Synthesis of highly fluorescent (E)-Stilbene derivatives containing polysubstituted imidazole scaffold

Article abstract of DOI:10.1007/s11164-016-2815-1

Abstract: A series of 2-(4-styrylphenyl)-1,4,5-triaryl imidazole derivatives have been efficiently synthesized through Wittig reactions starting with 2-[(4-bromomethyl)phenyl]-1,4,5-triarylimidazoles and been verified by spectroscopic data. The optical ab

Full text of DOI:10.1007/s11164-016-2815-1

Res Chem Intermed
DOI 10.1007/s11164-016-2815-1
Synthesis of highly fluorescent (E)-Stilbene derivatives
containing polysubstituted imidazole scaffold
Zarrin Ghasemi1 Zahra Fathi1
•
Received: 16 June 2016 / Accepted: 12 November 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract A series of 2-(4-styrylphenyl)-1,4,5-triaryl imidazole derivatives have been
efficiently synthesized through Wittig reactions starting with 2-[(4-bromomethyl)phenyl]-
1,4,5-triarylimidazoles and been verified by spectroscopic data. The optical absorption and
emission properties ofthe synthesized novel (E)-stilbene derivatives wereexaminedindilute
ethanol solution. Most of the products exhibit a strongly enhanced fluorescence intensity.
Graphical Abstract
N
N
N
CH2Br
N
Ar
1) PPh3 / Toluene
2) K2CO3 / DMF
Ar-CHO , r.t.
R
3a-p
R=H, Cl
Keywords Wittig reaction Á Fluorescent materials Á (E)-stilbenes Á
Highly substituted imidazoles
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2815-1)
contains supplementary material, which is available to authorized users.
&
Zarrin Ghasemi
z.ghasemi@tabrizu.ac.ir
1
Department of Organic and Biochemistry, Faculty of Chemistry, University of Tabriz,
Tabriz 5166614766, Iran
123

Z. Ghasemi, Z. Fathi
Introduction
The design and synthesis of fluorescent organic compounds have always been of
extreme interest because of their high technology applications [1–5]. p-Conjugated
molecules containing aromatic heterocycles have also attracted particular attention
due to their widespread use in optical systems such as organic light-emitting diodes
(OLEDs) [6–8], optical sensors [9, 10], dye-sensitized solar cells [11], fluorescent
colorants [12–14] and fluorescent biological labels [15, 16]. The imidazole ring with
desirable electron-rich characteristic and unique structural feature is widely present
in natural and synthetic molecules [17]. 1,2,4,5-Tetraarylimidazoles, which can be
prepared by four-component condensation of aromatic aldehydes, benzil, primary
amines and ammonium acetate [18–20], have been considered as light-emitter
scaffolds [21, 22]. Furthermore, pharmaceutical and biological activities of these
polyaromatic compounds should not be ignored [23–28]. On the other hand, long p-
conjugation, which can increase electron mobility and thermal stability of the
chromophores, plays an important role in their photophysical properties [29].
Functionalized styryl groups are widely introduced in photoactive structures to
increase the resonance and improve the quantum yields of luminescents [30, 31].
Indeed, the stilbene-core derivatives are of utmost importance to researchers owing
to diverse biological activities [32], high quantum yield, large Stokes shift and
bright light emission [33–35]. Through our interest in the synthesis of polyaromatic
frameworks bearingan imidazole core [36, 37], the present work is directed towards
the Wittig synthesis of a novel series of (E)-Stilbene derivatives containing tetraaryl
imidazole moiety, and study of their UV–Vis absorption and fluorescence emission
properties.
Experimental
Starting compounds 1a, b and 2a, b were prepared according to the literature
[38–40]. All materials were obtained from Merck or Fluka companies and were used
without purification. Melting points were determined on an Electrothermal MEL-
TEMP apparatus (model 1202D). FT-IR spectra were obtained with a Tensor
1
27-Bruker spectrometer as KBr pellets; m in cm-1. H and 13C NMR spectra were
recorded with a Bruker Spectrospin Avance 400 spectrometer, at 400 MHz (1H
NMR) and 100 MHz (13C NMR), in CDCl3 as solvent and TMS as internal
standard. UV–Vis absorption spectra were obtained on analytikjena SPECORD 250
spectrometer. Fluorescence emission spectra were acquired on a Jasco FP-750
spectrofluorimeter.
General procedure for the synthesis of target compounds 3a–p
A mixture of bromide 2a or 2b (1 mmol) and triphenylphosphine (0.4 g, 1.5 mmol)
in toluene (2 mL) was stirred at room temperature for 12 h. Then, the mixture was
filtered and the white solid (phosphonium salt) was washed several times with 6:1 n-
123

