Synthesis and electronic absorption and fluorescence of 2-arylbenzothiazole derivatives

Article abstract of DOI:10.1016/j.saa.2006.11.038

A series of new 2-arylbenzothiazoles have been prepared in high yields by Jacobson's cyclization condensation of 2-aminobenzenethiol with benzoyl chloride or benzaldehyde derivatives under three different routes. These compounds have been fully characterized by EA, IR, NMR and MS. The electronic absorption and fluorescence of these compounds have been systematically investigated for the first time. The relationships between their photophysical properties and structures have been discussed. The alteration of absorption and emission wavelengths can be elucidated by Hammett's substituent constants.

Full text of DOI:10.1016/j.saa.2006.11.038

Spectrochimica Acta Part A 68 (2007) 317–322
Synthesis and electronic absorption and fluorescence
of 2-arylbenzothiazole derivatives
Lianqing Chen, Chuluo Yang∗, Suyue Li, Jingui Qin∗
Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials,
Wuhan University, Wuhan 430072, China
Received 3 September 2006; accepted 27 November 2006
Abstract
A series of new 2-arylbenzothiazoles have been prepared in high yields by Jacobson’s cyclization condensation of 2-aminobenzenethiol with
benzoyl chloride or benzaldehyde derivatives under three different routes. These compounds have been fully characterized by EA, IR, NMR and
MS. The electronic absorption and fluorescence of these compounds have been systematically investigated for the first time. The relationships
between their photophysical properties and structures have been discussed. The alteration of absorption and emission wavelengths can be elucidated
by Hammett’s substituent constants.
© 2006 Elsevier B.V. All rights reserved.
Keywords: 2-Arylbenzothiazole; Structure; Electronic absorption; Fluorescence; Hammett’s substituent constants
1. Introduction
tives. Understanding the relationship will be benefit to design
new fluorescent agent and benzothiazole-based ligands to tune
luminescent properties of their corresponding complexes.
In this article, a series of 2-arylbenzothiazole compounds
have been synthesized by condensation of 2-aminobenzenethiol
with benzoyl chloride derivatives or benzaldehyde derivatives,
and fully characterized by EA, NMR and MS. We will discuss
how the substitutes with different electronic effect or position
or number on the aryl ring of 2-arylbenzothiazole tune their
electronic absorption and photoluminescent properties.
Benzothiazole derivatives have been well known for their
biological and pharmaceutical activities, such as antitumor,
antiviral, antimicrobial activities and potent inhibitory activity
[1–3]. Recently, benzothiazole derivatives have also attracted
increasing attention due to their application in the area of organic
optoelectronic materials, such as second-order nonlinear optical
(NLO)materials[4], two-photoabsorption(TPA)chromophores
[5], photoconducting materials [6], liquid crystals [7] and flu-
orophores [8]. Most recently, 2-arylbenzothiazoles have been
used as cyclometalated ligands for heavy metal ions, such as Ir
[9] and Pt [10], because of their strong chelating capability, and
these complexes have been proven to be good phosphorescent
dyes for organic light-emitting diodes (OLEDs).
2. Experimental
2.1. General information
Though benzothiazole derivatives have been widely inves-
tigated, surprisingly, the studies on their photoluminescent
properties are very limited [8]. Moreover, to our knowledge,
there are not any systematic surveys on the relationship between
their structure and photophysical properties, which are closely
related with the electronic characters of benzothiazole deriva-
Melting points (uncorrected) were performed on WRS-2
Melting Point apparatus. 1H NMR and 19F NMR were recorded
on Varian Mercury VX-300 MHz spectrometer in CDCl3.
Infrared spectra were obtained on a Nicolet SX Fourier trans-
form spectrometer. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on a Carlorerba-1106 microanalyzer.
Mass Spectra (FAB-MS) were determined by VJ-ZAB-3F Mass
Spectrometer. UV–vis absorption spectra were recorded on
Schimadzu 160A spectrophotometer. PL spectra were recorded
on Perkin-Elmer LS 55 luminescence spectrophotometer.
∗
Corresponding authors. Tel.: +86 27 68756757; fax: +86 27 68756757.
