The influence of fluorination on the structure and properties of azo pigments

Article abstract of DOI:10.1016/j.dyepig.2016.01.027

The synthesis and single crystal X-ray structures of two fluorinated azo pigments and their nonfluorinated analogues derived from N-acetoacetanilide and N,N′-(1,4-phenylene) bis-(acetoacetamide) as coupling components were described. These two kinds of pi

Full text of DOI:10.1016/j.dyepig.2016.01.027

Dyes and Pigments 128 (2016) 246e255
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The influence of fluorination on the structure and properties of azo
pigments
,
,
,
, c, *
Zheng Li a c, Dongjun Lv b **, Xianggao Li a c, Shirong Wang a
a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
b School of Science, Tianjin University, Tianjin 300072, China
c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and single crystal X-ray structures of two fluorinated azo pigments and their non-
fluorinated analogues derived from N-acetoacetanilide and N,N0-(1,4-phenylene) bis-(acetoacetamide) as
coupling components were described. These two kinds of pigments existed in the ketohydrazone and
bisketohydrazone tautomeric form with intramolecular hydrogen bondings respectively. Fluorination
had a profound influence on the structure of molecules and crystals and consequently altered the
morphology and pigmentary performance. The results demonstrated that the solvent resistance was
Received 22 November 2015
Received in revised form
4 January 2016
Accepted 25 January 2016
Available online 3 February 2016
improved upon fluorination due to enhanced intermolecular interactions and efficient
molecules. However, in both cases fluorination gave rise to pigments with poorer stability to heat and
light.
p-packing of
Keywords:
Influence
Fluorination
Structure
© 2016 Elsevier Ltd. All rights reserved.
Properties
Azo pigments
Fastness
1. Introduction
microcrystalline form. However, due to the limited solubility of
organic pigments in solvent, it is difficult to obtain suitable crystals
Fluorine is the most electronegative non-metallic element
widely distributed in nature. As a result of the negative inductive
for X-ray single crystal diffraction. Previous work on crystal engi-
neering of organic pigment using fluorine has suggested that
fluorination modifies the crystal packing of the molecules [6e8].
For example, CuPc and CuF16Pc adopt significantly different stack-
effect, a fluorinated organic compound exhibits different
p-cloud
compared to that of its nonfluorinated analogue and thereby the
structure of molecules and crystals is changed [1]. The special
electronic character of fluorine allows it to act as a hydrogen bond
acceptor, which is of particular interest to the study of organic
compounds [2,3]. The CeF bond is very robust and is thought to
enhance thermal stability, which is important in terms of heat-
resisting materials [4,5]. Furthermore, due to the similar Van der
Waals radii of fluorine to that of hydrogen (1.35 Å and 1.20 Å,
respectively), there is no gross distortion of geometry upon fluo-
rination which is essential in biocompatibility.
ing patterns, the former possesses one-dimensional
pep stacking
similar to alpha type metal phthalocyanine, whereas the latter
takes a herringbone-type stacking and subsequently the electron
mobility is improved. The other cases include fluorinated perylene
bisimide (PBI) dyes and quinacridone derivatives. Nonfluorinated
PBI adopts a slipped stack arrangement while the difluoro-
substituted PBI takes a typical “herringbone” structure and also
owns higher mobility. All of these changes may originate from
intermolecular interactions involving fluorine.
Fluorination has a significant effect on the crystal structure and
technical performance of organic pigments which are used in
However, systematic study on the crystal structure and prop-
erties of fluorinated azo pigments and their nonfluorinated ana-
logues is rare. In order to clarify the influence of fluorine
substitution on the structure and properties of azo pigments, two
fluorinated pigments and their nonfluorinated analogues derived
from N-acetoacetanilide and N,N0-(1,4-phenylene)bis-(acetoaceta-
mide) as coupling components have been synthesized and their
crystal structures described, both of which is shown in Fig. 1.
* Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, China. Tel.: þ86 22 2740 4208.