Synthesis of highly fluorescent (E)-Stilbene derivatives…
hexane and ethyl acetate. A mixture of the dried phosphonium salt (0.3 mmol),
aromatic aldehyde (0.3 mmol) and K2CO3 (0.06 g, 0.45 mmol) in N,N-dimethyl-
formamide (2 mL) was then stirred at room temperature for 24 h. Cold water
(10 mL) was then added and the precipitated solid was separated by filtration.
Recrystallization of the crude product (mixture of (Z)- and (E)- isomers) in ethanol
(96%) gave the pure (E)-stilbene derivative.
(E)-2-(4-Styrylphenyl)-1,4,5-triphenyl-1H-imidazole (3a) White crystal, Yield:,
69% (0.098 g), mp 228–229 °C. 1H NMR (400 MHz, CDCl3) d 7.06–7.62 (m, 26H,
Ar–H), 13C NMR (100 MHz, CDCl3) d 125.2, 125.5, 125.6, 126.4, 126.7, 126.9,
127.0, 127.1, 127.3, 127.4, 127.7, 128.0, 128.1, 128.2, 128.5, 129.6, 130.1, 133.4,
136.0, 136.2, 145.5. FTIR (KBr)/cm-1 3024, 3059, 1625, 1416, 1389, 695. Anal.
Calc. for C35H26N2: C 88.58, H 5.52, N 5.90. Found: C 88,29, H 5.68, N 5.63%.
(E)-2-[4-(4-Methylstyryl)phenyl]-1,4,5-triphenyl-1H-imidazole (3b) White crys-
1
tal, Yield: 70% (0.102 g), mp 230–131 °C. H NMR (400 MHz, CDCl3) d 2.35 (s,
3H, CH3), 6.96–7.40 (m, 22H, Ar–H), 7.59–7.61 (m, 3H, Ar–H). 13C NMR
(100 MHz, CDCl3) d 20.2, 125.0, 125.4, 125.6, 125.9, 126.4, 126.9, 127.1, 127.3,
127.4, 128.0, 128.1, 128.2, 128.4, 129.5, 129.9, 130.1, 133.3, 133.4, 136.1, 136.2,
136.6, 137.4, 145.6. FTIR (KBr)/cm-1 2922, 3028, 1491, 1674, 1597, 759, 697.
Anal. Calc. for C36H28N2: C 88.49, H 5.78, N 5.73. Found: C 88.22, H 5.89, N
5.55%.
(E)-2-[4-(4-Chlorostyryl)phenyl]-1,4,5-triphenyl-1H-imidazole (3c) Yellow crys-
tal, Yield: 63% (0.096 g), mp 238–240 °C. 1H NMR (400 MHz, CDCl3) d
7.02–7.44 (m, 23H, Vinyl, Ar–H), 7.60 (d, J = 7.3 Hz, 2H, Ar–H). 13C NMR
(100 MHz, CDCl3) d 125.2, 125.6, 126.4, 126.6, 126.8, 126.9, 127.2, 127.3, 127.4,
127.6, 127.8, 128.1, 128.14, 128.8, 129.5, 130.1, 132.3, 133.4, 134.6, 135.7, 136.1,
137.4, 145.5. FTIR (KBr)/cm-1 3055, 1598, 698, 1492, 1088, 963. Anal. Calc. for
C35H25ClN2: C 82.58, H 4.95, N 5.50. Found: C 82.27, H 5.26, N 5.77%.
(E)-2-[4-(4-Isopropylstyryl)phenyl]-1,4,5-triphenyl-1H-imidazole
(3d) White
1
crystal, Yield: 71% (0.110 g), mp 228–230 °C. H NMR (400 MHz, CDCl3) d
1.25 (d, J = 6.3 Hz, 6H, CH(CH3)2), 2.89–2.92 (m, 1H, CH(CH3)2), 7.06–7.59 (m,
25 H, Vinyl, Ar–H). 13C NMR (100 MHz, CDCl3) d 22.9, 32.9, 125.1, 125.2, 125.5,
125.6, 125.7, 126.1, 126.4, 126.9, 127.1, 127.3, 127.4, 127.6, 127.7, 127.8, 128.0,
128.1, 128.3, 129.6, 130.1, 133.4, 133.7, 136.1, 136.2, 147.7. FTIR (KBr)/cm-1
2870, 3028, 1493, 1599, 697. Anal. Calc. for C38H32N2: C 88.34, H 6.24, N 5.42.
Found: C 88.09, H 6.53, N 5.69%.
(E)-N,N-Dimethyl-4-[4-(1,4,5-triphenyl-1H-imidazol-2-yl)styryl]aniline (3e) Yel-
1
low soild, Yield: 65% (0.100 g), mp 266–268 °C. H NMR (400 MHz, CDCl3) d
2.97 (s, 6H, N(CH3)2), 6.69 (d, J = 8.6 Hz, 2H, Ar–H), 6.84 (d, J = 16.2 Hz, 1H,
Vinyl-H), 7.00–7.39 (m, 20H, Ar–H), 7.60 (d, J = 7.4 Hz, 2H, Ar–H). 13C NMR
(100 MHz, CDCl3) d 40.4, 111.8, 112.4, 123.6, 125.6, 126.5, 127.4, 127.6, 127.9,
128.1, 128.2, 128.3, 128.4, 128.5, 129.0, 129.06, 129.09, 129.1, 129.4, 130.7, 130.9,
131.1, 137.9, 146.5, 146.8, 150.2. FTIR (KBr)/cm-1 3059, 2802, 2923, 1600, 1496,
123

