Azo-hydrazone tautomerism observed from UV-vis spectra by pH control and metal-ion complexation for two heterocyclic disperse yellow dyes

Article abstract of DOI:10.1039/c2dt31102j

The azo-hydrazone tautomerism of two pyridine-2,6-dione based Disperse Yellow dyes has been achieved by pH control and metal-ion complexation, respectively, which is evidenced by UV-visible spectra using pH-titration, ¹H NMR and X-ray single-crystal diffraction techniques for two dyes and one neutral dinuclear dye-metal complex. pH-titration experiments under strong and weak acidic conditions (HCl and HOAc) as well as strong and weak alkaline conditions (NaOH and ammonia) demonstrate that there is an equilibrium between the azo (HL_1-A and HL_2-A) and hydrazone (HL _1-H and HL_2-H) tautomers for two dyes in solution but the hydrazone form is dominant under conventional conditions. The hydrazone proton is also observed in the ¹H NMR spectra of HL_1-H and HL_2-H which can be verified by the hydrogen-deuterium exchange and the presence of cooperative six-membered intramolecular hydrogen rings involving the hydrazone proton in their X-ray single-crystal structures. Moreover, the azo-hydrazone tautomerism is evidenced by the formation of a novel neutral dinuclear dye-metal complex Cu₂(L_2-A)₄, where all the ligands are in the azo form and two types of coordination modes are present for four L_2-A ligands. Namely, the side two ligands serve as the bidentate capping ligands, while the middle ones act as the quadridentate bridging ligands linking adjacent Cu^II centers in a reverse fashion.

Full text of DOI:10.1039/c2dt31102j

View Online / Journal Homepage
Dynamic Article Links
Dalton
Transactions
Cite this: DOI: 10.1039/c2dt31102j
www.rsc.org/dalton
PAPER
Azo-hydrazone tautomerism observed from UV-vis spectra by pH control and
metal-ion complexation for two heterocyclic disperse yellow dyes†
Xiao-Chun Chen,a Tao Tao,a Yin-Ge Wang,a,b Yu-Xin Peng,a Wei Huang*a and Hui-Fen Qian*a,b
Received 22nd May 2012, Accepted 6th July 2012
DOI: 10.1039/c2dt31102j
The azo-hydrazone tautomerism of two pyridine-2,6-dione based Disperse Yellow dyes has been achieved
by pH control and metal-ion complexation, respectively, which is evidenced by UV-visible spectra using
pH-titration, 1H NMR and X-ray single-crystal diffraction techniques for two dyes and one neutral
dinuclear dye–metal complex. pH-titration experiments under strong and weak acidic conditions (HCl and
HOAc) as well as strong and weak alkaline conditions (NaOH and ammonia) demonstrate that there is an
equilibrium between the azo (HL1-A and HL2-A) and hydrazone (HL1-H and HL2-H) tautomers for two
dyes in solution but the hydrazone form is dominant under conventional conditions. The hydrazone
proton is also observed in the 1H NMR spectra of HL1-H and HL2-H which can be verified by the
hydrogen–deuterium exchange and the presence of cooperative six-membered intramolecular hydrogen
rings involving the hydrazone proton in their X-ray single-crystal structures. Moreover, the azo-hydrazone
tautomerism is evidenced by the formation of a novel neutral dinuclear dye–metal complex Cu2(L2-A)4,
where all the ligands are in the azo form and two types of coordination modes are present for four L2-A
ligands. Namely, the side two ligands serve as the bidentate capping ligands, while the middle ones act as
the quadridentate bridging ligands linking adjacent CuII centers in a reverse fashion.
of functional groups, the presence of different tautomeric forms,
Introduction
and crystallographic arrangements help decide their properties.11
Remarkable progress has been achieved on the studies of func-
tional dyes during the past decades in both academic and indus-
trial fields,1,2 not only because of their various topologies and
intriguing structures, but also their interesting physical and
chemical properties such as textile dyeing and polyamide fiber
coloring,3 non-linear optics,4 optical data storage,5 two photon
absorptions,6 and dye-sensitized solar cells.7 Specifically, azo-
functionalized dyes bearing aromatic heterocyclic components
have shown brilliant color and chromophoric strength, especially
for their excellent properties on light and sublimation fastness.8,9
In some cases, azo dyes are also referred to as hydrazone dyes
since they can be either in the azo form or in the tautomeric
hydrazone one.10 It is worthwhile to mention that subtle change
Tautomerism is one of the most important structural isomer-
isms, which is significant not only to the dyestuff manufacturers
but also to other areas of chemistry.12 Comparisons between the
azo dyes and corresponding hydrazone ones in the tautomeric
system appear to be quite interesting from structures to proper-
ties.13 On the one hand, X-ray single-crystal diffraction has been
proved to be the most powerful tool to characterize various struc-
tural isomerisms by analyzing the data of bond lengths and
angles, dihedral and torsion angles, and supramolecular inter-
actions in the solid state.14 On the other hand, tautomers in dis-
tinctive isomeric forms have not only different colors, but also
different tinctorial strengths (and hence economics) and proper-
ties such as color fastness to washing, weathering, rubbing, light,
sublimation, perspiration, and so on.15
In our previous work, a series of Disperse Yellow dyes
bearing the pyridine-2,6-dione or quinoline-2,4-dione backbone
have been systematically investigated16–20 where the structural
and computational results demonstrated the presence of the
hydrazone forms for this family of heterocyclic Disperse Yellow
dyes. Furthermore, we have studied the azo-hydrazone tautomer-
ism of these dyes by means of NiII and CuII complexation to
form mononuclear and one-dimensional coordination poly-
mers21,22 as well as in situ CuII catalysis, oxidation, and com-
plexation,22 where deprotonation of dyes and subsequent
coordination with the metal ions are proved to be effective for
the azo–hydrazone tautomerism.
aState Key Laboratory of Coordination Chemistry, Nanjing National
Laboratory of Microstructures, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, P. R. China.
