Synthesis and optoelectronic investigation of triarylamines based on imidazoanthraquinone as donor–acceptors for n-type materials

Article abstract of DOI:10.1007/s12039-018-1443-2

Abstract: A series of five new donor–acceptor molecules based on imidazo-anthraquinone-amines have been synthesized and characterized. The structure-property relationship in these molecules is systematically examined by absorption-emission spectroscopy, c

Full text of DOI:10.1007/s12039-018-1443-2

J. Chem. Sci. (2018) 130:49
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1443-2
REGULAR ARTICLE
Synthesis and optoelectronic investigation of triarylamines based
on imidazoanthraquinone as donor–acceptors for n-type materials
BHARAT K SHARMAa, AZAM M SHAIKHa, SAJEEV CHACKOb and RAJESH M KAMBLEa,∗
aDepartment of Chemistry, University of Mumbai, Mumbai, Maharashtra 400 098, India
bDepartment of Physics, University of Mumbai, Mumbai, Maharashtra 400 098, India
E-mail: kamblerm@chem.mu.ac.in
MS received 3 December 2017; revised 21 February 2018; accepted 1 March 2018
Abstract. A series of five new donor–acceptor molecules based on imidazo-anthraquinone-amines have been
synthesized and characterized. The structure-property relationship in these molecules is systematically examined
by absorption-emission spectroscopy, cyclic voltammetry and theoretical studies. Optical properties of these
molecules have been studied in solvents of varying polarity as well as in neat solid film and found to be affected
by the nature of triarylamine substituent with broad absorption windows, strong charge transfer transitions
(425–502 nm), high molar extinction coefficients and emission in green light (500–568 nm). Electrochemical
data indicated that the dyes possess relatively low-lying LUMO values (−3.18 to −3.42 eV) while TGA studies
revealed good thermal stability. The donor-acceptor architecture and HOMO–LUMO energies were further
rationalized using DFT calculations. Experimental studies along with theoretical calculations suggest that these
compounds have potential to be used as n-type materials in optoelectronic devices.
Keywords. Imidazoanthraquinone–triarylamines; intramolecular charge transfer; donor–acceptor
architecture; HOMO–LUMO energy levels; n-type materials.
1. Introduction
fluoranthene, pyrene, etc., around the trigonal amine
nitrogen often hinders the aggregation of molecules
in the solid state and found to enhance thermal sta-
Triarylamine-containing heteroaromatics are widely
investigated and applied as electro-optical materials
in applications such as organic light emitting diodes
(OLEDs),1 organic field effect transistors (OFETs),2
organic photovoltaic cells (OPVs),3 nonlinear optical
devices4 and in chemo-sensing applications.5 The tri-
arylamine unit is a well-known substituent used for
the synthesis of diverse organic molecules for appli-
cations in organic electronics because of its ability to
alter their optoelectronic properties due to its ease of
oxidation of nitrogen centre and its ability to transport
charge carriers via radical cation species with high sta-
bility.6 The introduction of electron-rich triarylamine
unit into the electron-deficient core molecule creates
an electron donor-acceptor (D–A) type of molecular
architecture, which tunes the absorption and emis-
sion properties of the molecule through intramolecular
charge transfer transitions (ICT) and extends the con-
jugation in the molecular structure. Further, the pres-
ence of bulkier polyaromatic groups such as carbazole,
bility.7 -conjugated small organic molecules with
π
D–A architecture is currently of interest in optoelec-
tronic devices because of their well-defined structures,
ease of purification, reliable reproducibility and bet-
ter solubility in most organic solvents.8 A variety
of molecules based on D–A approach derived from
electron-rich triarylamine unit and electron-deficient
core such as borane-amine,9 quinoxaline-fluorine,10
triazole-amine,11 bathophenanthroline-amine,12 acri-
done-amine,13 tribenzophenazine-amine14 and oxadia-
zole-carbozole15 have been synthesized and used for
applications in organic electronics.
Imidazoanthraquinone derivatives attracted a lot of
research attention because of their successful bio-
logical application as anticancer16 drugs and as ion
sensors.17In our recent publication, we have explored
imidazoanthraquinone aryl derivatives for application in
organic electronics.18 However, imidazoanthraquinone-
triarylamines are not yet synthesized and explored for
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1443-2) contains
supplementary material, which is available to authorized users.

49 Page 2 of 13
J. Chem. Sci. (2018) 130:49
Figure 1. Molecular structure of imidazoanthraquinone-amine derivatives.
their ability to accept electron in connection with n-type further purification. The organic solvents were of analytical
or spectroscopic grade and were dried and freshly distilled
organic semiconductor application. Keeping the above
mentioned benefits of triarylamines and the D–A archi-
tecture therein in mind, we report the synthesis of new
D–A molecules based on 1H–anthra[1,2–d]imidazole–
6,11–dione substituted with triarylamines, 1–5 (Fig-
ure 1) and their detailed photophysical, electrochemical
and thermal studies.
