Synthesis and photophysical properties of fluorescent arylstyrylimidazo[1,2-a]pyridine-based donor-acceptor chromophores

Article abstract of DOI:10.1016/j.dyepig.2014.09.016

A series of novel fluorescent arylstyrylimidazo[1,2-a]pyridines was synthesized and fully characterized. All styryl derivatives have an E-configuration of the vinyl double bond as unequivocally shown by ¹H NMR spectroscopy. It was observed that

Full text of DOI:10.1016/j.dyepig.2014.09.016

Accepted Manuscript
Synthesis and photophysical properties of fluorescent arylstyrylimidazo[1,2-a]pyridine-
based donor-acceptor chromophores
Zeynel Seferoğlu, Heiko Ihmels, Ertan Şahin
PII:
S0143-7208(14)00375-1
10.1016/j.dyepig.2014.09.016
DYPI 4529
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 July 2014
Revised Date: 1 September 2014
Accepted Date: 9 September 2014
Please cite this article as: Seferoğlu Z, Ihmels H, Şahin E, Synthesis and photophysical properties of
fluorescent arylstyrylimidazo[1,2-a]pyridine-based donor-acceptor chromophores, Dyes and Pigments
(2014), doi: 10.1016/j.dyepig.2014.09.016.
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Graphical Abstract (for review)
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*Highlights (for review)
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Highlights

A series of novel fluorescent arylstyrylimidazo[1,2-a]pyridine-based dyes was
efficiently synthesized and characterized.


The arylstyrylimidazo[1,2-a]pyridine dyes exhibit fluorosolvatochromism.
These compounds may be used as pH-sensitive fluorescent light-up probe because
they display strong changes of absorption and emission properties with decreasing pH.

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Synthesis and photophysical properties of fluorescent
arylstyrylimidazo[1,2-a]pyridine-based donor-acceptor
chromophores
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a,b
b
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Zeynel Seferoğlu *, Heiko Ihmels and Ertan ꢀahin
a
Gazi University, Department of Chemistry, 06500 Ankara, Turkey
b
University of Siegen, Organic Chemistry II, Adolf-Reichwein-Str. 2, D-57068 Siegen,
Germany
c
Atatürk University, Department of Chemistry, Erzurum, Turkey
Abstract
A series of novel fluorescent arylstyrylimidazo[1,2-a]pyridines was synthesized and fully
characterized. All styryl derivatives have an E-configuration of the vinyl double bond as
1
unequivocally shown by H NMR spectroscopy. It was observed that the E isomers are stable
in the solid state; however, the derivatives with strong electron donating dialkylamino
substituents underwent partial E-Z isomerization in solution at room temperature. The styryl
derivatives absorb in the UV or visible region and emit light with moderate Stokes shifts.
These compounds exhibit fluorosolvatochromism, namely the emission band is red shifted
with increasing solvent polarity. Moreover, the absorption and emission properties of the
styryl derivatives change drastically upon acidification, as the protonation of the nitrogen
atoms of the imidazo[1,2-a]pyridine ring increases the donor-acceptor interplay of the π
system.
Keywords: Styryl dyes, heterocycles, imidazo[1,2-a]pyridine, pH sensitive dyes, fluorescent
probes.
*
Corresponding author Tel.: +90 312 2021525; fax: +90 312 2122279
*E-mail addresses: znseferoglu@gazi.edu.tr (Z. Seferoğlu).
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1. Introduction
Functional dyes with appropriate absorption and emission color are useful components for the
development of optical chemosensors, photonic materials or photosensitizers [1-4].
Specifically, fluorescent chemosensors are versatile tolls in analytical chemistry, biochemistry
and medicine [5]. These probe molecules may be employed for the fluorimetric
characterization of their surounding environment, because the latter may affect the emission
energy, emission quantum yield, or emission lifetime [6,7]. Along these lines, styryl dyes
have been established as a promising class of compounds that exhibit a great potential to be
employed in advanced materials such as laser dyes, optical or electro-optical devices or sensor
materials [8-31] etc. due to their excellent optical-electronic properties. Also, many potential
metal-free solar cell sensitizers are neutral styryl dyes [32]. However, some neutral styryl
dyes also show thermally induced or photo-induced E-Z isomerization and dimerization
especially in solution [33]. In addition, the cationic styryl dyes are used as DNA probes and
voltage-sensitive dyes in cardiac tissue [34-37]. Overall, the class of styryl dyes exhibits a
remarkable structural diversity. And among the substructures that are frequently used for the
development of functional styryl dyes, nitrogen-containing heterocyles constitute a highly
variable and versatile component [26]. In this context it is surprising that imidazopyridine
rings have not been explored systematically as heteroaromatic unit in styryl dyes.
Imidazopyridine rings systems are well-known heterocyclic compounds with a large potential
to exhibit biological activity [38-43]. In addition these heterocycles are readily available and
functionalized. Therefore we proposed that the integration of imidazopyridine units into
styryl-type structures leads to novel dye structures with promising properties that may be
considered for the development of functional dyes. Herein we present the synthesis and and
investigation of photophysical properties of arylstyrylimidazo[1,2-a]pyridine dyes.
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2. Experimental Section
2.1.
General Instrumentations and Materials
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All commercially available chemicals were reagent grade and used without further
purification. The melting points were determined with a Büchi 510K melting point apparatus
and are not corrected. Mass spectra (ESI in the positive-ion mode, source voltage 6 kV) were
recorded with a Finnigan LCQ Deca instrument; only m/z values in the range of 100-2000
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units were analyzed. NMR spectra were measured on Bruker Avance 400 ( H: 400 MHz, C:
00 MHz) spectrometers at 20 °C (293 K); chemical shifts are given in ppm (δ) relative to
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TMS (δ = 0.00 ppm). Elemental microanalysis of the compounds was performed with a
HEKAtech EuroEA combustion analyzer by Mr. H. Bodenstedt (Organische Chemie I,
Universität Siegen). The X-ray diffraction data collection was performed on a Rigaku R-
AXIS RAPID-S diffractometer equipped with Mo Kα (λ=0.71073 Å) radiation using w scans.
