Red Emitting Monoazo Disperse Dyes with Phenyl(1H-benzoimidazol-5-yl) Methanone as Inbuilt Photostabilizing Unit: Synthesis, Spectroscopic, Dyeing and DFT Studies

Article abstract of DOI:10.1007/s10895-018-2226-3

Synthesis of three novel phenyl(1H-benzoimidazol-5-yl)methanone based fluorescent monoazo disperse dyes and their characterization by spectroscopic methods (¹H NMR, ¹³C NMR, IR and MS) are presented. Insertion of phenyl(1H-benzoimida

Full text of DOI:10.1007/s10895-018-2226-3

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2226-3
ORIGINAL ARTICLE
Red Emitting Monoazo Disperse Dyes
with Phenyl(1H-benzoimidazol-5-yl) Methanone as Inbuilt
Photostabilizing Unit: Synthesis, Spectroscopic, Dyeing and DFT Studies
1
1
1
Amol G. Jadhav & Suvidha S. Shinde & Nagaiyan Sekar
Received: 6 February 2018 /Accepted: 27 March 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Synthesis of three novel phenyl(1H-benzoimidazol-5-yl)methanone based fluorescent monoazo disperse dyes and their charac-
1
13
terization by spectroscopic methods ( H NMR, C NMR, IR and MS) are presented. Insertion of phenyl(1H-benzoimidazol-5-
yl)methanone moiety bring about induced fluorescence properties and enhanced photostability as compared to the previously
reported analogues (CI Solvent Yellow 14, 4-diethylamino-2-hydroxy-1-diazobenzene and 7-(diethylamino)-4-hydroxy-
3
-(phenyldiazenyl)-2H-chromen-2-one). Synthesized phenyl(1H-benzoimidazol-5-yl)methanone based dyes exhibited red-
shifted absorption maxima (497–516 nm), high molar extinction coefficients and are emitting in the far-red region (565–
27 nm). Moreover, naphthalene-comprising dyes showed negative solvatochromism while N,N-diethylamine comprising dyes
6
showed positive solvatochromism and are in good agreement with solvent polarity graphs and the computed energy levels of
highest occupied and lowest unoccupied molecular orbitals. Synthesised dyes have better photostability (light fastness) and
sublimation fastness on dyed polyester and nylon compared to reported analogues. DFT calculated energies, electrophilicity
index and Frontier Molecular Orbitals (FMO’s) enabled to evaluate the stabilities of azo and hydrazone forms of the dyes.
.
.
.
Keywords Phenyl(1H-benzoimidazol-5-yl)methanone Fluorescentmonoazodyes Solvatochromism Aza-hydrazoneandDFT
study
Introduction
made these dyes very successful [3–5]. Economy and ease of
preparation of azo dyes have made them highly applicable to
many colour industries [6, 7]. But fastness improvements are
vital in the textile industry since dyed fabrics have found wide
applications and are exposed to direct sunlight [8].
Azo dyes cover the single largest group of dyes with respect to
number and production scales with many industrial applica-
tions to introduce new and efficient colours to the substrates
[
1]. They are most versatile and robust having high tinctorial
Benzophenone based dyes have found applications in
photoinitiators and photosensitizers [9], thermal stabilizers
[10], UV absorbers [11], oxidants in photo-induced electron
transfer (PET) [12], fluorescent chemosensors [13], biological
probes [14] and Excites State Intramolecular Proton Transfer
strengths with a full-color range from yellow to blue-green
shades [2]. The high molar extinction coefficient, structural
diversity, good fastness properties and ease of synthesis have
(
ESIPT) based fluorescent chemosensors [15, 16]. In general,
benzophenone unit is photostabilizing and UV blocking entity
17]. Stability is influenced by appropriate substitutions in the
Amol G. Jadhav and Suvidha S. Shinde contributed equally to this work.
Electronic supplementary material The online version of this article
[
(https://doi.org/10.1007/s10895-018-2226-3) contains supplementary
material, which is available to authorized users.
aromatic ring at various positions of benzophenone [18].
Substitution at meta-position to the carbonyl group of benzo-
phenone derivatives show good intramolecular redox process
*
Nagaiyan Sekar
n.sekar@ictmumbai.edu.in; nethi.sekar@gmail.com
[19]. Similarly, substitution at para-position remarkably mod-
ifies photochemistry of the molecule [20]. Such appropriately
modified few promising benzophenone based mordant and
acid azo dyes are available in the literature [21, 22]. Some of
1
Department of Dyestuff Technology, Institute of Chemical
Technology, Nathalal Parekh Marg, Matunga, Mumbai 400 019,
India

J Fluoresc
the reported benzophenone derivatives are emissive [23–26].
In the same line Barsotti et al. recently did extensive studies on
fluorescence properties of 4-hydroxy benzophenone [23, 27].
Similarly, dyes containing imidazole unit have found wide
applications in dye sensitizer solar cells (DSSC) [28], hole
transporting materials [29], ESIPT based dyes [30] fluorescent
sensor [31] and solid-state emissive dyes [32]. Dyes with imid-
azole unit are also used in singlet oxygen generation [33] and
radical photopolymerization reactions [34]. Due to strong
accepting capacity [35] and high molar extinction coefficients
Solvatochromic properties are in good agreement with solvent
polarity graphs. Dyes 5a, 5b and 5c on dyed polyester and nylon
showed excellent light and sublimation fastness compared to
parent dyes 5a’, 5b’ and 5c’ respectively.
Experimental Section
Materials and Methods
[
36] of imidazole-based dyes, photostability gets enhanced [37].
3,4-Diaminobenzophenone and p-nitrobenzaldehyde were pro-
cured by Spectrochem Pvt. Ltd. Mumbai.Naphthalen-2-oland
were obtained from Sigma-Aldrich.3-(diethylamino)phenol, so-
dium nitrate, sodium carbonate, sodium hydroxide, conc. HCl,
urea, metamol (dispersing agent), sodium chloride and all or-
ganic solvents were purchased from S. D. Fine Chemicals Ltd.,
Mumbai, India. All reagents and solvents were characterised by
melting or boiling point and used without further purification.
Readymade dyeing polyester (100%) substrate (weight70 gm/
An important use of fluorescent dyestuffs is in the coloration
of synthetic fibres like polyesters, acrylics and polyamides.
Fluorescent textiles not only increases visibility but also pro-
vides high design options [38]. Many dyes for acrylic fibres,
particularly methines such as CI Basic Violet 7, CI Basic Red
1
3 and CI Basic Red 74 are fluorescent and very bright resulting
into important fluorescent textile fibres [38]. The dominating
structural classes for fluorescent textiles dyes include coumarins,
perylene and methines [38, 39]. But the available dyes do not
meet the simultaneous requirement of both good fluorescent
properties and light fastness properties. So, in order to develop
synthetic dyes for textile applications, it is advisable that modern
colourants should have both photostabilities as well as improved
spectroscopic characteristics [40].
2
m ) was purchased from Piyush Syndicate, Mumbai, India.
Melting points were recorded on the instrument from Sunder
1
13
Industrial Product, Mumbai. H NMR and C NMR spectra
were recordedat 25 °C on Agilent NMR vnmrs 500 MHz and
125 MHz respectively. Chemical shifts were expressed in ppm
using TMS as an internal standard. IR spectra were recorded on
Perkin Elmer spectrum-100 FTIR spectrometer. Mass spectra
were recorded on FINNIGAN LCQ ADVANTAGE MAX in-
strument from Thermo Electron Corporation (USA). All dying
were performed on Flexi dyer machine (RossariLabtech,
Mumbai, India).
Considering combined advantages of both benzophenone
core and imidazole unit we developed in-built stabilising
phenyl(1H-benzo[d]imidazol-5-yl)methanone group.
Presence of donors (–N(Et) , -OH groups), acceptors (imidaz-
2
ole, >C=O) and long π-conjugation [41] in the present
phenyl(1H-benzo[d]imidazol-5-yl)methanone based azo dyes
enable them to act as good candidates for attractive fluorescent
compounds. However, very few reports are available on fluo-
rescent monoazo disperse dyes and the strategies to make
them emissive [42–46]. Recently reported fluorescent dyes
contained phenyl(1H-benzo[d]imidazol-5-yl)methanone as a
core moiety [47]. So, we are interested to study the effect of
incorporation of phenyl(1H-benzo[d]imidazol-5-
yl)methanone moiety into few selected azo dyes available
commercially. Moreover, as benzophenone core is well
photostabilizing unit [17], it is expected that the designed dyes
not only become red emission in nature but can also show
good fastness properties.
Spectroscopic Instruments
Absorption and emission spectra of the compounds were re-
corded on Perkin Elmer Lambda 25 UV-Visible spectropho-
tometer and Varian Inc. Cary Eclipse spectrofluorometer re-
spectively. 5 μM solutions of the dyes were prepared by using
7 different polarity solvents, viz. toluene, 1,4-dioxane, CHCl3
(chloroform), EtOAc (ethyl acetate), MeOH (Methanol), ace-
tonitrile and N,N-dimethylformamide (DMF).
Fastness and Color Assessment Instruments
In the present work, we are reporting three red emitting dis-
perse dyes (5a, 5b and 5c) containing in-built photostabilising
and emission enhancing unit i.e. phenyl(1H-benzo[d]imidazol-
Light fastness of dyed samples was tested on Q-Sun Xenon
Test Chamber (Q-Lab Corporation, Ohio, USA) by the
AATCC 16–2004 method. Sublimation fastness of the dyed
samples were tested on Sublimation fastness tester (RBE
Electronics Engg. Pvt. Ltd., Mumbai, India) by the standard
method ISO 105-F04. Shade change and staining of adjacent
fabrics were rated according to appropriate Society of Dyers
& Colourists (SDC) grey scales. Colour properties of the dyed
samples were measured using Spectra Scan 5100+ under the
illuminant D65 using 10° standard observers.
5-yl)methanone. 5a, 5b and 5c disperse dyes exhibited red-
shifted absorption and emission compared to corresponding par-
ent analogues CI Solvent Yellow 14 (5a’) (CI 12055), 4-
diethylamino-2-hydroxy-1-diazobenzene (5b’) and
7
-(diethylamino)-4-hydroxy-3-(phenyldiazenyl)-2H-chro-
men-2-one) (5c’) dyes, respectively. Dye 5a and its parent an-
alogue dye 5a’ showed negative solvatochromism.

