Phenanthroimidazole derivatives showing mild intramolecular charge transfer and high quantum yields and their applications in OLEDs

Article abstract of DOI:10.1039/d1nj02134f

Phenanthroimidzole (Phen-I) has attracted the attention of researchers for applications in materials science including organic electronics and biological applications. Herein, we designed small Phen-I derivatives showing mild bipolar characters and strong emissions in the blue region and discussed their applications in OLEDs. Designing of materials is based on the presence of electron-deficient nitrogen (sp²) (acting as the mild electron-withdrawing core) and the availability of other nitrogen (N1) and carbon (C2) atoms for substitution at the imidazole ring. Earlier studies on the structural properties of Phen-I and our theoretical studies show that substitution at N1 is orthogonal to the plane of the imidazole ring, thus limiting the effect of extended conjugation. The aryl groups at the C2-carbon of the imidazole moiety caused considerable alteration in the electronic properties which is evident from the absorption and emission spectra. Absorption and emission in different solvents show slight alterations in the peak maxima suggesting mild intramolecular charge transfer in these molecules. These materials were also studied in thin films when doped in blends with polyvinylcarbazole (PVK). Furthermore, one of these Phen-I derivatives is used for OLED applications. A high luminance of ～ 2.5 × 10⁴Cd m^?2at a current density of 35 mA cm^?2was found for Phen-I based OLEDs. This gave a high current efficiency of ～ 60 Cd A^?1(@ 15 V) and an external quantum efficiency of ～3.2%.

Full text of DOI:10.1039/d1nj02134f

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Phenanthroimidazole derivatives showing mild
intramolecular charge transfer and high quantum
yields and their applications in OLEDs†‡
Cite this: DOI: 10.1039/d1nj02134f
Swati J. N. Dixit,a Chandan Gupta,b Tanveer Habib Tadavi,a
c
b
a
K. R. S. Chandrakumar,
Sangita Bose
and Neeraj Agarwal
*
Phenanthroimidzole (Phen-I) has attracted the attention of researchers for applications in materials
science including organic electronics and biological applications. Herein, we designed small Phen-I
derivatives showing mild bipolar characters and strong emissions in the blue region and discussed their
applications in OLEDs. Designing of materials is based on the presence of electron-deficient nitrogen
(sp2) (acting as the mild electron-withdrawing core) and the availability of other nitrogen (N1) and
carbon (C2) atoms for substitution at the imidazole ring. Earlier studies on the structural properties of
Phen-I and our theoretical studies show that substitution at N1 is orthogonal to the plane of the
imidazole ring, thus limiting the effect of extended conjugation. The aryl groups at the C2-carbon of the
imidazole moiety caused considerable alteration in the electronic properties which is evident from
the absorption and emission spectra. Absorption and emission in different solvents show slight
alterations in the peak maxima suggesting mild intramolecular charge transfer in these molecules. These
materials were also studied in thin films when doped in blends with polyvinylcarbazole (PVK).
Furthermore, one of these Phen-I derivatives is used for OLED applications. A high luminance of
B 2.5 ꢀ 104 Cd mꢁ2 at a current density of 35 mA cmꢁ2 was found for Phen-I based OLEDs. This gave
a high current efficiency of B 60 Cd Aꢁ1 (@ 15 V) and an external quantum efficiency of B3.2%.
Received 1st May 2021,
Accepted 27th July 2021
DOI: 10.1039/d1nj02134f
rsc.li/njc
stability for solid-state device applications is considered to be
difficult.5 Several rigid conjugated systems, starburst systems
Introduction
The design and development of organic materials showing composed of various emitters, including fused aryl/heteroaryl
narrow luminescence in the red, green and blue regions has derivatives of pyrene,6 fluorene,7 carbazole,8 anthracene,9 and
been an active research area for about three decades.1–3 Their phenanthroimidazole10 (Phen-I), and donor–acceptor (D–A)
applications in organic electronics including organic light- systems11 have been reported as emitters of different colors.
emitting diodes (OLEDs) for lighting, display technologies, In the design and development of blue emitters, challenges
amplified spontaneous emissions, in organic solid-state lasers, arise from (i) broad emissions due to the extended conjugation
etc. are the driving force for extensive research on suitable and on the substitution of aromatic groups resulting in mixing of
efficient luminophores.4 Compared to green and red emitters, colors, (ii) poor oxidative stability of blue emitters causing keto
the development of blue emitters with high color purity and defects, and (iii) strong aggregation in thin films which leads to
band broadening and/or quenching of fluorescence.12,13 A mild
charge transfer character is advantageous for solid-state devices
a School of Chemical Sciences, UM-DAE, Centre for Excellence in Basic Sciences,
as it brings a slight bipolar nature which helps in the balanced
transport of charge carriers to the respective electrodes.14
University of Mumbai, Kalina campus, Santacruz (E), Mumbai 400098, India.
E-mail: na@cbs.ac.in
Imidazole and phenanthroimidazole have been known for
few decades and their derivatives have been reported for
various studies and applications.15 Phen-I based materials are
luminescent materials which show high fluorescence quantum
yield, thermal stability, and good thin film forming properties.
