Comparisons between azo dyes and Schiff bases having the same benzothiazole/phenol skeleton: Syntheses, crystal structures and spectroscopic properties

Article abstract of DOI:10.1016/j.dyepig.2011.09.008

Three pairs of heterocyclic azo dyes and their corresponding Schiff bases were prepared by diazotization and Schiff-base condensation reactions between substituted 2-aminobenzothiazoles and either 3-(diethylamino)phenol or 3,5-dichloro-2-hydroxybenzaldehy

Full text of DOI:10.1016/j.dyepig.2011.09.008

Dyes and Pigments 92 (2012) 916e922
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Comparisons between azo dyes and Schiff bases having the same benzothiazole/
phenol skeleton: Syntheses, crystal structures and spectroscopic properties
*
Tao Tao, Feng Xu, Xiao-Chun Chen, Qian-Qian Liu, Wei Huang , Xiao-Zeng You
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2011
Received in revised form
9 September 2011
Accepted 12 September 2011
Available online 17 September 2011
Three pairs of heterocyclic azo dyes and their corresponding Schiff bases were prepared by diazo-
tization and Schiff-base condensation reactions between substituted 2-aminobenzothiazoles and
either 3-(diethylamino)phenol or 3,5-dichloro-2-hydroxybenzaldehyde in order to obtain some high
performance Disperse Red dyes and compare the structural and spectral differences between the azo
dyes and Schiff bases. All azo dyes and Schiff bases in this work have the same benzothiazole/phenol
skeleton but different substituent group in the phenyl ring. X-ray single-crystal diffraction analyses
of selected compounds reveal that they have a similar planar conformation between the benzo-
thiazole and phenol units but dissimilar dimeric crystal packing. Electronic spectra of the dyes
demonstrate that the presence of N]N and C]N double bond chromophore units as well as
substituent effects of different auxochrome groups in the benzothiazole backbone leads to significant
alterations of bathochromic and hypsochromic shifts despite only slight differences in their molec-
ular structures.
Keywords:
Benzothiazole
Azo dyes
Schiff bases
Electronic spectra
Crystal structures
pep Stacking interactions
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
because of the low band gap of pep* transition in azo dye
systems. On the other hand, molecular conformation and
packing fashion of these two kinds of compounds in the solid
state and in the bulk, which may be influenced by the supra-
molecular interactions among molecules, are of great signifi-
cance and worth discussing [9e12].
Azo-functionalized dyes bearing aromatic heterocyclic compo-
nents [1] have attracted ever increasing attention in recent years
because of their wide applications in metallochromic indicators [2],
optical data storage [3], non-linear optics [4], and dye-sensitized
solar cells [5]. Specifically, aromatic azo derivatives containing
In our previous work, several Disperse Yellow azo dyes
crystallizing in the hydrazone form in the solid state and their azo-
hydrazone tautomerisms have been investigated [13e17]. At the
same time, we have studied four heterocyclic Disperse Red azo dyes
having the same benzothiazole/azo/benzene skeleton in crystallo-
graphic terms [18]. As an extensive study in this area, we report
herein three pairs of heterocyclic azo dyes and their corresponding
Schiff bases 1e6 bearing the same benzothiazole/phenol skeleton.
Among them, single-crystal structures of compounds 2 and 5 are
included for comparison. They have an electron-donating methyl
group (1 and 4) or an electron-withdrawing nitro group (3 and 6) as
well as an electron-donating p-positioned diethylamino unit (1e3)
or an electron-withdrawing dichloro unit (4e6) in their molecular
structures. The aim of combing various donor-acceptor substituents
is to finely tune their electronic structures and compare the spec-
troscopic properties between azo dyes and Schiff bases bearing
similar aromatic moieties linked by nitrogen-nitrogen and carbon-
nitrogen double bonds.
intramolecular De
excellent photo-physical properties [6e8] since they have exten-
sive -systems delocalized between the acceptor and donor units
peA charge-transfer chromophores show
p
across the azo linkage.
It is also known that amino groups can react reversibly with
aldehyde groups to form Schiff bases having conjugated
carbon-nitrogen double bonds. Comparisons between the
nitrogenenitrogen double bonds in azo dyes with the
carbon-nitrogen double bonds in Schiff bases appear to be quite
interesting from structures to related properties. On the one
hand, electronic spectra of azo dyes show that they have
maximum absorptions (lmax) at red or near infrared wavelengths
with extraordinarily large molar absorption coefficient
(x)
* Corresponding author. Tel.: þ86 25 83686526; fax: þ86 25 83314502.
