Quinoxaline based amines as blue-orange emitters: Effect of modulating donor system on optoelectrochemical and theoretical properties

Article abstract of DOI:10.1016/j.dyepig.2019.04.005

A series of eight novel donor?acceptor based quinoxaline?amine derivatives 2–9 were prepared by modulating donor species on quinoxaline core with Buchwald?Hartwig coupling amination reaction. The synthesized molecules were fully characterized and studied

Full text of DOI:10.1016/j.dyepig.2019.04.005

Accepted Manuscript
Quinoxaline based amines as blue-orange emitters: Effect of modulating donor
system on optoelectrochemical and theoretical properties
Deepali N. Kanekar, Sajeev Chacko, Rajesh M. Kamble
PII:
S0143-7208(19)30183-4
https://doi.org/10.1016/j.dyepig.2019.04.005
DYPI 7460
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 January 2019
Revised Date: 27 February 2019
Accepted Date: 2 April 2019
Please cite this article as: Kanekar DN, Chacko S, Kamble RM, Quinoxaline based amines as blue-
orange emitters: Effect of modulating donor system on optoelectrochemical and theoretical properties,
Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.04.005.
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Graphical Abstract

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Quinoxaline based amines as Blue-orange emitters: Effect of modulating
donor system on Optoelectrochemical and theoretical properties
Deepali N. Kanekara, Dr. Sajeev Chackob and Dr. Rajesh M. Kamblea*
aDepartment of Chemistry, University of Mumbai, Santacruz (E), Mumbai 400 098, India
bDepartment of Physics, University of Mumbai, Santacruz (E), Mumbai 400 098, India
e-mail: kamblerm@chem.mu.ac.in
Abstract
A series of eight novel donor‒acceptor based quinoxaline‒amine derivatives 2−9 were
prepared by modulating donor species on quinoxaline core with Buchwald‒Hartwig coupling
amination reaction. The synthesized molecules were fully characterized and studied for
impact of D−A interaction on optoelectrochemical properties of derivatives. Absorption
spectra of 2−9 display intramolecular charge transfer (ICT) transitions in the range of
377−456 nm. Dyes 2−9 show positive solvatochromism in emission and emit in blue‒orange
region with emission maxima 472–592 nm on excitation at their respective ICT maxima in
toluene, chloroform, DCM and neat solid film. Herein, dyes 2, 3 and 7 which possess intense
emission in solid state were further studied for Aggregation Induced Emission (AIE) effect.
The HOMO and LUMO energy level for compound 2−9 obtained by CV were found in the
range of ‒5.23 to ‒5.83 eV and ‒3.55 to ‒3.74 eV. Theoretical studies of molecules were also
carried out by using
TD‒DFT calculations. The comparable HOMO and LUMO
energy level of 2−9 with reported ambipolar materials and efficient solid state emission make
synthesized compounds potential candidate for solid state emissive, ambipolar materials in
organic electronics.
Keywords: Donor‒Acceptor (D−A) architecture, Quinoxaline‒amines, Intramolecular charge
transfer (ICT) transitions, Aggregation Induced Emission (AIE) effect, HOMO and LUMO
energy levels, Ambipolar materials.

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1. Introduction
The development in the field of organic semiconductors is of immense interest for
researchers due to its widespread applications in organic light emitting diodes (OLEDs) [1,2],
organic field effect transistors (OFETs) [3], dye sensitized solar cells (DSSCs) [4‒6] , sensors
[7,8], etc. Organic materials are designed and synthesized with incorporation of
Donor‒Acceptor (D–A) system offers the unique properties such as well–defined structures,
ease of purification, solubility in wide range of organic solvents, tuneable photophysical and
electrochemical properties, thermal stability and low cost solution processing techniques [9].
This D–A design approach give choice of suitable D/A unit to obtain required luminescence
and charge transporting properties in molecule. In the past, variety of D–A type of molecules
having combination of electron deficient core like quinoxaline [10,11], indoloquinoxaline
[12], pyarazine [13], phenazine [14,15], pyrazino‒phenazine [16] and electron rich
triarylamine/heterocyclic amines have been synthesized and explored in optoelectronic
devices. The luminescence in such D–A system mainly arises due to ICT and can be tuned by
structure modification. It has observed that efficient luminescence in dilute solutions obtained
for many fluorophores is generally weakened or quenched in aggregation/ solid state leads to
aggregation-caused quenching (ACQ) due to non‒radiative relaxation taking place in the
molecule [17‒20]. However, organic materials with efficient solid state emission are essential
for various optoelectronic applications. Hence, there is need to govern the factors which
cause ACQ in order to obtain solid state emissive materials. In 2001, Tang et.al discovered
propeller-shaped, non-planar luminogen that were almost nonfluorescent in solution, became
highly fluorescent on aggregation. This phenomenon of enhanced emission on aggregation is
known as aggregation-induced emission (AIE) [21]. The use of AIE active groups in

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conventional ACQ building block has been effective approach to increase solid state emission
[22].
At same time, literature witnessed successful development of hole transporting (p–
type) materials (HTM) whereas the development of n–type and ambipolar materials is still
lagging behind due to their difficult synthesis and instability under ambient conditions.
However, electron transporting materials are indispensable component of integrated circuit
and has huge requirement in optoelectronic applications. According to reports, p–type
materials can be converted to n–type by introducing N-atom in structural core of molecule or
by introducing electron withdrawing groups such as ‒F, ‒CN, ‒Cl, ‒NO2, ‒COOH, etc [23].
On the other hand, there is great demand for ambipolar materials to achieve high
performance optoelectronic device, as it facilitates both (p-channel) hole and (n-channel)
electron transport, thus improves recombination frequency of excitons. Since, the charge
transfer process in the small molecule is chain of redox processes, dependant on embedded
electron donating and withdrawing moieties in the system. The incorporation of appropriate
donor and acceptor moiety in the same molecule can help to build ambipolar transporting
materials [24,25].
With this clues of ambipolar and AIE properties dependency on D–A architecture, we
have chosen electron deficient 2,3-bis(4-bromophenyl)quinoxalin-6-yl(phenyl)methanone (1)
having quinoxaline core as an acceptor. It has outstanding electron accepting properties due
to presence of two ‘N’ atoms in ring and one electron withdrawing benzoyl group attached to
this quinoxaline ring. Along with optoelectronic properties, quinoxaline is also well known
for broad range of biological and pharmaceutical activities such as antibacterial [26],
antiinflammetry [27], anticancer [28,29], antimicrobial [30], etc. Considering the importance
of structure–property relationship, electron deficient quinoxaline incorporated with different
donor moieties in order to study changes occurring in photophysical, electrochemical and