Owing to these properties, Phen-I attracts the attention of
researchers for applications in materials science. Phen-I has
been used in solid-state devices such as emitters, electron
b School of Physical Sciences, UM-DAE, Centre for Excellence in Basic Sciences,
University of Mumbai, Kalina Campus, Santacruz (E), Mumbai 400098, India
c Theoretical Chemistry Division, Bhabha Atomic Research Center, Trombay,
Mumbai, India
† This research work is part of PhD thesis of Ms Swati Dixit to be submitted to the
University of Mumbai.
‡ Electronic supplementary information (ESI) available: NMR, absorption, emis-
sion, and MALDI-TOF spectra, and cyclic voltammograms for 1–7. See DOI:
10.1039/d1nj02134f
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transporters, hole blockers, sensitizers in solar cells, sensors,
piezomaterials, etc.16 Materials consisting of phenanthrene
and imidazole moieties, e.g., BCP, 4,7-diphenyl-1,10-phenan-
throline (BPhen), and 2,20,200-(1,3,5-benzenetriyl)-tris-(1-phenyl-
1H-benzimidazole) (TPBi), are being used as electron-transport
facilitating materials in OLED devices.17 Designing of Phen-I
derivatives is based on (i) the availability of one sp2 hybridized
nitrogen which makes it moderately electron deficient thus acting
as the mild electron-withdrawing core and (ii) the other nitrogen
is in tetrahedral geometry, therefore, substitution on it is expected
to be out of the Phen-I core. Therefore, substitution at tetrahedral
N will have a limited effect of extended conjugation. Considering
these two important aspects, we synthesized Phen-I derivatives
aiming for strong blue emission and mild ICT properties. We
introduced aryl substituents of varying electron densities and
conjugation lengths at imidazole (C and N) and studied their
detailed photophysical and electroluminescence properties.
Results and discussion
Synthesis
Scheme 1 Synthesis of phenanthroimidazole derivatives (1–7)
A synthetic procedure is adopted for Phen-I derivatives using
single-step condensation reactions between 9,10-phenanthren-
equinone, aromatic amine and aromatic aldehyde in the
presence of glacial acetic acid and ammonium acetate.18 In
this reaction, an aryl-substituted imidazole ring is formed. The
aryl substituent at the nitrogen and carbon (sp2) of the imida-
zole ring is obtained from the aromatic amine and aromatic
aldehyde, respectively, used in this reaction. We believe that
aryl substitution at the nitrogen will not be in the plane of the
core phenanthroimidazole and will be somewhat orthogonal to
the plane (see molecular geometry obtained from DFT calcula-
tions). Thus, we expect a minimum effect of this aryl group on
p-conjugation. However, the aryl substituent on the carbon is
expected to be in a similar plane to that of the phenanthroi-
midazole core and hence should affect the electronic properties
considerably. Taking this into account, we have designed and
synthesized phenanthroimidazole derivatives 1–7 as shown in
Scheme 1. In 1–5, N1 substitution is kept as phenyl while
the substituent at the C2-carbon of the imidazole moiety is
varied as anisyl (1), pyridyl (2), benzothienyl (3), p-N,N0-
dimethylphenyl (4) and thienyl (5). In 5–7, thiophene is at the
C2-carbon position while phenyl (5), benzothiazole (6) and
thiazole (7) are placed at the N1 position of imidazole. These
compounds were characterized by 1H-NMR, 13C-NMR, and
mass spectrometry. Analysis of the 1H and 13C NMR spectra
suggests the proposed structures of the synthesized Phen-I
derivatives. The MALDI-TOF spectra were consistent with the
corresponding molecular weight of 1–7, and the associated
NMR and mass spectra are given in the ESI.‡
spectra of 1–7 recorded in dichloromethane are shown in
Fig. 1(a) and (b), respectively, and a few photophysical observa-
tions obtained for 1–7 are summarized in Table 1. The absorp-
tion spectra of 1–7 revealed that these compounds have more or
less similar absorption properties and show two or three
absorption peaks between 320 and 380 nm. These peaks are
assigned to the p–p* transitions of Phen-I and aromatic
groups.14 A slight shift towards the lower energy region in the
absorption spectra is attributed to the delocalized p–p* transi-
tion extending from the phenanthroimidazole core to the
substituted aryl groups. The extinction coefficients of 3, 6 and
7 are observed to be higher than those of other derivatives. It
has been reported earlier that an extinction coefficient related
to p–p* transitions increases with the increase of conjugation
length.19 Absorption peaks are somewhat broad which could be
due to the intramolecular charge transfer from the electron-
donating moiety to the electron-accepting core of Phen-I.
Different aryl groups (part of aryl aldehyde during the synth-
esis) at the C2-carbon of the imidazole moiety caused consider-
able alteration in the electronic properties which is evident
from the absorption spectra of 1–5. Interestingly, marginal
shifts in the absorption peaks were observed when the aryl
groups at N1 were varied (in the case of 5–7) (see the elaborated
figure in the ESI‡). Thus, we can infer that the electronic
properties altered as expected and proposed in our design of
Phen-I derivatives.