2.2.
Spectrophotometric measurements.
Absorption spectra were recorded on a Varian Cary 100 double-beam spectrophotometer;
steady-state fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence
spectrophotometer. All spectrophotometric measurements were performed in thermostated
quartz sample cells at 20 °C, using spectral grade solvents (Fluka, Riedel-de Haën). The
solution concentrations were 10 ꢁM for absorption spectroscopy and 1.0-5.0 ꢁM for
fluorescence spectroscopy. Spectrophotometer slit widths were set to bandwidths of 5 nm for
emission spectroscopy. The relative fluorescence quantum yields, Φfl, were determined by
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standard methods [44,45] with Coumarin 1 (laser grade, Acros Organics, Φfl = 0.73 in
ethanol) as a reference.
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2.3.
X-ray crystallography
Single crystal X-ray diffraction data were collected on Rigaku R-AXIS RAPID-S
diffractometer equipped with a IP detector, using monochromatized Mo K (λ=0.71073 A°)
α
radiation. Integration of the intensities, correction for Lorentz and polarization effects and cell
refinements were performed using CrystalClear (Rigaku/MSC Inc., 2005) software [46]. The
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structures were solved by direct methods SHELXS97 [47] and refined on F by full-matrix
least-squares using the programs SHELXL97 [47]. All non-hydrogen atoms were refined
anisotropically. Positions of H atoms bonded to C atoms were calculated at ideal positions
and refined by the riding model with Uiso -1.5Ueq(C) for methyl, and Uiso -1.2Ueq(C) for
remaining H atoms. The final difference Fourier maps showed no peaks of chemical
significance. Crystal data and details of the structure determination for the compounds are
summarized in Table S1 (in Supporting information page S5).
2.4.
General procedure for the synthesis of imidazo[1,2-a]pyridine derivatives
3a-c.
A mixture of respective 2-bromoacetophenone derivative (0.001 mol) the 2-amino-4-
methylpyridine (0.001 mol, 0.108 g) and NaHCO (0.84 g, 0.01 mol) in EtOH (95%, 20 mL)
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was stirred under reflux for 20 h. The reaction progress was monitored by TLC. Upon
completion of the reaction, the mixture was evaporated to dryness and the residue was
dissolved in CHCl and dried with MgSO . The solvent was evaporated under reduced
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pressure. The precipitate was filtered and dried. The crude product was recrystallized from
ethanol to yield the pure compound as crystal. Characterization data for compounds is
presented here.
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2-phenyl-7-methylimidazo[1,2-a]pyridine (3a). Light colorless crystals were obtained after
recrystalization by ethanol in 73% yield (0.15 g); mp. 167-168 °C (lit. mp: 165-167 °C [48]);
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H NMR (CDCl ): δ 2.38 (s, 3H), 6.59 (dd, 1H), 7.31-7.33 (m, 1H), 7.40-7.44 (m, 3H), 7.76
+
+
12 2
(s, 1H), 7.92-7.98 (m, 3H); MS (ESI ): m/z = 209.5 [M+H] ; Anal. Calcd for C14H N : C,
80.74; H, 5.81; N, 13.45. Found: C, 80.88; H, 6.04; N, 13.34.
2-(4-Methoxyphenyl)-7-methylimidazo[1,2-a]pyridine (3b). Light yellow crystals were
obtained after recrystalization by ethanol in 82% yield (0.20 g); mp. 163-165 °C ((lit. mp:
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164-165 °C [49]); H NMR (CDCl ): δ 2.39 (s, 3H), 3.82 (s, 3H), 6.58 (dd, 1H), 6.94-6.97 (d,
3
2H, J = 8.8 Hz), 7.38 (br s, 1H), 7.68 (s, 1H), 7.85-7.87 (d, 2H, J = 8.8 Hz), 7.96 (d, 1H); MS
+
+
(
ESI ): m/z = 239 [M+H] ; Anal. Calcd for C15
14 2
H N O: C, 75.61; H, 5.92; N, 11.76. Found:
C, 75.76; H, 5.89; N, 11.83.
2-(4-Bromophenyl)-7-methylimidazo[1,2-a]pyridine (3c). Light colorless crystals were
obtained after recrystalization by ethanol in 79% yield (0.23 g); mp. 219-220 °C ((lit. mp.
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16-217 °C [50]); H NMR (CDCl
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): δ 2.39 (s, 3H), 6.59 (dd, 1H), 7.36 (br s, 1H), 7.51-7.54
+
(
d, 2H, J = 8.6 Hz), 7.73 (s, 1H), 7.73-7.94 (d, 2H, J = 8.6 Hz), 7.96 (d, 1H); MS (ESI ): m/z
81
+
=
11 2
289.6 [M+H, Br] ; Anal. Calcd for C14H N Br: C, 58.56; H, 3.86; N, 9.76. Found: C,
58.42; H, 3.82; N, 9.61.
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2.5.
General procedure for the synthesis of the imine derivatives 4a-f.
Equimolar mixture of various substituted aldehyde (10 mmol) and 4-chloroaniline (10 mmol)
in methanol (15 ml) was refluxed for 4 h. After cooling, filtration and drying. The crude
product was recrystallized from methanol or ethanol to yield the pure compound as powder or
crystal.