Z. Ghasemi, Z. Fathi
1524, 697. Anal. Calc. for C37H31N3: C 85.85, H 6.04, N 8.12. Found: C 85.58, H
6.23, N 8.39%.
(E)-2-{4-[2-(5-Methylfuran-2-yl)vinyl]phenyl}-1,4,5-triphenyl-1H-imidazole (3f)
1
Pale yellow soild, Yield: 71% (0.102 g), mp 220–222 °C. H NMR (400 MHz,
CDCl3) d 2.33 (s, 3H, CH3), 5.99 (s, 1H, Fur-H), 6.21 (d, J = 2.4 Hz, 1H, Fur-H),
6.79 (d, J = 16.2 Hz, 1H, Vinyl-H), 6.88 (d, J = 16.2 Hz, 1H, Vinyl-H), 7.06 (d,
J = 6.2 Hz, 2H, Ar–H), 7.12–7.46 (m, 15H, Ar–H), 7.60 (d, J = 7.2 Hz, 2H, Ar–
H). 13C NMR (100 MHz, CDCl3) d 12.8, 106.9, 109.3, 116.1, 117.3, 123.7, 124.5,
124.8, 125.6, 126.4, 126.9, 127.1, 127.3, 127.4, 127.9, 128.1, 129.6, 129.9, 130.1,
133.4, 136.1, 136.2, 137.4, 145.6, 150.6, 151.5. FTIR (KBr)/cm-1 3035, 1627,
1597, 1495, 955. Anal. Calc. for C34H26N2O: C 85.33, H 5.48, N 5.85. Found: C
85.09, H 5.69, N 5.57%.
(E)-2-{4-[2-(Naphthalen-2-yl)vinyl]phenyl}-1,4,5-triphenyl-1H-imidazole
(3g)
1
White soild, Yield: 66% (0.104 g), mp 248–250 °C. H NMR (400 MHz, CDCl3)
d 7.07 (d, J = 6.4 Hz, 2H, Ar–H), 7.13 (d, J = 5.6 Hz, 2H, Ar–H), 7.18–7.46 (m,
16H, Ar–H), 7.61 (d, J = 7.3 Hz, 2H, Ar–H), 7.70 (d, J = 8.2 Hz, 2H, Ar–H),
7.79–7.81 (m, 4H, Ar–H). 13C NMR (100 MHz, CDCl3) d 122.4, 124.9, 125.2,
125.3, 125.6, 125.8, 126.4, 126.7, 126.9, 127.0, 127.2, 127.3, 127.4, 127.7, 128.0,
128.1, 128.3, 128.5, 129.5, 130.1, 130.9, 131.0, 131.1, 132.0, 132.6, 133.4, 133.6,
136.0, 145.6. FTIR (KBr)/cm-1 3047, 1685, 1596, 1496, 696. Anal. Calc. for
C39H28N2: C 89.28, H 5.38, N 5.34. Found: C 88.97, H 5.56, N 5.62%.
(E)-2-[4-(2-(Thiophen-2-yl)vinyl)phenyl]-1,4,5-triphenyl-1H-imidazole
(3h)
1
White soild, Yield: 70% (0.100 g), mp 262–264 °C. H NMR (400 MHz, CDCl3)
d 6.86 (d, J = 16.1 Hz, 1H, Vinyl-H), 6.98–7.00 (m, 1H, Ar–H), 7.04–7.07 (m, 3H,
Ar–H), 7.13 (d, J = 6.2 Hz, 2H, Ar–H), 7.19–7.34 (m, 13H, Ar–H), 7.40 (d,
J = 8.1 Hz, 2H, Ar–H), 7.60 (d, J = 7.3 Hz, 2H, Ar–H). 13C NMR (100 MHz,
CDCl3) d 121.2, 123.5, 124.9, 125.3, 125.6, 126.4, 126.5, 126.6, 126.9, 127.1,
127.3, 127.4, 127.8, 128.0, 128.1, 128.4, 129.5, 129.9, 130.0, 133.3, 135.6, 136.1,
137.4, 141.7, 145.5. FTIR (KBr)/cm-1 3057, 1598, 1492, 958, 700. Anal. Calc. for
C33H24N2S: C 82.47, H 5.03, N 5.83, S 6.67. Found: C 82.16, H 5.22, N 5.61, S
6.38%.
(E)-2-[4-(3,4-Dimethoxystyryl)phenyl]-1,4,5-triphenyl-1H-imidazole (3i) White
1
crystal, Yield: 68% (0.108 g), mp 228–230 °C. H NMR (400 MHz, CDCl3) d
3.89 (s, 3H, OCH3), 3.92 (s, 3H, OCH3), 6.84 (d, J = 8.1 Hz, 1H, Ar–H), 6.90 (d,
J = 16.3 Hz, 1H, Vinyl-H), 7.01–7.08 (m, 5H, Ar–H), 7.13 (d, J = 5.9 Hz, 2H, Ar–
H), 7.17–7.29 (m, 9H, Ar–H), 7.36 (d, J = 8.3 Hz, 2H, Ar–H), 7.41 (d, J = 8.3 Hz,
2H, Ar–H), 7.60 (d, J = 7.2 Hz, 2H, Ar–H). 13C NMR (100 MHz, CDCl3) d 55.8,
55.9, 108.8, 111.3, 119.9, 125.9, 126.2, 126.6, 127.4, 127.9, 128.1, 128.2, 128.3,
128.5, 129.0, 129.1, 129.3, 130.3, 130.7, 130.9, 131.1, 134.5, 137.2, 137.3, 138.4,
146.7, 149.1, 149.2. FTIR (KBr)/cm-1 3051, 1600, 1488, 697. Anal. Calc. for
C37H30N2O2: C 83.12, H 5.66, N 5.24. Found: C 82.78, H 5.81, N 5.56.
123