Emission. The emission spectra of 1–7 (Fig. 1(b)) show
maxima between B390 and 411 nm. Vibronic features were
observed in low polarity solvents which vanish in highly polar
solvents. A shoulder in the lower energy region was observed
Photophysical studies
Absorption. The photophysical properties of Phen-I deriva- at 425–440 nm in compounds 3–7. Compounds 3 and 4
tives 1–7 were studied in solution and thin films using absorp- showed the most red-shifted emission which is assigned to
tion and emission spectroscopies. Absorption and emission the extended p-conjugation in 3 and the effect of strong
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Fig. 1 (a) Absorption and (b) emission spectra of 1–7 in dichloromethane; (c) absorption and (d) emission spectra of 5 in different solvents.
Table 1 Photophysical properties of 1–7 in dichloromethane and thin films
In dichloromethane
In thin films
Comp.
labs in nm (log e)
lem (nm)
Stokes shift (cmꢁ1
)
fa (%)
t (ns)
labs (nm)
lem (nm)
1
2
3
4
5
6
7
315 (4.98), 363 (4.48)
377, 392
396
391, 411
396, 411
376, 397
376, 396
375, 396
2030
2370
2050
2051
1770
1771
1774
45
73
64
90
55
54
50
5.11
2.11
1.18
1.58
1.90
1.90
1.96
315, 367
339, 368
349, 368, 388
339, 372
344, 377
344. 378
348, 376
403
465
461
451
430
407
406, 436
331 (4.85), 345 (3.82), 362 (4.78)
343 (6.17), 361 (6.20), 379 (6.19)
359 (5.39), 379 (4.29)
338 (5.34), 352 (5.27), 371 (5.25)
338 (6.34), 370 (6.25)
337 (6.03), 370 (5.95)
a
W.r.t. 9,10-diphenylanthracene in cyclohexane (f = 90%).
electron-donating group in 4. Compounds 5–7 showed almost maxima do not change or change very slightly with a varying
identical emission spectra as observed in the absorption spec- solvent polarity which indicates the non-polar nature of the
tra too. This observation further emphasizes that N1 substitu- ground state. Broadening of spectra with loss of vibronic
tion causes a minimal effect on the electronic properties. To features in polar solvents was observed. The interaction of
compare the emission properties of 1–7, phenanthroimidazole the excited state with polar solvents may be the reason behind
was taken as standard which shows emission at 375 nm in the loss of vibronic features in high polarity solvents. A mar-
dichloromethane.14b A bathochromic shift of B20 nm for 1, 2, ginal, though considerable, red-shift is observed in high polar-
5–7 and B35 nm for 3, 4 was observed. This red-shift is much ity solvents which indicates the polar nature of the excited
smaller as compared to other literature-reported donor–accep- state. Compound 1 showed emission maxima at 386 nm in
tor systems of phenanthroimidazole core which suggests that, hexane while 391 nm in DMSO, a shift of 5 nm. Compound 2
in our system, donor and acceptor moieties are of mild showed peaks at 381 and 414 nm (a shift of 33 nm), 3 showed
nature.20
peaks at 403 and 414 nm (a shift of 11 nm), and 4 showed peaks
To understand the donor–acceptor nature of 1–7, we studied at 396 and 429 nm (a shift of 33 nm). Compounds 5–7 showed a
the absorption and emission properties in different solvents of negligible red-shift in emission in hexane and DMSO. Emission
varying polarity (Fig. 1c, d and ESI‡). It is found that absorption spectra arising from locally excited (LE) S0–S1 do not show
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Table 2 Quantum yield, f (%), fluorescence lifetimes, and t (ns) of 1–7 in different solvents
Compound
Solvent
1
2
3
4
5
6
7
f
t
f
t
f
t
f
t
f
t
f
t
f
t
Hexane
Toluene
DCM
Methanol
Acetonitrile
DMSO
33
41
38
45
40
53
4.21
4.99
5.11
6.05
5.76
7.59
71
73
72
83
84
89
2.43
2.03
2.11
1.07
2.23
2.37
55
68
64
68
72
73
1.00
1.06
1.18
1.22
1.26
1.30
98
86
90
94
87
93
1.63
1.37
1.58
1.59
2.08
2.02
48
58
53
53
53
57
1.60
1.62
1.90
1.91
2.52
2.29
51
65
54
66
56
68
1.59
1.87
1.90
2.82
3.58
3.95
48
62
50
61
56
63
1.83
1.96
1.83
2.79
3.51
—
dependency on solvent polarity whereas emission arising due
Emission studies in thin films. Fluorescent materials some-
to charge transfer (CT) states alters with the polarity of the times exhibit different properties in thin films than in solution.
solvent medium.21 The solvatochromic effect is considered to They show interesting properties such as energy transfer, triplet
be the measure of intramolecular charge transfer (ICT), where state population, exciplex formation, quenching of emission,
more shift indicates stronger ICT. Here, our studied com- etc. when doped in host materials consisting of small molecules
pounds 5–7 showed smaller bathochromic shifts than com- or polymers.23 Thus, it can be envisaged that emission can be
pounds 1–4 which resulted in mild ICT in 5–7. Nevertheless, the controlled by having suitable host materials. The emission
solvatochromic shifts in 1–4 are considerably smaller than intensity and color can be tuned to some extent by the varying
many other donor–acceptor systems known in the literature.
amount of interactions between the host and guest molecules.