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N-Benzylidene-4-chloroaniline (4a): Light yellow crystals were obtained after
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recrystalization from ethanol; 74% yield (1.60 g); mp. 63-64 °C (lit. mp. 53-56 °C [51]); H
NMR (CDCl
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): δ 7.14-7.16 (d, 2H, J = 8.8 Hz), 7.34-7.36 (d, 2H, J = 8.8 Hz), 7.47-7.49 (d,
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H, J = 8.6 Hz), 7.88-7.90 (d, 2H, J = 8.6 Hz), 8.43 (s, 1H).
4
-Chloro-N-[4-(piperidinylamino)benzylidene]aniline (4b): Yellow crystals were obtained
after recrystalization from ethanol; yield 86% (2.56 g); m.p. 136-137 °C (lit. mp. 135-137 °C
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[52]); H NMR (CDCl
3
): δ 1.66-1.69 (m, 6H), 3.32-3.34 (m, 4H), 6.91-6.93 (d, 2H, J = 8.6
Hz), 7.11-7.13 (d, 2H, J = 9.1 Hz), 7.30-7.32 (d, 2H, J = 9.1 Hz), 7.74-7.76 (d, 2H, J = 8.6
Hz), 8.43 (s, 1H).
4-Chloro-N-[4-(diethylamino)benzylidene]aniline (4c): Light yellow crystals were obtained
after recrystalization from ethanol; yield 89% (2.55 g); mp. 93-94 °C, (lit. mp. 94-95 °C [53]);
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H NMR (CDCl ): δ 1.18-1.22 (t, 4H), 3.39-3.44 (q, 6H), 6.67-6.70 (d, 2H, J = 9.1 Hz), 7.10-
7.13 (d, 2H, J = 9.1 Hz), 7.29-7.31 (d, 2H, J = 9.1 Hz), 7.72-7.74 (d, 2H, J = 9.1 Hz), 8.24 (s,
H).
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4-Chloro-N-[4-(dimethylamino)benzylidene]aniline (4d): Yellow powder were obtained
after recrystalization from methanol; yield 82% (2.12 g); mp. 156-157 °C (lit. mp. 135-137 °C
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[
52]); H NMR (CDCl
3
): δ 3.12 (s, 6H), 6.71-6.73 (d, 2H, J = 8.7 Hz), 7.12-7.14 (d, 2H, J =
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8.7 Hz), 7.30-7.32 (d, 2H, J = 8.5 Hz), 7.76-7.78 (d, 2H, J = 8.6 Hz), 8.28 (s, 1H).
4-Chloro-N-[4-methylsulfanylbenzylidene]aniline (4e): Yellow powder were obtained after
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recrystalization from ethanol; yield 88% (2.30 g); mp. 126-127 °C; H NMR (CDCl
3
): δ 2.52
(s, 3H, SCH
3
), 7.13-7.15 (d, 2H, J = 8.7 Hz), 7.28-7.30 (d, 2H, J = 8.4 Hz), 7.32-7.34 (d, 2H,
J = 8.5 Hz), 7.78-7.80 (d, 2H, J = 8.6 Hz), 8.36 (s, 1H).
4-Chloro-N-[4-methoxybenzylidene]aniline (4f): White crystals were obtained after
recrystalization from methanol; yield 85% (1.98 g); mp: 101-102 °C (lit. mp: 90-94 °C [52]);
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H NMR (CDCl ): δ 3.89 (s, 3H, OCH ), 6.97-6.98 (d, 2H, J = 8.8 Hz), 7.16-7.18 (d, 2H, J =
8.7 Hz), 7.33-7.35 (d, 2H, J = 8.7 Hz), 7.87-7.89 (d, 2H, J = 8.8 Hz), 8.36 (s, 1H).
4-Chloro-N-thiophene2-ylbenzylideneaniline (4g): Light brown powder were obtained after
1
recrystalization from ethanol; yield 81% (1.79 g); mp: 74-75 °C; H NMR (CDCl
3
): δ 7.10-
7.33 (d, 2H, J = 8.8 Hz), 7.48-7.61 (d, 2H, J = 8.7 Hz), 7.57 (m, 1H), 7.73 (m, 1H), 7.92 (d,
1H), 8.33 (s, 1H).
2.6.
General procedure for the synthesis of styrylimidazo[1,2-a]pyridine
derivatives.
A stirred mixture of various 2-substituted-7-methylimidazo[1,2-a]pyridine (0.01 mol) and
t
K BuO (0.25 mol) and the corresponding imine derivatives (0.01 mol) in DMF (20 mL) was
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heated at 90 °C for 4 h. The mixture was allowed to cool and methanol/water mixture (10:1)
was added. The precipitate was filtered and dried. The crude product was recrystallized from
EtOAc or EtOAc/MeOH to yield the pure compound as crystalline solid or as fine powder.
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7-[(E)-2-(phenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine (5a): The formed solid was
collected by filtration and recrystallized from EtOAc/DME to give compound as light brown
1
solid in 74% yield (2.19 g); mp: 211-212 °C (lit. mp 207-208 °C [54]); H NMR (CDCl
3
) δ
7.10-7.14 (m, 2H), 7.17-7.21 (d, 1H, Jtrans = 16.0 Hz), 7.31-7.49 (m, 6H), 7.56-7.58 (d, 1H),
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7
1
1
.68 (s br 1H), 7.85 (s, 1H), 7.98-8.0 (d, 2H), 8.0-8.1 (d, 1H); C NMR (100 MHz, CDCl ): δ
3
09.2, 111.1, 115.9, 113.1, 116.4, 126.1, 126.9, 127.6, 128.9, 129.1, 129.6, 129.7, 131.4,
+
+
16 2
37.5, 144.5, 146.4, 159.6; MS (ESI ): m/z = 297.8 [M] ; Anal. Calcd for C21H N : C, 85.11;
H, 5.44; N, 9.45 Found: C, 85.01; H, 5.21; N, 9.28.
