Two novel blue phosphorescent host materials containing phenothiazine-5,5-dioxide structure derivatives

Article abstract of DOI:10.3762/bjoc.14.73

Two novel D-A bipolar blue phosphorescent host materials based on phenothiazine-5,5-dioxide: 3-(9H-carbazol-9-yl)-10-ethyl- 10H-phenothiazine-5,5-dioxide (CEPDO) and 10-butyl-3-(9H-carbazol-9-yl)-10H-phenothiazine-5,5-dioxide (CBPDO) were synthesized and characterized. The photophysical, electrochemical and thermal properties were systematically investigated. CEPDO and CBPDO not only have a high triplet energy but also show a bipolar behavior. Moreover, their fluorescence emission peaks are in the blue fluorescence region at 408 nm and the fluorescence quantum efficiency (φ) of CEPDO and CBPDO were 62.5% and 59.7%, respectively. Both CEPDO and CBPDO showed very high thermal stability with decomposition temperatures (Td) of 409 and 396 °C as well as suitable HOMO and LUMO energy levels. This preferable performance suggests that CEPDO and CBPDO are alternative bipolar host materials for the PhOLEDs.

Full text of DOI:10.3762/bjoc.14.73

Two novel blue phosphorescent host materials containing
phenothiazine-5,5-dioxide structure derivatives
Feng-Ming Xie1,2, Qingdong Ou3, Qiang Zhang1,2, Jiang-Kun Zhang1,2, Guo-Liang Dai1,2,
Xin Zhao*1,2 and Huai-Xin Wei*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 869–874.
1College of Chemistry, Biology and Material Engineering, Suzhou
doi:10.3762/bjoc.14.73
University of Science and Technology, Suzhou, Jiangsu 215009, P.R.
China, 2Jiangsu Key Laboratory of Environmental Functional
Materials, Suzhou, Jiangsu 215009, P.R. China and 3Department of
Materials Science and Engineering, Monash University, Clayton,
Victoria 3800, Australia
Received: 12 November 2017
Accepted: 03 April 2018
Published: 17 April 2018
This article is part of the Thematic Series "Recent advances in materials
for organic light emitting diodes".
Email:
Xin Zhao* - zhaoxinsz@126.com; Huai-Xin Wei* - hxwei@usts.edu.cn
Guest Editor: E. Zysman-Colman
*
Corresponding author
©
2018 Xie et al.; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
host materials; phenothiazine; phosphorescence; synthesis
Abstract
Two novel D–A bipolar blue phosphorescent host materials based on phenothiazine-5,5-dioxide: 3-(9H-carbazol-9-yl)-10-ethyl-
1
0H-phenothiazine-5,5-dioxide (CEPDO) and 10-butyl-3-(9H-carbazol-9-yl)-10H-phenothiazine-5,5-dioxide (CBPDO) were syn-
thesized and characterized. The photophysical, electrochemical and thermal properties were systematically investigated. CEPDO
and CBPDO not only have a high triplet energy but also show a bipolar behavior. Moreover, their fluorescence emission peaks are
in the blue fluorescence region at 408 nm and the fluorescence quantum efficiency (Φ) of CEPDO and CBPDO were 62.5% and
5
9.7%, respectively. Both CEPDO and CBPDO showed very high thermal stability with decomposition temperatures (Td) of 409
and 396 °C as well as suitable HOMO and LUMO energy levels. This preferable performance suggests that CEPDO and CBPDO
are alternative bipolar host materials for the PhOLEDs.
Introduction
Since 1987, the Tang group [1] firstly reported double organic angle (≥170°), fast response (1 μs scale), high luminous effi-
light-emitting diodes (OLEDs) with ultra-thin film by using ciency, low drive voltage (3–10 V), thickness (less than 2 mm),
vacuum evaporation technology. It has attracted much attention lightweightness and flexibility [2-6]. Compared with traditional
to develop it. In recent years, OLEDs have been rapidly de- fluorescence OLEDs which only utilize singlet (25%) excitons
veloped and widely used in lighting and display because of for electroluminescence, PhOLEDs can simultaneously harvest
many unique performance advantages such as wide viewing both the singlet and triplet (75%) excitons through spin-orbit
869