E-mail: whuang@nju.edu.cn, qhf@njut.edu.cn; Fax: +86-25-83314502;
Tel: +86-25-83686526
bCollege of Sciences, Nanjing University of Technology, Nanjing,
210009, P. R. China
†Electronic supplementary information (ESI) available: FT-IR, 1H and
13C NMR, ESI-TOF-MS, additional UV-vis absorption spectra, powder
X-ray diffraction patterns and molecular orbital diagrams of heterocyclic
Disperse Yellow dyes HL1-H, HL2-H and dinuclear dye–metal complex
Cu2(L2-A)4. CCDC 853389, 853390 and 882917 for HL1-H, HL2-H and
Cu2(L2-A)4. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt31102j
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

View Online
As an extensive study in this area, we try herein another
deprotonation method by the subtle pH control with the addition
of a weak base (ammonia) to explore the azo–hydrazone tauto-
merism of two new Disperse Yellow dyes having the same
benzene/pyridine-2,6-dione skeleton (HL1-H and HL2-H), where
the alteration of ratios of the two isomers can be monitored by
their characteristic UV-visible (UV-vis) spectral absorptions. The
results indicate that both of the dye molecules exist as an
equilibrium mixture of azo and hydrazone tautomers in metha-
nol, but the hydrazone form is dominant under conventional con-
ditions. Furthermore, X-ray single-crystal structures and proton
nuclear magnetic resonance (1H NMR) spectra before and after
the hydrogen–deuterium exchange of two dyes manifest the pres-
ence of dominant hydrazone forms both in the solid state and in
solution. In addition, a novel neutral dinuclear dye–metal
complex Cu2(L2-A)4 has been described where the L2-A ligands
in the azo form are classified as bidentate capping and quadri-
dentate bridging ligands, respectively. The alterations of UV-vis
spectra and related molecular geometry before and after CuII ion
complexation have been involved, too.
under vigorous mechanical stirring (0–5 °C). After additional
stirring for 2 h, the mixture was neutralized with ammonia to pH
5–6 for 30 min. The precipitate was filtered and dried after a
thorough wash with acetone and ethanol. The crude product was
recrystallized and the microcrystals of HL1-H were obtained in a
yield of 2.23 g (65%). Single-crystal samples suitable for X-ray
diffraction measurement were grown from DMF by slow evapor-
ation in air at room temperature for two weeks. M.p. >250 °C.
1H NMR (500 MHz, DMSO-d6, ppm): δ 15.61 (s, 1H, hydra-
zone), 8.17 (d, 1H, benzo), 7.77 (d, 1H, benzo), 7.56 (dd, 1H,
benzo), 3.91 (s, 3H, OCH3), 3.23 (s, 3H, NCH3), 2.57 (s, 3H,
CH3); 13C NMR (125 MHz, CDCl3, ppm): δ 160.7, 160.0,
158.0, 157.5, 137.0, 131.1, 125.5, 124.3, 119.3, 114.0, 108.6,
56.3, 26.6, 16.7; Main FT-IR absorptions (KBr pellets, ν, cm−1):
3417 (b), 2225 (m), 1679 (m), 1637 (s), 1481 (vs), 1421 (s),
1184 (s), 1130 (m), and 1031 (m); ESI-TOF-MS (positive): m/z
344.3 [M + H]+; Anal. calcd for C15H13N5O5: C, 52.48; H, 3.82;
N, 20.40%. Found: C, 52.16; H, 3.91; N, 20.11%. UV-vis: (λmax
in MeOH), 465 and 282 nm.
Synthesis of (Z)-1-ethyl-5-(2-(4-methoxy-2-nitrophenyl)hydra-
zono)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbo-
nitrile (HL2-H). The synthesis of HL2-H was similar to that
described for HL1-H except that 1-ethyl-4-methyl-3-cyano-6-
hydroxypyrid-2-one was used as the starting material. Yield,
2.43 g (68%). Single-crystal samples suitable for X-ray diffrac-
tion measurements were also grown from DMF by slow evapor-
ation in air at room temperature for two weeks. M.p. >250 °C.
1H NMR (500 MHz, DMSO-d6, ppm): δ 15.61 (s, 1H, hydra-
zone), 8.16 (d, 1H, benzo), 7.76 (d, 1H, benzo), 7.55 (dd, 1H,
benzo), 3.91 (s, 3H, OCH3), 3.90 (m, 2H, NCH2), 2.56 (s, 3H,
CH3), 1.15 (t, 3H, NCH2CH3); 13C NMR (125 MHz, CDCl3,
ppm): δ 160.4, 159.5, 158.0, 157.4, 136.9, 131.1, 125.7, 124.3,
119.3, 114.0, 108.6, 104.2, 56.3, 35.4, 16.8, 13.0; Main FT-IR
absorptions (KBr pellets, ν, cm−1): 3406 (b), 2223 (m),
1679 (m), 1637 (m), 1479 (s), 1182 (s), 1128 (m) and 1049 (m);
ESI-TOF-MS (positive): m/z 380.2 [M + Na]+; Anal. calcd
for C16H15N5O5: C, 53.78; H, 4.23; N, 19.60%. Found: C,
53.66; H, 4.51; N, 19.45%. UV-vis: (λmax in MeOH), 466
and 284 nm.
Experimental
Materials and measurements
All melting points were measured without corrections. 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 per-
formed on a Perkin-Elmer 1400C analyzer. Infrared (IR) spectra
(4000–400 cm−1) were recorded using a Nicolet FT-IR 170X
spectrophotometer on KBr disks. Electrospray ionization mass
spectra (ESI-TOF-MS) were recorded on a Finnigan MAT SSQ
1
710 mass spectrometer in a scan range of 200–2000 amu. H
and 13C NMR spectra were measured with a Bruker dmx
500 MHz NMR spectrometer at room temperature in DMSO-d6
with tetramethylsilane as the internal reference. UV-vis spectra
were recorded with a Shimadzu UV-3150 double-beam spectro-
photometer using a quartz glass cell with a path length of
10 mm. Fluorescence measurements were performed at room
temperature on Shimadzu RF-5301PC spectrophotometer.
Powder X-ray diffraction (PXRD) measurements were performed
on a Philips X’pert MPD Pro X-ray diffractometer using Cu Kα
radiation (λ = 0.15418 nm), in which the X-ray tube was
operated at 40 kV and 40 mA at room temperature. All pH
measurements were made with a pH-10C digital pH meter.