7-[(E)-2-(phenyl)vinyl]-2-(4-methoxyphenyl)imidazo[1,2-a]pyridine (5b): The formed
solid was collected by filtration and recrystallized from EtOAc to give compound as light
1
yellow crystals in 82% yield (2.67 g); mp: 239-240 °C (lit. mp 232-233 °C [54]); H NMR
(CDCl
3
) δ 3.83 (s, 3H), 6.99-7.02 (d, 2H, J = 8.8 Hz), 7.07-7.09 (dd, 1H), 7.09-7.14 (d, 1H,
J
trans = 16.3 Hz), 7.15-7.19 (d, 1H, Jtrans = 16.3 Hz), 7.30-7.34 (m, 1H), 7.39-7.42 (m, 2H),
7.56-7.58 (d, 2H, J = 8.6 Hz), 7.62 (s br 1H), 7.71 (s, 1H), 7.90-7.92 (d, 2H, J = 8.8 Hz), 8.06-
13
8
1
.08 (d, 1H); C NMR (100 MHz, CDCl
3
): δ 56.1, 108.3, 110.9, 115.0, 115.7, 121.2, 125.9,
+
27.5, 127.6, 128.1, 129.0, 131.2, 133.4, 134.8, 137.4, 142.8, 144.5, 160.5; MS (ESI ): m/z =
+
327.8 [M] ; Anal. Calcd for C22
18 2
H N O: C, 80.96; H, 5.56; N, 8.58. Found: C, 80.89; H, 5.53;
N, 8.44.
8

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7-[(E)-2-(4-Piperidinylaminophenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine (5c): The
formed solid was collected by filtration and recrystallized from EtOAc/MeOH to give
1
compound as yellow powder in 88% yield (3.34 g); mp: 293-294 °C; H NMR (CDCl
3
) δ
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
1.61-1.62 (m, 2H), 1.70-1.72 (m, 4H), 3.23-3.26 (m, 4H), 6.91-6.94 (m, 3H), 7.10-7.14 (m,
2H), 7.35-7.36 (m, 1H), 7.42-7.47 (m, 4H), 7.68 (s, 1H), 7.79 (s, 1H), 7.96-7.98 (d, 2H, J =
13
9
1
.1 Hz), 8.05-8.06 (d, 1H); C NMR (100 MHz, CDCl
3
): δ 24.6, 25.4, 49.1, 108.0, 115.6,
24,1, 125.4, 126.2, 128.2, 128.9, 131.9, 134.0, 138,4, 144.6, 146.6, 148.3, 156.1, 159.4,
+
+
25 3
160.6; MS (ESI ): m/z = 380.8 [M] , 759.6, 760.6 (Dimer); Anal. Calcd for C26H N : C,
82.29; H, 6.64; N, 11.07 Found: C, 82.11; H, 6.52; N, 11.01.
7-[(E)-2-(4-Piperidinylaminophenyl)vinyl]-2-(4-methoxyphenyl)imidazo[1,2-a]pyridine
(5d): The formed solid was collected by filtration and recrystallized from EtOAc/MeOH to
1
give compound as brillant yellow powder in 88% yield (3.40 g); mp: 292-293 °C; H NMR
(
CDCl ) δ 1.60-1.63 (m, 2H), 1.69-1.72 (m, 4H), 3.24-3.27 (m, 4H), 3.84 (s, 3H, OCH ),
3
3
6.88-6.92 (m, 3H), 6.96-6.98 (d, 2H, J = 8.8 Hz), 7.11-7.15 (m, 2H), 7.41-7.43 (d, 2H, J = 8.8
13
Hz), 7.70 (s, 1H), 7.72 (s, 1H), 7.89-7.92 (d, 2H, J = 8.7 Hz), 8.05-8.07 (d, 1H); C NMR
100 MHz, CDCl ): δ 24.7, 25.9, 50.5, 54.9, 107.5, 115.3, 112.5, 114.8, 116.6, 122.4, 125.7,
26.7, 127.1, 128.0, 128.5, 132.6, 144.5, 145.6, 155.4, 160.1; MS (ESI ): m/z = 410.7 [M] ,
819.1, 820.1 (Dimer) Anal. Calcd for C H N O: C, 79.19; H, 6.65; N, 10.26 Found: C,
(
3
+
+
1
27
27
3
78.96; H, 6.59; N, 10.11.
7-[(E)-2-(4-Diethylaminophenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine (5e): The formed
solid was collected by filtration and recrystallized from EtOAc/MeOH to give compound as
1
dark yellow crystals in 84% yield (3.08 g); mp: 174-175 °C; H NMR (CDCl ) δ 1.15-1.20 (t,
3
4H), 3.36-3.41 (q, 6H), 6.65-6.67 (d, 2H, J = 8.8Hz), 6.84-6.88 (d, 1H, Jtrans = 16.1 Hz), 7.02-
9

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7.08 (m, 2H), 7.32-7.34 (m, 1H), 7.39-7.45 (m, 4H), 7.54 (s, 1H), 7.77 (s, 1H), 7.94-7.96 (d,
1
3
2
1
1
H, J = 8.4 Hz), 7.98-8.0 (d, 1H); C NMR (100 MHz, CDCl
3
): δ 12.6, 44.4, 108.0, 110.4,
11.6, 113.3, 121.4, 123.7, 125.0, 125.9, 128.0, 128.2, 128.3, 128.7, 130.4, 131.1, 133.3,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
3
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5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
+
+
36.0, 146.1, 147.8; MS (ESI ): m/z = 368.6 [M] , 735.1, 736.2 (Dimer); Anal. Calcd for
C H N : C, 81.71; H, 6.86; N, 11.43 Found: C, 81.86; H, 6.84; N, 11.43.