The fluorescence quantum yield of these compounds was Many times small organic materials show red-shifted emission
calculated with respect to 9,10-diphenylanthracene (f = 90% in in thin films and these spectral shifts are believed to be due to
cyclohexane).22 Compounds 1–7 showed high to very high the aggregation behaviour. The emission spectra of 1–7 in spin-
quantum efficiency. Interestingly, these compounds showed coated thin films are shown in the ESI.‡ A slight red-shift was
high quantum yield (B50–98%) in polar as well as non-polar observed in these compounds which is attributed to the varying
solvents (Table 2). To understand the excited state properties, amounts of aggregation in thin films. We further studied the
the fluorescence decay profiles of compounds 1–7 were effect of host molecules on the emission properties of 1–7. The
recorded using the TCSPC method and their decay traces are interaction of 1–7 in the polymeric host matrix of polyvinyl
shown in the ESI.‡ The lifetime decay of 1–7 is found to be carbazole (PVK) was studied in thin films of their blends.
mono-exponential in solution. Compounds 2–7 showed fluores- Blends of 1 : 2 and 1 : 4 (w/w) were prepared and thin films
cence decay time of o2 ns in dichloromethane and, for 1, it is were spin-coated on quartz plates. Fig. 3 shows the emission of
B5 ns. The fluorescence lifetimes of 1–7 in different solvents neat 2, 3 and 5 and their blends with PVK. Neat PVK shows
are summarized in Table 2. It is also observed that lifetimes emission at B406 nm while 2 shows emission at 466 nm, 3 at
increase slightly as we increase the polarity of solvents 462 nm and 5 at 466 nm (Fig. 3) in thin films. The thin films of
(Table 2). The fluorescence decay profiles of 5 recorded in PVK and 2, 3 and 5 (blends in 1 : 2 and 1 : 4 ratios) show new
different solvents are shown in Fig. 2. From the photophysical emission peak maxima at 398 nm for 2, at 415 and 394 nm for
studies, we conclude that these chromophores have mild ICT 3, and at 403 and 381 nm for 5. These new peaks in blends do
and high fluorescence quantum yields.
not match with the thin film emission of the individual
material. Interestingly, new emission features in PVK blends
of 2, 3 and 5 match the solution spectra closely. The resem-
blance with solution spectra suggests the suppression of the
intermolecular interaction of molecules in thin films. With the
aggregation behaviour of the PVK blends of these Phen-I
derivatives, red-shifting of emission could be minimized.
Electrochemical studies
The redox behaviour of Phen-I derivatives 1–7 was studied
using cyclic voltammetry in an acetonitrile/DCM mixture. Tet-
rabutylammonium hexafluorophosphate (0.1M) was used as a
supporting electrolyte. Ferrocene was used as an internal
standard and the oxidation potential of Fc/Fc+ is considered
as a reference potential. Two irreversible oxidation waves were
observed for 1–7. The first oxidation potentials were found in
the range of 0.96–1.47 V for 1–7. The oxidation potential for 4
was the lowest among them which suggests that it could be
oxidized easily as compared to others. Compound 2 having the
Fig. 2 Fluorescence decay profiles of 1–7 in DCM.
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Fig. 3 Emission studies in thin films for 2, 3 and 5 and their PVK blends.
electron-accepting pyridine group was difficult to oxidize and initially optimized and the corresponding structures are shown
showed higher oxidation potential. The cyclic voltammograms in Fig. 4. It can be seen that the phenyl group is perpendicular
of 1–7 are shown in the ESI.‡ The highest occupied molecular to the phenanthroimidazole ring and the planar angle differ-
orbital (HOMO) and the lowest unoccupied molecular orbital ence is found to be 96 degrees. Similarly, the calculated
(LUMO) energy levels for 1–7 were estimated using the first HOMO–LUMO gap of molecules 3 and 5 is 3.56 and 3.71 eV,
oxidation potentials and optical band gaps.24 The HOMO and which is found to be in good agreement with the experimental
LUMO energy levels for 1–7 are summarized in Table 3. It is values of 3.21 and 3.30 eV, respectively, obtained from the
worth noting here that the EHOMO of these compounds varies optical band gap measurement (Table 3). Although the optical
with the aryl group. For example, the EHOMO of 2 (having band gap is found to be very close to the computed values, the
pyridine) is ꢁ6.09 eV and that of
4
(having N,N- HOMO and LUMO values are observed to be slightly
dimethylphenyl) is ꢁ5.57 eV. We could not observe the underestimated.
reduction peaks for these compounds under our experimental
conditions. The optical band gap of 1–7 was measured from the HOMO, LUMO and LUMO+1) are shown in Fig. 5, indicating
intersection of excitation and emission spectra. ELUMO was that the HOMO is exclusively contributed by the phenanthroi-
The frontier molecular orbital density plots (HOMOꢁ1,
calculated using EHOMO and optical band gaps. The EHOMO
optical band gaps and ELUMO are summarized in Table 3.
,
midazole core units whereas the LUMO is mostly contributed
by the thienyl group. Also, it can be seen from Fig. 5 that the
HOMOꢁ1 is mostly localized on the phenanthroimidazole as
well as thienyl groups from where the electronic transitions are
assumed to take place, i.e., HOMOꢁ1 to LUMO as well as
LUMO+1 instead of HOMO–LUMO.