E-mail address: whuang@nju.edu.cn (W. Huang).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.008

T. Tao et al. / Dyes and Pigments 92 (2012) 916e922
917
2. Experimental section
(500 MHz, CDCl3, ppm):
d
14.05 (s, 1H, OH), 8.70 (d, 1H, benzo), 8.28
(s, 1H, benzo), 7.96 (d, 1H, benzo), 7.50 (d, 1H, phenol), 6.54 (dd, 1H,
phenol), 6.13 (d, 1H, phenol), 3.53 (q, 4H, CH2), and 1.25 (t, 6H,CH3);
2.1. Materials and measurements
Main FTeIR absorptions (KBr pellets, n
, cmꢀ1): 3410 (vs), 1640 (m),
All melting points were measured without correction. The
reagents of analytical grade were purchased from commercial
sources and used without any further purification. Elemental
analyses (EA) for carbon, hydrogen, and nitrogen were performed
1527 (m), 1332 (m), 1297 (m), 1124 (s), and 617 (m). ESIeTOFeMS
(positive): m/z 372.2 [M þ H]þ, 395.1 [M þ Na]þ; Elemental anal-
ysis: calcd (%) for C17H17N5O3S: C, 54.97; H, 4.61; N, 18.86; found: C,
55.11; H, 4.77; N, 18.56.
on
a PerkineElmer 1400C analyzer. Infrared (IR) spectra
(4000e400 cmꢀ1) were recorded using a Nicolet FTeIR 170X
spectrophotometer on KBr disks. Electrospray ionization mass
spectra (ESIeTOFeMS) were recorded on a Finnigan MAT SSQ 710
mass spectrometer in a scan range of 200e2000 amu. Electro-
ionization mass spectra (EIeTOFeMS, electron energy 70 eV) were
recorded by GCT TOF mass spectrometry (Micromass, Manchester,
UK). 1H NMR spectra were measured with a Bruker DMX500 MHz
NMR spectrometer at room temperature in CDCl3 with tetrame-
thylsilane as the internal reference. UVeVis spectra were recorded
with a Shimadzu UV-3150 double-beam spectrophotometer using
a Pyrex cell with a path length of 10 mm.
2.2.3. 2,4-Dichloro-6-((6-methylbenzo[d]thiazol-2-ylimino)methyl)
phenol (4)
ꢀ
About 10 g of 4 A molecular sieves were added into a toluene
solution (30 mL) of 2-amino-6-methylbenzothiazole (0.50 g,
3.0 mmol) and 3,5-dichloro-2-hydroxybenzaldehyde (0.57 g,
3.0 mmol), and the reaction mixture was heated under reflux for
24 h with vigorous mechanical agitation. The mixture was then
filtered from the molecular sieves which were washed with solvent.
The solvent was removed from the filtrate and washings by rotary
evaporation, and the product purified by either vacuum distillation
or crystallization. Yield, 0.89 g (88%), m.p. 193e195 ꢁC; 1H NMR
(500 MHz, CDCl3, ppm): d 13.02 (s,1H, OH), 9.25 (s,1H, CH]N), 7.87
2.2. Synthesis
(d, 1H, benzo), 7.66 (s, 1H, benzo), 7.54 (d, 1H, phenol), 7.42 (d, 1H,
phenol), 7.33 (d, 1H, benzo), and 2.51 (s, 3H, CH3); Main FTeIR
2.2.1. 5-(Diethylamino)-2-((6-methylbenzo[d]thiazol-2-yl)
absorptions (KBr pellets, n
, cme1): 3415 (s), 1650 (m), 1588 (s), 1557
diazenyl)phenol (1)
(m), 1481 (m), 1448 (s), 1379 (m), 1354 (m), 1280 (m), 1225 (s), 1184
(s), 1154 (vs), 1099 (s), 862 (m), 808 (m), 775 (m), 742 (m), 681 (m),
564 (w), 465 (w), and 437 (w). EIeTOFeMS: m/z 335.9, 337.9, 340.0
[M]þ; Elemental analysis: calcd (%) for C15H10Cl2N2OS: C, 53.42; H,
2.99; N, 8.31; found: C, 53.58; H, 3.11; N, 8.56.
