The synthesis and optical properties of novel 1,3,4-oxadiazole derivatives containing an imidazole unit

Article abstract of DOI:10.1016/j.dyepig.2010.01.011

A series of 1,3,4-oxadiazole derivatives containing an imidazole unit were synthesized and characterized using ¹H NMR, ¹³C NMR, mass spectrometry (or high-resolution mass spectrometry) and elemental analysis. The crystal structure of 2,5-bis(4-(1-n-butyl-4,5-diphenylimidazol-2-yl)phenyl)-1,3,4-oxadiazole was determined as monoclinic, space group C2/c type, using single crystal X-ray crystallography. For nine samples, UV-visible absorption coefficient (e{open}), maximum wavelength (λ_max), fluorescence excitation wavelength (λ_ex), fluorescence emission wavelength (λ_em), fluorescence quantum yield (Φ_F), fluorescence lifetime (T) were measured in dichloromethane and in the solid state. For these selected compounds, thermogravimetric analysis was also employed and structure:optical behaviour characteristics were discussed.

Full text of DOI:10.1016/j.dyepig.2010.01.011

Dyes and Pigments 86 (2010) 249e258
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and optical properties of novel 1,3,4-oxadiazole derivatives
containing an imidazole unit
*
Yun-Nan Yan, Wen-long Pan, Hua-Can Song
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 1,3,4-oxadiazole derivatives containing an imidazole unit were synthesized and characterized
using 1H NMR, 13C NMR, mass spectrometry (or high-resolution mass spectrometry) and elemental
analysis. The crystal structure of 2,5-bis(4-(1-n-butyl-4,5-diphenylimidazol-2-yl)phenyl)-1,3,4-oxadia-
zole was determined as monoclinic, space group C2/c type, using single crystal X-ray crystallography. For
Received 3 September 2009
Received in revised form
19 January 2010
Accepted 20 January 2010
Available online 10 February 2010
nine samples, UVevisible absorption coefficient (3), maximum wavelength (lmax), fluorescence excitation
wavelength (lex), fluorescence emission wavelength (lem), fluorescence quantum yield (FF), fluorescence
lifetime (T) were measured in dichloromethane and in the solid state. For these selected compounds,
thermogravimetric analysis was also employed and structure:optical behaviour characteristics were
discussed.
Keywords:
Synthesis
Fluorescence
Optical property
Crystal structure
1,3,4-Oxadiazoles
Imidazole
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
benzoimidazole unit were prepared so as to investigate their
UVevisible absorption and fluorescence character. The synthetic
Imidazole derivatives are important five-membered nitrogen-
containing heterocyclic compounds which are widely used in many
fields, such as P38 MAP kinase [1], antivascular disrupting, anti-
tumour activator [2], ionic liquids [3], anion sensors [4], as well as
electrical and optical materials [5e7]. 1,3,4-Oxadiazoles are a class
of significant heterocyclic compounds used in medicinal and
pesticide chemistry [8,9], asymmetric organic synthesis [10], and
polymer and materials science [11,12]. 2,5-Diaryl-1,3,4-oxadiazoles
are important materials used as red luminescent emitters with
carrier-transporting ability [13], efficient electron transporting and
hole blocking materials in organic light-emitting diodes (OLEDs)
[14e28].
The work described by the current authors relates to the
synthesis and optical properties of heterocycle-based chromo-
phores [29e31]. There are a few reports of the synthesis and optical
properties of such compounds that contain 1,3,4-oxadiazole and
imidazole or benzoimidazole units within a conjugated, long chain
molecule [28,32]. Hence, novel fluorescent dyes using a benzene
ring to link an aryl-1,3,4-oxadiazole unit and arylimidazole unit or
pathway and the structures of the target molecules are shown in
Figs. 1 and 2.
2. Experimental
2.1. Reagents and instruments
All melting points were determined on an X4 melting point
microscope. 1H NMR and 13C NMR spectra were measured on
a Bruker AVANCE-300 NMR spectrometer. MS were taken with an
SHIMADZU LCMS-2010A. High-resolution mass spectra were
obtained on Thermo MAT-95XP. Element analyses were taken with
an Elementar Analysensysteme GmbH Vario EL. Single crystal was
characterized by Bruker Smart 1000 CCD X-ray single crystal
diffractometer. Ultraviolet and fluorescence spectra were recorded
on an SHIMADZU UV-1601 and SHIMADZU RF-5301PC spectro-
fluorophotometer. Fluorescence lifetimes were determined on
EDINBURGH FLSP920. Thermogravimetric analysis (TGA) was
carried out up to 900 ꢀC with a heating speed of 10.0 K/min in
nitrogen atmosphere on a Netzsch TG-209 thermogravimetric
analyzer. Reagents and solvents used for the synthesis were all
synthetic grades and used without further purification.
* Corresponding author. Tel.: þ86 20 84110918; fax: þ86 20 84112245.
E-mail address: yjhxhc@mail.sysu.edu.cn (H.-C. Song).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.01.011

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Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
NH2
NH2
OHC
O
CO2CH3
O
N
C4H9Br, 50%K2CO3
TBAB, butanone
CO2CH3
N
H
Reflux
EtOH, reflux
1a
N
N
N
NH2NH2 H2O
CH3OH, reflux
CO2CH3
CONHNH2
N
C4H9
C4H9
2a
3a
R
N
O
1) Pyridine, RCOCl, reflux
2) POCl3, reflux
N
N
N
C4H9
4a-4c
OHC
NH4OCOCH3
CH3CO2H, reflux
CO2CH3
C4H9Br, 50%K2CO3
TBAB, butanone
Reflux
O
O
N
CO2CH3
N
H
1b
N
N
N
NH2NH2 H2O
CH3OH, reflux
CONHNH2
CO2CH3
N
C4H9
C4H9
2b
3b
R
O
N
N
1) Pyridine, RCOCl, reflux
2) POCl3, reflux
N
N
C4H9
4d-4f
OHC
CO2CH3
NH4OCOCH3
CH3CO2H, ref lux
C4H9Br, 50%K2CO3
TBAB, butanone
Reflux
O
O
N
CO2CH3
N
H
1c
N
N
N
NH2NH2 H2O
CH3OH, ref lux
CONHNH2
CO2CH3
N
C4H9
C4H9
2c
3c
R
O
N
1) Pyridine, RCOCl, reflux
2) POCl3, reflux
N
N
N
C4H9
4g-4i
4a, 4d, 4g R =
4b, 4e, 4h R =
4c, 4f, 4i R =
S
Fig. 1. The synthetic route of unsymmetrical 1,3,4-oxadiazole derivatives.
2.2. Synthesis
2.2.1.1. Methyl
solid, yield 50%; m.p. 220e221 ꢀC (Lit. [35] 220e221 ꢀC). 1H
NMR (300 MHz, DMSO-d6): 3.89 (s, 3H, OCH3), 7.18e7.27 (m,
4-(1H-benzimidazol-2-yl)benzoate
(1a). White
2.2.1. Preparation of 1aec
d
Compound 1a was prepared according to the reported method
[33], while compounds 1b and 1c were obtained using reported
procedures [34].