The absorption spectra calculated by the TD-DFT method for
3 and 5 are shown in Fig. 6. The calculated oscillator strengths
and the maximum wavelengths (l) along with their orbital
assignments for compounds 3 and 5 are presented in Table 4.
For 3, at the equilibrium geometry, the spectrum shows one
major peak at 370.3 and other small peaks at 352.3 and
286.7 nm. Similarly, for 5, three dominant peaks are observed
at 360.9, 348.1 and 273.9 nm. By inspecting Table 4, it can be
seen that for both 3 and 5, the first two peaks are the result of
the electronic transitions from the HOMO to LUMO as well as
Theoretical studies
Quantum chemical calculations were performed on 3 and 5 to
understand the effect of the phenyl group on the Phen-I
derivatives and the origin of the photophysical properties.
The geometry optimization and time-dependent DFT (TDDFT)
based calculations for the evaluation of electronic excitation
spectra were performed using Becke’s three-parameter
exchange functional and the Lee–Yang–Parr correlation func-
tional (B3LYP exchange–correlation functional).25,26 The geo-
metry of 3 and 5, having the phenanthroimidazole unit with
one phenyl group situated at the N1 of the imidazole ring, is
Table 3 Electrochemical data of 1–7 in acetonitrile
HOMOb
(eV)
LUMOc
(eV)
Optical band
gapd (eV)
a
Eox
(V)
Compound
1
2
3
4
5
6
7
0.76, 1.11
0.98, 1.20
0.73, 0.96
0.47, 0.84
0.69
0.59, 0.95
0.59, 1.08
ꢁ5.86
ꢁ6.09
ꢁ5.83
ꢁ5.57
ꢁ5.79
ꢁ5.69
ꢁ5.69
ꢁ2.43
ꢁ2.75
ꢁ2.62
ꢁ2.37
ꢁ2.49
ꢁ2.38
ꢁ2.35
3.43
3.34
3.21
3.20
3.30
3.31
3.34
a
b
c
W.r.t. Fc/Fc+. EHOMO = ꢁ(E[ox vs. Fc/Fc ]
+
+ 5.1) eV. ELUMO = EHOMO +
d
optical band gap eV. Obtained from the intersection of excitation and
emission spectra in thin films.
Fig. 4 Optimized geometry of compounds 3 and 5.
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Table 4 Dominant optical transition characters for the lowest excitation
states of 3 and 5, calculated by the TDDFT-based B3LYP method
abs
Oscillator
strength
l
max
Compound
(nm)
Orbital transitions
3
0.950
0.112
0.201
0.242
0.361
370.3
352.3
286.7
360.9
348.1
HOMO - LUMO
HOMO - LUMO+1
HOMOꢁ2 - LUMO
HOMO - LUMO
HOMO - LUMO and
HOMO - LUMO+1
HOMOꢁ1 - LUMO and
HOMOꢁ1 - LUMO+1
5
0.269
273.9
less. This is illustrated in terms of isodensity molecular orbital
plots as presented in Fig. 5.
Organic light-emitting devices
A suitable amount of intramolecular charge transfer in fluor-
escent materials is considered beneficial for high efficiency
OLEDs. Materials with very small ICT are expected to show
bipolar charge transport imparting balanced holes and elec-
trons in the device. Compound 5 was used to study the
electroluminescence properties and OLEDs were fabricated
with the following device geometry: ITO/PEDOT:PSS (50 nm)/
NPD (50 nm)/5 (70 nm)/Bphen (25 nm)/LiF (2–5 nm)–Al
(120 nm). N,N0-Di(1-naphthyl)-N,N0-diphenyl-(1,10-biphenyl)-
4,40-diamine (NPD) was used as the hole transporting layer
near the anode and 4,7-diphenyl-1,10-phenanthroline (Bphen)
was used as the hole blocking layer (HBL) near the cathode. A
few prominent results obtained for OLEDs are presented in
Fig. 7. The electroluminescence (EL) spectra of OLEDs made of
5 showed a emission peak at B460 nm (see Fig. 7a) which
shows a close resemblance with the photoluminescence of 5 in
thin films. A weaker peak at B600 nm is also observed in the EL
spectra which may be due to the electromer formation during
the electrical excitation. We are currently investigating this in
detail. The electroluminescence spectra of OLEDs at different
voltages are being recorded now. The EL spectra at different
voltages are found to be identical (see the ESI‡). The turn-on
voltage, VON, was found to be 7 V (Fig. 7b). The devices showed a
high luminance of B2.4 ꢀ 104 cd mꢁ2 at a current density of
Fig. 5 The frontier orbitals (HOMOꢁ1, HOMO, LUMO and LUMO+1) of 3
and 5.
from the HOMO to LUMO+1. However, the third peak origi-
nates from the excitation of HOMOꢁ2/HOMOꢁ1 to LUMO/
LUMO+1. A general analysis of these frontier orbitals indicates
that the electron density is transferred predominantly from the
phenanthroimidazole-based aromatic groups to the thienyl or
benzothienyl rings. Interestingly, it can also be observed that
the low intensity peak located at 352.3 nm in 3 and at 273.9 nm
in 5 is mostly associated with the HOMO to LUMO+1 transition,
which shows the dominant charge transfer from the
phenanthroimidazole-based aromatic groups to the aryl
groups. It may be noted that such transition probabilities are
35 mA cmꢁ2. The current efficiency was also high (B60 cd Aꢁ1
)
and the external quantum efficiency was found to be B3.2%
(Fig. 7c).