25
25
3
7-[(E)-2-(4-Diethylaminophenyl)vinyl]-2-(4-methoxyphenyl)imidazo[1,2a]pyridine (5f):
The formed solid was collected by filtration and recrystallized from EtOAc/MeOH to give
1
compound as yellow powder in 83% yield (3.30 g); mp: 230-231 °C; H NMR (CDCl ) δ
3
3
1.17-1.20 (t, 4H), 3.36-3.41 (q, 6H), 3.85 (s, OCH ), 6.65-6.68 (d, 2H, J = 8.8 Hz), 6.84-6.88
(d, 1H, Jtrans = 16.1 Hz), 6.95-6.98 (d, 2H, J = 8.7 Hz), 7.0-7.20 (m, 1H), 7.02-7.07 (d, 1H,
Jtrans = 16.1 Hz), 7.39-7.41 (d, 2H, J = 8.6 Hz), 7.49 (s, 1H), 7.69 (s, 1H), 7.86-7.89 (d, 2H, J
13
=
8.8 Hz), 7.97-7.99 (d, 1H); C NMR (100 MHz, CDCl
3
): δ 12.6, 44.4, 55.3, 107.1, 109.9,
111.6, 113.6, 114.1, 121.7, 123.8, 124.8, 126.6, 127.2, 128.1, 130.5, 135.3, 146.1, 147.7,
+
+
159.5; MS (ESI ): m/z = 398.6 [M] , 795.2, 796.2 (Dimer); Anal. Calcd for C26
H
27
N
3
O: C,
78.56; H, 6.85; N, 10.57 Found: C, 78.44; H, 6.75; N, 10.55.
7-[(E)-2-(4-Dimethylaminophenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine
(5g):
The
formed solid was collected by filtration and recrystallized from EtOAc/MeOH to give
1
compound as yellow needle crystals in 84% yield (2.85 g); mp: 291-292 °C; H NMR
(CDCl
3
) δ 3.0 (s, 6H), 6.69-6.73 (d, 2H, J = 8.8 Hz), 6.86-6.91 (d, 1H, Jtrans = 16.2 Hz), 7.10-
7.14 (m, 2H), 7.30-7.32 (m, 1H), 7.41-7.68 (m, 4H), 7.68 (s br 1H), 7.78 (s, 1H), 7.96-7.98 (d,
1
3
2
1
H, J = 8.5 Hz), 8.04-8.06 (d, 1H); C NMR (100 MHz, CDCl
3
): δ 40.3, 108.4, 112.6, 112.6,
+
21.9, 125.5, 126.1, 128.2, 128.5, 128.9, 130.5, 131.8, 135.3, 145.9, 147.8; MS (ESI ): m/z =
10

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4
5
+
340.6 [M] , 679.1, 680.2 (Dimer); Anal. Calcd for C23
21 3
H N : C, 81.38; H, 6.24; N, 12.38
Found: C, 81.19; H, 6.25; N, 12.39.
1
1
1
1
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6
7
-[(E)-2-(4-Dimethylaminophenyl)vinyl]-2-(4-methoxyphenyl)imidazo[1,2a]pyridine
(5h): The formed solid was collected by filtration and recrystallized from EtOAc/MeOH to
1
give compound as light yellow needle crystals in 88% yield (3.25 g); mp: 280-281 °C; H
NMR (CDCl
3
) δ 3.0 (s, 6H), 3.85 (s, 3H) 6.70-6.72 (d, 2H, J = 8.8 Hz), 6.86-6.90 (d, 1H, Jtrans
= 16.2 Hz), 6.96-6.98 (d, 2H, J = 8.8 Hz), 7.06-7.12 (m, 2H), 7.42-7.44 (d, 2H, J = 8.8 Hz),
1
3
7
.61 (s br 1H), 7.69 (s, 1H), 7.88-7.90 (d, 2H, J = 8.8 Hz), 8.01-8.02 (d, 1H); C NMR (100
MHz, CDCl ): δ 40.5, 55.8, 107.2, 110.5, 112.3, 114.6, 121.9, 124.7, 125.0, 127.3, 128.0,
30.6, 135.8, 144.8, 145.0, 146.2, 150.5, 160.8; MS (ESI ): m/z = 370.9 [M] , 739.6 (Dimer);
Anal. Calcd for C H N O: C, 78.02; H, 6.27; N, 11.37 Found: C, 77.83; H, 6.18; N, 11.21.
3
+
+
1
2
4
23
3
7
-[(E)-2-(4-Methylsulfanylphenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine (5i): The formed
solid was collected by filtration and recrystallized from EtOAc/MeOH to give compound as
1
light yellow powder in 79% yield (2.70 g); mp: 250-251 °C; H NMR (CDCl
3
) δ 2.56 (s, 3H,
SCH
3
), 7.03-7.06 (d, 1H, Jtrans = 16.0 Hz), 7.09-7.13 (d, 1H, Jtrans = 16.1 Hz), 7.23-7.26 (d,
2
H, J = 8.4 Hz), 7.32-7.36 (m, 1H), 7.42-7.47 (m, 4H), 7.63 (s br 1H), 7.82 (s, 1H), 7.94-7.97
13
(d, 2H, J = 8.6 Hz), 8.05-8.07 (d, 1H); C NMR (100 MHz, CDCl ): δ 16.4, 109.1, 111.0,
3
1
15.8, 126.0, 126.8, 127.4, 127.9, 128.9, 129.5, 130.5, 134.2, 139.6 (The some signals are not
+
+
18 2
observed); MS (ESI ): m/z = 343.5 [M+H] , 685.0 (Dimer); Anal. Calcd for C22H N S: C,
77.16; H, 5.30; N, 8.18, S, 9.36 Found: C, 77.03; H, 5.22; N, 8.11, S, 9.33.