Synthesis of neutral dinuclear copper(II) complex Cu2(L2-A)4.
Cu(CH3COO)2·H2O (0.042 g, 0.21 mmol) and HL2-H (0.075 g,
0.21 mmol) were dissolved in 15 mL N,N′-dimethylformamide
and the mixture was stirred at room temperature for 5 h. Blue
single-crystal samples suitable for X-ray diffraction measure-
ments were grown from the filtered solution by slow evaporation
in air at room temperature for two weeks. The microcrystals of
complex Cu2(L2-A)4 were obtained in a yield of 0.038 g (47%
based on HL2-H). M.p. >250 °C. Main FT-IR absorptions (KBr
pellets, ν, cm−1): 3436 (b), 2219 (m), 1644 (m), 1571 (m),
1531 (s), 1376 (s), 1270 (m) and 1201 (m); ESI-TOF-MS (nega-
tive): m/z 356.3 [L2]−, m/z 735.2 [2L2 + Na]−; Anal. calcd for
C64H56Cu2N20O20: C, 49.52; H, 3.64; N, 18.05%. Found: C,
49.66; H, 3.91; N, 18.34%. UV-vis: (λmax in MeOH), 432, 316
and 278 nm.
Synthesis of (Z)-5-(2-(4-methoxy-2-nitrophenyl)hydrazono)-
1,4-dimethyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile
(HL1-H). 4-Methoxy-2-nitroaniline (1.68 g, 10.0 mmol) was dis-
solved in a mixture of concentrated hydrochloric acid (5 mL)
and water (5 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 30 min under stirring. The diazonium
salt was obtained and used for the next coupling reaction. 1,4-
Dimethyl-3-cyano-6-hydroxypyrid-2-one (1.64 g, 10.0 mmol)
was added to 80 mL methanol–water (1 : 1,v/v) solution in a
three-necked flask immersed in an ice bath. Freshly prepared dia-
zonium salt was added dropwise for 1 h to the reaction mixture
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

View Online
Table 1 Crystal data and structural refinements for HL1-H, HL2-H and
X-Ray data collection and solution
Single-crystal samples of HL1-H, HL2-H and Cu2(L2-A 4
Cu2(L2-A 4
)
)
were
Compound
HL1-H
HL2-H
Cu2(L2-A)4
covered in glue and mounted on glass fibers for data collection
on a Bruker SMART 1K CCD area detector at 173(2) K, respect-
ively, using graphite mono-chromated Mo Kα radiation (λ =
0.71073 Å). The collected data were reduced by using the
program SAINT23 and empirical absorption corrections were
done by the SADABS24 program. The crystal systems were
determined by Laue symmetry and the space groups were
assigned on the basis of systematic absences by using 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 dis-
placement parameters, while hydrogen atoms were inserted in
the calculated positions assigned fixed isotropic thermal para-
meters 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 the cases of
HL1-H and HL2-H, there is an obviously large Q peak in electron
density maps for each atom near N4 when we tried to locate the
hydrogen atom in the difference synthesis, and almost no Q peak
is found around atom O1, indicative of the presence of the hydra-
zone form instead of the azo one. However, the final N–H bond
length seems to be a little shorter than the normal ones when we
refined the hydrogen atom isotropically. As a result, the hydra-
zone proton is added theoretically in HL1-H and HL2-H instead
of the difference synthesis in order to avoid using bond length
restraints.
Empirical
formula
Formula
weight
Temperature/K
Wavelength/Å
C15H13N5O5
343.30
C16H15N5O5 C64H56Cu2N20O20
357.33
1552.39
173(2)
0.71073
173(2)
0.71073
291(2)
0.71073
Crystal size (mm) 0.10 × 0.12 × 0.10 × 0.11 × 0.10 × 0.12 ×
0.12
0.12
0.12
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
Orthorhombic Monoclinic
Pca21
16.875(2)
8.018(1)
11.219(1)
90
90
90
1517.9(2)
4/1.502
712
0.116
−17/20
−9/9
−13/13
2625/229
R1 = 0.0446
Triclinic
ˉ
P21/c
P1
6.425(1)
13.544(2)
19.535(2)
90
105.364(1)
90
1639.3(2)
4/1.448
744
11.654(1)
12.926(1)
13.271(2)
76.991(1)
72.708(1)
65.931(1)
1730.3(2)
1/1.490
γ/°
V/Å3
Z/Dcalcd (g cm−3
F(000)
μ/mm−1
hmin/hmax
kmin/kmax
)
798
0.111
−7/7
−16/10
−23/23
2881/239
R1 = 0.0360
0.704
−13/12
−15/15
l
min/lmax
−12/15
Data/parameters
Final R indices
[I > 2σ(I)]
6060/478
R1 = 0.0498
wR2 = 0.0945 wR2 = 0.1032 wR2 = 0.1376
R1 = 0.0444 R1 = 0.0653
wR2 = 0.0995 wR2 = 0.1079 wR2 = 0.1443
R indices (all data) R1 = 0.0646
S
0.89
1.10
0.18/−0.17
0.98
1.09/−1.01
Max/min Δρ/e Å−3 0.18/−0.16
2 2
2 2 1/2
R1 = ΣkFo| − |Fck/Σ|Fo|, wR2 = [Σ[w(Fo2 − Fc ) ]/Σw(Fo ) ]
.
All calculations were carried out on a PC with the SHELXTL
PC program package25 and molecular graphics were drawn by
using XSHELL, Diamond and ChemBioDraw softwares. Details
of the data collection and refinement results for HL1-H, HL2-H
and Cu2(L2-A)4 are listed in Table 1, while selected bond dis-
tances and bond angles are given in Table 2.
Typical absorptions corresponding to the free CuN group in
HL1-H and HL2-H are observed at 2225 and 2223 cm−1, respect-
ively, in their FT-IR spectra (Fig. SI1 and SI2†). In addition, two
single peaks at 1679 and 1637 cm−1 are suggested to be the
CvO and CvN stretching vibrations of HL1-H and HL2-H
respectively. In contrast, the absorption peak of CuN group in
the dinuclear copper(II) complex Cu2(L2-A 4 has shifted to
Computational details
,
All calculations were carried out with Gaussian03 programs.26
The geometries of dyes were fully optimized and calculated by
B3LYP method and 6-31G* basis set without any symmetry con-
straints. The CIFs of HL1-H and HL2-H were used as the starting
geometries.