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thermal properties of derivatives. Thus, we have designed and synthesized derivatives 2–9 by
incorporating a Buchwald–Hartwig coupling amination [31,32] reaction between electron
deficient 1 and various electron donating diarylamines/heterocyclic amines and aliphatic

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Figure 1. Molecular structure of compounds 2‒9.

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amine morpholine. Further, the effect of various diarylamine substituted with electron
donating (–OCH3) and withdrawing (–NO2, aza) group on their optoelectrochemical
properties such as intramolecular charge transfer transition (ICT), HOMO−LUMO energy
levels and band gap has been studied and discussed.
2. Experimental Section
2.1 Chemicals and materials
All the starting materials and reagents were purchased from commercial sources (Sigma
Aldrich and Alfa Aesar) and were used without any further treatment and purification unless
otherwise mentioned. 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 chromatography 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.
2.2 Synthesis and Characterization
1
Compound 1 is prepared according to the reported procedure and confirmed by H and
13C NMR. Compounds 2−9 were obtained by Buchwald-Hartwig coupling of 1 with the
corresponding diarylamines/heterocyclic amines in the presence of Pd2(dba)3 catalyst, SPhos
ligand and t-BuONa as strong base in toluene. The synthesized compounds 2–9 were
obtained in moderate 42%–77% yield as yellow‒green solids soluble in common organic
solvents including toluene, dichloromethane (DCM), chloroform, Tetrahydrofuran (THF),
dimethyl sulfoxide (DMSO). However, they are completely insoluble in water. The identity
1
and purity of all the target compounds were confirmed by FT−IR, H, 13C−NMR
spectroscopy,
MALDI‒TOF mass spectrometry and elemental analysis. The
synthetic route of compound 2−9 is shown in Scheme 1.

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Scheme 1. Synthetic route of 2–9.
2.2.1 Synthesis of (2,3-bis(4-bromophenyl)quinoxalin-6-yl)(phenyl)methanone (1)
A mixture of 3,4-diaminobenzophenone ( 0.8 g, 3.77 mmol) and 4,4′-dibromobenzil
(1.38 g, 3.77 mmol) was dissolved in 25 mL of glacial acetic acid and refluxed for 5 hours.
The reaction mixture was allowed to cool and then poured over crushed ice to obtain white
coloured solid. The obtained solid was then dried under vacuum and purified using column
chromatography (eluent: n-hexane:chloroform ratio as 90:10), white solid, yield 1.84 g
(90%); mp 200 0C.

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1H NMR (300 MHz, CDCl3): δ 8.50 (s, 1H), 8.26 (s, 2H), 7.89 (d, J = 7.1 Hz, 2H),
7.72–7.33 (m, 11H); 13C NMR (75 MHz, CDCl3): δ 195.54 (C=O), 153.53, 153.00, 142.87,
140.20, 138.67, 137.20, 137.17, 137.02, 132.92, 132.47, 132.25, 131.77, 131.45, 131.34,
130.26, 130.11, 129.67, 128.54, 124.26, 124.09.
MALDI-TOF: mass calcd for C27H16Br2N2O [M+]: 543.96, found: 543.96; FT−IR,
(KBr, νmax cm-1): 3054, 1650, 1575, 1072, 713; elemental anal. calcd for C27H16Br2N2O: C
59.59,
2.86.
H 2.96, Br 29.36, N 5.15, O 2.94. found: C 59.07, H 3.28, Br 29.46, N 5.33, O
2.2.2 (2,3-bis(4-(diphenylamino)phenyl)quinoxalin-6-yl)(phenyl)methanone (2)
A mixture of compound 1 (0.2 g, 0.37 mmol) and diphenylamine (0.125 g, 0.74
mmol) was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3 (20 mg, 0.02
mmol), SPhos (25 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were added. The
reaction mixture was continuously stirred under nitrogen atmosphere at 100 0C for 15 hours.
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 80:20); yellow solid; 129 mg (48%); mp 254 0C.
1H NMR (300 MHz, CDCl3): δ 8.47 (s, 1H), 8.22 (d, J = 1.1 Hz, 2H), 7.90 (d, J = 7.1
Hz, 2H), 7.63 (t, J = 7.4 Hz, 1H), 7.56–7.42 (m, 6H), 7.31–7.25 (m, 9H) (1H extra due to
residual CHCl3 in CDCl3), 7.16‒7.04 (m,16H); 13C NMR (75 MHz, CDCl3): δ 195.91 (C=O),
154.82, 154.30, 149.02, 148.84, 147.18, 147.14, 142.99, 142.93, 139.97, 137.83, 137.35,
132.70, 132.35, 131.94, 131.74, 130.89, 130.69, 130.12, 129.48, 129.42, 129.39, 128.48,
125.20, 125.11, 123.72, 123.62, 121.93, 121.72.
MALDI-TOF: mass calcd for C51H36N4O [M+]: 720.29, found: 720.31; FT−IR (KBr,
νmax cm-1): 3034, 2921, 1658, 1586, 1278, 692; elemental anal. calcd for C51H36N4O : C
84.97, H 5.03, N 7.77, O 2.22 found: C 85.02, H 4.97, N 7.82, O 2.19.

