New fluorescent heterocyclic systems from imidazo[1,2-a]pyridine: Design, synthesis, spectral studies and quantum-chemical investigations

Article abstract of DOI:10.1016/j.molstruc.2016.08.085

Two new fluorescent heterocyclic systems dipyrido[1′,2′:1,2]imidazo[4,5-b:4,5-e]pyridine-13-carbonitrile and pyrido[1′,2′:1,2]imidazo[4,5-b]pyrido[2′,1′:2,3]imidazo[4,5-e]pyridine-13-carbonitrile were synthesized by one-pot reaction of imidazo[1,2-a]pyridine with 2-(imidazo[1,2-a]pyridin-3-yl)acetonitrile and 2-(imidazo[1,2-a]pyridin-2-yl)acetonitrile, respectively, in MeOH/KOH solution via the nucleophilic substitution of hydrogen in high yields. Spectral and analytical data have confirmed the structures of the synthesized dyes. The optical and solvatochromic properties of the compounds were investigated and the results showed that they exhibited interesting photophysical properties. Density functional theory (DFT) calculations of fluorescent dyes were performed to provide the optimized geometries and relevant frontier orbitals by using the B3LYP hybrid functional and the 6–311?++ G(d,p) basis set. Calculated electronic absorption spectra were also obtained by time-dependent density functional theory (TD-DFT) method. In addition, electron density iso-surface map, intra- and intermolecular interactions of these fluorescent heterocyclic systems were evaluated by AIM (Atoms in Molecules) analysis.

Full text of DOI:10.1016/j.molstruc.2016.08.085

Accepted Manuscript
New fluorescent heterocyclic systems from imidazo[1,2-a]pyridine: Design, synthesis,
spectral studies and quantum-chemical investigations
Mehdi Pordel, Hamed Chegini, Shirin Ramezani, Mohammadreza Daee
PII:
S0022-2860(16)30915-2
10.1016/j.molstruc.2016.08.085
MOLSTR 22911
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 2 May 2016
Revised Date: 29 August 2016
Accepted Date: 31 August 2016
Please cite this article as: M. Pordel, H. Chegini, S. Ramezani, M. Daee, New fluorescent heterocyclic
systems from imidazo[1,2-a]pyridine: Design, synthesis, spectral studies and quantum-chemical
investigations, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.08.085.
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ACCEPTED MANUSCRIPT
Graphical Abstract

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New fluorescent heterocyclic systems from imidazo[1,2-a]pyridine:
Design, synthesis, spectral studies and quantum-chemical
investigations
Mehdi Pordel *, Hamed Chegini, Shirin Ramezani and Mohammadreza Daee
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
*Corresponding author. Tel.: +98 0513 8414182; fax: +98 0513 8424020. E-mail:
mehdipordel58@mshdiau.ac.ir
Abstract- Two new fluorescent heterocyclic systems dipyrido[1',2':1,2]imidazo[4,5-b:4,5-
e]pyridine-13-carbonitrile and pyrido[1',2':1,2]imidazo[4,5-b]pyrido[2',1':2,3]imidazo[4,5-
e]pyridine-13-carbonitrile were synthesized by one-pot reaction of imidazo[1,2-a]pyridine with
2
-(imidazo[1,2-a]pyridin-3-yl)acetonitrile and 2-(imidazo[1,2-a]pyridin-2-yl)acetonitrile,
respectively, in MeOH/KOH solution via the nucleophilic substitution of hydrogen in high
yields. Spectral and analytical data have confirmed the structures of the synthesized dyes. The
optical and solvatochromic properties of the compounds were investigated and the results
showed that they exhibited interesting photophysical properties. Density functional theory (DFT)
calculations of fluorescent dyes were performed to provide the optimized geometries and
relevant frontier orbitals by using the B3LYP hybrid functional and the 6-311 ++ G(d,p) basis
set. Calculated electronic absorption spectra were also obtained by time-dependent density
functional theory (TD-DFT) method. In addition, electron density iso-surface map, intra- and
intermolecular interactions of these fluorescent heterocyclic systems were evaluated by AIM
(Atoms in Molecules) analysis.
Keywords: Imidazo[1,2-a]pyridine; Fluorescence; Emission and absorption spectra; Density
functional theory; AIM analysis.

