Hydroxy-benzimidazoles as blue-green emitters: Synthesis, structural and DFT studies

Article abstract of DOI:10.1016/j.jphotochem.2020.112751

The new benzimidazole ligands (3a-e) were synthesized using ethyl 4-(butylamino)-3-nitrobenzoate and substituted salicylaldehyde in the presence of sodium dithionite reagent which undergoes “one-pot’’ nitro reductive cyclization. Here benzimidazole moiety acts as an electron-acceptor (A) whereas substituted 2-hydroxyphenyl moiety acts as an electron-donor (D) unit. The molecular structures were characterised by FTIR, ¹H NMR, ¹³C NMR, single crystal XRD and MS analysis. Optoelectronic properties were determined by UV–vis, solution and solid photoluminescence, quantum yield and lifetime. The solvent-dependent absorption and emission were studied using both polar protic and polar aprotic solvents. All the derivatives exhibited ESIPT, especially compound ethyl 1-butyl-2-(3,5-dichloro-2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (3e) displayed dual emission in both polar protic and polar aprotic solvents. The compound stability and electrochemical property were determined by thermal gravimetric analysis (TGA) and cyclic voltammetry (CV) respectively. The compounds emit intense blue-green fluorescence with high to moderate quantum yield. Also, these derivatives exhibited a high Stokes shift. The computational studies like Density-Functional Theory (DFT) and Molecular Electrostatic Potential (MEP) were conducted to provide important insights into the structure-property relationships. The crystal packing is stabilized through intermolecular hydrogen bonds (C—H…O) and intermolecular interactions (π… π). The findings of results help in developing novel ligands in the field of organic optoelectronics.

Full text of DOI:10.1016/j.jphotochem.2020.112751

Journal Pre-proof
Hydroxy-Benzimidazoles as Blue-Green Emitters: Synthesis, Structural
and DFT Studies
Reshma Sathyanarayana, Vasantha Kumar, G H Pujar, Boja
Poojary, Madan Kumar Shankar, Sangappa Yallappa
PII:
S1010-6030(20)30550-5
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112751
JPC 112751
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
29 February 2020
27 June 2020
4 July 2020
Please cite this article as: Sathyanarayana R, Kumar V, H Pujar G, Poojary B, Shankar MK,
Yallappa S, Hydroxy-Benzimidazoles as Blue-Green Emitters: Synthesis, Structural and DFT
Studies, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112751
This is a PDF file of an article that has undergone enhancements after acceptance, such as
the addition of a cover page and metadata, and formatting for readability, but it is not yet the
definitive version of record. This version will undergo additional copyediting, typesetting and
review before it is published in its final form, but we are providing this version to give early
visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal
pertain.
©
2020 Published by Elsevier.

Hydroxy-Benzimidazoles as Blue-Green Emitters: Synthesis, Structural and
DFT Studies
a
b
c
a,*
d
Reshma Sathyanarayana , Vasantha Kumar , G H Pujar , Boja Poojary , Madan Kumar Shankar , Sangappa
Yallappae
aDepartment of Chemistry, Mangalore University, Mangalagangothri-574199, Karnataka, India. E-Mail:
bojapoojary@gmail.com. Tel.: +91 824 2287262; Fax: +91 824 2287367;
bDepartment of Chemistry, Sri Dharmasthala Manjunatheshwara College (Autonomous), Ujire, Karanataka –
5
74240;
cResearch Centre, Department of Physics, GM Institute of Technology, Davangere, Karnataka, India-577006
dIOE, Vijnana Bhavan, University of Mysore, Mysuru, India 570006
e
Department of Physics, Mangalore University, Mangalagangothri-574199, Karnataka, India.
Highlights

Five novel bipolar benzimidazole derivatives (3a-e) were synthesized via “one-pot”
nitro reductive cyclization as blue-green fluorescent emitters.
Simple and cost-effective synthetic routes. Products obtained with high yield.
All derivatives exhibited high stokes effect, high to moderate quantum yield and good
thermal stability.




All the derivatives exhibit ESIPT process
Computational studies like DFT, MEP were studied
Abstract
The new benzimidazole ligands (3a-e) were synthesized using ethyl 4-(butylamino)-3-
nitrobenzoate and substituted salicylaldehyde in the presence of sodium dithionite reagent
which undergoes “one-pot’’ nitro reductive cyclization. Here benzimidazole moiety acts as an
electron-acceptor (A) whereas substituted 2-hydroxyphenyl moiety acts as an electron-donor
1
13
(
D) unit. The molecular structures were characterised by FTIR, H NMR, C NMR, single
crystal XRD and MS analysis. Optoelectronic properties were determined by UV-vis, solution
and solid photoluminescence, quantum yield and lifetime. The solvent-dependent absorption
and emission were studied using both polar protic and polar aprotic solvents. All the derivatives
exhibited ESIPT, especially compound ethyl 1-butyl-2-(3,5-dichloro-2-hydroxyphenyl)-1H-
benzo[d]imidazole-5-carboxylate (3e) displayed dual emission in both polar protic and polar

aprotic solvents. The compound stability and electrochemical property were determined by
thermal gravimetric analysis (TGA) and cyclic voltammetry (CV) respectively. The
compounds emit intense blue-green fluorescence with high to moderate quantum yield. Also,
these derivatives exhibited a high Stokes shift. The computational studies like Density-
Functional Theory (DFT) and Molecular Electrostatic Potential (MEP) were conducted to
provide important insights into the structure-property relationships. The crystal packing is
stabilized through intermolecular hydrogen bonds (C---H...O) and intermolecular interactions
(π… π). The findings of results help in developing novel ligands in the field of organic
optoelectronics.
Key words: Benzimidazole; Photoluminescence; DFT; Cyclic voltammetry; MEP; Quantum
yield; Fluorescence lifetime; ESIPT
Introduction
Before organic light-emitting diodes (OLEDs) came to light, various display technologies such
as inorganic light-emitting diodes, liquid crystal displays, plasma displays and cathode ray
tubes were top in the market. Due to many drawbacks of these display technologies, OLEDs
emerged in present generation display technology with superior characteristics to overcome the
limitations like colour tunability, bulkiness and low viewing angle [1]. OLEDs are thin, flat
with large surface area, lightweight, flexible and bendable. It is possible to attain unbroken
light emission over a large surface area due to the organic nature of OLEDs. Energy saving,
low weight, brightness, reduction in thickness, large viewing angle, reduction in cost, colour
reproduction and low power consumption are some of the key advantages of OLEDs [2]. These
have also emerged in the field of commercial products such as digital cameras and cell phones
[
3]. Hence a great deal of work has been carried out in the development of visible light-emitting
OLEDs. The efficacy of fluorophores boosted when they undergo excited state intramolecular
proton transfer (ESIPT) [4, 5]. The molecules exhibiting ESIPT process have been used as
polymer photo-stabilizer [6], fluorescent probes [7], laser dyes [8] and light-emitting materials
for electroluminescent devices [9].
One of the major problems faced by the organic and polymer chromophore is the high emission
property in dilute solution but weak luminescent in solid state [10]. This is due to the
aggregation-caused quenching (ACQ), where aggregation of molecules takes place resulting in
less emission. But, for the practical applications luminophores should exist in solid films.
Henceforth, few researchers worked on aggregation-induced emission (AIE), which is the