Beilstein J. Org. Chem. 2018, 14, 869–874.
coupling (SOC), and obtain nearly 100% of the internal quan- bipolar host material. Kim et al. [17] reported that the bipolar
tum efficiency (IQE). Thus, most of researchers have focused host material 9-(4-(9H-pyrido[2,3-b]indol-9-yl)phenyl)-9H-3,9'-
on phosphorescent organic light-emitting diodes (PhOLEDs) all bicarbazole (pBCb2Cz) has a high triplet energy level
over the world [7,8]. With the deepening of this research, the (2.93 eV), which is the main material of blue PhOLEDs, and the
performance of red and green electroluminescent devices has EQE of the device is 23.0%. The Suh group [18] reported that
been able to meet the commercial requirements, but the blue the EQE of the prepared device of the bipolar host material
electroluminescent devices have several weaknesses such as N-(3,5-di(9H-carbazol-9-yl)phenyl)-N-(pyridin-2-yl)pyridin-2-
low efficiency and poor stability and so on, which hinder its de- amine (DCPPy) based on carbazole group is 21.6%. The Wang
velopment. It has been proved that the selection of proper mate- group [19] designed a host material with symmetrical structure
rials for each layer is very important for achieving highly effi- based on phenothiazine-5,5-dioxide. But the host materials with
cient PhOLEDs. In particular, the design of host materials plays asymmetric structure based on the phenothiazine-5,5-dioxide
a critical role in the determination of the devices performance. were rarely reported. For obtaining a high triplet energy level
Therefore, it is beneficial to develop new blue phosphorescent and good stability, herein, with phenothiazine-5,5-dioxide as
host materials with high-performance for blue PhOLEDs [9-12]. acceptor (A) and carbazole as donor (D), and introducing an
alkane chain group to the host materials for better film-forming
Generally, ideal host materials are required to fulfill several properties, two novel blue phosphorescent host materials,
requirements [13,14]: i) the triplet energy level (ET) should be CEPDO and CBPDO, were synthesized. At the same time, the
higher for efficient energy transfer to the guest; ii) suitable photophysical properties, electrochemical properties and their
energy levels appropriately aligned with those of the neigh- thermal stability were studied and the expected results were ob-
boring active layers for efficient charge carrier injection to tained.
achieve a low operating voltage; iii) good and balanced charge
Results and Discussion
carrier transport properties for the hole–electron recombination
process; iv) good thermal and morphological stability for the Synthesis and theoretical calculations
vacuum deposition method to prolong the device operational The synthesis route for CEPDO and CBPDO is shown in
lifetime.
Scheme 1. The detailed synthesis procedures and characteriza-
tions are given in Supporting Information File 1.
Carbazole groups are widely used in host materials because of
their high triplet energy levels and high hole mobility [15]. The In order to further understand the structural properties of the
Lee group [16] linked carbazolyl groups to diphenyl phosphor- materials and the possibility of charge transfer from donor to
amines to design asymmetric (9-phenyl-9H-carbazole-2,5- acceptor on electronic excitation, the electronic structure of the
diyl)bis(diphenylphosphine oxide) (PCPOs) with a higher triplet materials were analyzed by density functional theoretical (DFT)
energy level (2.80 eV) and a glass transition temperature calculations using the Gaussian 09 program package. The elec-
(
140 °C). The maximum external quantum efficiency (EQE) of tron density distributions and energy levels of the HOMO and
PhOLEDs was 31.4%, which was prepared by PCPO as a LUMO are displayed in Figure 1.
Scheme 1: Synthetic routes of CEPDO and CBPDO.
870

Beilstein J. Org. Chem. 2018, 14, 869–874.
carbazole moiety of the molecules, and the weaker absorptions
around 295 nm assign to the n→π* transition of the conjuga-
tion of the whole molecule.
The optical bandgap (Eg) of the two substances were all calcu-
lated to be 3.32 eV from the UV–vis absorption spectra of
CEPDO and CBPDO. Upon photoexcitation of 330 nm at room
temperature, both CEPDO and CBPDO exhibited a FL spec-
trum with peaks at 408 nm and emitted blue fluorescence. The
fluorescence quantum efficiencies (Φ) of CEPDO and CBPDO
were 62.5% and 59.7%, respectively, by using quinine sulfate
as a reference [20]. Compared with CEPDO, the longer alkyl
chain of CBPDO led to a corresponding increase in Φ value.
Therefore, CEPDO and CBPDO are promising photoelectric
materials. To obtain the triplet energy level, their low-tempera-
ture Phos spectra were measured in a 2-methyltetrahydrofuran
Figure 1: Structures and molecular orbitals of (a) CEPDO and
b) CBPDO.
(
The HOMOs of CEPDO and CBPDO are mainly distributed solution at 77 K which are also shown in Figure 2. The phos-
over the electron-donating carbazole moiety and slightly ex- phorescence emission peaks were at 440 nm and 439 nm, re-
tended to the phenyl ring. The LUMOs are mostly localized on spectively. According to the onset of their phosphorescence
the phenothiazine-5,5-dioxide, based on the DFT calculation. spectra, the calculated triplet energy levels (ET) of CEPDO and
There is a small degree of spatial overlap between the HOMO CBPDO were identical at 2.82 eV, which matched with the blue
and LUMO in these two molecules. The separated HOMO and phosphorescent guest material FIrpic (2.65 eV), dark blue phos-
LUMO resulted from the strong electron-donating nature of the phorescent guest material FCNIrpic (2.74 eV) and FIr6
carbazole unit and electron-withdrawing ability of the phenothi- (2.73 eV). The high ET was attributed to the insulated carbazole
azine-5,5-dioxide unit, thus realized the orbital separation of moieties. Hence, both CEPDO and CBPDO are expected to be
hole and electron transport in the same molecule. This indicat- applied to PhOLED as a blue phosphorescent host material.
ed CEPDO and CBPDO have bipolar characteristic.
Electrochemical properties
Photophysical properties
The electrochemical properties of CEPDO and CBPDO were
Figure 2 presents the UV–vis absorption, photoluminescence studied by cyclic voltammetry (CV) measurements in deoxy-
and phosphorescence (77 K) spectra of CEPDO (a) and CBPDO genated DCM solution with 0.1 M tetrabutylammonium hexa-
(
b) in solution, respectively. Obviously, the strong absorption fluorophosphate as the supporting electrolyte. The cyclic
peak at 236 nm can be ascribed to the π→π* transition of voltammograms are shown in Figure 3.
Figure 2: UV–vis absorption and photoluminescence spectra in DCM solution (1 × 10−5 M and 1 × 10−6 M, respectively) at room temperature, phos-
phorescence spectra in 2-MeTHF solution (1 × 10−3 M) at 77 K of CEPDO (a) and CBPDO (b).
871

