Novel Iminocoumarin Derivatives: Synthesis, Spectroscopic and Computational Studies

Article abstract of DOI:10.1007/s10895-015-1648-4

Three novel iminocoumarin derivatives with high quantum yield are synthesized from 3-benzimidazole substituted coumarin and different aromatic aldehydes. The photophysical behavior of the synthesized compounds was studied using UV-visible and fluorescence spectroscopy in polar and non-polar solvents. The compounds show absorption maxima at around 450 nm and emission maxima at around 500 nm. The quantum yields of compounds in dichloromethane and chloroform are more than 0.90. The absorption, emission and quantum yield of compounds are dependent on the polarity of solvents. Along with an intense absorption, these compounds show shoulder absorption peak in the studied solvents. The solvent polarity plots revealed the charge transfer process in the synthesized molecules from donor to acceptor. Density Functional Theory and Time Dependent Density Functional Theory computations have been used to have more understanding of the structural, molecular, electronic and photophysical parameters of the dyes. The dyes were characterized by FT-IR, ¹H NMR, ¹³C NMR and mass spectral analysis.

Full text of DOI:10.1007/s10895-015-1648-4

J Fluoresc
DOI 10.1007/s10895-015-1648-4
ORIGINAL ARTICLE
Novel Iminocoumarin Derivatives: Synthesis, Spectroscopic
and Computational Studies
Santosh B. Chemate1 & Nagaiyan Sekar1
Received: 1 June 2015 /Accepted: 18 August 2015
#
Springer Science+Business Media New York 2015
.
.
.
.
Abstract Three novel iminocoumarin derivatives with high
quantum yield are synthesized from 3-benzimidazole
substituted coumarin and different aromatic aldehydes. The
photophysical behavior of the synthesized compounds was
studied using UV-visible and fluorescence spectroscopy in
polar and non-polar solvents. The compounds show absorp-
tion maxima at around 450 nm and emission maxima at
around 500 nm. The quantum yields of compounds in dichlo-
romethane and chloroform are more than 0.90. The absorp-
tion, emission and quantum yield of compounds are depen-
dent on the polarity of solvents. Along with an intense absorp-
tion, these compounds show shoulder absorption peak in the
studied solvents. The solvent polarity plots revealed the
charge transfer process in the synthesized molecules from do-
nor to acceptor. Density Functional Theory and Time Depen-
dent Density Functional Theory computations have been used
to have more understanding of the structural, molecular, elec-
tronic and photophysical parameters of the dyes. The dyes
were characterized by FT-IR, 1H NMR, 13C NMR and mass
spectral analysis.
Keywords Coumarin Iminocoumarin DFT TD-DFT
Quantum yield
Introduction
In recent years, synthesis and applications of fluorescent
materials happen to be a fruitful research activity [1–4].
In the field of functional materials, designing and devel-
opment of innovative fluorescent molecules for high-tech
applications is very demanding [1–4]. The coumarin de-
rivatives have been one of the most widely studied clas-
ses of fluorescent dyes, and probably one of the most
frequently used fluorescent compounds [5–7]. Fluores-
cent coumarin derivatives have been widely used in
many applications such as fluorescent whitening agents
[8], metal sensing [9], cell biology [10, 11], laser dyes
[12, 13], pH sensors [14], dye sensitized solar cell
(DSSc) [15, 16] and in nonlinear optics [17]. Coumarin
itself is non fluorescent but it exhibits intense fluores-
cence after its substitution by various functional groups
at different positions [18]. The fluorescence properties of
coumarin can be tuned by incorporating electron donat-
ing and accepting groups at different positions on the
basic unit. A study of literature reveals that, electron
donor substitution at 7-position along with acceptor at
3-position gives red shifted absorption, because such
substitution pattern leads to an effective donor acceptor
chromophore [1, 19]. The strength of the electron donors
and acceptors are also helpful to change the fluorescence
properties of the coumarin derivatives. To avoid intersys-
tem crossing and achieve high fluorescence intensity
with large Stoke’s shift, fused coumarin derivatives are
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-015-1648-4) contains supplementary material,
which is available to authorized users.
* Nagaiyan Sekar
n.sekar@ictmumbai.edu.in
1
Tinctorial Chemistry Group, Department of Dyestuff Technology,
Institute of Chemical Technology, Mumbai 400 019, India

J Fluoresc
reported in the literature for different applications [20,
21]. Also the dye julolidine-coumarin has 3–5 times
greater photochemical stability than that of rhodamine
6G and high energy efficiency compared with the other
coumarins [22].
Result and Discussion
Chemistry
Three novel iminocoumarin derivatives 5a-b and 12 have
been synthesized from iminocoumarin 3 or 11 and aromatic
aldehydes 4a-b or 10. The aromatic aldehydes 4a-b were ob-
tained by Vilsmeier Haack formylation reaction of corre-
sponding aromatic amines [32]. 2-cyanomethyl benzimid-
azole 1 was prepared from o-phenylene diamine and
ethylcyanoacetate in o-xylene [33]. The iminocoumarin 3
and 11 was prepared from 2-cyano methyl benzimidazole 1
and o-hydroxy aldehyde 2 or 10 by a condensation followed
by cyclization reaction in the presence of catalytic amount of
piperidine. The structures of the dyes were confirmed by FT-
IR, 1H NMR, 13C NMR and mass spectral analysis. The ab-
sorption and emission spectra were measured to investigate
the photophysical properties and also the solvatochromism
and solvatofluorism behaviors of the molecule were studied
by measuring the absorption and emission spectra in the
solvents.
Iminocoumarin is a subclass of coumarin family [23], and
very few reports are available in the literature describing the
optical properties of iminocoumarins [24–26]. The reported
iminocoumarin derivatives bear an electron-releasing group
in the 7-position and an electron-acceptor group in the 3-
position, a choice probably justified by the fact that this sub-
stitution pattern leads to enhanced absorption and fluores-
cence efficiency in the closely related coumarin series. In the
known iminocoumarins, the electron donors are N,N-dialkyl
groups at 7 position and electron acceptors like benzothiazole
or benzimidazole or cyano group at 3-position which enhance
the fluorescence efficiency. The iminocoumarin derived
fluorophores are known for their high fluorescence ability
and efficient quantum yield and hence are used for metal sen-
sors [9, 26], biological system [27], pH sensor [28] and lasers
[24]. The attachment of an ester or amide group in the 3-
position leads to a dye with interesting lasing properties and
high sensitivity to solvent effects [29]. Most of the
iminocoumarin fluorophores emit in the blue green region
[19, 30]. As many biological samples also emit in the blue
green region of the spectrum, this would interfere with the
fluorescence signals generated from the coumarin or
iminocoumarin derived fluorophores [25]. This is a limitation
of iminocoumarin which emit in the blue-green region. Con-
sidering fluorescence properties of iminocoumarin, those
which emit beyond 500 nm are demanding in functional ap-
plications. This will be achieved by changing donor or accep-
tors units. The reaction between the aromatic aldehydes with
3-benzimidazol-2-yl-2-iminocoumarin leads to a rigid molec-
ular structure. In the present paper, instead of playing with
donor and acceptor units we have synthesized rigid
iminocoumarin from 3-benzimidazol-2-yl-2-iminocoumarin
and aromatic aldehydes (Schemes 1 and 2). The two different
aromatic aldehydes were used bearing different spacer
groups. The length of spacer groups plays important role
during the binding of fluorescent probes with the biomol-
ecules. The purpose of fused iminocoumarin designing
was to achieve the planarity which will enhance electron
flow within the conjugated ring system and to avoid in-
tersystem crossing due to free rotation of groups present
at 3-position on coumarin unit [25, 31].
Photophysical Properties
The UV-Vis absorption and fluorescence emission of the
fused iminocoumarin 5a, 5b and 12 in solvents of varying
polarities are reported in Table 1, and absorption and
emission spectra are shown in Figs. 1, 2 and 3. The com-
pounds 5a, 5b and 12 are fluorescent in solution. The
absorption and emission properties of the compounds
5a, 5b and 12 were evaluated in different solvents and
the results revealed that the compounds 5a, 5b and 12
are sensitive towards the solvent polarities. The com-
pound 5a shows short wavelength and long wavelength
absorption in the visible region. The short wavelength
absorption is intense between 425 to 443 nm and long
wavelength absorption (shoulder peak) is between 447 to
467 nm. Similarly, the compound 5b showed short wave-
length absorption maxima between 430 to 443 nm and
longer wavelength absorption maxima between 455 to
468 nm. The two absorption peaks observed, may be
due to the cross conjugation present in the molecule 5a
and 5b. The absorption and emission properties for 5a and
5b are almost similar. The spacer (R group) present on
nitrogen atom does not have any influence on spectro-
scopic properties. The compound 12 also shows dual ab-
sorption in chloroform, ethylacetate and 1,4 dioxane sol-
vents. The shorter wavelength maxima is at 452 nm in
chloroform, 430 nm in ethyl acetate and 443 nm in 1,4
dioxane and long wavelength absorption is at 471 nm in
chloroform, 453 nm in ethylacetate and 462 nm in 1,4
dioxane. The compound 12 shows red shifted absorption
and emission as compared to the compounds 5a and 5b.