opposite of ACQ where molecules will be luminesce in aggregation state rather than in solution
state [11-13].
Apart from benzimidazole moiety being a bioactive molecule in the pharmaceutical field [14-
1
8], this moiety was also found to display exciting fluorescent and photophysical properties
[
19-21]. One of the best methods to enhance fluorescence properties, device efficiency and
operational lifetime is to incorporate donor/acceptor strategy. This in turn depends on the
number of holes (HT) and electron transporting (ET) moieties [22, 23]. The electroluminescent
materials with high emission quantum yield, high glass-transition temperature and balance of
holes and electrons are successfully employed in single-layer OLEDs [24]. The design and
synthesis of small bipolar molecules combining both electron-donating and electron-
withdrawing groups in the same molecule enhance the fluorescence efficiency. Electron-
donating moieties such as carbazole and diphenylamine act as a hole-transporting component
whereas electron-withdrawing moieties such as benzimidazole, 1,3,4-oxadiazole, phosphine
oxide and triazine behave as electron – transporting acceptors [25].
Benzimidazole also played a significant role in the field of coordination chemistry as this
moiety is capable of forming stable complexes with metal ions. Due to the presence of a high
electron density zone, benzimidazole also behaves as a corrosion inhibitor [26]. This moiety
also made an impact in the field of flame retardants, liquid materials, CO
supramolecular chemistry [27].
2
chemosensors and
Given these findings, novel bipolar benzimidazole derivatives 3a-e as blue-green emitting
fluorescent material have been synthesized by inserting benzimidazole as an electron acceptor
(A) and substituted 2-hydroxyphenyl as an electron-donor (D) unit (Fig. 1.). The combination
of electron-donating and electron-withdrawing in the single molecular framework results in
strong intramolecular charge transfer. The blue-emitting fluorescent material can be used both
as blue-emitting OLEDs as well as host for white and other colours [28]. The optoelectronic
properties such as UV-vis/fluorescence spectra; thermal property like thermogravimetric
analysis (TGA); computational studies such as HOMO-LUMO calculations and molecular
electrostatic potential (MEP); electrochemical property by cyclic voltammetry (CV) have been
studied and analysed. Furthermore, the solvatochromic behaviour of the title compounds was
examined to study the strong intramolecular charge transfer. In addition, quantum yield and
fluorescence lifetime of the derivatives were also studied. The findings of these studies help in
discovering suitable OLEDs.

Fig. 1: Chemical structures of synthesized target molecules 3a-e
Experimental
Materials and methods
The reagents used for the synthesis of this series were analytical grade and purchased from S.
D. Fine, Sigma-Aldrich and Spectrochem, which were used directly without further
purification. Only the analytical solvents were used during the experiments. The completion of
the reaction was determined by thin-layer chromatography plate from Merck. Recrystallization
was used as a purification method either in dimethylformamide or ethanol as a solvent.
Procedure for the synthesis of ethyl-4-chloro-3-nitrobenzoate (1)
To a solution of 4-chloro-3-nitrobenzoic acid (10 g, 49.6 mmol) in ethanol (100 mL) added a
catalytic amount of conc. H
2
SO and heated to reflux for 16 h. The completion of the reaction
4
was monitored by TLC. After the completion of the reaction, excess of solvent was removed
under high vacuum and the residue was poured into ice-cold water. The precipitate formed was
filtered and dried to afford intermediate 1 as white solid. Yield: 92%
Procedure for the synthesis of ethyl 4-(butylamino)-3-nitrobenzoate (2)
Ethyl-4-chloro-3-nitrobenzoate (10.5 g, 45.8 mmol) was taken in dry THF. To this mixture
butylamine (45.8 mmol) and TEA (45.8 mmol) were added at room temperature and stirred

overnight at the same temperature. The completion of the reaction was monitored by thin-layer
chromatography. Excess of solvent was removed under a high vacuum and the residue left was
added into ice-cold water. The precipitate formed was filtered, dried and recrystallized from
ethanol to yield intermediate 2 as yellow solid. Yield: 82%
1
Yellow solid; Yield: 82%; M.p.: 70-72 °C; H NMR (400 MHz, DMSO-d6, δ ppm ): 0.98-1.019
(
t, 3H, J = 12 Hz, 4-CH
3
), 1.37-1.40 (t, 3H, J = 12 Hz, ester CH
.70-1.76 (quint, 2H, 2-CH ), 3.33-3.38 (q, 2H, J = 4 Hz, ester CH
.85-6.87 (d, 1H, J = 8 Hz, H of benzyl), 8.061-8.067 (d, 1H, J = 2.4 Hz, H
3
) 1.46-1.52 (quint, 2H, 3-CH
2
),
),
1
6
2
2
) 4.33-4.38 (t, 2H, 1-CH
2
5
6
of benzyl), 8.33
of benzyl) ppm; C NMR (100 MHz): δ =
65.4, 147.9, 136.5, 131.3, 129.6, 117.5, 113.6, 61.2, 43.1, 31.08, 20.3, 14.5, 13.9; MS:
1
3
(bs, 1H, NH), 8.872-8.877 (d, 1H, J = 2 Hz, H
2
1
+
calculated for C13
H
18
N O
2 4
is 266.13; found [M+H] 267.15 m/z; Anal. calcd. for C13
H
18
N
2
4
O :
C, 58.63; H, 6.81; N, 10.52%. Found: C, 58.60; H, 6.79; N, 10.50%.
General procedure for the synthesis of ethyl 1-butyl-2-(2-hydroxyphenyl)-1H-
benzo[d]imidazole-5-carboxylate (3a-e)
To a stirred solution of intermediate 2 (0.5 g, 19.8 mmol) in dry DMSO (5 mL), substituted
salicylaldehyde (19.8 mmol) and sodium dithionite (39.6 mmol) was added and heated to 90
°
C with stirring for 3 h. After completion of the reaction, the reaction mass was poured into
ice-cold water. The precipitate formed was filtered, dried and recrystallized from DMF to
afford the target compound 3a-e. Yield: 90-95%.
Ethyl 1-butyl-2-(2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (3a)
Off white solid; yield: 92%; melting point (°C): 98-100; FT IR (ATR, υmax/cm): 3477 (-OH),
1
1
3
4
7
701 (C=O of ester), 1292 (C-O of ester). H NMR (400 MHz, DMSO-d
6
): δ = 0.99-1.03 (t,
), 1.93-2.01 (quint, 2H, 2-CH ),
), 6.96-6.99 (m, 1H, H of 2-hydroxy-phenyl)
of 2-hydroxy-phenyl), 7.36-7.44 (m, 1H, H4, of 2-hydroxy-
of benzimidazole), 8.05-8.07 (d, 1H, J = 8 Hz, H of
benzimidazole) ppm; C NMR (100 MHz): δ = 13.8, 14.6,
0.2, 32.0, 46.0, 61.2, 109.7, 113.0, 118.6, 119.0, 121.2, 125.0, 125.7, 126.7, 132.1, 138.4,
40.2, 153.0, 159.3, 167.0 ppm; MS: calculated for C20
is 338.16; found [M+H]+
39.05 m/z; Anal.calc. for C20 : C, 70.99; H, 6.55; N, 8.28%. Found: C, 70.95; H, 6.53;
H, J = 16 Hz, 4-CH
.39-4.45 (q, 4H, J = 8 Hz, ester CH
.15-7.17 (d, 1H, J = 8 Hz, H
3
), 1.44-1.50 (m, 5H, ester CH
3
and 3-CH
2
2
2
and 1-CH
2
3
5
H
6
phenyl), 7.61-7.63 (d, 1H, J = 8 Hz, H
6
7
13
benzimidazole), 8.47 (s, 1H, H
4
2
1
3
H
22
N
2
O
3
H
22
N
2
O
3
N, 8.20%; Uv-Vis: λmax (ACN) = 322 nm, Fluorescence: λex = 322 nm, λem (ACN) = 474 nm, Φ
0.14.
=

