Synthesis, spectroscopic and DFT studies of novel fluorescent dyes: 3-Aminoimidazo[1,2-a]pyridines possessing 4-pyrone moieties

Article abstract of DOI:10.1016/j.saa.2013.09.056

A series of novel imidazo[1,2-a]pyridines possessing 4-pyrone ring were synthesized by three-component condensation of 4-pyrone carbaldehydes, 2-aminopyridines and isocyanides. Bismuth (III) chloride was used as a catalyst in these reactions and desired products were synthesized in good yields at a very short period of time under solvent free conditions. UV-Vis absorption and fluorescence emission spectra of these compounds were investigated. It shown that two of these compounds (10f and 10g) exhibit intense fluorescence in dichloromethane. Optimized ground-state molecular geometries and orbital distributions of these two fluorescent dyes were obtained using density functional theory (DFT). Thermogravimetric analysis and electrochemical properties of these compounds were also studied.

Full text of DOI:10.1016/j.saa.2013.09.056

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 614–621
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic and DFT studies of novel fluorescent dyes:
3-Aminoimidazo[1,2-a]pyridines possessing 4-pyrone moieties
⇑
Aziz Shahrisa , Kazem Dindar Safa, Somayeh Esmati
Department of Organic and Biochemistry, Faculty of Chemistry, University of Tabriz, Tabriz 5166614766, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
New bis-heterocyclic compounds
were synthesized using GBB
multicomponent reaction.
ꢀ
UV–Vis absorption and fluorescence
emission spectra, TGA and CV of
compounds were studied.
ꢀ
ꢀ
Two of these novel compounds
showed high fluorescence emissions.
Optimized molecular geometries and
orbital distributions were calculated
using DFT.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2013
Received in revised form 4 August 2013
Accepted 7 September 2013
Available online 19 September 2013
A series of novel imidazo[1,2-a]pyridines possessing 4-pyrone ring were synthesized by three-compo-
nent condensation of 4-pyrone carbaldehydes, 2-aminopyridines and isocyanides. Bismuth (III) chloride
was used as a catalyst in these reactions and desired products were synthesized in good yields at a very
short period of time under solvent free conditions. UV–Vis absorption and fluorescence emission spectra
of these compounds were investigated. It shown that two of these compounds (10f and 10g) exhibit
intense fluorescence in dichloromethane. Optimized ground-state molecular geometries and orbital dis-
tributions of these two fluorescent dyes were obtained using density functional theory (DFT). Thermo-
gravimetric analysis and electrochemical properties of these compounds were also studied.
Ó 2013 Published by Elsevier B.V.
Keywords:
Multicomponent reactions
4-Pyrone derivatives
UV–Vis spectra
Fluorescence
Density functional theory (DFT)
Introduction
able and there are some reports on the use of MCRs for the synthe-
sis of fluorescent dyes [22,23].
Design and synthesis of novel organic fluorescent compounds is
one of interesting and active areas in the organic chemistry.
Fluorescent dyes have wide applications in the biological labels,
photovoltaic cells, light emitting diodes (LEDs), optical sensors,
materials for collecting solar energy, fluorescent films for green-
houses, fluorescent colorants and etc [1–15]. In the last decade
synthesis of various organic compounds via multicomponent reac-
tions (MCRs) have attracted a great interest, because most of these
reactions are simple, one-pot, fast and high yielding [16–21].
Therefore synthesis of fluorescent compounds via MCRs is so favor-
One of isocyanide based multicomponent reaction (IMCR) is
Groebke–Blackburn–Bienaymé multicomponent reaction (GBB
MCR) [24–26] in which three-component condensation of alde-
hyde, isocyanide and 2-aminoazine enables synthesis of imi-
dazo[1,2-a]azines. Compounds containing imidazo[1,2-a]azine
ring system have been shown to possess a broad range of useful
pharmacological and biological properties [27–29]. Moreover in
the recent years fluorescence properties of these compounds have
also been investigated [30].
On the other hand, pyran-containing fluorescent dyes are an
important class of organic light emitting diodes (OLEDs) and also
have been used in dye lasers, sensors, dye-sensitized solar cells
(DSSCs), fluorescent probes, logic gates and optical chemosensors
⇑
Corresponding author. Tel.: +98 411 339 3118; fax: +98 411 334 0191.
E-mail address: ashahrisa@yahoo.com (A. Shahrisa).
