Synthesis and properties of imidazole-p-phenothiazine derivatives as light-emitting and ambipolar materials

Article abstract of DOI:10.14233/ajchem.2014.17308

Novel phenothiazine derivatives were designed and synthesized for application as electroluminescent materials. They were characterized by means of 1H, 13C NMR and FTIR. The optical, electrochemical and thermal properties were also investigated. The relati

Full text of DOI:10.14233/ajchem.2014.17308

Asian Journal of Chemistry; Vol. 26, No. 6 (2014), 1637-1640
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.17308
Synthesis and Properties of Imidazole-π-phenothiazine
Derivatives as Light-Emitting and Ambipolar Materials†
1,2
1,2
1,2
JING SHAO , XIN ZHAO1,2,*, XING HE , ZHENGYIN ZHANG1,2 and RONGJIA ZHAI
1College of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou 215011, Jiangsu, P.R. China
2Jiangsu Key Laboratory of Environmental Functional Materials, Suzhou 215009, Jiangsu Province, P.R. China
*Corresponding author: E-mail: zhaoxinsz@126.com
Published online: 10 March 2014;
AJC-14851
Novel phenothiazine derivatives were designed and synthesized for application as electroluminescent materials. They were characterized
by means of 1H, 13C NMR and FTIR. The optical, electrochemical and thermal properties were also investigated. The relationship between
the chemical structure and the properties of these materials was evaluated. Due to the introduction of thiophene ring, these compounds
exhibited efficient emission from blue to green and red shifs in absorption and photoluminescence spectra. With respect to thermal
properties and the oxidation potentials, it can be turned by varying the conjugation length of compounds and the length of alkyl chains.
Mostly, all four compounds had suited HOMO and LUMO energy levels which can be used not only hole-transporting but also electron-
tansporting materials in organic light-emitting devices.
Keywords: Phenanthroimidazole, Phenothiazine, Light-emitting materials, Bipolar, Organic light-emitting diode.
nitrogen atoms with a butterfly conformation at the ground
state12. The additional electron rich sulfur atom renders
phenothiazine a stronger donor than other amines. In addition,
its “butterfly” non-planar structure impedes π-stacking
aggregation and intermolecular excimer formation, resulting
in diverse optoelectronic applications13,14. Thermally and
electrochemically stable phenothiazine is a strong electron
donor, even better than triphenylamine, tetrahydroquinoline,
carbazole and iminodibenzyles15,16.Not only phenothiazine is
low cost but commercially available. It can be easily tailored
by connecting soluble groups such as alkyl chains to the N
atom to improve solubility. Thus, it is important for developing
cost-effective and high properties organic light-emitting
materials leading to the commercial application.
INTRODUCTION
Since the pioneering work on organic light-emitting
devices (OLEDs) was developed by the Kodak group, extensive
research has been carried out to bring OLEDs into commercial
applications in flat-panel displays and solid-state lightings1,2.
Although the photoelectronic properties of OLEDs have been
investigated thoroughly and great advances have been made
in developing novel organic light-emitting materials, with high
stability, powerful brightness and high current efficiency remain
great interests3,4.
Charge transport and balancing are key factors for
obtaining high device efficiency. Recently some deep-blue
emitters were realized by simultaneously incorporating donor
and acceptor groups into one molecule and proved to be a
promising method for obtaining a high performance deep-blue
emitter5-9. Phenanthroimidazole is a class of molecules with
π-conjugated system and planar structure and the imidazole
group is an electron-deficient ring used as an acceptor unit.
And more, phenanthroimidazole derivatives have been proved
to be high efficient for electron injection or hole blocking10 as
well as blue emission11. Phenothiazine (PTZ) is a non-planar
heterocyclic compound which has electron rich sulfur and
In this work, we use the donor-π-acceptor approach to
design four new molecules by using phenanthroimidazole as
the acceptor for its good electron-transporting mobility and
phenothiazine (PTZ) as the donor for its good hole transport
mobility and connecting them with different linkers, such as
thiophene. The introduction of thiophene would allow a long
π-conjugated system to be achieved, while integration of the
alkyl chains could increase the solubility of materials. The
physical and photophysical properties of these compounds
†Presented at The 7th International Conference on Multi-functional Materials and Applications, held on 22-24 November 2013, Anhui
University of Science & Technology, Huainan, Anhui Province, P.R. China

