Novel electroluminescent donor-acceptors based on dibenzo[a,c]phenazine as hole-transporting materials for organic electronics

Article abstract of DOI:10.1039/C6NJ03553A

A series of novel donor-acceptor type of molecules (2-6) based on dibenzo[a,c]phenazine were synthesized by employing a palladium = catalyzed C-N bond forming amination reaction in a good yield and were then fully characterized, whereby the optical properties of 2-6 were determined by UV-vis absorption, fluorescence spectroscopy, and the electrochemical properties by cyclic voltammetry. The absorption spectra of 2-6 showed intramolecular charge transfer (ICT) transitions in the range of 447-513 nm in solution. The HOMO and LUMO energy levels of 2-6 were in the range from ?5.03 to ?5.29 eV and ?2.75 to ?2.87 eV, respectively, with an electrochemical band gap within 2.26-2.45 eV. The HOMO energy levels of 2-6 are comparable with those of the most commonly used hole-transporting materials, which makes them potential candidates for hole-transporting materials in organic electronics.

Full text of DOI:10.1039/C6NJ03553A

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Novel electroluminescent donor–acceptors based
on dibenzo[a,c]phenazine as hole-transporting
materials for organic electronics†
Cite this:DOI: 10.1039/c6nj03553a
Azam M. Shaikh,a Bharat K. Sharma,a Sajeev Chackob and Rajesh M. Kamble*a
A series of novel donor–acceptor type of molecules (2–6) based on dibenzo[a,c]phenazine were
synthesized by employing a palladium = catalyzed C–N bond forming amination reaction in a good
yield and were then fully characterized, whereby the optical properties of 2–6 were determined by
UV-vis absorption, fluorescence spectroscopy, and the electrochemical properties by cyclic
voltammetry. The absorption spectra of 2–6 showed intramolecular charge transfer (ICT) transitions in
the range of 447–513 nm in solution. The HOMO and LUMO energy levels of 2–6 were in the range
from ꢀ5.03 to ꢀ5.29 eV and ꢀ2.75 to ꢀ2.87 eV, respectively, with an electrochemical band gap within
2.26–2.45 eV. The HOMO energy levels of 2–6 are comparable with those of the most commonly used
hole-transporting materials, which makes them potential candidates for hole-transporting materials in organic
electronics.
Received (in Montpellier, France)
14th November 2016,
Accepted 2nd December 2016
DOI: 10.1039/c6nj03553a
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(e.g., Indium tin oxide (ITO)), a hole-transporting material
(HTM), a light-emitting layer, and electron-transporting material
1. Introduction
Over the last few decades, astonishing progress has been made (ETM), and a metallic cathode.1a The efficiency and durability of
in the field of organic electronics from industry to academics, organic electronic devices depend upon the stability of both the
including in organic light-emitting devices (OLEDs),1 organic hole- and electron-transporting materials.1a Generally HTMs
field effect transistors (OFETs),2 and dye-sensitized solar cells should possess good hole mobility and an appropriate HOMO
(DSSCs).3 Small organic p-conjugated molecules with a donor– level and LUMO level for smooth functioning of the device.
acceptor (D–A) architecture have attracted significant attention Continuous efforts are also being made with HTMs to decrease
due to their unique properties, such as bipolar charge (electron the energy barrier between the anode and emissive material
and hole) transport and high luminescence quantum yield,4 and to shift the recombination area (recombination of charge
well-defined structures, ease of purification, reliable reprodu- carriers) toward the middle of the device to prevent quenching
cibility, better solubility in most organic solvents, and easy of the excitons (electron–hole pair) in the device14 and thus
fabrication for large area photovoltaic devices.5 The luminescence enhance the stability and lifetime of the device. So far, a number
from such D–A systems arises from the intramolecular charge of aromatic amine derivatives have been utilized for HTMs,
transfer (ICT) excited states or due to excimers or exciplexes including a-NPD (N,N0-di(1-naphthyl)-N,N0-diphenyl-(1,10-
formation, and further their optoelectronic properties can be biphenyl)-4,40-diamine), TPD (N,N0-bis(3-methylphenyl)-N,N0-
tuned over a broad range by modifying the D–A moieties.6
diphenylbenzidine), spiro-OMe-TAD (2,20,7,70-tetrakis[N,N-di(4-
In the recent past, a variety of D–A types of molecules based methoxyphenyl)amino]-9,90-spirobifluorene),15 Shirota star shaped
on anthracene,7 benzimidazole,8 carbazole,9 dibenzothiophene,10 molecules,16 and oligomeric amines.17
fluorene,11 pyrene,12 and quinoxaline,13 etc. have been synthesized
However, there is still a large scope for other organic materials
and extensively utilized as electroluminescent materials in organic to act as alternatives to the above-mentioned commonly used
electronic devices. Typically an OLED consisting of an anode hole-transporting materials. In the literature, very few reports
can be been found on phenazine derivatives, which include
dihydrophenazine derivatives18 and polymers based on 2,7-dioctyl-
a Department of Chemistry, University of Mumbai, Santacruz (East),
substituted dibenzo[a,c]phenazine19 for solar cells. Very recently,
S. Yagi et al.20 (2015) reported 10,13-disubsitituted dipyrido[3,2-
a:20,30-c]phenazine (dppz) derivatives for organic electronics.