2-Amino-6-methylbenzothiazole (1.64 g, 10.0 mmol) was dis-
solved in a mixture of concentrated sulfuric acid (2 mL) and glacial
acetic acid (10 mL) at ꢀ5 ꢁC in an ice bath. Sodium nitrite (0.76 g,
11.0 mmol) was dissolved in cold water and added dropwise to the
reaction mixture for 0.5 h under stirring. The diazonium salt was
obtained and used for the next coupling reaction. 3-(Diethylamino)
phenol (1.65 g, 10.0 mmol) was added to a mixture of methanol/
water (90 mL, 2:1, v/v) solution in a three-necked flask immersed in
an ice bath. Freshly prepared diazonium salt was added dropwise
for 1 h to the reaction mixture under vigorous mechanical stirring
(0e5 ꢁC). After additional stirring for 1.5 h, the mixture was
neutralized with aqueous ammonia to pH 5e6 for 0.5 h. The
precipitate was filtered and dried after thorough washing with
acetone and ethanol. The crude product was recrystallized and
microcrystals of 1 were obtained. Further purification was per-
formed by column chromatography using CHCl3 as eluent. Yield,
2.67 g (79%), m.p. 232e234 ꢁC; 1H NMR (500 MHz, CDCl3, ppm):
2.2.4. 2-((Benzo[d]thiazol-2-ylimino)methyl)-4,6-dichlorophenol
(5) and 2,4-dichloro-6-((6-nitrobenzo[d]thiazol-2-ylimino)methyl)
phenol (6)
The syntheses of compounds 5 and 6 were similar to that
described for compound 4. Compound 5: Yield, 0.82 g (85%), m.p.
186e188 ꢁC; 1H NMR (500 MHz, CDCl3, ppm):
d 13.00 (s, 1H, OH),
9.28 (s, 1H, CH]N), 7.99 (d, 1H, benzo), 7.87 (d, 1H, benzo), 7.55
(s, 1H, phenol), 7.53 (t, 1H, benzo), 7.43 (s, 1H, phenol), and 7.42
(t, 1H, benzo); Main FTeIR absorptions (KBr pellets, n
, cmꢀ1): 3418
(vs), 2360 (w), 1646 (m), 1596 (s), 1558 (m), 1481 (m), 1434 (s), 1358
(m), 1295 (m), 1191 (m), 1166 (s), 1152 (s), 1102 (m), 874 (m), 806
(w), 753 (s), 742 (m), 725 (m), 680 (m), 626 (w), 563 (w), and 434
(w). EIeTOFeMS: m/z 321.9, 323.9, 326.0 [M]þ; Elemental analysis:
calcd (%) for C14H8Cl2N2OS: C, 52.03; H, 2.49; N, 8.67; Found: C,
52.21; H, 2.56; N, 8.54. Compound 6: Yield, 0.81 g (74%), m.p.
d
13.61 (s, 1H, OH), 7.84 (d, 1H, benzo), 7.60 (s, 1H, benzo), 7.53
(d,1H, phenol), 7.24 (d,1H, benzo), 6.46 (dd,1H, phenol), 6.14 (d,1H,
phenol), 3.48 (q, 4H, CH2), 2.48 (s, 3H, CH3-benzo), and 1.27
(t, 6H,CH3); Main FTeIR absorptions (KBr pellets,
n
, cmꢀ1): 3405
244e246 ꢁC; 1H NMR (500 MHz, CDCl3, ppm):
d 9.35 (s, 1H, CH]N),
(vs), 2975 (w), 1629 (m), 1527 (w), 1135 (vs), 817 (w), 639 (m), and
618(m). ESIeTOFeMS (positive): m/z 341.2 [M þ H]þ, 363.1
[M þ Na]þ; Elemental analysis: calcd (%) for C18H20N4OS: C, 63.50;
H, 5.92; N, 16.46; found: C, 63.72; H, 5.99; N, 16.23.
8.83 (s, 1H, benzo), 8.40 (d, 1H, benzo), 8.08 (d, 1H, benzo), 7.62
(s, 1H, phenol), and 7.50 (s, 1H, phenol); Main FTeIR absorptions
(KBr pellets, n
, cmꢀ1): 3396 (vs), 1647 (s), 1570(m), 1526(s), 1496(s),
1448(s), 1327(s), 1294(vs), 1206(m), 1158(m), 1124(s), 885(w),
825(w), 752(m), 553(w), and 432(w). ESIeTOFeMS (negative): m/z
366.2, 368.1, 370.0 [MeH]ꢀ; Elemental analysis: calcd (%) for
C14H8Cl2N2OS: C, 52.03; H, 2.49; N, 8.67; Found: C, 52.21; H, 2.56; N,
8.54.