J Fluoresc
O
H
Scheme 1 Synthesis of
iminocoumarin derivative 5a-b
N
H
N
CN
OH
Methanol, RT
N
H
+
N
N
O
NH
N
3
1
2
R
R
N
N
N
n-Butanol , Reflux
Piperidine
N
H
N
+
N
O
NH
N
O
N
R
O
H
N
R
5a: R: -CH2 - COOMe
5b: R: -CH2-CH 2-COOMe
4a: R: -CH2 - COOMe
4b: R: -CH2 - CH2 - COOMe
3
The red shifted absorption as well as emission of com-
pound 12 are due to the strength and rigidity of the elec-
tron donor unit. Due to planarity, the molecule 12 could
undergo aggregation which can be responsible for chang-
es in shape and intensity of absorption and emission spec-
tra. The iminocoumarin derivatives have more solubility
in polar solvents than the less polar solvents. The com-
pound 5a shows intense absorption in acetone, acetoni-
trile, methanol, ethanol and chloroform. In the case of
compound 5b, the absorption pattern is almost similar in
all the studied solvents except 1,4 dioxane. The fluores-
cence properties of the compounds in different solvents
were measured to study solvofluorism property. The exci-
tation wavelength used for the fluorescence measurements
was absorption maxima of the compounds in respective
solvents. The fluorescence emission of the compounds are
sensitive towards the solvent properties Table 1. The com-
pounds show positive solvotoflurosium. These coumarins
showed a blue-shifted fluorescence emission maxima in
non-polar solvents and red-shifted fluorescence emission
maxima in the polar solvent. The compound 5a shows a
red shifted emission in the polar solvents (methanol, eth-
anol, DMSO and acetonitrile) and the blue shifted emis-
sion in the less polar solvents (acetone, DCM, chloroform,
O
H
Scheme 2 Synthesis of
iminocoumarin 12
OH
OH
OCH3
HI
OCH3
Br
POCl3
DMF
Na2CO3
+
HCl
Cl
N
N
8
N
NH2
6
7
9
10
H
O
N
H
N
OH
CN
Methanol, RT
Piperidine
N
H
+
N
N
O
NH
11
N
1
10
R
R
N
N
N
n-Butanol , reflux
Piperidine
N
N
H
+
N
O
N
N
O
NH
R
N
R
O
12: R: -CH2-CH2-COOMe
4b: R: -CH2-CH2-COOMe
11

J Fluoresc
Table 1 The UV-Vis absorption and fluorescence emission of compounds 5a, 5b and 12 in solvents of varying polarities
Dye
Solvents
λabsnm
λemsnm
Stokes shift
(cm−1
Є
f
ϕf
)
(mol−1cm−1
)
5a
MeOH
EtOH
443a
467b
442a
467b
440a
462b
434a
458b
437a
461b
440a
464b
425a
447b
435a
455b
430a
453b
438a
460b
443a
468b
443a
467b
443a
462 b
434a
455 b
438a
463b
440a
463b
443a
465b
437a
456 b
430a
455 b
438a
456b
464
502a
502a
502a
492a
2653aa
1493ab
2653aa
1493ab
2807aa
1725ab
2716aa
1509ab
2391aa
2387bb
2319aa
2096bb
2739aa
5649bb
4707aa
3696ab
2335aa
2273ab
2710aa
1618ab
2573aa
1368ab
4258aa
3098ab
4811aa
3883ab
2675aa
1611ab
3783aa
2551bb
2151aa
2143bb
1783aa
715ab
21,000
23,000
21,000
23,000
23,000
20,000
20,000
21,000
22,000
19,000
33,000
37,000
33,500
37,000
40,000
380,000
37,000
38,000
29,000
38,000
0.8252
0.8120
0.6622
0.9764
0.7936
0.7241
0.7856
0.8255
0.1949
0.6753
2.8230
3.4292
1.4445
3.2304
0.7716
3.1265
1.1626
3.9317
4.1789
2.0235
0.67
0.61
0.48
0.45
0.93
0.94
0.70
0.77
0.27
0.82
0.63
0.53
0.39
0.49
0.92
0.94
0.73
0.81
0.43
0.82
DMSO
Acetone
DCM
488a
518b
490a
514b
481a
506b
494a
CHCl3
EA
ACN
1,4 Dioxane
DMF
478a
508b
497a
5b
MeOH
EtOH
500a
502a
506a
491a
DMSO
Acetone
DCM
490a
518b
499a
516b
481a
CHCl3
EA
ACN
494a
2640aa
1687ab
2247aa
2370bb
2629aa
1728ab
1982
1,4 Dioxane
DMF
476a
510 b
495a
12
MeOH
EtOH
505
510
512
501
498
496a
56,660
51,890
20,810
70,600
39,400
35,760
0.8531
0.8298
0.3002
1.5819
0.6351
0.5446
0.62
0.52
0.61
0.62
0.94
0.95
464
464
464
470
452a
471b
430a
453b
465
3270
3631
1592
2758
2044aa
1151ab
3175aa
1995ab
1898
DMSO
Acetone
DCM
CHCl3
EA
498a
17,660
0.2456
0.56
ACN
510
491a
75,680
16,600
1.1908
0.2734
0.89
0.42
1,4 Dioxane
443a
462b
464
2207aa
1278ab
2020
DMF
512
22,200
0.6531
0.77
a Intense peak, b Shoulder peak, aa Stokes shift for intense absorption wavelength and emission wavelength, ab Stokes shift between shoulder peak
bb
absorption wavelength and emission wavelength,
Stokes shift between shoulder peak absorption wavelength and longer emission wavelength,
ε = molar extinction coefficient, f = Oscillator Strength
MeOH: Methanol, EtOH: Ethanol, EA: Ethyl acetate, DMSO: Dimethylsulfoxide, DCM: Dichloromethane, CHCl3: Chloroform, DMF: n, n dimethyl
formamide, ACN: Acetonitrile