Mumbai–400 098, India. E-mail: kamblerm@chem.mu.ac.in
b Department of Physics, University of Mumbai, Santacruz (East),
Mumbai–400 098, India
Thus, in this work, considering the importance of D–A types of
molecules and hole-transporting materials in organic electronics,
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6nj03553a
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we designed and synthesized compounds 2–6 by incorporating derivatives with a single bond between the ortho positions.22 In
a Buchwald–Hartwig coupling amination21 reaction between compound 6, we used morpholine amine to see the rigid effect of
electron-deficient 3,6,11-tribromo-dibenzo[a,c]phenazine (1) and a cyclic aliphatic amine on the photo-electrochemical properties
various electron-donating diarylamines at the 3rd, 6th, and 11th of dibenzo[a,c]phenazine.
positions of 1 and studied their detailed optoelectronic and
Further, the studies also explored the effect of electron-
thermal properties. The design of compounds 2–6 was mainly donating and -withdrawing substituents on the diarylamine
based on the fact that the diarylamines are well-known electron moiety on the photophysical and electrochemical properties,
donors, while compound 1 was selected as an electron-deficient such as the intramolecular charge transfer transitions (ICT),
backbone due to its coplanar structure formed by connecting the HOMO–LUMO energy levels, and the energy band gap. The
two separated phenyl rings of phenyl-substituted quinoxaline structures of the synthesized molecules 2–6 are shown in Fig. 1.
Fig. 1 Molecular structure of compounds 2–6.
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2. Results and discussion
Synthesis and characterization
reaction was observed when diarylamines bearing a functional
group such as an amino (–NH2) and nitro (–NO2) and hetero-
amines, such as phenothiazine, phenoxazine, and carbazole, were
used in the reaction. The synthesized compounds 2–6 were
obtained in 53–67% yields as dark orange to dark red solids
soluble in common organic solvents, including dichloromethane,
toluene, methanol, and cyclohexane. However, they were comple-
tely insoluble in water. The identity and purity of all the target
compounds were confirmed by FT-IR, 1H, 13C NMR spectroscopy,
high resolution mass spectroscopy (HRMS), and elemental
analysis.
The synthetic route for the synthesis of compounds 2–6 is shown
in Scheme 1. Compound
1 (3,6,11-tribromodibenzo[a,c]-
phenazine) was synthesized in two steps. The first step involved
the bromination of 9,10-phenanthrenedione with molecular
bromine in nitrobenzene and it was done according to the
reported procedure with an 86% yield and then confirmed by
its melting point 288 1C (Lit. 286–287 1C).23 The second step
involved the synthesis of 1 by condensing 3,6,-dibromo-
phenanthrene-9,10-dione and 4-bromo-2,3-diaminobenzene in
glacial acetic acid with a 91% yield. Compounds 2–6 were
obtained by the Buchwald–Hartwig coupling21 of 1 with the
corresponding diarylamine (2–5) and morpholine (6) in the
Photophysical properties
The absorption and photoluminescence properties of compounds
presence of palladium catalyst, a ligand, and a strong base in 2–6 were studied by UV-visible and fluorescence spectroscopy.
tetrahydrofuran (THF) (Scheme 1). Other solvents, such as In order to understand the effect of solvent polarity on the
toluene and dimethyl sulfoxide (DMSO), were also tried, but
the best results were obtained in THF. Tris(dibenzylideneacetone)-
dipalladium (Pd2(dba)3) was the source of the catalyst. Ligands
such as 2-dicyclohexylphoshpino-20,60-dimethoxy-biphenyl (SPhos),
2-dicyclohexylphosphino-20,40,60-triisopropylbiphenyl (XantPhos),
and 1,10-bis(diphenyl phosphino)ferrocene (DPPF) were used.
Sodium tertiarybutoxide (t-BuONa) was used as a base. However,
photophysical properties of 2–6, their absorption and emission
spectra were recorded in solvents of different polarity (cyclo-
hexane, toluene, dichloromethane, methanol, and dimethyl
sulfoxide) and in a neat solid film. The synthesized molecules
showed similar UV-vis absorption spectra in all the above solvents
and in a neat solid film (Fig. 2).
Compound 1 showed absorption maxima at 288, 384, and
the use of a Pd2(dba)3/SPhos catalyst–ligand combination with the 406 nm in toluene, which were due to n–p* and p–p* transitions
base t-BuONa significantly reduced the reaction time but with a
(see the ESI†). The absorption spectra of 2–6 in all the
good yield. Further, it is worth mentioning that no coupling
mentioned solvents showed major bands around 297–341 nm,
Scheme 1 Synthetic route of compounds 2–6.