2.2.2. 2-(Benzo[d]thiazol-2-yldiazenyl)-5-(diethylamino)phenol (2)
and 5-(diethylamino)-2-((6-nitrobenzo[d]thiazol-2-yl)diazenyl)
phenol (3)
The syntheses of compounds 2 and 3 were similar to that
described for compound 1. Compound 2: Yield, 2.41 g (74%), m.p.
2.3. X-ray data collection and solution
228e229 ꢁC; 1H NMR (500 MHz, CDCl3, ppm):
d 13.65 (s, 1H, OH),
7.89 (d, 1H, benzo), 7.73 (d, 1H, benzo), 7.46 (d, 1H, phenol), 7.36
(t, 1H, benzo), 7.20 (t, 1H, benzo), 6.40 (dd, 1H, phenol), 6.07 (d, 1H,
phenol), 3.42 (q, 4H, CH2), and 1.20 (t, 6H,CH3); Main FTeIR
Single-crystal samples of 2 and 5 were glue-covered and
mounted on glass fibers for data collection on a Bruker SMART 1K
CCD area detector at 191 and 293 K, respectively, using graphite
absorptions (KBr pellets,
n
, cmꢀ1): 3400 (vs),1632 (m),1408 (s),1111
mono-chromated Mo Ka radiation (
l
¼ 0.71073 A). The collected
ꢀ
(vs), and 617 (m). ESIeTOFeMS: (positive) m/z 327.2 [M þ H]þ,
349.2 [M þ Na]þ; Elemental analysis: calcd (%) for C17H18N4OS: C,
62.55; H, 5.56; N, 17.16; found: C, 62.71; H, 5.79; N, 16.98.
Compound 3: Yield, 2.67 g (72%), m.p. 287e289 ꢁC; 1H NMR
data were reduced by using the program SAINT [19] and empirical
absorption corrections were done by SADABS [20] program. The
crystal systems were determined by Laue symmetry and the space
groups were assigned on the basis of systematic absences by using

918
T. Tao et al. / Dyes and Pigments 92 (2012) 916e922
Table 2
XPREP. The structures were solved by direct method and refined by
least-squares method. All non-hydrogen atoms were refined on F2
by full-matrix least-squares procedure using anisotropic displace-
ment parameters, while hydrogen atoms were inserted in the
calculated positions assigned fixed isotropic thermal parameters at
1.2 times of the equivalent isotropic U of the atoms to which they
are attached (1.5 times for the methyl groups) and allowed to ride
on their respective parent atoms. In compound 2, atoms C7 and S1
of the thiazole ring and atom O1 of phenyl ring are refined disor-
deredly over two positions by the free variable method and the final
site occupancy factors are given as 0.651(3) and 0.349(3), respec-
tively. All calculations were carried out using the SHELXTL PC
program package [21] and molecular graphics were drawn by using
XSHELL, XP and ChemDraw softwares. Details of the data collection
and refinement results for compounds 2 and 5 are listed in Table 1,
while selected bond distances and bond angles are given in Table 2.
ꢁ
ꢀ
Selected bond distances (A) and angles ( ) for compounds 2 and 5.
Bond distances
Bond angles
2
S1eC5
S1eC7
S10eC6
S10eC70
O1eC9
O10eC13
N1eC6
N1eC7
N10eC5
N10eC70
N2eN3
N2eC7
N20eN3
N20eC70
N3eC8
N4eC14
N4eC11
N4eC16
1.709(3)
1.768(7)
1.705(6)
1.737(13)
1.296(4)
1.208(6)
1.431(10)
1.325(14)
1.403(14)
1.347(14)
1.147(6)
1.406(5)
0.980(7)
1.366(10)
1.356(4)
1.469(5)
1.357(3)
1.468(4)
C5eS1eC7
C6eS10eC70
C6eN1eC7
C5eN10eC70
N3eN2eC7
N3eN20eC70
N2eN3eC8
N20eN3eC8
C14eN4eC16
C11eN4eC16
C11eN4eC14
N10eC5eC4
S1eC5eC4
N10eC5eC6
S1eC5eC6
N1eC6eC5
S10eC6eC1
S10eC6eC5
S1eC7eN2
89.2(2)
92.3(4)
104.5(8)
107.3(11)
116.8(5)
123.9(8)
123.6(4)
144.4(6)
116.4(2)
121.0(3)
122.2(2)
120.2(6)
130.5(3)
119.0(6)
108.7(2)
119.0(5)
132.9(4)
106.8(3)
120.1(5)
2.4. Computational details
All calculations were carried out with Gaussian03 programs
[22]. The geometries of 1e6 were fully optimized and calculated by
B3LYP method and 6-31G* basis set without any symmetry
constraints. The single-crystal structures of 2 and 5 were used as
the starting geometries, while other input files were obtained by
the substituent-modified approach based on earlier output files.