J Fluoresc
0.25
0.2
0.15
0.1
0.05
0
0.8
0.6
0.4
0.2
0
Acetone
Acetonitrile
EA
Methanol
Ethanol
DMSO
Acetone
Acetonitrile
EA
Methanol
Ethanol
DMSO
5a
12
DCM
DCM
Dioxane
Chloroform
DMF
Dioxane
Chloroform
DMF
340
390
440
490
540
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
350
300
250
200
150
100
50
600
500
400
300
200
100
0
Acetone
Acetonitrile
EA
Methanol
Ethanol
DMSO
5a
Acetone
Acetonitrile
EA
Methanol
Ethanol
DMSO
12
DCM
Dioxane
Chloroform
DMF
DCM
Dioxane
Chloroform
DMF
0
450
500
550
600
650
470
500
530
560
590
Wavelength (nm)
Wavelength (nm)
Fig. 1 Absorption and emission spectra of compound 5a in different
Fig. 3 Absorption and emission spectra of compound 12 in different
solvents
solvents
short wavelength emission and the shoulder peak due to
the long wavelength emission. The compound 5a shows
emission band at around 500 nm in polar solvents and
around 490 nm in nonpolar solvents. Similar emission
pattern is observed for the compound 5b. The compound
5b shows red shifted emission in methanol, and DMSO at
around 500 nm and blue shifted intense emission in meth-
anol, acetone, DCM, chloroform, ethylacetate, dioxane
and acetonitrile at around 490 nm with the shoulder peak
at around 510 nm. The fluorescence quantum yield of the
compound 5b is less in 1,4 dioxane as compared to other
solvents. The compound 12 shows a red shifted emission
as compared to the compounds 5a and 5b. The emission
spectra of the compound 12 in different solvents are also
broad as compared to the compound 5a and 5b. A red
shifted emission was observed at around 510 nm for the
compound 12 in DMSO, ethanol, acetonitrile and DMF
and a blue shifted emission at around 500 nm in acetone,
methanol, ethylacetate, DCM, chloroform was observed.
The molar extinction coefficient of the dyes varied from
19,000 to 75,680 mol−1 cm−1. The dye 5b have higher
molar extinction coefficient values than the dye 5a. How-
ever, the dye 12 shows higher molar extinction coefficient
values than the dye 5a and 5b.
ethylacetate and dioxane). Along with the intense emis-
sion, the shoulder peak is observed for the compound 5a
in the studied solvents. The intense peak is due to the
0.5
Acetone
5b
Acetonitrile
0.4
EA
Methanol
0.3
0.2
0.1
0
Ethanol
DMSO
DCM
Dioxane
Chloroform
DMF
340
390
440
490
540
Wavelength (nm)
500
400
300
200
100
0
Acetone
Acetonitrile
EA
Methanol
Ethanol
DMSO
DCM
Dioxane
Chloroform
DMF
5b
The fluorescence quantum yields of the synthesized cou-
marins were determined in different solvents and tabulated in
Table 1. The fluorescence quantum yields of the coumarins
largely depend on the solvent polarity shown in Table 1. The
450
500
550
600
650
Wavelength (nm)
Fig. 2 Absorption and emission spectra of compound 5b in different
solvents

J Fluoresc
three coumarins showed quantum efficiencies >0.90 in dichlo-
romethane, chloroform and the lowest quantum yield in 1, 4
dioxane. The quantum yield of the compound 5a is very high
in chlorinated solvents like dichloromethane, chloroform
(>0.90) while in other solvents such as DMF, ethyl acetate,
acetonitrile and methanol it is more than 0.6. The compound
5a shows low quantum yield in 1, 4-dioxane (0.27). A similar
trend is observed for compound 5b. The quantum yields of the
compound 12 in the solvents such as dichloromethane, chlo-
roform, acetonitrile, and 1,4 dioxane are high as compared to
the compounds 5a and 5b. In compound 12 rigid fixing of the
diethylamino group by two saturated six-membered heterocy-
cles leads to red shift in absorption and emission. From
photophysical study it is observed that rigid fixing of electron
donor group in iminocoumarin dyes not only shift absorption
and emission towards longer wavelength but also increases
the fluorescence intensity.
transition from ground to excited state. Oscillator strength
( f ) can be calculated using the following Eq. 1 [38]. Where
ε is the extinction coefficient (L mol−1 cm−1), and ν represents
the wavenumber (cm−1). From this equation we have calcu-
lated oscillator strength for iminocoumarin derivatives and
tabulated in Table S3.
Z
f ¼ 4:32 Â 10−9 εðνÞdν
ð1Þ
By using the value of f, we have calculated transition
dipole moment that was the differences in electric charge
distribution between a ground and excited state of the
molecule. The transition dipole moment for absorption
(μge) is a measure of the probability of radiative transi-
tions which have been calculated form of Debye in differ-
ent solvent environments using the Eq. 2 [38]. The tran-
sition dipole moment was increased with increase in the
oscillator strength.
Solvent Polarity Plots and Intramolecular Charge
Transfer
f
μ2ge
¼
ð2Þ
2:13 Â 10−30 Â ν
The Stokes shift increases with increase in the solvent polarity
which indicates an extensive structural re-organization in the
excited state [34]. This shows that these coumarins have polar
excited state and it is more stabilized in the polar solvent. This
confirms the intramolecular charge transfer characteristics of
these coumarins [34]. The solvatochromic behaviour of the
molecules was studied with the help of Lippert plot [35] and
Weller Plot [36]. Lippert’s plot is the plot of stokes shift in
cm−1 vs orientation polarizability. The Lippert function is con-
stituted of the polarity function f (ε) and polarizability function
f (η) shows good correlation which supports the intramolecu-
lar charge transfer in the synthesized dyes (Figure S1,
Table S1). The Weller plots shows good linear behaviour
which is indicative of charge transfer at excited state in these
molecules. Thus the emission is highly sensitive towards the
polarity of the solvents (Figure S2). All the dyes responds well
to the solvent polarity functions (ε, n), the similar equations
are introduced by Bilot-Kawski [37] for the estimation of ratio
of excited state dipole moment and ground state dipole mo-
ment i.e. μe/μg. The dipole moments calculated with the above
equations is given in Table S2. The compounds 5a and 5b has
μe/μg ratio higher than compound 12 which suggest that the
excited state is more polar than the ground state for these
molecules.
Where,
ge is transition dipole moment (D),
μ
f is oscillator strength,
ν is wavenumber (cm−1).
The molar extinction co-efficient (ε) and apparently the
transition dipole moments (μa) are higher for 12 than
compound 5a and 5b (Table S3). This clearly indicates
effective charge transfer in compound 12 due to rigidity
at donor part.
DFT and TD-DFT Computations
All the computations were performed with the Gaussian 09
package [39]. DFT method was used for the ground state
optimization, while for the excited state optimization, time-
dependent density functional theory (TD-DFT) was used.
The hybrid functionals namely B3LYP (Becke3-Lee-Yang-
Parr hybrid functional) [40–43] was used. The 6-31G(d) basis
set was used for all atoms and later was ascertained in the
literature [44]. The Polarizable Continuum Model (PCM)
[45] was used to optimize the ground and excited state geom-
etries. The excitation energies, oscillator strengths and orbital
contribution for the lowest 10 singlet-singlet transitions at the
optimized geometry in the ground state were obtained by TD-
DFT calculations using the same basis set as for the geometry
minimization. The solvents used were methanol (MeOH), ace-
tonitrile (ACN), N,N-dimethyl formamide (DMF),
dimethylsulfoxide (DMSO), ethanol (EtOH), 1,4 dioxane,
ethyl acetate (EA), chloroform (CHCl3), dichloromethane
(DCM) and acetone.
Oscillator Strength and Transition Dipole Moment
of the Dyes
Oscillator strength is dimensionless quantity that expresses the
probability of absorption and emission properties in energy
levels, which helps to understand charge transfer within the
molecules. It was simply described number of electron