E-mail address: clyang@whu.edu.cn (C. Yang).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.11.038

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L. Chen et al. / Spectrochimica Acta Part A 68 (2007) 317–322
All silica gel column chromatography was performed
2-(4-Chlorophenyl)-benzothiazole 4-Cl-bt (5): white crys-
tals, yield: 90%. H NMR: 8.03 (t, J = 8.4 Hz, 3H), 7.89 (d,
1
with use of silica gel (200–300 mesh). The intermediates,
i.e. 4-cyanobenzoyl chloride [11], 4-iodobenzaldehyde [12],
3-bromobenzaldehyde [13], 2-methoxybenzaldehyde [14], 3-
methoxybenzaldehyde [15], 2,4-dimethoxybenzaldehyde [16]
and 2,4,5-trimethoxybenzaldehyde [17] were prepared accord-
ing to literature procedure. Other materials were purchased and
used without further purification.
J = 8.1 Hz, 1H), 7.48 (q, J = 7.2 Hz, 3H), 7.38 (t, J = 7.5 Hz, 1H).
mp = 116–117 ◦C. Anal. Calcd. for C13H8NClS: C, 63.54; H,
3.28; N, 5.70. Found: C, 63.45; H, 3.19; N, 5.64%. MS (FAB):
m/e, 245 (M+).
2-(4-Bromophenyl)-benzothiazole 4-Br-bt (6): light green
needle crystals yield: 92%. 1H NMR: 8.04 (d, J = 7.8 Hz, 1H),
7.93 (m, 3H), 7.60 (d, J = 8.4 Hz, 2H), 7.44 (t, J = 8.4 Hz, 1H),
7.38 (t, J = 8.1 Hz, 1H). mp = 120–121 ◦C. Anal. Calcd. for
C13H8NBrS: C, 53.81; H, 2.78; N, 4.83. Found: C, 53.65; H,
2.92; N, 4.96%. MS (FAB): m/e, 289 (M+).
2.2. Preparation of 1–3
General procedure: 2-aminobenzenethiol (10 mmol) was
dissolved into 20 ml of N-methyl-pyrrolidinone under argon
atmosphere, and then the corresponding acid chloride (15 mmol)
was slowly added at room temperature. The mixture was heated
at 100 ◦C for 4 h. After cooling, the solution was poured into ice
water and the solution was adjusted to pH 8–9 with 7N aque-
ous ammonia. A white precipitate was separated out and the
crude product was filtered, washed with water several times,
and further purified by column chromatography.
2-(4-Fluorophenyl)-benzothiazole 4-F-bt (1), white solid,
yield:93%. 1HNMR: 8.05 (m, 3H), 7.88 (d, J = 7.8 Hz, 2H), 7.48
(t, J = 7.5 Hz, 1H), 7.38 (t, J = 7.5 Hz, 1H), 7.17 (t, J = 8.4 Hz,
1H). 19F NMR: −109.3. mp = 99–100 ◦C. Anal. Calcd. for
C13H8NFS: C, 68.10; H, 3.52; N, 6.11. Found: C, 68.20; H,
3.53; N, 6.23%. MS (FAB): m/e, 229 (M+).
2-(4-Iodophenyl)-benzothiazole 4-I-bt (7): green crystals,
yield: 86%. 1H NMR: 8.04 (t, J = 8.1 Hz, 3H), 7.90 (d, J = 7.8 Hz,
1H), 7.48 (m, 3H), 7.39 (t, J = 8.1 Hz, 1H). mp = 134–135 ◦C.
Anal. Calcd. for C13H8NIS: C, 46.31; H, 2.39; N, 4.15. Found:
C, 46.35; H, 2.56; N, 4.06%. MS (FAB): m/e, 337 (M+).
2-(3-Bromophenyl)-benzothiazole 3-Br-bt (8): green crys-
1
tals, yield: 82%. H NMR: 8.27 (s, 1H), 8.07 (d, J = 8.1 Hz,
1H), 7.98 (d, J = 8.1 Hz, 1H), 7.90 (d, J = 7.2 Hz, 1H), 7.61
(d, J = 7.2 Hz, 1H), 7.51 (t, J = 7.5 Hz, 1H), 7.38 (m, 2H).
mp = 124–126 ◦C. Anal. Calcd. for C13H8NBrS: C, 53.81; H,
2.78; N, 4.83. Found: C, 53.75; H, 2.89; N, 4.86%. MS (FAB):
m/e, 289 (M+).