The D–A architecture and HOMO-LUMO energies
in context with n–type materials were further ratio-
nalized using DFT calculations. Imidazoanthraquinone
acts as typical electron-accepting unit while the donor
using the standard procedures whenever anhydrous solvents
1
were required. H–NMR (500 MHz) spectra were recorded
on a Varian 600 MHz Ultrashield spectrometer with tetram-
ethylsilane (TMS) as internal reference and residual CHCl3
in CDCl3 as reference and 13C–NMR (75 MHz) spectra
were recorded on a Bruker Avance II 300 MHz Ultrashield
spectrometer with tetramethylsilane (TMS) as internal ref-
erence and residual CHCl3 in CDCl3 and DMSO–d6 as
reference. Fourier transform infrared (FT-IR) spectra were
recorded on a Perkin Elmer Frontier 91579. Mass spec-
trometric measurements were recorded using MALDI-TOF
unit comprises of various triarylamine units. Incorpo- (Bruker). Melting points of the products were determined by
differential scanning calorimetry. Elemental analyses were
obtained on Elemental Analyser-EURO Vector Italy EA-
3000. Cyclic voltammetry and differential pulse voltammetry
were carried out on a computer controlled Palmsens3 poten-
tiostat/galvanostat. Typically, a three-electrode cell equipped
with a glassy carbon as working electrode, Ag/AgCl (non-
aqueous) as reference electrode and platinum (Pt) wire as
counter electrode were employed. The measurements were
carried at room temperature in anhydrous acetonitrile with
tetrabutylammonium hexafluorophosphate solution (0.1 M)
as supporting electrolyte with a scan rate of 100 mVs−1. The
potential of Ag/AgCl reference electrode was calibrated by
using ferrocene/ferrocenium redox couple. Absorption and
fluorescence data were acquired using ∼ 2 × 10−6 M solu-
tions of Std and 1–5. UV-visible spectra were recorded on
SHIMADZU U.V-A114548 and fluorescence studies were
done on Horiba Fluorolog 3 at room temperature. The flu-
orescence quantum yields (ꢀF) were calculated relative to
ration of triarylamines into the imidazoanthraquinone
backbone is expected to affect the optoelectronic prop-
erties effectively. Morpholine amine was used to see the
effect of the rigid cyclic aliphatic amine on the opto-
electrochemical properties of Imidazoanthraquinone.
Further, the studies also explore the effect of elec-
tron donating (−OCH3) and withdrawing (−NO2)
substituents on the triarylamine moiety on the photo-
physical, electrochemical and thermal properties such
as ICT, HOMO–LUMO energy, and band gap.
2. Experimental
2.1 Materials and methods
All the reagents were purchased from commercial sources
(Sigma Aldrich and Alfa Aesar) and were used without any

J. Chem. Sci. (2018) 130:49
Page 3 of 13 49
fluorescein (ꢀF = 0.79 in 0.1 M NaOH). The neat solid ArH, J = 7.8 Hz), 7.34 (d,4H, ArH, J = 2.4 Hz), 7.73 (d,
films of compounds 2–6 were prepared by using a spin 2H, ArH, J = 8.4 Hz), 7.83 (m, 2H, ArH), 8.05 (2H, ArH,
coater (Holmarc HO-TH-05) at 1000 rpm for 2 min using- d, J = 7.8 Hz), 8.14 (1H, ArH, d, J = 8.4 Hz), 8.26 (d,
∼ −6 mg mL−1 of sample in chloroform. The area of the ArH,1H, J = 8.4 Hz), 8.30 (t,1H,ArH, J = 9.0 Hz), 8.36
neat solid film is 2 × 2 cm2 and approximately around 100- (d, 1H,ArH, J = 9.0 Hz), 11.32 (s, 1H, -NH); 13C–NMR
200 μL of the solution was used for coating. The AFM (75 MHz, DMSO–d6, 25 ◦C) δ, ppm: 111.30, 114.50, 121.13,
topographical images were recorded on a Bruker Dimen- 122.68, 124.70, 125.28, 126.20, 126.39, 126.75, 127.83,
sion Icon AFM instrument. Thermal studies were performed 128.06, 129.32, 131.02, 131.08, 131.76, 131.96, 132.34,
on Staree system Metler Toledo TGA 1 with a heating rate 133.34, 134.45, 141.47, 151.01, 182.38, 185.49; MALDI-
of 10 ◦C min−1 under nitrogen atmosphere. Density func- TOF: mass calcd. for C33H21N3O2 [M+]: 491.54; found:
tional theoretical (DFT) calculations were performed using 491.54; anal. Calcd. for C33H21N3O2: C, 80.64; H, 4.31; N,
Gaussian 03 software package using B3LYP as exchange- 8.55% Found: C, 80.45; H, 4.22; N, 8.36%.
correlation functional at 6-311 G basis set.