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2.2.3 (2,3-bis(4-(naphthalen-1-yl(phenyl)amino)phenyl)quinoxalin-6-yl)(phenyl)methanone
(3)
A mixture of compound 1 (0.2 g, 0.37 mmol) and N-Phenyl-1-napthylamine (0.169 g,
0.74 mmol) was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3 (20 mg,
0.02 mmol), SPhos (20 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were added. The
reaction mixture was continuously stirred under nitrogen atmosphere at 100 0C for 15 hours.
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 80:20); yellow solid; 157 mg (51%); mp 160 0C.
1H NMR (300 MHz, CDCl3): δ 8.44 (s, 1H), 8.19 (s, 2H), 7.94–7.75 (m, 7H),
7.63–6.96 (m, 26H), 6.89 (d, J = 6.8 Hz, 4H); 13C NMR (75 MHz, CDCl3): δ 196.07 (C=O),
154.97, 154.43, 149.51, 149.34, 147.56, 147.49, 142.92, 142.90, 142.87, 139.91, 137.68,
137.35, 135.29, 132.64, 132.31, 131.54, 131.04, 130.87, 130.66, 130.09, 129.41, 129.27,
129.25, 128.44, 127.35, 127.31, 126.91, 126.85, 126.49, 126.45, 126.30, 126.25, 124.09,
123.28, 123.17, 122.95, 122.84, 119.94, 119.75.
MALDI-TOF: mass calcd for C59H40N4O [M+]: 820.32, found: 820.55; FT−IR (KBr,
νmax cm-1): 3045, 2973, 1658, 1586, 1267, 692; elemental anal. calcd for C59H40N4O: C
86.32, H 4.91, N 6.82, O 1.95 found: C 86.12, H 5.04, N 6.98, O 1.86.
2.2.4 (2,3-bis(4-(bis(4-methoxyphenyl)amino)phenyl)quinoxalin-6-yl)(phenyl)methanone
(4)
A mixture of compound 1 (0.2 g, 0.37 mmol) and 4,4’-Dimethoxydiphenylamine
(0.169 g, 0.74 mmol) was dissolved in anhydrous toluene (50 ml). To this solution Pd2(dba)3
(20 mg, 0.02 mmol), SPhos (20 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were
0
added. The reaction mixture was continuously stirred under nitrogen atmosphere at 100 C
for 15 hours. Reaction mixture was then cooled to room temperature and extracted with

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chloroform. The solid thus obtained was then purified by column chromatography (eluent:
n−hexane:chloroform ratio as 70:30); red solid; 162 mg (52%); mp 154 0C.
1H NMR (300 MHz, CDCl3): δ 8.44 (s, 1H), 8.18 (d, J = 0.8 Hz, 2H), 7.89 (d, J = 7.1
Hz, 2H), 7.62 (t, J = 7.4 Hz, 1H), 7.55–7.37 (m, 6H), 7.14–7.04 (m, 8H), 6.91–6.81 (m, 12H),
3.81 (s, 12H, −OCH3); 13C NMR (75 MHz, CDCl3): δ 196.00 (C=O), 156.39, 156.31, 154.95,
154.44, 149.78, 149.60, 142.90, 140.24, 140.16, 139.86, 137.50, 137.44, 132.62, 132.33,
130.76, 130.53, 130.19, 130.11, 129.98, 129.36, 129.13, 128.44, 127.22, 127.13, 118.96,
118.75, 114.78, 114.76, 55.49 (−OCH3).
MALDI-TOF: mass calcd for C55H44N4O5 [M+]: 840.33, found: 840.79; FT−IR
(KBr, νmax cm-1): 2931, 1658, 1596, 1503, 1237, 1021, 815; elemental anal. calcd for
C55H44N4O5: C 78.55, H 5.27, N 6.66, O 9.51 found: C 77.98, H 5.47, N 6.74, O 9.81.
2.2.5 (2,3-bis(4-((4-nitrophenyl)(phenyl)amino)phenyl)quinoxalin-6-yl)(phenyl)methanone
(5)
A mixture of compound 1 (0.2 g, 0.37 mmol) and 4-Nitrodiphenylamine (0.157 g,
0.74 mmol) was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3 (20 mg,
0.02 mmol), SPhos (20 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were added. The
reaction mixture was continuously stirred under nitrogen atmosphere at 100 0C for 15 hours.
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); yellow solid; 158 mg (53%); mp 160 0C.
1H NMR (300 MHz, CDCl3): δ 8.52 (s, 1H), 8.28 (d, J = 1.0 Hz, 2H), 8.05 (dd, J =
9.2, 3.9 Hz, 4H), 7.96–7.87 (m, 2H), 7.73–7.48 (m, 7H), 7.44–7.34 (m, 4H), 7.33–7.24 (m,
3H) (1H extra due to residual CHCl3 in CDCl3), 7.23–7.14 (m, 7H), 7.02 (dd, J = 9.2, 5.3 Hz,
4H);
13C NMR (75 MHz, CDCl3): δ 195.77 (C=O), 154.12, 153.49, 152.85, 147.08,
146.93, 145.38, 142.94, 141.04, 140.99, 140.19, 138.54, 137.10, 134.97, 134.84, 132.83,