2H, ArH), 7.56 (d, J ¼ 7.5 Hz, 1H, ArH), 7.70 (d, J ¼ 7.2 Hz, 1H,
ArH), 8.12 (d, J ¼ 8.4 Hz, 2H, ArH), 8.31 (d, J ¼ 8.4 Hz, 2H,
ArH), 13.10 (s, 1H, NH). 13C NMR (75 MHz, DMSO-d6):
d 52.2,

Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
251
N
R
R
1) SOCl2, reflux
CO2H
NaOH, 95% C2H5OH
Reflux
O
2a
2b
N
2) 3a, pyridine, reflux
N
N
3) POCl3, reflux
C4H9
5a
5b
6a
R
R
N
N
1) SOCl2, reflux
O
NaOH, 95% C2H5OH
Reflux
CO2H
2) 3b, pyridine, reflux
N
N
C4H9
3) POCl3, reflux
6b
R
R
N
N
O
1) SOCl2, reflux
NaOH, 95% C2H5OH
Reflux
CO2H
2c
2) 3c, pyridine, ref lux
N
N
3) POCl3, reflux
C4H9
5c
6c
N
N
N
6a
6b
R =
R =
6c
R =
N
N
N
C4H9
C4H9
C4H9
Fig. 2. The synthetic scheme of symmetrical 1,3,4-oxadiazole derivatives.
126.4, 129.6, 130.1, 134.1, 149.8, 165.5. ESI-MS (m/z): 253
[M þ H]þ.
(t, J ¼ 7.2 Hz, 2H, NCH2), 7.09e7.21(m, 3H, ArH), 7.37e7.49 (m, 4H,
ArH), 7.50e7.57 (m, 3H, ArH), 7.90 (d, J ¼ 8.4 Hz, 2H, ArH), 8.11 (d,
J ¼ 8.4 Hz, 2H, ArH). 13C NMR (75 MHz, DMSO-d6,
d ppm): 13.0, 18.8,
2.2.1.2. Methyl4-(4,5-diphenylimidazol-2-yl)benzoate(1b). Yellowish
solid, yield 93%; m.p. 235e236 ꢀC (Lit. [36] 246e248 ꢀC). 1H NMR
31.7, 44.1, 52.2, 126.0, 126.2, 127.9, 128.6, 128.8, 128.9, 129.0, 129.2,
129.3, 130.5, 130.7, 134.2, 135.3, 136.9, 145.3, 165.7. ESI-MS (m/z):
411 [M þ H]þ.
(300 MHz, DMSO-d6):
d 3.88 (s, 3H, OCH3), 7.19e7.35 (m, 3H, ArH),
7.38e7.59 (m, 7H, ArH), 8.06 (d, J ¼ 8.4 Hz, 2H, ArH), 8.23 (d,
J ¼ 8.1 Hz, 2H, ArH), 12.93 (s, 1H, NH). ESI-MS (m/z): 355 [M þ H]þ.
2.2.2.3. Methyl 4-(1-n-butylphenanthroimidazol-2-yl)benzoate (2c).
Yellowish solid, yield 74%; m.p. 101e102 ꢀC. 1H NMR (300 MHz,
2.2.1.3. Methyl 4-(1H-phenanthroimidazol-2-yl)benzoate (1c).
Yellowish solid, yield 90%; m.p. >300 ꢀC. 1H NMR (300 MHz, DMSO-
CDCl3):
d
0.82 (t, J ¼ 7.2 Hz, 3H, CH3), 1.18e1.31 (m, 2H, CH2),
1.88e1.97 (m, 2H, CH2), 4.00 (s, 3H, OCH3), 4.64 (t, J ¼ 7.2 Hz, 2H,
NCH2), 7.61e7.73 (m, 4H, ArH), 7.87 (d, J ¼ 8.7 Hz, 2H, ArH),
8.22e8.28 (m, 3H, ArH), 8.72 (d, J ¼ 7.8 Hz, 1H, ArH), 8.79 (dd,
J ¼ 1.5 Hz, 7.8 Hz, 1H, ArH), 8.86 (dd, J ¼ 1.8 Hz, 7.8 Hz, 1H, ArH). ESI-
MS (m/z): 409 [M þ H]þ.
d6):
d 3.89 (s, 3H, OCH3), 7.54e7.76 (m, 4H, ArH), 8.12 (s, 2H, ArH),
8.41 (s, 2H, ArH), 8.51 (s, 1H, ArH), 8.61 (d, J ¼ 7.5 Hz, 1H, ArH),
8.66e8.87 (m, 2H, ArH), 13.51 (s, 1H, NH). ESI-MS (m/z): 353
[M þ H]þ.
2.2.2. General procedure for the synthesis of 2aec
2.2.3. General procedure for the preparation of 3aec
A mixture of methyl benzoate derivatives (5 mmol), hydrazine
hydrate (15 mmol), and methanol (15 mL) was refluxed for 12 h. The
reaction was monitored by TLC. After the solvent was removed, the
residue was thoroughly washed with water and dried completely to
give pure product.
A
mixture of methyl 4-(imidazol-2-yl)benzoate derivative
(2.5 mmol), n-butyl bromide (3.5 mmol), tetra-n-butylammonium
bromide (TBAB) (0.25 g), 50% potassium carbonate (10 mL) and
butanone (10 mL) was refluxed for 20e24 h and the reaction was
monitored by TLC. After butanone was removed, the residue was
poured into water (100 mL). The solid was collected and purified by
silica-gel column chromatography to offer the corresponding
product.
2.2.3.1. 4-(1-n-Butylbenzimidazol-2-yl)benzohydrazide (3a). White
solid, yield 80%; m.p. 62 ꢀC. 1H NMR (300 MHz, DMSO-d6):
d 0.75
(t, J ¼ 7.2 Hz, 3H, CH3), 1.06e1.19 (m, 2H, CH2), 1.59e1.69 (m, 2H,
CH2), 4.32 (t, J ¼ 6.6 Hz, 2H, NCH2), 4.59 (s, 2H, NH2), 7.21e7.34 (m,
2H, ArH), 7.67 (t, J ¼ 7.2 Hz, 2H, ArH), 7.85 (d, J ¼ 8.1 Hz, 2H, ArH),
8.00 (d, J ¼ 7.8 Hz, 2H, ArH), 9.93 (s, 1H, NH). 13C NMR (75 MHz,
2.2.2.1. Methyl 4-(1-n-butylbenzimidazol-2-yl)benzoate (2a). White
solid, yield 75%; m.p. 100e101 ꢀC. 1H NMR (300 MHz, DMSO-d6):
d
0.74 (t, J ¼ 7.2 Hz, 3H, CH3), 1.07e1.19 (m, 2H, CH2), 1.59e1.72 (m,
2H, CH2), 3.91 (s, 3H, OCH3), 4.33 (t, J ¼ 6.7 Hz, 2H, NCH2), 7.22e7.33
(m, 2H, ArH), 7.68 (t, J ¼ 7.5 Hz, 2H, ArH), 7.94 (d, J ¼ 7.5 Hz, 2H,
ArH), 8.14 (d, J ¼ 7.5 Hz, 2H, ArH). 13C NMR (75 MHz, DMSO-d6):
DMSO-d6): d 13.2, 19.2, 31.2, 43.8, 110.8, 119.1, 121.9, 122.4, 127.2,
128.9, 132.8, 133.9, 135.5, 142.4, 151.9, 165.0. ESI-MS (m/z): 309
[M þ H]þ.
d
13.1, 19.1, 31.1, 43.7, 52.1, 110.7, 119.1, 121.8, 122.5, 129.1, 129.2,
130.0, 134.7, 135.5, 142.3, 151.4, 165.4. ESI-MS (m/z): 309 [M þ H]þ.
2.2.3.2. 4-(1-n-Butyl-4,5-diphenylimidazol-2-yl)benzohydrazide (3b).