1638 Shao et al.
Asian J. Chem.
were investigated with the aim of understanding the structure-
property relationships and developing novel molecular π-
conjugated bipolar materials for OLEDs.
(DSC) analysis and thermogravimetry analysis (TGA) were
performed on a METTLER DSC823 thermal analyzer and a
Rigaku TG-DTA 8120 thermal analyzer, respectively, with
heating rate of 10 °C/min under nitrogen atmosphere. Cyclic
voltammetry (CV) measurements were carried out on anAutolab
Potentiostat PGSTAT 12 with a three-electrode system (platinum
counter electrode, glassy carbon working electrode and SCE
electrode) at scan rate of 50 mV/s in dichloromethane under
argon atmosphere. The concentrations of an analytical material
and supporting electrolyte TBAP were 10-3 and 0.1 M, respec-
tively.
EXPERIMENTAL
All reagents were commercially used without further
purification. Petroleum ether for column chromatography was
redistilled.
1H and 13C nuclear magnetic resonance (NMR) spectra
were recorded on a Bruker AVANCE 400 MHz spectrometer
with tetramethylsilane as the internal reference using CDCl3
as solvent. Infrared (IR) spectra were measured on a Perkine-
Elmer FTIR spectroscopy spectrum RXI spectrometer as
potassium bromide (KBr) disc. Ultraviolet visible (UV-VIS)
spectra were recorded as a diluted solution in spectroscopic
grade dichloromethane on a Perkine-Elmer UV Lambda 25
spectrometer. Photoluminescence spectra and the fluorescence
quantum yields (ΦF) were recorded with a Perkin-Elmer LS
50B Luminescence Spectrometer as a dilute solution in spectro-
scopic grade dichloromethane. Differential scanning calorimetry
RESULTS AND DISCUSSION
Synthesis: The molecular structures and synthetic route
of the target compounds are outlined in Scheme-I.As illustrated
in Scheme-I, we synthesize two series of four anthracenes
bearing phenanthroimidazole, thiophene and phenothiazine.
In the first series, we began the synthesis 1a with alkylation of
phenothiazine at N-10 position with ethyl chain followed by
Vilsmeier Reaction with POCl3 and DMF to give 2a in a fair
R
H
R
N
(i)
(ii)
N
N
S
S
CHO
S
2a:R=C H
2
1a:R=C H
2
5
9
5
9
2b:R=C H
4
1b:R=C H
4
(iii)
R
N
R
N
(iv)
S
S
CHO
S
Br
4a:R=C H
2
5
3a:R=C H
2
5
4b:R=C H
4
3b:R=C H
4
9
9
R
N
5a:R=C H
2
5
N
(v) 2
S
5b:R=C H
4 9
N
O
O
R
N
6a:R=C H
2
5
9
(v) 4
S
N
S
N
6b:R=C H
4
Scheme-I: Synthetic route to 5a, 5b, 6a and 6b. Reagents and conditions: (i) NaOH, DMSO, rt; (ii) DMF, POCl3, reflux; (iii) NBS, DMF, rt;
(iv) Pd(PPh3)4, 1 M Na2CO3, 1,2-dioxane, reflux; (v) phenanthraquinone, ammonium acetate, glacial acetic acid, 140 ºC

