.90, 130.49, 130.46,
130.36, 129.71, 126.70, 126.63, 125.46, 123.08, 122.87,
temperature range 21–350 °C. After heating again,
a
well-defined glass transition occurred at 211 °C (Tg) and
no exothermic peak related to crystallization was observed
at temperatures up to 350 °C. In addition, SF-DPSO exhib-
ited a decomposition temperature (Td, corresponding to
5% weight loss) of 438 °C. These observations indicate that
the introduction of the spirobifluorene substituents signif-
icantly enhances the thermal stability of the sulfone deriv-
atives. Furthermore, atomic force microscopy (AFM) was
used to investigate the morphology of the film prepared
from the bulky compound through spin-coating a 1,
122.68. MALDI-TOF-MS (m/z): calcd for
C62H38O2S:
846.26, found: 845.52. Anal. Calcd for C62H38O2S: C 87.91,
H 4.52, S 3.79; found: C 87.79, H 4.36, S 3.76.
3. Results and discussion
3.1. Molecular design and synthesis
The SF-DPSO was designed as a wide band gap emitter
for UV–violet fluorescent organic light-emitting device
and electron transport type host material for phosphores-
cent OLEDs because the sulfone core structure is an
electron deficient unit and the spirobifluorene can also
transport electrons due to the aromatic structure. The
sulfone group was selected as electron-acceptor for its
excellent electron injection and transporting properties.
Fig. 1. TGA trace of SF-DPSO recorded at a heating rate of 10 °C minꢀ1
Inset: DSC measurement recorded at a heating rate of 10 °C minꢀ1
.
.
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
5
2-dichloroethane solution onto the PEDOT:PSS layer. As we
can see from Fig. 2, the surface of the compound is free of
pinholes and quite smooth, and the root-mean-square
(RMS) value is 0.35 nm. In contrast, the film prepared from
mCP onto the PEDOT:PSS layer exhibited a kind of pin-
holes, or aggregation. The excellent thermal property and
small surface roughness readily indicate high-stability
OLEDs, which is favorable to improve the performance
and lifetime during operation (see Table 1).
show in Table 2, the emissions of SF-DPSO have a stepwise
red shift from toluene to dimethyl formamide (DMF) with
increasing solvent polarity, while the absorption spectra
are almost unchanged. Even in the highest polarity solu-
tion DMF, the compound gives UV–violet fluorescence
with maximum wavelength below 400 nm, which indi-
cates that the intramolecular charge transfer (ICT) trend
in such molecule is weak. To obtain more information
about the change in the dipole moment upon excitation,
we used the Lippert–Matega equation (1), which expressed
the Stokes shift as a function of the solvent polarity param-
3.3. Photophysical properties
eter
D
f (e, n) [46] (see Table 3).
Photophysical properties of the SF-DPSO were analyzed
using ultraviolet–visible (UV–Vis) and photoluminescence
(PL) spectrometer. UV–Vis, solution PL and low tempera-
ture PL spectra of the SF-DPSO are shown in Fig. 3. Compar-
ing to the absorption spectra of spirobifluorene (SF), [39].