5
Cl1eC11
Cl2eC13
S1eC5
S1eC7
O1eC14
N1eC6
N1eC7
N2eC7
N2eC8
C8eC9
1.737(2)
1.727(2)
1.725(2)
1.734(2)
1.340(2)
1.391(2)
1.296(2)
1.401(2)
1.278(2)
1.441(3)
C5eS1eC7
88.7(1)
109.8(2)
117.4(2)
129.1(2)
109.5 (1)
114.8(2)
125.6(2)
117.0(1)
126.0(2)
117.0(1)
122.7(2)
120.0(2)
119.1(1)
118.9(1)
119.5(1)
118.9(2)
122.4(2)
C6eN1eC7
C7eN2eC8
S1eC5eC4
S1eC5eC6
N1eC6eC5
N1eC6eC1
S1eC7eN2
N1eC7eN2
S1eC7eN1
N2eC8eC9
Cl1eC11eC10
Cl1eC11eC12
Cl2eC13eC12
Cl2eC13eC14
O1eC14eC13
O1eC14eC9
3. Results and discussion
3.1. Syntheses
Azo dyes 1e3 having the same benzothiazole/phenol skeleton
were prepared via classical diazotization reactions between 3-
(diethylamino)phenol and the corresponding diazonium salts,
while Schiff bases 4e6 were synthesized by the formation of the
carbon-nitrogen double bond between substitutional 2-amino-
benzothiazole compounds and 3,5-dichloro-2-hydroxybenzal-
dehyde using dehydration condensation reactions (Scheme 1).
Concentrated sulfuric acid and glacial acetic acid system was used
as the reaction mediator in the synthetic process of azo dyes
because of the low solubility of substitutional 2-aminobenzo-
Table 1
ꢀ
thiazole compounds. Molecular sieves 4 A were added as the
Crystal data and structural refinements for compounds 2 and 5.
catalyst in the synthetic process of Schiff bases due to low reactivity
of the aromatic amine components.
Compound
2
5
Empirical formula
Formula weight
Temperature/K
C17H17N4OS
325.42
191
C14H8Cl2N2OS
323.19
293
All three pairs of compounds 1e6 were fully characterized by 1H
NMR, FTeIR UVeVis, TOFeMS and elemental analyses. The
assignments of 1H NMR peaks of aromatic heterocyclic compounds
1e6 show the sequence of deshielding effects for the substituent
groups as: nitro > hydrogen > methyl. As can be seen in Scheme 2,
ꢀ
Wavelength/A
0.71073
0.10 ꢂ 0.11 ꢂ 0.12
Monoclinic
P21/c
7.114(2)
20.770(6)
12.721(4)
90
122.263(4)
90
1589.4(8)
4/1.360
0.71073
0.30 ꢂ 0.25 ꢂ 0.20
Monoclinic
P21/c
11.841(3)
8.046(2)
14.454(3)
90
102.613(3)
90
1343.8(5)
4/1.597
Crystal size (mm)
Crystal system
Space group
the chemical shifts (d) of hydrogen atoms of benzothiazole and
ꢀ
a/A
ꢀ
phenol rings resonate in the range 6.14e7.88 ppm. However, the
assignments of different protons of compounds 1 and 4 can be
analyzed by means of the deshielding effects and the splitting of
signals. More importantly, the coupling constant (J) analyses reveal
that the J values of protons of benzene ring from benzothiazole are
larger than those of the phenol ring, which can help us to further
confirm the final peak assignments. Additionally, X-ray single-
crystal diffraction method has been used for characterizing the
structures of representative compounds 2 and 5.