J Fluoresc
Computational Study
NH), 139.09 (Ar-C-N=), 136.50 (Ar-C-CO), 134.00 (Ar-C),
33.05 (Ar-C-CO), 131.79 (Ar-C), 130.42 (Ar-C), 129.39
1
All the DFT computations were performed using Gaussian 09
package [48] on an HP workstation XW 8600 with Xeon
processor, 4 GB RAM and Windows Vista as the operating
system. DFT and TD-DFT methods were employed for the
ground state and excited state optimisations respectively.
The hybrid functional B3LYP (Becke3-Lee-YangPar) as and
(Ar-C), 128.86 (Ar-C), 121.61 (Ar-C), 112.41 (Ar-C),
107.97 (Ar-C), 97.92 (Ar-C).
FT-IR: 3458 (N-H stretching), 1687 (C=O stretching),
1635 (Imine C=N stretch), 1281 (C-N stretching).
+
+
Mass (m/z): Calculated 314.12, [M + H] for C H N O
found 314.1, [M + H] .
2
0 16 3
+
6
-31G(d) basis set were used for all the atom [49]. Solvents
Elemental analysis (%) - Found: C, 76.6; H, 4.8; N,
used for Polarizable Continuum Model (PCM) [50] were tol-
13.4%; molecular formula C H N O calculated: C, 76.66;
2
0 15 3
uene, 1,4-dioxane, chloroform (CHCl ), ethyl acetate
H, 4.82; N, 13.41%.
3
(
EtOAc), methanol (MeOH), acetonitrile and N,N-
dimethylformamide (DMF).
General Procedure for Diazotization-Coupling Reactions
Synthesis and Characterization
Mixture of amine 3 (3.3 mmol), conc. HCl (3 ml, 35.0 mmol),
and water (15 mL) was boiled in reaction flask to get clear
solution. Solution was then cooled to 0 °C followed by gradual
addition of sodium nitrite (0.25 g, 3.6 mmol) with constant
stirring. Solution was allowed to stir below 5 °C for 30 min
and starch iodide paper was used as a process control test.
Reaction was monitored by spot test. After completion of reac-
tion urea was added to quench excess nitrous acid. Meanwhile,
the couplers (3.3 mmol) were dissolved in 10% sodium hydrox-
ide solution to get clear solution. Diazonium salt solution was
gradually added to the respective coupler solutions with contin-
uous stirring and maintaining the temperature below 5 °C.
During addition of diazonium salt pH was maintained in be-
Methanone (2-(4-nitrophenyl)-1H-benzo[d]imidazol-5-yl)
phenyl (2)
Mixture of 3, 4 diaminobenzophenone 1 (1.09 g, 5.2 mmol),
4
-nitrobenzaldehyde (0.71 g, 4.7 mmol) and water (25 mL)
was taken in a round bottom flask. Potassium ferro-cyanide
0.17 g, 10 mol%) was added to the mixture and stirred at
0 °C for 2 h. Reaction was monitored by using TLC (thin
(
3
layer chromatographic technique). After completion of the
reaction, solid obtained was filtered, washed with water, dried
and recrystallized from ethanol to obtain the desired product
(
Yield: 89%). Above said procedure and characterizations of
tween 7.5–8 using NaCO (10% w/v) solution.
3
compound 2 is reported in the literature [51].
After completion of reaction precipitated was formed
which was then filtered using a nutsche and washed thorough-
ly with water further. Recrystallized in ethanol afforded de-
sired final dyes 5a, 5b and 5c in good yields. Above said
procedure is reported in literature [1].
Methanone(2-(4-aminophenyl)-1H-benzo[d]imidazol-5-yl)
phenyl (3)
Compound 2 (2 g, 5.8 mmol) and Fe powder (0.81 g,
Couplers used for coupling reactions were Naphthalen-2-
ol, 3-(diethylamino)phenol and 7-(diethylamino)-4-hydroxy-
2H-chromen-2-one. Synthetic scheme for the preparations of
monoazo disperse dyes (5a-5c) is shown in Scheme 1.
Structural analogues of 5a is available in the literature [52].
7-(diethylamino)-4-hydroxy-2H-chromen-2-one was pre-
pared by reported procedure [45]. Dyes 5a’ (CI Solvent
Yellow 14), 5b’ (4-diethylamino-2-hydroxy-1-diazobenzene)
and 5c’ (7-(Diethylamino)-4-hydroxy-3-(phenyldiazenyl)-
2H-chromen-2-one)) are available in the literature [45,
53–55]. Structural variations of present synthesised
phenyl(1H-benzo[d]imidazol-5-yl)methanone based dyes are
compared with previously reported parent dyes (Scheme 2).
Characterizations (Supporting Information)
1
4.5 mmol) were added in the reaction-flask containing meth-
anol (50 mL). To this mixture AcOH (0.83 mL, 14.5 mmol)
was added slowly to keep temperature below 30 °C. The re-
action mixture was then further refluxed at 64 °C for 30 min.
Reaction was monitored by using thin layer chromatographic
technique. After completion of the reaction, reaction mass was
cooled and neutralized by using NaHCO . Precipitate formed
3
was filtered, washed with water and dried. Recrystallization in
EtOH afforded the desired product 3 (Yield: 70%). Above said
procedure is reported in the literature [13]. Compound 3 is
characterized and provided in supporting information.
Yield: 70%, Melting point: 196–199 °C.
1
H-NMR δ (500 MHz, DMSO, TMS)(ppm): 7.76 (2H, d,
H
J = 8.0 Hz, Ar-H), 7.35 (2H, d, J = 8.0 Hz, Ar-H), 7.23 (2H, d,
J = 7.5 Hz, Ar-H), 7.15 (2H, d, J = 7.5 Hz, Ar-H), 7.13 (1H, s,
Ar-H), 7.05 (2H, t, J = 7.5 Hz, Ar-H), 6.99 (1H, d, J = 8.0 Hz,
5a:Phenyl(2-phenyl 4-(2-hydroxynaphthalene-1-
diazene)1H-benzo[d]imidazol-5-yl)methanone.
Yield: 85%, Melting point: 129–131 °C.
1
Ar-H), 5.52 (1H, s, N-H), 4.50 (2H, s, -NH ).
H-NMR δH (500 MHz, DMSO, TMS) (ppm): δ13.40
2
1
3
C NMR δ (125 MHz, DMSO, TMS)(ppm): 196.51
(1H, s, Hydrogen bonding), 8.57 (1H, d, J = 8.5 Hz, Ar-H),
8.34 (2H, d, J = 8.5 Hz, Ar-H), 8.06–8.01 (2H, m, Ar-H), 7.97
C
(
>C=O), 160.47 (-C=N), 153.63 (Ar-C-NH ), 151.73 (Ar-C-
2