7
-[(E)-2-(4-Methylsulfanylphenyl)vinyl]-2-(4-methoxyphenyl)imidazo[1,2a]pyridine (5j):
The formed solid was collected by filtration and recrystallized from EtOAc/MeOH to give
11

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compound as light yellow powder in 77% yield (2.87 g); mp: 268-269 °C; H NMR (CDCl
δ 2.53 (s, 3H, SCH ), 3.87 (s, 3H, OCH ), 6.98-7.0 (d, 2H, J = 8.8 Hz), 7.03-7.07 (d, 1H, Jtrans
16.3 Hz), 7.16-7.20 (d, 1H, Jtrans = 16.3 Hz), 7.24-7.27 (d, 2H), 7.45-7.47 (d, 2H, J = 8.4
3
)
3
3
=
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
1
3
Hz), 7.73 (s br 1H), 7.86 (s, 1H), 7.92-7.94 (d, 2H, J = 8.7 Hz), 8.10-8.12 (d, 1H); C NMR
+
+
(100 MHz, CDCl ): It was not obtained good spectrum; MS (ESI ): m/z = 373.8 [M+H] ,
3
7
45.2 (Dimer); Anal. Calcd for C22
3.97; H, 5.38; N, 7.44, S, 8.55.
18 2
H N S: C, 74.16; H, 5.41; N, 7.52, S, 8.61 Found: C,
7
7
-[(E)-2-(4-Methoxyphenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine (5k): The formed solid
was collected by filtration and recrystallized from EtOAc/MeOH to give compound as light
1
yellow powder in 75% yield (2.70 g); mp: 261−263 °C (lit. mp. 243-244 °C [54]); H NMR
3
3
trans
(CDCl ) δ 3.04 (s, 3H, OCH ), 6.91-6.93 (d, 2H, J = 8.8 Hz), 6.95-6.99 (d, 1H, J = 16.4
Hz), 7.04-7.06 (dd, 1H), 7.08-7.12 (d, 1H, Jtrans = 16.3 Hz), 7.33-7.35 (m, 1H), 7.42-7.49 (m,
1
3
4
H), 7.57 (s br 1H), 7.82 (s, 1H), 7.94-7.97 (d, 2H, J = 8.7 Hz), 8.04-8.05 (d, 1H); C NMR
(100 MHz, CDCl ): δ 55.4, 108.2, 111.5, 114.6, 115.5, 120.6, 124.7, 125.2, 126.8, 128.2,
28.9, 129.4, 132.4, 134.3, 136.4, 144.2, 145.8, 159.8; MS (ESI ): m/z = 327.4 [M+H] , 652.9
Dimer); Anal. Calcd for C22 O: C, 80.96; H, 5.56; N, 8.58 Found: C, 81.03; H, 5.35; N,
3
+
+
1
(
18 2
H N
8.43.
7
-[(E)-2-(4-Methoxyphenyl)vinyl]-2-(4-methoxyphenyl)imidazo[1,2a]pyridine (5l):
The formed solid was collected by filtration and recrystallized from EtOAc to give compound
1
as pale yellow powder in 78% yield (2.78 g); mp: 275-276 °C (lit. mp. 266-267 °C [54]); H
NMR (CDCl
.99 (d, 1H, Jtrans = 16.3 Hz), 6.91-7.0 (d, 2H, J = 8.9 Hz), 7.06-7.08 (dd, 1H), 7.10-7.14 (d,
1H, Jtrans = 16.3 Hz), 7.47-7.50 (d, 2H, J = 8.7 Hz), 7.62 (s br 1H), 7.73 (s, 1H), 7.89-7.91 (d,
3 3 3
) δ 3.84 (s, 3H, OCH ), 3.86 (s, 3H, OCH ), 6.91-6.93 (d, 2H, J = 8.7 Hz), 6.95-
6
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1
3
2H, J = 8.8 Hz), 8.04-8.05 (d, 1H); C NMR (100 MHz, CDCl
3
): δ 55.2, 55.9, 109.1, 111.4,
1
13.2, 114.9, 121.8, 125.9, 127.3, 128.8, 129.3, 130.6, 131.7, 144.8, 145.1, 159.8, 161.6 (The
+
+
some signals are not observed); MS (ESI ): m/z = 357.7 [M+H] , 713.3 (Dimer); Anal. Calcd
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
for C H N O : C, 77.51; H, 5.66; N, 7.86 Found: C, 77.28; H, 5.58; N, 7.71.
23 20
2
2
7-[(E)-2-thienylvinyl]-2-(4-methoxyphenyl)imidazo[1,2a]pyridine (5m): The formed solid
was collected by filtration and recrystallized from EtOAc/MeOH to give compound as pale
1
yellow powder in 88% yield (2.78 g); mp: 227-228 °C (lit. mp: 222-223 °C [54]); H NMR
3 3
(CDCl ) δ 3.85 (s, 3H, OCH ), 6.87-6.91 (d, 1H, Jtrans = 16.0 Hz), 6.96-6.98 (d, 2H, J = 8.8
Hz), 6.98-6.99 (dd, 1H), 7.02-7.04 (m, 2H), 7.12-7.13 (d, 1H), 7.24-7.28 (d, 1H, Jtrans = 16.3
13
Hz), 7.57 (s br 1H), 7.73 (s, 1H), 7.87-7.89 (d, 2H, J = 8.8 Hz), 8.02-8.05 (d, 1H); C NMR
(
100 MHz, CDCl
127.3, 127.8, 133.1, 137.0, 142.1, 144.5, 145.6, 159.7; MS (ESI ): m/z = 333.7 [M+H] , 665.1
Dimer); Anal. Calcd for C20 OS: C, 72.26; H, 4.85; N, 8.43; S, 9.65 Found: C, 72.11; H,
3
): δ 55.3, 107.5, 110.1, 114.2, 114.3, 123.6, 125.1, 125.3, 126.1, 127.1,
+
+
(
16 2
H N
4.79; N, 8.33; S, 9.48.