Yellowish solid, yield 78%; m.p. 193e194 ꢀC. 1H NMR (300 MHz,
2.2.2.2. Methyl 4-(1-n-butyl-4,5-diphenylimidazol-2-yl)benzoate
DMSO-d6):
d
0.53 (t, J ¼ 7.2 Hz, 3H, CH3), 0.85e0.97 (m, 2H, CH2),
(2b). Yellowish solid, yield 70%; m.p. 134e135 ꢀC. 1H NMR
1.21e1.31 (m, 2H, CH2), 3.93 (t, J ¼ 7.2 Hz, 2H, NCH2), 7.09e7.21 (m,
3H, ArH), 7.39e7.55 (m, 7H, ArH), 7.84 (d, J ¼ 8.1 Hz, 2H, ArH), 7.98
(d, J ¼ 8.1 Hz, 2H, ArH). ESI-MS (m/z): 411 [M þ H]þ.
(300 MHz, DMSO-d6):
d
0.52 (t, J ¼ 7.2 Hz, 3H, CH3), 0.84e0.96
(m, 2H, CH2), 1.20e1.32 (m, 2H, CH2), 3.89 (s, 3H, OCH3), 3.94

252
Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
2.2.3.3. 4-(1-n-Butylphenanthroimidazol-2-yl)benzohydrazide (3c).
White solid, yield 80%; m.p. 223e224 ꢀC. 1H NMR (300 MHz, DMSO-
1H NMR (300 MHz, CDCl3):
d
0.64 (t, J ¼ 7.5 Hz, 3H, CH3), 0.98e1.05
(m, 2H, CH2), 1.32e1.43 (m, 2H, CH2), 3.96 (t, J ¼ 6.9 Hz, 2H, NCH2),
7.10e7.24 (m, 4H, ArH), 7.42e7.55 (m, 9H, ArH), 7.92 (d, J ¼ 8.1 Hz,
2H, ArH), 8.11e8.20 (m, 2H, ArH), 8.27 (d, J ¼ 7.2 Hz, 2H, ArH). 13C
d6):
d
0.69 (t, J ¼ 7.2 Hz, 3H, CH3), 1.03e1.18 (m, 2H, CH2), 1.73e1.87
(m, 2H, CH2), 4.57 (t, J ¼ 7.5 Hz, 2H, NCH2), 4.69 (br, 2H, NH2),
7.58e7.81 (m, 4H, ArH), 7.87 (d, J ¼ 6.9 Hz, 2H, ArH), 8.06 (d,
J ¼ 7.8 Hz, 2H, ArH), 8.33e8.44 (m, 1H, ArH), 8.59 (d, J ¼ 7.5 Hz, 1H,
ArH), 8.77e8.88 (m, 1H, ArH), 8.89e9.00 (m, 1H, ArH), 9.96 (s, 1H,
CONH). ESI-MS (m/z): 409 [M þ H]þ.
NMR (75 MHz, CDCl3):
d 13.4, 19.6, 32.7, 44.8, 123.8, 126.3, 126.7,
126.9, 127.0, 127.9, 128.7, 128.9, 129.4, 130.4, 130.9, 131.1, 131.7, 134.2,
134.6, 138.2, 146.1, 164.1, 164.6. HRMS (EI) (Mþ): Calcd. for
C33H28N4O: 496.2263; found: 496.2258. Anal. Calcd. (%) for
C33H28N4O: C, 79.81; H, 5.68; N, 11.28. Found: C, 79.61; H, 5.87; N,
11.33.
2.2.4. General procedure for the synthesis of 4aei
The benzohydrazide derivative (2 mmol) was added to a stirred
solution of the corresponding benzoyl chloride (or 2-naphthoyl
chloride, or thiophene-2-carbonyl chloride) (2 mmol) in anhydrous
pyridine (10 mL). The mixture was refluxed for 20e24 h, cooled and
washed with water and the ensuing crude product was filtered,
dried and then was added to phosphorus oxychloride (caution:
reacts violently with water; incompatible with many metals, alco-
hols, amines, phenol, DMSO, strong bases; 10 mL); the mixture was
refluxed overnight. The majority of phosphorus oxychloride was
distilled from the reaction mixture and the residue was gently
added to powdered ice. The resulting oxadiazole product was
filtered, washed with water, dried and recrystallized from the
mixture of chloroform and ethanol, or purified by silica-gel column
chromatography using petroleum (b.p. 60e90 ꢀC)/ethyl acetate
(2:1) as eluent.
2.2.4.5. 2-(4-(1-n-Butyl-4,5-diphenylimidazol-2-yl)phenyl)-5-
(naphthalen-2-yl)-1,3,4-oxadiazole (4e). White solid, yield 44%; m.
p. 206e208 ꢀC. 1H NMR (300 MHz, DMSO-d6):
d
0.57 (t, J ¼ 7.2 Hz,
3H, CH3), 0.90e1.02 (m, 2H, CH2), 1.28e1.38 (m, 2H, CH2), 4.01 (t,
J ¼ 7.5 Hz, 2H, NCH2), 7.11e7.24 (m, 3H, ArH), 7.41e7.60 (m, 7H,
ArH), 7.60e7.72 (m, 2H, ArH), 8.02e8.07 (m, 3H, ArH), 8.15e8.24
(m, 3H, ArH), 8.36 (d, J ¼ 8.7 Hz, 2H, ArH), 8.82 (s, 1H, ArH). 13C NMR
(75 MHz, CDCl3):
d 13.4, 19.6, 32.7, 44.8, 121.0, 123.2, 123.8, 126.3,
126.7, 127.0,127.1, 127.3,127.8, 127.9, 128.0, 128.7,128.8, 129.0, 129.5,
130.5, 130.9, 131.1, 132.8, 134.2, 134.6, 138.3, 146.1, 164.2, 164.8. ESI-
MS (m/z): 547 [M þ H]þ. Anal. Calcd. (%) for C37H30N4O$0.2H2O: C,
80.76; H, 5.57; N, 10.18. Found: C, 80.77; H, 5.59; N, 10.21.
2.2.4.6. 2-(4-(1-n-Butyl-4,5-diphenylimidazol-2-yl)phenyl)-5-(thi-
ophen-2-yl)-1,3,4-oxadiazole (4f). Yellowish solid, yield 41%; m.p.
2.2.4.1. 2-(4-(1-n-Butylbenzimidazol-2-yl)phenyl)-5-phenyl-1,3,4-
oxadiazole (4a). White solid, yield 50%; m.p. 144e146 ꢀC. 1H NMR
168e169 ꢀC. 1H NMR (300 MHz, CDCl3):
d
0.60 (t, J ¼ 6.9 Hz, 3H,
CH3), 0.90e1.06 (m, 2H, CH2), 1.23e1.40 (m, 2H, CH2), 4.09 (t,
J ¼ 7.2 Hz, 2H, NCH2), 7.14e7.32 (m, 4H, ArH), 7.33e7.48 (m, 4H,
ArH), 7.49e7.62 (m, 4H, ArH), 7.85 (s, 1H, ArH), 8.01 (d, J ¼ 7.2 Hz,
2H, ArH), 8.28 (d, J ¼ 7.2 Hz, 2H, ArH). 13C NMR (75 MHz, CDCl3):
(300 MHz, DMSO-d6):
d
0.77 (t, J ¼ 7.2 Hz, 3H, CH3), 1.11e1.23 (m,
2H, CH2), 1.64e1.74 (m, 2H, CH2), 4.38 (t, J ¼ 6.9 Hz, 2H, NCH2),
7.24e7.34 (m, 2H, ArH), 7.64e7.73 (m, 5H, ArH), 8.06 (d, J ¼ 8.1 Hz,
2H, ArH), 8.17 (d, J ¼ 7.5 Hz, 2H, ArH), 8.33 (d, J ¼ 8.1 Hz, 2H, ArH).
d
13.3, 19.5, 32.6, 44.8, 123.5, 124.9, 126.3, 126.7, 127.0, 127.9, 128.1,
13C NMR (75 MHz, DMSO-d6):
d
13.3, 19.2, 31.2, 43.9, 110.9, 119.1,
128.7, 128.9, 129.4, 129.8, 130.2, 130.4, 130.8, 131.0, 134.2, 134.6,
138.2, 146.0, 160.8, 163.5. ESI-MS (m/z): 503 [M þ H]þ. Anal. Calcd.