Vol. 26, No. 6 (2014)
Synthesis and Properties of Imidazole-π-phenothiazine Derivatives 1639
yield of 78 %. Target 5a was formed by Debus-Radziszewski
imidazole synthesis reaction with 2a, 9,10-phenanthrene-
quinone and ammonium acetate in 43 % yield as dark yellow
solids. In order to enhance the solubility of organic light-emitting
materials, we insert the butyl chain at N-10 position of pheno-
thiazine to get 5b, which is similar to 5a in all synthetic steps.
To study the employment of the thiophene ring in organic light-
emitting materials, we synthesize another series of two comp-
ounds containg a thiophene. A two-step synthesis was carried
out by regioselective bromination at C-3 position of 1a with
NBS in DMF followed by palladium-catalyzed Suzuki cross-
coupling of the obtained 3a with Pd(PPh3)4, Na2CO3 and
5-formyl-2-thiophen boronic acid in 1,2-dioxane at 100 ºC.
Anthracene 4a was obtained as a light yellow solid in an overall
yield of 35 %. Using a simlar approach for 5a by replacing 2a
with 4a, a yellow solid 6a was finally obtained in a good yield
of 41 %. In the same way, 6b was gained as a deep yellow
solid in 47 % yield. All newly synthesized compounds were
fully characterized by standard spectroscopic methods. They
had good solubility in most organic solvents at room tempe-
rature, resulting from the presence of ethyl and butyl groups
at the N-10 position of phenothiazine.
5a
UV-vis
PL
5b
6a
6b
250
300
350
400
450
500
550
600
650
700
Wavelength(nm)
Fig. 1. UV-visible absorption and photoluminescence spectra of 5a, 5b,
6a and 6b measured in dilute CH2Cl2 solution
For the photoluminescence spectra, compounds 5a, 5b,
6a and 6b in solution CH2Cl2 ranging from blue to bright green
colours with the peaks at 475, 483, 533 and 529 nm, respec-
tively. The solution photoluminescence spectrum of 6a, 6b
red-shifted with respect to that of 5a, 5b, whereas the photo-
luminescence spectra of 6a and 6b were duplicate. These
results were in agreement to what was observed from the
theoretical and UV-visible experiments.
Photophysical properties: The photophysical properties
of compounds 5a-6b were examined by UV-visible spectro-
scopy and fluorescence spectroscopy in dilute CH2Cl2 solution
at room temperature. All the data are summarized in Table-1
including compounds absorption maxima (λabs), photolumi-
nescence emission maxima (λem), fluorescence quantum yields
(ΦF) and the Stokes shifts.
The fluorescence quantum yields (ΦF) were measured in
CH2Cl2 using Rhodamine B (Φ = 0.69 ) as standard. The Φ
value of 0.36 is observed for 5a, which is higher than that of
5b. This might be due to the increase of alkyl chain at N-10
position of phenothiazine resulted in the photoluminescence
emission quenching of 5b. For 6a and 6b, this change was
also suitable. These materials showed a Stokes shift (109-132
nm) suggesting energy loss during the relaxation process
and efficient fluorescence.
Fig. 1 presents UV-visible absorption and photolumine-
scence spectra of the four compounds in dilute CH2Cl2 solutions.
For the absorption spectra, it can be seen that all compounds
show similar absorption bands at a wavelength of 260 nm,
which can be attributed to the π-π* transition of their common
thiophene ring and benzene ring. The longer wavelength
absorption bands vary in the range of 332-401 nm, which might
be originated from the π-π* transition from the substituent on
the 2-imidazole position to the phenanthroimidazole acceptor.
On the other hand, it is worth noting that the difference in
spectra of 5a and 6a is due to the introduction of the thiophene
ring between phenanthroimidazole and phenothiazine, which
enhances the extent of intramolecular charge transfer in 6a
and thus leads to a red shift (35 nm) of the absorption spectra.
Whereas the UV-visible spectra of 5a and 5b, 6a and 6b were
duplicate, it indicated that the effect of alkyl chain is slight.
Electrochemical properties: Electrochemical behaviours
of all four compounds were investigated by cyclic voltammetry
(CV) in CH2Cl2 containg 0.1 M TBAP as a supporting electro-
lyte. Electrochemical band gaps were calculated from onset
potentials of the anodic and cathodic waves. The HOMO
energy levels of all compounds were calculated from the onset
potential of oxidation (Eonset) with the formula expressed to
EHOMO= - [4.74 + Eonset(ox)], (-4.74 eV for SCE with respect to
TABLE-1
PHYSICAL AND PHOTOPHYSICAL DATA OF 5a, 5b, 6a AND 6b
f
g
f
Stokes shiftb
(nm)
Tg/Tdd
(ºC)
Ege
(eV)
EHOMO
(eV)
a
a
ELUMO
Eonset
λabs
λem
c
Compound
ΦF
(nm)
(nm)
(eV)
-2.67
-2.83
-2.96
-2.86
(V)
260, 317, 366
262, 311, 365
255, 401
475
483
533
529
109
118
132
128
0.36
0.19
0.34
0.17
79/251
134/298
–/374
2.94
2.85
2.71
2.71
-5.61
-5.68
-5.67
-5.57
0.87
0.94
0.93
0.83
5a
5b
6a
6b
257, 401
–/250
aMeasured in a dilute CH2Cl2 solution. bStokes shift calculated from the difference between λmax of the absorption and emission spectra. cDetermined
in CH2Cl2 solutions (A < 0.1) at room temperature using Rhodamine B solution in ethanol (ΦF = 0.69) as a standard. dObtained from DSC on the
second heating cycle and TGA measurements under N2 at a heating rate of 10 ºC/min. eEstimated from the onset of the absorption spectra (Eg =
1240/λonset). fCalculated using the empirical equation: EHOMO = -(4.74 + Eonset) and ELUMO = HOMO + Eg. gMeasured using a three-electrode systemfi
tted with a glassy carbon working electrode, a platinum rod counter electrode and SCE reference electrode in degassed CH2Cl2 containing 0.1 M
TBAP as a supporting electrolyte at a scan rate of 50 mV/s.