J Fluoresc
O
O
NH2
N
a
b
NO2
N
H
NH2
1
2
O
O
N
c
N
NH2
N N
N
H
N
H
3
4
OH
O
N
HO
N
N
N
H
N
HO
5a
O
N
O
N
N
HO
d
N N
OH
O
N
H
N
N
N
H
5b
4
N
O
O
N
HO
N
N
N
H
N
O
5c
O
o
Reagents and Conditions: a) 4-nitrobenzaldehyde, K [Fe(CN) ], H O, 30 C; b) Fe-powder, AcOH,
4
6
2
o
o
MeOH, 64 C; c) NaNO , HCl, 0-5 C; d)Na CO , pH=7-8.
2
2
3
Scheme 1 Synthesis of monoazo disperse 5a, 5b and 5c dyes
(
(
1H, d, J = 9.5 Hz, Ar-H), 7.90 (1H, d, J = 4.0 Hz, Ar-H), 7.81
1H, d, J = 8.5 Hz, Ar-H), 7.79–7.75 (2H, m, Ar-H), 7.70 (2H,
N=N), 147.75 (Ar-C-N=N), 146.02 (Ar-C-NH), 143.73 (Ar-
C), 138.57 (Ar-C-NH), 135.14 (Ar-C), 134.77 (Ar-C), 133.50
(Ar-C), 133.15 (Ar-C), 132.53 (Ar-C), 131.65 (Ar-C), 131.50
(Ar-C), 130.39 (Ar-C), 129.88 (Ar-C), 128.88 (Ar-C), 128.78
(Ar-C), 128.66 (Ar-C), 128.45 (Ar-C), 126.91 (Ar-C), 125.16
(Ar-C), 122.16 (Ar-C), 119.34 (Ar-C), 114.60 (Ar-C), 111.96
(Ar-C), 109.98 (Ar-C), 106.98 (Ar-C), 100.13 (Ar-C).
dd, J = 12.0, 7.5 Hz, Ar-H), 7.64 (2H, dd, J = 15.5, 7.5 Hz, Ar-
H), 7.58 (2H, t, J = 7.5 Hz, Ar-H), 7.48 (1H, t, J = 7.5 Hz, Ar-
H), 6.88 (1H, dd, J = 9.5, 4.0 Hz, Ar-H), 5.48 (1H, s, N-H).
1
3
C NMR δ (125 MHz, DMSO, TMS)(ppm): 195.88
C
(
>C=O), 156.02 (Ar-C-OH), 154.23 (-C=N), 153.34 (Ar-C-
Previously reported dyes
O
O
N
N
N
N
N
N
N
N
HO
HO
HO
5
b'
5c'
5
a'
O
O
O
O
O
N
N
N
N
N
N
N
N
N
N
N
H
HO
N
N
H
HO
N
H
HO
5c
5
a
5b
Present dyes
Scheme 2 Comparative structural variations of previously reported and present dyes

J Fluoresc
FT-IR: 3058 (O-H stretching phenolic), 1685 (C=O
stretching), 1643 (C=C stretching), 1528 (N=N stretching),
130.21 (Ar-C), 129.18 (Ar-C), 128.65 (Ar-C), 127.15 (Ar-C),
125.44 (Ar-C), 124.59 (Ar-C), 122.29 (Ar-C), 121.40 (Ar-C),
119.37 (Ar-C), 114.87 (Ar-C), 112.15 (Ar-C), 107.76 (Ar-C),
102.83 (Ar-C), 97.71 (Ar-C), 92.78 (Ar-C), 45.19 (N,N diethyl-
CH ), 13.49 (N,N diethyl-CH ).
−1
1
274 (C-N stretching) cm .
+
Mass (m/z): Calculated 469.17, [M + H]
for
+
2
+
C H N O found 469.1, [M + H] .
3
0
21
4
2
3
Elemental analysis (%) - Found: C, 76.9; H, 4.3; N,
FT-IR: 3549 (N-H stretch imidazole), 3064 (O-H
1
1.9%; molecular formula C H N O calculated: C, 76.91;
stretching phenolic), 1694 (C=O stretching), 1642 (C=C
3
0 20 4 2
−1
H, 4.30; N, 11.96%.
stretching), 1526 (N=N stretching), 1274 (C-N stretch) cm .
+
5
b: Phenyl(2-phenyl 4-(3-(diethylamino)phenol)1H-
Mass (m/z): Calculated 558.21, [M + H]
for
+
+
benzo[d]imidazol-5-yl)methanone.
C H N O found 558.2, [M + H] .
3
3 28 5 4
Yield: 71%, Melting point: 120–122 °C.
Elemental analysis (%) - Found: C, 71.1; H, 4.8; N,
12.5%; molecular formula C H N O calculated: C, 71.08;
1
H-NMR δ (500 MHz, DMSO, TMS) (ppm): 13.33 (1H,
H
33 27 5 4
s, Hydrogen bonding), 8.29 (2H, d, J = 8.5 Hz, Ar-H), 7.87
H, 4.88; N, 12.56%.
(
(
3H, d, J = 8.5 Hz, Ar-H), 7.76 (2H, d, J = 7.5 Hz, Ar-H), 7.68
2H, d, J = 7.5 Hz, Ar-H), 7.65 (1H, s, Ar-H), 7.58 (2H, d, J =
General Procedure of Dyeing
7
.5 Hz, Ar-H), 7.56 (1H, s, Ar-H), 7.51 (1H, d, J = 9.0 Hz, Ar-
H), 6.53 (1H, d, J = 9.0 Hz, Ar-H), 6.05 (1H, s, N-H), 3.46
Nylon and polyester fabrics dyeing were carried out using 2%
shade depth and material to liquor ratio of 1:30. Total dye
solution calculated on the weight of fabric. As azo disperse
dyes were insoluble in water, hence dissolved in 5 ml of N,N-
dimethylformamide followed by dilution with 15 ml of buff-
ered solution of pH 4 to 5 by using acetic acid in water.
Ultrasonication for 30 min resulted in thefine dispersion of
the dye in water. Saragen 50 was used as a dispersing agent.
Nylon and polyester fabrics were dyed using the above dye
solution. Dyeing was started at room temperature and raises to
130 °C (PET), 95 °C (Nylon) respectively temperatures for
50 min, and cooled to 60 °C. The dyed fabrics were rinsed
with warm & cold water. Reduction clearing treatment was
given only Polyester fabric using 2 g/l soda ash (Na CO ), 2 g/
(
4H, q, J = 7.0 Hz, N-(CH ) ), 1.15 (6H, t, J = 7.0 Hz, N-(C-
2 2
CH ) ).
3
2
1
3
C NMR δ (125 MHz, DMSO, TMS)(ppm): 195.98
C
(
>C=O), 159.94 (Ar-C-N(Et) ), 153.10 (Ar-C-OH), 151.20
2
2
1
1
3 (-C=N), 148.12 (Ar-C-N=N), 143.76 (Ar-C-N=N),
38.57 (Ar-C-NH), 135.97 (Ar-C), 133.47 (Ar-C-NH),
32.52 (Ar-C), 131.50 (Ar-C), 131.27 (Ar-C), 130.72 (Ar-
C), 129.89 (Ar-C), 128.86 (Ar-C), 128.33 (Ar-C), 126.84
Ar-C), 125.12 (Ar-C), 124.27 (Ar-C), 121.97 (Ar-C),
21.08 (Ar-C), 119.05 (Ar-C), 114.55 (Ar-C), 111.44 (Ar-
C), 107.44 (Ar-C), 97.39 (Ar-C), 44.87 (N,N diethyl-CH2),
(
1
1
3.18 (N,N diethyl-CH3).
FT-IR: 3286 (N-H stretch imidazole), 2973 (O-H
2
3
stretching phenolic).
688 (C=O stretching), 1640 (C=C stretching), 1521 (N=N
l Sodium hydrosulphite and 1 g/l soap solution at 70 °C for
30 min (1:50) then treated fabrics were rinsed with cold water
and allowed to dry in the open air.
1
−1
stretching), 1269 (C-N stretch) cm .
+
Mass (m/z): Calculated 490.22, [M + H]
for
+
+
C H N O found 490.2, [M + H] .
3
0 28 5 2
Elemental analysis (%) - Found: C, 73.6; H, 5.5; N,
4.3%; molecular formula C H N O calculated:C, 73.60;
Results and Discussion
1
3
0 27 5 2
H, 5.56; N, 14.31%.
Spectroscopic Characteristics
5c:Phenyl(2-phenyl4-(7-(diethylamino)-4-hydroxy-2H-
chromen-2-one)1H-enzo[d]imidazol-5-yl)methanone.
Absorption spectra of synthesised azo dyes were recorded in
different polarity solvents (Fig. 1). Tabulated results in Table 1
suggested that dye 5a (λmax = 504 nm to 516 nm) and its
Yield: 69%, Melting point: 139–142 °C.
1
H-NMR δ (500 MHz, DMSO, TMS) (ppm): 13.02 (1H, s,
H
Hydrogen bonding), 7.98 (2H, d, J = 8.5 Hz, Ar-H), 7.56 (3H, d,
J = 8.5 Hz, Ar-H), 7.45 (2H, d, J = 7.5 Hz, Ar-H), 7.36 (2H, d,
J = 7.5 Hz, Ar-H), 7.34 (1H, s, Ar-H), 7.27 (2H, d, J = 7.5 Hz,
Ar-H), 7.24 (1H, s, Ar-H), 7.20 (1H, d, J = 9.0 Hz, Ar-H), 6.22
parent analogue 5a’ (CI Solvent Yellow 14) (λmax
472 nm to 476 nm) exhibited negative solvatochromism in
absorption properties. On the other hand, dyes 5b (λmax
=
=
498 nm to 513 nm) and 5c (λmax = 497 nm to 511 nm) and
their respective parent analogues dyes 5b’ (λmax = 364 nm to
376 nm) and 5c’ (λmax = 456 nm to 460 nm) showed positive
solvatochromism in absorption properties. Similarly, 5a, 5b
and 5c on excitation from 497 nm to 516 nm exhibited signif-
icant solvatochromism in emission properties (Fig. 2). Dye 5a
and is emitting with negative solvatochromism from 569 nm
to 602 nm, while dyes 5b and 5c is emitting with positive
(1H, d, J = 9.0 Hz, Ar-H), 5.74 (1H, s, N-H), 3.15 (4H, q, J =
7
.0 Hz, N-(CH ) ), 0.84 (6H, t, J = 7.0 Hz, N-(C-CH ) ).
2
2
3 2
1
3
C NMR δ (125 MHz, DMSO, TMS)(ppm): 196.30
C
(
>C=O), 160.26 (Ar-C-N(Et) ), 157.37 (-CO-O), 153.42 (Ar-
2
C-OH), 151.52 (-C=N), 148.44 (Ar-C-N=N), 144.08 (Ar-C-
N=N), 138.89 (Ar-C-NH), 136.29 (Ar-C), 133.79 (Ar-C-NH),
132.84 (Ar-C), 131.81(Ar-C), 131.59 (Ar-C), 131.03 (Ar-C),