Synthesis of highly fluorescent (E)-Stilbene derivatives…
(E)-1-(4-Chlorophenyl)-4,5-diphenyl-2-(4-styryl
phenyl)-1H-imidazole
(3j)
White crystal, Yield: 69% (0.105 g), mp 176–178 °C. 1H NMR (400 MHz,
CDCl3) d 6.99 (d, J = 7.8 Hz, 2H, Ar–H), 7.07–7.42 (m, 19H, Vinyl, Ar–H), 7.50
(d, J = 6.9 Hz, 2H, Ar–H), 7.58 (d, J = 6.7 Hz, 2H, Ar–H). 13C NMR (100 MHz,
CDCl3) d 125.3, 125.5, 125.7, 126.3, 126.8, 127.2, 127.5, 127.7, 128.1, 128.4,
128.6, 129.2, 129.7, 130.0, 133.1, 133.2, 134.6, 136.0, 136.2, 137.6, 145.6. FTIR
(KBr)/cm-1 3024, 1488, 1090, 697. Anal. Calc. for C35H25ClN2: C 82.58, H 4.95, N
5.50. Found: C 82.29, H 5.18, N 5.83.
(E)-1-(4-Chlorophenyl)-4,5-diphenyl-2-[4-(4-methylstyryl)phenyl]-1H-imida-
zole (3k) White crystal, Yield: 71% (0.110 g), mp 232–234 °C. 1H NMR
(400 MHz, CDCl3) d 2.35 (s, 3H, CH3), 6.97–7.40 (m, 22H, Vinyl, Ar–H), 7.59 (d,
J = 7.2 Hz, 2H, Ar–H). 13C NMR (100 MHz, CDCl3) d 125.2, 125.5, 125.7, 125.9,
126.3, 127.2, 127.5, 127.7, 127.8, 127.9, 128.1, 128.3, 128.4, 128.6, 129.3, 129.7,
129.9, 130.1, 133.2, 133.3, 134.7, 136.5, 136.7, 137.6, 145.6. FTIR (KBr)/cm-1
3026, 2923, 2858,, 1741, 1701, 1494, 1094, 699. Anal. Calc. for C36H27ClN2: C
82.66, H 5.20, N 5.36. Found: C 82.34, H 5.46, N 5.05.
(E)-1-(4-Chlorophenyl)-2-[4-(4-chlorostyryl) phenyl]-4,5-diphenyl-1H-imidazole
(3l) Bright yellow crystal, Yield: 69% (0.112 g), mp 176–178 °C. 1H NMR
(400 MHz, CDCl3) d 6.54 (s, br, 1H, Ar–H), 6.94–7.41 (m, 21H, Vinyl, Ar–H), 7.57
(s, br, 2H, Ar–H). 13C NMR (100 MHz, CDCl3) d 125.3, 125.7, 126.3, 126.7, 127.1,
127.2, 127.4, 127.5, 127.7, 127.8, 127.9, 128.1, 128.3, 128.4, 128.5, 129.1, 129.2,
129.7, 129.8, 130.0, 134.5, 135.9, 145.6. FTIR (KBr)/cm-1 3051, 1600, 1489, 1059,
698. Anal. Calc. for C35H24Cl2N2: C 77.35, H 4.45, N 5.15. Found: C 77.58, H 4.62,
N 5.41.
(E)-1-(4-Chlorophenyl)-4,5-diphenyl-2-{4-[2-(2-thiophen-3-yl)vinyl]phenyl}-
1H-imidazole (3m) Yield: 71% (0.109 g), mp 246–248 °C. 1H NMR (400 MHz,
CDCl3) d 6.89 (d, J = 16.4 Hz, 1H, Vinyl-H), 6.98 (d, J = 8.5 Hz, 2H, Ar–H),
7.11–7.14 (m, 3H, Ar–H), 7.18–7.41 (m, 15H, Ar–H), 7.58 (d, J = 7.2 Hz, 2H, Ar–
H). 13C NMR (100 MHz, CDCl3) d 121.7, 121.75, 122.6, 123.8, 125.1, 125.2, 125.7,
126.3, 126.8, 127.2, 127.5, 127.9, 128.1, 128.4, 128.6, 129.3, 129.7, 130.1, 133.1,
133.2, 134.7, 136.4, 137.6, 138.9, 145.6. FTIR (KBr)/cm-1 3052, 1598, 1491, 700.
Anal. Calc. for C33H23ClN2S: C 76.95, H 4.50, N 5.44, S 6.23. Found: C 76.64, H
4.29, N 5.67, S 6.51.
(E)-1-(4-Chlorophenyl)-4,5-diphenyl-2-[4-(4-methoxystyryl)phenyl]-1H-imida-
1
zole (3n) Yield: 68% (0.109 g), mp 242–243 °C. H NMR (400 MHz, CDCl3) d
3.82 (s, 3H, OCH3), 6.88–6.94 (m, 3H, Vinyl, Ar–H), 6.98 (d, J = 8.3 Hz, 2H, Ar–
H),7.06 (d, J = 16.3 Hz, 1H, Vinyl-H), 7.12 (d, J = 6.3 Hz, 2H, Ar–H), 7.18–7.27
(m, 8H, Ar–H), 7.39 (s, br, 4H, Ar–H), 7.44 (d, J = 8.5 Hz, 2H, Ar–H), 7.58 (d,
J = 7.2 Hz, 2H, Ar–H). 13C NMR (100 MHz, CDCl3) d 54.3, 113.2, 124.8, 125.0,
125.7, 126.4, 126.8, 127.2, 127.5, 127.7, 128.0, 128.1, 128.4, 128.6, 128.9, 129.3,
129.7, 130.1, 133.2, 134.7, 136.7, 137.6, 145.7, 158.4. FTIR (KBr)/cm-1 3027,
2929, 1600, 1503, 1252, 875. Anal. Calc. for C36H27ClN2O: C 80.21, H 5.05, N
5.20. Found: C 80.53, H 5.29, N 5.48.
123