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Fig. 2 UV-vis absorption spectra of compounds 2–6 in toluene (a) and neat solid film (b) (for other solvents, see the ESI†).
Table 1 Absorption data of compounds 2–6
labsa, nm (log emax
Compd Toluene
M
ꢀ1cmꢀ1
)
labs, nm
Cyclohexane
CH2Cl2
CH3OH
DMSO
Film
2
308 (4.85), 339 (4.72),
496 (4.69)
307 (5.02), 338 (4.73),
491 (4.88)
307 (4.78), 341 (4.64),
491 (4.50)
303 (4.81), 339 (4.71),
494 (4.64)
307 (4.82), 339 (4.75),
503 (4.69)
340, 510
3
4
5
322 (4.85), 490 (4.76)
308 (4.85), 507 (4.48)
317 (4.91), 508 (4.96)
322 (5.04), 484 (5.01),
307 (5.02), 502 (4.56)
311 (4.94), 501 (5.01),
329 (4.74), 493 (4.63)
310 (4.77), 508 (4.52)
316 (4.91), 510 (4.98)
323 (4.79), 491 (4.78)
308 (4.57), 508 (4.41)
311 (3.89), 503 (3.98)
329 (4.99), 499 (4.87)
329 (4.60), 513 (4.45)
316 (4.90), 512 (4.93)
334, 497
341, 517
313, 439
514
6
299 (4.98), 457 (4.70)
300 (4.89), 447 (4.59)
297 (4.95), 458 (4.68)
297 (4.62), 456 (4.37)
307 (4.92), 472 (4.68)
300, 468
a
Recorded in 10ꢀ5 M solution.
corresponding to n–p* and p–p* transitions. This high energy
However no significant change was observed on increasing
transition originated from the entire dibenzo[a,c]phenazine the solvent polarity on the n–p*, p–p* transition, and ICT
skeleton. Apart from the n–p* and p–p* transitions, the synthe- transitions in compounds 2–6, indicating that the ground state
sized compounds also showed a lower energy intramolecular of the molecules are more stable than the excited state. Further,
charge transfer transition (ICT) around 447–513 nm (Table 1) a slight bathochromic shift in the ICT transition of around
from the various electron-donor arylamines in 2–5 and mor- 9–17 nm was observed in the case of compounds 4 and 5 due
pholine in 6 to the electron-acceptor dibenzo[a,c]phenazine to the presence of polar groups, such as methoxy (–OCH3) and
segment (Fig. 2). This lowest-energy absorption band is the diazene, respectively, on the diarylamine moiety.
most sensitive to the solvent polarity and to the donor moiety
(arylamines).
In compound 6, a blue shift of around 31–49 nm was
observed in the ICT transition and the intensities of the n–p*,
Fig. 3 Emission spectra of 2 in various solvents (a) and in a neat solid film of 2–6 (b) (for other solvents, see ESI†).
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p–p*, and ICT transitions were found to be weak compared to shift observed in the thin film absorption maxima and slight
other derivatives due to the presence of aliphatic cyclic amine broadening of the peak suggests the presence of intermolecular
and morpholine. The absorption of 2–6 in a neat solid film aggregation in the solid state.
was observed in the range of 300–341 and 468–517 nm. The red
All the compounds 2–6 emit in the yellow-green region on
excitation at the lmax obtained in the absorption spectra with
the emission maxima in the range of 520–583 nm in toluene,
except for compound 5. A significant red shift of around
Table 2 Emission data, Stokes shift, optical band gap, and quantum yield
of compounds 2–6
64–154 nm was observed for 2–6 as compared to 1 (lemi
=
lemia, nm
Compd Toluene Cyclohexane Film
lemi, nm
429 nm) (see the ESI†) in solution. Further, a bathochromic
shift of 50–62 nm was observed in compound 4 as compared to
the other derivatives in toluene due to the presence of an
electron-donating methoxy (–OCH3) on the diarylamine moiety.
No emission was observed in all of the solvents and in the neat
solid film for compound 5 due to the presence of an electron-
withdrawing diazene group on the diarylamine moiety, thus
showing the influence of peripheral amines on the emission
intensity. A blue shift of around 13–41 nm was observed in
cyclohexane for 2–6 as compared to toluene. On increasing the
polarity of the solvent from toluene to CH2Cl2 and DMSO,
compounds 2, 3, and 6 emitted with a red shift of around
Stokes shiftb,
cmꢀ1
Eogpt c (eV) fF
d
2
3
4
5
6
532
525
583
—
510
508
540
—
562
555
609
—
13 670
12 008
15 255
—
2.37, (2.38) 0.41
2.33, (2.34) 0.36
2.20, (2.21) 0.30
—
—
520
493
545
14 326
2.46, (2.48) 0.52
a
b
c
Recorded in 10ꢀ5 M solution. Recorded in 10ꢀ5 M toluene. Optical
band gap estimated using the emission and excitation spectra in a neat
1240:8
ꢀ
ꢁ
solid film form and toluene (in parentheses) Egopt
¼
eV.
lopt edge
d
Quantum yield with reference to fluorescein (f = 0.79 in ethanol) in
toluene.