2.4. Preparation of 9–13
2-(4-Cyanophenyl)-benzothiazole 4-CN-bt (2): light yellow
1
solid, yield: 91%. H NMR: 8.06 (d, J = 7.5 Hz, 1H), 7.96 (d,
General procedure: 2-aminothiophenol (10 mmol) and
methoxy substituted benzaldehyde (10 mmol) and p-
toluenesulfonic acid monohydrate (PTSA) (1.0 mmol) were
refluxed in chloroform (20 ml) under nitrogen for 24 h. After
cooling, the mixture was extracted with water as well as ether
and dried with Na2SO4. After removal of solvent, the crude
product was recrystallized with ethanol to give pure product.
2-(2-Methoxyphenyl)-benzothiazole 2-MeO-bt (9): light
J = 8.4 Hz, 2H), 7.90 (d, J = 7.5 Hz, 1H), 7.63 (d, J = 8.4 Hz, 2H),
7.50 (t, J = 7.5 Hz, 1H), 7.41 (t, J = 6.9 Hz, 1H). IR (KBr, cm−1):
2206. (s, –CN). mp = 100–101 ◦C. Anal. Calcd. for C14H8N2S:
C, 71.16;H, 3.41;N, 11.86. Found:C, 71.08;H, 3.45;N, 11.67%.
MS (FAB): m/e, 236 (M+).
2-(4-tert-Butylphenyl)-benzothiazole 4-tBu-bt (3): white
1
solid, yield: 87%. H NMR: 8.02 (m, 3H), 7.82 (d, J = 7.2 Hz,
1
1H), 7.43 (m, 3H), 7.29 (t, J = 7.5 Hz, 1H), 1.30 (s, 9H).
mp = 106–107 ◦C. Anal. Calcd. for C17H17NS: C, 76.36; H,
6.41; N, 5.24. Found: C, 76.24; H, 6.42; N, 5.26%. MS (FAB):
m/e, 267 (M+).
green crystals, yield: 85%. H NMR: 8.52 (d, J = 8.1 Hz, 1H),
8.08 (d, J = 8.1 Hz, 1H), 7.91 (d, J = 7.8 Hz, 1H), 7.46 (m,
2H), 7.36 (t, J = 7.5 Hz, 1H), 7.10 (m, 2H), 4.06 (s, 3H).
mp = 118–119 ◦C. Anal. Calcd. for C14H11ONS: C, 69.68; H,
4.59; N, 5.90. Found: C, 69.72; H, 4.63; N, 6.01%. MS (FAB):
m/e, 241 (M+).
2.3. Preparation of 4–8
2-(4-Methoxyphenyl)-benzothiazole 4-MeO-bt (10): white
crystals, yield: 86%. 1H NMR: 8.03 (d, J = 7.2 Hz, 3H), 7.86 (d,
J = 8.1 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.34 (t, J = 7.8 Hz, 1H),
7.00 (d, J = 8.1 Hz, 2H), 3.88 (s, 3H). mp = 122–123 ◦C. Anal.
Calcd. for C14H11ONS: C, 69.68; H, 4.59; N, 5.80. Found: C,
69.61; H, 4.45; N, 5.71%. MS (FAB): m/e, 241 (M+).
2-(3-Methoxyphenyl)-benzothiazole 3-MeO-bt (11): white
General procedure: 2-aminothiophenol (10 mmol) and the
corresponding benzaldehyde (10 mmol) were dissolved to 20 ml
of DMSO under argon atmosphere. The mixture was heated at
200 ◦C for 0.5 h. After cooling, the solution was poured into
ice water, and then adjusted the solution to pH 8–9 with 1N
NaHCO3 solution. The precipitate was filtered, washed with a
great deal of water several times. After dried under vacuum, the
crude product was recrystallized with ethanol.