The sharp peak at 309 and a broad one around 250–
2
2ð
D
l
Þ
D
v
¼
vabs
ꢀ
vf
u
D
fð
e; nÞ þ A
ð1Þ
hca3
and
D
f (e
, n) is calculated by:
e
ꢀ 1
n2 ꢀ 1
265 nm are assigned to the p–
pꢂ transition of SF unit, while
D
fð
e; nÞ ¼
ꢀ
ð2Þ
2e
þ 1 2n2 þ 1
the band around 330 nm should be attributed to the
diphenylsulfone-centered n–pꢂ transition [28]. Upon UV
excitation, the PL spectra lost vibronic structure and the
emission peak red-shifted to 388 nm with respect to
the spirobifluorene monomer emission. From the edge of
the UV–Vis absorption peak, the band gap of SF-DPSO is
determined to be 3.40 eV. Triplet energy was measured
from the first phosphorescent emission peak in dichloro-
methane at 77 K and was 2.85 eV for this compound, which
is high enough to serve as the appropriate hosts for FIrpic
(2.62 eV) and Ir(mppy)3 (2.55 eV) to effectively confine
the triplet excitons and prevent of back energy transfer
[40,41]. To further confirm their exciton confinement
property, the transient photoluminescence decay of
5 wt% FIrpic doped into SF-DPSO and 4,40-bis(9-carbazol-
y)-2,20-biphenyl (CBP), and 5 wt% Ir(mppy)3 doped into
SF-DPSO were measured (Fig. 4) [42]. The CBP:FIrpic film
exhibits a multi-exponential decay curve, which can be
attributed to the backward energy transfer from FIrpic to
CBP and the triplet exciton quenching [43]. In contrast,
Here,
D
v = vabs
ꢀ
vf corresponds to the Stokes shift (in
cmꢀ1) between the maxima of absorption and fluorescence
emission. The other terms: h, c and a respectively represent
Planck0s constant, the velocity of light and the solute cavity
radius. In Eq. (2), n and e are refractive index and the static
dielectric constant of the solvent, respectively. Fig. 5 shows
the linear Lippert–Mataga plots with the slope values of
12364 cmꢀ1 for SF-DPSO. Accordingly, the difference of
the dipole moment between the excited state and the
ground state (Dl) was estimated to be only 10.42 D, which
also indicates the compound has a weak ICT trend that is
the key point for the phosphor to be used as a high energy
violet or ultra-violet emitting material [18]. Moreover, it
was reported that the small molecular dipole moment
could weaken the intramolecular and intermolecular elec-
tric field and benefit the electroluminescent performance
of fluorescent OLEDs [46]. In addition, it should be noted
that the fluorescence quantum efficiency of the compound
is extremely high in spite of the extremely limited
p
-conjugation. Even in the neat film, the yield still remains
SF-DPSO: FIrpic or Ir(mppy)3 film clearly exhibits
a
above 0.92, which was measured by the integrating sphere
method under air condition. Such high quantum efficiency
can be assign to the bulky sprio-structure, which effec-
tively suppresses the fluorescence quenching routes in
the film. Furthermore, sulfone-bridged organic chro-
mophores are also well known for their enhanced
photoluminescence and stability, which is important
for high efficient OLEDs [32].
mono-exponential decay curve, indicating that the triplet
energy was well confined and the backward energy trans-
fer was completely suppressed [44,45]. To illustrate the
reasonable molecular design in this work, the UV–Vis
absorption and fluorescence spectra of the compound in
various solvents of different polarity were recorded. As
3.4. Electrochemical analysis and theoretical calculations
The electrochemical properties of SF-DPSO were stud-
ied in solution through cyclic voltammetry (CV) using
tetrabutylammonium hexafluorophosphate (TBAPF6) as
the supporting electrolyte and ferrocene as the internal
standard (Fig. 6). During the anodic scan in dichlorometh-
ane, an oxidation peak is observed at 1.42 V, which is
ascribed to the oxidation of the spirobifluorene unit of
SF-DPSO. No reduction waves were detected. On the basis
of the onset potentials for oxidation, we estimated the
highest occupied molecular orbital (HOMO) energy lever
Fig. 2. AFM topographic images of ITO/PEDOT:PSS/SF-DPSO and ITO/
PEDOT:PSS/mCP.
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

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X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
Table 1
The photophysical, electrochemical, and thermal data of the compound.
kem [nm] kem [nm] Eg [eV] kmax [77 K]d [nm] ET [eV] EOX [V] Uf
HOMOh [eV] LUMOh [eV] Tg/Td[°C]
ꢀ2.48 211/438
a,b
a
c
a
d
g
e,f
kabs (log
e) [nm]
SF-DPSP 309(4.62), 329(4.43) 388
394 3.4 435 2.85 1.42
ꢃ1 (0.92) ꢀ5.88
a
b
c
Measured in CH2Cl2 solutions (1 ꢁ 10ꢀ6 M).
e
denotes the molar extinction coefficients
Neat film data measured at 298 K.
In CH2Cl2 glass at 77 K.