)
2219 cm−1 (Fig. SI3†), indicative of the formation of a coordina-
tive bond after copper(II) ion complexation. Furthermore, only a
single peak is observed at 1644 cm−1 in Cu2(L2-A)4 correspond-
ing to the NvN stretching vibrations which reflect the alteration
of backbone originating from the hydrazone form to the azo one.
As can be seen in Scheme 2, the chemical shifts (δ) of the
hydrogen atoms of benzene rings in HL1-H and HL2-H are
closely located from 7.55 to 8.16 ppm. However, the assign-
ments of different protons can be easily analyzed by means of
the deshielding effects and the split of peaks. More importantly,
the active hydrazone proton (NH) reaches as high as δ =
15.61 ppm in HL1-H and HL2-H and only one peak can be
found, which are comparable with our previously reported
pyridine-2,6-dione based Disperse Yellow dyes.21 In the
1H NMR spectra of both HL1-H and HL2-H, the NMR integral
height of hydrazone proton is less than 1.00 (0.79 and 0.71 in
Fig. SI4 and SI5†). In general, the proton of phenolic group
(OH) in the azo form is an active one and it is sometimes very
difficult to be observed especially when it is in a small amount
Results and discussion
Syntheses and spectral characterizations
Dyes HL1-H and HL2-H were prepared via classical diazotization
reactions in satisfactory yields between 4-methoxy-2-nitroaniline
and 1,4-dimethyl-3-cyano-6-hydroxypyrid-2-one or 1-ethyl-
4-methyl-3-cyano-6-hydroxypyrid-2-one (Scheme 1). They have
the same nitrobenzene/pyridine-2,6-dione skeleton but different
N-substituent groups in the pyridone ring (methyl in HL1-H and
ethyl in HL2-H). The neutral dinuclear copper(II) complex
Cu2(L2-A)4 can be easily produced by mixing Cu(CH3COO)2
and HL2-H in the absence of a base, where the deprotonated
process takes place for the HL2-H ligand and each CuII cation is
countered by two L2-A dianionic ligands.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

View Online
Table 2 Selected bond distances (Å) and angles (°) for HL1-H, HL2-H
and Cu2(L2-A 4
)
Bond distances
Bond angles
HL1-H
O1–C1
O2–C2
O3–N5
O4–N5
O5–C9
O5–C12
N1–C1
N1–C2
N1–C15
N2–C14
N3–N4
N3–C5
N4–C6
N5–C11
C1–C5
C2–C3
1.228(4)
1.214(4)
1.241(4)
1.208(4)
1.351(4)
1.439(4)
1.376(4)
1.401(4)
1.477(4)
1.154(5)
1.314(3)
1.325(4)
1.395(4)
1.468(4)
1.478(4)
1.466(5)
C9–O5–C12
C1–N1–C2
C1–N1–C15
C2–N1–C15
N4–N3–C5
N3–N4–C6
O3–N5–O4
O3–N5–C11
O4–N5–C11
C6–N4–H4
N3–N4–H4
O1–C1–C5
N1–C1–C5
O1–C1–N1
N1–C2–C3
O2–C2–N1
O2–C2–C3
C3–C4–C5
N3–C5–C1
117.4(2)
124.2(2)
117.6(3)
118.2(3)
120.2(3)
119.1(3)
122.3(3)
118.8(3)
118.9(3)
120.4(3)
120.5(3)
122.5(3)
117.0(3)
120.5(3)
116.1(3)
121.3(3)
122.6(3)
118.5(3)
123.1(3)
Scheme 1 Schematic illustration for the preparation and azo–hydra-
zone tautomerism of two Disperse Yellow dyes and neutral dinuclear
copper(II) complex Cu2(L2-A)4.
HL2-H
O1–C1
O2–C2
O3–N5
O4–N5
O5–C9
O5–C12
N1–C1
N1–C2
N1–C15
N2–C14
N3–N4
N3–C5
N4–C6
N5–C11
C1–C5
C15–C16
1.229(2)
1.217(2)
1.230(2)
1.225(2)
1.361(2)
1.428(2)
1.382(2)
1.398(2)
1.483(2)
1.140(2)
1.313(2)
1.322(2)
1.397(2)
1.459(2)
1.468(2)
1.506(3)
C9–O5–C12
C1–N1–C2
C1–N1–C15
C2–N1–C15
N4–N3–C5
N3–N4–C6
O3–N5–O4
O3–N5–C11
O4–N5–C11
C6–N4–H4
N3–N4–H4
O1–C1–C5
N1–C1–C5
O1–C1–N1
N1–C2–C3
O2–C2–N1
O2–C2–C3
N1–C15–C16
117.9(2)
123.4(2)
117.9(2)
118.7(2)
120.1(2)
118.5(2)
122.0(2)
119.7(2)
118.4(2)
120.7(2)
120.8(2)
122.2(2)
117.7(2)
120.0(2)
116.6(2)
120.9(2)
122.5(2)
112.5(2)
Scheme 2 Schematic illustration for 1H NMR peaks of HL1-H and
HL2-H in DMSO-d6 as well as their D2O exchange diagrams.
Cu2(L2-A 4
)
Cu1–O1
Cu1–O4
Cu1–O9
Cu1–N2
Cu1–N7
Cu1–N5#1
O1–N1
O2–N1
O4–C9
O6–N6
O7–N6
N1–C1
N2–N3
N3–C8
N5–C15
N7–N8
N8–C24
N10–C31
C8–C9
2.538(4)
1.921(2)
1.938(2)
1.965(3)
1.970(3)
2.641(4)
1.231(5)
1.204(4)
1.262(4)
1.200(5)
1.231(7)
1.487(5)
1.299(3)
1.357(5)
1.124(5)
1.293(4)
1.352(4)
1.141(6)
1.430(5)
1.431(5)
O1–Cu1–O4
O1–Cu1–O9
O1–Cu1–N2
O1–Cu1–N7
O1–Cu1–N5#1
O4–Cu1–O9
O4–Cu1–N2
O4–Cu1–N7
O4–Cu1–N5#1
O9–Cu1–N2
O9–Cu1–N7
O9–Cu1–N5#1
N2–Cu1–N7
N2–Cu1–N5#1
N5#1–Cu1–N7
Cu1–O1–N1
Cu1–N2–N3
Cu1–N2–C6
Cu1–O9–C25
Cu1–N5#1–C15#1
103.4(2)
83.5(2)
71.6(2)
106.5(2)
158.1(2)
173.0(2)
88.5(2)
90.9(2)
91.7(2)
93.2(2)
87.6(1)
81.5(2)
177.8(2)
93.4(2)
88.7(2)
108.2(2)
126.6(3)
121.7(2)
119.9(2)
124.0(3)
the two compounds considering the general calculated errors of
integral area in the 1H NMR spectra.