J Fluoresc
To gain more insight into the electronic properties of the
to 15 nm deviation between short wavelength absorption and
vertical excitation (Table 2). Compound 5b also shows dual
absorption, short wavelength absorption and long wavelength
absorption about 437 and 460 nm respectively. The short
wavelength absorption in studied solvents is in good agree-
ments with vertical excitation. The compound 5b shows the
lower deviation in experimental short wavelength absorption
and vertical excitation in acetone (9 nm), DMF (11 nm), ace-
tonitrile (12 nm) and 1,4 dioxane (8 nm) (Table 3). In the case
compound 12 the deviation between experimental absorption
and vertical excitation is more in methanol, acetonitrile, DMF,
DMSO, ethanol, acetone, DCM and chloroform (~30 nm).
Compound 12 shows 2 nm deviation in absorption and verti-
cal excitation in ethyl acetate (Table 4).
The experimental fluorescence emissions were compared
with TD-DFT emission for compounds 5a-b and 12. The
compounds 5a and 5b shows duel emission in less polar sol-
vents. For compound 5a, deviation between experimental
emission and TD-DFT emission was very less for shorter
emission while higher deviation was observed for longer ex-
perimental emission and TD-DFT emission. The compound
5a shows lower deviation in shorter emission and TD-DFT
emission in 1,4 dioxane (8 nm) and ethyl acetate (13 nm) and
compounds, density functional theoretical (DFT) and Time-
dependent density functional theory (TD-DFT) calculations
were performed for the compounds 5a, 5b and 12 with the
Gaussian 09 program package. The structures of compounds
5a-b and 12 were optimized using the B3LYP functional and
6- 31G(d) basis set. The theoretical predictions about the ver-
tical excitation, oscillator strength and their orbital contribu-
tions are summarised in Tables 2, 3 and 4. The experimental
absorption and vertical excitation data are in good agreement
for compound 5b as compared to compounds 5a and 12. The
compound 5a shows dual absorption, short wavelength ab-
sorption around 438 nm and long wavelength absorption
was around 460 nm. The short wavelength absorption in stud-
ied solvents is in good agreements with vertical excitation. For
compound 5a the lower deviation in experimental absorption
and vertical excitation was observed in ethyl acetate 3 nm,
while for methanol 20 nm deviation is observed. This vertical
excitation is due to HOMO to LUMO transition. In ethanol
and methanol 5a shows large deviation between long wave-
length absorption and vertical excitation (44 nm). This vertical
excitation is due to HOMO to LUMO transition (98 %) with
high oscillator strength (1.23). In other solvents 5a shows 10
Table 2 Experimental and computed absorption and emission of the dye 5a in different solvents
Solvent
Experimental
TD-DFT
λabsa (nm)
λemsb
(nm)
Vertical
ev
f d
Assignment
% De (abs)
TD-DFT
Emission
% Df
(ems)
Excitationc (nm)
MeOH
EtOH
443
467
442
467
440
462
434
458
437
461
502
502
502
492
423
423
425
423
424
2.9343
2.9293
2.9169
2.9308
2.9268
1.2210
1.2316
1.2511
1.2302
1.2476
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
4.5
9.4
4.3
9.4
3.4
8.0
2.5
7.6
3.0
8.0
467
468
471
468
470
7.0
6.8
6.2
4.9
DMSO
Acetone
DCM
488
518
3.7
9.3
CHCl3
EA
440
464
490
514
423
422
2.9341
2.9420
1.2492
1.2282
H-L = 98 %
H-L = 98 %
3.9
8.8
471
468
3.9
8.4
425
447
481
506
0.7
5.6
2.7
7.5
ACN
435
455
430
453
494
423
419
2.9309
2.9581
1.2267
1.2295
H-L = 98 %
H-L = 98 %
2.8
7.0
2.6
7.5
469
470
5.1
1,4 Dioxane
478
508
1.7
7.5
DMF
438
460
497
425
2.9153
1.2548
H-L = 98 %
3.0
7.6
471
5.2
a Experimental absorption wavelength
b Experimental emission wavelength
c Computed absorption wavelength
d Oscillator strength
e % Deviation between experimental absorption and vertical excitation computed by DFT
f % Deviation between experimental emission and computed (TD-DFT) emission

J Fluoresc
Table 3 Experimental and computed absorption and emission of the dye 5b in different solvents
Solvent
Experimental
TD-DFT
λabsa (nm)
λemsb (nm)
Vertical
ev
f d
Assignment
% De (abs)
TD-DFT
Emission
% Df
(ems)
Excitationc (nm)
MeOH
EtOH
443
443
443
434
438
500
502
506
491
424
425
425
425
425
2.9229
2.9184
2.9218
2.9199
2.9167
1.1194
1.1332
1.1301
1.1307
1.1509
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
4.3
4.1
4.1
2.1
3.1
471
472
473
472
475
5.8
6.2
6.5
3.9
DMSO
Acetone
DCM
490
518
3.1
8.3
CHCl3
440
499
516
425
2.9165
1.0738
H-L = 98 %
3.5
507
1.6
1.7
EA
443
437
430
481
494
424
425
422
2.9229
2.9198
2.9402
1.0385
1.1276
1.0529
H-L = 98 %
H-L = 98 %
H-L = 98 %
4.5
2.8
1.9
475
472
526
1.2
4.5
ACN
1,4 Dioxane
476
510
9.5
3.0
DMF
438
495
427
2.9053
1.1656
H-L = 98 %
2.5
475
4.0
a Experimental absorption wavelength
b Experimental emission wavelength
c Computed absorption wavelength
d Oscillator strength
e % Deviation between experimental absorption and vertical excitation computed by DFT
f % Deviation between experimental emission and computed (TD-DFT) emission
38 nm deviation was observed longer emission (Table 2). The
shorter wavelength emission deviation for other solvents was
slightly higher (such as for methanol 35 nm, Ethanol 34 nm,
DMSO 31 nm, Acetone 24 nm, DCM 18 nm, CHCl3 19 nm,
Acetonitrile 25 nm, DMF 26 nm) while emission deviation in
longer wavelength was higher for DCM 48 nm. The deviation
Table 4 Experimental and computed absorption and emission of the dye 12 in different solvents
Solvent
Experimental
TD-DFT
λabsa (nm)
λemsb (nm)
Vertical
ev
f d
Assignment
% De
TD-DFT
Emission
% Df
(ems)
Excitationc (nm)
MeOH
EtOH
DMSO
Acetone
DCM
464
464
464
464
470
452
430
465
443
464
505
510
512
501
498
496
498
510
491
512
435
435
437
435
435
433
432
435
428
436
2.8536
2.8494
2.8362
2.8514
2.8512
2.8628
2.8684
2.8502
2.8938
2.8429
1.1137
1.1267
1.1506
1.1250
1.1463
1.1483
1.1222
1.1207
1.1328
1.2214
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
H-L = 98 %
6.3
6.3
5.8
6.3
7.4
4.2
0.5
6.5
3.4
6.0
476
477
479
477
479
480
477
477
481
480
5.7
6.5
6.4
4.8
3.8
3.2
4.2
6.5
2.0
6.3
CHCl3
EA
ACN
Dioxane
DMF
a Experimental absorption wavelength
b Experimental emission wavelength
c Computed absorption wavelength
d Oscillator strength
e % Deviation between experimental absorption and vertical excitation computed by DFT
f % Deviation between experimental emission and computed (TD-DFT) emission