Fig. 4 Tapping mode AFM topographical images (3D) of compounds 2 and 3 neat solid film coated (1000 rpm) from chloroform solution on quartz
coverslips.
Fig. 5 The cyclic voltammogram (anodic sweep) of compounds 2–4 (a) and a full scan of compound 5 (b) measured in anhydrous CH2Cl2.
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Table 3 Electrochemical and thermal data of compounds 2–6
the ground state in polar solvents due to the presence of polar
groups, such as –OCH3 and diazene.
f
Tm
peak a
ox
peak b
red
g
HOMOc LUMOd EgEC e (1C) Td (1C)
A red shift of around 25–30 nm was observed in the neat
solid film as compared to in toluene (Fig. 3(b)), which may be
due to intermolecular aggregation in the solid state. The optical
band gaps of 2–6 were determined by the intersection of
the emission and excitation spectra. A large Stokes shift was
observed for 2–6, which suggest a considerable change in the
ground and excited state geometry (Table 2). The quantum
yields of 2–6 were calculated using fluorescein (f = 0.79 in
ethanol) as a reference. The quantum yields were found to
be high and to depend on the electron-donating nature of
diarylamines at the 3rd, 6th, and 11th positions of the
dibenzo[a,c]phenazine backbone. The morphological charac-
Compd
E
E
2
3
4
5
6
0.95, 1.18, 1.35 ꢀ1.64 ꢀ5.23 ꢀ2.78 2.45 272 330 (495)
0.94, 1.18, 1.39 ꢀ1.59 ꢀ5.23 ꢀ2.85 2.38 238 291 (481)
0.72, 0.93, 1.47 ꢀ1.69 ꢀ5.03 ꢀ2.77 2.26 240 280 (350)
1.01, 1.30, 1.64 ꢀ1.59 ꢀ5.29 ꢀ2.87 2.42 262 366 (399)
0.87, 1.02, 1.23 ꢀ1.71 ꢀ5.15 ꢀ2.75 2.40 223 270 (340)
Epoxeiak oxidation peak potential (V). Erpeedak reduction peak potential (V).
a
b
c
peak
oxi
HOMO energy level calculated from EHOMO = ꢀ[E
ꢀ Eredox(Fc/Fc+) +
d
peak
red
4.8] eV. LUMO energy level calculated from ELUMO = ꢀ[E
ꢀ
Eredox(Fc/Fc+) + 4.8] eV. EgEC calculated from EEgC = [HOMO–LUMO] eV.
e
f
g
Melting point determined by DSC. Decomposition temperature at
5% and 10% (in parentheses) weight loss derived by TGA.
18–55 nm, thus showing positive solvatochromism. However, teristics of the neat solid film of compounds 2–6 were studied
quenching in the emission was observed for 2–6 in methanol, by atomic force microscopy (AFM) in the tapping mode. Fig. 4
while compounds 4 and 5 did not emit in CH2Cl2 and DMSO, shows the AFM results of compounds 2 and 3, while for other
suggesting a nonradiative relaxation from the excited state to compounds, see ESI.† The AFM results show a smooth film with
Fig. 6 Frontier molecular orbital of compounds 2–6 based on the DFT (B3LYP/6-311G) calculations.
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a root mean square (rms) roughness value of 0.94–8.72 nm, while range of ꢀ5.03 to ꢀ5.29 eV and ꢀ2.75 to ꢀ2.87 eV, respectively.
micrometer-sized phase separation was not observed.
The HOMO energy levels of 2–6 are comparable to those of the
commonly used/reported hole-transporting materials.
The energy band gap calculated from the cyclic voltammetry
measurements were in the range of 2.26–2.45 eV and closely
matched with the optical band gap. A decrease in band gap was
observed in compound 4 (2.26 eV), indicating the influence of
the electron-donating –OCH3 group on the diarylamine moiety
at the 3rd, 6th, and 11th positions of the dibenzo[a,c]phenazine
moiety (Table 3). Thus, the presence of an electron-donating
substituent on the diarylamine moiety results in a decrease in
the HOMO and LUMO energy levels and the energy band gap.
Electrochemical properties
The redox properties and molecular orbital energy levels (HOMO
and LUMO) of compounds 2–6 were studied by cyclic voltam-
metry (CV) (Fig. 5) in anhydrous dichloromethane solution using
ferrocene as an internal standard to calibrate the redox potentials,
and the pertinent data is presented in Table 3.