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1
. Introduction
The heterocycles constitute the largest group and most varied family of organic compounds and
are becoming ever more important in all aspects of pure and applied chemistry. Heterocyclic
compounds count among their number many natural products, such as hormones, vitamins,
alkaloids, antibiotics, as well as pharmaceuticals, herbicides, dyes, and other products of
technical importance (corrosion inhibitors, antiaging drugs, sensitizers, stabilizing agents, etc.).
Among various heterocycles, imidazo[1,2-a]pyridines are an important class of heterocyclic
compounds. They are found in many biologically active compounds such as antiviral [1],
anticancer [2], anxiolytic [3], antimalarial [4], hypnotic [5], antiprotozoal [6] and anti-
inflammatory [7] agents, which has rendered this ring system quite an attractive target. This
heterocycle continues to be a very popular scaffold for the development of new bioactive
molecules, which is maybe due to its simple preparation together with the success of some drugs
possessing this skeleton. The best example continues to be Sanofi-Synthe´labo zolpidem
®
®
®
(
Stilnox , Ambien , Myslee ), a blockbuster for the treatment of sleeping disorders. Recently,
imidazo[1,2-a]pyridine moieties became of interest as dye [8,9] and fluorescent compounds [8–
0]. Furthermore, the analytical properties of some imidazo[1,2-a]pyridine derivatives
1
demonstrate they can use as an acid–base indicator. The results show these compounds are
superior to well-known indicators such as Quinaldine Red, Malachite Green, and Methyl Green
in pH range 2–2.8 [11].
Taking these facts into consideration, in the present study, we have synthesized two new
heterocyclic systems dipyrido[1',2':1,2]imidazo[4,5-b:4,5-e]pyridine-13-carbonitrile and
pyrido[1',2':1,2]imidazo[4,5-b]pyrido[2',1':2,3]imidazo[4,5-e]pyridine-13-carbonitrile with
interesting fluorescence properties. Density functional theory calculations (DFT) of fluorescent
dyes were performed to provide the optimized geometries and relevant frontier orbitals.
Furthermore, calculated electronic absorption spectra of the new fluorophores were obtained by
TD-DFT method. Electron density iso-surface map, intra- and intermolecular interactions in
these fluorophores were also studied by AIM analysis.

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2
. Experimental
2
.1. Materials
Methanol, N,N-dimethylformamide (DMF), chloroform, 1,4-dioxane, acetonitrile (MeCN),
cyclohexane, 2-bromo-1,1-dimethoxyethane, 1,3-dichloroacetone and 2-amino-pyridine were
purchased from Merck. Potassium hydroxide was purchased from Sigma-Aldrich. All solvents
were dried according to standard procedures. Compounds 1 and 2a,b were synthesized as in
literature [12–14].
2
.2. Equipment
Absorption and fluorescence spectra were recorded on a Varian 50-bio UV–visible
spectrophotometer and a Varian Cary Eclipse spectrofluorophotometer. UV–vis and fluorescence
scans were recorded from 200 to 1000 nm. Melting points were measured on an
Electrothermaltype-9100 melting-point apparatus. The IR (as KBr discs) spectra were obtained
1
3
on a Tensor 27 spectrometer and only noteworthy absorptions are listed. The C NMR (100
1
MHz) and the H NMR (400 MHz) spectra were recorded on a Bruker Avance DRX-400
3
spectrometer in CDCl . Chemical shifts are reported in parts per million downfield from TMS as
the internal standard; coupling constant J is given in hertz. The mass spectra were recorded on a
Varian Mat, CH-7 at 70 eV. Elemental analysis was performed on a Thermo Finnigan Flash EA
microanalyzer. All measurements were carried out at room temperature.
2
.3. Computational methods
DFT calculations were performed with the Gaussian 98 software package [15] by using the
B3LYP hybrid functional [16] and the 6-311++G(d,p) basis set. Firstly, geometry of the
compounds 3 and 4 was fully optimized in the chloroform solution. The optimized geometry was
confirmed to have no imaginary frequency. Then, their optimized geometry was used for
frequency calculations.
Here, one of self-consistent reaction field methods, the sophisticated Polarized Continuum
Model (PCM) [17] has been used for investigation of the solvent effects. The PCM calculations
have been performed in the chloroform solution and the zero-point corrections were considered