Beilstein J. Org. Chem. 2018, 14, 869–874.
fore, we successfully synthesized two novel bipolar host materi-
als with higher triplet energy by choosing suitable donor and
acceptor units.
Thermal properties
The thermal properties of CEPDO and CBPDO were deter-
mined by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under nitrogen atmosphere at a
scanning rate of 10 °C/min and the results are shown in
Figure 4. Both CEPDO and CBPDO show very high thermal
stability with decomposition temperatures (Td) of 409 and
3
1
96 °C and glass transition temperatures (Tg) of 167 and
38 °C, respectively. Compared to the introduction of ethyl
groups, the introduction of normal butyl groups to the phenothi-
azine-5,5-dioxide moieties appears to decrease the Td and Tg of
CEPDO by 13 °C and 29 °C, respectively, relative to those of
CBPDO. The reason may be the n-butyl chain is longer. It
reduces the polarity and intermolecular forces of molecules.
Figure 3: Cyclic voltammograms for CEPDO and CBPDO in DCM
solution.
Cyclic voltammetry was measured with a glassy carbon The high thermal values ensure high thermostability and that
working electrode, a platinum wire counter electrode, a satu- the amorphous structure can form homogeneous and stable
rated Ag/AgCl reference electrode, ferrocenium-ferrocene films by vacuum deposition to improve the lifetime of the
(
Fc+/Fc) as the internal standard and tetrabutylammonium hexa- PhOLEDs. The photophysics, electrochemical and thermal
fluorophosphate (0.1 M) as the supporting electrolyte. properties of CEPDO and CBPDO are summarized in Table 1.
The onset potential (Eonsetox) of the first oxidation wave for Conclusion
CEPDO and CBPDO are utilized to estimate the HOMO energy In summary, we have designed and synthesized two bipolar host
level according to the equation EHOMO= −e(Eonsetox + 4.8) as materials CEPDO and CBPDO. CEPDO and CBPDO not only
ca. −5.63 and −5.64 eV, respectively. And both HOMO energy have a high triplet energy but also show a bipolar behavior.
levels are matched with the functional function of the anode Moreover, their fluorescence emission peaks are blue fluores-
ITO (−4.5 to −5.0 eV). The LUMO energy levels of CEPDO cence at 408 nm and the fluorescence quantum efficiency (Φ) of
and CBPDO are estimated from the half-potential to be CEPDO and CBPDO are 62.5% and 59.7%, respectively. Both
−
2.31 eV and −2.32 eV, respectively, which are matched with CEPDO and CBPDO show very high thermal stability with Td
the LUMO energy level of electron injection material TAZ and of 409 and 396 °C, Tg of 167 and 138 °C, respectively, and also
favorable for electron injection and transmission [21]. There- appear suitable HOMO and LUMO energy levels. Hence,
Figure 4: DSC and TGA curves of CEPDO and CBPDO.
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Table 1: Photophysics, electrochemical and thermal properties of CEPDO and CBPDO.
compound λabsa
nm
λonsetb
/nm
λemtc
/nm
λphosd
/nm
Φe
/%
Eonsetf
/eV
Egg
/eV
Eth
/eV
EHOMO/LUMOi
/eV
Td/gj
/°C
/
CEPDO
CBPDO
236
236
373
372
408
408
440
439
49
69
1.23
1.24
3.32
3.32
2.82
2.82
−5.63/−2.31
−5.64/−2.32
409/167
396/138
aThe absorption maximum of the UV–vis spectrum; bestimated from the onset of the UV–vis spectrum; cemission fluorescence maximum at room
temperature; dphosphorescence emission peak at 77 K; efluorescence quantum yield; ffirst oxidation peak potential; gEg = 1240/λonset; hET = 1240/
λphos; iEHOMO: measured from the oxidation potential in 10−3 M DCM solution by cyclic voltammetry, ELUMO = Eg + EHOMO; jdecomposition tempera-
ture (Td) with 5% loss, glass transition temperature (Tg).
7.
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Acknowledgements
We acknowledge financial supports from the National Natural
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