2.2c 2-(4-(naphthalen-1-yl(phenyl)amino)phenyl)-
1H-anthra[1,2-d]imidazole-6,11-dione (2): A mixture
of 6 (0.40 g, 1 mmol) and N–phenylnapathylamine (0.26 g,
1.2 mmol) were reacted in toluene as mentioned in the gen-
eral method. The crude solid thus obtained was purified by
column chromatography using hexane/chloroform to obtain
a bright yellow solid. Yield: 0.29 g (54%); M.p.: 244 ◦C; FT-
IR (KBr, vmax/cm−1): 3599.72, 3391.24, 1659.07, 1589.87,
1474.29, 1295.20, 1008.33, 798.92, 715.04, 597.72; 1H–
NMR (600 MHz, CDCl3, 25 ◦C) δ ppm: 7.04 (d, 1H, ArH,
J = 9.0 Hz), 7.09 (t, 1H, ArH, J = 7.8 Hz), 7.24 (d, 1H,
ArH, J = 7.8 Hz), 7.32 (t,1H, ArH, J = 7.8 Hz), 7.41 (t,
2H, ArH, J = 7.2 Hz), 7.52 (m, ArH, 2H), 7.76 (d, 2H, ArH,
J = 8.4 Hz), 7.83 (m, 4H, ArH), 7.94 (m, ArH, 1H), 8.07 (d,
2H, ArH, J = 8.4 Hz), 8.16 (d, 1H, ArH, J = 8.4 Hz), 8.23
(d,1H, ArH, J = 8.4 Hz), 8.28 (d,1H, ArH, J = 8.4 Hz),
8.30 (d,1H,ArH, J = 9.0 Hz), 8.38 (d,1H,ArH, J = 9.0 Hz),
11.34 (1H, s, -NH); 13C–NMR (75 MHz, CDCl3, 25 ◦C) δ,
ppm: 115.17, 120.00, 119.38, 122.24, 132.03, 123.83, 124.13,
125.70, 126.45, 127.06, 127.51, 127.62, 128.39, 128.50,
128.64, 129.23, 129.56, 129.78, 130.98, 131.30, 132.61,
132.78, 133.27, 133.44, 133.72, 134.12, 134.42, 135.21,
135.36, 141.32, 142.24, 181.35, 185.28; MALDI-TOF: mass
calcd. for C37H23N3O2 [M+]: 541.60; found: 542.45; anal.
Calcd. for C37H23N3O2: C, 82.05; H, 4.28; N, 7.76% Found:
C, 81.89; H, 4.32; N, 7.64%.
2.2 Synthesis
The Std molecule (2-phenyl-anthra[1,2-d]imidazole- 6,11-
dione)and2-(4-bromo)phenyl-anthra[1,2-d]imidazole-6,11-
dione (6) were synthesized according to the reported proce-
dure and confirmed by their reported melting point.16b,17d
A mixture of 1,2-diaminoanthraquinone (3 g, 12.6 mmol)
and 4-bromobenzaldehyde (2.78 g, 15 mmol) was heated in
nitrobenzene (20 mL) at 140 ◦C for 18 h. Then the mixture
was cooled to room temperature and hexane was added to
obtain a solid precipitate which was washed several times
with hexane. The crude solid thus obtained was purified by
column chromatography using hexane/chloroform to obtain a
bright yellow solid. Yield: 4.53 g (90%); M.p. 303 ◦C.16b, 17d
2.2a General method for the synthesis of compounds
1-5: In a three-necked round bottom flask equipped with
a reflux condenser and argon inlet and outlet ports, 2-
(4-bromo)phenyl-anthra[1,2-d]imidazole-6,11-dione (6) (1.0
mmol) and diarylamine (1.2 mmol) were dissolved in anhy-
drous toluene (20 mL) under argon atmosphere. The palla-
dium catalyst [Pd2(dba)3] (5–8 mol%), 2-dicyclohexylpho-
shpino-2’,6’-dimethyl biphenyl (SPhos) (10–15 mol%) and
sodium-t-butoxide (3.1 mol) were added to the reaction
mixture. The reaction mixture was thoroughly stirred under
argon atmosphere while the temperature was slowly raised to
100 ◦C. Reaction mixture was stirred at this temperature for
24 h. Reaction mixture was cooled to room temperature and
extracted with chloroform (3 × 50 mL) followed by water
wash (3 × 50 mL). All the organic layers were combined
and dried over anhydrous Na2SO4 and evaporated to get the
crude product which was further purified by silica gel column
chromatography.
2.2d 2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1H-
anthra[1,2-d]imidazole-6,11-dione (3): A mixture of 6
(0.40 g, 1 mmol) and bis(4-methoxyphenyl)amine (0.27 g,
1.2 mmol) were reacted in toluene as mentioned in the gen-
eral method. The crude solid thus obtained was purified by
column chromatography using hexane/chloroform to obtain a
dark red solid. Yield: 0.34 g (62%); M.p.: 197 ◦C; FT-IR (KBr,
vmax/cm−1): 2849.41, 1661.79, 1582.76, 1470.89, 1280.89,
1031.76, 826.67, 715.34, 575.59; 1H–NMR (600 MHz,
2.2b 2-(4-(diphenylamino)phenyl)-1H-anthra[1,2-d]
imidazole-6,11-dione(1): Amixtureof6(0.40g, 1mmol) CDCl3, 25 ◦C) δ ppm: 3.83 (s, 6H, −OCH3), 6.90 (d, 4H,
and diphenylamine (0.20 g, 1.2 mmol) were reacted in ArH, J = 9.0 Hz), 6.99 (d, 2H, ArH, J = 8.4 Hz), 7.14 (d,
toluene as mentioned in the general method. The crude solid 4H, ArH, J = 8.4 Hz), 7.80 (m, ArH, 2H), 7.93 (d, 2H, ArH,
thus obtained was purified by column chromatography using
hexane/chloroform to obtain a bright yellow solid. Yield:
0.27 g (56%); M.p. : 295 ◦C; FT-IR (KBr, vmax/cm−1):
J = 8.4Hz), 8.05(d, 1H, ArH, J = 8.4Hz), 8.21(d,1H, ArH,
J = 8.4 Hz), 8.27 (t, 1H, ArH, J = 8.4 Hz), 8.34 (d,1H, ArH,
J = 8.4Hz), 11.14(s,1H, -NH); 13C–NMR(75MHz, CDCl3,
3390.18, 1656.45, 1589.56, 1472.69, 1289.71, 1006.84, 25 ◦C) δ, ppm: 55.51, 115.02, 117.51, 118.93, 118.66, 122.06,
834.99, 715.43, 593.32, 510.83; 1H–NMR (600 MHz, CDCl3, 124.70, 126.44, 127.55, 127.67, 127.91, 128.15, 128.49,
25 ◦C) δ, ppm: 7.15 (t,2H, ArH, J = 5.4 Hz), 7.19 (d, 4H, 133.28, 133.49, 133.63, 134.13, 134.32, 139.43, 151.71,

49 Page 4 of 13
J. Chem. Sci. (2018) 130:49
156.95, 157.09, 157.09, 182.57, 185.27; MALDI-TOF: mass 817.21, 717.58, 575.75; 1H–NMR (600 MHz, CDCl3, 25 ◦C)
calcd. for C35H25N3O4 [M+]: 551.59; found: 551.26; anal. δ ppm: 3.33 (t, 4H, ArH, J = 4.8 Hz, -NCH2), 3.89 (t,4H,
Calcd. for C35H25N3O4: C, 79.21; H, 4.57; N, 7.62% Found: ArH, J = 5.4 Hz, −OCH2), 7.03 (d, 2H,ArH, J = 9.0 Hz),
C, 78.89; H, 4.51; N, 7.91%.