(%) for C31H26N4OS: C, 74.08; H,5.21; N, 11.15. Found: C, 73.83; H,
5.27; N, 11.19.
122.0, 122.6, 123.1, 123.9, 126.6, 126.8, 129.2, 129.8, 131.9, 133.4,
135.6, 142.2, 151.3, 163.3, 163.9. ESI-MS (m/z): 395 [M þ H]þ. Anal.
Calcd. (%) for C25H22N4O$1.5H2O: C, 71.24; H, 5.98; N, 13.29. Found:
C, 71.40; H, 5.80; N, 13.39.
2.2.4.7. 2-(4-(1-n-Butylphenanthroimidazol-2-yl)phenyl)-5-phenyl-1,
2.2.4.2. 2-(4-(1-n-Butylbenzimidazol-2-yl)phenyl)-5-(naphthalen-2-
3,4-oxadiazole (4g). Yellowish solid, yield 40%; m.p. 209e210 ꢀC. 1H
yl)-1,3,4-oxadiazole (4b). White solid, yield 46%; m.p. 216e218 ꢀC.
NMR (300 MHz, CDCl3):
d
0.85 (t, J ¼ 6.6 Hz, 3H, CH3), 1.19e1.36 (m,
1H NMR (300 MHz, DMSO-d6):
d
0.79 (t, J ¼ 7.2 Hz, 3H, CH3),
2H, CH2), 1.90e2.07 (m, 2H, CH2), 4.70 (t, J ¼ 6.9 Hz, 2H, NCH2),
7.51e7.61 (m, 3H, ArH), 7.61e7.77 (m, 4H, ArH), 7.99 (d, J ¼ 7.8 Hz,
2H, ArH), 8.12e8.30 (m, 3H, ArH), 8.37 (d, J ¼ 7.8 Hz, 2H, ArH), 8.72
(d, J ¼ 8.1 Hz, 1H, ArH), 8.76e8.91 (m, 2H, ArH). 13C NMR (75 MHz,
1.13e1.25 (m, 2H, CH2), 1.66e1.75 (m, 2H, CH2), 4.39 (t, J ¼ 7.2 Hz,
2H, NCH2), 7.25e7.35 (m, 2H, ArH), 7.65e7.73 (m, 5H, ArH),
8.03e8.08 (m, 3H, ArH), 8.14e8.23 (m, 3H, ArH), 8.38 (d, J ¼ 8.1 Hz,
2H, ArH), 8.80 (s, 1H, ArH). 13C NMR (75 MHz, CDCl3):
d
13.6, 20.0,
CDCl3): d 13.5, 19.6, 29.7, 46.9, 120.6,122.4, 122.9, 123.2, 123.6, 124.3,
31.9, 44.7, 110.1, 120.0, 120.8, 122.5, 123.0, 123.1, 124.7, 127.0, 127.1,
127.3, 127.8, 127.9, 128.7, 128.9, 129.8, 132.7, 133.7, 134.6, 135.6,
142.9, 152.0, 163.9, 164.8. ESI-MS (m/z): 445 [M þ H]þ. Anal. Calcd.
(%) for C29H24N4O$0.4H2O: C, 77.11; H, 5.53; N, 12.40. Found: C,
77.03; H, 5.40; N, 12.30.
124.8,125.5,126.4,126.7,126.8,126.9,127.1,127.2,128.1,128.9,129.1,
130.5, 131.7, 134.0, 138.1, 151.1, 163.8, 164.6. HRMS (EI) (Mþ): Calcd.
for C33H26N4O: 494.2107; found: 494.2102. Anal. Calcd. (%) for
C33H26N4O$0.5H2O: C, 78.99; H, 5.38; N, 11.17. Found: C, 78.93; H,
6.06; N, 10.98.
2.2.4.3. 2-(4-(1-n-Butylbenzimidazol-2-yl)phenyl)-5-(thiophen-2-yl)-
2.2.4.8. 2-(4-(1-n-Butylphenanthroimidazol-2-yl)phenyl)-5-(naph-
thalen-2-yl)-1,3,4-oxadiazole (4h). Yellowish solid, yield 34%; m.p.
1,3,4-oxadiazole (4c). White solid, yield 45%; m.p. 111e112 ꢀC. 1H
NMR (300 MHz, DMSO-d6):
d
0.77 (t, J ¼ 7.2 Hz, 3H, CH3), 1.10e1.23
259e260 ꢀC. 1H NMR (300 MHz, CDCl3):
d
0.86 (t, J ¼ 7.2 Hz, 3H,
(m, 2H, CH2), 1.64e1.73 (m, 2H, CH2), 4.38 (t, J ¼ 7.2 Hz, 2H, NCH2),
7.23e7.36 (m, 3H, ArH), 7.70 (t, J ¼ 7.2 Hz, 2H, ArH), 7.99e8.01 (m,
2H, ArH), 8.05 (d, J ¼ 8.7 Hz, 2H, ArH), 8.28 (d, J ¼ 8.7 Hz, 2H, ArH).
CH3), 1.23e1.35 (m, 2H, CH2), 1.90e2.03 (m, 2H, CH2), 4.69 (t,
J ¼ 7.2 Hz, 2H, NCH2), 7.58e7.75 (m, 6H, ArH), 7.91e7.94 (m, 1H,
ArH), 7.99 (t, J ¼ 9.0 Hz, 4H, ArH), 8.27 (t, J ¼ 8.7 Hz, 2H, ArH),
8.40 (d, J ¼ 8.1 Hz, 2H, ArH), 8.67e8.72 (m, 2H, ArH), 8.81 (d,
J ¼ 7.2 Hz, 1H, ArH), 8.86 (d, J ¼ 7.8 Hz, 1H, ArH). 13C NMR
13C NMR (75 MHz, DMSO-d6):
d 13.3, 19.2, 31.3, 43.9, 110.9, 119.2,
122.0, 122.6, 122.7, 123.7, 124.0, 126.8, 128.6, 129.9, 130.6, 131.7,
133.4, 135.6, 142.3, 151.4, 160.4, 162.8. ESI-MS (m/z): 401 [M þ H]þ.
Anal. Calcd. (%) for C23H20N4OS$1$1H2O: C, 65.72; H, 5.32; N, 13.33.
Found: C, 65.50; H, 5.25; N, 13.29.
(75 MHz, CDCl3):
d 13.2, 19.3, 30.9, 48.9, 120.2, 120.6, 121.9, 122.2,
122.6, 123.9, 124.2, 124.3, 124.8, 126.7, 126.8, 126.9, 126.9, 126.9,
127.1, 127.6, 127.7, 127.8, 128.3, 128.7, 129.6, 132.1, 132.4, 134.5,
146.5, 162.7, 164.7. ESI-MS (m/z): 545 [M þ H]þ. Anal. Calcd. (%)
for C37H28N4O$0.7H2O: C, 79.75; H, 5.32; N, 10.05. Found: C,
79.72; H, 5.42; N 9.72.
2.2.4.4. 2-(4-(1-n-Butyl-4,5-diphenylimidazol-2-yl)phenyl)-5-phenyl-
1,3,4-oxadiazole (4d). Yellowish solid, yield 45%; m.p. 147e148 ꢀC.

Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
253
2.2.4.9. 2-(4-(1-n-Butylphenanthroimidazol-2-yl)phenyl)-5-(thio-
phen-2-yl)-1,3,4-oxadiazole (4i). Yellowish solid, yield 35%; m.p.