Conclusions
Phen-I derivatives showing mild bipolar characters and strong
emissions in the blue region were synthesized and character-
ized. The aryl groups at the C2-carbon of the imidazole moiety
caused considerable alteration while the aryl groups at N1
showed marginal shifts in the electronic properties as evident
from the absorption and emission spectra. The photophysical
properties in different solvents show evidence of mild
intramolecular charge transfer. Also, a very high fluorescence
quantum yield (490%) is found for some of these compounds.
Fig. 6 Simulated absorption spectra of 3 (above) and 5 (below).
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Fig. 7 Characteristics of OLEDs (ITO/PEDOT:PSS (50 nm)/NPD (50 nm)/5 (70 nm)/Bphen (25 nm)/LiF (2–5 nm)–Al (120 nm)) fabricated with compound 5.
The Phen-I derivatives were blended with PVK to study their supporting electrolyte at a scan rate of 100 mV sꢁ1. Ferrocene
interactions in the excited state. It was observed that the was used as an internal standard reference. UV-vis spectra were
intermolecular interactions are suppressed in thin films in obtained using a Shimadzu 1800 and the steady-state fluores-
the presence of PVK and the emissions were found to be similar cence spectra were recorded using a Horiba Fluoromax-4. A
to those in solution. Furthermore, compound 5 is used in OLED diode laser or a nano-LED-based time-correlated single-photon
applications. For OLEDs, the luminance is B2.5 ꢀ 104 cd mꢁ2 counting (TCSPC) spectrometer (Deltaflex, HORIBA) was used
at a current density of 35 mA cmꢁ2, with an external quantum to obtain the time-resolved fluorescence measurements. The
efficiency of B3.2%. Our results convincingly show that these fluorescence transients were collected at the magic angle (54.71)
new Phen-I derivatives are highly emissive in the blue region configuration. To obtain the instrument response function
and are promising materials for efficient OLED applications.
(IRF), the scattered excitation light from the suspended SiO2
particles in water was monitored. All the theoretical calcula-
tions were performed using the ORCA electronic structure-
based program. The RHF types of Kohn–Sham based density
functional theory (DFT) methods have been employed. The
geometry optimization and time-dependent DFT (TDDFT)
Experimental
Methods
Chemicals were purchased from SD Fine Chemicals, Sigma based calculations for the evaluation of electronic excitation
Aldrich, etc. and were used as received. Organic solvents of spectra have been performed using Becke’s three-parameter
analytical and spectroscopic grades were used. 1H and 13C NMR exchange functional and the Lee–Yang–Parr correlation func-
were recorded using a Varian 600 or 800 MHz spectrometer. tional (B3LYP exchange–correlation functional). The grid-based
Residual chloroform in CDCl3 was used as an internal refer- DFT has been used which employs a typical grid quadrature for
ence. Mass spectra were recorded using a Bruker MALDI-TOF. the computation of the integrals. Typically, the grid consists of
Cyclic voltammetry was carried out on a CH Instruments (620D 96 radial shells with 36 and 72 angular points during the SCF
model) electrochemical workstation. For a CV three-electrode procedure. OLED devices were fabricated using thermal
cell with glassy carbon as the working electrode, Ag/AgCl (non- vacuum deposition which was performed at a base vacuum of
aqueous) as the reference electrode and Pt wire as the counter 2 ꢀ 10ꢁ6 mbar. An ITO-coated substrate (15–25 O sqꢁ1, Sigma
electrode were used. Electrochemical studies were performed in Aldrich) of dimensions 22 ꢀ 12 cm was initially etched to
a mixture of dichloromethane and acetonitrile with tetrabuty- incorporate four independent active devices with the help of
lammonium hexafluorophosphate solution (0.1M) as
a
Zn powder and 10% HCl. Substrate cleaning was performed in
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three simple steps. It was first cleaned with soap solution and HRMS: mass calcd for C26H17N3: 372.1495, found m/z: 372.1495
rinsed with distilled water. Furthermore, it was sonicated with (M)+.
distilled water and propanol. The sonication time was 10 min
3.
A
mixture of 9,10-phenanthrenequione (0.20 g,
for each step. This was followed by cleaning with trichloroethy- 0.96 mmol), aniline (90 mL, 0.96 mmol), benzo[b]thiophene-2-
lene (TCE) vapors and drying using nitrogen. The cleaned carbaldehyde (0.16 g, 0.96 mmol) and ammonium acetate
substrates were kept under a UV lamp for 30 minutes. The (0.11 g, 0.96 mmol) in acetic acid was refluxed for 24 h. A buff
layer of PEDOT:PSS was spin-coated onto the UV-treated sub- white solid compound was obtained. Yield: (0.306 g, 72%).