7.80 (m,2H, ArH,), 8.06 (m, 3H, ArH,), 8.21 (d, 1H, ArH,
J = 8.4 Hz), 8.25 (d, 1H, ArH, J = 9.0 Hz), 8.34 (d, 1H,
ArH, J = 9.0 Hz), 11.18 (s,1H, -NH); 13C–NMR (75 MHz,
CDCl3, 25 ◦C) δ ppm: 47.78, 66.60, 114.56, 117.55, 118.51,
122.04, 124.74, 126.42, 127.54, 128.00, 128.45, 133.26,
133.37, 133.64, 134.09, 134.32, 149.59, 153.19, 156.91,
182.52, 185.21; MALDI-TOF: mass calcd. for C25H19N3O3
[M+]: 409.14; found: 410.24; anal. Calcd. for C25H19N3O_3:
C, 73.34; H, 4.68; N, 10.26% Found: C, 73.62; H, 4.24; N,
9.95%.
2.2e 2-(4-((4-nitrophenyl)(phenyl)amino)phenyl)-1H-
anthra[1,2-d]imidazole-6,11-dione (4): A mixture of
6 (0.40 g, 1 mmol) and 4–nitro–N–phenylaniline (0.25 g,
1.2 mmol) were reacted in toluene as mentioned in general
method. The crude solid thus obtained was purified by column
chromatography using hexane/chloroform to obtain a deep
yellow solid. Yield: 0.22 g (43%); M.p.: 281 ◦C; FT-IR (KBr,
vmax/cm−1): 3374.84, 1640.14, 1572.78, 1453.19, 1272.57,
987.21, 814.84, 710.85, 582.26, 491.54; 1H–NMR (600 MHz,
CDCl3, 25 ◦C) δ ppm: 7.10 (d, 2H, ArH, J = 9.0 Hz), 7.22 (d,
2H, ArH, J = 8.4 Hz), 7.29 (t, 2H, ArH, J = 7.2 Hz), 7.32 (d,
2H,ArH, J = 8.4 Hz), 7.43 (t, 2H, ArH, J = 7.2 Hz), 7.73 (d,
2H, ArH, J = 8.4 Hz), 7.82 (s, ArH, 2H), 8.12 (m, 3H, ArH),
8.29(d, 1H, ArH, J = 9.0 Hz), 8.36(d,1H, ArH, J = 9.0 Hz),
11.31 (s,1H, -NH); 13C–NMR (75 MHz, CDCl3, 25 ◦C) δ,
ppm: 114.72, 117.21, 118.09, 118.36, 121.76, 124.40, 126.14,
127.25, 127.37, 127.61, 127.85, 128.19, 128.68, 132.27,
133.19, 133.33, 133.83, 134.02, 139.13, 140.64, 149.50,
151.41, 156.65, 156.79, 182.27, 184.97; MALDI-TOF: mass
calcd. for C33H20N4O4 [M+]: 536.54; found: 537.62; anal.
Calcd. for C33H20N4O4: C, 73.87; H, 3.76; N, 10.44% Found:
C, 73.54; H, 3.81; N, 10.73%.
3. Results and Discussion
3.1 Synthesis and characterization
The synthesis of the target compounds 1–5 based on
2-phenyl-anthra[1,2-d]imidazole-6,11-dione, is illus-
trated in Scheme 1.
The starting bromo derivative 2-(4-bromo)phenyl-
anthra[1,2−d]imidazole-6,11-dione (6) required for the
present study was synthesized by the reported pro-
cedure.17d4-bromobenzaldehyde was oxidatively cou-
pled with 1,2-diaminoanthraquinone to obtain the pre-
cursor compound 6. The compound 6 was conve-
niently converted to triarylamine derivatives 1–5 by
Buchwald-Hartwig coupling reaction in the presence
of Pd2(dba)3 (5–8 mol%) as the source of the palla-
dium catalyst, SPhos (2-dicyclohexylphoshpino-2ꢀ,6ꢀ-
2.2f 2-(4-morpholinophenyl)-1H-anthra[1,2-d]
imidazole-6,11-dione(5): Amixtureof6(0.40g, 1mmol)
and morpholine (0.10 g, 1.2 mmol) were reacted in toluene
as mentioned in the general method. The crude solid thus
obtained was purified by column chromatography using hex-
ane/chloroform to obtain a deep red solid. Yield: 0.26 g
(65%); M.p.: 314 ◦C; FT-IR (KBr, vmax/cm−1): 3435.61,
dimethylbiphenyl) as co-ligand and sodium t-butoxide
1
2850.57, 1666.20, 1490.33, 1249.78, 1292.25, 1121.33, as a base. Characterization methods which include H
Scheme 1. Synthesis of imidazoanthraquinone-amine derivatives 1–5.