1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.saa.2013.09.056

A. Shahrisa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 614–621
615
[31–39]. Most of these compounds are dicyanomethylene pyran
derivatives and the fluorescence properties of 4H-pyran-4-ones
are rarely investigated.
Thermogravimetric analysis (TGA) was conducted under nitro-
gen at a heating rate of 10 °C minꢁ1 with a TGA/SDTA851 (Mettler
Toledo) from 50 °C to 600 °C under 20 ml minꢁ1 of argon flow.
Electrochemical experiments were performed using Autolab
PGSTAT 30 electrochemical analyzer system and GPES 4.9 software
package (Eco Chemie, The Netherlands). The utilized three-elec-
trode cell was composed of a glassy carbon electrode with 2 mm
diameter as the working electrode, a platinum wire as auxiliary
electrode, and SCE (saturated calomel electrode) as reference elec-
trode (0.01 M AgNO3) in CH2Cl2 solution (10ꢁ3 mol Lꢁ1) in the pres-
ence of TBAPF6 (tetrabutylammonium hexafluorophosphate)
(0.10 mol Lꢁ1) as supporting electrolyte. Mass spectra (MS) were
measured by a Shimadzo (70 eV) spectrometer and elementary
analyses (C,H,N) were performed on a Vario EL III analyzer. Thin-
layer chromatography was done with prepared glass-backed plates
Because of the wide applicability of these types of compounds,
the synthesis and studies of their chemical and photophysical
properties is an interesting research field. Therefore, as a part of
our ongoing interest in the multicomponent reactions and synthe-
sis of novel derivatives of 4-pyrones [40–43], in this investigation
we used GBB MCR for the synthesis of novel 3-aminoimi-
dazo[1,2-a]pyridines possessing 4-pyrones by three-component
reaction of aldehydes, isocyanides and 2-aminopyridines catalyzed
by BiCl3 under solvent free conditions and then UV–Vis absorption
spectra, fluorescence emission spectra, DFT structure optimization,
thermogravimetric analysis and electrochemical properties of
these compounds were also studied.
(20 ꢂ 20 cm2,500
l) using silica gel (Merk Kieselgel 60 HF254, Art.
7739). The chemical reagents used in synthesis were purchased
from Merck and Sigma–Aldrich CO. Synthetic details and charac-
terizations of compounds 3a, 6 and 9 are given in Supplementary
material.
Experiments
General
Melting points were determined with a MEL-TEMP model
1202D and are uncorrected. FT-IR spectra were recorded on a Bru-
ker Tensor 27 spectrometer as KBr disks. The 1H NMR spectra were
recorded with a Bruker Spectrospin Avance 400 spectrometer with
CDCl3 as solvent and TMS as internal standard. 13C NMR spectra
were determined on the same instrument at 100 MHz. All chemical
shifts were reported as d (ppm) and coupling constants (J) are gi-
ven in Hz. UV–Vis spectra were recorded on analytikjena SPECORD
250 spectrometer. Fluorescence spectra were obtained on a Jasco
General procedure for the synthesis of 3-aminoimidazo[1,2-
a]pyridines 10a–g
To a mixture of 4-pyrone carbaldehyde (0.5 mmol), 2-amino-
pyridine or 2-amino-6-methyl pyridine (0.5 mmol) and isocyanide
(0.5 mmol) was added BiCl3 (5 mol%) and the reaction mixture was
stirred on a preheated oil bath at 110 °C. After completion of the
reaction (monitored by TLC), the crude residue was either treated
with ethyl acetate/n-hexane (1:3) to afford the product as a precip-
itate, or was subjected to silica gel preparative layer chromatogra-
phy (ethyl acetate: n-hexane; 1:3).