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to obtain energies. Based on the optimized geometries and using time-dependent density
functional theory (TD-DFT) [18–20] methods, the electronic spectra of the compounds 3 and 4
were predicted.
2
.3. General procedure for the synthesis of 3 and 4.
3
-Nitroimidazo[1,2-a]pyridine 1 (3.26 g, 20 mmol) and 2a,b (25 mmol) were added with stirring
to a solution of KOH (20 g, 357 mmol) in MeOH (80 mL). The mixture was stirred at rt for 24 h.
After concentration at reduced pressure, the precipitate was collected by filtration, washed with
water, following with cold EtOH and acetone respectively, and then air dried to give 3 and 4.
More purification was achieved by recrystallization from suitable solvent such as MeOH or
EtOH.
Dipyrido[1',2':1,2]imidazo[4,5-b:4,5-e]pyridine-13-carbonitrile (3). Compound 3 was
obtained as shiny yellow needles (EtOH), (69%), m.p.: 339–341 °C; IR (KBr disk): CN 2243
-1 1
cm ; H NMR: δ 7.21 (td, 1 H, J = 6.8 Hz, J = 1.9 Hz, Ar H); 7.24 (td, 1 H, J = 6.7 Hz, J = 2.1
1
2
1
2
Hz, Ar H); 7.83 (td, 1 H, J = 6.6 Hz, J = 2.2 Hz, Ar H); 7.90 (td, 1 H, J = 6.9 Hz, J = 2.4 Hz,
1
2
1
2
Ar H); 7.95–8.04 (m, 2 H, Ar H); 9.21 (d, 1 H, J = 6.7 Hz, Ar H); 9.43 (d, 1 H, J = 6.8 Hz, Ar H)
1
3
ppm; C NMR (100 MHz): δ 97.3, 112.3, 119.4, 120.8, 121.9, 122.3, 125.4, 127.9, 129.6, 136.7,
+
1
39.5, 141.6, 146.1, 148.5, 154.7, 156.9 ppm; MS (EI, 70 eV), m/z (Irel (%)): 284 [М] (3);
Found: C, 67.49; H, 2.82; N, 29.43. C H N (284.3) Calculated (%): C, 67.60; H, 2.84; N,
16
8
6
2
9.56.
Pyrido[1',2':1,2]imidazo[4,5-b]pyrido[2',1':2,3]imidazo[4,5-e]pyridine-13-carbonitrile (4).
Compound 4 was obtained as shiny yellow needles (MeOH), (75%), m.p.: 355–357 °C; IR (KBr
-1 1
disk): CN 2240 cm ; H NMR: δ 7.35 (td, 2 H, J = 6.8 Hz, J = 1.8 Hz, Ar H); 7.86 (td, 2 H, J =
1
2
1
7
2
.1 Hz, J = 2.1 Hz, Ar H); 8.05 (d, 2 H, J = 7.1 Hz, Ar H); 9.25 (d, 2 H, J = 6.8 Hz, Ar H) ppm;
1
3
C NMR (100 MHz): 1091.8, 117.3, 121.2, 122.3, 131.4, 137.9, 141.5, 144.2, 156.6 ppm; MS
+
(
EI, 70 eV), m/z (I (%)): 284 [М] (5); Found: C, 67.45; H, 2.81; N, 29.77. C H N (284.3)
rel
16
8
6
Calculated (%): C, 67.60; H, 2.84; N, 29.56.

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3
. Results and Discussion
3
.1. Synthesis and structures of the new compounds 3 and 4
As depicted in Scheme 1, the precursor of 3-nitroimidazo[1,2-a]pyridine 1 was obtained in two
steps from the reaction of 2-aminopyridine with 2-bromo-1,1-dimethoxyethane. Nitration in
H SO and HNO was accomplished by adopting a literature method [12]. Other precursor 2-
2
4
3
(imidazo[1,2-a]pyridin-3-yl)acetonitrile 2a was prepared by the reaction of imidazo[1,2-
a]pyridine with Mannich reagent, MeI and KCN, respectively, in good yield [13]. The new
heterocyclic systems dipyrido[1',2':1,2]imidazo[4,5-b:4,5-e]pyridine-13-carbonit 3 was
synthesized via the nucleophilic substitution of hydrogen [21] of 3-nitroimidazo[1,2-a]pyridine 1
with 2-(imidazo[1,2-a]pyridin-3-yl)acetonitrile 2a in basic MeOH solution and following with
subsequent cyclization and dehydration at room temperature in high yield (Scheme 1). A
tentative mechanism to explain the formation of compound 3 is shown in Scheme 2 [8,22].
<
Scheme 1>
Scheme 2>
<
Other new heterocyclic system pyrido[1',2':1,2]imidazo[4,5-b]pyrido[2',1':2,3]imidazo[4,5-
e]pyridine-13-carbonitrile 4 was obtained in a similar mechanism. Nucleophilic substitution of
hydrogen of 3-nitroimidazo[1,2-a]pyridine 1 with 2-(imidazo[1,2-a]pyridin-2-yl)acetonitrile 2b
which was readily obtained from the reaction of 2-(chloromethyl)H-imidazo[1,2-a]pyridine with
2
KCN in H O under reflux using a literature method [14], led to the formation of new fluorophore
4
in excellent yield (Scheme 3). The work-up procedure was very simple, performed by filtration
of the precipitated product after the mixture was concentrated at reduced pressure. Washing the
precipitated product with suitable solvents (water and then acetone) gives practically pure
compound 4.
<Scheme 3>