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132.27, 131.47, 131.32, 130.13, 129.64, 128.56, 126.75, 126.71, 126.26, 126.22, 125.45,
125.11, 124.99, 119.64, 119.54.
MALDI-TOF: mass calcd for C51H34N6O5 [M+]: 810.26, found: 810.36; FT−IR
(KBr, νmax cm-1): 3404, 1647, 1575, 1483, 1288, 1103, 692; elemental anal. calcd for
C51H34N6O5 : C 75.54, H 4.23, N 10.36, O 9.87 found: C 75.27, H 4.25, N 10.41, O 10.07.
2.2.6 (2,3-bis(4-(phenyl(4-((E)-phenyldiazenyl)phenyl)amino)phenyl)quinoxalin-6-
yl)(phenyl)methanone (6)
A mixture of compound 1 (0.2 g, 0.37 mmol) and 4-(Phenylazo)diphenylamine
(0.202 g, 0.74 mmol) was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3
(20 mg, 0.02 mmol), SPhos (20 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were
0
added. The reaction mixture was continuously stirred under nitrogen atmosphere at 100 C
for 15 hours. 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 80:20); yellow solid; 147 mg (43%); mp 254 0C.
1H NMR (300 MHz, CDCl3): δ 8.51 (s, 1H), 8.26 (s, 2H), 7.95–7.80 (m, 10H),
13
7.67–7.30 (m, 17H), 7.28–7.13 (m, 15H) (1H extra due to residual CHCl3 in CDCl3); C
13
NMR (75 MHz, CDCl3 ): C NMR (75 MHz, CDCl3) δ 195.84 (C=O), 154.03, 152.87,
149.84, 148.28, 148.11, 147.99, 147.93, 146.51, 142.96, 140.07, 138.13, 137.27, 133.28,
133.11, 132.79, 132.34, 131.13, 130.96, 130.45, 130.13, 129.73, 129.04, 128.52, 126.07,
124.85, 124.79, 124.29, 123.56, 123.39, 123.16, 123.05, 122.62.
MALDI-TOF: mass calcd for C63H44N8O [M+]: 928.36, found: 928.97; FT−IR
(KBr, νmax cm-1): 3054, 2921, 1658, 1586, 1483, 1257, 692; elemental anal. calcd for
C63H44N8O:
C 81.44, H 4.77, N 12.06, O 1.72 found: C 81.57, H 4.72, N 11.97, O 1.74.
2.2.7 (2,3-bis(4-(9H-carbazol-9-yl)phenyl)quinoxalin-6-yl)(phenyl)methanone (7).

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A mixture of compound 1 (0.2 g, 0.37 mmol) and carbazole (0.123 g, 0.74 mmol)
was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3 (20 mg, 0.02 mmol),
SPhos (20 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were added. The reaction
mixture was continuously stirred under nitrogen atmosphere at 100 0C for 15 hours. 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
80:20); yellow‒green solid; 178 mg (68%); mp > 280 0C.
1H NMR (300 MHz, CDCl3): δ 8.61 (s, 1H), 8.41–8.30 (m, 2H), 8.14 (d, J = 7.7 Hz,
13
4H), 7.98–7.83 (m, 6H), 7.75–7.62 (m, 5H), 7.61–7.35 (m, 10H), 7.31–7.29 (m, 4H); C
NMR (75 MHz, CDCl3): δ 195.69 (C=O), 154.16, 153.59, 143.08, 140.44, 140.37, 139.03,
138.90, 138.75, 137.27, 137.20, 137.10, 132.94,131.61, 131.48, 130.16, 129.79, 128.58,
126.81, 126.76, 126.17, 123.68, 123.66, 120.43, 120.39, 120.36, 109.62.
MALDI-TOF: mass calcd for C51H32N4O [M+]: 716.26 found: 716.39; FT−IR
(KBr, νmax cm-1): 3065, 2921, 1658, 1596, 1452, 1237, 713; elemental anal. calcd for
C51H32N4O:
C 85.45, H 4.50, N 7.82, O 2.23 found: C 85.54, H 4.63, N 7.92, O 1.91.
2.2.8 (2,3-bis(4-(10H-phenothiazin-10-yl)phenyl)quinoxalin-6-yl)(phenyl)methanone (8)
A mixture of compound 1 (0.2 g, 0.37 mmol) and phenothiazine (0.147 g, 0.74
mmol) was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3 (20 mg, 0.02
mmol), SPhos (20 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were added. The
reaction mixture was continuously stirred under nitrogen atmosphere at 100 0C for 15 hours.
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 80:20); brown solid, yield: 121 mg (42%); mp 151 0C.
1H NMR (300 MHz, CDCl3): δ 8.58 (s, 1H), 8.33 (s, 2H), 7.93 (d, J = 7.0 Hz, 2H),
7.77–7.57 (m, 5H), 7.56–7.51 (m, 2H), 7.35 (d, J = 8.4 Hz, 4H), 7.15–7.09 (m, 4H), 6.91–

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13
6.82 (m, 8H), 6.52–6.42 (m, 4H); C NMR (75 MHz, CDCl3): δ 195.66 (C=O), 154.44,
153.91, 143.36, 143.29, 143.27, 143.09, 142.99, 140.31, 138.64, 137.13, 136.94, 136.58,
132.93, 132.40, 132.12, 132.01, 130.24, 130.16, 129.77, 128.58, 127.83, 127.34, 127.25,
127.24, 127.02, 126.99, 123.74, 123.52, 123.38, 123.23, 118.59, 118.19.
MALDI-TOF: mass calcd for C51H32N4OS2 [M+]: 780.20 found: 780.17; FT−IR
(KBr, νmax cm-1): 3053, 2921, 1658, 1575, 1452, 1257, 740; elemental anal. calcd for
C51H32N4OS2 : C 78.44, H 4.13, N 7.17, O 2.05, S 8.21 found C 78.34, H 4.18, N 7.27, O
2.03, S 8.18.
2.2.9 (2,3-bis(4-morpholinophenyl)quinoxalin-6-yl)(phenyl)methanone (9).
A mixture of compound 1 (0.2 g, 0.37 mmol) and morpholine (0.064 g, 0.74 mmol)
was dissolved in anhydrous toluene (50 mL). To this solution Pd2(dba)3 (20 mg, 0.02 mmol),
SPhos (25 mg, 0.048 mmol) and t-BuONa (75 mg, 0.75 mmol) were added. The reaction
mixture was continuously stirred under nitrogen atmosphere at 100 0C for 15 hours. 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
60:40); brown solid, yield: 157 mg (77%); mp 162 0C.
1H NMR (300 MHz, CDCl3): δ 8.45 (s, 1H), 8.19 (s, 2H), 7.89 (d, J = 7.2 Hz, 2H),
7.72–7.27 (m, 10H), 3.86 (m, 8H, −OCH2), 3.24 (m, 8H, −NCH2); 13C NMR (75 MHz,
CDCl3): δ 195.97 (C=O), 154.67, 154.14, 151.78, 151.66, 142.88, 139.89, 137.58, 137.41,
132.65, 132.32, 131.07, 130.86, 130.12, 130.00, 129.83, 129.36, 129.19, 128.45, 114.71,
114.60, 66.73(−O(CH2)2), 48.50 (−N(CH2)2).
MALDI-TOF: mass calcd for C35H32N4O3 [M+]: 556.25, found: 556.25; FT−IR
(KBr, νmax cm-1): 2921, 1647, 1596, 1237, 1123, 928, 692; elemental anal. calcd for
C35H32N4O3 :
8.00.
C 75.52, H 5.79, N 10.06, O 8.62 found: C 75.68, H 5.88, N 10.44, O