Nevertheless, the peak of the hydrazone proton cannot be
observed any more after adding one drop of D2O into the
DMSO-d6 solutions of both HL1-H and HL2-H 27 The hydrogen–
.
deuterium exchange experiments clearly exhibit the disappear-
ance of the hydrazone proton in HL1-H and HL2-H, as can be
seen in Scheme 2. An extensive experiment has been made by
adding solid NaOH to the DMSO-d6 solution of HL1-H, and the
1H NMR spectrum (Fig. SI6†) clearly demonstrates the azo–
hydrazone tautomerism where a large amount of the hydrazone
tautomer converts to the azo tautomer in the presence of a strong
base. However, one can still see some remnants of the hydrazone
proton which is in agreement with its electronic spectra in the
strong basic conditions. We believe this is very clear proof that
two compounds exist as an equilibrium of hydrazone ⇌ azo tau-
tomerism in solution and the hydrazone form is the dominant
one. The presence of the dominating hydrazone isomer can also
be verified by the evidence provided from the following pH-titra-
tion experiments monitored by their UV-vis spectra as well as
their X-ray single-crystal diffraction data in the solid state.
C24–C25
Symmetry code: #1, 1 − x, −y, 1 − z.
(less than 30% in these two cases). In our case, the proton of the
phenolic group in the azo form of both HL1-H and HL2-H cannot
be observed. So it is difficult to give the exact tautomer ratios of
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

View Online
UV-Vis spectra of HL1-H and HL2-H having the same
benzene/pyridine-2,6-dione skeleton have been recorded in their
methanol solutions with the same concentration of 4.0 ×
10−5 mol L−1. In addition, the pH driving azo–hydrazone tauto-
merism experiments are carried out by adding different amounts
of ammonia to their methanol solutions at room temperature, as
illustrated in Fig. 1a and 1b, respectively. For the two dye
solutions only, they show similar spectral behavior and there are
two obvious absorption peaks centered at 282 and 465 nm
(ξ = 6800 and 18 400 L cm−1 mol−1) in HL1-H and 284 and
466 nm (ξ = 10 600 and 28 900 L cm−1 mol−1) in HL2-H, which
are ascribed to the n–π* transitions between the phenyl rings and
the middle hydrazone units and the π–π* transitions between the
pyridine rings and the phenyl rings in the hydrazone form,
respectively.27,28 Additionally, another weak peak is observed
around 409 nm corresponding to the π–π* transitions between
the pyridine ring and the phenyl ring in the azo form.
HL1-H and HL2-H at 465 and 409 nm changes accordingly,
namely, the former is weakened and the latter is enhanced.
Furthermore, at the high-energy band, two new peaks emerge at
318 and 249 nm. The former is attributed to the n–π* transitions
between the azo and the phenyl rings, while the latter is ascribed
to the n–π* transitions between the azo and the pyridine rings in
the azo form.
It is concluded that there is a hydrazone ⇌ azo tautomeric
equilibrium in solution and the ratio of the hydrazone form
divided by the azo one continuously decreases with the increase
of pH values. The hydrazone isomers of HL1-H and HL2-H are
dominating under neutral pH conditions and the amounts of the
deprotonated azo species (L1-A and L2-A) increase with the
increase of pH values. L1-A and L2-A become the preponderant
ones after the pH values of solutions are higher than 10 in the
former and 9 in the latter. The structures of L1-A and L2-A are
suggested to be similar to those previously reported deprotonated
azo ligands in the metal complexes.21 Further experiments per-
formed by adding different amounts of acetic acid exhibit almost
no conversion to the hydrazone form of two dyes (Fig. 2). Actu-
ally, only dilute effects of dye solutions are observed in their
UV-vis spectra because the volume of whole dye solution is
nearly doubled after adding acetic acid at the end of our exper-
iments. However, as illustrated in Fig. 3, by dropping excess
hydrochloric acid or sodium hydroxide into the two dye solu-
tions, respectively, one can still see a small amount of conversion
to the hydrazone form in strong acidic conditions and no com-
plete conversion to the deprotonated azo form is attained even
under the strong alkaline medium. Additionally, the recorded
electronic spectra of dye in methanol by dropping excess NaOH
and then different amounts of HCl prove that the azo–hydrazone
tautomerism is reversible which is shown in Fig. SI7.† So the
deprotonation effect herein is obviously responsible for the con-
version from the hydrazone tautomer to the azo one.
By adding ammonia to their methanol solutions in order to
finely adjust the pH values, the strength of absorption peaks of
1
In conclusion, in the H NMR spectra, the hydrazone form of
HL1-H and HL2-H is preponderant under neutral conditions and
the presence of the hydrazone proton can be verified by the
hydrogen–deuterium exchange and the addition of NaOH
Fig. 1 UV-Vis absorption spectra for HL1-H (a) and HL2-H (b) in their
methanol solutions (4.0 × 10−5 mol L−1) at room temperature. pH values
are finely adjusted by dropping different amounts of ammonia.
Fig. 2 UV-Vis absorption spectra for HL1-H in their methanol solu-
tions at room temperature. pH values are finely adjusted by dropping
different amounts of acetic acid.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

View Online
Fig. 5 ORTEP drawings of HL1-H 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 hydrogen atoms are shown as small
spheres of arbitrary radii.
Fig. 3 UV-Vis absorption spectra for HL2-H in their methanol solu-
tions at room temperature. pH values are adjusted by dropping an excess
amount of hydrochloric acid or sodium hydroxide.