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The structural assignments of compounds 3 and 4 were based on the analytical and spectral data.
1
For example, in the H NMR spectrum of compound 4 revealed two triplet of doublet signals at
δ 7.35 ppm and 7.86 ppm and two doublet peaks at δ 8.05 ppm and 9.25 ppm assignable to
H
H
eight protons of aromatic rings. Moreover, the FT-IR spectrum of 4 in KBr showed an absorption
-1
band at 2240 cm corresponding to cyanide group. Furthermore, the mass spectrum of this
+
product shows the molecular ion m/z 284 (M ), confirming its presumed structure. Analytical
data are also in accordance with the proposed pentacyclic structure of compound 4.
3
.2. Photophysical properties and quantum-chemical investigations of 3 and 4
These compounds were spectrally characterized by using an UV-Vis spectrophotometer and a
fluorescence spectrophotometer. The wavelength range of both spectrophotometers is 200 nm-
1
000 nm. The electronic absorption spectra of compounds 3 and 4 were recorded at
-5
-1
concentration of 5 × 10 mol L and their fluorescence emission spectra were obtained at
-6
-5
-1
concentrations of 1 × 10 and 5 × 10 mol L in chloroform, respectively. The solvatochromic
properties of compounds 3 and 4 in different solvents were also investigated (Figures 1 and 2).
The absorption and emission spectra of 3 and 4 in polar solvents undergo a relatively modest red
shift. Increasing the solvent polarity stabilizes the excited-state molecule relative to the ground-
state molecule with the observed red shift of the absorption maximum as the experimentally
observed result (Tables 1). For example, in the absorption and emission spectra of compound 3,
λabs and λ shift from 455 to 490 nm and 485 to 540 nm respectively, as the solvent changes
flu
from cyclohexane to DMF (Table 1).
Values of extinction coefficient (ε) were calculated as the slope of the plot of absorbance vs
concentration. The fluorescence excitation (λ ) wavelength at 380 nm (λ /nm) was used for
ex
ex
compounds 3 and 4.

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<Table 1>
<
<
Figure 1>
Figure 2>
As can be seen from Table 1, Figures 1 and 2, dye 3 showed the higher maximum absorption
wavelength (λabs) at 475 nm than λabs of 4 (425 nm) in chloroform. Furthermore, in the UV-
visible absorption spectrum of compound 3, there are two absorption peaks into the visible
region compared to 4 which has an absorption band in this area. To gain a deeper insight into the
role of HOMO and LUMO frontier orbitals in the UV-visible absorption spectra of compounds 3
and 4, we performed DFT calculations at the B3LYP/6-311++G(d,p) level and obtained the
optimized geometries and HOMO and LUMO frontier orbitals of fluorescent dyes 3 and 4. The
optimized geometries of compounds 3 and 4 can be found in Figure 3. In the optimized
geometries of the compounds 3 and 4, all of these rings and cyano group are essentially planar
and the C=C bond lengths (1.38-1.44 Ångstrom) of the aromatic rings are in the expected range
(Tables S1 and S2; see Supplementary data) [24].
The energy difference between the HOMO and LUMO frontier orbitals is one of the important
characteristics of molecules, which has a determining role in such cases as electric properties,
electronic spectra and photochemical reactions. The HOMO and LUMO maps of 3 and 4 are
shown in Figure 4. Separation energies between the HOMO and LUMO (∆ε = εLUMO – εHOMO) in
dyes 3 and 4 are 2.95 and 3.25 eV (420 and 381 nm), respectively. It can be seen from Figure 4
that dye 3 has more π-system overlap in its HOMO and LUMO frontier orbitals which led to the
lower separation energy between the HOMO and LUMO compared to compound 4. In the
Supporting information (Scheme S1), neutral and some charge-separated mesomeric structures
of compounds 3 and 4 are presented.
The TD-DFT electronic spectra calculations on 3 show that there are two electronic transition
bands: a relatively sharp peak at 349 nm (oscillator strength: 0.3425), which can be attributed to
π – π* transitions (donor endocyclic nitrogen (N16) to the acceptor CN group), and a relatively
broad band at 489 nm with an oscillator strength of 0.3097, which can be linked to π – π*
transitions from the donor N3 to the acceptor CN group. These electronic transition bands can be
compared with the experimental values of 380 and 475 nm, respectively.