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2.3 Instrumentation and methods
UV-visible spectra were recorded in 10‒6 mol L‒1 solutions in a 1 cm path length quartz
cuvette as well as the neat solid films on a SHIMADZU UV-2401PC instrument at room
temperature. The neat solid films of compounds 2–9 were prepared by using a spin coater
(Holmarc
HO-TH-05) at 1000 rpm for 2 min ∼6 mg mL‒1 of the sample in
chloroform. Quartz plates were used for the neat solid film studies. The excitation and
emission spectra were carried out on a Perkin Elmer LS 55 Fluorescence spectrophotometer.
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 temperature in
anhydrous dichloromethane with tetra-butyl ammonium hexafluorophosphate solution (0.1
M) as the supporting electrolyte with a scan rate of
100 mVs‒1. The potential of the
Ag/AgCl reference electrode was calibrated by using a ferrocene/ferrocenium redox couple,
which has the known oxidation potential of +5.1 eV. The thermogravimetric analysis (TGA)
were performed using a Metler Toledo instrument (TG) under nitrogen atmosphere. 1H NMR
13
spectra and C NMR spectra were recorded using CDCl3 on Varian 300 MHz Ultrashield
spectrometer with tetramethylsilane (TMS) as internal reference at working frequency 300
MHz and 75 MHz respectively. Fourier trans-form infrared (FT–IR) spectra were recorded on
a Perkin Elmer Frontier 91579. Mass spectrometric measurements were recorded using
MALDI–TOF (Bruker) and elemental analysis was carried out on EA Euro–elemental
analysis instrument. To confirm formation of nanoparticle in AIE studies, sample of
synthesized dye subjected to field emission gun scanning electron microscopy (JEOL JSM-
7600F FEG−SEM).
2.4 Preparation of Nanoaggregates

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Stock THF solution of the dyes 2, 3 and 7 with concentration 10-4 M were prepared. Aliquots
of stock solution were transferred to 5 mL volumetric flask. After appropriate amount of THF
were added, water was added drop wise under vigorous shaking to obtain 10-5 M solution
with different water contents (0−100 vol %). The photoluminescence (PL) measurement of
resultant solution were recorded. Also, size and morphology of nanoparticles of dye 2 was
observed by field emission scanning electron microscopy (FEG−SEM).
2.5 Theoretical Calculation
DFT and time dependant density functional theoretical (TD–DFT) calculations were
performed using Gaussian 03 software package using B3LYP as exchange correlation
functional at
6–311G, 6–311G + G* and 6–311G + G** basis set.
3. Results and Discussion
3.1 Photophysical Studies
Absorption and photoluminescence (PL) spectra of synthesized molecules were recorded in
increasing polarity of solvents (toluene, chloroform, dichloromethane, and dimethyl
sulphoxide) to compare their photophysical properties. The UV‒Vis absorption spectra of
compounds 2−9 in toluene and chloroform are presented in Figure 2 while pertinent data
summarized in Table 1.

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Figure 2. UV−Vis absorption spectra of compounds 2−9 in toluene (a) and chloroform (b)
(for other solvents and neat solid film see supporting information).
On comparison of optical properties of 2−9 with the parent 2,3-bis(4-
bromophenyl)quinoxalin-6-yl)phenyl methanone (1) (see supporting information for
absorption spectra of 1) have observed that in case of compound 1 absorption peaks at 279–
334 nm arises due to
overlapping of n‒π* and π‒π* transitions whereas absorption
maxima at 360−364 nm could be attributed to inbuilt D–A system where phenyl rings joined
by single bond to quinoxaline core.
Table 1. Absorption data of 1‒9.
a
λabsa nm, ( log εmax M−1cm−1)
λabs
Compd
Toluene
CHCl3
DCM
DMSO
Solid Film
278 (5.02),
293 (4.99),
364 (4.94)
259 (5.73),
364 (5.23)
259 (5.28)
361 (4.78),
256 (5.64),
331(5.04),
360 (5.18)
262, 373
327, 480
1
2
256 (5.48),
305 (5.51),
441 (5.17)
259 (5.33),
296 (5.35),
434 (4.95)
251 (5.30),
305 (5.29),
435 (4.92)
286 (5.73),
431 (5.00)
286 (5.65),
337 (5.05),
430 (4.93)
238 (5.59),
260 (5.60),
436 (5.14)
252 (5.51),
312 (5.29),
427 (5.06)
260 (5.22),
427 (4.80)
273, 377,
487
3
4
5
243 (5.08),
291 (5.32),
446 (4.94)
288 (5.40),
332 (4.93),
411 (5.29)
259 (5.53),
292 (5.54),
456 (5.18)
239 (5.39),
257 (5.35),
416 (5.32)
258 (5.14),
288 (5.12),
447 (4.73)
251 (5.53),
294 (5.47),
453 (5.12)
251 (5.15),
314 (4.62),
412 (5.11)
361, 501
260, 460
259 (5.23),
417 (5.19)
279 (5.25),
306 (5.21),
435 (5.36)
264 (5.46),
296 (5.39),
442( 5.41)
230 (5.41),
306 (5.27),
439 (5.31)
258 (5.22),
306 (5.18),
441 (5.22)
269, 485
296, 386
6
287 (5.94),
339 (5.05),
390 (4.94)
243 (6.07),
291 (5.80),
389 (5.29)
274 (6.20),
339 (5.11),
385 (5.00)
252 (5.36),
293 (5.14),
377 (4.92)
7
8
232 (4.78),
289 (5.22),
408 (4.50)
251 (6.03),
307 (5.28),
402 (4.41)
258 (6.27),
410 (5.07)
258 (5.64),
397 (4.41)
274, 371,
447