b/A
ꢀ
c/A
ꢁ
ꢁ
a
/
b
/
ꢁ
g
/
3
ꢀ
V/A
Z/Dcalcd (g/cm3)
F(000)
684
0.214
656
0.633
m
/mmꢀ1
hmin/hmax
kmin/kmax
lmin/lmax
Data/parameters
Final R indices [I > 2ó(I)]
ꢀ8/8
ꢀ15/11
ꢀ22/24
ꢀ10/10
ꢀ15/13
ꢀ18/18
2796/250
R1 ¼ 0.0452
wR2 ¼ 0.1216
R1 ¼ 0.0578
wR2 ¼ 0.1278
1.081
3223/182
R1 ¼ 0.0397
wR2 ¼ 0.1041
R1 ¼ 0.0549
wR2 ¼ 0.1122
0.994
3.2. Structural description of compound 2
R indices (all data)
The molecular structure of 2 with atom-numbering scheme is
shown in Fig. 1a. It crystallizes in the monoclinic P21/c space group
without the presence of any solvent molecule. The bond length of
S
ꢀꢀ3
Max./min. Dr/e A
0.47/ꢀ0.49
0.34/ꢀ0.26
ꢀ
the azo unit (N2eN3) is 1.147(6) A, indicative of the typical double-

T. Tao et al. / Dyes and Pigments 92 (2012) 916e922
919
HO
N
N
N
S
(1) NaNO2, H2SO4/CH3COOH
R
R
S
N
NH2
HO
N
(2)
R
R
1 R=CH3
2 R=H
3 R=NO2
N
R = CH3, H, NO2
HO
Cl
Cl
S
N
N
toluene, molecular sieves
NH2
N
Cl
HO
S
4
5
6
R=CH3
R=H
R=NO2
OHC
R = CH3, H, NO2
Cl
Scheme 1. Schematic illustration for the preparation of compounds 1e6.
ꢀ
bond character. In contrast, the N1eC7 bond length is 1.325(14) A
which is shorter than those of N1eC6, N2eC7 and N3eC8 in the
reported methyl or methoxyl substituted benzothiazole azo
dyes [18,23,24]. The torsion angles for S1eC7eN2eN3 (3.4(5)ꢁ),
(176.6(6)ꢁ),
N2eN3eC8eC9
(2.2(5)ꢁ)
ꢀ
lengths of 1.431(10), 1.406(5) and 1.356(4) A, exhibiting predomi-
N1eC7eN2eN3
nantly
p
-conjugated double-bond character. It is worthwhile to
N2eN3eC8eC13 (177.2(3)ꢁ), and C7eN2eN3eC8 (178.8(3)ꢁ)
exhibit that all the 16 non-hydrogen atoms in the chromophore
skeleton, except the N-substituted ethyl groups, are essentially
coplanar with the mean deviation from least-squares plane of
0.023 A (Fig. 1b), which is the typical structural feature for most azo
dyes.
mention that strong intramolecular OeH/N hydrogen bonding
interactions are found with the H/A distances of 1.82 and 1.87 A in
ꢀ
2 forming six-membered C2N2OH hydrogen bonded rings (Table 3).
The thiazole sulfur atom and the azo unit lie at the same side of
N2eC7 single bond with the related bond angles of S1eC7eN2
(120.1(5)ꢁ), N1eC7eN2 (121.3(7)ꢁ), C7eN2eN3 (116.8(5)ꢁ) and
N2eN3eC8 (123.6(4)ꢁ) which are consistent with previously
ꢀ
The N-substituted ethyl groups of 2 are found to point to the
same side of the above-mentioned molecular least-squares plane,
Scheme 2. Schematic illustration for the assignments of 1H NMR peaks of compounds 1 and 4.