J. Chem. Sci. (2018) 130:49
Table 1. Photophysical data of Std and 1–5.
Page 5 of 13 49
a
b
a
Compound
, nm (log
,nm
, nm Stokes shifta, cm−1 Egc eV ꢀ (%)d
λabs
λabs
λem
F
/M−1cm−1 a
)
ε
Std
1
273 (4.72), 339
(3.63), 402 (4.10)
278 (4.66), 339
(4.16), 403 (4.42),
500 (3.53)
266 (4.82), 334
(4.43), 409 (4.24),
479 (4.11)
—
512
5382
469
2.68
2.70
0.30
0.36
274, 347, 415, 514
284, 350, 413, 540
274, 347, 526
514
524
540
2
3
1793
1402
2.66
2.69
0.36
0.91
261 (4.66),
339(4.37),
502(3.81)
4
5
261 (4.22), 321
(4.10), 436 (4.18)
265 (4.39), 293
(4.64), 312 (4.61),
460 (4.18)
263, 303, 326, 444
271, 308, 481
560
540
5078
3221
2.71
2.49
0.46
0.95
Std molecule is 2-phenyl-anthra[1,2-d]imidazole-6,11-dione (See Figure 1). [a]In DCM. [b] Neat solid film.
[c]Optical band gap estimated using emission and excitation spectra in DCM. [d]Quantum yield with reference
to fluorescein (ꢀF = 0.79 in 0.1 M NaOH) in toluene.
NMR, 13C NMR, MALDI-TOF mass, FTIR, etc. Fur- 3.2 Photophysical properties
ther, it was observed that the yield of the reaction was
significantly affected by the substituents attached to the UV–Vis absorption and fluorescence emission spectra
diarylamine; when electron-donating –OCH3 group was of dyes 1–5 were carried out in solvents of varying
present on diarylamine resulted in better yield as com- polarity and in neat solid film. Absorption and emission
pared to electron-withdrawing -NO2 group. The five spectra in dichloromethane solution of dyes 1–5 along
target compounds were obtained in quantitative yield with parent compound for comparison are presented in
as yellow to red solids. Compounds 1–5 showed good Figure 2(a) and 3(a) while the pertinent data are summa-
solubility in common organic solvents.The identity and rized in Table 1. The absorption spectra of core molecule
purity of all new compounds were confirmed by var- 2-phenyl-anthra[1,2-d]imidazole-6,11-dione (Std) pos-
1
ious spectroscopic techniques. H NMR (600 MHz) sesses two absorption maxima at 273 nm which arises
and 13C NMR(75 MHz) of 2-(4-bis(4- methoxy phenyl) due to overlap of n- *and - * transitions within the
π
π π
amino)phenyl)−1H-anthra[1,2-d]imidazole-6,11-dio-
entire molecule, while a broad and low-intensity peak
ne (3) recorded in CDCl3 (see Supporting Informa- at 405 nm arises due to intramolecular charge trans-
tion, Figure S3). A characteristic singlet peak due to fer transition (ICT).19 The absorption spectra of 1–5
imidazole proton was observed at 11.14 ppm. Two are dominated by multiple overlapping bands owing
doublets due to two protons of phenyl ring (b,bꢀ) to the presence of different chromophoric segments.
appeared at 8.34 and 8.27 ppm confirming the phenyl Dyes 1–5 display a high intensity band in the range
substitution on the imidazole ring. Two doublets due of 261–293 nm and a new distinguished band in the
to two protons (c and d) of the anthraquinone moi- range of 312–339 nm emerges in 1–5 compared to Std
π
π π
ety appear at 8.21 and 8.05 ppm while a doublet and these are assigned to n- * and - *transitions
of 2 protons (e,eꢀ) was observed at 7.93 ppm and a from entire molecule due to extension in conjugation
multiplet due to two protons (f,fꢀ) at 7.80 ppm show- by the introduction of triarylamines into imidazoan-
ing the presence of the anthraquinone moiety in the thraquinone core.
molecule. The presence of singlet at 3.83 ppm indicates
The lowest absorption maximum for all the dyes
the presence of –OCH3 in the molecule. Character- appear in the range of 436–502 nm which is assigned
istic signal of two carbonyl groups was observed at to intramolecular charge transfer (ICT) transitions from
185.27 and 182.57 ppm and methoxy carbon appeared electron–donor triarylamine subunit to electron–
at 55.51 ppm in 13C–NMR. All other peaks observed acceptor imidazoanthraquinone core. ICT transitions in
in 13C–NMR spectra can be assigned to aromatic 1–5 were found to be red-shifted with an increase in the
carbons.
absorption intensity as compared to Std. This increase

49 Page 6 of 13
J. Chem. Sci. (2018) 130:49
Figure 2. (a) Normalized absorption spectra of compounds 1–5 in dichloromethane (2 × 10−6 M) and (b) neat solid film
ε
(thickness of the film was not measured). Molar extinction coefficients ( ) of 1–5 are given in Table 1. Std molecule is
2-phenyl-anthra[1,2−d]imidazole-6,11-dione (Figure 1).
in absorbance indicates that the transition probability for up to 700 nm for dyes 3, 4 and 5 containing substitution
charge transfer increased upon substitution of amines on on triarylamine ring indicating advantageous absorption
the imidazoanthraquinone segment while the red-shift is window proliferation.
justified considering the auxochromic effect exerted by
the diarylamine unit.