FP-750 spectrofluorometer. The fluorescence quantum yields (Uf
)
were determined in CH2Cl2 dilute solutions by using quinine sul-
fate (Us = 0.546 in 0.05 M H2SO4) as standard. Calculations are
done by Eq. (1), where Uf is the fluorescence quantum yield of
the sample, Us the fluorescence quantum yield of the standard, F
and FS are the areas under the fluorescence emission curves of
the samples and the standard, respectively. A and AS are the rela-
tive absorbance of the samples and standard at the excitation
2-[4-(3-(Cyclohexylamino)imidazo[1,2-a]pyridin-2-yl)phenyl]-6-
phenyl-4H-pyran-4-one (10a)
Yellow solid. Yield: 97%. M.p: 180–182 °C. FT-IR (KBr): 3249
(NAH), 3072 (aromatic CAH), 2924 (aliphatic CAH), 1646 (pyrone
C@O). 1H NMR (400 MHz, CDCl3): d 1.19–1.90 (m, 10H); 3.04 (m,
1H); 3.17 (bs, 1H); 6.82–6.87 (m, 2H); 6.90 (d, J = 2 Hz, 1H);
7.18–7.22 (m, 1H); 7.55–7.59 (m, 4H); 7.88–7.94 (m, 2H); 7.97
(d, J = 8.4 Hz, 2H); 8.13 (d, J = 6.8, 1H); 8.31 (d, J = 8.4 Hz, 2H). 13C
wavelength, respectively.
g and gs are the refractive indices of sol-
vents for the sample and standard, respectively [44]:
2
Uf
¼
UsðF=FsÞðAs=AÞð
g
=gs
Þ
ð1Þ
Scheme 1. Synthesis of 4-pyrone carbaldehydes. Reagents and conditions: (i) NBS, BPO, CCl4, reflux, 48 h; (ii) HMTA, CHCl3, reflux, 30 min; (iii) EtOH/H2O (3:2), reflux, 24 h;
(iv) NaBH4 (4eq), MeOH, reflux, 3 h; (v) Active MnO2 (8eq), CH2Cl2, rt., 3 days; and (vi) PhCH2Br (1eq), NaOH (1eq), MeOH/H2O: 10/1, 60 °C, 4 h.

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Table 1
Synthesis of 3-aminoimidazo[1,2-a]pyridines possessing 4-pyrones.
Entry
1
Ar
R
X
Product
Yield (%)
97
Cyclohexyl
H
2
Cyclohexyl
CH3
96
3
4
t-Butyl
H
H
94
95
Cyclohexyl
5
Cyclohexyl
H
75
6
7
Cyclohexyl
Cyclohexyl
H
72
74
CH3
NMR (100 MHz, CDCl3): d 24.9; 25.7; 34.3; 57.1; 111.1; 111.5;
111.9; 117.7; 122.7; 124.4; 125.8; 126.0; 126.1; 127.4; 129.2;
129.8; 131.4; 131.6; 135.4; 137.9; 141.9; 163.3; 180.3. MS (m/
z,%): 461 (M+, 12), 385 (90), 303 (100), 275 (53), 250 (77), 157
(58), 102 (38), 78 (89). Anal. Calcd. For C30H27N3O2: C, 78.07; H,
5.90; N, 9.10; Found: C, 77.84; H, 5.92; N, 8.98%.
J = 2 Hz, 1H); 6.90 (d, J = 2 Hz, 1H); 7.04–7.08 (m, 1H); 7.45 (d,
J = 9.2 Hz, 1H); 7.57–7.59 (m, 3H); 7.91–7.96 (m, 4H); 8.22 (d,
J = 8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3): d 20.1; 24.9; 25.7;
33.3; 59.2; 111.1; 111.5; 113.9; 116.1; 124.7; 125.9; 126.0;
127.2; 128.2; 129.2; 129.8; 131.4; 131.6; 136.2; 137.7; 138.5;
143.6; 163.3; 180.3. MS (m/z, %): 475 (M+, 3), 392 (9), 368 (11),
278 (31), 250 (68), 92 (74), 55 (100). Anal. Calcd. For
C
31H29N3O2: C, 78.29; H, 6.15; N, 8.84; Found: C, 77.97; H, 6.17;
2-(4-(3-(Cyclohexylamino)-5-methylimidazo[1,2-a]pyridin-2-
yl)phenyl)-6-phenyl-4H-pyran-4-one (10b)
N, 8.76%.
Yellow solid. Yield: 96%. M.p: 184–186 °C. FT-IR (KBr): 3245
(NAH), 3068 (aromatic CAH), 2926 (aliphatic CAH), 1644 (pyrone
C@O). 1H NMR (400 MHz, CDCl3): d 1.09–1.73 (m, 10H); 2.83 (m,
1H); 2.97 (s, 3H); 3.16 (bs, 1H); 6.48 (d, J = 6.4 Hz, 1H); 6.84 (d,
2-[4-(3-(tert-Butylamino)imidazo[1,2-a]pyridin-2-yl)phenyl]-6-
phenyl-4H-pyran-4-one (10c)
Yellow solid. Yield: 94%. M.p: 217–219 °C. FT-IR (KBr): 3238
(NAH), 3071 (aromatic CAH), 2925 (aliphatic CAH), 1642 (pyrone

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617
(a)
(b)
Wavelength (nm)
Fig. 3. Photographs of dyes 10a–g (left to right) in daylight and long UV (365 nm).