Z. Ghasemi, Z. Fathi
(E)-1-(4-Chlorophenyl)-4,5-diphenyl-2-[4-(4-nitrostyryl)phenyl]-1H-imidazole
(3o) Yellow crystal, Yield: 71% (0.118 g), mp 292–294 °C. 1H NMR (400 MHz,
CDCl3) d 7.00 (d, J = 8.5 Hz, 2H, Ar–H), 7.12–7.32 (m, 12H, Ar–H), 7.46 (s, br,
4H, Ar–H), 7.58–7.63 (m, 4H, Ar–H),8.22 (d, J = 8.8 Hz, 2H, Ar–H). 13C NMR
(100 MHz, CDCl3) d 124.1, 126.4, 126.6, 127.0, 127.3, 127.4, 128.2, 128.6, 129.3,
130.0, 130.1, 130.6, 131.2, 131.5, 132.3, 133.4, 134.2, 135.6, 136.3, 137.2, 143.8,
145.6, 146.3. FTIR (KBr)/cm-1 3061, 2924, 1590, 1512, 1338, 653. Anal. Calc. for
C35H24ClN3O2: C 75.87, H 4.37, N 7.58. Found: C 75.59, H 4.60, N 7.29.
(E)-1-(4-Chlorophenyl)-4,5-diphenyl-2-[4-(3-nitrostyryl)phenyl]-1H-imidazole (3p)
1
white crystal, Yield: 67% (0.111 g), mp 236–238 °C. H NMR (400 MHz, CDCl3)
d 7.00 (d, J = 8.5 Hz, 2H, Ar–H), 7.11–7.32 (m, 12H, Ar–H), 7.46–7.54 (m, 5H,
Ar–H), 7.59 (d, J = 7.2 Hz, 2H, Ar–H), 7.77 (d, J = 7.7 Hz, 1H, Ar–H), 8.10 (dd,
J = 8.1, 1.0 Hz, 1H, Ar–H), 8.35 (s, 1H, Ar–H). 13C NMR (100 MHz, CDCl3) d
119.9, 121.2, 125.7, 125.9, 126.4, 127.2, 127.3, 127.6, 128.3, 128.4, 128.5, 128.6,
128.8, 129.1, 129.9, 129.97, 130.1, 131.3, 133.4, 137.9, 145.2, 147.7. FTIR (KBr)/
cm-1 3063, 1607, 1528, 1490, 1350, 703. Anal. Calc. for C35H24ClN3O2: C 75.87,
H 4.37, N 7.58. Found: C 76.16, H 4.09, N 7.70.
Results and discussion
First, 2-(4-tolyl)-1,4,5-triphenylimidazole 1a was prepared by four-component con-
densation of 4-methylbenzaldehyde, benzil, aniline and ammonium acetate in the
presenceofPTSA(p-toluenesulfonic acid) inethanol [38]. Benzylic bromination ofthis
compound with N-bromosuccinimide (NBS) and catalytic amounts of AIBN (azabi-
sisobutyronitrile) in CCl4 gave the bromide 2a (Scheme 1) [39, 40]. Reaction of 2a with
triphenylphosphine in toluene, and consequently treatment of the obtained phospho-
niumsalt withbenzaldehydeinthe presenceofa base, affordedthe(E)-product3awhich
was readily separated from minor (Z)-isomer by recrystallization.
Table 1 represents the yield and E/Z selectivity of the reactions of benzaldehyde
and phosphonium salt (obtained from 2a) by using of different bases and solvents at
room temperature. As seen, by using of NaOH in various solvents, the best
efficiency was achieved in DMF as solvent (Entry 3). Examination of the other
bases, resulted in the higher yield of 3a from the reaction promoted by K2CO3 in
DMF (Entry 5).
In order to diversify the products using optimized conditions, we then applied
various aldehydes as well as bromide 2b which was prepared in a manner similar to
2a. As a result, the products 3a–p were obtained in good yields (Table 2) and easy
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
N
1) PPh3
N
N
NBS/AIBN
CH3
CH2Br
CCl4, 70 oC, 24 h
2) Base, Solvent, Ph-CHO
r.t.
Ph
Ph
Ph
1a
3a
2a
Scheme 1 The sequence reactions for synthesis of (E)-stilbene 3a
123