Fig. 4 UV-Vis absorption spectra for HL2-H and Cu2(L2-A 4
) in their
methanol solutions at room temperature.
experiments. In contrast, the active azo proton cannot be found
in the H NMR spectra, but it can be observed in the UV-vis
spectra and the strength of these peaks can be obviously
changed by the fine adjustment of pH values.
Fig. 6 ORTEP drawings of HL2-H 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 hydrogen atoms are shown as small
spheres of arbitrary radii.
1
In order to explore and compare the difference of HL2-H and
Cu2(L2-A)4 before and after CuII ion complexation, UV-vis spec-
trum of Cu2(L2-A)4 in the methanol solution is determined. As
shown in Fig. 4, compared with the free HL2-H dye, the π–π*
complexation and the ligands in the dye–metal complex should
be in the azo form, which can also be verified by the single-
crystal structure of a dinuclear copper(II) complex Cu2(L2-A)4.
As illustrated in Fig. SI8–SI10,† the pure phase of dyes
HL1-H, HL2-H and Cu2(L2-A)4 is also confirmed by PXRD pat-
terns which are in good agreement with their single-crystal
diffraction simulative data that will be discussed below.
transition absorption peak of purple Cu2(L2-A 4 complex is
)
shifted to 432 nm exhibiting a hypochromic shift of 34 nm. The
alteration of π–π* transition absorption is ascribed to the increas-
ing dihedral angles between the phenyl and pyridyl rings in the
L2-A ligands, which will be discussed in the following structural
description section. Furthermore, two n–π* transition absorption
peaks observed at 316 and 278 nm, which are obviously differ-
ent from the free HL2-H dye, agree well with the aforementioned
characteristic absorptions of azo isomers. So it is deduced that
the azo–hydrazone tautomerism takes place after metal-ion
Structural description of Disperse Yellow dyes HL1-H and HL2-H
The molecular structures of dyes HL1-H and HL2-H with atom-
numbering scheme are shown in Fig. 5a and 6a, respectively.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

View Online
They both have the same nitrobenzene/pyridine-2,6-dione skel-
eton but different N-substituent groups in the pyridone ring.
X-Ray structural analyses reveal that HL1-H crystallizes in the
orthorhombic Pca21 space group and HL2-H in the monoclinic
P21/c space group. All the non-hydrogen atoms except one
carbon atom of the N-substituted ethyl group of HL2-H are essen-
tially coplanar with the mean deviations from the least-squares
planes as 0.0893(4) Å in HL1-H and 0.1739(3) Å in HL2-H
(Fig. 5b and 6b). The dihedral angles between the pyridine-
2,6-dione ring and the phenyl ring in the molecular structures of
HL1-H and HL2-H are 7.3(3) and 6.9(3)°, respectively.
Both HL1-H and HL2-H exist in the hydrazone forms which
can be deduced by related N–N, C–N and C–O bond lengths,
(Table 2) just like our previously reported structures16–18,21 with
the same backbone but different coupling components. It is
worthwhile to note that strong cooperative intramolecular
N–H⋯O hydrogen bonds can be observed in both HL1-H and
HL2-H (Fig. 5a and 6a). The nitro group of the phenyl ring and
the carbonyl group of the pyridine ring in HL1-H and HL2-H are
fixed on the same side of N–H group where two fused six-
membered hydrogen-bonded rings are formed. It is suggested
that the formation of these intramolecular N–H⋯O hydrogen
bonds should contribute to stabilizing the hydrazone form in the
solid state.
Fig. 8 Perspective view of the crystal packing of HL2-H
.
Table 3 Hydrogen bonding parameters (Å, °) in HL1-H, HL2-H and
Cu2(L2-A 4
)
D–H⋯A
D–H H⋯A D⋯A
∠DHA Symmetry code
HL1-H
N4–H4⋯O1
N4–H4⋯O3
C10–H10⋯N2
0.88 1.92
0.88 1.98
0.95 2.56
2.595(4) 133
2.607(4) 127
3.510(5) 179
3.333(5) 160
x, 1 + y, 1 + z
1/2 + x, 1 − y,
1 − z
C12–H12B⋯O2 0.98 2.40
By comparing the two structures, it is found that a subtle
alteration of the substituted groups (N-methyl versus N-ethyl) in
the pyridine-2,6-dione backbone results in different packing
modes in their crystal packing. Two sets of molecules are found
in HL1-H with the dihedral angle of 70.2(3)°. There are typical
π–π stacking interactions between one pyridine-2,6-dione ring
and another adjacent phenyl ring with the centroid-to-centroid
separation of 3.580(1) Å, as shown in Fig. 7. In contrast, a
slipped layer packing structure is observed in HL2-H with the
interlayer contact of 2.859(2) Å (Fig. 8). Moreover, weak inter-
molecular C–H⋯N and C–H⋯O hydrogen bonds (Table 3) and
the hydrophobic effects of the N-substituted ethyl group of
HL2-H are suggested to play an important role in the formation
of this extended layer stacking.
HL2-H
N4–H4⋯O1
N4–H4⋯O3
C7–H7⋯N2
0.90 1.92
0.88 2.00
0.95 2.55
2.590(2) 132
2.620(2) 126
3.284(2) 135
−1 − x, −1/2 + y,
1/2 − z
C10–H10⋯O4
0.95 2.44
3.351(2) 162
2 − x, −y, 1 − z
Cu2(L2-A 4
C4–H4⋯O10
)
0.93 2.47
3.349(5) 158
−x, 1 − y, 1 − z
As we mentioned before, two dyes in the hydrazone form are
subjected to cooperative intramolecular hydrogen bonding inter-
actions. So the planarity of the hydrazone form is better than the
azo one (λmax = 465 or 466 nm versus λmax = 409 nm) and
hence has a usually higher tinctorial strength.22 However, when
a base is added, the deprotonation process takes place and the
cooperative intramolecular hydrogen bonds are destroyed. As a
result, bathochromic shifts from the hydrazone form to the azo
one are observed because of the decrease of conjugacy of the
whole dye molecules.