Measured versus quinine sulfate (Uf = 0.577 in 0.1 M H2SO4).
Measured by the integrating sphere method under air condition.
The onset potential of oxidation as referenced to ferrocene (Fc).
The HOMO and LUMO energies were determined from cyclic voltammetry and absorption data.
d
e
f
g
h
Lee–Yang–Parr exchange functional (B3LYP) with 6-31G(d)
basis sets. The optimized structure and the HOMO/LUMO
distribution for SF-DPSO are given in Fig. 6 inset. The
LUMO level of the compound is localized predominantly
on the electron-deficient diphenylsulfone core, while the
HOMO level is distributed mostly over the spirobifluorene
fragments. This separation could effectively maintain the
individual electronic properties of these functional groups
and benefit the transport of both holes and electrons [47].
3.5. Electroluminescent properties
To assess the utility of this solution-processable com-
pound as a UV–violet emitter, non-doped OLED with the
simple configuration: ITO/PEDOT:PSS/SF-DPSO/TPBI/Cs2
CO3/Al was fabricated. The schematic energy level diagram
of the device is shown in Fig. 8 (Device A). In this device,
ITO (indium tinoxide) and Al (aluminum) served as the an-
ode and cathode, respectively. PEDOT:PSS was used as the
hole-injection layer, TPBI as the electron-transport layer
and Cs2CO3 (caesium carbonate) as the electron-injection
layer. As shown in Fig. 7(a), the EL spectra of the device
display a UV–violet light emission around 402 nm, which
correspond well with the PL spectra (kmax = 394 nm) of
the solid film. Thus, it clearly demonstrates that the peak
from the device originated from compound SF-DPSO. The
slightly red shift and broader peak width at half height of
the EL spectra may be result from molecular aggregation
and excimer emission in the excited state. Fig. 7(c) shows
the luminous efficiency-current density curves, the UV–
violet emitting device has a maximum luminous efficiency
(LE) of 0.32 cd Aꢀ1, and a maximum external quantum effi-
ciency (EQE) of 0.2%. It should be emphasized that such EL
performance is achieved with a neat small molecule emit-
ting layer without emissive dopants, and the EL spectra
with the maximum emission around 400 nm indicates
rather effective hole and electron recombination in the
large-energy-gap UV–violet emitting material, which also
provide evidence that the UV–violet emitting OLED is suc-
cessfully achieved by using such a solution-processable
emitter. As far as we know, this is the first report of using
sulfone-based compound as an efficient emitter for solu-
tion-processed OLED with peak emission approaching
400 nm. Nonetheless, it is noted that the external quantum
efficiency is very poor which might be ascribed to the week
hole transporting property and the low carrier utilization
in the emitting layer. Therefore, the device performance
Fig. 3. Normalized absorption, emission and phosphorescence (77 K)
spectra of SF-DPSO in CH2Cl2.
Fig. 4. Transient photolumine scence decay of SF-DPSO:Firpic or
Ir(mppy)3 and CBP:FIrpic flims. Inset: energy levels and energy-transfer
schemes for SF-DPSO, CBP and FIrpic.
of SF-DPSO to be ꢀ5.88 eV. Through subtraction of the
optical energy gap from the HOMO energy level, we
calculated the energy level of the lowest unoccupied
molecular orbital (LUMO) to be ꢀ2.48 eV. To further
understand the effect of the topology structure on the elec-
tronic properties, theoretical calculation was performed
using a suite of Gaussian programs and the chemical struc-
tures of the host material was fully optimized by density
functional theory (DFT) using Becke’s three parameterized
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
7
Table 2
Spectral properties of SF-DPSO in different solvents.
,n)a
kabs,max (nm)
kf,max (nm)
D
Vd (cmꢀ1
)
b
c
Solvents
D
f (
e
N-hexane
Toluene
Chloroform
Tetrahydrofuran
Dichloromethane
Dimethyl formanide
0.001
0.013
0.147
0.210
0.217
0.274
329
330
328
331
327
325
372
380
384
385
388
396
3513
3987
4446
4421
4807
5328
a
b
c
D
f is the orientation polarizability parameter of the solvent.