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Also, the TD-DFT electronic spectra calculations on 4 show a relatively sharp peak at 414 nm
(oscillator strength: 0.4606), which corresponds to the experimental data (425 nm). This peak
located in the visible light range can be attributed to π – π* transitions from the donor endocyclic
nitrogens (N3 and N14) to the acceptor CN group. The calculated electronic absorption spectra
of compounds 3 and 4 are shown in the Supporting information.
<
<
Figure 3>
Figure 4>
The fluorescence quantum yields (Φ ) of compounds 3 and 4 were determined via comparison
F
methods, using fluorescein as a standard sample in 0.1 M NaOH and MeOH solution [23]. The
used value of the fluorescein emission quantum yield is 0.79 and the emission quantum yields of
the compounds 3 and 4 are 0.69 and 0.51, respectively. The intramolecular charge transfer (ICT)
states from the donor endocyclic nitrogens to the acceptor CN group can be considered as the
main reason for the fluorescence intensity of green fluorescent dyes 3 and 4 [8–10]. Compounds
3
and 4 are structural isomers and difference in their fluorescence intensity and fluorescence
quantum yield can be attributed to the energy difference between the HOMO and LUMO frontier
orbitals (It was discussed by DFT calculations) and the concentration of the electron density of
two fluorophores. A more detailed discussion on the electron density of 3 and 4 is examined by
AIM analysis.
The Bader theory comes of ages as a very useful tool to analyze intra- and intermolecular
interactions.
The main difference between the studied 3 and 4 dyes and other similar dyes, in which the
structures are planar, is principally associated to the π-electronic structure. The cyano fragment,
having a completely planar structure, plays an acceptor role of the photo excited electron
phenomenon in these dyes [25].
The presence of the intramolecular interactions C⋯H differs concentration of electron density
along the C-N bond in the cyano fragment in two isomers. The AIM analysis can prove that there
is more fluorescence property in compound 3 compared to 4.

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The electron density map, ρ, of two isomers are shown in Figure 5. In this figure, the negative
region is located throughout of two isomers and shown in red. At the first glance, the negative
region is reliable evidence for the formation of resonance along the structures. With comparison
of two the electron density maps of isomers, it illustrates that the negative region of dye 4 is
weaker than that of dye 3, explaining that the resonance in 3 is stronger than 4. In other hand, the
negative region which is located at the substituent CN part of 3 is more than 4 indicating that the
electron withdrawing of CN at 3 is considerable.
It seem that CN as an electron withdrawing group has greater effect on resonance strength of
aromatic ring in 3, because hydrogen bonding between H30 and C21 of CN group can be affect
the strength of CN electron withdrawing. Since it makes aromatic rings active, so the resonance
considerably locates on the structure of 3.
All of these data are reliable evidence for strength of the intramolecular hydrogen bond and
define that formation of the dye 3 is much probable to show fluorescence effect than the dye 4.
<
Figure 5>
ꢀ
The parameters such as the Laplacian of the electron density∇ ꢁ, the electron energy density H ,
C
the sum of the kinetics electron energy density (G
C
) and the potential electron energy density
(
V ), and as well as -G /V ,ꢂꢃ(ꢄ, ꢅ) derive from the Bader theory and imply the interaction type
C
C
C
and changing stability of bond after molecular rotate, intermolecular or intra-molecular
interactions.
ꢀ
For a negative value of a Laplacian,ꢂ∇ ꢁ < 0ꢂꢂbeyond doubt the interaction is completely
ꢀ
ꢀ
covalent. If both ∇ ꢁ and H be simultaneously positive ( ꢆ ꢁ(ꢇ)ꢂ> ꢂ0, ꢈ ꢂ> ꢂ0) which can be
C
ꢉ
categorized as the interactions of closed electron shells, to which, in particular, most hydrogen
ꢀ
bonds and van der Waals interactions belong. If ∇ ꢁ be positive while H is negative, and the -
C
G /V
C C
is smaller than 1, then the nature of interaction is considered as partly covalent [26].
ꢀ
The topological parameters, such as ꢁBCP, ∇ ꢁ, G , V , (−ꢊ /ꢋ ) and H at the BCP (bond
C
C
C
C
C
critical point) of C⋯H , C−N and C−C for two isomers are represented in Table 2. It appears
from the molecular graphs that there are a BCP between the C⋯H atoms, which are connected
through two bond paths. The topological structure shows that the intramolecular C⋯H bond