On an anodic sweep, three quasi-reversible waves were observed
for all compounds 2–6, which was attributed to the oxidation of
the diarylamine moiety at the 3rd, 6th, and 11th positions of the
dibenzo[a,c]phenazine moiety, suggesting three electrons were
removed successfully from three nitrogen atoms. The Eoxi
potential of 2–6 decreased on increasing the donor strength
(4 4 6 4 3 4 2 4 5). On a cathodic sweep, one reversible wave
was observed for 2–6 (see the ESI†), corresponding to reduction
of the dibenzo[a,c]phenazine core. The energy levels of the
highest occupied and lowest orbitals included TPD (HOMO =
ꢀ5.12 eV),15b a-NPD (HOMO = ꢀ5.20 eV),1a,15a spiro-OMe-TAD
(HOMO = ꢀ5.22 eV),15c–f DMFL-NPB (9,9-dimethyl-N,N0-di-
(1-naphthyl)-N,N0-diphenyl-9H-fluorene-2,7-diamine)24 (HOMO =
ꢀ5.20 eV), 1,3,5-tris(2-(9-ethylcabazyl-3)ethylene)benzene
(HOMO = ꢀ5.20 eV).25 Thus, with a lower HOMO energy level,
compounds 2–6 may act as hole-transporting materials.
Unoccupied molecular orbital’s (HOMO and LUMO) were calcu-
lated from the first oxidation and reduction potentials. The
HOMO and LUMO energy levels of 2–6 were found to be in the
Theoretical properties
In order to understand the molecular orbital levels and elec-
trical properties of compounds 2–6 theoretically, the structures
and frontier molecular orbital profiles of these molecules were
optimized using DFT calculations at the B3LYP/6-311G level
in the Gaussian 03 suite of programs.26 Fig. 6 show the
theoretically calculated molecular orbitals of 2–6. The theoreti-
cally calculated HOMO and LUMO of 2–6 were in the range of
ꢀ4.74 to ꢀ5.27 eV and ꢀ1.98 to ꢀ2.58 eV, respectively, and were
in close agreement with those calculated by CV (Table 4).
The HOMO energy levels of 2–6 were found to be localized
throughout the molecule (Fig. 6), indicating that the HOMO
energy levels were contributed by both the electron-donating/
acceptor moieties, while the LUMO energy levels of 2–6 were
mainly located on the dibenzo[a,c]phenazine backbone.
Thus, HOMO–LUMO transitions occur from the electron-
donor diarylamines to the electron-acceptor dibenzo[a,c]phenazine
backbone. The first ionization potential, the electron affinity, the
energies of the HOMO and LUMO levels, the HOMO–LUMO gap,
and the ground-state dipole moment computed for 2–6 are listed
in Table 4. The dipole moment calculated for 2–6 were found to be
low as the dipole vectors lie in opposite directions and cancel each
other out, whereas the dipole moment of 4 (3.11) was high,
compared to the other compounds, due to the presence of an
electron-withdrawing diazene group on the diarylamine moiety.
Table 4 Computed electron affinities, ionization potentials, HOMO–LUMO
energies, energy band gap, and dipole moments of compounds 2–6
Compd EA (eV) IP (eV) HOMO (eV) LUMO (eV) ETgH mg (debye)
2
3
4
5
6
1.13
1.06
0.91
1.81
0.84
5.94
5.90
5.58
6.02
6.25
ꢀ5.06
ꢀ5.04
ꢀ4.74
ꢀ5.27
ꢀ5.14
ꢀ2.23
ꢀ2.11
ꢀ1.98
ꢀ2.58
ꢀ2.08
2.82 0.88
2.93 1.30
2.75 0.89
2.69 3.11
3.06 0.22
Fig. 7 (a) TGA and (b) DSC thermograms of compounds 2–6 under a nitrogen atmosphere at normal pressure. Heating rate, 10 1C minꢀ1
.
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Thermal properties
a SHIMADZU UV-2401PC instrument at room temperature. The
neat solid films of compounds 2–6 were prepared by using a
spin coater (Holmarc HO-TH-05) at 1000 rpm for 2 min using
B6 mg mlꢀ1 of sample in chloroform. Quartz coverslips were
used for the neat solid film studies. The AFM topographical
images were recorded on a Bruker Dimension Icon AFM
instrument. The excitation and emission spectra were carried
out on a Perkin Elmer LS 55 Fluorescence spectrophotometer.
The fluorescence quantum yield of the derivatives 2–6 were
calculated using fluorescein (fF = 0.79 in ethanol). Cyclic
voltammetry studies were carried out on a computer-controlled
Palmsens3 potentiostat. Typically, a three electrode cell equipped
with a glassy carbon working electrode, a Ag/AgCl (non-aqueous)
reference electrode, and platinum (Pt) wire as the counter electrode
was employed. The measurements were carried at room tem-
perature in anhydrous acetonitrile with tetra butyl ammonium
hexafluorophosphate solution (0.1 M) as the supporting electro-
lyte with a scan rate of 100 mV sꢀ1. The potential of the Ag/AgCl
reference electrode was calibrated by using a ferrocene/ferrocenium
redox couple, which has the known oxidation potential of +4.8 eV.27
The melting point of the products were determined by differential
scanning calorimetry (DSC). The thermogravimetric analysis (TGA)
and DSC were performed using a Metler Toledo instrument
The thermal investigations of compounds 2–6 were carried out
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The melting points of 2–6 were determined
by DSC (Fig. 7b) and were in the range of 223–272 1C.