The emission peak for 1–5 in dichloromethane solu-
tions appear in the green region in the range of 512–
A bathochromic and hyperchromic shift in ICT tran- 560 nm (Figure 3a) upon excitation ( ) at 405 nm.
λex
sition was observed in the absorption spectra of 3 The emission characteristics of the dyes were found to
and 5 compared with the other derivatives indicating be affected by the auxochromic effect of diarylamines
the influence of electron donating –OCH3 substituent incorporated in Std. The red shift by 2–48 nm in the
on diarylamine and cyclic aliphatic morpholine amine, emission profiles of dyes was observed compared to
respectively. While a blue and more hyperchromic shift Std. The emission colour of 1–5 are greenish in nature
for ICT was found in 4 compared to that in 3 and 5, while the shift in emission depends on the nature of
which is due to the strong electron acceptor -NO2 sub- triarylamine unit. Dyes 1–5 do not show emission in
stituent on diphenylamine, thus showing the effect of neat solid film form, this can be attributed to aggrega-
electron donating/withdrawing substituent present on tion based quenching of emission in the solid state by
the triarylamine moieties in absorption spectra. Fur- fast interchain electron transfer from donor to acceptor
ther slight blue shift were also observed in 1 and 2 as subunit via close spatial contact.20
compared to 3 and 5, which again confirms the effect
No specific solvent-dependent absorption and emis-
of substituent on absorption profile of triarylamines. sion behaviour was observed for the dyes but showed
Bathochromic shift of the ICT absorption maxima was 5–30 nm shift in the absorption ICT peak on varying
observed in the more polar solvent, which indicates that the nature of solvents irrespective of their polarity (see
the excited state is more polar than the ground state, Supporting Information). Considerable Stokes shift of
suggesting an ICT nature in 1–5.8 In the absorption about 467–5078 cm−1 was observed for 1–5 indicating
spectra of 1–5 as neat solid film form (Figure 2(b)), the change in the ground and excited state geometry,
the ICT absorption band is slightly broadened and red and also suggesting the possibility of formation of a
shifted by 14, 61, 24, 8 and 21 nm for the dyes 1, 2, charge–transfer state. The Stokes shifts recorded for the
3, 4 and 5, respectively, in comparison to that observed dyes 1–5 were lower than that of Std and observed to
in dichloromethane, which could be due to the inter- increase on substituting the respective diarylamine unit
molecular aggregation. The absorption onset extended in Std. The quantum yields of 1–5 were calculated using

J. Chem. Sci. (2018) 130:49
Page 7 of 13 49
Figure 3. (a) Normalized emission spectra of Std and 1–5 in dichloromethane (2 x 10−6 M) excited at
Tapping mode AFM topographical 3D images of compounds 1 and 4 as neat solid film coated (1000 rpm) from chloroform
= 405 nm. (b)
λ
ex
solution on quartz coverslips.
Table 2. Electrochemical and thermal data of Std and 1–5.
Compound
Eaox (eV)
Ebred (eV)
HOMOc (eV) LUMOd (eV) Ege (eV) Tmf (0 C) Tgd(0 C)
Std
1
2
3
4
1.96, 2.19
1.83, 2.37
1.85, 2.25
−1.58, −2.31
−1.50, −2.51
−1.55, −2.24
−6.69
−6.56
−6.58
−6.01
−6.62
−5.67
−3.15
−3.23
−3.18
−3.20
−3.42
−3.18
3.54
3.33
3.40
2.81
3.20
2.49
—
258
247
278
187
314
263
293
250
198
281
316
1.28, 1.84, 2.24 −1.53, −2.21
1.89
−1.31, −1.65
5
0.94, 1.84, 2.25 −1.55, −2.28
Std molecule is 2-phenyl-anthra[1,2-d]imidazole-6,11-dion[ed(]Fig. 1), [a] Oxidation peak potential. [b]Reduction
= −(E[ox vs Fc/Fc+] + 5.1)eV. ELUMO = −(E[red vs Fc/Fc+] + 5.1)eV, Eox for
[c]
peak potential.
E
HOMO
Fc/Fc+ = 0.37 V. [e]Eg = EHOMO − ELUMO
.
Melting points determined by DSC. [g]Decomposition tem-
[f]
perature obtained from the onset of TGA.
fluorescein (ꢀF = 0.79 in 0.1 M NaOH)21 as a refer- 3.3 Electrochemical properties
ence. The relatively low quantum yield observed for 1–5
In order to estimate the charge transport capability
suggest a prominent nonradiative deactivation pathway
for the excited state and were found to be dependent
upon the electron donating/withdrawing nature of sub-
stituents on triarylamines and on the polarity of solvent
(see Supporting Information). The morphological char-
acteristics of the neat solid film of compounds 1–5
were studied by atomic force microscopy (AFM) in the
tapping mode. Figure 3(b) shows the AFM results of
compound 1 and 4 (for other compounds, see Support-
ing Information). The AFM results show a smooth film
with a root mean square (rms) roughness value of 7.69–
33.2 nm, while micrometre-sized phase separation was
not observed.
and feasibility of electron injection and transport of
1–5, cyclic voltammetry (CV) measurements were per-
formed. The corresponding redox potentials of 1–5 are
listed in Table 2 and selected CVs are displayed in
Figure 4. The CV wave did not undergo substantial
modification after the repeated scans. Cyclic voltam-
metry of Std showed two irreversible oxidation peaks
at 1.96 and 2.19 V, which are expected due to the
oxidation of the imidazole moiety. While on cathodic
sweep, one quasi-reversible and one irreversible reduc-
tion peaks were observed at −1.58 and −2.31 V,
respectively.