1640 Shao et al.
Asian J. Chem.
the zero vacuum level). Moreover, we calculated the LUMO
energy levels from the HOMO energy levels and their respec-
tive band gap energy obtained from the tail-end of their UV-
VIS absorptions. Fig. 2 presents the CV traces of 5a, 5b, 6a
and 6b, whereas Table-1 lists their band gap energies and
LUMO and HOMO energy levels. The results suggest that
compounds 5a, 5b, 6a and 6b may be capable of hole- and
electro-transporting.
thermally stable with 5 % weight loss temperature were over
250 ºC. During the first heating DSC scan of sample 5a, only
endothermic melting peaks (Tm) at 135 ºC were detected, while
under a second heating cycle, only endothermic baseline shift
owing to glass transition (Tg) at 79 ºC was detected with no
crystallization and melting peaks at higher temperature being
observed. For compound 5b, Tg was found at 134 ºC, which
investigated that 5a and 5b all have good film-forming ability.
However, it is worth noting that, for 6a and 6b, no obvious Tg
could be detected.
5a
150
Conclusion
5b
6a
Two series of four novel compounds containing phenan-
throimidazole and phenothiazine have been synthesized. Their
optical, electrochemical, thermal properties were studied. All
of them were thermally stable with degradation temperature
well above 250 ºC, in addition, 5a and 5b also had good film-
forming ability due to the fact that their Tg were 79 and 134 ºC,
respectively. As believed, these compounds showed efficient
emission from blue to green. Moreover, the compounds exhibit
suited HOMO ranges (-5.68 to -5.57 eV) and LUMO ranges
(-2.96 to -2.67 eV), which have promising potential for appli-
cation in OLEDs as hole- and electron-transporting bipolar
materials.
100
6b
50
0
-50
-100
-150
ACKNOWLEDGEMENTS
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential vs. calomel electrode (V)
This work was supported by the Postgraduate Innovation
Fund of Jiangsu Province (CXZZ12_0877).
Fig. 2. CV curves of 5a, 5b, 6a and 6b measured in CH2Cl2 at a scan rate
of 50 mV/s
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of indium tin oxide electrodes. The HOMO energy levels are
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most widely used hole-transport materials in OLEDs. Thus,
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and phenothiazine moiety, all compounds exhibit good thermal
stability. These results suggest that all compounds were