7
-[(E)-2-thienylvinyl]-2-(4-bromophenyl)imidazo[1,2a]pyridine (5n): The formed solid
was collected by filtration and recrystallized from EtOAc/MeOH to give compound as yellow
1
powder in 83% yield (3.16 g); mp: 255-256 °C; H NMR (CDCl
3
) δ 6.87-6.91 (d, 1H, Jtrans
=
16.1 Hz), 7.02-7.05 (m, 2H), 7.13-7.14 (d, 1H), 7.27-7.30 (d, 1H, Jtrans = 16.1 Hz), 7.54-7.56
(
d, 2H, J = 8.6 Hz), 7.58 (s, br, 1H), 7.81 (s, 1H), 7.81-7.83 (d, 2H, J = 8.7 Hz), 8.03-8.05 (d,
1
3
1
H); C NMR (100 MHz, CDCl
3
): δ 108.5, 110.5, 114.3, 122.2, 124.1, 125.4, 125.6, 125.8,
+
127.3, 127.5, 127.8, 131.9, 133.1, 137.0, 141.9, 144.6, 145.8; MS (ESI ): m/z = 381.7 [M+H
7
9
+
81
+
Br] , 383.6 [M+H Br] ; Anal. Calcd for C19
13 2
H N SBr: C, 59.85; H, 3.44; N, 7.35; S, 8.41
Found: C, 59.40; H, 3.34; N, 7.25; S, 8.44.
13

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3. Results and Discussion
3.1.
Preparation of arylstyrylimidazo[1,2-a]pyridine based chromophores
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The aryl-substituted methylimidazo[1,2-a]pyridine derivatives 3a-c were synthesized from 2-
bromoacetophenone derivatives 2a–c and 4-methyl-2-aminopyridine in good yields (Scheme
1
). Light colorless and yellow crystals respectively for 3a and 3b of compounds suitable for
X-ray diffraction analysis were obtained by slow evaporation from methanol (Figures S1 and
S2, in Supporting information page S2). The imine derivatives 4a–f were prepared by the
reaction of aromatic aldehydes and the corresponding 4-chloroaniline derivative [54]. The
arylstyrylimidazo[1,2-a]pyridine derivatives 5a–n were obtained by the base-catalyzed
condensation of the imine derivatives 4a–f and the 2-aryl-substituted-7-methylimidazo[1,2-
o
a]pyridine derivatives 3a–c at 90 C in DMF (Scheme 1). The experimental protocol is
straightforward and offers easy access to a wide variety of arylstyrylimidazo[1,2-a]pyridines
containing electron-donating groups in good yields. All compounds 5a–n were characterized
by NMR spectroscopy, mass spectrometry, and elemental analysis. NMR spectroscopy
proved to be very useful to assess the configuration of the vinylic double bond. Within the
limit of NMR-spectroscopic analysis only the formation of the E isomers could be detected.
The assignment of the E-configuration of the vinylic double-bond was based on the analysis
of the vicinal coupling constants of the olefinic protons, because the latter exhibit the
characteristic coupling constant of J ≈ 16 Hz. Although the arylstyrylimidazo[1,2-a]pyridines
are persistent and can be stored in the solid state, the styryl double bond of the alkylamino-
substituted derivatives isomerizes in solution, presumably induced by exposure to room light
1
or sun light. The partial E-Z isomerization and amount of percent were detected by H NMR
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spectroscopic analysis of compound 5h (Figures S3 and S4 in Supporting information page
S3,S4).
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Scheme 1 is here
3.2.
Absorption and emission properties
The styryl derivatives 5a-n are colored in solid state and in solution. To assess the
solvatochromic behavior the absorption and emission data of the styryl derivatives were
2 2
determined in DMSO, acetonitrile, THF, CH Cl and toluene (Table 1).
Table 1 is here
The compounds 5a–n show absorption maxima (λmax) in the UV/visible region with long
wavelength absorption maxima at 376-390 nm (in CH Cl ). Although the compounds 5a,b
2
2
and 5i–n exhibit two absorption bands in all solvents used, the derivatives 5c–h showed just
one absorption band (Figure 1). In addition, the compounds 5e-h exhibit one shoulder in long
wavelength in toulene used. The dyes which have two absorption bands or one absorption
band with shoulder, may exhibit E-Z isomerization [26]. In addition these styryl derivatives
are fluorescent (416–473 in CH Cl ). Generally, the dyes 5c–n show lower fluorescence
2
2
2
quantum yields than derivatives 5a,b with R = H. It may be proposed that the relative low
fluorescence quantum yields are the result of an photo-induced intramolecular electron
transfer (PET) [55-57] reaction between the excited fluorophore (imidazopyridine ring as an
electron acceptor) and the electron donating dialkylamino, thiomethyl, methoxy groups and
thienyl ring. In general, larger Stokes shifts are obtained for alkylamino derivatives 5c-h
–1
–1
under investigation (e.g., 5d: 5319 cm ; 5g: 4972 cm in CH Cl ).
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Regarding the absorption the compounds 5a–n showed only weak solvatochromic properties.
In contrast, all compounds exhibit fluorosolvatochromism, i.e. a bathochromic shift of the
emission band was observed with increasing solvent polarity (Figure 1). Such a behavior is
typical of donor-acceptor π systems that undergo an internal charge transfer upon excitation,
[58] because the polar excited state is significantly stabilized by polar solvents.
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Figure 1 is here
The absorption maxima of derivatives with an electron-donating substituent, such as the
dialkylamino, methylsulfanyl and methoxy substituents, or of the thienylvinylimidazo[1,2-
a]pyridines where the phenyl ring is substituted with the electron rich thiophene unit, showed
significant bathochromic shifts as compared to the ones of the reference compounds 5a,b. The
same trend was observed in the emission spectra of compounds 5a–n. The bathochromic shift
is most likely the result of the more pronounced donor-acceptor interplay between the donor
functionalities and the eletron-poor imidazopyridine ring. Notably, this effect is more
dominant in compounds 5c–h than for compounds 5i–n as the former carry the stronger donor
substituents. Hence, when the dialkylamino groups are attached at the para position of phenyl
ring, a more pronounced donor-acceptor system is established between dialkylamino
functionality and the imidazopyridine ring resulting in a larger red shift.