920
T. Tao et al. / Dyes and Pigments 92 (2012) 916e922
Fig. 1. ORTEP drawings of compound 2 with the atom-numbering scheme (top view
for a and side view for b). Displacement ellipsoids are drawn at the 30% probability
level and the H atoms are shown as small spheres of arbitrary radii.
which may help to form a head-to-tail dimeric packing mode
between adjacent molecules in its crystal structure. As illustrated in
Fig. 2, p-p stacking interactions are observed between the phenyl
Fig. 2. Perspective view and schematic illustration of the packing structures of
compound 2.
rings and the thiazole rings of neighboring molecules with the
ꢀ
centroid-to-centroid separation of 3.752 A, where the alkyl groups
S1eC7eN2eC8
(177.6(1)ꢁ),
N1eC7eN2eC8
(2.1(3)ꢁ),
point outside of the dimeric units. It is noted that there are two sets
of molecules in the crystal packing of 2, and each set packs in
a herringbone fashion and the dihedral angle between them is
86.9ꢁ. Furthermore, weak intermolecular CeH/O hydrogen
bonding interactions are found between the phenyl hydrogen atom
H12 and its adjacent oxygen atom O1.
N2eC8eC9eC10 (178.2(2)o) N2eC8eC9eC14 (2.3(3)o), and
C7eN2eC8eC9 (179.5(2)o) demonstrate that all the 19 non-
hydrogen atoms in the chromophore skeleton in 5 are essentially
coplanar with the same mean deviation from least-squares plane as
that in 2 (Fig. 3b).
In the crystal packing of 5,
pep stacking interactions are
observed between the phenyl rings and the benzothiazole rings of
contiguous molecules with dissimilar centroid-to-centroid sepa-
rations of 3.507 and 3.694 A, respectively (Fig. 4). A head-to-tail
3.3. Structural description of compound 5
ꢀ
The molecular structure of 5 with atom-numbering scheme is
shown in Fig. 3a. It also crystallizes in the monoclinic P21/c space
group without the presence of any solvent molecule. The bond
dimeric packing fashion between adjacent planar molecules in its
crystal structure is also observed. Nevertheless, the aromatic rings
between adjacent planar dimeric units are severely offset and no
ꢀ
length of the imine unit (N2eC8) in 5 is 1.278(2) A which is a little
pep stacking interactions can be found in this case, even if there is
ꢀ
longer than the NeN azo unit (1.147(6)) A in 2, indicative of the
no steric crowding of alkyl groups and the interlayer separation
ꢀ
difference between nitrogen and carbon atoms. In contrast, the
between them is only 3.298 A. Additionally, there are also two sets
ꢀ
N1eC7 bond length is 1.296(2) A, which is shorter than those of
of molecules and each set packs in a herringbone mode and the
dihedral angle between them is 67.4ꢁ.
N1eC6, N2eC7 and C8eC9 in the lengths of 1.391(2), 1.401(2) and
ꢀ
1.441(3) A too, exhibiting predominantly
p-conjugated double-
bond character. Intramolecular NeH/O hydrogen bonding inter-
ꢀ
actions are found as well with the H/A distance of 1.93 A in 5
constituting a six-membered C3NOH hydrogen bonded rings. The
thiazole nitrogen atom and the imine unit are positioned at the
same direction of N2eC7 single bond with the related bond angles
of S1eC7eN2 (117.0(1)ꢁ), N1eC7eN2 (126.0(2)ꢁ), C7eN2eC8
(117.4(2)ꢁ) and N2eC8eC9 (122.7(2)ꢁ). The torsion angles for
Table 3
Hydrogen bonding parameters (A, ꢁ) in compounds 2 and 5.
ꢀ
DeH/A
DeH
H/A
D/A
:DHA
Symmetry
code
2
O1eH1A/N2
O10eH1A0N20
C12eH12/O1
0.82
0.85
0.93
1.82
1.87
2.45
2.547(4)
2.671(8)
3.240(5)
147.0
156.5
143.0
1 þ x, y, z
5
Fig. 3. ORTEP drawings of compound 5 with the atom-numbering scheme (top view
for a and side view for b). Displacement ellipsoids are drawn at the 30% probability
level and the H atoms are shown as small spheres of arbitrary radii.
O1eH1O/N2
C8eH8/N1
0.82
0.93
1.93
2.39
2.653(2)
2.753(3)
146.0
103.0

T. Tao et al. / Dyes and Pigments 92 (2012) 916e922
921
Table 4
Spectroscopic properties of azo dyes 1e3 and Schiff bases 4e6. Measurements are
made at room temperature in methanol solutions.
a
Compound
lmax(ε)
Egap [eV]
Egap (calcd.b)
[eV]
[nm (L molꢀ1 cmꢀ1)]
1
2
3
4
5
6
532 (49600)
528 (48300)
538 (34600)
273 (44700), 355 (9660)
270 (45200), 352 (6070)
249 (51700), 353 (68400)
2.33
2.35
2.30
3.49
3.52
3.51
2.91
2.95
2.85
3.44
3.50
3.36
a
Optical band gap determined from the long-wavelength of maximum
absorption in the solution UVeVis spectra.