strates. Spin coating was performed at a speed of 6000 rpm for M.p.: 4 250 1C; 1H-NMR (600 MHz, CDCl3, d ppm): 8.89 (d, 1 H,
45 seconds which gave a 50-nm-thick layer. The substrates were J = 8.4 Hz), 8.76 (d, 1 H, J = 8.4 Hz), 8.69 (d, 1 H, J = 8.4 Hz), 7.81–
subsequently annealed at 160 1C for 30 minutes to give a hard, 7.73 (m, 5 H), 7.67–7.65 (m, 3 H), 7.59 (d, 1 H, J = 7.8 Hz), 7.52–
uniform layer of PEDOT:PSS. All the subsequent layers of NPD, 7.50 (t, 1 H, J = 7.2 Hz), 7.32–7.29 (m, 3 H), 7.16 (d, 1 H,
compound 5, BPhen, LiF and Al were thermally evaporated in J = 8.4 Hz), 6.96 (s, 1 H); 13C NMR (200 MHz, CDCl3): 145.65,
vacuum. The first three layers were grown without breaking the 140.28, 139.79, 138.43, 137.84, 133.27, 130.78, 130.76, 129.52, 129.39,
vacuum to ensure clean interfaces with the active layer of 128.79, 128.46, 127.39, 127.05, 126.49, 125.24, 124.53, 124.22, 124.53,
compound 5.
124.22, 123.49, 123.19, 123.05, 122.86, 122.03, 120.80; HRMS: mass
calcd for C29H18N2S: 427.1623, found: 427.1268 (M)+.
General method for the synthesis of 1–7
4.
A
mixture of 9,10-phenanthrenequione (0.20 g,
The compounds (1–7) were synthesized by refluxing the mixture 0.96 mmol), aniline (90 mL, 0.96 mmol), 4-(dimethylamino)ben-
of 9,10-phenanthrenequione (1 mmol), primary aromatic zaldehyde (0.14 g, 0.96 mmol) and ammonium acetate (0.11 g,
amine (1 mmol), ammonium acetate (1 mmol) and aromatic 0.96 mmol) in acetic acid was refluxed for 24 h. A pure yellowish
aldehyde (1 mmol) in glacial acetic acid for 24 h. The comple- white solid compound was obtained. Yield: (0.289 g, 70%).
tion of the reaction was monitored by the TLC method. After M.p.: 4250 1C; 1H-NMR (600 MHz, CDCl3, d ppm): 8.76–8.73 (t,
completion, the reaction mixture was cooled to room tempera- 3 H, J = 9.00, 9.60 Hz), 8.63 (d, 1 H, J = 7.8 Hz), 8.33 (d, 1 H, J =
ture and water was added to precipitate the organic solids. The 7.8 Hz), 8.24 (d, 3 H, J = 9 Hz), 7.75–7.64 (m, 6 H), 6.83 (d, 3 H,
crude solid products were purified by silica gel column chro- J = 8.4 Hz), 3.09 (s, 6 H); 13C NMR (200 MHz, CDCl3, d ppm):
matography to obtain the pure 1–7 in 70–80% yield.
163.61, 152.17, 144.30, 135.84, 128.91, 128.76, 127.34, 127.25,
126.41, 125.91, 125.89, 123.83, 123.52, 123.10, 121.40, 120.77,
115.10, 111.82, 40.31; MALDI-TOF: mass calcd for C29H23N2S:
Characterization data
1. 9,10-Phenanthrenequione (0.2 g, 0.96 mmol), aniline 413.19, found: 413.51 (M)+.
(90 mL, 1.0 mmol), ammonium acetate (1.0 mmol) and anisal-
5. 9,10-Phenanthrenequione (0.20 g, 0.96 mmol), aniline
dehyde (1.0 mmol) in glacial acetic acid were used for synthesis (90 mL, 0.96 mmol), ammonium acetate (0.11 g, 0.96 mmol)
as described in the general method. A pure white solid com- and thiophene-2-carbaldehyde (0.11 g, 0.96 mmol) in glacial
pound was obtained. Yield: (0.321 g, 80%). M.p.: 4250 1C; acetic acid were used for synthesis as described in the general
1H-NMR (600 MHz, CDCl3, d ppm): 8.87 (d, 1 H, J = 7.8 Hz), 8.77 method. A pure white solid compound was obtained. Yield:
(d, 1 H, J = 8.4 Hz), 8.71 (d, 1 H, J = 8.4 Hz), 7.75–7.72 (t, 1 H, J = (0.319 g, 85%). M.p.: 4 250 1C; 1H-NMR (600 MHz, CDCl3,
7.8 Hz), 7.65–7.58 (m, 4 H), 7.51–7.48 (m, 5 H), 7.26–7.24 (m, d ppm): 8.86 (d, 1 H, J = 8.4 Hz), 8.79 (d, 1 H, J = 8.4 Hz), 8.69 (d,
1 H), 7.17 (d, 1 H, J = 8.4 Hz), 6.82 (d, 2 H, J = 9 Hz), 3.79 (s, 3 H); 1 H, J = 8.4 Hz), 7.76–7.61 (m, 4 H), 7.66–7.63 (t, 1 H, J = 7.8 Hz),
13C NMR (200 MHz, CDCl3): 160.10, 151.07, 139.00, 137.39, 7.62 (d, 2 H, J = 7.8 Hz), 7.50–7.48 (t, 1 H, J = 7.8 Hz), 7.30 (d, 1
130.89, 130.21, 129.78, 129.26, 129.19, 128.29, 128.03, 127.31, H, J = 5.4 Hz), 7.24–7.23 (d, 1 H, J = 7.8 Hz), 7.14 (d, 1 H,
126.31, 125.58, 124.78, 124.17, 123.19, 123.17, 123.10, 122.83, J = 8.4 Hz), 6.91–6.90 (t, 1 H, J = 4.2 Hz), 6.86 (d, 1 H, J = 3.6 Hz);
120.84, 113.76, 55.31; MALDI-TOF: mass calcd for C18H20N2O: 13C NMR (200 MHz, CDCl3): 145.99, 138.42, 137.54, 133.12,
400.16, found: 400.37 (M)+.