Ethyl 1-butyl-2-(5-fluoro-2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (3b)
Off white solid; yield: 90%; melting point (°C): 146-148; FT IR (ATR, υmax/cm): 3412 (-OH),
1
1
1
2
715 (C=O of ester), 1021 (C-F), 1215 (C-O of ester); H NMR (400 MHz, DMSO-d
6
): δ =
), 1.95-1.99 (quint,
), 7.10-7.12 (dd, 2H, H and H
of benzimidazole), 7.43-7.45 (d, 1H, J = 8 Hz,
of benzimidazole), 8.07-8.073 (dd, 1H, J = 1.2 Hz, H of benzimidazole), 8.472-8.474 (d,
.007-1.04 (t, 3H, J = 13 Hz, 4-CH
3
), 1.42-1.47 (m, 5H, ester CH
3
and 3-CH
2
H, 2-CH ), 4.40-4.45 (q, 4H, J = 8 Hz, ester CH
2
2
and 1-CH
2
3
4
of 5-fluoro-2-hydroxyphenyl), 7.32-7.35 (m, H
7
H
8
5
1
3
1
2
1
H, J = 0.8 Hz, H of 5-fluoro-2-hydroxyphenyl) ppm; C NMR (100 MHz): δ = 13.7, 14.6,
6
0.2, 32.0, 45.9, 61.2, 109.8, 112.7, 112.75 (J = 40 Hz), 119.0 (J = 92 Hz), 119.6 (J = 32 Hz),
21.4, 125.3, 126.0, 138.3, 140.0, 152.0, 154.3, 156.1 (J = 480 Hz), 166.94 ppm; MS:
+
calculated for C20
H
21
N
2
FO
3
is 356.15; found [M+H] 357.10 m/z; Anal.calc. for C20
H
21
N
2
FO :
3
C, 67.40; H, 5.94; N, 7.86%. Found: C, 67.3; H, 5.90; N, 7.50%; Uv-Vis: λmax (ACN) = 332
nm, Fluorescence: λex = 332 nm, λem (ACN) = 487 nm, Φ = 0.03.
Ethyl 1-butyl-2-(5-chloro-2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (3c)
Off white solid; yield: 95%; melting point (°C): 120-122; FT IR (ATR, υmax/cm): 33412 (-OH),
1
1
1
2
709 (C=O of ester), 1209 (C-O of ester), 835 (C-Cl); H NMR (400 MHz, DMSO-d
6
): δ =
), 1.94-1.98 (quint,
), 7.12-7.14 (dd, 2H, H and H
of benzimidazole), 7.45-7.47 (d, 1H, J =
of benzimidazole), 8.08-8.084 (dd, 1H, J = 1.6 Hz, H of benzimidazole), 8.574-8.576
.008-1.05 (t, 3H, J = 13 Hz, 4-CH
3
), 1.44-1.49 (m, 5H, ester CH
3
and 3-CH
3
H, 2-CH ), 4.42-4.47 (q, 4H, J = 8 Hz, ester CH
2
2
and 1-CH
2
3
4
of 5-chloro-2-hydroxyphenyl), 7.33-7.37 (m, 1H, H
Hz, H
d, 1H, J = 0.8 Hz, H
7
8
8
5
1
3
(
6
of 5-chloro-2-hydroxyphenyl) ppm; C NMR (100 MHz): δ = 13.9,
1
1
4.1, 20.1, 30.1, 49.6, 61.2, 110.2, 110.8, 115.7, 118.3, 128.7, 129.1, 130.3, 130.5, 130.6,
34.3, 141.0, 150.8, 158.7, 167.2 ppm; MS: calculated for C20
H
21
N
2
ClO is 372.12; found
3
+
[
M+H] 373.05 m/z; Anal.calc. for C20
H
21
N
2
ClO : C, 64.43; H, 5.68; N, 7.51%. Found: C,
3
6
4.0; H, 5.45; N, 7.25%; Uv-Vis: λmax (ACN) = 332 nm, Fluorescence: λex = 332 nm, λem (ACN)
486 nm, Φ = 0.30.
=
Ethyl 2-(5-bromo-2-hydroxyphenyl)-1-butyl-1H-benzo[d]imidazole-5-carboxylate (3d)
Off white solid; yield: 93%; melting point (°C): 128-130; FT IR (ATR, υmax/cm): 3465 (-OH),
1
1
705 (C=O of ester), 1210 (C-O of ester), 650 (C-Br); H NMR (400 MHz, DMSO-d
6
): δ =

1
2
.007-1.04 (t, 3H, J = 13 Hz, 4-CH
3
), 1.42-1.47 (m, 5H, ester CH
and 1–CH ), 7.13-7.15 (dd, 2H, H
of benzimidazole), 7.44-7.46 (d, 1H, J =
of benzimidazole), 8.06-8.09 (dd, 1H, J = 1.6 Hz, H of benzimidazole), 8.336-8.338
3
and 3-CH
2
), 1.95-1.99 (quint,
H, 2-CH ), 4.40-4.45 (q, 4H, J = 8 Hz, ester CH
2
2
2
3
and H
4
of 5-bromo-2-hydroxy phenyl), 7.31-7.34 (m, 1H, H
Hz, H
d, 1H, J = 0.8 Hz, H
7
8
8
5
1
3
(
6
of 5-bromo-2-hydroxy phenyl) ppm; C NMR (100 MHz): δ = 13.7,
1
1
4.2, 20.4, 30.1, 49.4, 61.0, 110.2, 110.9, 116.7, 118.2, 120.1, 129.1, 130.2, 130.7, 133.3,
34.3, 141.5, 151.9, 159.5, 168.2 ppm; MS: calculated for C20
H
21
N
2
BrO is 416.07; found
3
+
[
M+H] 417.15 m/z; Anal.calc. for C20
H
21
N
2
BrO : C, 57.56; H, 5.07; N, 6.71%. Found: C,
3
5
7.45; H, 5.05; N, 6.65%; Uv-Vis: λmax (ACN) = 332 nm, Fluorescence: λex = 332 nm, λem
(ACN) = 497 nm, Φ = 0.16.
Ethyl 1-butyl-2-(3,5-dichloro-2-hydroxyphenyl)-1H-benzo[d]imidazole-5-carboxylate (3e)
Off white solid; yield: 92%; melting point (°C): 120-122; FT IR (ATR, υmax/cm): 3390 (-OH),
1
1
707 (C=O of ester), 1219 (C-O of ester), 831, 748 (C-Cl); H NMR (400 MHz, DMSO-d
6
):
), 1.93-2.0
), 7.43-7.47 (m, 2H, H of
of benzimidazole), 7.536-7.542 (d, 1H, J = 2.4 Hz, H
of benzimidazole), 8.08-8.10 (dd, 1H, J = 1.6 Hz, 10 Hz, H of benzimidazole), 8.427-8.30 (d,
δ = 1.03-1.06 (t, 3H , J = 12 Hz, 4-CH
quint, 2H, 2-CH ), 4.40-4.46 (q, 4H, J = 8 Hz, ester CH
,5-dichloro-2-hydroxyphenyl and H
3
), 1.42-1.51 (m, 5H, ester CH
3
and 3-CH
2
(
2
2
and 1-CH
2
4
3
7
4
6
1
3
1
1
1
H, J = 1.2 Hz, H
4.6, 20.2, 31.9, 46.1, 61.3, 109.9, 114.7, 121.4, 123.4, 124.4, 124.7, 125.7, 126.4, 131.8,
38.2, 139.4, 151.0, 154.2, 166.7 ppm; MS: calculated for C20 Cl is 406.09; found
Cl : C,
8.98; H, 4.95; N, 6.88%. Found: C, 58.95; H, 4.90; N, 6.70%; Uv-Vis: λmax (ACN) = 336 nm,
6
of 3,5-dichloro-2-hydroxyphenyl) ppm; C NMR (100 MHz): δ = 13.7,
H
20
N
2
2
O
3
+
+
+
[
M+H] 407.05, [M+H+2] 409.05, [M+H+4] 411.05 m/z; Anal.calc. for C20
H
20
N
2
2
O
3
5
Fluorescence: λex = 336 nm, λem (ACN) = 500 nm, Φ = 0.87.
Characterization details
The determination of the melting point was done by an open capillary method and was
1
13
uncorrected. Bruker Avance III, 400 MHz was used to record H NMR and C NMR spectra.
The chemical shift values were stated in parts per million (ppm) and coupling constants in
Hertz (Hz). TMS was used as an internal standard. FT (ATR)-IR absorption spectra were
-1
recorded on Shimadzu, in the range 4000–400 cm . Mass spectra were recorded on Schimadzu
LCMS-8030 and are uncorrected. Thin layer chromatography on a silica-coated aluminium
sheet was used to check the completion of the reaction and for checking the purity. Ethylacetate
and hexane (3:7, v/v) were used as mobile phase. The UV-vis spectra of the derivatives were