J Fluoresc
Fig. 1 Normalized Absorption spectra of 5a’, 5a, 5b and 5c dyes in different solvent
solvatochromism from 565 nm to 627 nm in varying polarity
solvents. 5a, 5b and 5c showed very high molar extinction
coefficients (ε) as compared to parent dye 5a’, 5b’ and 5c’,
respectively suggested increased in conjugation and the chro-
mophore area.
these dyes with acid titrations [56]. Further it is also described
that azo bridge get protonated in acidic medium resulted into
formation of azonium cation [57]. So in order to get into the
details of possible forms of the dyes, trifluoroacetic acid (acid)
was added to dyes solutions (in methanol) resulting into a new
strong red shifted absorption band in absorption spectra at
around 582 nm to 592 nm region (Supporting Information
Fig. S1 (a)). In the acidic medium highly expected protonated
azonium cation is formed [57].
Stability study was performed under Ultraviolet irradiation
(at 254 nm) for better understanding of stabilities of azo and
protonated forms of the dyes. Stabilities studies of protonated
forms carried out at pH = 3 (in methanol) suggested that pro-
tonated forms of 5b and 5c are stable under irradiations. Azo
forms of 5a, 5b and 5c dyes are also found to be stable under
ultraviolet irradiation (Supporting Information Fig. S1 (b)).
Dyes 5a, 5b and 5c exhibited a small peak at around
3
40 nm which corresponds to the benzophenone unit. Stokes
−
1
shift (Δϑ) of 5a decreases from 2769 cm (toluene) to
1
2
267 cm− (DMF). On the other hand, Stokes shift (Δϑ) of
−1
−1
5
b and 5c increases from 2444 cm (toluene) to 2937 cm
−1
−1
(
DMF) and from 2422 cm (toluene) to 3621 cm (DMF),
respectively (Table 1). All the synthesised dyes have a
significant effect of solvent polarity on Stokes shifts (Δϑ).
−
1
5
a exhibited minimum Δϑ in DMF (2267 cm ) while
−1
maximum in toluene (2769 cm ). On contrary, 5b and 5c
exhibited minimum Δϑ in toluene (2444 cm ) and
2422 cm ) while maximum Δϑ in DMF (2927 cm ) and
−
1
−1
−1
(
(
−1
3621 cm ) respectively.
Estimations of Photo-Physical Properties
Among all the parent dyes, only 5c’ is weakly emissive in
nature and show positive solvatochromism. The dye 5c’ ex-
hibited significantly larger Stokes shifts having minimum Δϑ
The role of solvent polarity and different substituents for al-
tering spectroscopic processes have been estimated in terms of
−
1
in toluene (3377 cm ) while maximum in acetonitrile
the radiative rate constant (k ), then on-radiative rate constant
r
−
1
(
5662 cm ) (Table 1). Hence, it can be concluded that
(k ) and life time τ (ns) evaluations. Relative fluorescence
nr
significant effect of substituents on spectroscopic
characteristics of the dyes have been observed.
quantum yields (Ф ) of 5a, 5b and 5c were obtained by using
f
Rhodamine 6G (Ф = 0.94 in Ethanol) as standard. k > > k
r
f
nr
It is reported that tautomer forms of monoazo dyes in equi-
librium can be shifted towards hydrazone form by treating
for all dyes indicate so much loss of energy during non-
radiative processes. k are found to be in good agreement with
r

J Fluoresc
Table 1 Spectroscopic characteristics of 5a, 5b, 5c, 5a’, 5b’ and 5c’ dyes in different solvents
Dye
Entry
Toluene
Dioxane
CHCl3
EtOAc
MeOH
Acetonitrile
DMF
5
5
a’
λ
max (nm)
476
7.4
476
9.2
475
9.8
474
9.6
473
10.4
507
14.2
579
72
473
10.2
506
12.8
576
70
472
7.6
ε (mM− cm−1
1
)
a
λmax (nm)
516
8.8
515
15.0
599
84
513
12.1
593
80
510
12.0
587
77
504
8.4
ε (mM− cm−1
1
)
λemm (nm)
602
86
569
65
Δϑ (nm)
Δϑ (cm−
1
)
2769
364
9.4
2723
366
9.1
2630
367
9.0
2572
370
9.5
2453
372
8.9
2402
373
9.3
2267
376
9.6
5
5
b’
λmax (nm)
ε (mM− cm−1
1
)
b
λmax (nm)
498
24.0
567
69
500
29.0
571
71
503
27.0
577
74
506
27.0
585
79
509
32.0
594
85
510
29.0
597
87
513
22.0
604
91
ε (mM− cm−1
1
)
λemm (nm)
Δϑ (nm)
Δϑ (cm−
1
)
2444
456
9.9
2487
457
10.9
581
124
4670
499
30.0
570
71
2550
453
9.8
2669
457
9.5
2811
463
10.2
613
150
5285
508
12.0
605
97
2857
460
10.1
622
162
5662
509
15.0
616
107
3413
2937
460
10.4
621
161
5636
511
19.0
627
116
3621
5
5
c’
λmax (nm)
ε (mM− cm−1
1
)
λemm (nm)
539
83
588
135
5068
502
23.0
586
84
582
125
4700
505
24.0
594
89
Δϑ (nm)
Δϑ (cm−
1
)
3377
497
22.0
565
68
c
λmax (nm)
ε (mM− cm−1
1
)
λemm (nm)
Δϑ (nm)
Δϑ (cm−
1
)
2422
2496
2855
2967
3156
low fluorescence quantum yield (Ф ). Life time (τ) has not
shown significant variations with the varying polarity of the
solvents (Table 2).
also absorption is also sensitive to the solvent polarity. So
we utilised Bakhshiev plot which depends on additive
frequencies of absorption and emission with solvent polarity
functions [60]. Bakhshiev plot showed significant positive
f
2
Solvent Polarity Graphs
slope for dyes 5a (R = 0.9939), and significant negative
2
2
slope for dyes 5b (R = 0.9940) and 5c (R = 0.9958) (Fig.
3c). Hence we can reveal that significant effect of
naphthalene and N,N-diethylamine substituents were
observed, resulting into negative solvatochromism for dye
5a in both absorption and emission while opposite trend of
positive solvatochromism was observed for 5b and 5c in both
absorption and emission.
Statistical evalution for solvatochromic properties of 5a, 5b
and 5c is provided by solvent polarity functions like Lippert-
Mataga, Weller and Bhakshiev. Lippert-Mataga plot [58] has
shown very good linearity of Stokes shift vs. ƒ
func-
LM(ε,η)
2
tions with excellent regression coefficients (R ≥ 0.9834) sug-
gested a significant effect of solvent polarity on Stokes shift.
Stokes shift decreases for 5a, while increases for 5b and 5c
with increasing solvent polarity (Fig. 3a). Weller’s equation
allows estimation of the excited state dipole moments [59] and
Colour Properties of the Dyes
−1
gave the plot of emission frequency (cm ) versus Weller’s
Dyes 5a, 5b, 5c, 5a’, 5b’and 5c’were applied at 2% shade
with MLR 1:30 on nylon and polyester respectively. Dyeing
was evaluated using the CIELAB system in terms of L*, a*
and b*.
function [60]. Weller plot showed distinct positive slope with
2
excellent regression coefficients (R = 0.9853) for 5a indicat-
ed negative solvatochromism. On the other hand, anegative
2
slope with excellent regression coefficients of R = 0.9868
K/S values were determined by using below eq. 1 [61].
2
and R ≥ 0.9960 for 5b and 5c indicated positive
2
K
S
ð1−RÞ
2R
solvatochromism (Fig. 3b). Not only Stokes shift and
emission properties were affected by solvent polarity, but
¼
ð1Þ