130.59, 130.55, 129.35, 129.25, 128.32, 128.19, 127.51, 127.43,
2. 9,10-Phenanthrenequione (0.2 g, 0.96 mmol), aniline 127.27, 127.24, 127.02, 126.38, 125.71, 124.95, 124.13, 123.11,
(90 mL, 0.96 mmol), ammonium acetate (0.11 g, 0.96 mmol) 122.93, 122.87, 120.60.; HRMS: mass calcd for C25H16N2S:
and 4-pyridinealdehyde (0.10 g, 0.96 mmol) in glacial acetic 377.1107, found: 377.1106 (M)+.
acid were used for synthesis as described in the general
6. 9,10-Phenanthrenequione (0.20 g, 0.96 mmol), benzo[b]
method. A pure buff white solid compound was obtained. Yield: thiazole-2-amine (145 g, 0.96 mmol), ammonium acetate
(0.274 g, 75%). M.p.: 4250 1C; 1H-NMR (600 MHz, CDCl3, (0.11 g, 0.96 mmol) and thiophene-2-carbaldehyde (0.11 g,
d ppm): 8.86 (d, 1 H, J = 8.4 Hz), 8.77 (d, 1 H, J = 8.4 Hz), 8.71 0.96 mmol) in glacial acetic acid were used for synthesis as
(d, 1 H, J = 8.4 Hz), 8.53 (d, 2 H, J = 6 Hz), 7.77–7.74 (t, 1 H, J = described in the general method. A pure buff white solid
7.2 Hz), 7.73–7.70 (t, 1 H, J = 7.2, 7.2 Hz), 7.69–7.66 (m, 3 H), compound was obtained. Yield: (0.35 g, 83%). M.p.: 4250 1C;
7.55–7.52 (t, 3 H, J = 7.2, 7.2 Hz), 7.48 (d, 2 H, J = 6 Hz), 7.28 (d, 1H-NMR (600 MHz, CDCl3, d ppm): 8.73 (d, 3 H, J = 8.4 Hz),
1 H, J = 7.8 Hz), 7.17 (d, 1 H, J = 7.8 Hz); 13C NMR (200 MHz, 7.69–7.66 (m, 4H), 7.64–7.61 (t, 4 H, J = 7.8 Hz), 7.43 (d, 2 H, J =
CDCl3):) 174.73, 149.31, 138.28, 138.22, 137.72, 130.61, 130.49, 4.8 Hz), 7.16–7.15 (t, 2 H, J = 4.2 Hz).
129.75, 129.00, 128.82, 128.50, 127.54, 126.97, 126.50, 126.11,
7. 9,10-Phenanthrenequione (0.20 g, 0.96 mmol), thiazole-2-
125.57, 124.22, 123.21, 122.97, 122.73, 121.00; m.p.: 4250 1C; amine (0.1 g, 0.96 mmol), ammonium acetate (0.11 g,
New J. Chem.
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Paper
( f ) Y. Jiang, Y.-Y. Liu, X. Liu, H. Lin, K. Gao, W.-Y. Lai
and W. Huang, Chem. Soc. Rev., 2020, 49, 5885; (g) Y. Jiang,
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W. Huang, Angew. Chem., Int. Ed., 2021, 60, 10007.
0.96 mmol) and thiophene-2-carbaldehyde (0.11 g, 0.96 mmol)
in glacial acetic acid were used for synthesis as described in the
general method. A pure buff white solid compound was
obtained. Yield: (0.303 g, 83%). 1H-NMR (600 MHz, CDCl3, d
ppm): 8.73 (d, 3 H, J = 8.4 Hz), 7.69–7.61 (m, 8 H), 7.43 (d, 1 H,
J = 4.8 Hz), 7.15–7.14 (t, 1 H, J = 4.2 Hz); m.p.: 4 250 1C; MALDI-
TOF: mass calcd for C22H13N3S: 383.50, found: 383.50 (M)+.
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Conflicts of interest
There are no conflicts to declare.
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K. Mu¨llen, J. Mater. Chem. C, 2014, 2, 9083–9086;
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Acknowledgements
SD carried out the synthesis and photophysical studies and CG
fabricated the OLEDs. SD thanks DST SERB for fellowship. We
thank Mrs Mamata Joshi and Mrs Gitanjali Dhotre of Tata
Institute of Fundamental Research, Mumbai, for NMR and
MALDI-TOF, respectively. We are thankful to Indian Institute
of Technology, Bombay, for providing the HRMS facility. NA
and SB thank the Department of Science and Technology for
partial financial support (EMR/2017/000805).
Notes and references
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