J Fluoresc
between shorter wavelength emission and TD-DFT emission
was less than longer wavelength emission (Table 4). For com-
pound 12 the deviation in emission was observed more in
polar solvents around 33 nm while in less polar solvents emis-
sion deviation was around 12 nm (Table 4).
delocalized at the electron acceptor unit of benzimidazole
(Table 5). Also from bond length and bond angle data of 5a,
5b and 12 clearly suggested that the charge transfer from elec-
tron donor to acceptor unit (Table S4). This indicates that the
charge separation increases in the excited state which results in
a larger dipole moment than that in the ground state. It also
explains the sensitivity of the fluorescence emission spectra of
these push-pull dipolar dyes to the solvent polarities.
The dihedral angles optimized at the ground state (GS) and
excited state (ES) suggests that planarity increases in the ex-
cited state (Table S4). The dihedral angle between 7-N, N
diethyl amino group and phenyl ring of coumarin in the
optimised structure of 5a is 179.9 (GS) and dihedral angle
between coumarin unit and benzimidazole group is 178.9
(GS) indicates the structure is coplanar (Fig. 4). Based on
these geometrical optimizations, we found that the coumarin
unit is almost coplanar with the benzimidazole, leading to ef-
fective π-conjugation throughout the molecules (Fig. 4). The
dihedral angle between iminocoumarin unit and n-
substituted benzene (receptor) is −125.2 (GS) which in-
dicates the iminocoumarin unit and n-substituted ben-
zene is nonplanar. The n-substituted benzene part is at-
tached to tetrahedral carbon and expected dihedral angle
in 109. Similar observation is observed for 5b (Fig. S4).
For compound 12 is the dihedral angle between 7-
position electron donor nitrogen atom and phenyl ring
of coumarin in the optimised structure is 172.7 (GS)
(Fig. S4). The contribution of electronic transition from
HOMO to LUMO is about 98 % in 5a, 5b and 12.
Therefore, we confirm that electronic transitions of both
compounds are originated from HOMO to LUMO. As
shown in Table 5, all the compounds show that electron
transfer from the donor to the acceptor segment occurs
effectively. The HOMO levels of 5a, 5b and 12 are
mainly dominated by π-orbital contribution of the N, N diethyl
electron donor unit, while the LUMO levels are largely
Experimental
Methods and Materials
All the commercial reagents and solvents were pur-
chased from S. D. Fine Chemicals Pvt. Ltd. and used
without purification and all the solvents were of spec-
troscopic grade. The absorption spectra of the dyes
were recorded on a Perkin-Elmer spectrophotometer,
Lambda 25, and emission spectra were recorded on
Varian Cary Eclipse fluorescence spectrophotometer
using freshly prepared solutions in solvents of different
polarities at a concentration of 1 × 10−6 mol L−1. The
photophysical properties were investigated using the
solvatochromic and solvatofluoric behaviors of the
dyes. The excitation wavelength used for the fluores-
cence measurements was absorption maxima of the
compounds in respective solvents. The FT-IR spectra
were recorded on a Jasco 4100 Fourier Transform IR
instrument (ATR accessories). 1H NMR and 13C NMR
spectra were recorded on Varian 500 MHz instrument
using TMS as an internal standard. Mass spectra were
recorded on Finnigan mass spectrometer.
Fig. 4 Optimized geometry
parameters of dye 5a in methanol
solvent in the ground state and
excited state (dihedral angles are
in degree)

J Fluoresc
Table 5. FMO diagrams for compound 5a-b and 12 calculated with B3LYP/6-31G(d) method in methanol
HOMO
LUMO
5a
5b
12
Relative Quantum Yield Calculations
emission characteristics of the standards and for the com-
pounds in polar as well as non-polar solvents were measured
at different concentrations at room temperature. The fluores-
cence measurements were performed with solution containing
molecular oxygen. The emission intensity values were plotted
against the absorbance values and linear plots were obtained.
The gradients were calculated for the compounds in each sol-
vent and for the standards. All the measurements were done
The quantum yields of the compounds 5a-b and 12 in differ-
ent solvents were evaluated. Fluorescence quantum yields
were evaluated with respect to 3-(2-benzothiazolyl)-7-
diethylaminocoumarin (Coumarin-6) in ethyl alcohol
(ϕ = 0.78) [46]. The quantum yield values were calculated
using the comparative method [47–49]. The absorption and