C@O). 1H NMR (400 MHz, CDCl3): d 1.09 (s, 9H); 3.13 (bs, 1H);
6.78–6.82 (m, 2H); 6.86 (s, 1H); 7.14–7.18 (m, 1H); 7.54–7.56 (m,
4H); 7.87–7.93 (m, 4H); 8.18–8.22 (m, 3H). 13C NMR (100 MHz,
CDCl3): d 29.4; 55.6; 110.0; 110.4; 110.6; 116.5; 122.4; 123.2;
123.5; 124.7; 124.9; 127.5; 128.1; 128.8; 130.3; 130.5; 137.1;
137.6; 141.3; 162.2; 162.3; 179.2. MS (m/z, %): 435 (M+, 28), 379
(100), 351 (80), 102 (16), 78 (92). Anal. Calcd. For C28H25N3O2: C,
77.22; H, 5.79; N, 9.65; Found: C, 77.14; H, 5.81; N, 9.59%.
Wavelength (nm)
Fig. 1. (a) UV–Vis absorption spectra of 10a–d and (b) 10e–g (10ꢁ5 M) in CH2Cl2 at
room temperature.
2-(4-(3-(Cyclohexylamino)imidazo[1,2-a]pyridin-2-yl)phenyl)-6-
methyl-4H-pyran-4-one (10d)
Yellow solid. Yield: 95%. M.p: 156–158 °C. FT-IR (KBr): 3251
(NAH), 3072 (aromatic CAH), 2928 (aliphatic CAH), 1659 (pyrone
C@O). 1H NMR (400 MHz, CDCl3): d 1.17–1.87 (m, 10H); 2.41 (s,
3H); 3.00 (m, 1H); 3.16 (bs, 1H); 6.19 (s, 1H); 6.77 (d, J = 2 Hz,
1H); 6.82 (t, J = 8 Hz, 1H); 7.18 (t, J = 8 Hz, 1H); 7.57 (d, J = 8 Hz,
1H); 7.85 (d, J = 8 Hz, 2H); 8.11 (d, J = 8 Hz, 1H); 8.24 (d, J = 8 Hz,
2H). 13C NMR (100 MHz, CDCl3): d 19.9; 24.9; 25.7; 34.3; 57.0;
110.4; 111.9; 114.3; 117.6; 122.7; 124.4; 125.8; 125.9; 127.3;
129.6; 135.3; 137.7; 141.9; 163.5; 165.4; 180.3. MS (m/z, %): 399
(M+, 32), 316 (87), 289 (95), 92 (22), 78 (100). Anal. Calcd. For
10a
10b
10c
10d
10e
10f
10g
C25H25N3O2: C, 75.16; H, 6.31; N, 10.52; Found: C, 75.03; H, 6.35;
N, 10.45%.
Wavelength (nm)
2-[3-(Cyclohexylamino)imidazo[1,2-a]pyridin-2-yl]-6-phenyl-4H-
pyran-4-one (10e)
Fig. 2. The emission spectra of compounds 10a–g (10ꢁ5 M) in CH2Cl2 at room
temperature.
Pale yellow solid. Yield: 75%. M.p: 146 °C (decom.). FT-IR (KBr):
3382 (NAH), 3108 (aromatic CAH), 2926 (aliphatic CAH), 1647
(pyrone C@O). 1H NMR (400 MHz, CDCl3): d 1.11–186 (m, 10H);
3.03 (m, 1H); 3.86 (d, J = 8 Hz, 1H); 6.78 (d, J = 2 Hz, 1H); 6.86 (t,
J = 8 Hz, 1H); 7.17 (d, J = 2 Hz, 1H); 7.22 (t, J = 8 Hz, 1H); 7.56–
7.58 (m, 4H); 7.82–7.88 (m, 2H); 8.06 (d, J = 8 Hz, 1H). 13C NMR
(100 MHz, CDCl3): d 24.8; 25.5; 33.9; 56.9; 111.8; 112.6; 118.3;
123.2; 125.4; 126.1; 127.0; 129.2; 129.5; 131.4; 131.9; 142.1;
161.2; 163.2; 179.7. MS (m/z, %): 385 (M+, 75), 303 (100), 275
(55), 157 (82), 105 (28), 78 (86). Anal. Calcd. For C24H23N3O2: C,
74.78; H, 6.01; N, 10.90; Found: C, 74.62; H, 6.06; N, 10.82%.