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existed in the structure of molecule 3. From the data shown in Table 2, the following results can
ꢀ
be drawn: for the BCP of the C⋯H, the ∇ ꢁ, H are positive, and the ratio of G to V , −ꢊ /ꢋ ,
C
C
ꢌ
ꢌ
ranges up than 1; therefore, all values of topological parameters imply that the intramolecular
C⋯H bond, is rather van der Waals in dye 3 [27–30]. From Table 2, it seems that due to
hydrogen intramolecular interactions and increase in electron donating of nitrogen in pyridine
ring, affected by N14 of imidazol ring, ρꢍꢎꢏ at the BCP of C7-N10, C11-N14, N14-C15
increased. It is deserve to mention that intramolecular interactions bond could help to improve
resonance by decrease efficiency of withdrawing electron CN substituent in the structures.
<Table 2>
The RCP (ring critical point) is a point of minimum electron density within the ring surface and a
maximum on the ring line [31–33]. Table 3 gives the information about electron density ρꢐꢎꢏ at
ꢀ
the RCP and ꢆ ꢂꢁ
of the pyridine, imidazole and chelate ring which produced by the intra-
ꢐꢎꢏ
intermolecular interaction.
<Table 3>
From Table 3, the value of the electron densityꢂꢁꢑꢌꢒ,ꢂꢆꢀꢁꢑꢌꢒ and K reveal this fact that pyridine
ring is unstable with decreasing the electron withdrawing strength of CN substituent.
On other hand, it can be seen that -C1 - C2 - N3 - C4 - C5 - C6- and -N3 - C4 - N9 - C8 - C7-
ring in affected of efficiency the electron withdrawing CN substituent could be stable in 3 and
the -C11 - C12 - N16 - C15 - N14-ring and -N14 - C15 - C22 - C21 - C20 - C19-ring is stable in
4
.
As mentioned before, dye 4 is much stable than dye 3, which assures that resonance increasing in
structure due to deactivation of CN substituent is the reason of this variation.
4
. Conclusion
In conclusion, we have synthesized two new green fluorescent heterocyclic systems
dipyrido[1',2':1,2]imidazo[4,5-b:4,5-e]pyridine-13-carbonitrile and pyrido[1',2':1,2]imidazo[4,5-
b]pyrido[2',1':2,3]imidazo[4,5-e]pyridine-13-carbonitrile in excellent yields which demonstrate

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interesting optical properties including high extinction coefficients and high quantum yields.
DFT and TD-DFT calculations on these fluorophores were performed to gain a deeper insight
into their spectroscopic properties by using the B3LYP hybrid functional and the 6-311++G(d,p)
basis set. The results showed that the geometries of two dyes were rigid and planar and the
separation energies between the HOMO and LUMO in dyes 3 and 4 were 2.95 and 3.25 eV,
respectively. In addition, AIM analysis was conducted to investigate the QTAIM molecular
graph, electron density iso-surface map, intra- and intermolecular interactions in these
fluorophores. The results show that concentration of electron density through the C-N bond in
the cyano fragment of two dyes is different, because of the intramolecular interactions
H(30)⋯C(CN) and increase in electron donating of nitrogen in pyridine ring in fluorophore 3 which
could help to improve resonance by decrease efficiency of withdrawing electron CN substituent
in the structure. Comparing the quantum-chemical investigations with the experimental results
reveals that they are in good agreement and DFT and TD-DFT calculations and AIM analysis
can prove the higher maximum absorption wavelength and the fluorescence intensity in
fluorophore 3 compared to 4.
Acknowledgment
We would like to express our sincere gratitude to Research Office, Mashhad Branch, Islamic
Azad University, Mashhad-Iran, for financial support of this work.