ACCEPTED MANUSCRIPT
238 (5.42),
259 (5.40),
261 (5.48),
416 (5.01)
415 (5.03)
251 (5.51),
377 (5.03),
425 (5.09)
285 (5.79),
418 (4.79)
263, 434
9
aRecorded in10-6 M solution
Similarly, the absorption spectra of 2−9 show high absorption band between 232–339 nm in
different polarity of solvents corresponds to n‒π*, π‒π* transitions. These high energy
transitions obtained due to entire molecule. The lower energy transition band found in the
range of 377−456 nm attributed to intramolecular charge transfer (ICT) transition from
electron donating triarylamine, heterocyclic and aliphatic amines to electron accepting
quinoxaline core. These ICT transitions show considerable red shift for derivatives 2−9 due
to modified D−A architecture as compared to 1.
Almost all the synthesized molecules in different solvents show similar pattern of
absorption spectra with varying intensities and display better absorption and high molar
absorptivity. The absorption spectra show that lowest intensity ICT band are more sensitive
to donor moiety [33]. The bathochromic shift of 16−26 nm in ICT transition has observed in
case of 4 attributed to presence of electron donating ‒OCH3 group on triarylamine whereas
the blue shift of around 19−25 nm in 5 corresponds to presence of electron withdrawing ‒
NO2 group on triarlyamine moiety. However, heterocyclic amine like carbazole,
phenothiazine and morpholine show hypsochromic shift of 39−58 nm in 7, 22−37 nm in 8
and 10−26 nm in 9 in ICT transitions compared to triphenylamine derivatives. In all the
solvents, dye 7 display more blue shifted ICT band than 8 could be due to the weak electron
donating strength of carbazole as compared to phenothiazine. This confirm the fact that extra
sulfur atom present in the phenothiazine ring makes it strong electron donating moiety than
carbazole [34]. Further, the absorption of 2−9 in neat solid film showed significant red shift
of 9−60 nm in absorption maxima with broadening of peaks attributed to the molecular
aggregation in molecules [35].

ACCEPTED MANUSCRIPT
The other photophysical factors such as emission data, optical band gap, stokes shift
and quantum yield were calculated and summarized in Table 2. Compound 2−9 emit in blue‒
orange region with emission maxima 472–592 nm on excitation at their respective ICT
maxima in toluene, chloroform, DCM and neat solid film. (Figure 3)
Compound 2−9 display blue‒green fluorescence with emission maxima (λEmax) of
472−552 nm in toluene. The occurrence of positive solvatochromism on using polar protic
solvents such
Figure 3. Emission spectra of 2−9 in toluene (a), neat solid film (b) (for other solvents and
neat solid film of 1 see supporting information).
as chloroform and DCM witnessed by red shift of 5−53 nm and 21−68 nm in emission
correspondingly drawn fluorescence to yellow‒orange region in polar solvents (Figure 4).

ACCEPTED MANUSCRIPT
Figure 4. Fluorescence image of 1−9 in toluene and Chloroform (10-6 M) under 365 nm UV
illumination.
This noteworthy solvatochromism, in the emission spectra attributed to polarized excited
state. The enhanced dipole moment of the polarized excited states due to charge transfer
transitions stabilized with reorientation of solvent molecule. This process of reorientation
helps solvent molecule to achieve high dipole moment, thereby reduces energy of system
leading to bathochromic shift in emission [36].
The emission spectra presented in Figure 3 (a), show compound 4 in toluene own
bathochromic shift of 41 nm due to presence of electron donating ‒OCH3 group on
triarlyamine moiety. On the other hand, it has observed that molecule 7 having carbazole as a
weak donor unit display fluorescence with blue shift of 39−58 nm in toluene, chloroform and
DCM. No emission was observed for compound 6 and 8 in all of the solvents. Such non‒
radiative relaxation in 6 could be due to presence of electron withdrawing diazene moiety
whereas as quenching of emission in 8 could be attributed to photo induced electron transfer
(PET) from strong donor phenothiazine to acceptor quinoxaline core leading to charge

ACCEPTED MANUSCRIPT
separated state [37,38]. The possibility of photo induced electron transfer in 8 were further
validated by density functional theory in theoretical discussion. However, dye 4 and 5
bearing polar functional group show quenching in emission in more polar solvents like
chloroform and DCM. Furthermore, dyes 2−9 does not emit in DMSO. Such quenching of
luminescence in polar solvent indicates manifestation of nonradiative relaxation from excited
state to ground state [39].
The larger stokes shift in 2−9 indicates that the origin of emission is mainly based on
intramolecular charge transfer between donor and acceptor unit and ensures that there is no
reabsorption of the emitted photons. Further increase in stokes shift in more polar solvent like
chloroform and DCM suggest interaction between charge transfer complex and polar solvents
[40]. The lone pair of electrons in non
in dye molecules interact strongly with polar solvents, leads to stabilization of polarized
excited state by polar solvent. Owing to this decrease in band gap for 2 9 has observed on
−bonding orbitals of species like carbonyl and amines
−
increase in polarity of solvent from toluene (2.17−1.94 eV) to chloroform and DCM
(2.06−1.83 eV).
The quantum yield of 2−9 were calculated using fluorescein (Ф = 0.79 in 0.1 N NaOH) as
reference and are presented in Table 2. The quantum yield of these dyes are in the range
0.15‒0.52 in toluene and were found to be decreased due to presence of electron donating (‒
OCH3)
Table 2. Emission data, Stokes shift, Optical band gap and Quantum yield of 1−9.
a
λemi , nm,
Thin
Film
Stokes shift, cm-1,
Solid
Film
(ꢇF)c
( ꢀꢁꢂꢃꢄ, ꢅꢆ)b
Compd
Toluene CHCl3
DCM
Toluene CHCl3 DCM
−
−
−
−
−
−
−
−
1
2
515
(2.09)
559
(1.91)
575
(1.88)
541
3784
(0.46)
4786
(0.20)
5650
(0.06)
2349