2.2.6.1. 2,5-Bis(4-(1-n-butylbenzimidazol-2-yl)phenyl)-1,3,4-oxadia-
zole (6a). White solid, yield 10%; m.p. 213e215 ꢀC. 1H NMR
231e232 ꢀC. 1H NMR (300 MHz, CDCl3):
d
0.63 (t, J ¼ 7.5 Hz, 3H,
(300 MHz, DMSO-d6):
d
0.79 (t, J ¼ 7.2 Hz, 6H, CH3), 1.23e1.25 (m,
CH3), 0.93e1.05 (m, 2H, CH2), 1.73e1.82 (m, 2H, CH2), 5.06 (t,
J ¼ 6.9 Hz, 2H, NCH2), 7.17e7.23 (m, 1H, ArH), 7.24e7.29 (m, 1H,
ArH), 7.40 (t, J ¼ 7.8 Hz, 1H, ArH), 7.56e7.82 (m, 4H, ArH), 8.09 (d,
J ¼ 8.1 Hz, 2H, ArH), 8.28 (d, J ¼ 8.1 Hz, 1H, ArH), 8.36 (t, J ¼ 8.4 Hz,
3H, ArH), 8.49 (d, J ¼ 8.1 Hz, 1H, ArH), 8.91 (d, J ¼ 7.8 Hz, 1H, ArH).
4H, CH2), 1.66e1.75 (m, 4H, CH2), 4.40 (t, J ¼ 7.2 Hz, 4H, NCH2),
7.24e7.35 (m, 4H, ArH), 7.71 (t, J ¼ 7.5 Hz, 4H, ArH), 8.07 (d,
J ¼ 8.4 Hz, 4H, ArH), 8.36 (d, J ¼ 8.4 Hz, 4H, ArH). 13C NMR (75 MHz,
CDCl3):
d 13.6, 20.0, 31.9, 44.7, 110.1, 120.0, 122.6, 123.1, 124.5, 127.1,
129.9, 133.9, 135.6, 142.9, 151.9, 164.1. HRMS (EI) (Mþ): Calcd. for
C36H34N6O: 566.2794; found: 566.2792. Anal. Calcd. (%) for
C36H34N6O: C, 76.30; H, 6.05; N, 14.83. Found: C, 76.13; H, 6.18; N,
14.97.
13C NMR (75 MHz, CDCl3):
d 13.2, 19.3, 31.0, 48.9, 119.6, 120.6, 121.9,
122.3,124.1,124.3,124.4,124.8,126.5,127.0,127.1,127.8,127.9,128.2,
128.5, 129.7, 130.2, 130.6, 132.0, 146.4, 160.9, 162.0. ESI-MS (m/z):
501 [M þ H]þ. Anal. Calcd. (%) for C31H24N4OS$0.4H2O: C, 73.32; H,
4.92; N 11.03. Found: C, 73.76; H, 4.96; N, 10.90.
2.2.6.2. 2,5-Bis(4-(1-n-butyl-4,5-diphenylimidazol-2-yl)phenyl)-
1,3,4-oxadiazole (6b). White solid, yield 12%; m.p. 217e219 ꢀC. 1H
2.2.5. General procedure for the synthesis of 5aec
NMR (300 MHz, DMSO-d6):
d
0.57 (t, J ¼ 7.2 Hz, 6H, CH3), 0.90e1.02
The mixture of corresponding compound 2 (5 mmol) and
sodium hydroxide (10 mmol) in 95% methanol (10 mL) was refluxed
for about 1 h. After most solvent was evaporated, the residue was
dissolved in water (20 mL), the pH was adjusted to 1.0 with
concentrated hydrochloric acid, and then the resulting solid was
filtered, washed, and dried to give pure 5a, 5b and 5c.
(m, 4H, CH2), 1.27e1.37 (m, 4H, CH2), 4.00 (t, J ¼ 7.2 Hz, 4H, NCH2),
7.11e7.23 (m, 6H, ArH), 7.42e7.51 (m, 8H, ArH), 7.53e7.59 (m, 6H,
ArH), 8.04 (d, J ¼ 8.4 Hz, 4H, ArH), 8.33 (d, J ¼ 8.4 Hz, 4H, ArH). 13C
NMR (75 MHz, CDCl3):
d 13.3, 19.5, 32.6, 44.8, 123.6, 126.3, 126.7,
127.1,127.9,128.7,128.9,129.4,130.5, 130.8,131.0,134.2,134.7,138.2,
146.0, 164.2. ESI-MS (m/z): 771 [M þ H]þ. Anal. Calcd. (%) for
C52H46N6O$0.6H2O: C, 79.89; H, 6.09; N, 10.75. Found: C, 79.79; H,
6.37; N 10.29.
2.2.5.1. 4-(1-n-Butylbenzimidazol-2-yl)benzoic acid (5a). White
solid, yield 75%; m.p. 235e236 ꢀC. 1H NMR (300 MHz, DMSO-d6):
d
0.76 (t, J ¼ 7.2 Hz, 3H, CH3), 1.11e1.23 (m, 2H, CH2), 1.65e1.75 (m,
2.2.6.3. 2,5-Bis(4-(1-n-butylphenanthroimidazol-2-yl)phenyl)-1,3,4-
2H, CH2), 4.40 (t, J ¼ 7.2 Hz, 2H, NCH2), 7.42e7.51 (m, 2H, ArH), 7.81
(d, J ¼ 7.8 Hz,1H, ArH), 7.91 (t, J ¼ 7.5 Hz,1H, ArH), 7.99 (d, J ¼ 7.8 Hz,
2H, ArH), 13.39 (s, 1H, COOH). ESI-MS (m/z): 293 [M ꢁ H]ꢁ.
oxadiazole (6c). Yellowish solid, yield 15%; m.p. ＞300 ꢀC. 1H NMR
(300 MHz, CDCl3):
d
0.85 (t, J ¼ 6.6 Hz, 6H, CH3), 1.20e1.36 (m, 4H,
CH2), 1.89e2.06 (m, 4H, CH2), 4.70 (t, J ¼ 6.0 Hz, 4H, NCH2),
7.59e7.76 (m, 8H, ArH), 8.00 (d, J ¼ 7.5 Hz, 4H, ArH), 8.30 (d,
J ¼ 7.2 Hz, 2H, ArH), 8.39 (d, J ¼ 7.5 Hz, 4H, ArH), 8.72 (d, J ¼ 7.8 Hz,
2H, ArH), 8.80 (d, J ¼ 7.2 Hz, 2H, ArH), 8.86 (d, J ¼ 7.5 Hz, 2H, ArH).
2.2.5.2. 4-(1-n-Butyl-4,5-diphenylimidazol-2-yl)benzoic acid (5b).
Yellowish solid, yield 80%; m.p. 124e125 ꢀC (lit. [30] 124e126 ꢀC).
1H NMR (300 MHz, DMSO-d6):
d
0.53 (t, J ¼ 7.5 Hz, 3H, CH3),
13C NMR (300 MHz, CDCl3):
d 13.6, 19.7, 32.5, 47.1, 120.7, 122.5,
0.85e0.97 (m, 2H, CH2), 1.22e1.32 (m, 2H, CH2), 3.94 (t, J ¼ 7.5 Hz,
2H, NCH2), 7.09e7.21 (m, 3H, ArH), 7.39e7.57 (m, 7H, ArH), 7.88 (t,
J ¼ 8.4 Hz, 2H, ArH), 8.09 (d, J ¼ 8.1 Hz, 2H, ArH),13.10 (s,1H, COOH).