J Fluoresc
Fig. 2 Normalised Emission spectra of dyes 5a (excited at 504–516 nm), 5b (excited at 498–513 nm) and 5c (excited at 497–511 nm) in different
solvents
where, R is the reflectance of coloured samples and K and S
are the absorption and scattering coefficients respectively.
Comparative K/S values of Dyes all the dyes dyed on nylon
are tabulated in Table 3. 5c showed highest K/S compared to
other dyes due to presence of presence of extended conjugated
fluorescent coumarin core. Phenyl(1H-benzo[d]imidazol-5-
yl)methanone based 5a, 5b and 5c dyes have good colour
strengths compared to respective parent analogues 5a’, 5b’
Table 2 Estimated spectroscopic (emission) characteristics of 5a, 5b, 5c and 5c’ in different solvent
Dye
Entry
Toluene
Dioxane
CHCl3
EtOAc
MeOH
Acetonitrile
DMF
5
5
5
5
a
Ф
f
0.0127
0.12
0.0211
0.13
0.0101
0.12
0.0184
0.13
0.0173
0.14
0.0176
0.13
0.0149
0.15
τ (ns)
8
8
8
9
8
8
8
8
Kr (s-1)
1.0 × 10
1.6 × 10
0.8 × 10
8.3 × 10
0.0219
0.09
1.4 × 10
1.2 × 10
1.3 × 10
1.0 × 10
nr(s−
1
)
)
)
)
8.1 × 10
0.0183
0.09
9
7.8 × 10
0.0367
0.13
9
7.6 × 10
0.0288
0.12
9
7.2 × 10
0.0321
0.12
9
7.4 × 10
9
6.6 × 10
0.0177
0.09
9
K
b
c’
c
Ф
f
0.0214
0.09
τ (ns)
K
K
r
(s−
1
)
2.0 × 10
1.1 × 10
0.0094
0.11
8
2.9 × 10
0.7 × 10
0.0108
0.14
8
2.3 × 10
1.0 × 10
0.0131
0.10
8
2.5 × 10
0.8 × 10
0.0157
0.09
8
2.8 × 10
0.8 × 10
0.0011
0.12
8
2.3 × 10
8
2.0 × 10
1.2 × 10
0.0024
0.14
8
nr(s−
1
10
10
10
10
10
1.0 × 10
10
10
Ф
f
0.0028
0.14
τ (ns)
K
K
r
(s−
1
)
1.8 × 10
8.1 × 10
0.0367
0.13
8
2.1 × 10
7.6 × 10
0.0501
0.11
8
1.6 × 10
8.6 × 10
0.0346
0.13
8
1.4 × 10
7.8 × 10
0.0411
0.12
8
1.7 × 10
8.4 × 10
0.0239
0.13
8
1.2 × 10
8
2.5 × 10
7.1 × 10
0.0339
0.14
8
nr(s−
1
9
9
9
9
9
8.1 × 10
9
9
Ф
f
0.0278
0.13
τ (ns)
K
K
r
(s−
1
)
2.7 × 10
7.2 × 10
8
4.7 × 10
8.9 × 10
8
2.6 × 10
7.3 × 10
8
3.4 × 10
8.0 × 10
8
1.9 × 10
7.6 × 10
8
2.1 × 10
8
2.5 × 10
7.1 × 10
8
nr(s−
1
9
9
9
9
9
7.3 × 10
9
9

J Fluoresc
Fig. 3 Solvent polarity plots: a Lippert-Mataga, b Weller plot and c Bakhshiev plot of 5a, 5b and 5c dyes for different polarity solvents
and 5c’. Good K/S strength of a dyeing and higher values of
present dyes represent darker and more saturated colours.
Comparative K/S values of all the dyes dyed on
polyester are tabulated in Table 4. 5a showed highest
K/S compared to other dyes due to presence of presence
of extended conjugated fluorescent coumarin core. Dyes
Light Fastness Properties
Evaluations of light fastness properties of dyes were per-
formed by comparative light fastness measurements of before
and after exposure of dyed polyester and nylon samples to the
xenon lamp. Same dyed sample was used for measurement in
order to have more precision and better comparison of light
fastness values. Half part of the sample was exposed to a
xenon lamp for 48 h while the other half was covered during
the light fastness determination. The samples were then
5
a, 5b and 5c are having higher K/S values compared
to their respective analogues 5a’, 5b’ and 5c’. Good
K/S strength of a dyeing and higher values of present
dyes represent darker and more saturated colours.
Table 3 Color coordinates (CIELAB) of 5a, 5b, 5c, 5a’, 5b’ and 5c’
Table 4 Color coordinates (CIELAB) of 5a, 5b, 5c, 5a’, 5b’ and 5c’
dyes for Nylon dyeing
dyes for Polyester dyeing
0
0
Dye
L*
a*
b*
c*
h
K/S
Dye
L*
a*
b*
c*
h
K/S
5
5
5
5
5
5
a’
41.1
31.2
39.2
41.1
43.1
57.9
38.9
26.3
33.9
16.9
41.5
35.1
39.5
18.4
37.8
45.6
44.4
65.7
55.4
32.1
52.5
48.7
56.7
74.5
45.4
34.9
42.3
69.6
49.3
61.8
19.06
27.61
13.87
22.56
24.06
36.60
5a’
5a
57.9
41.2
53.8
41.3
49.9
43.1
46.0
54.5
47.2
46.8
43.3
44.7
76.6
43.7
73.7
41.5
70.8
49.7
89.4
69.9
89.6
62.6
85.7
66.9
58.9
38.6
57.9
41.6
55.8
48.0
32.65
52.59
29.95
44.69
26.82
39.09
a
b’
b
5b’
5b
c’
c
5c’
5c