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.

ACCEPTED MANUSCRIPT
Schemes
Scheme 1. Synthesis of new fluorescent dye 3.
Scheme 2. The proposed reaction mechanism for the formation of 3.
Scheme 3. Synthesis of new fluorescent dye 4.

ACCEPTED MANUSCRIPT
Figures
-5
-1
-6
-
Figure 1. Visible absorption (left) (5 × 10 mol L ) and emission spectra (right) (1 × 10 mol L
1
)
of compound 3 in different solvents.
-5
-1
-5
-
Figure 2. Visible absorption (left) (5 × 10 mol L ) and emission spectra (right) (5 × 10 mol L
1
)
of compound 4 in different solvents.

ACCEPTED MANUSCRIPT
Figure 3. Optimized geometries of the compounds 3 and 4
Figure 4. The HOMO and LUMO frontier orbitals of the compounds 3 (left) and 4 (right).

ACCEPTED MANUSCRIPT
4
3
Figure 5. The QTAIM molecular graph and electron density iso-surface map of two isomers.

ACCEPTED MANUSCRIPT
Tables
Table 1. Spectroscopic data for green fluorescent dyes 3 and 4 at 298 K in different solvents.
3
4
3
ε
4
ε
3
4
3
4
-
4
-4
Solvent
DMF
λ
abs
λ
abs
× 10
× 10
λ
ex
λ
ex
λ
flu
λ
flu
a
-1 -
-1 -
c
d
(nm)
(nm)
[(mol L ) [(mol L ) (nm)
(nm)
(nm)
(nm)
1
-1 b
1
-1
cm ]
cm ]
490, 400 435
480,385 430
5.2
5.4
4.6
3.6
2.6
5.0
4.8
3.6
3.2
2.5
490
480
475
460
455
435
430
425
425
540
525
510
505
485
515
495
475
470
460
CH CN
3
Chloroform 475,380 425
,4-Dioxane 460,375 425
Cyclohexane 455,370 410
1
410
a
b
c
Wavelengths of maximum absorbance (
λ
abs); Extinction coefficient; Wavelengths of
d
fluorescence excitation (λex); Wavelengths of fluorescence emission (λflu).