ACCEPTED MANUSCRIPT
511
(2.09)
552
(1.94)
518
554
(1.92)
557
(1.90)
−
562
(1.91)
−
542
592
547
−
3686
(0.50)
4336
(0.17)
5025
4883
(0.19)
3976
(0.08)
−
5625
(0.03)
−
(0.01)
5427
(0.01)
−
2083
3068
3457
−
3
4
5
6
7
8
9
−
−
(1.99)
(0.15)
−
−
−
−
472
(2.17)
525
(2.06)
540
(2.00)
−
480
−
588
4454
(0.52)
−
6650
(0.42)
−
7446
(0.38)
−
5073
4765
4990
−
−
521
(2.07)
548
(1.97)
559
(1.83)
554
3585
(0.35)
5850
(0.16)
6141
(0.06)
aRecorded in 10-6 M solution, Optical band gap (in parentheses) estimated using emission
b
and
ꢉꢊꢋꢌ.ꢍ
excitation spectra ꢈꢀꢁꢂꢃꢄ
=
ꢐ ꢅꢆ. Quantum yield (in parentheses) with reference to
ꢂꢃꢄꢅꢏꢁꢅ
c
ꢎ
fluorescein (ꢑ = 0.79 in 0.1 M NaOH).
or withdrawing (‒NO2) groups on diarylamine moiety in 4 and 5 respectively. Further,
compound 2−9 showed red shift in emission with decrease in quantum yields on increase in
solvent polarity can be explained by energy gap law, which says that, internal conversion
efficiency of excited state and luminescence is reduced with decrease in energy gap between
ground and excited states.
Almost all the compounds showed more or less intense emission in neat solid film except
6. This quenching in emission of 6 could be due to aggregation-based quenching (ACQ) in
solid state by fast interchain electron transfer from donor to acceptor subunit via close spatial
contact [41]. However, the bathochromic shift in emission of around 8−40 nm for other
derivatives in neat solid film was observed as compared to toluene. It could be attributed to
intermolecular aggregation in the solid state. Amazingly, some of the derivatives (2, 3, 5 and
7) emits intensely in the neat solid film. These results revealed the existence of aggregation
induced emission (AIE) effect [42]. Hence, we have studied AIE properties of some dyes in
detail for better understanding.
3.2 AIE Phenomenon

ACCEPTED MANUSCRIPT
To evaluate AIE characteristic, we chose molecule 2, 3 and 7 which show intense emission in
neat solid film. Figure 5 show fluorescent image of 2, 3 and 7 dissolved in different
THF/water mixtures (0‒100%). The photoluminscence (PL) spectra of these molecules were
recorded in different THF/water fractions for the comparision of PL intensities and presented
in Figure 6. The synthesized molecules are highly soluble in THF but insoluble in water.
Hence, we prepared nanoaggregates of luminophores 2, 3 and 7 by increasing percentage of
water in THF solution of molecules. This is the simple precipitation method reported to
prepare nanoparticles without surfactants [43].
All the three luminophores show emission in 100% THF due to intramolecular charge
transfer from the donor amines to acceptor quinoxaline moiety. The more or less intensity of
fluorescence in pure THF is attributed to chromophores attached as donor unit in D−A
system. It has observed that, luminophore 3 and 7 show large intensity in 100% THF whereas
luminophore 2 possess comparatively less fluorescence in pure THF which rapidly increases
on aggregation formation. This phenomenon could be explained on the basis of
conformational stability of the molecule. The presence of bulky group like napthyl phenyl
and closed ring in

ACCEPTED MANUSCRIPT
Figure 5. Photograph of 2 (a), 3 (b) and 7 (c) in THF‒water mixture with subsequently
increasing water fractions (10-5 M) by 10% under UV‒Visible light.

ACCEPTED MANUSCRIPT
carbazole hinders the intramolecular rotations, making luminophore 3 and 7 intrinsically
fluorescent in the solution state while the less fluorescence of 2 in pure THF could be
attributed to intramolecular twisting by rotatable phenyl ring in diphenyl amine [44]. Later,
on addition

ACCEPTED MANUSCRIPT
Figure 6. Fluorescence spectra of 2 (a), 3 (b) and 7 (c) (10-5 M) in THF−water mixture with
increasing water fractions (ƒw) and 2 (A), 3 (B) and 7 (C) plots of PL intensity and
peak wavelength versus ƒw.
of water in THF solutions of luminophores 2, 3 and 7, quenching in emission has found for
mixtures with gradual increase of water up to 50% water content (ƒw) due to insolubility of
luminophores in water. The improved PL intensity of luminogen after 50% (ƒw) validate
formation of nanoaggregates. In the aggregate form originated restricted intramolecular
rotation (RIR), owing to physical constraint blocks the non-radiative pathway and activate
radiative channel, enabling the molecules to emit strongly [45]. Thus, molecules 2, 3 and 7
emits strongly in aggregate form. Luminophore 2 and 3 emits yellow‒green fluorescence
whereas luminophore 7 displays blue‒green fluorescence in 60 to 90 % of water fraction (ƒw).
This distinct red shift or blue shift in aggregation induced emission (AIE) were probably
attributed to morphological changes in aggregates due to different conformations and packing
modes [46].
The AIE behavior of 2, 3 and 7 were further explored using absorption spectroscopy.
The level of tails at longer wavelength of absorption spectra of luminophores above 50% ƒw
in THF/water mixture was caused due to Mie effect or light scattering by the nanoparticles
formed during aggregation. This verifies presence of AIE activity in molecules [47]. Thus,
molecules 2, 3 and 7 can also termed as AIEgen. (for Absorption spectra see supporting
information)
Further, the size and morphology of nanoparticles of luminophore 2 formed with
non‒solvent water in THF determined by field emission scanning electron microscopy
(FEG−SEM). Figure 7 show that nanoparticles of luminophore 2 obtained in 70% water
fraction (ƒw) are spherical in shape with a mean diameter of around 30−50 nm. The
nanoparticle suspension of 2, 3 and 7 formed on increasing water fraction after 60% ƒw are