-5
measured in ACN, EtOH, DMF, DMSO and THF solvents (concentration 2.66 x 10 M and 2
-6
x 10 M) at room temperature using Shimandzu spectrometer. The photoluminescence (PL)
spectra were recorded in Hitachi, Japan F-7000 Fluorescence Spectrophotometer. The
florescence quantum yield was calculated using equation 1.
퐈 퐎퐃퐑 풏ퟐ
Φ = Φ
R
(1)
퐈퐑 퐎퐃 풏ퟐ
푹
Where I is the integrated intensity, OD is the optical density and n is the refractive index, the
subscript R refers to the reference fluorophore of known quantum yield [29]. The fluorescence
lifetimes were measured employing picosecond time domain spectrometer based on Time
Correlated Single Photon Counting (TCSPC) technique (IBH Jobin Yvon 6.1). All the samples
were excited at around 330 nm in ACN solvent using pulsed excitation source (NanoLED) in
an IBH Fluorocube apparatus. apparatus. The fluorescence emission was counted by a
Hamamatsu Micro Channel Plate Photo-multiplier tube (R3809 MCP-PMT). The
thermogravimetric analysis (TGA) was carried out to determine the thermal stability using
−
1
STA-2500, NETZSCH, Germeny at a heating rate of 10 °C min under an inert atmosphere.
The cyclic voltammetry (CV) was carried out by using three-electrode system. The experiment
-1
was conducted in Ivium vertex electrochemical work station with 100 mV s scan rate, in
which platinum wire used as a counter electrode, working electrode used was material coated
glassy carbon electrode and saturated calomel electrode (SCE) as a reference electrode. The
supporting electrode used was tetrabutylammoniumhexafluorophosphate (0.1 M) in
acetonitrile solvent. The measurements were calibrated using ferrocene/ferrocenium (Fc/Fc+)
standard. The density functional theory (DFT) and molecular electrostatic potential (MEP)
were carried out using Acer Veriton i5 Workstation using Materials Science Suite 2017-1,
Schrödinger LLC, New York, NY, 2017. The X-ray intensity data for compounds 3a and 3e
were collected at a temperature of 293 K on a Rigaku Saturn724 diffractometer using graphite
monochromated Mo-Kα radiation. A complete data set was processed using CrystalClear [30].
The structures were solved by SHELXS and SHELXL programs [31]. The geometrical
calculations were carried out using the program PLATON [32]. The molecular and packing
diagrams were generated using software MERCURY [33].
RESULTS AND DISCUSSIONS
Synthesis and Characterization

The synthetic route for compounds 3a-e is depicted in the Scheme 1 given below. The synthesis
of the target compound was obtained in three simple steps according to the reported procedure
[
34]. Firstly, 4-chloro-3-nitrobenzoic acid in ethanol was refluxed in the presence of catalytic
amount conc. H SO to afford ethyl 4-chloro-3-nitrobenzoate (1). Intermediate 1 undergoes
2
4
nucleophilic substitution reaction upon treatment with n-butylamine in the presence of
triethylamine as a base in dry THF at room temperature to give ethyl 4-(butylamino)-3-
nitrobenzoate (2). Intermediate 2 on treatment with substituted salicylaldehyde in the presence
of sodium dithionite as a reducing agent in DMSO undergoes one-pot nitro reductive
cyclisation to yield the title compounds 3a-e. The obtained products were purified by
recrystallisation in DMF. All these compounds can be stored for a longer time under the
ambient conditions without any detectable degradation. The formation of the products was
1
13
confirmed by H NMR, C NMR, MS, FTIR, single crystal XRD and elemental analyses.
3
a: R = H, 3b: R = 5-F, 3c: R = 5-Cl, 3d: R = 5-Br, 3e: R = 3,5-Cl
2
Scheme 1: The synthesis of target compounds (3a-e). Reagents and conditions: (a) H
2
SO , dry
4
ethanol, 16 h; (b) n-Butylamine, triethylamine, room temperature; (c) substituted
salicaldehydes, sodium dithionite, 90 °C, 3 h.

Fig. 2: Synthetic strategy for designing target molecule
1
Formation of 3b to the consideration, H NMR spectrum showed triplet at 1.00-1.04, multiplet
at 1.42-1.47, quintet at 1.95-1.99 and quartet at 4.40-4.45 ppm confirmed the presence of N-
butyl and ethyl ester group. The signal doublet of doublet at 7.10-7.12 ppm with J = 4 Hz, 1.6
Hz was attributed for H
Whereas multiplet at 7.32-7.35 ppm was attributed for H
benzimidazole showed a doublet with J = 8 Hz at 7.43-7.45 ppm due to ortho coupling.
Furthermore, signals in the region 8.06-8.09 and 8.472-8.474 were assigned for H of
benzimidazole with J = 1.6 Hz represented meta coupling and H of 5-fluoro-2-hydroxy phenyl
having J = 0.8 Hz showed para coupling respectively. The formation of the product was
3
and H
4
of 5-fluoro-2-hydroxy phenyl due to meta and para coupling.
7
of benzimidazole, H of
8
5
6
1
3
supported by C NMR spectrum with a characteristic peak at 166.9 ppm for carbonyl carbon
of ester and 154.3 ppm for carbon attached to hydroxy group.
Crystal structures and Thermal Properties
The crystals suitable for X-ray analysis of 3a and 3e were obtained using slow evaporation
method. The molecular and crystal structure of 3a and 3e confirmed using X-ray
crystallographic studies (Fig. 3. and Fig. 5.). In the title compounds 3a and 3e, (Fig. 4, Fig. 6
and Table 1), the molecules are connected through C11---H11A…O2 and C17---H17B…O2
intermolecular hydrogen bonds. The intramolecular hydrogen bonds of the type O3---H3…N2
are observed in both the compounds. The Cg…Cg short interactions in molecule 3a are,
Cg1…Cg1 (3.747 (2) Å, slippage = 1.281 Å, -X,-Y,-Z), Cg1…Cg3 (3.6532 Å), -X,1-Y,-Z) and
Cg2…Cg3 (3.696(2) Å, -X,-Y,-Z). Where, Cg1: N1/C7/N2/C6/C8, Cg2:C4-C10 and Cg3:

C15-C20. The molecule 3e has short interactions of the type Cg2…Cg2 (3.712(2) Å, 1-X,-Y,1-
Z, slippage = 0.722), Cg2…Cg4 (3.683(2) Å, -X,-Y,1-Z) and Cg3…Cg4 (3.767(2) Å, 1-X,-
Y,1-Z). Where, Cg2: N1/C7/N2/C6/C8, Cg3: C4-C10and Cg4: C11-C16. The bond length and
bond angles of 3a and 3e are included in ESI (Table S4-S9).
Thermogravimetric analysis (TGA) was carried out to determine the thermal stability of the
molecules (3a-e). The TGA plots are shown in Fig. S15 (ESI). The compounds 3a, 3b, 3c, 3d
and 3e show high decomposition temperature (T ) of 259, 221, 252, 270 and 266 °C,
d
respectively and 5%, 19%, 34%, 2% and 17 % of compounds 3a-e left as residual matter.
Among five synthesized compounds (3a-e), 3d showed the highest decomposition temperature.
Whereas, the amount of compound left as the residual matter is highest for 3c indicating highest
stability among the five synthesized compounds (3a-e) (Table 2).
Table 1: Intramolecular and Intermolecular interactions parameters
D---H...
D---H H...A/Cg/X D....A/CgX
D---H... A/Cg
A/Cg/X
)
Symmetry
x,1/2+y,1/2-z
1-x,-y,1-z
(Å)
(Å)
(Å)
(
3
a
O3---H3…N2
0.82
0.97
1.79
2.56
2.524(5)
3.493(6)
149
161
C11---H11A…O2
3
e
O3---H3…N2
0.82
0.97
1.80
2.51
2.524(5)
3.422(9)
146
156
C17---H17B…O2

Fig. 3: ORTEP diagram of Ethyl 1-butyl-2-(2-hydroxyphenyl)-1H-benzo[d]imidazole-5-
carboxylate (3a)
Fig. 4: Packing of the 3a molecules when viewed down along the b-axis. The dotted lines
represent intermolecular hydrogen bonds.