J Fluoresc
compared with the Blue wool standard scale and fastness rat-
ings were given. On comparing light fastness values of dyed
polyester and dyed nylon for any dye, it can be concluded that
dyed polyester sample have slightly better light fastness com-
pared to dyed nylon sample. Moreover, light fastness of all the
synthesized dyes (5a, 5b and 5c) varies from good to very
good for dyed polyester and dyed nylon samples (Table 5).
It can be further concluded that synthesized dyes showed bet-
ter light fastness as compared to commercially available re-
spective analogues 5a’, 5b’ and 5c’ dyes.
corresponding hydrazone forms. On the other hand hydrazone
forms of 5b and 5c are stable than azo forms (Table 6).
Frontier Molecular Orbital Energies
Energy levels of frontier molecular orbitals i.e. HOMO, LUMO
and their spatial distributions can give insight into excitation
properties and the idea of the ability of hole or electron injection,
which in turn allowed to understand photostability and spectro-
scopic characteristics of the dyes. In order to get into the stability
details of azo and hydrazone forms of the dyes, comparative
energy levels and electronic densities need to study. Pictorial
diagram of FMO’s suggested that the electron densities at
HOMOs of CI Solvent Yellow 14 (5a’) and 5a dyes were
located on the N=N motif, while electron densities on the
LUMOs were found to be localised on N=N motif periphery
of the azo forms of the dyes (Fig. 4a). On the other hand, elec-
tron densities at HOMOs of 5b and 5c dyes were located on
N,N-diethylamino, -N=N- and imidazole motif. Electron densi-
ties at the LUMOs were found to be localised mostly on the
Sublimation Fastness
In order to get the sublimation fastness, dyed samples were
sandwiched between two undyed cloth pieces (cotton and poly-
ester/nylon) and were subjected to 150 °C, 180 °C and 210 °C for
30 s in a sublimation fastness tester. Two parameters, i.e. colour
change (cc) and colour staining (on undyed cloth) [cs] were rated.
Synthesized dyes 5a, 5b and 5c showed better sublimation fast-
ness as compared to previously available 5a’, 5b’ and 5c’ deriv-
ative at all the temperatures (Table 5) on polyester and nylon.
Dyes 5a, 5b and 5c exhibited excellent sublimation fastness for
>C=O motif of the azo forms of the dyes. This suggested that
there is good charge transfer was observed in 5b and 5c dyes
resulting into significant positive solvatochromism. HOMO-
LUMO energy gap is significantly reduced in 5a compared to
CI Solvent Yellow 14. Moreover, there is lowering of HOMO
and LUMO energy levels of 5a compared to CI Solvent Yellow
14 clearly suggested red-shifted absorption maxima in 5a. Dyes
5b and 5c are also having similar energy levels as compared to
(150 °C, 180 °C, 210 °C) on both polyester and nylon.
DFT Study
Geometry Optimisation and Calculated Energies of Tautomer
Forms
5a. So, improved spectroscopic characteristics were observed
There is a possibility of the existence of tautomer forms that is
evident from shoulder peaks in the absorption spectra of these
dyes (Fig. 1). Optimised structures of Azo and Hydrazone forms
of dyes 5a at B3LYP/6-31G(d) in chloroform have shown in
supporting information Fig. S2. It is clearly observed that both
the forms have a significant difference at the 47H atom. An azo
form of CI Solvent Yellow 14 (5a’) and 5a are stable than the
for dyes 5a, 5b and 5c compared to 5a’ dye.
HOMO-LUMO electronic densities of hydrazone forms of
5a’ and 5a showed truncated delocalization of electronic den-
sities compared to corresponding azo forms of dyes (Fig. 4b).
Moreover, HOMO energy levels of azo forms of 5a’ and 5a
are more stabilized than hydrazone forms. On the other hand,
HOMO and LUMO electronic densities of hydrazone forms
Table 5 Light and sublimation fastness ratings of 5a, 5b, 5c, 5a’, 5b’ and 5c’ dyes for dyed nylon and polyester
Dye
Polyester
Nylon
Light fastness rating
Sublimation fastness rating
Light fastness rating
(1–8)*
Sublimation fastness rating
(1–8)*
(1–5)*
(1–5)*
1
50 °C
180 °C
210 °C
150 °C
180 °C
210 °C
5
5
5
5
5
5
a’(Solvent Yellow 14)
5/6
7
3
2/3
4
2
5
2/3
4
2
1/2
3
a
4/5
2/3
4/5
2/3
4/5
3/4
1/2
3/4
2
6/7
5/6
6/7
5
3/4
2/3
3/4
2/3
3/4
b’
b
6
2
3
2
7
4
4
3
c’
c
6
2/3
4
3
2
7
3/4
6/7
4
3/4

J Fluoresc
Table 6 Comparative energies of azo and hydrazone forms of 5a’, 5a,
of 5b and 5c dyes are comparatively same as that of their azo
forms, but HOMO energy levels of hydrazone forms are more
stabilized than their corresponding azo (Fig. 4b).
5
b and 5c dyes in Chloroform at B3LYP/6-31G(d)
Compound name
E/Hartree
ΔE/kJ mol-1
ΔG/Hartree
Calculated HOMO and LUMO energy levels for the azo
forms of dyes in solvents of varying polarity suggested that there
were an increase in the energy gap for dyes 5a (2.850 to
5
5
5
5
5
5
5
5
a’Azo
−801.388745
−801.384091
−1524.261226
−1524.256309
−1583.128132
−1583.128454
−1847.882662
−1847.893409
0
−801.440571
−801.445106
−1524.345413
−1524.35064
−1583.22673
−1583.227169
−1847.988914
−1848.000234
a’Hydrazone
a Azo
12.221404
0
2
3
.880 eV) and CI Solvent Yellow 14 (5a’) (3.723 to
.752 eV) with increasing solvent polarity, correlated with neg-
ative solvatochromism. On the other hand, 5b (2.729 to
.704 eV) and 5c (2.766 to 2.745 eV) showed decrease in the
a Hydrazone
b Azo
12.912042
0.845572
b Hydrazone
c Azo
0
2
28.221622
0
energy gap with increase in the solvent polarities correlated with
the positive solvatochromism (Supporting Information Fig.
S3a). Similar trends were observed for hydrazone forms of the
dyes. 5a’ and 5a dyes showed slight elevation of HOMO energy
levels while 5b and 5c showed lowering of HOMO levels
c Hydrazone
Fig. 4 (a) Comparative molecular orbitals and energy gaps of azo forms of 5a’, 5a, 5b and 5c dyes optimized at B3LYP/6-31G(d) in chloroform (b)
Comparative molecular orbitals and energy gaps of hydrazone forms of 5a’, 5a, 5b and 5c dyes optimised at B3LYP/6-31G(d) in chloroform

J Fluoresc
compared to their corresponding azo forms. LUMO energy
levels and HOMO-LUMO band gap is lowered in all the
hydrazone forms of the dyes compared to their corresponding
azo forms (Supporting Information Fig. S3b).
TD-DFT
Ground state optimised geometry of dyes at ground state in sol-
vents of various polarities was subjected to TD using B3LYP/6-
31G(d) function. At least 10 excited states were calculated for
each molecule. Comparative absorption maxima, computed ver-
tical excitations, oscillator strength and their orbital contributions
in chloroform are listed in Table 7. The results of DFT and TD-
DFT suggest that there was the observable influence of (1H-
benzo[d]imidazol-5-yl)(phenyl)methanone group on the absorp-
tion spectra of the dyes. Computed values for these dyes are in
good agreement with the experimental observations.
Electrophilicity Index
We have calculated the stability of the dyes mathematically
using computationally deduced energies at B3LYP/6-31G(d).
For this reason, the electrophilicity index (ω) was utilised. As
defined by Parr et al. [62, 63], this electrophilicity index mea-
sures the propensity of the moiety to absorb electrons and is
mathematically formulated as:
2
μ
ω ¼ 2
ð2Þ
η
where,
ω
μ
η
electrophilicity index
chemical potential
chemical hardness
ðIP þ EAÞ
μ ¼ −
ð3Þ
ð4Þ
2
ðIP−EAÞ
η ¼
2
where, IP = Ionization Potential, i.e., the change in the energy
when an electron is removed from the system.
EA = Electron Affinity, i.e., the change in the energy when
an electron is added to the system
ðELUMO þ EHOMOÞ
μ ¼
η ¼
ð5Þ
ð6Þ
2
ðELUMO−EHOMO
Þ
2

J Fluoresc
where,ELUMO is the energy of the lowest unoccupied molec-
ular orbital and EHOMO is the energy of the highest occupied
molecular orbital.
Æ
þ
−
ω ¼ ω þ ω
ð7Þ
ð8Þ
where,
2
ðELUMO
Þ
ωþ ¼
2
ðE
−E
Þ
LUMO
HOMO
and
2
ðEHOMOÞ
−
ω ¼
ð9Þ
2
ðELUMO−EHOMOÞ
Among azo and hydrazone forms of the dyes the stable con-
formation of dyes were justified by calculating net electrophi-
±
±
licity index (ω ) [1]. The net electrophilicity index (ω ) allowed
us to predict comparative stabilities of azo and hydrazone tau-
±
tomer forms of the dyes. Net electrophilicity index (ω ) sug-
gested that azo form of 5a’ and 5a dye are slightly more stable
than hydrazone forms. On the other hand, the exactly reverse
trend was observed for 5b and 5c dyes with hydrazone forms
were slightly more stable than azo forms (Table 8).
Conclusion
Three novel phenyl(1H-benzoimidazol-5-yl)methanone based
fluorescent monoazo disperse dyes were successfully synthe-
sised. The dyes 5a, 5b and 5c exhibited red-shifted absorption
maxima from 497 nm to 516 nm and much higher molar ex-
tinction coefficient as compared to compared to parent dyes 5a’,
5b’ and 5c’ respectively. The dyes 5a, 5b and 5c are emitting in
the far-red region (565–627 nm) while only 5c’ is weakly emit-
ting (539 to 621 nm). Dye 5a and 5a’ showed negative
solvatochromism, while dyes 5b, 5c, 5b’ and 5c’ showed posi-
tive solvatochromism. Solvent polarity graphs are in good
agreement with solvatochromic data. Dyes 5a, 5b and 5c on
dyed polyester and nylon showed very good light and sublima-
tion fastness. DFTcalculated energies, electrophilicity index and
Frontier Molecular Orbitals calculations of 5a, 5b and 5c are in
good agreements with the experimental observations. 5a, 5b
and 5c are emission in nature and has good fastness properties
on fabrics, so can find the potential applications in high-
visibility colour dyeing of textile products [64].
Acknowledgements One of the author Amol G. Jadhav is thankful to
UGC for financial assistance in terms of SRF.
Suvidha Shinde is thankful to the Centre of Advanced Studies (UGC)
for JRF and SRF under the Special Assistance Programme (SAP).