b
The geometries of 1e6 were optimized and calculated by B3LYP method and
6-31G* basis set.
electronic spectral investigations on three pairs of compounds with
slight differences in molecular structures demonstrate that the
presence of distinguishing azo and Schiff-base units (chromophore
groups) as well as substituents (auxochrome groups) in the
benzothiazole backbone in 1e6 lead to significant alterations of
bathochromic and hypsochromic shifts.
Fig. 4. Perspective view and schematic illustration of the packing structures of
compound 5.
3.4. Spectral characterizations and density function theory (DFT)
computations
To further reveal the essential distinction of 1e6, DFT
computational studies are carried out where the fixed atom
coordinates of compounds 1e6 are used for the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) gap (band gap) calculations. DFT computational
results given in Table 4 reveal that the resultant HOMO-LUMO
gaps for azo dyes 1e3 are 2.91, 2.95 and 2.85 eV, respectively,
while those for the corresponding Schiff bases 4e6 are much
bigger at 3.44, 3.50 and 3.36 eV, respectively, which are in
agreement with their UVevis absorptions. As can be seen in Fig. 6,
subtle changes are observed in the calculated spatial representa-
tions of HOMOs and LUMOs because of the influences of intro-
ducing different electron-donating or electron-withdrawing
group in the phenyl ring. Compared with compounds 2 and 5,
introducing an electron-withdrawing nitro group in 3 and 6 is
more remarkable than introducing an electron-donating methyl
group in 1 and 4.
UVeVis spectra of azo dyes 1e3 and their corresponding Schiff
bases 4e6 in their methanol solutions with the same concentration
of 2.0 ꢂ 10ꢀ5 mol/L were recorded at room temperature, respec-
tively, in order to compare the differences originated from their
molecular structures. As illustrated in Fig. 5, strong absorption
bands at 532, 528, and 538 nm are assigned to the pep* transition
between the aromatic rings and the azo units in compounds 1e3,
which are analogous to our previously reported benzothiazole
Disperse Red azo dyes (512e536 nm) [18]. In contrast, the corre-
sponding absorptions of compounds 4e6 shows obvious hyp-
sochromic shifts to 355, 352 and 353 nm because of the presence of
different chromophores in their molecular structures (ꢀC]Ne
versus eN]Ne groups), where compound 6 has a very strong
absorption peak and the other two are relatively weak. Compared
with the absorptions of compounds 2, the
pep* transition of
compound 3 shows a bathochromic shift of 10 nm due to the
electron-attracting effects of NO2 substituent group. In summary,
Furthermore, not only from the geometry of X-ray diffraction
results, but also from that of optimized structures by DFT calcula-
tions, the dihedral angles between the benzothiazole and the
phenol units are very close to zero showing excellent planarity. In
addition to the presence of delocalized
p
system of molecules,
stack-
intramolecular hydrogen-bonding and intermolecular
pep
ing interactions are believed to play important roles, which are also
analogous to those in literature [25]. In addition, the alteration of
torsion angles by introducing different substituent groups (CH3, H,
or NO2) in the phenyl ring can be neglected in this case.
Fig. 5. UVeVis absorption spectra for azo dyes 1e3 and Schiff bases 4e6 at room
temperature in their methanol solutions with the same concentration of
2.0 ꢂ 10ꢀ5 mol/L.
Fig. 6. Calculated spatial representations of HOMOs and LUMOs for azo dyes 1e3 and
Schiff bases 4e6 with the DFT B3LYP/6-31(d) level.

922
T. Tao et al. / Dyes and Pigments 92 (2012) 916e922
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X-ray single-crystal diffraction analyses of compounds 2 and 5
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We acknowledge the Major State Basic Research Development
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the National Natural Science Foundation of China (Nos. 21171088,
21021062 and 20871065) and the Jiangsu Province Department of
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Appendix. Supplementary material
CCDC reference numbers 831567 and 831568 for compounds 2
and 5 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge at www.ccdc.cam.
ac.uk/conts/retrieving.html [or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax:
(internat.) þ44 1223/336 033; E-mail: deposit@ccdc.cam.ac.uk].
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