Fig. 5: ORTEP diagram of Ethyl 1-butyl-2-(3,5-dichloro-2-hydroxyphenyl)-1H-
benzo[d]imidazole-5-carboxylate (3e)
Fig. 6: Packing of the 3e molecules when viewed down along the b-axis. The dotted lines
represent intermolecular hydrogen bonds.
Photophysical Properties
UV-vis studies Photoluminescence (PL) spectral studies
The photophysical properties of the benzimidazole derivatives were measured using UV-vis
absorption and photoluminescence (PL) spectroscopy. The UV-vis absorption of the
synthesized compounds (3a-e) was studied at room temperature. Fig. 7 shows UV-vis
absorption spectra of 3a-e in acetonitrile. The synthesized benzimidazole derivatives have a
hydroxy-substituted aromatic ring with continuous conjugation with a benzimidazole-5-
carboxylate moiety which shows two main groups of bands in absorption spectra observed at

2
20-290 nm and 330-335 nm. The strong absorption band at a shorter wavelength is due to the
π-π* of the hydroxy-substituted aromatic ring towards benzimidazole moiety [35]. The
relatively weak absorption band at higher wavelength is due to π-π* transition within the whole
molecule. The maximum absorption wavelengths (휆 푎m 푏a푠x⁡) of 3a, 3b, 3c, 3d and 3e were
observed at 322, 332, 332, 332 and 336 nm respectively.
Photoluminescence (PL) spectral studies
All chromophores exhibit dual emission bands around 500 nm (i.e., strong main band) and 369
nm (i.e., weak sub band) in weak H-bonding solvent such as acetonitrile at room temperature.
This dual emission originates from phototautomerization (i.e., excited-state intramolecular
proton transfer (ESIPT)) that changes enol form (E) to keto form (K) upon excitation, by
migrating a proton to the neighbouring electronegative atom through intramolecular H-bonding
(
see Scheme 2). After relaxation of the K-form to the ground state, the E-form is again
recovered by ground-state intramolecular proton transfer (GSIPT) i.e., reverse proton transfer
36-39]. Further, Fig. 3 and Fig. 5 confirms that 2-hydroxyphenyl benzimidazole derivatives
[
exhibit intramolecular hydrogen bonding of type O3---H3…N2. The weak shorter wavelength
emission (369 nm) is the normal emission from excited enol whereas strong longer wavelength
emission (474-500 nm) is from tautomer due to ESIPT [40, 41]. Usually the tautomer emission
has highest Stokes shift [42].
Scheme 2: Typical schematic representation of the Four Level ESIPT Photocycle for 3a
compound

The tautomer emission found to be red shifted on changing the substitution on 2-hydroxyphenyl
moiety. Whereas normal emission did not display any changes. The slight red shift in tautomer
emission spectra of compounds 3b-e compared to 3a (Fig. 7) was due to the presence of
halogen atom in the fifth position of the hydroxy phenyl moiety. It was observed that as the
size of the halogen atom increases emission maxima showed bathochromic shift. The molecule
3
e with 3,5-Cl substitution in the hydroxyphenyl moiety showed the highest emission maxima
2
at 500 nm. The emission maxima of compounds 3a-e is in the order 3e > 3d > 3b > 3c > 3a
with wavelength 500, 497, 486, 487and 474 nm respectively (Table 2). Even though the
marginal difference was observed concerning for to emission maxima, but there was a huge
difference in intensity of these compounds due to variation in the structural property. The
moiety with 3,5-Cl (3e) substituted in hydroxy phenyl showed the highest intensity compared
2
to other moieties in the series followed by 3c, 3d, 3b and 3a.
Solid state photoluminescence studies
The emission spectra of compounds 3a-e in solid-state films were obtained by exciting at 300
nm (Fig. 7) and the 휆푒푚푖 being 481-509 nm which fall within blue to green region. The results
max⁡
revealed that there were about 9-14 nm of the bathochromic shift in a solid state when compared
to the solution phase which explains the existence of aggregation induced emission. The
absorption and emission maxima of compounds 3a-e were summarised in Table 2.
The degree of a bathochromic shift from absorption maxima (휆 푎m 푏a푠x⁡) to emission maxima (휆 푒m 푚a 푖x ⁡
is measured by the Stokes shift which is the range of 151-165 nm. Among five title compounds,
a and 3e showed smaller and larger stokes effect respectively (Table 2).
)
3
3
3
3
3d
3e
a
b
c
3
3
3
3
3
a
b
c
d
e
6
00
0
0
0
.10
.05
.00
400
2
00
0
350
2
00
250
300
350
400
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(a)
(b)

3
3
3
a
b
c
6
6
5
5
5
1
1
8
5
3
x10
x10
x10
x10
x10
3d
3e
0
4
25
450
475
500
525
550
575
Wavelength (nm)
(c)
Fig. 7: (a) UV-Vis absorption spectra of compounds 3a-e in ACN at room temperature, at 2
µM concentration
(
b) Emission spectra of compounds 3a-e in ACN at room temperature, at 2 µM concentration
excited at λex = 330 nm.
c) Emission spectra of compounds 3(a-e) in solid state film when excited at λex = 300 nm.
(
Quantum Yield (Φ) calculations
Fluorescence quantum yield (ΦPL) were measured in ACN at room temperature and compared
with 2-napthylamine taken as a reference of known quantum yield (Fig. S18). The radiative
(kr) and non-radiative rate constants were calculated using ΦPL and lifetime values. It was
observed that moieties with chlorine substituent showed highest quantum yield compared to
other halogen atoms. The highest ΦPL value of 0.87 was observed for 3e, which is due to
decrease of non-radiative rate constant based on tight packing structure with hydrogen-bonds
(see Table 2). The comparison between previously reported benzimidazole derivatives [35,
4
3-46] and 3e has been summarised in Table S10 (ESI). The introduction of the ehtylester and
alkyl chain greatly enhanced the optoelectronic property, quantum yield and Stokes shift.
Alkyl group greatly affects the solid-state fluorescence behaviour of small organic
luminophore [47] and introduction of ethylester which is in continuous conjugation with
benzimidazole nucleus leads to an intramolecular charge transfer due to which emission peaks
are redshifted depending on the nature of electron-acceptor unit [48]. (Fig. 2). The moderate
Φ
PL was observed for the rest of the derivatives which was in the range of 0.03-0.3 (Table 2).
The compound with fluorine substituent (3b) showed the least fluorescence quantum yield of
.03 because of increased non-radiative rate constant.
0

Table 2: Summary of Optical and thermal properties of compounds (3a-e)
Comp. Absorption
Emission
Stoke
shift
∆λ
Φ
PL
T
d
[°C][e] Average Radiativ
Non-
radiative
rate
풂풃풔
푒푚푖
푒푚푖
흀
휆
휆
max⁡
lifetime
(ns)
e
퐦퐚퐱⁡
max⁡
nm)[a]
(nm)[b]
Tautomer
emission emission
(nm)
(
rate
[
c]
(nm)
constant constant
Normal
kr
knr
8
-1
9
-1
(
10 S ) (10 S )
3
a
322
332
332
332
336
369
369
369
369
369
369
474
487
486
497
500
400
483
500
495
498
510
-
152
155
154
156
165
55
0.14
0.03
0.30
0.16
0.87
0.91
259
221
252
270
266
-
0.31
0.26
0.21
0.26
1.21
-
4.52
1.15
14.3
6.15
7.19
-
2.77
3.73
3.33
3.23
0.11
-
3
b
3
c
3
d
3
e
Std[d]
345
[
[
a]
-6
The absorption spectra were measured in acetonitrile at 2 x 10 M concentration.
b]
-6
The emission spectra were measured in acetonitrile at 2 x 10 M concentration.
[c]The emission spectra were measured in solid state film.
[
d]
2
-Napthylamine was used as a standard compound.
[e]Obtained TGA measurements.
Solvent Effect
The role of solvent is important in spectroscopic analysis as it brings major changes in intensity,
shape and position of absorbance and fluorescence band. Hence to study the solvent-induced
shift in absorption and emission band and to understand the intramolecular charge transfer
(
ICT) effect, the UV-vis and photoluminescence study of molecules 3a-e were studied in
-5
solvents of different dielectric constant with a set concentration of 2.66 x 10 M. The chosen
solvents are tetrahydrofuran (THF), N,N-dimethylformamide (DMF), ethanol (EtOH) and
dimethyl sulfoxide (DMSO). The results are summarised in Table S11 and depicted in Figs.
S10-14 (ESI). The spectral shifts with respect to solvent polarity reflects on the molecular
interactions between solute and solvent. The spectral curves revealed that molecules 3a-e
displayed an absorption peak in the region 274-337 nm. Finite bathochromic shifts were
observed differing by 2-25 nm on varying substitution on hydroxyphenyl moiety. Further, it
was observed that on increasing the solvent polarity absorbance maxima shifted to blue region.
A similar trend in absorption maxima of compounds 3a-e on varying solvent polarity was