1
crystals, yield: 82%. H NMR: 8.06 (d, J = 7.8 Hz, 1H), 7.90
(d, J = 7.8 Hz, 1H), 7.65 (m, 2H), 7.49 (t, J = 7.2 Hz, 1H),
7.39 (t, J = 7.2 Hz, 2H), 7.04 (d, J = 7.8 Hz, 1H), 3.92 (s, 3H).
mp = 120–121 ◦C. Anal. Calcd. for C14H11ONS: C, 69.68; H,
4.59; N, 5.80. Found: C, 69.57; H, 4.56; N, 5.76%. MS (FAB):
m/e, 241 (M+).
2-(2,4-Dimethoxyphenyl)-benzothiazole 2,4-MeO-bt (12):
yellow green crystals, yield: 87%. 1H NMR: 8.45 (d, J = 8.1 Hz,
1H), 8.02 (d, J = 8.4 Hz, 1H), 7.87 (d, J = 8.1 Hz, 1H), 7.45 (t,
2-(4-tolyl)-benzothiazole 4-Me-bt (4): white crystals, yield:
1
89%. H NMR: 8.02 (d, J = 8.1 Hz, 1H), 7.95 (d, J = 8.1 Hz,
2H), 7.85 (d, J = 8.1 Hz, 1H), 7.45 (t, J = 7.5 Hz, 1H), 7.33
(t, J = 7.2 Hz, 1H) 7.26 (d, J = 8.1 Hz, 2H), 2.42 (s, 3H).
mp = 85–86 ◦C. Anal. Calcd. for C14H11NS: C, 74.63; H, 4.92;
N, 6.22. Found: C, 74.45; H, 4.82; N, 6.24%. MS (FAB): m/e,
225 (M+).

L. Chen et al. / Spectrochimica Acta Part A 68 (2007) 317–322
319
availability of precursors, three synthetic routes were adopted
as shown in Schemes 1–3.
In route A, 4-substituted benzoic acid was converted to
corresponding acid chlorides, which subsequently underwent
condensation with 2-aminobenzenethiol to afford the desired
compounds 1–3. The active acid chlorides make the reaction
proceed under mild condition. The solvent of N-methyl-
pyrrolidinone (NMP) provides a weak basic circumstance,
which serves for the absorption reagent of hydrogen chloride
produced during the reaction. It is noteworthy that the route A
avoids the use of expensive 4-substitued benzaldehyde deriva-
tives, such as 4-fluoro-benzaldehyde, 4-cyano-benzaldehyde
and 4-tert-butyl-benzaldehyde.
Scheme 1. Synthesis of 2-arylbenzothiazoles 1–3 by route A.
In route B, the easy available substituted benzaldehydes
were used as the precursors for cyclization reaction with 2-
aminobenzenethiol. The solvent of dimethyl sulfoxide (DMSO)
can provide not only the high temperature that speeds the
reaction, but also its bibulous capability promotes the reac-
tion process. The two factors make the reaction complete very
quickly.
In route C, 1,3-benzenediol was converted to 2,4-dihydroxy-
benzaldehyde via Vilsmeier reactions, which then underwent
bromination to give 5-bromo-2,4-dihydroxy-benzaldehyde. The
bromine was subsequently replaced by methoxy under the
CuCl as catalyst to afford the intermediate 2,4-dihydroxy-5-
methoxy-benzaldehyde. All hydroxyls of these compounds were
methylated by use of dimethyl sulfate. The methoxyl-substituted
benzaldehydes finally reacted with 2-aminobenzenethiol under
the p-toluenesulfonic acid (PTSA) as catalyst to afford the
desired products 9–13.
J = 7.2 Hz, 1H), 7.32 (t, J = 7.2 Hz, 1H), 6.65 (d, J = 8.7 Hz, 1H),
6.56 (s, 1H), 4.01 (s, 3H), 3.86 (s, 3H). mp = 139–140 ◦C. Anal.
Calcd. for C15H13O2NS: C, 66.40; H, 4.83; N, 5.16. Found: C,
66.32; H, 4.76; N, 5.02%. MS (FAB): m/e, 271 (M+).