J Fluoresc
by keeping the parameters such as solvent and slit width
constant. The relative quantum yields of the synthesized
compounds in different solvents were calculated using the
Eq. 3 [47–49].
precipitate was collected by filtration, washed with dry meth-
anol, and dried under high vacuum to obtain a yellow solid of
3-benzimidazole iminocoumarin 3. Yield = 82 %, m.p.238 °C
(lit.239 °C) [28].
2
Gradx
ηx
Synthesis of Coumarin 5a and 5b
ϕx ¼ Φst Â
Â
ð3Þ
2
Gradst ηst
A mixture of the iminocoumarin 3 (0.332 g 3 mmol) and the
aromatic aldehyde 4a-b (3 mmol), piperidine (0.1 mL) in n-
butanol (15 ml) was heated to reflux for 4 h, cooled, and left
overnight. The precipitated solid was filtered off, washed with
ethanol, and dried. The yellow colored solid 5a-b obtained
was purified by column chromatography.
Where:
Φx
Quantum yield of compound
Φst
Quantum yield of standard sample
Gradx Gradient of compound
Gradst Gradient of standard sample
ηx
Refractive index of solvent used for synthesized
compound
1
5a: Yield = 78 %, Orange red solid, H NMR (CDCl3,
ηst
Refractive index of solvent used for standard sample
500 MHz) δ ppm : 1.18 (t, 6H), 3.37 (s, 6H), 3.71 (s, 4H),
4.34 (m, 4H), 6.34(d, J = 2.5 Hz, 1H), 6.4 (dd, J = 2.5 Hz
& 9 Hz, 1H), 6.54 (d, J = 9 Hz, 2H), 7.04 (d, J = 9 Hz,
2H), 7.10 (d, J = 8 Hz, 1H), 7.20(dd, J = 1.5 Hz & 5 Hz,
1H), 7.21(dd, J = 1.5 Hz & 5 Hz, 1H), 7.23 (d, J = 6 Hz,
1H), 7.26 (s,1H), 7.74 (d, J = 8 Hz, 1H), 8.07 (s, 1 H), 13C
NMR (CDCl3, 125 MHz) δ ppm: 12.5, 30.5, 44.9, 65.0,
73.6, 97.8, 105.3, 108.0, 108.4, 111.2, 112.7, 118.9,
122.6, 122.9, 127.2, 129.6, 129.7, 130.0, 133.0, 144.3,
145.4, 148.0, 151.1, 154.1, 155.6, 171.1, FT-IR (cm−1) :
2966 (−C-H, aromatic), 1731 (carbonyl), 1677, 1606
(−C=C-, aromatic), 1551, 1521 (−C=C-), Mass: m/z
581.7 [M + 2]+.
Experimental Procedure
Synthesis of Intermediate 1
A mixture of o-phenylene diamine (16.2 g, 0.15 mol) and
ethyl cyanoacetate (25.4 g, 0.22 mol) and o-xylene (160 ml)
was refluxed for 10–12 h in flask equipped with dean-stark
trap. Then the reaction mixture was allowed to stand overnight
at room temperature. The obtained solid was filtered and
dried. The compound was recrystalised by 95 % ethyl alcohol.
Yield =60 %, m.p. 206 °C (lit. 207 °C) [33].
5b: Yield = 71 %, Orange red solid, 1H NMR (CDCl3,
500 MHz) δ ppm: 1.22 (t, 6H), 2.56(t, 4H), 3.42 (m, 4H),
3.64 (t, 4H), 3.67 (s, 6H), 6.42 (d, J = 2 Hz, 1H), 6.50 (dd,
J = 2 Hz & 7.5 Hz, 1H), 6.64 (d, J = 7.5 Hz, 2H), 7.03 (d,
J = 6.5 Hz, 1H), 7.08–7.15 (m, 4H), 7.27 (dd, J = 7 Hz &
7.5 Hz, 2H), 7.77 (d, J = 7 Hz, 1H), 8.11 (s, 1H), 13C
NMR (125.6 MHz, DMSO-d6, ppm, Me4Si): 12.4, 32.1,
44.8, 46.8, 51.7, 73.6, 97.7, 106.3, 107.9, 108.3, 111.1,
112.5, 118.9, 122.5, 122.8, 127.3, 128.1, 129.6, 129.9,
133.0, 144.4, 145.4, 146.7, 151.0, 154.1, 155.6, 172.4,
FT-IR(cm−1): 2966 (−C-H, aromatic), 1738 (carbonyl),
1692, 1666 (−C=C-, aromatic), 1572, 1549 (−C=C-),
Mass: m/z 608.6 [M + 1]+.
Synthesis of 4-(N,N-diethyl amino)-2-hydroxybenzaldehyde 2
Phosphorous oxychloride (POCl3) (2.75 ml, 0.03 mol) was
slowly added to N, N dimethyl formamide (DMF) (3.65 mL,
0.05 mol) at 5–10 °C under constant stirring. To this cooled
reagent 3-(N,N-diethyl amino) phenol (0.01 mmol) in DMF
(6 mL) was added slowly under constant stirring and the
resulting mixture was heated at 75 °C for 4 h. The reaction
mixture was cooled to room temperature and then poured into
ice cold water (60 mL). The reaction mass was neutralized
with sodium carbonate, brown colored solid separated out.
The solid product was filtered and washed with cold water,
dried and crystallised from ethanol to obtain the pure product
2, Yield =80 %, m.p. 62 °C (lit. 62 °C) [50].
Synthesis of 8-methoxy julolidine 8
Synthesis of 3-Benzimidazole Iminocoumarin 3
3-Methoxyaniline 6 (12.3 g, 0.10 mol), 1-bromo-3-
chloropropane 7 (235 g,1.5 mol), and anhydrous sodium car-
bonate (42.7 g, 0.4 mol) were combined in a 500 mL three
necked round bottomed flask equipped with an overhead me-
chanical stirrer, thermometer, and a pressure equalizing addi-
tion funnel. The top of the addition funnel was fitted with a
condenser. The mixture was heated to 70 °C for 1 h and
100OC for 2 h and then reflux for 11 h. The progress of the
To a solution of 2-cyano methyl benzimidazole 1 (0.55 g,
2.8 mmol), piperidine (0.1 mL, 1.4 mmol) was added in dry
methanol (50 mL), and the solution was stirred at room tem-
perature for 30 min. Then 4-(N, N-diethyl amino)-2-
hydroxybenzaldehyde (0.49 g, 2.8 mmol) was added. The
mixture was stirred for 5 h at room temperature, and the

J Fluoresc
reaction was monitored by TLC. After the completion of re-
action, the mixture was cooled to room temperature and
150 mL of concentrated HC1 and 50 mL of water were slowly
added. Upon dissolution of all solids, the phases were sepa-
rated and the organic layer was washed with 10 % HCl to
remove remaining product. This washing was added to the
aqueous phase, which was washed with ether to remove 1-
bromo-3-chloropropane. The aqueous phase was basified with
50 % aqueous sodium hydroxide and extracted with ethyl
acetate until the organic phase was no longer colored. The
ethyl acetate solution was dried over sodium sulphate, and
the solvent was removed under reduced pressure. The
resulting brown oil was purified by column chromatography
to get pure product. Yield = 64 %, Viscous yellow oil which
turned red on exposure to air [20].
Synthesis of Iminocoumarin 11
2-cyano methyl benzimidazole 1 (0.55 g, 2.8 mmol), piperi-
dine (0.1 mL, 1.4 mmol) was added in 50 mL of dry methanol,
and the solution was stirred at room temperature for 30 min.
Then 8-hydroxy-2,3,6,7-tetrahydro-1 H,5 H-pyrido[3,2,1-ij]-
quinoline-9-carbaldehyde 10 (0.49 g, 2.8 mmol) was added.
The mixture was stirred for 5 h at room temperature, and the
precipitate was collected by filtration. The compound was
washed with dry methanol to remove the impurities or
unreacted starting material, and dried under high vacuum to
obtain a yellow solid 3-benzimidazole iminocoumarin 11.
Yield = 81 % [51].
Synthesis of Coumarin 12
8-Hydroxyjulolidine 9
A mixture of the iminocoumarin 11 (0.332 g 3 mmol) and the
aromatic aldehyde 10 (3 mmol), piperidine (0.01 ml) in n-
butanol (15 ml) was heated to reflux for 3 h, cooled, and left
overnight. The precipitated solid was filtered off, washed with
ethanol, and dried. Obtained yellow colored solid 12 was pu-
rified by column chromatography, m.p. 235 °C.
8-Methoxyjulolidine 8 (10 g, 50 mmol) was dissolved in a
solution consisting of 50 mL of 47 % HI, 80 mL of concen-
trated HC1, and 200 mL of water. The reaction mixture was
refluxed and the progress of the reaction was monitored by
TLC. After 15 h another 50 mL concentrated HCl was added
to the reaction. The reaction completed within 60 h. The so-
lution was cooled in an ice bath and neutralized to pH 6 first
using 50 % NaOH and then phosphate buffer was added (6.9 g
of NaH2PO4.H2O and 1.4 g of Na2HPO4 in 100 mL of H2O).
The product was extracted with dichloromethane, the organic
phase was washed with brine and dried over Na2SO4, and the
solvent was removed under reduced pressure. The obtained
crude product was taken up in dichloromethane and extracted
in 10 % NaOH until the aqueous phase remained colorless.
The organic phase was then acidified, dried, and extracted,
and the solvent was removed as above to get product 9 to yield
6.24 g. Yield = 67 %, m.p. 126–130 °C [20].
1
12: Yield = 71 %, Orange red color solid, H NMR
(CDCl3, 500 MHz) δ ppm :1.60 (t, 4H), 1.99 (t, 4H), 2.63 (t,
4H), 2.78(t, 4H), 2.85 (broad, 4H), 3.69 (s, 6H), 6.61–6.72
(broad, 4H, −Ar), 7.02 (s, 1H), 7.69–7.75 (broad, 4H, −Ar),
8.83 (s, 1H), 9.76 (s, 1H), 13C NMR (CDCl3, 125 MHz) δ
ppm: 21.2, 22.7, 29.7, 46.6, 49.8, 50.2, 64.8, 106.0, 106.6,
108.7, 111.2, 111.1, 119.7, 122.5, 126.0, 126.4, 128.8, 129.3,
132.2, 142.9, 147.5, 148.5, 151.3, 151.8, 162.3, 171.5, 171.8,
FT-IR(cm−1): 2929 (−C-H), 2851 (aliphatic, −CH), 1730 (car-
bonyl), 1687 (−C=N), 1592, 1560 (−C=C-, aromatic), 1522
(−C=C-), Mass: m/z 632.3 [M + 1]+.
Synthesis of 8-hydroxy-2,3,6,7-tetrahydro-1 H,5
H-pyrido[3,2,1-ij]quinoline-9-carbaldehyde 10
Conclusion
In summary, synthesised iminocoumarin have interesting
fluorescence properties. A large stokes shift and high quantum
yield, good solubility in a wide range of solvents make these
dyes more demanding and attractive for applications as fluo-
rescent marker. The iminocoumarin 12 shows red shifted ab-
sorption and emission as compared to compounds 5a and 5b.
In dye 12 rigid fixing of the electron donor nitrogen by two
saturated six-membered heterocycles leads to red shift in ab-
sorption and emission. All the dyes are sensitive towards the
solvent polarity, they show good quantum yield in polar and
moderately polar solvents. The photophysical properties of
the synthesized dyes were supported by DFT and it was ob-
served that computational results are good agreement with the
theoretical observations.
Phosphorous oxychloride (2.7 mL, 4.4 g, 29 mmol) was added
dropwise in flask containing 10 mL DMF for a period of
15 min at 4 °C. A solution of 8-hydroxyjulolidine 7 (5.15 g,
26.9 mmol) in DMF (5 mL) was then added dropwise to the
complex over period of 10 min. When the addition was com-
plete, the reaction was stirred at room temperature for 30 min,
and then heated at 100 °C for 30 min. After cooling to room
temperature, 30 mL of water was added to the stirred dark
solution. The aqueous mixture was stirred for 1.5 h resulting
in the formation of a blue-green precipitate. The precipitate was
isolated by filtration, washed with water and dried. The crude
product was purified by column chromatography on silica gel
(Toluene/EtOAc, 2:1) Yield = 94 %, m.p. 73–74 °C [20].