observed that is, EtOH < DMF = DMSO < THF. About 25-51 nm of a shift to blue region was
observed on increasing solvent polarity.
The solution photoluminescence (PL) spectra of all the derivatives showed an emission band
in the violet-green region (352-508 nm). The compound 3a, 3b and 3c exhibited dual emission
in THF solvent, whereas single emission in EtOH, DMF and DMSO solvent which is attributed
to intramolecular hydrogen bonding dominates over solvent-solute interaction in THF solvent
resulting in dual emission. Whereas compound 3d and 3e displayed dual emission in all chosen
solvents. The absence of dual emission in few solvents suggests the absence of ESIPT process
due to the presence of intermolecular hydrogen bonding with solvent molecule resulting in the
stabilization of solvated isomer [36] (Fig. S19, ESI). Usually the tautomer emission has highest
Stokes shift. Hence it can be inferred that single emission at longer wavelength is tautomer
emission. Also, photoluminescence spectra were intense in the EtOH solvent which indicates
the stabilization of the dipole moment of the molecules in the excited state by the solvent.
Among the series 3a-e, compound 3e showed the highest Stokes shift in EtOH solvent.
Furthermore, it was observed that on increasing the solvent polarity emission maxima shifted
to red region.
Fluorescence lifetime studies
The fluorescence decay of 3a-e was measured in ACN solvent (Fig. S20 in ESI) and the data
were tabulated in Table 2. When excited around 330 nm, dual lifetimes were noticed for all
compounds (see Table S13 in ESI). The biexponential decay was perhaps due to dual emission
bands. The shorter lifetime values (i.e., 1) corresponds to the excited E-form and the longer
lifetime values (i.e., 2) corresponds to K-form of 3a-e compounds. The excited state lifetimes
vary from 0.21 ns (least for 3c) to 1.21 ns (highest for 3e).
Electrochemical Properties
To explore electrochemical properties and practical energy levels of the molecules (3a-e),
cyclic voltammetry studies were performed (Table S12, ESI). The cyclic voltammograms of
molecules (3a-e) is depicted in Fig. S16 (ESI). The molecules 3a-e exhibited one oxidation
potential peak in the anodic region at 0.99, 0.92, 0.99, 0.96, 1.00 V vs SCE respectively. The
highest occupied molecular orbital (HOMO) was calculated using equation (2)
표푥
EHOMO = -[퐸
⁡+ 4.8eV - EFOC
]
(2)
표푛푠푒푡

표푥
where, 퐸
and EFOC are the onset oxidation potentials of the compounds and ferrocene (EFOC
표푛푠푒푡
=
0.29 V vs SCE (Fig. S17, ESI)), respectively, −4.8 eV is the HOMO energy level of ferrocene
against vacuum.
Whereas, lowest unoccupied molecular orbital (LUMO) energy levels were calculated using
equation (3).
E
LUMO = EHOMO + E
g
(3)
The optical bandgap value (E
g
) estimated from the absorption edge of 3a-e were 3.51, 3.42,
3
.42, 3.41, 3.37 respectively, calculated using equation (4) which varied in the order 3a > 3b
3c = 3d > 3e.
=
1
240
퐸푔 = ꢀ
(4)
표푛푠푒푡
The calculated HOMO-LUMO energy gap of 3a-e using equation (5) are summarised in Table
S12. The results revealed that there was a marginal difference of HOMO-LUMO energy gap
of benzimidazole ligands.
∆
E = EHOMO - ELUMO
(5)
DFT calculation and electronic structures
The quantum parameters like highest occupied molecular orbital (HOMO), lowest unoccupied
molecular orbital (LUMO) and HOMO-LUMO bandgap are essential to determine the
chemical stability as well as molecular electrical transport properties of the synthesized
molecule [49]. To obtain the knowledge about the electronic structure of newly synthesized
benzimidazole derivatives, theoretical computational studies were carried out by density
function theory (DFT) with basis set ps-321G, ps-631G and ps-6311G for compounds 3a, 3b,
3
c and 3e. Whereas ps-LAV3P basis set for compound 3d. The optimized structure, HOMO
and LUMO distribution of 3a-e is depicted in Fig. 8. The HOMO of the title compounds is
localized on the whole conjugated molecular backbone excluding the ester group and n-butyl
group. Whereas, LUMO is localized on the whole conjugated molecular backbone as well as
on ester moiety but excluding n-butyl group. The calculated energies of HOMO are in the range
of -6.15 to -6.49 eV whereas LUMO in the range of -1.62 to -2.00 eV. The incorporation of
two chlorine atom in 3e effectively lowered the LUMO energy level which leads to the
bathochromic shift in electronic absorption. DFT-based reactive descriptors resulted from
HOMO-LUMO energies are summarized in Table S12 (ESI).

The perception of electronegativity (χ), global hardness (η), global softness (S) and
electrophilicity index (ω) can be calculated [50] according to Koopman’s theorem using
HOMO and LUMO energies (Table 3). The chemical behaviour of the synthesized compound
can be predicted by these parameters. The moiety is defined as acidic with higher χ value.
Whereas, the moiety is considered as basic with smaller χ value. The acidic character of the
synthesized molecule follows the order 3c > 3b > 3a > 3d > 3e. The stability of the molecule
is determined by the global hardness which is the order 3e > 3c > 3a > 3b > 3d. Whereas global
softness in the order 3d > 3a = 3b = 3c > 3e indicating 3d is soft and more reactive compared
to other moieties. The electrophilicity index values are in the order 3c > 3b > 3a > 3d > 3e
indicating all are highly electrophilic.

OPTIMISED
GEOMETRY
HOMO
LUMO
COMPOUNDS
3
a
3
b
3
c
3
d
3
e
Fig. 8: Calculated molecular orbital plots of HOMO and LUMO levels and optimized
molecular structures of compounds 3a-e.

Table 3: Conceptual DFT-based reactivity descriptors for the synthesized compounds 3a-e
Parameter
3a
3b
3c
3d
3e
Derivation
Ionization potential (I)
6.34
6.40
6.50
6.15
6.23
I = -EHOMO
(eV)
Electron affinity (A)
eV)
Chemical potential (µ)
eV)
Electronegativity (χ)
eV)
Global hardness (η)
eV)
Global softness (S)
2.00
-4.17
4.17
4.34
0.23
2.00
2.12
-4.26
4.26
4.28
0.23
2.12
2.15
1.90
1.63 A = -ELUMO
(
-4.32 -4.03 -3.93
µ = -(I +
A)/2
(
4.32
4.35
0.23
2.14
4.03
4.25
0.24
1.91
3.93
4.60
0.22
1.68
χ = - µ
(
η = I - A
S = 1/η
(
-1
(
eV )
Electrophilicity index
ω) (eV)
2
ω = µ /2η
(
Molecular Electrostatic Potential (MEP)
MEP surface reveals the dispersal of charges across the surface of a molecule and it is a very
useful tool for understanding hydrogen bonding interactions, nucleophilic and electrophilic
sites of the synthsized molecule [51]. The prediction of reactive sites of the compounds 3a-e
was obtained using the ps-321G, ps-631G and ps-6311G basis sets for compounds 3a, 3b, 3c
and 3e. Whereas ps-LAV3P basis set for compound 3d which are shown in Fig. 9. The colours
represent the magnitude of electrostatic potential. Blue colour represents the strongest
attraction; electron-deficient region while red colour represents the strongest repulsion;
electron-rich region. In contrast, red regions are prone to electrophilic attack while blue regions
to nucleophilic attack. These results give us evidence about which region of the molecule
undergoes intermolecular interaction when the reaction is carried out. From MEP surface it is
evident that the red region is localised on the N of benzimidazole, oxygen atom of ester,
3
fluorine atom and hydroxy group while the blue region is localised on the carbonyl carbon,
hydrogen atoms of the aromatic region and the hydrogen atom of the hydroxy group. The dark
blue colour region indicates the high reactivity towards nucleophiles while light blue regions
are less reactive.