On considering the aforementioned UV-vis spectra, we can
explain different absorptions of the azo and hydrazone forms of
HL1-H and HL2-H and bathochromic shifts from the hydrazone
form to the azo one after adding different amounts of ammonia.
Structural description of the neutral dinuclear dye–metal
complex Cu2(L2-A 4
)
The molecular structure of the neutral dye–metal complex
Cu2(L2-A)4 with atom-numbering scheme is shown in Fig. 9. It
ˉ
crystallizes in the triclinic P1 space group and the asymmetric
unit consists of one CuII cation countered by two L2-A dianionic
ligands. The CuII ion adopts a six-coordinate elongated octa-
hedral configuration. The basal coordination plane is composed
of two azo nitrogen atoms (N2 and N7) and two phenol oxygen
atoms (O4 and O9) from each L2-A ligand with the Cu–O and
Cu–N bond lengths in the range of 1.921(2)–1.970(4) Å.
However, the apical positions are occupied by one nitro O atom
Fig. 7 Perspective view of the crystal packing of HL1-H
.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

View Online
The crystal packing view of Cu2(L2-A 4
)
is also shown in
Fig. 10. Weak π–π stacking interactions are found between the
pyridyl and phenyl rings of the side bidentate L2-A ligands from
neighboring dinuclear copper(II) molecules with a centroid-to-
centroid separation of 4.097(1) Å, forming a slipped layer
packing structure.
Density function theory (DFT) computations for dyes HL1-H and
HL2-H
Fig. 9 Drawings of Cu2(L2-A)4 with the atom-numbering scheme. Dis-
placement ellipsoids are drawn at the 30% probability level and the
hydrogen atoms are shown as small spheres of arbitrary radii.
To further reveal the essential distinction of HL1-H and HL2-H
,
DFT computational studies are carried out where the fixed atom
coordinates of the two dyes are used for the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) gap calculations. DFT computational results
given in Fig. SI11,† reveal that the resultant HOMO–LUMO
gaps of dyes HL1-H and HL2-H are equal at 2.96 eV, which are in
accordance with their UV-vis absorptions. As can be seen in
Fig. SI11,† there are intramolecular charge transfers in the two
pyridine-2,6-dione based Disperse Yellow dyes. Little changes
are observed in the calculated spatial representations of the
HOMOs and LUMOs because of the influences of introducing
different N-substituent groups in the pyridone ring. Compared
with compounds HL1-H and HL2-H, the levels of HOMOs and
LUMOs are similar because the electronic structure of the mo-
lecules remains unchanged, which are consistent with previously
reported N-substituted diamide dyes.29 More importantly, differ-
ent substituent groups at the imide position do not have any sig-
nificant effect on the whole chromophore properties.
Fig. 10 Perspective view of the crystal packing of Cu2(L2-A)4.
(O1) from one L2-A ligand and one cyano N atom (N5#1, 1 − x,
−y, 1 − z) from the third L2-A ligand. These two axial Cu–O and
Cu–N bond lengths are much longer at 2.538(4) and 2.641(5) Å,
respectively, exhibiting a typical Jahn–Teller distortion. At the
same time, the nitro O atom (O1#1, 1 − x, −y, 1 − z) of the third
L2-A ligand is coordinated to another copper(II) center (Cu1#1,
1 − x, −y, 1 − z) forming a novel dinuclear copper(II) complex
Cu2(L2-A)4.
It is noted that two types of coordination modes are observed
for the four L2-A ligands in this case. Namely, the side two
ligands serve as the bidentate capping ligands, while the middle
ones act as the quadridentate bridging ligands linking adjacent
CuII centers in a reverse fashion. Because of the fixation of co-
ordinative bonds and the geometric requirement for the central
CuII polyhedron, each set of L2-A ligands could not maintain the
planar structure and the dihedral angles between the phenyl and
pyridyl rings of the middle and side ligands turn out to be
22.8(2) and 26.4(2)°, respectively. Furthermore, face-to-face π–π
stacking interactions are observed between the pyridyl rings of
two middle quadridentate bridging L2-A ligands in the dinuclear
copper(II) molecules with a centroid-to-centroid separation of
3.700(1) Å (Fig. 10).
It is worthwhile to mention that an interconversion between
the hydrazone and the azo tautomers occurs for each set of L2-A
ligands after metal-ion complexation. The related N–N (N2–N3
and N7–N8) and C–C (C8–C9 and C24–C25) bond lengths are
shortened from 1.313(2) Å and 1.468(2) Å in HL2-A to 1.299(3),
1.293(4) Å and 1.430(5) and 1.431(5) Å in Cu2(L2-A)4, exhibit-
ing more double-bond character. In contrast, their neighboring
C–N (C6–N2, C22–N7 and C8–N3, C24–N8) bond lengths are
lengthened from 1.397(2) and 1.322(2) Å in HL2-H to 1.426(5),
1.422(5) Å and 1.357(5), 1.352(4) Å in Cu2(L2-A)4, displaying
predominantly single-bond character.
Conclusions
In summary, two heterocyclic Disperse Yellow dyes HL1-H and
HL2-H having the same benzene/pyridine-2,6-dione skeleton,
have been synthesized by typical diazotization reactions and
fully characterized. UV-Vis spectra using the pH-titration
method, 1H NMR and X-ray single-crystal diffraction techniques
have been used to study the azo–hydrazone tautomerism
between these two new dyes and one neutral dinuclear dye–
metal complex Cu2(L2-A)4. The hydrazone proton is observed in
1
the H NMR spectra of HL1-H and HL2-H which can be verified
by the hydrogen–deuterium exchange and X-ray single-crystal
structures. Structural analyses of HL1-H and HL2-H demonstrate
that they have similar planar conformation between the benzene
and the pyridone units stabilized by cooperative six-membered
intramolecular hydrogen rings but dissimilar crystal packing
fashions (herringbone packing in HL1-H but slipped layer stack-
ing in HL2-H).
pH-titration experiments under strong and weak acidic con-
ditions (HCl and HOAc) as well as strong and weak alkaline
conditions (NaOH and ammonia) demonstrate that there is an
equilibrium between the azo and the hydrazone tautomers in the
solutions of HL1-H and HL2-H. For example, by adding
ammonia to their methanol solutions to finely adjust the pH
values, the amounts of the deprotonated azo forms of two dyes
(L1-A and L2-A) increase and those of the hydrazone forms
decrease accordingly. The hydrazone isomer is dominant when
the pH value is less than 9 and only a slight increase of the
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

View Online
6 H. P. Zhou, F. X. Zhou, S. Y. Tang, P. Wu, Y. X. Chen, Y. L. Tu, J. Y. Wu
and Y. P. Tian, Dyes Pigm., 2012, 92, 633.