No glass phase transitions were observed in all the com-
pounds 2–6 (see the ESI†). The TGA thermograms (Fig. 7(a)) of
compounds 2–6 revealed that these derivatives had good thermal
stability with no weight loss at low temperature. The decomposi-
tion temperature corresponding to 5% and 10% weight losses
were in the range of 270–366 1C and 340–495 1C, respectively
(Table 3). The order of thermal stability among the synthesized
derivatives was 5 4 2 4 3 4 4 4 6. The high thermal stability of
2–6 makes them suitable for device fabrication and to facilitate an
enhanced lifetime of the devices.
3. Conclusions
A series of 3,6,11-trisubstituted-dibenzo[a,c]phenazine derivatives
were synthesized via a Buchwald–Hartwig palladium-catalyzed
C–N bond forming reaction using a Pd2(dba)3/SPhos catalyst
system in good yields. The absorption, electrochemical, and thermal
properties of the synthesized materials were significantly influenced
by the nature of the peripheral amine segment attached to the
dibenzo[a,c]phenazine core. Compounds 2–6 emitted in the green
region (545–609 nm) in a neat solid film, except for compound 5.
The ionization potentials (HOMO energy levels) of 2–6 were found in
the range of ꢀ5.03 to ꢀ5.29 eV and were comparable to those
of the commonly used/reported hole-transporting materials. The
theoretically obtained results were found to be consistent with the
experimental results. The thermal properties revealed that the
derivatives had high melting points and good thermal stability.
Thus, on the basis of the experimental and theoretical results,
we believe that these compounds have the potential to be used as
hole-transporting materials in optoelectronic devices.
1
(TG/DSC) under a nitrogen atmosphere. H NMR spectra and
13C NMR spectra were recorded using CDCl3 on a Bruker 300
Ultrashield spectrometer with tetramethylsilane (TMS) as the
internal reference at a working frequency of 300 MHz and
75 MHz, respectively. Fourier transform infrared (FT-IR) spectra
were recorded on a Perkin Elmer Frontier 91579. The spectra
of solid compounds were performed in KBr pellets. High
resolution mass spectrometric measurements were recorded
on a Maxis Impact 282001.00081 instrument using Bruker
Compass data analysis 4.1, while elemental analysis was carried
on an EA Euro-elemental analysis instrument.
Synthetic procedures
Synthesis of 3,6,11-tribromodibenzo[a,c]phenazine (1).
A mixture of 3,6,-dibromophenanthrene-9,10-dione (0.92 g,
2.5 mmol) and 4-bromo-2,3-diaminobenzene (0.47 g, 2.5 mmol)
was dissolved in 25 ml of glacial acetic acid and refluxed for 5 h.
The reaction mixture was allowed to cool and then poured over
crushed ice to obtain a light green colored solid. The obtained
solid was then dried under vacuum and purified using column
chromatography (eluent: n-hexane : chloroform ratio as 60 : 40)
to obtain a light green solid. Yield: 1.17 g (91%), mp: 4360 1C.
FT-IR (KBr, nmax cmꢀ1): 3055.89, 1589.09, 1285.89, 1045.15,
824.23, 723.56; 1H NMR (300 MHz, CDCl3): d (ppm) = 8.09
(d, J = 9.3 Hz, 1H), 7.77 (d, J = 1.8 Hz, 2H), 7.54 (d, J = 2.7 Hz, 1H),
7.51(d, J = 2.7 Hz, 1H), 7.43 (d, J = 2.7 Hz, 1H), 7.32 (d, J = 1.8 Hz,
1H), 7.29 (d, J = 1.8 Hz, 1H), 7.26 (s, 1H); 13C NMR (75 MHz,
CDCl3): d (ppm) = 152.49, 152.13, 151.39, 143.26, 142.26, 139.96,
137.73, 133.19, 132.37, 129.63, 127.54, 127.03, 123.80, 123.38,
121.75, 116.78, 116.49, 109.23, 107.32, 107.16; MS: C20H9Br3N2,
m/z: 518.87 [M + 1]+; elemental anal. calcd for C20H9Br3N2: C 46.46,
4. Experimental section
Chemicals and materials
All the starting materials and reagents were purchased from com-
mercial sources (Sigma Aldrich and Alfa Aesar) and were used
without any further treatment and purification unless otherwise
noted. The organic solvents were of HPLC and spectroscopic grade
and were dried and freshly distilled using the standard procedures
and handled in a moisture-free atmosphere. Column chromato-
graphy was carried out using SD-fine silica gel (60–120 mesh),
eluting with n-hexane and chloroform. The progress of the reaction
and the purity of the compound were checked by thin layer
chromatography (TLC) on silica-gel-coated glass plates, in which
the spots were visualized with UV light (365 nm) and in an iodine
chamber.