J Fluoresc
1
2
2
9. Mitchell D, Lukeman M, Lehnherr D, Wan P (2005) Formal
Intramolecular Photoredox Chemistry of Meta-Substituted
Benzophenones. Org Lett 7:3387–3389. https://doi.org/10.1021/
ol051381u
0. Beckett A, Porter G (1963) Primary photochemical processes in
aromatic molecules. Part 10.-Photochemistry of substituted benzo-
phenones. Trans Faraday Soc 59:2051. https://doi.org/10.1039/
tf9635902051
1. Dixit B, Patel H, Dixit R, Desai D (2010) Synthesis, characteriza-
tion and dyeing assessment of novel acid azo dyes and mordent acid
azo dyes based on 2- hydroxy-4-methoxybenzophenone on wool
and silk fabrics. J Serbian Chem Soc 75:605–614. https://doi.org/
10.2298/JSC090704039D
References
1
.
Bhide R, Jadhav AG, Sekar N (2016) Light fast monoazo dyes with
an inbuilt photostabilizing unit: Synthesis and computational stud-
ies. Fibers Polym 17:349–357. https://doi.org/10.1007/s12221-
0
16-5717-3
2
.
Sharma RK, Gulati S, Pandey A, Adholeya A (2012) Novel, effi-
cient and recyclable silica based organic–inorganic hybrid Nickel
catalyst for degradation of dye pollutants in a newly designed
chemical reactor. Appl Catal B Environ 125:247–258. https://doi.
org/10.1016/j.apcatb.2012.05.046
3.
4.
5.
Sekar N (2014) Natural colorants versus synthetic colorants.
Colourage 61:54–56
Bafana A, Devi SSCT (2011) Azo dyes: past, present and the future.
Environ Rev 19:350–370
Seesuriyachan P, Takenaka S, Kuntiya A et al (2007) Metabolism of
azo dyes by Lactobacillus casei TISTR 1500 and effects of various
factors on decolorization. Water Res 41:985–992. https://doi.org/
22. Tsatsaroni EG, Eleftheriadis IC (2004) UV-absorbers in the dyeing
of polyester with disperse dyes. Dyes Pigments 61:141–147. https://
doi.org/10.1016/j.dyepig.2003.10.002
23. Barsotti F, Brigante M, Sarakha M et al (2015) Photochemical pro-
cesses induced by the irradiation of 4-hydroxybenzophenone in
different solvents. Photochem Photobiol Sci 14:2087–2096.
https://doi.org/10.1039/c5pp00214a
10.1016/j.watres.2006.12.001
6
7
.
.
Al-Sheikh M, Medrasi HY, Usef Sadek K, Mekheimer RA (2014)
Synthesis and Spectroscopic Properties of New Azo Dyes Derived
from 3-Ethylthio-5-cyanomethyl-4-phenyl-1,2,4-triazole.
Molecules 19:2993–3003. https://doi.org/10.3390/
molecules19032993
24. Bhasikuttan a C, Singh a K, Palit DK et al (1998) Laser Flash
Photolysis Studies on the Monohydroxy Derivatives of
Benzophenone. Laser Flash Photolysis Studies on the
Monohydroxy Derivatives of Benzophenone J Phys Chem 102:
3470–3480. https://doi.org/10.1021/jp972375l
2
2
2
5. Das PK, Encinas MV, Scaiano JC (1981) Laser flash photolysis
study of the reactions of carbonyl triplets with phenols and photo-
chemistry of p-hydroxypropiophenone. J Am Chem Soc 103:4154–
Abdou MM (2013) Thiophene-Based Azo Dyes and Their
Applications in Dyes. Chemistry 3:126–135. https://doi.org/10.
5
923/j.chemistry.20130305.02.
Athalye A (2015) Automotive Textiles. Int J Text Eng Process 1:
2–52
4
162. https://doi.org/10.1021/ja00404a029
8
.
.
6. Palit DK (2005) Photophysics and excited state relaxation dynam-
ics of p-hydroxy and p-amino-substituted benzophenones: a review.
Res Chem Intermed 31:205–225. https://doi.org/10.1163/
4
9
Ravve A (2006) Photosensitizers and Photoinitiators. In: Light
React Synth Polym. Springer New York, New York, pp 23–122
1
568567053147020
1
0. Jiang X, Rui Y, Chen G (2009) Improved Properties of Cotton by
Atmospheric Pressure Plasma Polymerization Deposition of
Sericin. J Vinyl Addit Technol 21:129–133. https://doi.org/10.
7. Barsotti F, Ghigo G, Berto S, Vione D (2017) The nature of the light
absorption and emission transitions of 4-hydroxybenzophenone in
different solvents. A combined computational and experimental
study. Photochem Photobiol Sci. https://doi.org/10.1039/
C6PP00272B
1
002/vnl
1
1. Patel HM (2014) Synthesis, Structure Investigation and Dyeing
Assessment of Novel Bisazo Disperse Dyes Derived from UV
Absorbing Material. IOSR. J Appl Chem 6:51–55
2
2
8. Kumar D, Justin Thomas KR, Lee C, Ho K (2014) Organic Dyes
Containing Fluorene Decorated with Imidazole Units for Dye-
Sensitized Solar Cells. J Org Chem 79:3159–3172. https://doi.org/
1
1
2. Bochet CG (2014) 9.13 Organic Photochemistry. In: Compr Org
Synth II. Elsevier, pp 330–350
1
0.1021/jo500330r
3. Jadhav AG, Shinde SS, Lanke SK, Sekar N (2017) Benzophenone
based fluorophore for selective detection of Sn2+ ion: Experimental
and theoretical study. Spectrochim Acta Part A Mol Biomol
Spectrosc 174:291–296. https://doi.org/10.1016/j.saa.2016.11.051
4. Dorman G, Prestwich GD (1994) Benzophenone Photophores in
Biochemistry. Biochemistry 33:5661–5673. https://doi.org/10.
9. Aulakh RK, Sandhu S, Tanvi, et al (2015) Designing and synthesis
of imidazole based hole transporting material for solid state dye
sensitized solar cells. Synth Met 205:92–97. doi: https://doi.org/
1
0.1016/j.synthmet.2015.03.030
1
1
3
3
3
0. Skonieczny K, Ciuciu AI, Nichols EM et al (2012) Bright, emission
tunable fluorescent dyes based on imidazole and π-expanded imid-
azole. J Mater Chem. https://doi.org/10.1039/c2jm33891b
1. Zhang X, Chen Y (2013) Synthesis and fluorescence of
dicyanoisophorone derivatives. Dyes Pigments 99:531–536.
https://doi.org/10.1016/j.dyepig.2013.05.031
2. Li W, Lin W, Wang J, Guan X (2013) Phenanthro[9,10- d ]imidaz-
ole-quinoline Boron Difluoride Dyes with Solid-State Red
Fluorescence. Org Lett 15:1768–1771. https://doi.org/10.1021/
ol400605x
1
021/bi00185a001
5. Sen GA, Paul K, Luxami V (2015) Ratiometric fluorescent
chemosensor for fluoride ion based on inhibition of excited state
intramolecular proton transfer. Spectrochim Acta, Part A Mol
Biomol Spectrosc 138:67–72. https://doi.org/10.1016/j.saa.2014.
11.026
1
6. Sen GA, Paul K, Luxami V (2016) Benzimidazole based
ratiometric chemosensor for detection of CN− and Cu2+ ions in
protic/aqueous system: Elaboration as XOR logic operation.
Inorganica Chim Acta 443:57–63. https://doi.org/10.1016/j.ica.
33. Prostota Y, Kachkovsky OD, Reis LV, Santos PF (2013) New un-
symmetrical squaraine dyes derived from imidazo[1,5-a]pyridine.
Dyes Pigments 96:554–562. https://doi.org/10.1016/j.dyepig.2012.
10.006
34. Fouassier J, Allonas X, Burget D (2003) Photopolymerization re-
actions under visible lights: principle, mechanisms and examples of
applications. Prog Org Coatings 47:16–36. https://doi.org/10.1016/
S0300-9440(03)00011-0
2
015.11.024
1
7. Kiguchi M, Evans PD (1998) Photostabilisation of wood surfaces
using a grafted benzophenone UVabsorber. Polym Degrad Stab 61:
3
3–45. https://doi.org/10.1016/S0141-3910(97)00124-9
1
8. Chou P-T, Chen Y-C, Yu W-S et al (2001) Excited-State
Intramolecular Proton Transfer in 10-Hydroxybenzo[h]quinoline.
J Phys Chem A 105:1731–1740. https://doi.org/10.1021/
jp002942w
35. Wan Z, Zhou L, Jia C et al (2014) Comparative study on photovol-
taic properties of imidazole-based dyes containing varying electron