49 Page 8 of 13
J. Chem. Sci. (2018) 130:49
Figure 4. Cyclic Voltammograms in acetonitrile. (a) (anodic sweep) of Std and 1–5, and (b) full scan of compound 1.
These reduction peaks are attributed to the reduction and reduction potential and were found in the range
of carbonyl groups of anthraquinone moiety.22 Anodic of − 5.67 to − 6.62 eV and − 3.18 to − 3.42 eV,
sweep of compounds 1–5, except compound 4, show respectively, by setting Fc/Fc+ at − 5.1 eV vs. vac-
similar oxidation pattern as that of 1 but the first oxi- uum.23 It is clearly evident that the nature of the
dation peak of 1–5 appear at a slightly lower potential triarylamine unit significantly alters the HOMO/LUMO
than Std and found in the range of 0.94–1.89 V, while energies. The low-lying LUMO energy level in the
the second peak is found to be shifted more positive range − 3.0 to − 4.0- eV is essential for the effi-
potential than Std with potentials in the range 2.24– ciency and stability of n-type materials.24 The LUMO
2.37 V. This shift in the oxidation potential is attributed energy levels of the dyes are found to be lower
to introduction of diarylamine moiety in the Std. An than that of the most widely used electron-transport
additional oxidation peak at lower potential appears (small n-type) materials; such as metal chelates like
for compound 3 and 5 at 1.28 V and 0.94 V, respec- Alq3 (tris(8-hydroxyquinoline)aluminium) (LUMO=-
tively. This is due to the oxidation of electron-rich 3.10 eV),25 organic materials such as 2,4,7,9-tetra-
–OCH3 substituent in 3 and morpholine in 5. Com- phenylpyrido[2,3-g]quinoline (LUMO = –3.10 eV)26
pound 4 shows only one oxidation peak at 1.89 V and 2,5-di([1,10-biphenyl]-4-yl)-1,1-dimethyl-3,4-dip-
which is probably due to the effect of -NO2 substituent henyl-1H-silole (LUMO = –3.30 eV)27 and some poly-
on diphenylamine ring. The oxidation potential of 1– mers such as polyquinoxalines (LUMO = –3.0 to
5 increases in the following order 5<3<1<2<4. This –3.24 eV)28 polyphenylquinoxalines and thiophene
trend is attributed to the electron donating nature of tri- linked polyphenylquinoxalines (LUMO=-3.0 to –3.30
arylamines in 1–5. On a cathodic sweep, 1–5 showed eV)29 and thus making these molecules as potential
similar two-wave reduction pattern with a slight change candidates for n-type materials. The energy band gap
in potentials as compared to Std, one quasi-reversible calculated from the cyclic voltammetry measurements
and second an irreversible reduction peak due to the were in the range of 2.49–3.40 eV. A decrease in
reduction of carbonyl groups of the anthraquinone
moiety.
band gap was observed in compound 3 (2.81 eV)
and 5 (2.49 eV), compared with the other derivatives
The HOMO and LUMO energy levels of the materi- indicating the influence of electron donating –OCH3
als are crucial parameters for most of the electro-optical substituent on diarylamine and cyclic aliphatic mor-
devices, which guides the interfacial charge transport pholine amine moiety, respectively (Table 2). Thus, the
kinetics. The energies of highest occupied and low- strength of electron-donating nature of the amine in 1–5
est unoccupied (HOMO and LUMO) energy levels results in a decrease in the HOMO and LUMO energy
of 1–5 (Table 2) were calculated from first oxidation levels and the energy band gap.

J. Chem. Sci. (2018) 130:49
Page 9 of 13 49
Figure 5. Schematic representation of theoretically calculated frontier molecular orbitals of Std and 1–5.
Table 3. Computed Electron affinity, Ionization potential, HOMO-LUMO energy
levels, Energy band gap and Dipole moment of Std and 1–5.
Comp EA (eV) IP (eV) HOMO (eV) LUMO (eV) EgT h(eV) μg (Debye)
Std
1
2
3
4
2.065
1.857
1.835
1.817
2.240
1.805
7.756
6.613
6.597
6.319
7.124
6.941
−6.502
−5.435
−5.464
−5.183
−5.991
−5.621
−3.358
−3.226
−3.201
−3.178
−3.462
−3.205
3.144
2.209
2.263
2.005
2.529
2.416
3.443
5.131
5.259
4.871
9.027
4.614
5
3.4 Theoretical studies
HOMO and LUMO of Std and 1–5 are shown in
Figure 5. The optimized structure, frontier molecular
orbitals (FMOs) along with the Cartesian coordinates
for all the molecules are shown in Supporting Infor-
mation. The energies of HOMO and LUMO levels,
HOMO-LUMO gap, first ionization potential, electron
affinity and ground-state dipole moment computed for
Std and 1–5 are collected in Table 3. In the parent
compound (Std), HOMO is delocalized over the
To gain more insight into the electronic structure, dyes
1–5 along with Std were modelled using density func-
tional theoretical (DFT) calculations using Gaussian
03 software package.30 The geometry of the dyes was
optimized by using B3LYP as exchange-correlation
functional and 6-311 G (d,p) basis set. The elec-
tronic distribution in the frontier molecular orbitals

49 Page 10 of 13
J. Chem. Sci. (2018) 130:49
Figure 6. Thermogram (a) and DSC plot (b) of Std and 1–5.