123.0, 123.3, 124.3, 124.4, 124.9, 125.6, 126.5, 126.8, 127.2, 127.3,
128.2, 129.3, 130.7, 134.4, 138.3, 151.2, 164.3. ESI-MS (m/z): 768
[M þ H]þ. Anal. Calcd. (%) for C52H42N6O: C, 81.44; H, 5.52; N, 10.96.
Found: C, 81.14; H, 5.64; N, 10.85.
13C NMR (75 MHz, DMSO-d6):
d 12.9, 18.8, 31.6, 44.0, 125.9, 126.0,
127.8, 128.4, 128.7, 128.8, 128.9, 129.4, 130.3, 130.4, 130.5, 130.6,
134.1, 134.8, 136.7, 166.6. ESI-MS (m/z): 395 [M ꢁ H]ꢁ.
Table 1
2.2.5.3. 4-(1-n-Butylphenanthrimidazol-2-yl)benzoic acid (5c).
Yellowish solid, yield 82%; m.p. 261e262 ꢀC. 1H NMR (300 MHz,
Crystal data and structure refinement for 6b.
Empirical formula
Formula weight
Temperature (K)
C55.50H54Cl4.50N6O2
996.57
110(2) K
DMSO-d6):
d
0.67 (t, J ¼ 7.5 Hz, 3H, CH3), 1.05e1.11 (m, 2H, CH2),
1.75e1.84 (m, 2H, CH2), 4.70 (t, J ¼ 6.6 Hz, 2H, NCH2), 7.62e7.71 (m,
3H, ArH), 7.74e7.81 (m, 1H, ArH), 7.94 (d, J ¼ 8.1 Hz, 2H, ArH), 8.18
(d, J ¼ 7.8 Hz, 2H, ArH), 8.42 (d, J ¼ 8.1 Hz, 1H, ArH), 8.59 (d,
J ¼ 7.8 Hz, 1H, ArH), 8.87 (d, J ¼ 8.4 Hz, 1H, ArH), 8.98 (d, J ¼ 8.1 Hz,
1H, ArH), 13.25 (s, 1H, COOH). ESI-MS (m/z): 393 [M ꢁ H]ꢁ.
Crystal system, space group
Unit cell dimensions
Monoclinic, C2/c
ꢀ
a (A)
46.662(6)
9.6597(12)
24.814(3)
90.000
116.996(2)
90.000
9966(2)
8
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
2.2.6. General procedure for the synthesis of 6aec
Compound 5a or 5b or 5c (2 mmol) was added to thionyl
chloride (caution: reacts violently with water; incompatible with
most common metals, strong reducing agents, strong bases,
alcohols; 15 mL), and the mixture was refluxed for 4 h. After
thionyl chloride was evaporated, compound 3 and anhydrous
pyridine (5 mL) was added. The mixture was refluxed for 20e24 h,
cooled and treated with water, and then the crude product
completely precipitated. The crude product was filtered, dried
completely and added to phosphorus oxychloride (10 mL), the
mixture was refluxed overnight. Most of the phosphorus oxy-
chloride was distilled from the reaction mixture, and the residue
was gently added to powdered ice. The oxadiazole was filtered,
washed with water, dried and recrystallized from the mixture of
chloroform and ethanol, then purified by silica-gel column chro-
matography using petroleum (b.p. 60e90 ꢀC)/ethyl acetate (2:1) as
eluent.
3
ꢀ
Volume (A )
Z
DCalc (Mg/m3)
Absorption coefficient (mmꢁ1
F (0 0 0)
1.328
0.314
4172
)
Crystal size (mm)
0.47 ꢂ 0.24 ꢂ 0.19
q
range for data collection (ꢀ)
0.98e26.00
Limiting indices
Reflections collected/unique
57 ꢃ h ꢃ 57, ꢁ11 ꢃ k ꢃ 7, ꢁ29 ꢃ l ꢃ 30
27 929/9727 [R(int) ¼ 0.0582]
99.4%
Completeness to theta ¼ 26.00
Refinement method
Full-matrix least-squares on F2
9727/0/632
Data/restraints/parameters
Absorption correction
Semi-empirical from equivalents
1.025
R1 ¼ 0.0769, wR2 ¼ 0.2055
R1 ¼ 0.1425, wR2 ¼ 0.2639
0.864 and ꢁ1.253
738503
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole (eA
CCDC
ꢀꢁ3
)

254
Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
Table 2
limits their use as intermediate in synthesis. But the solubility of
ꢀ
ꢀ
Selected bond lengths (A) and angles ( ) for 6b.
these compounds for common organic solvents could be improved
by introducing some alkyls into the molecules of these compounds.
Many methods were reported for the alkylation of imidazole
compounds, several deprotonation reagents, such as sodium,
potassium or sodium hydride [37,38], 50% sodium hydroxide in
DMF [39], solid potassium hydroxide [40], 1,8-diaza-bicyclo[5,4,0]
undec-7-ene [41] were tried in this work. However, theses reported
procedures were unsuitable for 1aec. We ever reported the alkyl-
ation reaction of imidazole compounds [30,31], using TBAB as
phase-transfer catalyst in butanone and using 50% sodium
hydroxide as base. At the beginning of this work, compound 3a was
prepared as the procedure in Fig. 3.
Our group attempted to obtain compound 2a not using 2d as
intermediate. In this process, a lot of alkylation reaction conditions
were tested by varying base reagents, reaction temperature and
reaction time. As shown in Fig. 1, this idea eventually could be
realized by replacement of 50% NaOH with 50% K2CO3 in alkylation
reaction shown in Fig. 3.
Bond lengths
N(2)eC(49)
C(5)eC(6)
C(19)eC(20)
C(27)eC(28)
C(35)eC(36)
1.464(6)
1.467(6)
1.473(6)
1.461(6)
1.473(6)
N(3)eC(4)
1.462(5)
1.469(6)
1.460(6)
1.474(6)
1.478(6)
C(12)eC(13)
C(23)eC(26)
C(31)eC(34)
C(42)eC(43)
Bond angles
N(1)eC(34)eC(31)
N(2)eC(34)eC(31)
N(3)eC(5)eC(6)
N(4)eC(12)eC(13)
N(5)eC(27)eC(28)
O(1)eC(26)eC(23)
C(4)eN(3)eC(5)
121.2(4)
126.9(4)
125.0(4)
121.1(4)
128.2(4)
118.5(4)
125.1(3)
121.8(4)
128.7(4)
121.9(4)
118.2(4)
119.7(4)
121.5(4)
118.1(4)
126.1(4)
122.3(4)
131.3(4)
121.5(4)
N(1)eC(35)eC(36)
N(2)eC(42)eC(44)
N(3)eC(19)eC(20)
N(4)eC(19)eC(20)
N(6)eC(26)eC(23)
O(1)eC(27)eC(28)
C(4)eN(3)eC(19)
C(5)eC(6)eC(11)
119.0(4)
122.5(4)
126.3(4)
123.3(4)
129.3(4)
119.2(4)
127.1(3)
120.0(4)
129.5(4)
119.4(4)
122.9(4)
120.7(4)
118.9(4)
122.9(4)
119.3(4)
131.7(4)
120.4(4)
125.5(4)
C(5)eC(6)eC(7)
C(5)eC(12)eC(13)
C(12)eC(13)eC(14)
C(19)eC(20)eC(21)
C(22)eC(23)eC(26)
C(27)eC(28)eC(33)
C(30)eC(31)eC(34)
C(34)eN(2)eC(49)
C(35)eC(36)eC(41)
C(42)eC(35)eC(36)
C(42)eC(43)eC(48)
C(6)eC(5)eC(12)
C(12)eC(13)eC(18)
C(19)eC(20)eC(25)
C(24)eC(23)eC(26)
C(27)eC(28)eC(29)
C(32)eC(31)eC(34)
C(35)eC(36)eC(37)
C(35)eC(42)eC(43)
C(42)eC(43)eC(44)
C(42)eN(2)eC(49)
It is noteworthy that such a procedure for alkylation reaction of
imidazole derivatives could be suitable to methyl 4-(4,5-dipheny-
limidazol-2-yl)benzoate and methyl 4-(phenanthroimidazol-2-yl)
benzoate, and this reaction could afford a satisfied yield (70e75%).