Maximum absorption wavelength.
Maximum fluorescent wavelength.
The Stokes shift between the maxima of absorption and fluorescence emission.
d
Table 3
Device performances of the OLEDs.
gc,max (cd Aꢀ1
)
Lmax (cd mꢀ2
)
kem (nm)
gext,
(%)
a
b
c
d
e
Device
Von (V)
max
A(SF-DPSO)
B(FIrpic)
B(Ir(mppy)3)
6.0
5.5
5.6
0.32
13.7
30.2
1100
11,900
21,200
402
472
514
0.2
6.8
8.8
a
b
c
Recorded at 1 cd mꢀ2
Maximum luminous efficiency.
Maximum luminance.
Data were measured at 8 V.
Maximum external quantum efficiency.
.
d
e
Fig. 5. PL spectra of SF-DPSO in solvents of different polarities. Inset:
Lipperte–Mataga plot of SF-DPSO in various solutions.
Fig. 6. Cyclic voltammogram of SF-DPSO with a concentration of 10ꢀ3
M
in CH2Cl2 solution. Inset: simulated electronic distributions of SF-DPSO0s
HOMO and LUMO.
can be further improved by the device optimization, such
as the use of hole transporting materials to facilitate the
hole injection property from PEDOT:PSS to SF-DPSO and
confine the electron carriers in the emitting layer.
and green devices are almost identical with the emission of
FIrpic and Ir(mppy)3, corresponding to the CIE coordinates
of (0.16, 0.32) and (0.29, 0.60). Moreover, no additional
emission coming from SF-DPSO is observed, indicative of
efficient energy transfer from the host to the dopants. As
presented in Fig. 7(b), the turn on voltages (defined as
the voltage at which device emission is 1 cd mꢀ2) of the
two devices are approximately the same (5.5 V for blue
and 5.6 V for green) irrespective of doping complexes,
but the luminance of the green device is higher than that
of blue one at the same driving voltage. For instance, the
maximum luminance of the blue and green devices is
11,900 and 21,200 cd mꢀ2, respectively. Fig. 7(c) presents
the luminous efficiency–current density curves, the
In view of the high triplet energy (ET = 2.85 eV) of SF-
DPSO, it is applications as solution-processable host mate-
rial was investigated by doping FIrpic and Ir(mppy)3 into
SF-DPSO as the emitting layers (EML). The devices have a
simple configuration of ITO/PEDOT:PSS/SF-DPSO:Ir-based
complex/TPBI/Cs2CO3/Al. TPBI was used as the hole-block-
ing and exciton-confining layer. The schematic energy
level diagrams of the two devices are shown in Fig. 8
(Device B). The best EL performances were achieved with
10 wt% FIrpic for blue devices, and 8 wt% Ir(mppy)3 for
green devices. As shown in Fig. 7(a), the EL spectra of blue
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

8
X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
Fig. 7. (a) EL Spectra of the SF-DPSO based devices (b) luminance–voltage characteristics and (c) luminance efficiency versus current density plots of the
devices (d) current density–voltage characteristics.
Fig. 8. Schematic energy-level diagram of the devices A and B.
FIrpic-based blue device has a maximum LE of 13.7 cd Aꢀ1
and a maximum external quantum efficiency (EQE) of 6.8%,
while the green-emitting device exhibited high perfor-
mance with a maximum LE of 30.2 cd Aꢀ1 and a maximum
EQE of 8.8%. The efficiency of SF-DPSO based solution-
processed blue and green PhOLEDs shows significantly
high performance compared with the corresponding
devices based on mCP, which is commonly used as host
Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035

X. Ban et al. / Organic Electronics xxx (2014) xxx–xxx
9
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p-conjugated lengths of the molecular is effectively
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We are grateful for the grants from the National Basic
Research Program China (2013CB932902), National
Natural Science Foundation of China (51103023 and
21173042), the science and technology support program
(industry) Project of Jiangsu province (BE 2013118).
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Please cite this article in press as: X. Ban et al., Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet
electrofluorescence, blue and green phosphorescent OLEDs, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.035