Synthesis of highly fluorescent (E)-Stilbene derivatives…
Table 1 Effect of base and
Entry
Base
Solvent
Yield of 3a (%)
(E)/(Z)
solvent on yield and
stereoselectivity of Wittig
production of 3a
1
2
3
4
5
6
7
NaOH
NaOH
NaOH
NaOH
K2CO3
KOH
THF
0
48
64
35
69
53
56
–
DMSO
DMF
60/40
80/20
50/50
80/20
60/40
70/30
Acetonitrile
DMF
DMF
tBuOK
DMF
Table 2 The Wittig synthesis of various (E)-stilbene derivatives 3a–p containing polysubstituted
imidazoles
1) PPh3
N
N
N
N
CH2Br
Ar
2) K2CO3 / DMF /
Ar-CHO , r.t.
R
R
2a: R=H
2b: R=Cl
3a-p
Entry
R
Ar
Ph
Products 3a–p
Yield (%)
1
H
3a
3b
3c
3d
3e
3f
69a
70
63
71
65
71
66
70
68
69
71
69
71
68
71
67
2
H
4-H3CC6H4
4-ClC6H4
3
H
4
H
4-(H3C)2CHC6H4
4-(H3C)2NC6H4
5-Methy-2-furyl
2-Naphthyl
2-Thienyl
5
H
6
H
7
H
3g
3h
3i
8
H
9
H
3,4-(OMe)2C6H3
Ph
10
11
12
13
14
15
16
a
Cl
Cl
Cl
Cl
Cl
Cl
Cl
3j
4-H3CC6H4
4-ClC6H4
3k
3l
3-Thienyl
3m
3n
3o
3p
4-OMeC6H4
4-O2NC6H4
3-O2NC6H4
See Ref. [42]: for previous report about the preparation of compound 3a see
123