Table 2
Photophysical properties of compounds 10a–g.
a
Compound
kabs (nm)
kem (nm)
D
mst (nm)
Uf
10a
10b
10c
10d
10e
10f
283, 359
284, 356
282, 355
282, 357
273, 353
265, 357
267, 352
508
506
515
500
475
478
476
149
150
160
143
122
121
124
0.028
0.032
0.024
0.019
0.082
0.209
0.231
5-(Benzyloxy)-2-[3-(cyclohexylamino)imidazo[1,2-a]pyridin-2-yl]-
4H-pyran-4-one (10f)
Pale yellow solid. Yield: 72%. M.p: 188–190 °C. FT-IR (KBr): 3311
(NAH), 3081 (aromatic CAH), 3036 (aromatic CAH), 2925
10g
a
Measured in CH2Cl2 solution using quinine sulfate ðUs ¼ 0:546Þ as a standard at
room temperature.

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A. Shahrisa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 614–621
(aliphatic CAH), 2852 (CAH), 1634 (pyrone C@O). 1H NMR
(400 MHz, CDCl3): d 1.20–1.88 (m, 10H); 2.92 (m, 1H); 3.54 (d,
J = 8 Hz, 1H); 5.15 (s, 2H); 6.82 (t, J = 6.8, 1H); 7.16–7.20 (m, 2H);
7.33–7.44 (m, 5H); 7.51 (d, J = 9.2 Hz, 1H); 7.60 (s, 1H); 7.99 (d,
J = 6.8, 1H). 13C NMR (100 MHz, CDCl3): d 23.9; 24.5; 33.2; 55.9;
70.9; 110.6; 111.5; 117.2; 122.0; 124.3; 126.9; 127.3; 127.6;
128.5; 134.8; 139.7; 140.9; 146.1; 159.2; 173.5. MS (m/z, %): 415
(M+, 100), 332 (33), 242 (35), 91 (84). Anal. Calcd. For
z,%): 429 (M+, 59), 346 (70), 319 (26), 256 (57), 91 (100). Anal.
Calcd. For C26H27N3O3: C, 72.71; H, 6.34; N, 9.78; Found: C,
72.48; H, 6.28; N, 9.73%.
Results and discussion
Synthesis
C
25H25N3O3: C, 72.27; H, 6.06; N, 10.11; Found: C, 72.08; H, 6.14;
Recently we reported the synthesis of some 3-aminoimi-
dazo[1,2-a]pyridines via BiCl3 catalyzed GBB MCR [45]. As a part
of our ongoing interest in the MCRs and synthesis of novel deriva-
tives of 4-pyrones, we describe the synthesis of 3-aminoimi-
dazo[1,2-a]pyridines possessing 4-pyrones by three-component
reaction of 4-pyrone carbaldehydes, isocyanides and 2-aminopyri-
dines catalyzed by BiCl3 (5 mol%) under solvent free conditions.
Therefore four 4-pyrone carbaldehydes were prepared and used
in these investigations. The syntheses of 4-pyrone carbaldehydes
3a and 3b have been reported in our recent works [46]. As shown
in Scheme 1 bromination of 4-pyrones 1 and 2 with N-bromosuc-
cinimide (NBS) produced the corresponding bromopyrones which
then reacted with hexamethylenetetramine (HMTA) in dry chloro-
form to afford hexaminium salts 2a,b. Treatment of 2a,b with
N, 9.95%.