3.2. Crystal structure
2.3. Determination of crystal structure of 6b
As can be seen from Fig. 4 and from Table 1, the bond lengths of
C5eC6, C12eC13, C19eC20, C23eC26, C27eC28, C31eC34,
C35eC36, C42eC43, N3eC4, and N2eC49 are, as expected, shorter
than a normal carbonecarbon single bond and nitrogen-carbon
single bond due to a conjugation effect. The dihedral angle between
the N3/C5/C12/N4/C19 ring and C6/C7/C8/C9/C10/C11 ring is
60.24ꢀ, 34.73ꢀ for the C13/C14/C15/C16/C17/C18 ring, and 43.10ꢀ for
C20/C21/C22/C23/C24/C25 ring. The dihedral angle between the
N1/C34/N2/C42/C35 ring and C31/C32/C33/C28/C29/C30 ring is
41.55ꢀ, 23.72ꢀ for the C36/C37/C38/C39/C40/C41 ring, and 86.02ꢀ
for C43/C44/C45/C46/C47/C48 ring. The dihedral angle between the
O1/C26/N6/N5/C27 ring and the C20/C21/C22/C23/C24/C25 ring is
3.19ꢀ, and 12.66ꢀ for the ring C31/C32/C33/C28/C29/C30. It could be
found from all the dihedral data that compound 6b is not a coplanar
molecule.
The needle-shaped single crystals of 6b suitable to X-ray
structural analysis were obtained by evaporation of 50% ethanol
and chloroform solution. The diffraction data for structure was
collected with an Bruker Smart 1000 CCD X-ray single crystal
diffractometer using a graphite monochromated Mo Ka radiation
ꢀ
(l
¼ 0.71073 A) at 110(2) K. The structures were solved by direct
methods with SHELXS-97 program and refinements on F2 were
performed with SHELXL-97 program by full-matrix least-squares
techniques with anisotropic thermal parameters for the non-
hydrogen atoms. All H atoms were initially located in a different
Fourier map. A summary of the crystallographic data and structure
refinement details is given in Table 1, and the selected bond lengths
and angles are presented in Table 2. The crystal structure and cell
structure of 6b are shown in Figs. 4 and 5, respectively.
3.3. Spectral properties
3. Results and discussion
3.3.1. UVevisible absorption spectra
3.1. Synthesis
For UVevisible absorption measurements, the dye concentra-
tion was 1 ꢂ 10ꢁ6 M, and the absorption data are summarized in
Table 3. The UVevisible absorption spectra of compounds 4g and
4i are given as an example in Fig. 6. Several absorption peaks
could be observed in the linear absorption spectra of the two
molecules in the wavelength range from 237 to 407 nm, while
almost no linear absorption was observed beyond 425 nm. It can
Twelve aromatic substituted 1,3,4-oxadiazoles were successfully
obtained in 6e23% yields. The key stage in the synthesis of these
target molecules is the preparation of 2aec. Compounds 1aec
possess a larger conjugated-system structure, it is hard for these
compounds to be dissolved in most of common solvents, which
N
N
C4H9Br, 50% NaOH
TBAB, butanone
CO2CH3
CO2Na
1) HCl
N
N
H
Reflux
2) SOCl2, CH3OH
C4H9 2d
1a
N
N
N
N
NH2NH2 H2O
CH3OH, reflux
CONHNH2
CO2CH3
2a
C4H9
C4H9
3a
Fig. 3. The synthesis of compound 3a.

Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
255
Fig. 4. The molecular structures of 6b, H atoms and the molecular structures of solvents are omitted for clarity.
be seen that they display very similar absorptions and the strong
absorption band at 260 nm, which should originate from the
benzene ring and thiophene ring of the molecules. The relatively
weak absorption bands between 279 and 407 nm are assigned to
possesses a large-radius sulfur atom, which would make the elec-
tron-pair on the sulfur atom absorb lower energy to shift into p*
orbit from n orbit.
Interestingly, in the solid state, there is a consequence for the lex
values: 4i (420 nm) > 4h (399 nm) > 4g (394 nm) > 4f
(368 nm) > 4e (364 nm) > 4d (361 nm). However, for their lem
values, the order is 4g (487 nm) > 4i (478 nm) > 4h (462 nm) > 4f
(439 nm) > 4d (430 nm) > 4e (414 nm). This is due to the stronger
intermolecular associating interactions for dyes including thio-
phene ring, which contains a large-radius sulfur atom.
the
p-
p* electronic transition focusing on the conjugated 1,3,4-
oxadiazole, and phenanthroimidazole units.
3.3.2. Fluorescence spectra
The excitation and emission spectra of these compounds were
investigated in diluted dichloromethane as well as in the solid state
(Tables 3 and 4). The maximum wavelengths of their excitation and
emission in dichloromethane are longer than that in solid state,
respectively, because the chromophores are completely dispersed
in the dilute solution whereas among the chromophores in solid
state exist intermolecular associating interactions.
If a comparison is drawn among these compounds in dichloro-
methane, it can be seen from Table 3 that the excitation spectra and
emission spectra slightly shift to the longer wavelength with the
increase of conjugated-system. However, for the lex and lem values
3.3.3. Fluorescence quantum yields
Fluorescence quantum yields
(FF) were determined by
a comparative method [42,43] (Equation (1)), using coumarin 1
(purchased from SigmaeAldrich) as a standard sample with
FF ¼ 0.99 in ethyl acetate [44] as the reference.
FX,AS,n2X
FS,AX,n2S
FF
¼
FFðSÞ
(1)
of compounds 4dei, there is
a consequence: 4e (359 nm,
where Fx and FS are the areas under the fluorescence emission
curves of the sample and standard, respectively. Ax and AS are the
absorbance of the sample and standard, respectively; and nx and
445 nm) > 4f (358 nm, 444 nm) > 4d (355 nm, 441 nm); 4h
(369 nm, 448 nm) z 4i (368 nm, 448 nm) > 4g (367 nm, 446 nm),
respectively. This can be attributed to the thiophene ring which
Fig. 5. A packing diagram for 6b, H atoms and the molecular structures of solvents are omitted for clarity.

256
Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
Table 3
Table 4
The lmax, log
3,
lex,
lem, Stokes shift, FF values, and the lifetime of fluorescence of
The lex, lem, and the lifetime of fluorescence of some samples in solid sate.
some samples in dichloromethane.