2-(2,4,5-Trimethoxyphenyl)-benzothiazole 2,4,5-MeO-bt
1
(13): green crystals, yield: 86%. H NMR: 8.05 (t, J = 7.2 Hz,
2H), 7.88 (d, J = 7.8 Hz, 1H), 7.45 (t, J = 7.5 Hz, 1H), 7.33 (t,
J = 7.2 Hz, 1H), 6.61 (s, 1H), 4.04 (s, 3H), 4.01 (s, 3H), 3.96
(s, 3H). mp = 198–199 ◦C. Anal. Calcd. for C16H15O3NS: C,
63.77; H, 5.01; N, 4.65. Found: C, 63.66; H, 4.96; N, 4.62%.
MS (FAB): m/e, 301 (M+).
3. Results and discussion
3.1. Synthesis and characterization
A lot of synthetic methods have been developed for preparing
the benzothiazole derivatives, such as solid or liquid-phase com-
binatorial synthesis [18], C–C coupling reaction [19], radical
cyclization [20] and microwave mediated reaction [21], etc. In
this study, the 2-arylbenzothiazoles were synthesized by Jacob-
son’s cyclization reaction of 2-aminobenzenethiol with benzoyl
chloride or benzaldehyde derivatives [22]. In view of the easy
Elemental analyses of the compounds are consistent with the
expected formulation of their structures. The mass spectra all
give corresponding molecular ion peaks, respectively. 1H NMR
spectra show well-resolved multilets for the aryl protons. The
protons of methoxyl appear a single peak at about 4.0 ppm. 19
F
NMR of 4-F-bt (1) shows a single peak at −109.3 ppm. 4-CN-bt
Scheme 2. Synthesis of 2-arylbenzothiazoles 4–8 by route B.

320
L. Chen et al. / Spectrochimica Acta Part A 68 (2007) 317–322
Scheme 3. Synthesis of 2-arylbenzothiazoles 9–13 by route C.
(2) displays a characteristic CN infrared stretching vibration at
2206 cm−1
maximum shifts hyposochromically, while the electron-
accepting CN group at the 4-position shifts the absorption
maximum towards the red (refer to the corresponding data for
compounds 4-MeO-bt (10), 4-tBu-bt (3), 4-Me-bt (4), 4-CN-
bt (2) in Table 1). Similarly, the maximum absorption peak
of stronger electron-donating methoxy at 3-position (3-MeO-bt
(11)) blue shifts 12 nm comparing to the electron-withdrawing
bromine at 3-position (3-Br-bt (8)). When changing the posi-
tion of substitute from 4-position to 3- or 2-position, a marked
bathochromic shift of the maximum absorption wavelengths is
observed (refer to the corresponding data for compounds 4-
.
3.2. Electronic absorption spectra
Figs. 1 and 2 show the absorption spectra of 2-
arylbenzothiazoles in the solution of dichloromethane. All
compounds show a wide absorption band in the region
of 290–350 nm, with molar extinction coefficient at about
10,000 mol−1 cm−1. With increasing the electron-donating abil-
ity of the 4-substituents on the phenyl ring, the absorption
Fig. 1. Absorption spectra of 2-arylbenzothiazoles with different electronic
effect of substitutes.
Fig. 2. Absorption spectra of 2-arylbenzothiazoles with different positions and
number of substitutes.

L. Chen et al. / Spectrochimica Acta Part A 68 (2007) 317–322
321
Table 1
Photophysical data of 2-arylbenzothiazole derivatives
Compounds
λmax (Abs)a
λmax (Em) (nm)
Stoke’s shift (nm)
Hammett’s substituent constants, σ
Φf (%)c
nm
log ε
Solutiona
Filmb
4-MeO-bt (10)
4-tBu-bt (3)
4-Me-bt (4)
4-F-bt (1)
4-Cl-bt (5)
4-Br-bt (6)
4-I-bt (7)
4-CN-bt (2)
3-Br-bt (8)
2-MeO-bt (9)
3-MeO-bt (11)
2,4-MeO-bt (12)
2,4,5-MeO-bt (13)
295
298
307
300
303
306
309
322
333
308
321
338
347
4.0
3.9
3.8
4.1
3.8
4.3
3.9
3.9
4.0
4.2
3.8
4.1
4.3
355
357
360
368
371
375
382
419
400
367
380
389
401
358
360
362
371
376
379
386
421
405
372
385
391
422
60
59
53
68
68
69
73
97
67
59
59
51
54
−0.28
−0.19
−0.17
0.062
0.227
0.232
0.28
22.6
22.4
21.0
24.1
26.4
33.6
38.2
25.5
30.1
23.2
20.1
18.5
16.3
0.66
0.39
0.11
a
In CH2Cl2 solution at 298 K.