Z. Ghasemi, Z. Fathi
1000
800
600
400
200
0
3k (Ethanol)
3k(chloroform)
3d(chloroform)
3d (Ethanol)
Product
3d
X (Ethanol)
X (Chloroform)
0.57
0.76
0.88
3k
0.65
300
350
400
450
500
550
600
650
Wavelength(nm)
Fig. 1 Comparison of emission intensity and quantum yield in ethanol and chloroform solutions
1
work-up and fully characterized by FT IR, H NMR, 13C NMR and elemental
analysis data.
We then studied the optical properties of the synthesized compounds. To evaluate
the effect of solvent on the emission intensity, we firstly recorded the fluorescence
spectra of two chosen products, 3d and 3k, in dilute (1.0 9 10-6 M) solutions of
ethanol and chloroform (Fig. 1). As seen, although the emission quantum yields
(/x) of both solutions were higher than standard (quinine sulfate, /s = 0.546e) [41],
the ethanol solutions revealed more fluorescence. The emission quantum yields (/x)
were determined by using Eq (1).
À
Á
/X ¼ ðAS Â FX Â n2X Â /SÞ= AX Â FS Â n2S
ð1Þ
According to the equation, x is the symbol of the samples and s is the symbol of
the standard, where / is the fluorescence quantum yield, F is the area under the
fluorescence emission curves and A shows the absorbance at the excitation
wavelength, and nx and ns are the refractive indexes of ethanol and water (1.36 and
1.33 respectively).
Figures 2 and 3 show the electronic absorption and fluorescence spectra of all
(E)-stilbene derivatives 3a–p in dilute (1.0 9 10-6 M) ethanol solution, respec-
tively. The related descriptions are also presented in Table 3. The absorption spectra
of target compounds (Fig. 2) exhibit two peaks: one around 270–290 nm and the
other broad one at 322–367 nm. Calculated molar extinction coefficients for the
latter broad peaks (Table 3) indicate the strong p–p* transitions. As shown in
Table 3, the expanded p-conjugation in the products 3a–p has resulted in the longer
absorption wavelength kmax (bathochromic shifts) and also a longer emission
wavelength kem relative to starting materials 1a and 1b. The fluorescence quantum
yields of a large number of products have also been increased, compared to the
starting imidazoles (1a, b). While the compounds containing nitro groups (3o and
3p) exhibited no fluorescence, the electron donors such as methoxy and
dimethylamino groups did not have a good effect on emissions (about 3e, 3i and
3n). According to Table 3, the products containing 4-chlorophenyl group at the N-1
position (obtained from 2b) have higher emission quantum yields (3j–l compared
123