3.3.
Acidochromic properties
Considering the alkaline functionality of the nitrogen atom of the dialkyl-aminophenyl and of
the imidazopyridine ring we examined the dependence of the absorption and emission
properties of the derivatives 5a–n (Table 2). Most notably, the absorption and emission
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properties of aminophenyl-substituted arylstyrylimidazo[1,2]pyridine derivatives 5c–h change
significantly upon protonation (Table 2).
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Table 2 is here
The general trend shall be illustrated exemplarily with the representative derivative 5h. Thus,
the addition of trifluoroacetic acid (TFA) to a solution of 5h in dichloromethane led to a shift
of the absorption maximum from 381 nm to 418 nm (Figures 2 and 3).
Figures 2 and 3 are here
The observed bathochromic shift (37 nm) of the absorption maximum may be explained by
initial protonation of the imidazopyridine ring that leads to an increased donor-acceptor
interplay between the amino functionality and the imidazonium ion (Scheme 2).
Scheme 2 is here
Further addition of TFA (1 mM) to the solution of 5h in dichloromethane led to a shift of the
absorption maximum from 418 nm to 349 nm, i.e. a hypsochromic shift of 32 nm with respect
to the neutral form 5h. Apparantly, with excess of TFA the compound is also protonated at
the amino-functionality which leads to a collapse of the donor-acceptor system (Scheme 2,
Figure 4). The same hypsochromic shifts were observed for the dialkylamino-substituted
derivatives 5c–g, and such a phenomenon has been already observed upon protonation of
alkylamino-substituted arylvinyldiazine derivatives [59].
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Figure 4 is here
Upon adition of acid the emission properties of the derivatives 5a–n also change significantly
(
Table 2). Hence, the emission intensity of 5i–n was completely or partially quenched upon
protonation, whereas the emission intensity of for 5c–h increased in the presence of the acid.
For example, upon addition of acid to 5h in CH Cl the fluorescence intensity increased 3 fold
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with a small red shift of the emission maxima from 457 nm to 465 nm (Figure 5,6). As
discussed above the low emission quantum yield of the amino-substituted derivatives
originates from a deactivating PIET between the amino functionality and the excited
imidazopyridine unit. Therefore the light-up effect of 5c–h on acidification is most likely the
result of a suppressed PIET by protonation of the amino functionality [54].
Figure 5 and 6 are here
Conclusion
In summary, fourteen arylstyrylimidazo[1,2-a]pyridine-based donor-acceptor chromophores
were prepared via facile methods. All chromophores have an E-configuration of the vinyl
1
double bond as unequivocally shown by H NMR spectroscopy. It was observed that the E
isomers are stable in the solid state; however, the derivatives with strong electron donating
dialkylamino substituents underwent partial E-Z isomerization in solution at room
temperature. The synthesized all compounds exhibit fluorosolvatochromism, namely the
emission band is red shifted with increasing solvent polarity.
The probes had high fluorescent in acidic media but low fluorescent in neutral environments.
PIET is observed with addition excess of TFA to the solution of 5c-h in dichloromethane.
After double protonation of compounds 5c-h light up effect is observed. This behavior may
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be used for the development of light up probe for pH differences in biological and
enviromental media.
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Acknowledgements
This investigation was supported by The Scientific and Technological Research Council of
Turkey (TUBITAK).
Supplementary material
1
The detailed spectroscopic data (Copies of H and Mass spectra for all new compounds) of all
new compounds are available. The crystallographic data for 3a and 3b can be seen therein.
CCDC-984716 (3a) and 984220 (3b) contain the all supplementary crystallographic data for
this paper. This data can be obtained free of charge from The Cambridge Crystallographic
Data Centre viahttp://www.ccdc.cam.ac.
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Figure Captions
1
. Figure 1. Absorption and emission spectra of compound 5h in different solvents;
absorption: c =10 ꢁM; emission: c: 1.0 ꢁM.
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2
. Figure 2. Spectrophotometric titration of TFA to compound 5h (10 ꢁM) in CH Cl
2
2.
Arrows indicate the increase and decrease of the absorbance intensity upon addition 1
mM TFA in CH Cl
. Figure 3. Pictures of absorption and emission color of 5h in CH
ꢁM; λex = 365 nm) in the absence (right) presence (left) of 1 mM TFA in CH
. Figure 4. Spectrophotometric titration of TFA to compound 5h (10 ꢁM) in CH Cl
2
2
.
3
4
2
Cl
2
solutions (c = 10
Cl
2
2
.
2
2.
Arrows indicate the increase and decrease of the absorbance intensity upon addition 1
mM TFA in CH Cl
5. Figure 5. Spectrofluorimetric titration of (10 ꢁM) 5h in CH Cl Arrow indicate the
2
2
.
2
2.
2 2
increase of the fluorescence intensity upon addition 1 M TFA in CH Cl
6
. Figure 6. Pictures of emission color of 5h (10 ꢁM) at neutral (left) and acidic (right)
conditions (λex = 365 nm).
Scheme Captions
1
. Scheme 1. Synthesis of arylstyrylimidazo[1,2-a]pyridine derivatives 5a-n.
. Scheme 2. Prototropic equilibria of 5h.
Table Captions
2
1
. Table 1. Photophysical data of arylstyrylimidazo[1,2-a]pyridine based dyes 5a–n
. Table 2. pH-Dependent Absorption and Emission Properties of arylstyrylimidazo[1,2-
a]pyridine based dyes 5a-n.
2
2
7