J Fluoresc
Acknowledgments Santosh B. Chemate is thankful for JRF and SRF
fellowship from the Principal Scientific Adviser (PSA), Government of
India for project stand-off detection of explosive based on immunoassay
technique.
17. Southgate PD (1971) Nonlinear optical susceptibility of a crystal-
line coumarin dye. J Appl Phys 42:4480. doi:10.1063/1.1659797
18. Ranjith C, Vijayan KK, Praveen VK, Kumar NSS (2010)
Photophysical investigation of 3-substituted 4-alkyl and/or 7-
acetoxy coumarin derivatives–a study of the effect of substituents
on fluorescence. Spectrochim Acta A Mol Biomol Spectrosc 75:
1610–1616. doi:10.1016/j.saa.2010.02.027
19. Khemakhem K, Ammar H, Abid S, et al. (2013) Spectroscopic
study of 3-aryl-7-methoxy-coumarin, iminocoumarin and bis-
iminocoumarin derivatives in solution. Dyes Pigments 99:594–
598. doi:10.1016/j.dyepig.2013.06.003
20. Van Gompel J, Schuster GB (1987) Chemiluminescence of organic
peroxides: intramolecular electron-exchange luminescence from a
secondary perester. J Org Chem 52:1465–1468. doi:10.1021/
jo00384a015
21. Lemieux G a, De Graffenried CL, Bertozzi CR (2003) A
fluorogenic dye activated by the staudinger ligation. J Am Chem
Soc 125:4708–4709. doi:10.1021/ja029013y
References
1. Li H, Cai L, Li J, et al. (2011) Novel coumarin fluorescent dyes:
synthesis, structural characterization and recognition behavior to-
wards Cu(II) and Ni(II). Dyes Pigments 91:309–316. doi:10.1016/j.
dyepig.2011.05.011
2. Bochkov AY, Akchurin IO, Dyachenko OA, Traven VF (2013)
NIR-fluorescent coumarin-fused BODIPY dyes with large stokes
shifts. Chem Commun (Camb) 49:11653–11655. doi:10.1039/
c3cc46498a
3. Signore G, Nifosì R, Albertazzi L, Bizzarri R (2009) A novel cou-
marin fluorescent sensor to probe polarity around biomolecules. J
Biomed Nanotechnol 5:722–729. doi:10.1166/jbn.2009.1089
4. Valeur B (2001) Molecular fluorescence Principles and
Applications.
5. Kumar S, Singh P, Srivastava R, et al. (2014) Engineering fused
coumarin dyes: a molecular level understanding of aggregation
quenching and tuning electroluminescence via alkyl chain substitu-
tion. J Mater Chem C. doi:10.1039/C4TC00807C
6. Liu Y, Zhang S, Lv X, et al. (2014) Constructing a fluorescent probe
for specific detection of cysteine over homocysteine and glutathi-
one based on a novel cysteine-binding group but-3-yn-2-one.
Analyst 139:4081–4087. doi:10.1039/c4an00639a
7. Widmer S, Dorrestijn M, Camerlo A, et al. (2014) Coumarin meets
fluorescein: a förster resonance energy transfer enhanced optical
ammonia gas sensor. Analyst 139:4335–4342. doi:10.1039/
c4an00061g
8. Micali G, Curro P, Calabro G (1984) High-performance liquid chro-
matographic separation and determination. Analyst 109:155–158
9. Komatsu K, Urano Y, Kojima H, Nagano T (2007) Development of
an iminocoumarin-based zinc sensor suitable for ratiometric fluo-
rescence imaging of neuronal zinc. J Am Chem Soc 129:13447–
13454. doi:10.1021/ja072432g
10. Sun Q, Qian J, Tian H, et al. (2014) Rational design of biotinylated
probes: fluorescent turn-on detection of (strept)avidin and
bioimaging in cancer cells. Chem Commun (Camb) 50:8518–
8521. doi:10.1039/c4cc03315a
22. Anufrik SS, Tarkovsky VV, SG G, Asimov MM (2012) New laser
dyes based on 3-imidazopyridylcoumarin derivatives. J Appl
Spectrosc 79:46–52
23. Gorobets NY, Abakumov VV (2002) 3-(benzimidazolyl-2)-2-
iminocoumarins in reactions with aromatic aldehydes. Chem
Heterocycl Compd 38:1518–1520. doi:10.1023/A:1022605930640
24. Maslov VV, Gorobets NY, Borisov AV, Nikitchenko VM (2003)
New series of dyes for flashlamp-excited lasers. J Appl Spectrosc
70:794–799. doi:10.1023/B:JAPS.0000008880.76919.05
25. Huang S-T, Jian J-L, Peng H-Z, et al. (2010) The synthesis and
optical characterization of novel iminocoumarin derivatives. Dyes
Pigments 86:6–14. doi:10.1016/j.dyepig.2009.10.020
26. Ahamed BN, Ghosh P (2011) Selective colorimetric and fluoromet-
ric sensing of Cu(II) by iminocoumarin derivative in aqueous buff-
er. Dalton Trans 40:6411–6419. doi:10.1039/c1dt10177c
27. Kim T, Jeong MS, Chung SJ, Kim Y (2010) An iminocoumarin-
based fluorescent probe for the selective detection of dual-specific
protein tyrosine phosphatases. Chem - A Eur J 16:5297–5300. doi:
10.1002/chem.201000154
28. Fan X, Sen LYZ, Zhang XY, Qu GR (2006) A n Efficient and Green
Synthesis of 1, 4-Dihydropyridine Derivatives through Multi-
Component Reaction in Ionic Liquid. Heteroat Chem 17:382–
388. doi:10.1002/hc
29. Turki H, Abid S, Fery-Forgues S, El Gharbi R (2007) Optical prop-
erties of new fluorescent iminocoumarins: part 1. Dyes Pigments
73:311–316. doi:10.1016/j.dyepig.2006.01.001
30. Kand D, Sahebrao P, Datar A, Talukdar P (2014) Iminocoumarin
based fl uorophores: Indispensable scaffolds for rapid, selective and
sensitive detection of thiophenol. Dyes Pigments 106:25–31. doi:
10.1016/j.dyepig.2014.02.001
11. Kim J, Park J, Lee H, Kim Y (2014) A boronate-based fluorescent
probe for the selective detection of cellular peroxynitrite. chem
comm. doi: 10.1039/c4cc02943g
12. Jones G, Rahman MA (1994) Fluorescence properties of coumarin
laser dyes in aqueous polymer media. Chromophore isolation in
poly(methacry1ic acid) hypercoils. J Phys Chem 8:13028–13037
13. Wlodarczyk P, Komarneni S, Roy R, White WB (1996) Enhanced
fluorescence of coumarin laser dye in the restricted geometry of a
porous nanocomposite. J Mater Chem 6:1967. doi:10.1039/
jm9960601967
14. Hong SW, Ahn CH, Huh J, Jo WH (2006) Synthesis of a PEGylated
polymeric pH sensor and its pH sensitivity by fluorescence reso-
nance energy transfer. Macromolecules 39:7694–7700. doi:10.
1021/ma061175h
15. Hara K, Sato T, Katoh R, et al. (2003) Molecular design of couma-
rin dyes for efficient dye-sensitized solar cells. J Phys Chem B 107:
597–606. doi:10.1021/jp026963x
16. Sánchez-de-Armas R, San Miguel MÁ, Oviedo J, Sanz JF (2012)
Coumarin derivatives for dye sensitized solar cells: a TD-DFT
study. Phys Chem Chem Phys 14:225–233. doi:10.1039/
c1cp22058f
31. Fakhfakh M, Turki H, Fery-Forgues S, El Gharbi R (2010) The
synthesis and optical properties of novel fluorescent
iminocoumarins and bis-iminocoumarins: investigations in the se-
ries of urea derivatives. Dyes Pigments 84:108–113. doi:10.1016/j.
dyepig.2009.07.003
32. Chemate S, Sekar N (2015) Highly sensitive and selective
chemosensors for Cu 2+ and Al 3+ based on photoinduced electron
transfer (PET) mechanism. RSC Adv 5:27282–27289. doi:10.
1039/C5RA00123D
33. Bokanov AI, Turchin KF, Sedov IAL, Granik VG (1999)
Cyanoethylation of 2-cyanomethylbenzimidazole. Pharm Chem J
33:94–97
34. Detert H, Schmitt V (2006) Acidochromism of stilbenoid chromo-
phores with ap-aminoaniline centre. J Phys Org Chem 19:603–607.
doi:10.1002/poc.1091
35. Lippert E (1957) Spektroskopische bestimmung des
dipolmomentes aromatischer verbindungen im ersten angeregten
singulettzustand. Zeitschrift für Elektrochemie, Berichte der