5-(Benzyloxy)-2-(3-(cyclohexylamino)-5-methylimidazo[1,2-
a]pyridin-2-yl)-4H-pyran-4-one (10g)
Pale yellow solid. Yield: 74%. M.p: 190-192 °C, FT-IR (KBr): 3307
(NAH), 3080 (aromatic CAH), 2926 (aliphatic CAH), 2853 (CAH),
1631 (pyrone C@O). 1H NMR (400 MHz, CDCl3): d 1.08–1.85 (m,
10H); 2.77 (m, 1H); 2.92 (s, 3H); 3.32 (d, J = 8 Hz, 1H); 5.18 (s,
2H); 6.48 (d, J = 6.8 Hz, 1H); 7.07 (dd, J = 6.8 Hz, J = 9.2 Hz, 1H);
7.19 (s, 1H); 7.33–7.42 (m, 4H); 7.45 (d, J = 7.2 Hz, 2H); 7.62 (s,
1H). 13C NMR (100 MHz, CDCl3): d 19.6, 25.3; 25.7; 33.2; 60.3;
72.1; 112.6; 114.4; 116.5; 125.7; 128.0; 128.4; 128.7; 129.7;
130.9; 135.9; 136.8; 141.0; 143.8; 147.2; 160.4; 174.6. MS (m/
Fig. 4. Optimized ground-state geometry and calculated spatial distributions of the HOMO, LUMO levels of compounds 6 (left) and 7 (right) with B3LYP/6-31G⁄.

A. Shahrisa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 614–621
619
EtOH:H2O (3:2) under reflux afforded the corresponding 4-pyrone
carbaldehydes 3a,b. Aldehyde 6 was synthesized from the corre-
sponding ethoxycarbonyl derivative 4 according to the reported
procedure [47]. Ester group was reduced to the corresponding
alcohol with NaBH4 and then oxidized to the aldehyde group using
active MnO2 to produce 4-pyrone carbaldehyde 6. Aldehyde 9 was
obtained from commercially available Kojic acid 7 by benzylation
of phenolic OH [48], followed by oxidation of the hydroxymethyl
group with active MnO2. These 4-pyrone carbaldehydes were used
in GBB MCRs and the corresponding derivatives 10a–g were syn-
thesized in good to high yields in a very short period of time
(Table 1).
HOMO–LUMO gaps for 10f and 10g are 3.918 eV (317 nm) and
3.884 eV (320 nm) respectively that are in good agreement with
the measured UV–Vis bands (357 and 352 nm).
Thermal properties
Thermal stabilities of the organic dyes play an important role in
the performance and lifetime of OLEDs during operation. The ther-
mal properties of compounds 10a–g were investigated by thermo-
gravimetric analysis (TGA). As shown at the thermogravimetric
curves in Fig. 5, these compounds demonstrated relatively high
thermal stability. Compounds 10a–d which decomposed at about
360–400 °C are more stable than compounds 10e–g. Compound
10e showed the least decomposition temperature (Td) at 150 °C
and compounds 10f and 10g with similar structures showed rela-
tively similar TGA curves with Td at about 300 °C.
UV–Vis absorption and fluorescence emission spectra
The UV–Vis absorption spectra of all seven 3-aminoimidazo[1,2-
a]pyridines possessing 4-pyrones were measured in CH2Cl2 solu-
tions with a concentration of about 1.0 ꢂ 10ꢁ5 mol Lꢁ1 (Fig. 1).
According to the structure pattern of these compounds we could
investigate them at two groups. At compounds 10a–d the 4-pyrone
ring is connected to imidazo[1,2-a]pyridine via a para-phenylene
group and at compounds 10e–g, 4-pyrone is directly connected to
imidazo[1,2-a]pyridine. Each group of these compounds exhibit
similar UV–Vis absorption spectra. Compounds 10a–d display
intense absorption bands at about 283 nm that should be attributed
Electrochemical properties
The electrochemical properties of compounds 10a–g are ex-
plored by the cyclic voltammetry in the CH2Cl2 solutions in the
presence
of
tetrabutylammonium
hexafluorophosphate
(0.10 mol Lꢁ1) as the supporting electrolyte. Cyclic voltammogram
of ꢃ10ꢁ3 M solutions of these compounds are shown in Fig. 6.
Interestingly, similar voltammograms were obtained for the each
group of compounds. It demonstrates that compounds with similar
structures are oxidized at almost the same potential. Compound
10a–d exhibited two oxidation peaks at about 0.5 and 1.2 V and
two reduction peaks at about 0.2 and 1.0 V. The cyclic voltammo-
gram of compound 10e exhibited one cathodic peak at 1.2 V, with-
out any anodic peak at reverse scan, indicating that the cathodic
peak is an irreversible electron transfer processes. According to
the molecular structure, compounds 10f and 10g showed similar
to
p ? p
* type transitions and weak broad bands centered at about
356 nm which are attributed to n ? p* type transitions. The
maximum UV–Vis absorptions of the compounds 10e–g are located
in the range of 265–273 nm, ascribed to the
p ? p
* type transition
of the conjugated molecular backbone and broad bands in the
352–357 nm attributed to n ? p* type transitions. These similar
absorption spectra are owing to their similar molecular structures.