2-phenyl-benzimidazole ring while the LUMO is local- as compared to other derivatives due to the presence of
ized on the entire anthraquinone moiety and extended polar -NO2 group attached to diphenylamine.
to the imidazole ring.
In case of 1–5, the introduction of triarylamine redis-
tributes the HOMO/LUMO energy level. HOMO in 1–5
3.5 Thermal properties
is mainly constituted by the triarylamine segment while
Thermal properties of 1–5 were investigated by thermo-
LUMO is localized on the anthraquinone and spread
gravimetric analysis (TGA) and differential scanning
over the imidazole ring as well. The visual representa-
calorimetry (DSC) as these phenomena play a crucial
tion of HOMO and LUMO of the dyes clearly support
role in the stability and lifetime of photo-electronic
the formation of the push-pull system in the molecule
devices. Materials possessing good thermal stability and
that has been commonly observed in the molecules
high decomposition temperature found to improve the
featuring donor-acceptor architecture31 and is consid-
device performance. The optoelectronic devices should
withstand temperature expeditions as high as 80 C.32
ered an indication of the prominent HOMO to LUMO
charge transfer absorption band in the compounds 1–
5. The calculated HOMO and LUMO energy of the
compounds are found in the range − 5.18 to − 5.99
and − 3.17 to − 3.46 eV, respectively, and the HOMO-
LUMO gap of the dyes are ranged in 2.00–2.52 eV.
The calculated LUMO energies were very close to those
estimated from CV. The first vertical ionization poten-
tial (VIP) and the electron affinity (EA) have been
computed as the difference in the total energies of neu-
tral and cationic/anionic species respectively. Thus, the
computed LUMO energy values suggest that these com-
pounds may be effective as electron-transport layer and
can be used as n-type materials for electroactive devices.
The ionization potential and electron affinity computed
for the dyes are also in agreement with the above obser-
vation.
◦
Observed thermal data of 1–5 is listed in Table 2
and thermograms are displayed in Figure 6. Thermo-
gravimetric analysis revealed that compounds 1–5 are
thermally stable materials, and the onset of the decom-
◦
position occurred above 250 C with no weight loss at
low temperature.
Melting points of 1–5 were determined by DSC and
◦
are found in the range of 198−316 C. No glass phase
transition temperature was observed in 1–5. It is found
that these compounds are thermally stable and could be
used in optoelectronic devices.
4. Conclusions
We have synthesized a new series of imidazoanthra-
quinone based triarylamines, donor-acceptor derivatives
via palladium-catalysed Buchwald–Hartwig C–N bond
The dipole moment calculated for the derivatives in
gaseous phase is small but found to be high for 4 (9.02)

J. Chem. Sci. (2018) 130:49
Page 11 of 13 49
formation amination reaction. These compounds were
thoroughly analyzed by routine spectral methods and
subjected to photophysical, electrochemical, thermal
and theoretical studies. The photophysical and electro-
chemical properties of the dyes are significantly influ-
enced by the nature of triarylamine segment attached
to the imidazoanthraquinone core displaying a broad
absorption ICT band attributed to charge transfer transi-
tions from the electron donating triarylamine to electron
acceptor imidazoanthraquinone core. These molecules
were found to emit in the green region. Electrochemical
studies reveal the stability of these compounds under
redox conditions and low-lying LUMO energy levels in
the range of –3.18 to –3.42 eV which are very simi-
lar to well-known n-type materials. The donor-acceptor
architecture and HOMO-LUMO energies were further
rationalized using DFT calculations. Optoelectronic
properties and thermal stability of these compounds
along with theoretical calculations suggest that these
molecules have potential to be used as n-type materials
in optoelectronic devices. Further, we are studying the
ion sensing ability of these molecules as chemosensors.
materials—molecular design, syntheses, reactions, prop-
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All additional information pertaining to compounds 1–5,
namely 1H and 13C–NMR spectra (Figures S1–S5), MALDI-
TOF (Figures S6–S10), mass spectra, FTIR spectra (Fig-
ures S11–S15), absorption and emission spectra in Toluene,
CH2Cl2, Methanol, Acetonitrile and DMSO (Figures S16–
S20) and photophysical data, AFM images of 1–5 (Figures
S21–S25), Cyclic voltammograms of Std and 1–5 (Figures
S26–S31), and optimized structures and Cartesian coordi-
nates of Std and 1–5 are given in the Supporting Information.
Supplementary Information is available at www.ias.ac.in/
chemsci.
Acknowledgements
The authors thank the Science and Engineering Research
Board (SERB), New Delhi, India for financial support [SERB
Scheme No. SB/EMEQ–507/2014]. BKS and AS thank the
University Grant Commission, India for research fellowships.
We thank the Micro-Analytical Laboratory, Department of
Chemistry, University of Mumbai, Mumbai for providing
instrumentation facility. We also thank the Tata Institute of
1
Fundamental Research, Mumbai for MALDI-TOF and H–
NMR facilities.
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