Synthesis of highly fluorescent (E)-Stilbene derivatives…
Fig. 2 Absorption spectra of
(E)-stilbene derivatives 3a–p in
ethanol solution (1.0 9 10-6 M)
123

Z. Ghasemi, Z. Fathi
Fig. 3 Fluorescence spectra of
(E)-stilbene derivatives 3a–p in
ethanol solution (1.0 9 10-6 M)
123

Synthesis of highly fluorescent (E)-Stilbene derivatives…
Table 3 Fluorescent and UV–
Dmb
c
/
X
Compound
kmax
kem
emax
Vis parameters of products 3a–
p and starting compounds 1a
and b
(nm)a
(nm)a
(mol-1 Lcm-1
)
9 104
3a
3b
3c
3d
3e
3f
331
336
340
332
364
365
342
350
344
339
335
331
331
342
280
282
423
415
429
418
447
418
429
421
420
421
410
422
404
410
383
379
2.56
2.19
2.23
2.26
1.25
1.03
1.76
1.35
2.21
2.38
2.44
3.13
3.39
1.83
1.46
2.26
6570
5665
6101
6197
5101
3474
5929
4818
5260
5745
5460
6514
5459
4849
9604
9075
0.79
0.71
0.64
0.78
0.26
0.15
0.82
0.14
0.38
0.98
0.87
0.95
0.98
0.27
0.55
0.45
3g
3h
3i
3j
3k
3l
a
Solution in 10-6 molL-1
ethanol
b
Stokes shift in cm-1
3m
3n
1a
1b
c
The fluorescence quantum
yields were measured in ethanol
using quinine sulfate in 0.5 M
sulfuric acid as the standard
Fig. 4 The dilute (1.0 9 10-6 M) solutions of target compounds 3a–p under UV lamp (366 nm)
with 3a–c). Stokes shifts of synthesized compounds, calculated from the difference
between emission wavelength kem and absorption wavelength kmax, are relatively
good, indicating low overlap between their absorption and emission spectra.
Figure 4 shows the photographs of target compounds 3a–p under UV lamp
(366 nm).
Conclusion
In conclusion, novel (E)-stilbene derivatives containing 1,2,4,5-tetraarylimidazole
moiety were synthesized by Wittig reactions in good yields. The products were
easily purified by recrystallization from ethanol. The optical properties of target
compounds were evaluated in dilute (1.0 9 10-6 M) ethanol solutions which
123

Z. Ghasemi, Z. Fathi
exhibited blue to violet emission (/x ¼ 0:14 À 0:98). The study of UV–Vis
absorption spectra help us to investigate our high p-conjugated systems.
Acknowledgements The authors thank the research affairs of the University of Tabriz for financial
support. We are also grateful to Prof. Mohammad Amjadi (Department of Analytical Chemistry, Faculty
of Chemistry, University of Tabriz, Tabriz 5166614766, Iran) for his valuable assistance.
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