ACCEPTED MANUSCRIPT
transparent and stable for longer period without precipitation. Similarly, the past reports have
witnessed long period stability of many organic nanoparticles bearing carbonyl group in
surfactant free water mixtures. This outstanding stability of organic nanoparticle suspension
accredited to polar groups such as carbonyl which induces electronic repulsion and
apparently prevents flocculation/coalescence of nanoparticles in water [48].
Figure 7: SEM image of nanoparticles of 2 obtained in nanoparticle suspension containing
70% volume fraction of water (ƒw) in THF.
3.3 Electrochemical Properties
The HOMO−LUMO energy levels and electrochemical band gap are the important aspects of
organic molecules for their application in organic electronics. Hence, the electrochemical
behaviour of parent 1 and its derivatives 2−9 investigated by cyclic voltammetry in DCM
with ferrocene as an internal standard to calibrate the redox potential and the pertinent data

ACCEPTED MANUSCRIPT
preserved in Table 3. The cyclic voltammogram of 4 and 5 are presented in Figure 8 (for
other compounds see supporting information).
Figure 8. Cyclic voltammogram (full scan) of compound 4 and 5 measured in anhydrous
dichloromethane (for other compounds see supporting information).
On anodic sweep, for parent 1 no oxidation peak was observed. Further, the substitution with
different chromophoric group as a donor in 2−9 extends the D−A interaction and improves
the oxidation peaks. In cyclic voltammogram of 2−9 all the dyes exhibited two quasi
reversible oxidation wave except dye 4, 8 and 9. Dye 4 and 8 display one reversible and one
quasireversible oxidation wave. These two oxidation peak in 2−8 attributed to oxidation of
triarylamine moiety in 2−6 and oxidation of carbazole and phenothiazine moiety in 7 and 8
respectively corresponds to two electron processes while a single quasireversible wave was
observe for dye 9 assigned to oxidation of aliphatic morpholine. It has found that the
oxidation potential is sensitive to nature of donor group and Eoxi potential of 2−9 decreases on
increasing donor strength (4 > 8 > 2 > 9 > 3 >7 > 6 > 5).
Further, on cathodic sweep all the derivatives along with parent 1 predominately show two
quasi reversible waves corresponding to reduction of quinoxaline core and carbonyl group
present in acceptor unit of designed D−A system. Beside this, compound 5 and 6 display one

ACCEPTED MANUSCRIPT
additional reduction peak could be assigned to reduction of ‒NO2 group and diazene segment
attached to triarylamine moiety respectively.
Table 3. Electrochemical and thermal data of 1‒9.
HOMOc
LUMOd
Td (0C)f
e
ꢀꢁꢀꢗ
ꢃꢅꢔꢕ
ꢂꢒꢓ
ꢃꢅꢔꢕ
ꢀ
ꢖꢅꢏ
a
b
Compd
ꢀ
−
‒1.02, ‒1.39
−
‒3.71
‒3.55
‒3.68
‒3.72
‒3.74
‒3.68
‒3.64
‒3.72
‒3.70
−
274 (312)
432 (480)
223 (487)
432 (505)
200 (401)
258 (370)
451 (510)
225 (398)
242 (305)
1
2
3
4
5
6
7
8
9
0.92, 1.12 ‒1.16, ‒1.46
1.12, 1.31 ‒1.05, ‒1.45
0.82, 1.44 ‒1.02, ‒1.51
‒5.45
‒5.63
‒5.30
1.90
1.95
1.58
2.09
1.98
1.92
1.51
1.88
1.35, 1.44 ‒1.00, ‒1.26, ‒1.54 ‒5.83
1.14, 1.65 ‒1.04, ‒1.24, ‒1.49 ‒5.66
1.11, 1.45 ‒1.12, ‒1.43
0.81, 1.53 ‒1.05, ‒1.37
‒5.56
‒5.23
‒5.58
1.08
‒1.06, ‒1.51
aEꢘꢛꢙꢜꢚꢝꢞoxidation peak potential (V). bEꢟꢛꢜꢜꢠꢝꢞreduction peak potential (V). cHOMO energy level
calculated from EHOMO = −[ Eꢛꢜꢝꢞ– Eredox(Fc/Fc+) + 5.1 ] eV. dLUMO energy level calculated
ꢘꢙꢚ
ꢛꢜꢝꢞ
e
from ELUMO = −[E
– Eredox(Fc/Fc+) + 5.1] eV. Eꢡꢢꢣcalculated from Eꢡꢢꢣ
=
ꢟꢜꢠ
f
[HOMO ‒ LUMO] eV. Decomposition temperature at 5% and 10% (in parentheses) weight
loss derived by TGA.
The first potential of this oxidation and reduction waves was used to calculate HOMO
and LUMO energy levels of 1−9 respectively. The HOMO energy level found in the range of
‒5.23 to ‒5.83 eV whereas LUMO energy level lies is in the range of ‒3.55 to ‒3.74 eV for
compound 2−9. According to previous reports on charge transporting properties of
molecules, ambipolar transport arises when molecules have HOMO energy level below or
equal to
‒5.0 eV and LUMO levels less than ‒3.15 eV [49,50]. Also, the HOMO
and LUMO values reported for ambipolar materials in literature for small molecules are
6,13-bis(triisopropylsilylethynyl)-1,4,8,11-tetraazapentacene (TIPS-PEN) (HOMO = −5.49