The maximum absorption peaks of compounds 10a–d are relatively
red-shifted in comparison with the absorption of compounds 10e–g
that can be attributed to more delocalized and extended
gated systems in these compounds.
p-conju-
Furthermore, the fluorescence spectra of these compounds
were investigated at room temperature in CH2Cl2 solutions. As
shown in Fig. 2, fluorescence emission spectra of compounds
10a–g with the excitation wavelength at 370 nm, exhibiting a blue
to green fluorescence emission with the maximum emission peaks
varying from 500 to 515 nm for compounds 10a–d and 475–
478 nm for compounds 10e–g. The maximum emission band of
compounds 10e–d are blue shifted compared to compounds 10e–
g. Compounds 10f and 10g with 5-benzyloxy-4H-pyran-4-one
moiety at imidazo[1,2-a]pyridine showed strong cyan emission
compared with the other compounds.
The photophysical properties including absorption and emis-
sion maxima (kabs and kem), Stokes’ shift (Dmst) and fluorescence
quantum yields (Uf) of the compounds 10a–g were summarized
in Table 2. Interestingly, compounds 10f and 10g showed high
fluorescence quantum yields (Uf = 0.209 and 0.231) and relatively
large Stokes shifts (about 120 nm) were observed for these two
compounds. Photographs of dyes in daylight and under laboratory
UV lamp (365 nm) are presented in Fig. 3.
Theoretical calculations
The geometrical structures and electronic configurations of the
new dyes were examined using density functional theory (DFT)
calculations with B3LYP functional and 6-31G* basis set on the
compounds 10f and 10g with Gaussian03 program package [49].
The orbital distributions of HOMO-1, HOMO, LUMO and LUMO+1
for these two fluorescent dyes are shown in Fig. 4. It is obvious that
both of the HOMO and LUMO of them localize at the 4-pyrone and
imidazo[1,2-a]pyridine rings. The theoretical estimation of the
Fig. 5. Thermogravimetric analysis graph for (a) compound 10a–d and (b)
compound 10e–g.

620
A. Shahrisa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 614–621
(a)
(c)
(e)
(b)
E (V)
E (V)
(d)
E (V)
E (V)
(f)
-1
0
1
2
E (V)
E (V)
2.5
2
(g)
1.5
1
0.5
0
-0.5
-1
-1
0
1
2
E (V)
Fig. 6. Cyclic voltammogram of (a) compound 10a, (b) compound 10b, (c) compound 10c, (d) compound 10d, (e) compound 10e, (f) compound 10f and (g) compound 10g
(10ꢁ3 mol Lꢁ1) in 0.1 mol Lꢁ1 Bu4NPF6–CH2Cl2, scan rate 50 mV/s.
voltammogram with one oxidation (at 1.42 and 1.24 V respec-
tively) and one reduction peak (at 0.32 and 0.15 V respectively).
ric analysis (TGA) of compounds 10a–g showed that these
compounds have relatively high thermal stability.
Acknowledgments
Conclusions
The authors thank research affairs of the University of Tabriz for
financial support. We also would like to thank Dr. Abolghasem
Jouyban (Faculty of Pharmacy, Tabriz University of Medicinal
Sciences) for his help in providing fluorescence emission spectra.
In conclusion, a novel series of 3-aminoimidazo[1,2-a]pyridines
possessing 4-pyrones were designed and synthesized by three-
component condensation of 4-pyrone carbaldehydes, 2-aminopyri-
dines and isocyanides in the presence of bismuth chloride (5 mol%)
under solvent free conditions. This method for synthesis of 3-
aminoimidazo[1,2-a]pyridines offers several advantages including
operational simplicity, short reaction times and high yields of
products. The results obtained by the absorption and emission
spectra of these compounds indicated that two of them (10f and
10g) are highly fluorescent in CH2Cl2 and can be used as blue light
emitting materials. The DFT calculations indicated that HOMO and
LUMO of these compounds localize at the 4-pyrone and imi-
dazo[1,2-a]pyridine rings and they possess a relatively high HOMO
energy levels (ꢁ5.39 and ꢁ5.24 eV). The electrochemical proper-
ties of these compounds were also investigated. Thermogravimet-
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.056.
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