Instrumentation and methods
UV-visible spectra were recorded in 10ꢀ5 mol Lꢀ1 solutions in a H 1.75, Br 46.36, N5.42. found: C 46.40, H 1.74, Br 46.39,
1 cm path length quartz cuvette as were the neat solid films on N 5.46.
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General procedure for the synthesis of compounds 2–6. C62H51N5O6:
Paper
C
77.40,
H
5.34,
N
7.28, O, 9.98. found:
A mixture of compound 1 (0.122 g, 0.25 mmol) and various C 77.42, H 5.33, N 7.19, O 10.06.
diarylamines (0.6 mmol) was dissolved in anhydrous tetra-
N3,N6,N11-Triphenyl-N6,N11,N11-tris(4-(E)-(phenyldiazenyl)phenyl)-
dibenzo[a,c]phenazine-3,6,11-triamine (5). Dark red solid, yield: 145
mg (53%), mp: 262 1C. FT-IR (KBr, nmax cmꢀ1): 3035.43, 1586.07,
hydrofuran (20 ml). To this solution, Pd2(dba)3 (20 mg,
0.02 mmol), SPhos (13 mg, 0.03 mmol) and t-BuONa (75 mg,
0.75 mmol) were added. The reaction mixture was continuously
stirred under a nitrogen atmosphere at 80 1C for 6 h. The reaction
mixture was then cooled to room temperature and extracted with
chloroform. The solid thus obtained was then purified by column
chromatography (eluent: n-hexane : chloroform ratio as 70 : 30) to
obtain a dark red to dark orange solid.
1
1488.55, 1269.42, 686.22; H NMR (300 MHz, CDCl3): d (ppm) =
9.12 (d, J = 9.0 Hz, 1H), 9.04 (d, J = 9.0 Hz, 1H), 8.08 (d, J = 9.0 Hz,
2H), 7.90–7.74 (m, 14H), 7.63 (d, J = 2.7 Hz, 1H), 7.60 (d, J = 2.7 Hz,
2H), 7.51–7.15 (m, 30H); 13C NMR (75 MHz, CDCl3): d (ppm) =
156.30, 152.89, 149.66, 148.93, 148.58, 148.41, 147.98, 147.90,
146.46, 146.34, 146.28, 143.15, 142.28, 141.95, 140.67, 139.42,
133.35, 132.85, 132.32, 131.88, 130.43, 129.91, 129.71, 129.05,
127.50, 127.38, 127.15, 126.39, 126.21, 125.80, 125.48, 125.23,
125.07, 124.40, 124.31, 123.75, 123.25, 123.14, 122.67, 122.62,
119.34, 116.48, 116.29, 116.28; HRMS: calcd for C75H52N11
[M + H]+ 1094.4323; found 1094.4411; elemental anal. calcd for
C75H51N11: C 81.22, H 4.70, N 14.08. found: C 81.32, H 4.63,
N 14.05.
N3,N3,N6,N6,N11,N11-Hexaphenyldibenzo[a,c]phenazine-3,6,11-
triamine (2). Dark orange solid, yield: 129 mg (66%), mp: 272 1C.
FT-IR (KBr, nmax cmꢀ1): 3033.43, 1585.98, 1485.72, 748.39,
691.76; 1H NMR (300 MHz, CDCl3): d (ppm) = 9.06 (d, J =
9.0 Hz, 1H), 8.99 (d, J = 9.0 Hz, 1H), 8.02 (d, J = 9.0 Hz, 1H),
7.62–7.63 (m, 3H), 7.55 (d, J = 9.0 Hz, 2H), 7.38–7.03 (m, 31H);
13C NMR (75 MHz, CDCl3): d (ppm) = 149.60, 149.31, 149.21,
147.98, 147.09, 147.07, 146.97, 143.18, 142.34, 142.28, 140.34,
138.83, 132.88, 132.24, 129.59, 129.40, 127.15, 125.58, 124.48,
124.21, 123.89, 123.79, 122.51, 122.19, 117.09, 115.59, 114.87,
114.62; HRMS: calcd for C56H40N5 [M + H]+; 782.3278 found
782.3293; elemental anal. calcd for C56H39N5: C 86.02, H 5.03,
N 8.96. found: C 86.12, H 4.93, N 8.95.
4,40,400-(Dibenzo[a,c]phenazine-3,6,11-triyl)trimorpholine (6).