ACCEPTED MANUSCRIPT
Table2: The topological properties at the BCP for two isomers.
ꢄꢓꢔꢕꢖ
ꢅꢗꢘ
|ꢙ(ꢄ, ꢅ)| ꢃ(ꢄ, ꢅ)
ꢁ
ꢆꢀꢂꢁ
ꢊ
ꢋ
ꢈ
−ꢊ/ꢋ
3
C11 - C12 2.7139
C11 - N14 2.5782
C12 - C13 2.6488
C12 - N16 2.6176
C13 - C21 2.6872
C15 - N16 2.6763
C21 - N22 2.1854
0.0200
0.4517
0.0767
0.2831
0.2438
0.3628
1.1313
0.6388
0.0419
0.6165
0.0758
0.4285
0.5112
0.6309
0.3639
0.2614
0.0199
1.0962 0.293628 -0.76866 0.089275 -0.37071 -0.28144 0.240819
1.1620 0.329337 -0.98884 0.176797 -0.6008
-0.42401 0.294268
1.2835 0.300357 -0.80403 0.098383 -0.39777 -0.29939 0.247334
1.0359 0.296774 -0.71618 0.225097 -0.62924 -0.40414 0.357729
1.1021 0.281924 -0.77272 0.086125 -0.36543
-0.2793
0.235683
0.33033
0.9585 0.284414 -0.74123 0.180386 -0.54608 -0.36569
2.3450 0.477002 -0.31516 0.779748 -1.63829 -0.85854 0.475953
1.2916 0.352293 -1.08969 0.205309 -0.68304 -0.47773 0.300582
C4 - N9
C7 - C8
2.5209
2.7024
1.0935 0.297812 -0.79642
1.2967 0.357278 -1.04229 0.257817 -0.77621 -0.51839 0.332149
1.2278 0.299025 -0.81115 0.09278 -0.38835 -0.29557 0.238909
0.09031
-0.37973 -0.28942
0.23783
C7 - N10 2.4861
C8 - C13
C8 - N9
2.6647
2.5744
1.2190 0.328508 -0.97089 0.179208 -0.60114 -0.42193 0.298114
1.1914 0.336696 -0.98265 0.229049 -0.70376 -0.47471 0.325465
N10 - C11 2.5358
N14 - C15 2.5053
1.3116 0.358186
-1.1129
0.219063 -0.71635 -0.49729 0.305804
-0.3828 0.335255
N3 - C4
N3 - C7
2.6573
2.6315
0.9703 0.290396 -0.75896 0.193058 -0.57585
0.9634 0.295408 -0.75201 0.209221 -0.60645 -0.39723 0.344995
C21 - H30 5.1053
0.0192 0.008844 0.029026 0.005884 -0.00451 0.001373 1.304367
4
C11 - C12 2.7030
C11 - N14 2.6267
C12 - C13 2.6652
C12 - N16 2.5765
C13 - C17 2.6845
C15 - N16 2.5202
C17 - N18 2.1853
0.0609
0.2730
0.0963
0.4239
0.2660
0.6417
1.1451
0.6415
0.0608
0.5698
0.0963
0.4241
0.5700
0.3598
0.3597
0.2733
1.0996 0.297427 -0.79346 0.090518
-0.3794
-0.28889
0.23858
0.9710 0.296368 -0.74393 0.214891 -0.61576 -0.40087 0.348983
1.2388 0.299182 -0.81094 0.092454 -0.38764 -0.29519 0.238504
1.2159 0.327987
1.1050 0.282855
-0.972
0.177973 -0.59895 -0.42097 0.297144
0.086425 -0.36725 -0.28082 0.235331
-0.7776
1.2942 0.352931 -1.09361 0.204648
-0.6827
-0.47805 0.299763
2.3500
0.47703 -0.31816 0.778999 -1.63754 -0.85854 0.475714
C4 - N9
C7 - C8
2.5202
2.7030
1.2944 0.352931 -1.09361 0.204648
1.0995 0.297427 -0.79346 0.090518
-0.6827
-0.3794
-0.47805 0.299764
-0.28888 0.238581
C7 - N10 2.5005
1.2604 0.348816 -0.98546
1.2388 0.299181 -0.81094 0.092454 -0.38764 -0.29519 0.238504
1.2158 0.327987 -0.972 0.177973 -0.59895 -0.42097 0.297144
0.25785
-0.76207 -0.50422 0.338357
C8 - C13
C8 - N9
2.6652
2.5765
N10 - C11 2.5005
N14 - C15 2.6586
1.2603 0.348816 -0.98546 0.257849 -0.76206 -0.50421 0.338357
0.9694 0.289609 -0.75228 0.193787 -0.57564 -0.38186 0.336644
0.9695 0.289609 -0.75228 0.193789 -0.57565 -0.38186 0.336645
0.9710 0.296368 -0.74393 0.214889 -0.61576 -0.40087 0.348981
N3 - C4
N3 - C7
2.6586
2.6267

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Table 3. The topological properties at the RCP for two isomers.
ꢀ
Atoms
ꢁ
ꢆ ꢂꢁ
ꢚ
-
C1 - C2 - N3 - C4 - C5 - C6-
0.021377 0.158132
0.052564 0.369192
0.023285 0.161113
0.052430 0.367555
-0.007667
-0.006408
-0.006892
-0.006482
-0.001962
-0.007581
-0.007660
-0.006428
-0.006862
-0.006428
-0.007660
-
N3 - C4 - N9 - C8 - C7-
C7 - C8 - C13 - C12 - C11 - N10-
C11 - C12 - N16 - C15 - N14-
-
3
4
-
-
C12 - C13 - C21 - H30 - C20 - N16- 0.006912 0.033775
C15 - N16 - C20 - C19 - C18 - C17- 0.021062 0.155754
-
-
C1 - C2 - N3 - C4 - C5 - C6-
N3 - C4 - N9 - C8 - C7-
-C7 - C8 - C13 - C12 - C11 - N10-
C11 - C12 - N16 - C15 - N14-
0.021361 0.158005
0.052520 0.368736
0.023456 0.162075
0.052520 0.368736
-
-
-
N14 - C15 - C22 - C21 - C20 - C19- 0.021361 0.158005

ACCEPTED MANUSCRIPT
Highlights
•
•
•
Two new fluorescent heterocyclic systems were synthesized from imidazo[1,2-
a]pyridine.
The optical and solvatochromic properties of these fluorophores were
investigated.
DFT calculations of two structures are performed at the B3LYP/6-311++G(d,p)
level.
•
Their intra- and intermolecular interactions were investigated by AIM analysis.