J Fluoresc
acceptors in dye-sensitized solar cells. Synth Met 196:193–198.
50. Zhao Y, Truhlar DG (2008) The M06 suite of density functionals
for main group thermochemistry, thermochemical kinetics,
noncovalent interactions, excited states, and transition elements:
two new functionals and systematic testing of four M06-class func-
tionals and 12 other function. Theor Chem Accounts 120:215–241.
https://doi.org/10.1007/s00214-007-0310-x
51. Shaikh KA, Patil VA, Shaikh PA (2012) An Efficient and
Convenient Synthesis of Imidazolines and Benzimidazoles via
Oxidation of Carbon-Nitrogen Bond in Water Media. Chinese J
Chem 30:924–928. https://doi.org/10.1002/cjoc.201100210
52. Devi TC, Krishnan R, Kunju AS (2004) Synthesis and
Characterization of Copper(II), Cobalt(II) and Manganese(II)
C o m p l e x e s o f 2 - ( 2 ′ - h y d r o x y n a p h t h y l a z o ) - 5 -
benzoulbenzimidazole. Asian J Chem 16:1611–1617
53. Munro CH, Smith WE, Armstrong DR, White PC (1995)
Assignments and Mechanism of SERRS of the Hydrazone Form
for the Azo Dye Solvent Yellow 14. J Phys Chem 99:879–885.
https://doi.org/10.1021/j100003a008
54. Olson DH, Camblor MA, Villaescusa LA, Kuehl GH (2004) Light
hydrocarbon sorption properties of pure silica Si-CHA and ITQ-3
and high silica ZSM-58. Microporous Mesoporous Mater 67:27–
33. https://doi.org/10.1016/j.micromeso.2003.09.025
https://doi.org/10.1016/j.synthmet.2014.08.005
3
6. Tsai M, Hsu Y-C, Lin JTet al (2007) Organic Dyes Containing 1 H -
Phenanthro[9,10- d ]imidazole Conjugation for Solar Cells. J Phys
Chem C 111:18785–18793. https://doi.org/10.1021/jp075653h
7. Shank NI, Zanotti KJ, Lanni F et al (2009) Enhanced Photostability
of Genetically Encodable Fluoromodules Based on Fluorogenic
Cyanine Dyes and a Promiscuous Protein Partner. J Am Chem
Soc 131:12960–12969. https://doi.org/10.1021/ja9016864
3
3
8. Bamfield P, Britain) RS of C (Great (2001) Chapter 3. Phenomena
Involving Absorption of Energy Followed by Emission of Light. In:
Chromic Phenom. Royal Society of Chemistry, Cambridge, pp
2
34–365
3
9. Szuster L, Kaźmierska M, Król I (2004) Fluorescent dyes destined
for dyeing high-visibility polyester textile products. Fibres Text
East Eur 12:70–75
4
0. Youssef BM, Ahmed MHM, Arief MMH, Mashaly HM (2014)
Synthesis and Application of Functional (Anti-UV) Azo-dyes
based on γ -acid on Wool Fabrics. Indian J Sci Technol 7:1005–
1
013
4
1. Kano N, Yamamura M, Kawashima T (2015) 2,2′-
Disilylazobenzenes Featuring Double Intramolecular
Nitrogen⋯Silicon Coordination: A Photoisomerizable
Fluorophore. Dalt Trans 44:16256–16265. https://doi.org/10.
55. Maximilian Paul Schmidt W-B, and Hermann Neuroth, Wiesbaden
G (1934) LIGHT-SENSITIVE LAYER. 3–4.
1
039/C5DT02038G
56. Chen X-C, Tao T, Wang Y-G et al (2012) Azo-hydrazone tautom-
erism observed from UV-vis spectra by pH control and metal-ion
complexation for two heterocyclic disperse yellow dyes. Dalt Trans
41:11107. https://doi.org/10.1039/c2dt31102j
57. Peters AT (1995) Freeman HS. Modern Colorants, Synthesis and
Structure. https://doi.org/10.1007/978-94-011-1356-4
58. Lanke SK, Sekar N (2016) Aggregation induced emissive
carbazole-based push pull NLOphores: Synthesis, photophysical
properties and DFT studies. Dyes Pigments 124:82–92. https://
doi.org/10.1016/j.dyepig.2015.09.013
59. Leu WCW, Fritz AE, Digianantonio KM, Hartley CS (2012) Push-
pull macrocycles: Donor-acceptor compounds with paired linearly
conjugated or cross-conjugated pathways. J Org Chem 77:2285–
2298. https://doi.org/10.1021/jo2026004
4
2. Satam MA, Raut RK, Sekar N (2013) Fluorescent azo disperse dyes
from 3-(1,3-benzothiazol-2-yl)naphthalen-2-ol and comparison
with 2-naphthol analogs. Dyes Pigments 96:92–103. https://doi.
org/10.1016/j.dyepig.2012.07.019
3. Yoshino J, Kano N, Kawashima T (2013) Fluorescent azobenzenes
and aromatic aldimines featuring an N–B interaction. Dalt Trans 42:
4
1
5826. https://doi.org/10.1039/c3dt51689j
4
4. Deshmukh MS, Sekar N (2015) Chemiluminescence properties of
isoluminol related mono azo disperse dyes: Experimental and DFT
based approach to photophysical properties. Dyes Pigments 115:
1
27–134. https://doi.org/10.1016/j.dyepig.2014.12.019
4
5. Tathe AB, Sekar N (2016) Red Emitting Coumarin—Azo Dyes :
Synthesis, Characterization, Linear and Non-linear Optical
Properties-Experimental and Computational Approach. J Fluoresc
60. Tathe AB, Sekar N (2016) Red-emitting NLOphoric carbazole-
coumarin hybrids - Synthesis, photophysical properties and DFT
studies. Dyes Pigments 129:174–185. https://doi.org/10.1016/j.
dyepig.2016.02.026
2
6:1279–1293. https://doi.org/10.1007/s10895-016-1815-2
4
6. Warde U, Sekar N (2017) NLOphoric mono-azo dyes with negative
solvatochromism and in-built ESIPTunit from ethyl 1,3-dihydroxy-
2
-naphthoate: Estimation of excited state dipole moment and pH
61. Mashaly HM, Abdelghaffar RA, Kamel MM, Youssef BM (2014)
Dyeing of Polyester Fabric using Nano Disperse Dyes and
Improving their Light Fastness using ZnO Nano Powder. Indian J
Sci Technol 7:960–967
62. Parr RG, Szentpály LV, Liu S (1999) Electrophilicity index. J Am
Chem Soc 121:1922–1924. https://doi.org/10.1021/ja983494x
63. Gupta VD, Tathe AB, Padalkar VS et al (2013) Red emitting solid
state fluorescent triphenylamine dyes: Synthesis, photo-physical
property and DFT study. Dyes Pigments 97:429–439. https://doi.
org/10.1016/j.dyepig.2012.12.024
study. Dyes Pigments 137:384–394. https://doi.org/10.1016/j.
dyepig.2016.10.032
4
7. Jadhav AG, Sekar N (2017) Substituent Modulation from ESIPT to
ICT Emission in Benzoimidazolphenyl-methanones Derivatives:
Synthesis, Photophysical and DFT Study. J Solut Chem 46:777–
7
97. https://doi.org/10.1007/s10953-017-0602-2
4
4
8. Frisch MJ, Trucks GW, Schlegel HB, et al (2009) Gaussian 09,
Revision C.01. Gaussian 09, Revis B01, Gaussian, Inc,
Wallingford CT
9. Becke AD (1988) Density-functional exchange-energy approxima-
tion with correct asymptotic behavior. Phys Rev A 38:3098–3100.
https://doi.org/10.1103/PhysRevA.38.3098
64. Hamdaoui M, Lanouar A, Halaoua S (2015) Study of Fluorescent
Dyeing Process and Influence of Mixture Dyes on High-visibility. J
Eng Fiber Fabr 10:89–96