Orange solid, yield: 82 mg (61%), mp: 223 1C. FT-IR (KBr,
nmax cmꢀ1): 3479.23, 2920.34, 2848.93, 1604.23, 1362.36,
1106.52, 685.90; 1H NMR (300 MHz, CDCl3): d (ppm) = 8.09
(d, J = 9.0 Hz, 1H), 7.79 (s, 2H), 7.55 (d, J = 2.4 Hz, 1H), 7.52
(d, J = 2.4 Hz, 1H), 7.43 (d, J = 2.1 Hz, 1H), 7.32 (d, J = 8.1 Hz,
2H), 7.25 (s, 1H), 3.97 (t, J = 4.5 Hz, 12H,), 3.42 (t, J = 4.5 Hz,
12H); 13C NMR (75 MHz, CDCl3): d (ppm) = 152.48, 152.15,
151.23, 143.49, 142.19, 140.01, 132.40, 132.31, 129.65, 127.21,
121.88, 121.57, 116.51, 116.35, 116.20, 109.38, 108.93, 108.28,
107.19, 107.03, 66.92, 66.81, 49.41, 49.14; HRMS: calcd for
N3,N6,N11-Tri(naphthlen-1-yl)-N6,N11,N11-triphenyldibenzo[a,c]-
phenazine-3,6,11-triamine (3). Red solid, yield: 139 mg (60%),
mp: 238 1C. FT-IR (KBr, nmax cmꢀ1): 3055.03, 1590.24, 1488.12,
1
1357.10, 770.23, 693.04; H NMR (300 MHz, CDCl3): d (ppm) =
9.00 (d, J = 9.0 Hz, 1H), 8.97 (d, J = 9.0 Hz, 1H), 7.96–7.73
(m, 10H), 7.51–6.97 (m, 33H); 13C NMR (75 MHz, CDCl3):
d (ppm) = 150.06, 149.65, 149.06, 147.45, 147.38, 147.26,
145.42, 143.17, 142.82, 142.71, 142.61, 142.26, 140.15, 138.43,
135.40, 135.16, 132.91, 132.20, 131.13, 131.08, 131.01, 129.42,
129.18, 128.51, 128.42, 127.56, 127.33, 127.21, 127.03, 126.99,
126.72, 126.53, 126.38, 126.30, 126.14, 125.23, 124.30, 123.97,
123.56, 123.41, 123.11, 123.04, 122.97, 122.90, 120.90, 120.60,
115.07, 112.75; HRMS: calcd for C68H46N5 [M + H]+ 932.3646;
found 932.3781; elemental anal. calcd for C68H45N5: C 87.62,
H 4.87, N 7.51. found: C 87.52, H 4.97, N 7.52.
C
32H34N5O3 [M + H]+ 536.2656; found 536.2650; elemental anal.
calcd for C32H33N5O3: C 71.75, H 6.21, N 13.07, O, 8.96. found:
C 71.70, H 6.23, N 13.19, O 8.98.
Acknowledgements
The authors are greatly thankful to Micro-Analytical Laboratory,
Department of Chemistry, University of Mumbai for providing
Instrumental facilities. We are sincerely thanks to Central
facility Centre, Department of Chemistry, IIT Bombay for
providing HRMS Spectrum and National centre for Nanoscience
and Nanotechnology, University of Mumbai for AFM. Two of the
authors (Azam M. Shaikh and Bharat K. Sharma) is grateful to
University Grants Commission, India for providing Senior
Research Fellowship (UGC-SRF).
N3,N3,N6,N6,N11,N11-Hexakis(4-methoxyphenyl)dibenzo[a,c]-
phenazine-3,6,11-triamine (4). Dark red solid, yield: 161 mg
(67%), mp: 240 1C. FT-IR (KBr, nmax cmꢀ1): 3037.72, 3000.07,
1
2928.33, 1598.68, 1502.71, 1236.60, 1030.52, 824.80; H NMR
(300 MHz, CDCl3): d (ppm) = 8.96 (d, J = 9.0 Hz, 1H), 8.91 (d, J =
9.0 Hz, 1H), 7.93 (d, J = 3.9 Hz, 1H), 7.91 (d, J = 3.9 Hz, 1H), 7.51
(s, 3H), 7.45 (d, J = 2.7Hz, 2H), 7.17 (d, J = 9.0 Hz, 6H), 7.06 (d, J =
9.0 Hz, 6H), 6.88 (d, J = 9.0 Hz, 6H), 6.80 (d, J = 9.0 Hz, 6H), 3.81
(s, 18H); 13C NMR (75 MHz, CDCl3): d (ppm) = 156.66, 156.36,
156.27, 150.20, 149.74, 149.21, 142.38, 140.25, 140.21, 140.10,
139.92, 132.93, 132.14, 129.12, 127.38, 127.13, 127.00, 126.89,
126.47, 124.77, 123.27, 120.34, 119.93, 114.97, 114.72, 113.63,
112.15, 111.82, 55.51, 55.40; HRMS: calcd for C62H52N5O6
[M + H]+ 962.3912; found 962.3932; elemental anal. calcd for
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