3
a
3b
3
c
3d
3
e
Fig. 9: Molecular Electrostatic Potential (MEP) of compounds 3a-e
Conclusions
Five novel benzimidazole derivatives have been synthesized via “one-pot” nitro reductive
cyclization using one of the cheaply available reagent sodium dithionite. The three-dimensional
crystal structures of 3a and 3e were confirmed using single-crystal X-ray diffraction method.
The introduction of halogen atoms to 2-hydroxyphenyl group attached to benzimidazole moiety
resulted in a bathochromic shift in UV-Vis and PL spectra, as well as resulted in increased
molecular stability. These moieties also exhibit blue-green fluoresce in solid-state. The
HOMO-LUMO bandgap energy obtained from DFT and CV are fairly in agreement. The

fluorescence quantum yield was found to be appreciable for benzimidazole derivative
compared to reference (2-Napthylamine). Especially compound 3e showed green emission,
dual emission in both polar protic and aprotic solvents, high quantum yield, highest excited
state lifetime and good thermal stability. The findings of results help in developing novel ligand
in the field of organic optoelectronics.
Supporting information
Supporting information includes spectral characterization of compounds 3a-e, TGA graph
solvent-dependant, cyclic voltammetry graphs and tables. Also, bond length and bond angle
tables of 3a and 3e.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgement
Authors are thankful to Nithin Pathoor, IIT Bombay for helping in conducting solid state
fluorescence; B. Ramachandra Bhat, NITK Surathkal for providing lab facility for carrying out
cyclic voltammetry; DST-PURSE Laboratory, Mangalore University and CeNS Bangalore for
analytical data; USIC, Karnataka University Dharwad for fluorescence lifetime and
Department of Biochemistry, Mangalore University for computational studies. MKS thank IOE
and DST PURSE, Vijnana Bhavan, University of Mysore, Mysuru.

References
. N. T. Kalyani, S. J. Dhoble, Organic light emitting diodes: Energy saving lighting
technology—A review, Renew Sust. Energ. Rev. 16 (2012) 2696-2723.
. V. C. Bender, T. B. Marchesan, J. M. Alonso, Solid-state lighting: A concise review of the
state of the art on LED and OLED modelling, IEEE Ind. Electron. Mag. 9 (2015) 6-16.
. A. P. Kulkarni, C. J. Tonzola, A. Babel, S. A. Jenekhe, Electron transport materials for
organic light-emitting diodes, Chem. Mater. 16 (2004) 4556-4573.
. M. Kasha, Perturbation of the tautomerization potential, J. Chem. Soc. Faraday Trans. 2, 82
1986) 2379-2392.
. J. E. Kwon, S.Y. Park, Advanced organic optoelectronic materials: Harnessing excited‐
state intramolecular proton transfer (ESIPT) process. Adv. Mater. 23 (2011) 3615-3642.
. D. B. O'Connor, G. W. Scott, D. R Coulter, A. Yavrouian, Temperature dependence of
1
2
3
4
(
5
6
electronic energy transfer and quenching in copolymer films of styrene and 2-(2'-hydroxy-5'-
vinylphenyl)-2H-benzotriazole, J. Phys. Chem. 95 (1991) 3615-3642.
7
. S. Santra, S. K. Dogra, Spectral characteristics of 2-(2′-aminophenyl) benzimidazole in β-
cyclodextrin. J. Photochem. Photobiol. A. 101 (1996) 221-227.
. P. T. Chou, M. L. Martinez, J. H. Clements, Reversal of excitation behavior of proton-
transfer vs. charge-transfer by dielectric perturbation of electronic manifolds. J. Phys. Chem.
8
9
9
7 (1993) 2618-2622.
. S. Park, O.H. Kwon, Y.S. Lee, D.J. Jang, S.Y. Park, Imidazole-Based Excited-State
Intramolecular Proton-Transfer (ESIPT) Materials: Observation of Thermally Activated
Delayed Fluorescence (TDF), J. Phys. Chem. A 111 (2007) 9649–9653.
1
0. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. D. D. C.
Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund, W. R. Salaneck,
Electroluminescence in conjugated polymers, Nature. 397 (1999) 121-128.
1
1. J. Luo, Z. Xie, J. W. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D.
Zhu, B. Z. Tang, Aggregation-induced emission of 1-methyl-1, 2, 3, 4, 5-
pentaphenylsilole, ChemComm. 18 (2001) 1740-1741.
1
2. J. Huang, N. Sun, Y. Dong, R. Tang, P. Lu, P. Cai, Q. Li, D. Ma, J. Qin, Z. Li, Similar or
totally different: the control of conjugation degree through minor structural modifications, and
deep‐ blue aggregation‐ induced emission luminogens for non‐ doped OLEDs, Adv. Funct.
2
1
3 (2013) 2329-2337.
3. W. Z. Yuan, P. Lu, S. Chen, J. W. Lam, Z. Wang, Y. Liu, H. S. Kwok, Y. Ma, B. Z. Tang,
Changing the behavior of chromophores from aggregation‐ caused quenching to aggregation‐

induced emission: development of highly efficient light emitters in the solid state, Adv. Mater.
2
1
2 (2010) 2159-2163.
4. I. Celik, G. Ayhan-Kilcigil, B. Guven, Z. Kara, A. S. Gurkan-Alp, A. Karayel, A. Onay-
Besikci, Design, synthesis and docking studies of benzimidazole derivatives as potential EGFR
inhibitors, Eur. J. Med. Chem. 173 (2019) 240-249.
1
5. K. Deepak, P. Arora, Review on: Benzimidazole derivatives as potent biological
agent, Curr. Pharm. Res. 9 (2018) 2676-2694.
6. H. Bektas, C. Albay, B. B. Sokmen, S. Aydin, E. Menteşe, G. Aydin, D. Şen, Synthesis,
1
Antioxidant, and Antibacterial Activities of Some New 2‐ (3‐ fluorobenzyl)‐ 1H‐
benzimidazole Derivatives, J. Heterocycl. Chem. 55 (2018) 2400-2407.
1
7. N. T. Gowda, C. V. Kavitha, K. K. Chiruvella, O. Joy, K. S. Rangappa, S. C. Raghavan,
Synthesis and biological evaluation of novel 1-(4-methoxyphenethyl)-1H-benzimidazole-5-
carboxylic acid derivatives and their precursors as antileukemic agents, Bioorg. Med. Chem.
lett. 19 (2009) 4594-4600.
1
8. H. J. Kwak, Y. M. Pyun, J. Y. Kim, H. S. Pagire, K. Y. Kim, K. R. Kim, S. Dal Rhee, W.
H. Jung, J. S. Song, M. A. Bae, D. H. Lee, Synthesis and biological evaluation of
aminobenzimidazole derivatives with a phenylcyclohexyl acetic acid group as anti-obesity and
anti-diabetic agents. Bioorg. Med. Chem. Let. 23 (2013) 4713-4718.
1
9. T. R. Chen, Luminescence and electroluminescence of bis (2-(benzimidazol-2-yl)
quinolinato) zinc. Exciplex formation and energy transfer in mixed film of bis (2-
benzimidazol-2-yl) quinolinato) zinc and N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1, 1′-
biphenyl-4, 4′-diamine, J. Mol. Struct. 737 (2005) 35-41.
0. M. Hranjec, E. Horak, M. Tireli, G. Pavlovic, G. Karminski-Zamola, Synthesis, crystal
(
2
structure and spectroscopic study of novel benzimidazoles and benzimidazo [1, 2-a] quinolines
as potential chemosensors for different cations, Dyes Pigm. 95 (2012) 644-656.
2
1. E. Horak, P. Kassal, M. Hranjec, I. M. Steinberg, Benzimidazole functionalised Schiff
bases: novel pH sensitive fluorescence turn-on chromoionophores for ion-selective optodes,
Sensor Actuat. B-Chem. 258 (2018) 415-423.
2
2. A. P. Kulkarni, C. J. Tonzola, A. Babel, S. A. Jenekhe, Electron transport materials for
organic light-emitting diodes, Chem. Mater. 16 (2004) 4556-4573.
3. N. Deshapande, N. S. Belavagi, M. G. Sunagar, S. Gaonkar, G. H. Pujar, M. N. Wari, S. R.
2
Inamdar, I. A. M. Khazi, Synthesis, characterization and optoelectronic investigations of
bithiophene substituted 1, 3, 4-oxadiazole derivatives as green fluorescent materials. RSC Adv.
5
(2015) 86685-86696.