7 Y. T. Li, C. L. Chen, Y. Y. Hsu, H. C. Hsu, Y. Chi, B. S. Chen,
W. H. Liu, C. H. Lai, T. Y. Lin and P. T. Chou, Tetrahedron, 2010,
66, 4223.
8 H. S. Freeman and J. C. Posey, Dyes Pigm., 1992, 20, 171.
9 S. Wang, S. Shen and H. Xu, Dyes Pigm., 2000, 44, 195.
10 P. Ball and C. H. Nicholls, Dyes Pigm., 1982, 3, 5.
11 J. A. Farrera, I. Canal, P. Hidalgo-Fernandez, M. L. Perez-Garcia,
O. Huertas and F. J. Luque, Chem.–Eur. J., 2008, 14, 2277.
12 M. Amaike, H. Kobayashi, K. Sakurai and S. Shinkai, Supramol. Chem.,
2002, 14, 245.
hydrazone isomer can be observed under strong acidic medium.
Under the high pH value conditions, L1-A and L2-A become the
preponderant ones and no complete conversion to the azo tauto-
mers can be achieved even under strong alkaline medium. Fur-
thermore, different UV-vis absorption peaks and bathochromic
shifts for the hydrazone and azo forms of HL1-H and HL2-H can
be explained by their X-ray single-crystal structures and the
destruction of cooperative intramolecular hydrogen bonds in the
process of deprotonation.
On the other hand, conversion from the hydrazone tautomer to
the azo one has been evidenced by the formation of a novel
neutral dinuclear dye–metal complex Cu2(L2-A)4. The structural
analyses of Cu2(L2-A)4 reveal that all the ligands are in the azo
form and two types of coordination modes are present for four
L2-A ligands. Namely, the side two L2-A ligands serve as the
bidentate capping ligands, while the middle ones act as the quad-
ridentate bridging ligands linking adjacent CuII centers in a
reverse fashion. Because of the fixation of coordinative bonds
and the geometric requirement for the central CuII polyhedron,
each set of L2-A ligands could not maintain the planar structure
and the dihedral angles between the phenyl and pyridyl rings of
the middle and side ligands turn out to be 22.8(2) and 26.4(2)°,
respectively, which agrees well with a hypochromic shift of
34 nm in its UV-vis spectrum compared with the free L2-A
ligand. So it is concluded that the pH control and metal-ion com-
plexation are two effective approaches in the process of the inter-
conversion between the hydrazone tautomers and the azo ones
for pyridine-2,6-dione based Disperse Yellow dyes and their
metal complexes.
13 H. Y. Lee, X. L. Song, H. Park, M. H. Baik and D. Lee, J. Am. Chem.
Soc., 2010, 132, 12133.
14 G. Pavlovic, L. Racane, H. Cicak and V. Tralic-Kulenovic, Dyes Pigm.,
2009, 83, 354.
15 M. H. Habibi, A. Hassanzadeh and A. Zeini-Isfahani, Dyes Pigm., 2006,
69, 111.
16 W. Huang and H. F. Qian, Dyes Pigm., 2008, 77, 446.
17 W. Huang, Dyes Pigm., 2008, 79, 69.
18 H. F. Qian and W. Huang, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 2006, C62, o62.
19 T. Tao, F. Xu, X. C. Chen, Q. Q. Liu, W. Huang and X. Z. You, Dyes
Pigm., 2012, 92, 916.
20 J. Geng, T. Tao, S. J. Fu, W. You and W. Huang, Dyes Pigm., 2011, 90,
65.
21 W. You, H. Y. Zhu, W. Huang, B. Hu, Y. Fan and X.-Z. You, Dalton
Trans., 2010, 39, 7876.
22 B. Hu, G. Wang, W. You, W. Huang and X. Z. You, Dyes Pigm., 2011,
91, 105.
23 Siemens, SAINT v4 Software Reference Manual, Siemens Analytical
X-Ray Systems, Inc., Madison, Wisconsin, USA, 2000.
24 G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction
of Area Dectector Data. Univ. of Gottingen, Germany, 2000.
25 Siemens, SHELXTL, Version 6.10 Reference Manual, Siemens Analytical
X-Ray Systems, Inc., Madison, Wisconsin, USA, 2000.
26 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. N. Kudin,
J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaus-
sian 03, Gaussian Inc., Pittsburgh, PA, 2003.
Acknowledgements
We acknowledge the Major State Basic Research Development
Program (Nos. 2011CB933300 and 2011CB808704) and the
National Natural Science Foundation of China (Nos. 21021062,
and 21171088) for financial aids.
References
1 A. D. Towns, Dyes Pigm., 1999, 42, 3.
2 Z. J. Chen, A. Lohr, R. S. M. Chantu and F. Wuerthner, Chem. Soc. Rev.,
2009, 38, 564.
27 M. H. Habibi, A. Hassanzadeh and A. Zeini-Isfahani, Dyes Pigm., 2006,
69, 93.
3 S. Kobayashi, T. Wakida, S. H. Niu, S. Hazama, C. Doi and Y. Sasakit,
J. Soc. Dyers Colour., 1995, 111, 111.
28 D. Nedeltcheva, L. Antonov, A. Lyčka, B. Damyanova and S. Popov,
4 A. Abbotto, L. Beverina, N. Manfredi, G. A. Pagani, G. Archetti,
H. G. Kuball, C. Wittenburg, J. Heck and J. Holtmann, Chem.–Eur. J.,
2009, 15, 6175.
Curr. Org. Chem., 2009, 13, 217.
29 F. Wüthner, S. Ahmed, C. Thalacker and T. Debaerdemaeker, Chem.–Eur.
J., 2002, 8, 4742.
5 S. Kawata and Y. Kawata, Chem. Rev., 2000, 100, 1777.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.