J Fluoresc
Bunsengesellschaft für Phys Chemie 61:962–975. doi: 10.1002/
bbpc.19570610819.
36. Valeur B, Berberan-Santos MN (2012) Molecular Fluorescence:
Principles and Applications 569.
45. Zhao Y, Truhlar D (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.
doi:10.1007/s00214-007-0310-x
46. Fakhfakh M, Turki H, Abid S, et al. (2007) Preparation and optical
properties of new fluorescent iminocoumarins: study of N-acyl-de-
rivatives. J Photochem Photobiol A Chem 185:13–18. doi:10.1016/
j.jphotochem.2006.04.036
37. Bilot L, Kawski A (1962) Zur theorie des einflusses von
lösungsmitteln auf die elektronenspektren der moleküle.
Zeitschrift für Naturforsch A. doi: 10.1515/zna-1962-0713.
38. Carlotti B, Flamini R, Kikaš I, et al. (2012) Intramolecular charge
transfer, solvatochromism and hyperpolarizability of compounds
bearing ethenylene or ethynylene bridges. Chem Phys 407:9–19.
doi:10.1016/j.chemphys.2012.08.006
39. Frisch MJ, Trucks GW, Schlegel HB, et al. (2009) Gaussian 09,
Revision C.01. Gaussian 09, Revis. B.01, Gaussian, Inc.,
Wallingford CT.
47. Ware WR, Rothman W (1976) Relative fluorescence quantum
yields using an integrating sphere. The quantum yield of 9,10-
diphenylanthracene in cyclohexane. Chem Phys Lett 39:449–453.
doi:10.1016/0009-2614(76)80301-6
40. Stephens PJ, Devlin FJ, Chabalowski CF, Frisch MJ (1994) Ab
initio calculation of vibrational absorption and circular dichroism
spectra using density functional force fields. J Phys Chem 98:
11623–11627. doi:10.1021/j100096a001
48. Williams ATR, Winfield SA, Miller JN (1983) Relative fluo-
rescence quantum yields using a computer-controlled lumines-
cence spectrometer. Analyst 108:1067. doi:10.1039/
an9830801067
41. Becke AD (1988) Density-functional exchange-energy approxima-
tion with correct asymptotic behavior. Phys Rev A 38:3098–3100
42. Becke AD (1993) A new mixing of hartree–fock and local density
functional theories. J Chem Phys 98:1372–1377
49. Dhami S, De M a J, Rumbles G, et al. (1995) Phthalocyanine
fluorescence at high concentration: dimers or reabsorption effect?
Photochem Photobiol 61:341–346. doi:10.1111/j.1751-1097.1995.
tb08619.x
43. Lee C, Yang W, Parr RG (1988) Development of the colle-salvetti
correlation-energy formula into a functional of the electron density.
Phys Rev B 37:785–789. doi:10.1103/PhysRevB.37.785
44. Saranya G, Kolandaivel P, Senthilkumar K (2011) Optical absorp-
tion and emission properties of fluoranthene, benzo[k]fluoranthene,
and their derivatives. A DFT study. J Phys Chem A 115:14647–
14656. doi:10.1021/jp208617s
50. Dalapati S, Jana S, Guchhait N (2011) Colorimetric Bnaked-eye^
highly selective detection of Cu(II) ion by a simple chemosensor: an
experimental and theoretical modeling. Chem Lett 40:279–281.
doi:10.1246/cl.2011.279
51. Frath D, Poirel A, Ulrich G, et al. (2013) Fluorescent boron(III)
iminocoumarins (boricos). Chem Commun (Camb) 49:4908–
4910. doi:10.1039/c3cc41555d