2
4. X. Chen, H. Yang, Y. Ge, L. Feng, J. Jia, J. Wang, Synthesis, X‐ ray crystal structure and
optical properties of novel 2‐ aryl‐ 3‐ ethoxycarbonyl‐ 4‐ phenylpyrido [1,2‐ a]
benzimidazoles, Luminescence. 27 (2012) 382-389.
2
5. S. H. Cheng, S. H. Chou, W. Y. Hung, H. W. You, Y. M. Chen, A. Chaskar, Y. H. Liu, K.
T. Wong, Fine-tuning the balance between carbazole and oxadiazole units in bipolar hosts to
realize highly efficient green PhOLEDs, Org. Electron. 14 (2013) 1086-1093.
2
6. M. Bouchouit, M. E. Said, M. K. Ali, S. Bouacida, H. Merazig, N. K. Chaouche, A.
Chibani, B. Zouchoune, A. Belfaitah, A. Bouraiou, Synthesis, X-ray structure, theoretical
investigation, corrosion inhibition and antimicrobial activity of benzimidazole thioether and
theirs metal complexes, Polyhedron. 119 (2016) 248-259.
2
7. J. F. Xiong, J. X. Li, G. Z. Mo, J. P. Huo, J. Y. Liu, X. Y. Chen, Z. Y. Wang, Benzimidazole
derivatives: selective fluorescent chemosensors for the picogram detection of picric acid, J.
Org. Chem. 79 (2014) 11619-11630.
2
8. C. H. Chen, W. S. Huang, M. Y. Lai, W. C. Tsao, J. T. Lin, Y. H. Wu, T. H. Ke, L. Y.
Chen, C. C. Wu, Versatile, Benzimidazole/Amine‐ Based Ambipolar Compounds for
Electroluminescent Applications: Single‐ Layer, Blue, Fluorescent OLEDs, Hosts for Single‐
Layer, Phosphorescent OLEDs, Adv. Funct. 19 (2009) 2661-2670.
2
9. F. D. Lewis, J. L. Hougland, S. A. Markarian, Formation and Anomalous Behavior of
Aminonaphthalene− Cinnamonitrile Exciplexes, J. Phys. Chem. A 104 (2000) 3261-3268.
0. Rigaku CrystalClear SM Expert 2.0 r15. Software for data collection and processing,
Rigaku Corporation, Tokyo, Japan, 2011.
3
3
3
1. G. M. Sheldrick, A short history of SHELX, Acta Cryst. A 64 (2008) 112-122.
2. A. L. Spek, Structure validation in chemical crystallography, Acta Crystallogr. Sect. D
Biol. Crystallogr. 65 (2009) 148-155.
3. C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edging-ton, P. McCabe, E. Pidcock, L.
3
Rodriguez-Monge, R. Taylor, J. van de Streek, P. A. Wood, Mercury CSD 2.0 – new features
for the visualization and investigation of crystal structures, J. Appl. Crystallogr. 41 (2008) 466-
4
3
70.
4. K. Vasantha, G. Basavarajaswamy, M. V. Rai, P. Boja, V. R. Pai, N. Shruthi, M. Bhat,
Rapid ‘one-pot’ synthesis of a novel benzimidazole-5-carboxylate and its hydrazone
derivatives as potential anti-inflammatory and antimicrobial agents, Bioorg. Med. Chem. 25
(2015) 1420-1426.

3
5. M. Barwiolek, A. Wojtczak, A. Kozakiewicz, M. Babinska, A. Tafelska-Kaczmarek, The
synthesis, characterization and fluorescence properties of new benzimidazole
derivatives, J. Lumin. 211 (2019) 88-95.
3
6. J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Synthesis of some fluorescent
benzimidazole derivatives using cobalt (II) hydroxide as highly efficient catalyst–spectral and
physico-chemical studies and ESIPT process. Photochem. Photobiol. Sci. 12 (2013) 1761-
1
773.
3
7. A. Douhal, F. Amat-Guerri, M. P. Lillo, A. U. Acuna, Proton transfer spectroscopy of 2-
(
2'-hydroxyphenyl) imidazole and 2-(2'-hydroxyphenyl) benzimidazole dyes. J. Photochem.
Photobiol. A. 78 (1994) 127-138.
8. P. Majumdar, J. Zhao, 2-(2-hydroxyphenyl)-benzothiazole (HBT)-rhodamine dyad: acid-
switchable absorption and fluorescence of excited-state intramolecular proton transfer
ESIPT). J. Phys. Chem. B. 119 (2015) 2384-2394.
9. V. S. Padalkar, A. Tathe, V. D. Gupta, V. S. Patil, K. Phatangare, N. Sekar, Synthesis and
3
(
3
photo-physical characteristics of ESIPT inspired 2-substituted benzimidazole, benzoxazole and
benzothiazole fluorescent derivatives. J. Fluoresc. 22 (2012) 311-322.
4
0. F. A. Chipem, N. Dash, G. Krishnamoorthy, Role of nitrogen substitution in phenyl ring
on excited state intramolecular proton transfer and rotamerism of 2-(2′-hydroxyphenyl)
benzimidazole: A theoretical study. J. Chem. Phys. 134 (2011) 104308.
4
1. K. Das, N. Sarkar, D. Majumdar, K. Bhattacharyya, Excited-state intramolecular proton
transfer and rotamerism of 2-(2′-hydroxyphenyl) benzimidazole. Chem. Phys. Lett. 198 (1992)
4
4
43-448.
2. S. K. Behera, G. Sadhuragiri, P. Elumalai, M. Sathiyendiran, G. Krishnamoorthy, Exclusive
excited state intramolecular proton transfer from a 2-(2′-hydroxyphenyl) benzimidazole
derivative. RSC Adv. 6 (2016) 59708-59717.
4
3. M. Hranjec, E. Horak, D. Babic, S. Plavljanin, Z. Srdovic, I. M. Steinberg, R. Vianello, N.
Perin, Fluorescent benzimidazo [1, 2-a] quinolines: synthesis, spectroscopic and computational
studies of protonation equilibria and metal ion sensitivity, New J. Chem. 41 (2017) 358-371.
4
4. T. Zhang, R. Zhang, Y. Zhao, Z. Ni, A new series of N-substituted tetraphenylethene-based
benzimidazoles: Aggregation-induced emission, fast-reversible mechanochromism and blue
electroluminescence, Dyes Pigm. 148 (2018) 276-285.
4
5. D. C. Hu, C. H. Xiao, G. Z. Guo, Y. X. Yang, J. C. Liu, A one-dimensional silver (I)
coordination polymer based on the 2-[2-(pyridin-4-yl)-1H-benzimidazol-1-ylmethyl] phenol
ligand exhibiting photoluminescence, Acta Crystallogr. E 69 (2013) 356-359.

4
6. X. Ban, W. Jiang, K. Sun, X. Xie, L. Peng, H. Dong, Y. Sun, B. Huang, L. Duan, Y. Qiu,
Bipolar host with multielectron transport benzimidazole units for low operating voltage and
high power efficiency solution-processed phosphorescent OLEDs,
ACS Appl. Mater. Interfaces. 7 (2015) 7303-7314.
7. S. Xue, X. Qiu, Q. Sun, W. Yang, Alkyl length effects on solid-state fluorescence and
mechanochromic behavior of small organic luminophores, J. Mater. Chem. 4 (2016) 1568-
4
1
4
578.
8. D. Ades, V. Boucard, E. Cloutet, A. Siove, C. Olivero, M. C. Castex, G. Pichler,
Photoluminescence of donor–acceptor carbazole-based molecules in amorphous and powder
forms, J. Appl. Phys. 87 (2000) 7290-7293.
4
9. E. Ermis, Synthesis, spectroscopic characterization and DFT calculations of novel Schiff
base containing thiophene ring, J. Mol. Struct. 1156 (2018) 91-104.
0. H. Bhakhoa, L. Rhyman, P. Ramasami, Theoretical study of the molecular aspect of the
suspected novichok agent A234 of the Skripal poisoning, R. Soc. Open Sci. 6 (2019) 181831.
1. C. Y. Panicker, H. T. Varghese, B. Narayana, K. Divya, B. K. Sarojini, J. A. War, C. Van
Alsenoy, H. K. Fun, FT-IR, NBO, HOMO–LUMO, MEP analysis and molecular docking study
of Methyl N-({[2-(2-methoxyacetamido)-4-(phenylsulfanyl) phenyl]
amino}[(methoxycarbonyl) imino] methyl) carbamate, Spectrochim. Acta A 148 (2015) 29-42.
5
5