In PMMA film (5% weight ratio).
b
c
Quantum yield of fluorescence was measured in CH2Cl2 solution relative to quinine bisulfate (10−5 M in 1.0N H2SO4 Φf = 0.546, as standard).
MeO-bt (10), 2-MeO-bt (9), 3-MeO-bt (11), 4-Br-bt (6) and
3-Br-bt (8) in Table 1). For example, by changing the positions
of methoxy substitute according to the order of 4-, 2-, and 3-
position, the absorption peaks bathochromic shift 13 and 26 nm,
respectively. It is obvious that the meta substitutes with respect
to benzothiazole ring result a larger red shift on the absorption
maximum than the ortho substitutes. This may be related to the
electron-deficient property of benzothiazole moiety. The num-
bers of substitutes have also effect on their absorption maximum.
The three methoxy substituted compound exhibits the maximum
absorption peak at 347 nm.
yields in solutions at room temperature, and large Stoke’s shifts
(50–97 nm) (Table 1). All compounds exhibit a broad structure-
less emission in the range 355–419 nm. Their emission spectra
show the same trends with the absorption spectra when altering
the kinds, positions or numbers of substitutes. On changing the
kinds of 4-substituted group on the phenyl, the emission peaks
hypsochromically shift with increasing the electron-donating
ability of substitutes. 4-MeO-bt (10) shows emission peak at
355 nm, blueshifting64 nmrelatedtothe4-CN-bt(2). Bychang-
ing the position of methoxy substitutes, according to the order
of 3-, 2-, and 4-position, a systematically hypsochromic shift
(∼25 nm) of the emission peaks is observed. As concerning the
numbers of substitutes, the more the substitutes are, the lower
energy emission occurs. The emission of these compounds in
PMMA film displayed a little bit red shift comparing to their
emission in solution, except the 2,4,5-MeO-bt (13), which has
an significantly larger red shift (21 nm), presumably due to the
strong ␲–␲ stacking interactions in its solid structure.
3.3. Fluorescent properties
The photoluminescence spectra of 2-arylbenzothiazoles were
measured in CH2Cl2 solution (Figs. 3 and 4) and in PMMA
(polymethylmethacrylate) film (5% weight ratio) at 298 K,
respectively. All compounds have good fluorescence quantum
Fig. 3. PL spectra of 2-arylbenzothiazoles with different electronic effect of
substitutes.
Fig. 4. PL spectra of 2-arylbenzothiazoles with different positions and number
of substitutes.

322
L. Chen et al. / Spectrochimica Acta Part A 68 (2007) 317–322
3.4. Relationship between photophysical properties and
structures
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order: –CN > –I > –Br > –Cl > –F > –Me > –tBu > –MeO, which
are correlated well with the maximum wavelengths of both elec-
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the larger the maximum absorption and emission are (refer to the
corresponding data for compounds 4-MeO-bt (10), 3-MeO-bt
(11), 4-Br-bt (6), 4-Br-bt (8) in Table 1).
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4. Conclusions
In summary, we have synthesized a series of 2-
arylbenzothiazoles by use of Jacobson’s cyclization of
2-aminobenzenethiol with benzoyl chloride or benzaldehyde
derivatives in view of the easy availability of the precursors.
The studies revealed that the maximum absorption and emis-
sion peaks could be systematic tuned by changing the kinds,
positions and/or numbers of substituted groups on the phenyl
ring of 2-arylbenzothiazoles. The alterations of absorption and
emission wavelengths coincide well with the order of Hammett’s
substituent constants. This provides some clue to design and
develop more fluorescent materials and novel cyclometalated
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Acknowledgments
We are grateful for financial support from the National Natu-
ral Science Foundation of China (20371036 and 20474047), the
program for New Century Excellent Talents in University of the
Ministry of Education of china, and the Hubei Province Science
Fund for Distinguished Yong Scholars (No. 2003ABB008).
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