Sample
lex/nm
lem/nm
Stokes shift/nm
T1/ns (Rel.%)
T2/ns (Rel.%)
Sample lmax/nm (log
3
)
lex
/
lem
/
Stokes
FF
T1/ns
4d
4e
4f
4g
4h
4i
6a
6b
6c
361
364
368
394
399
420
381
415
442
430
414
439
487
462
478
442
465
486
69
50
71
93
63
58
61
50
44
0.83 (87.92)
0.90 (73.09)
0.52 (78.25)
1.09 (75.97)
0.93 (86.45)
1.52 (75.21)
0.65 (83.05)
0.48 (95.97)
1.01 (84.63)
2.82 (12.08)
2.89 (26.91)
1.85 (21.75)
5.53 (24.03)
7.39 (13.55)
5.71 (24.79)
2.32 (16.95)
3.55 (4.03)
nm nm shift/nm
4d
4e
278 (4.51), 339 (4.43)
276 (4.48), 293 (4.37), 311 (4.36), 359 445 86
340 (4.42)
355 441 86
0.21 1.76
0.62 1.74
4f
4g
4h
294 (4.45), 308 (4.42), 340 (4.50) 358 444 86
260 (4.62), 335 (1.56), 363 (1.43) 367 446 81
260 (4.87), 308 (4.51), 340 (4.47), 369 448 79
363 (4.41)
0.33 1.65
0.61 1.84
0.75 1.78
6.78 (15.37)
4i
260 (4.75), 301 (4.43), 341 (4.39), 368 448 80
363 (4.36)
0.62 1.76
6a
6b
6c
331 (4.71)
273 (4.51), 352 (4.57)
260 (5.06), 347 (4.60), 366 (4.67) 374 451 77
346 407 61
369 445 76
0.63 0.99
0.69 1.56
0.78 1.64
Interestingly, compound 4f was more emissive than 4d
FF ¼ 0.33 and 0.21, respectively), and compound 4i was very
(
similar to 4g (FF ¼ 0.62 and 0.61, respectively). The reason is also
attributed to thiophene ring, which would make the electron-pair
in the highest occupied molecular orbit possess a lower energy;
the electron-pair could be excited easily to transit into a higher
orbit.
nS are the refractive indices of the solvents used for sample and
standard, respectively. Both the sample and the reference were
excited at the same wavelength. The absorbance of the solutions
at the excitation wavelength ranged between 0.04 and 0.05. [45]
The FF values for 4d, 4e, 4f, 4g, 4h, 4i, 6a, 6b and 6c were listed
in Table 3.
Concerning the fluorescence quantum yields, the compounds
can be obviously divided in two groups. The two-imidazole-
substituted compounds (6aec) exihited high fluorescence efficiency
3.3.4. Fluorescence lifetimes
The fluorescence lifetime values were determined [48e50] in
dichloromethane and in solid state, the obtained data were shown
in Tables 3 and 4. T1 means that lifetime of fluorescence diminished
in first-order-progression, T2 means that lifetime of fluorescence
diminished in second-order-progression, and the data in brackets is
the ratio of the corresponding decreasing. First-order-progression
of samples in dichloromethane was gained. However, both first-
order-progression and second-order-progression of samples in
solid state were obtained. We could find the following results from
Tables 3 and 4:
(
FF ¼ 0.63, 0.69, and 0.78, respectively), while the mono-imidazole-
substituted compounds (4dei) were less emissive. The main reason
for high fluorescence quantum yields of these reported compounds
is that imidazole structure (1-n-butylbenzimidazolyl,1-n-butyl-4,5-
diphenylimidazol, and 1-n-butylphenanthroimidazolyl) restricts
the free rotation between benzene ring and oxadiazole [46,47].
Otherwise, it can be found that the FF values for 6aec increase
with the increase of conjugated-system. Compound 6c exhibited
higher fluorescence efficiency than 6b because Compound 6c
possesses a larger conjugated-system structure than 6b does.
Actually, this deduction could be confirmed by X-ray diffraction
data. The three ring planes: N1/C34/N2/C42/C35 ring, C36/C37/C38/
C39/C40/C41 ring, and C43/C44/C45/C46/C47/C48 ring are not
coplanar. This occurs to the other three planes: N3/C5/C12/N4/C19
ring, C6/C7/C8/C9/C10/C11 ring, and C20/C21/C22/C23/C24/C25
ring.
(1) In solid state, for 4dei and 6aec, the T1 values are all smaller
than their corresponding T2 value.
(2) Comparing the T1 values of these compounds in dichloro-
methane with T1 values of these corresponding compounds in
solid state, respectively, we found that the T1 values in
dichloromethane are generally larger than that in solid state.
In addition, some samples showed a shorter fluorescence life-
time in solution and some samples could show a longer fluores-
cence lifetime in solid state. For example, the lifetime of 6a is
0.99 ns in dichloromethane, and the T2 value of 4h is 7.39 ns in solid
state.
0.10
4g
4i
3.4. Thermal stability
0.08
An organic phosphor applied in the fabricated light-emitting
diodes (LEDs) and OLEDs is required to possess a high thermal
stability. In order to investigate the thermal stability of the
synthesized compounds, the thermogravimetric analysis (TGA) and
differential thermogravimetric (DTG) techniques were employed
and nine samples (4dei, and 6aec) were measured (see Supporting
information), and the decomposition temperature and thermal
weightlessness percentage for these nine samples are listed in
Table 5.
0.06
0.04
0.02
0.00
It could be found from Table
5 that the decomposition
temperature for nine samples are all more than 400 ꢀC, and the
starting temperature for thermal weightlessness for nine samples is
more than 165 ꢀC, which demonstrate that these samples are
thermally stable to be used in LEDs and OLEDs.
250
300
350
400
450
500
Wavelength (nm)
Fig. 6. UVevis absorption spectra of compounds 4g and 4i in dichloromethane.

Y.-N. Yan et al. / Dyes and Pigments 86 (2010) 249e258
257
Table 5
The decomposition temperature and thermal weightlessness percentage for 4dei, and 6aec.
Sample
4d
4e
4f
4g
4h
4i
6a
6b
6c
480.9
4.34
236.7
Decomposition temperature (ꢀC)
Weightlessness (%)
Weightlessness temperature (ꢀC)
426.2
6.13
176.5
462.2
1.87
251.7
443.3
1.90
241.3
457.0
20.19
326.2
474.8
7.77
319.5
459.6
2.55
267.7
442.3
1.37
252.5
479.5
8.39
301.4
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4. Conclusions
The approach reported here for the alkylation of methyl 4-
(benzimidazol-2-yl)benzoate derivatives could be an efficient
method, especially for industry, as the reaction condition is mild,
the yield is satisfied, solvent could be recycled, the product is easy
to be purified.
The procedures reported here could afford an efficient route to
introduce imidazole units to 2-position or 5-position of 1,3,4-oxa-
dizole ring, which could provide an attractive and even an efficient
approach to the preparation of some useful dyes and pigments.
Herein, 1,3,4-oxadiazole derivatives containing imidazole unit
could possess a medium strong fluorescence-emitting ability with FF
values in the region of 0.21e0.78. Otherwise, TGA shows that these
nine compounds are thermally stable to be used in LEDs and OLEDs.
However, it should be pointed out that more experiments must be
carried out to estimate their potential application in LEDs and OLEDs.
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and X-ray crystal structure of novel 1,3,4-oxadiazole derivatives carrying
a thiophene unit. Dyes and Pigments 2009;80:163e7.
5. Supplementary material
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The crystallographic data (excluding structure factors) of 6b
have been deposited with the Cambridge Crystallographic Center as
supplementary publication no. CCDC 738503. Copy of this infor-
mation may be obtained free of charge via www: http://www.ccdc.
cam.ac.ukor from The Director, CCDC, 12 Union Road, Cambridge
CB221EZ, UK (fax: þ44 1223 336033; email: deposit@ccdc.cam.ac.
uk). Structural factors are available on request from the authors.
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blend of two blue luminescent materials. Applied Surface Science 2008;254
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Synthesis of multi-functional POSS emitters for OLED applications. Chemistry
of Materials 2007;19(20):4991e7.
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Acknowledgments
This project is supported by Scientific and Technical Project
Foundation of Guangdong Province (2003C103006 and
2007B060401068).
[21] Lee JH, Tsai HH, Leung MK, Yang CC, Chao CC. Phosphorescent organic light-
emitting device with an ambipolar oxadiazole host. Applied Physics Letters
2007;90(24):243501/1e243501/3.
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quantum well structure on the characteristics of OLEDs. Displays 2007;28
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