** Corresponding author. Tel.: þ86 22 2740 4208.
E-mail addresses: lvdongjun@163.com (D. Lv), wangshirong@tju.edu.cn
(S. Wang).
http://dx.doi.org/10.1016/j.dyepig.2016.01.027
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
247
Fig. 1. Molecular structure of as-synthesized compounds.
A comparison of the pigmentary properties in relation to crystal
structures provides some clues for molecular design of organic
pigments in future.
provide a yellow filtrate. The diazonium liquor was then added to
the suspension of N-acetoacetanilide (9.30 g, 0.0525 mol) in water
with mechanical agitation over 30 min and maintained for 1 h at
room temperature. The terminal was detected using H-acid test.
The slurry was heated at 70 ꢀC for 1 h, then filtered and washed salt
free with water. The residue was dried under vacuum at 60 ꢀC to
obtain a greenish yellow solid (3.2 g, 93.6%). Crystals 1-a suitable
for X-ray analysis were obtained by slow evaporation of an ethanol
2. Experimental section
2.1. Materials and methods
solution of the complex at room temperature. IR (KBr, n
/cmꢁ1):
All chemicals used were of analytical reagent (Aladdin or Sigma
products). All the measurements were carried out at room tem-
perature (298 K) except the single crystal X-ray diffraction (113
(2) K).
3446 (amide I, NeH bond), 1662 (ketone, C]O stretch), 1523
(amide II, NeH bond); ES-MS: calcd for C16H15N3O2 Mþ 282.1237,
found 282.1260; 1H NMR (400 MHz, CDCl3)
d
14.82 (d, J ¼ 47.4 Hz,
1H), 11.49 (s, 1H), 7.64 (t, J ¼ 10.2 Hz, 2H), 7.50e7.33 (m, 6H),
7.25e7.14 (m, 2H), 2.61 (s, 3H); 13C NMR (101 MHz, CDCl3)
199.50
The infrared spectrum were obtained as KBr disks on a Thermo
Nicolet 380 spectrometer. The UVevis spectra were recorded in the
range of 200e600 nm on a Thermo Spectronic spectrometer using
1 cm matched quartz cells. All MS data were collected using a
microTOF-Q II mass spectrometer (Bruker Daltonic Inc.). NMR
spectra were obtained on a Bruker AVANCE III 400 NMR spec-
trometer with CDCl3 as solvent. Thermogravimetric analysis was
carried out on a Mettler TGA/DSC thermal analyzer at a heating rate
of 10 ꢀC/min under a flowing nitrogen atmosphere. Scanning
electron microscope (SEM) were collected on a Rigaku XtaLab P200
diffractometer. Light fastness testing was carried out on a Solar
Simulator PL-X500C (Xe-lamp, 400e1100 nm). The particle size
measurement was taken on a Delsa Nano S particle size analyzer.
d
(s), 163.06 (s), 141.73 (s), 137.20 (s), 129.64 (s), 129.05 (s), 126.09 (s),
125.40 (s), 124.80 (s), 120.94 (s), 115.90 (s), 26.12 (s).
Compound 1-b was obtained as a bright yellow solid (14.4 g,
96.3%) according to Section 2.2.1. Crystals 1-b suitable for X-ray
analysis were obtained by slow evaporation of a mixed petroleum
ether/chloroform (1:3) solution of the complex at room tempera-
ture. IR (KBr,
stretch), 1516 (amide II, NeH bond); ES-MS: calcd for
16H14N3O2F Mþ 300.1161, found 300.1143; 1H NMR (400 MHz,
n
/cmꢁ1): 3444 (amideⅠ, NeH bond), 1660 (ketone, C]
O
C
CDCl3)
d
14.81 (s, 1H), 11.47 (s, 1H), 7.63 (d, J ¼ 7.7 Hz, 2H), 7.39 (dd,
J ¼ 10.5, 5.1 Hz, 4H), 7.15 (dt, J ¼ 17.1, 8.0 Hz, 3H), 2.59 (s, 3H); 13C
NMR (101 MHz, CDCl3) d 199.34 (s), 163.09 (s), 138.07 (s), 137.11 (s),
129.05 (s), 126.08 (s), 124.86 (s), 120.94 (s), 117.22 (d, J ¼ 8.1 Hz),
2.2. Synthesis
116.62 (s), 116.39 (s), 26.07 (s); 19F NMR (377 MHz, CDCl3)
(s).
d 116.76
2.2.1. Synthesis and characterization of 3-oxo-N-phenyl-2-
(phenyldiazenyl)butanamide (1-a) and 2-((4-fluorophenyl)
diazenyl)-3-oxo-N-phenylbutanamide (1-b)
2.2.2. Synthesis and characterization of N,N0-(1,4-phenylene)bis(3-
oxo-2-(phenyldiazenyl)butanamide) (2-a) and N,N0-(1,4-
phenylene)bis(2-((4-fluorophenyl)diazenyl)-3-oxobutanamide) (2-
b)
Aniline (3.73 g, 0.04 mol) was stirred with concentrated hy-
drochloric acid (9.9 mL) in water (19 mL). A solution of sodium
nitrite (2.90 g, 0.042 mol) in water (6.8 mL) was added with stirring
over 30 min keeping the temperature below 5 ꢀC with ice cooling
for 30 min. The pH was then adjusted to 6 by addition of sodium
acetate anhydrous. The presence of a nitrous acid excess of sodium
Aniline (4.66 g, 0.05 mol) was stirred with concentrated hy-
drochloric acid (12.4 mL) in water (24 mL). A solution of sodium
nitrite (3.62 g, 0.0525 mol) in water (8.5 mL) was added with stir-
ring over 30 min keeping the temperature below 5 ꢀC and held it for
30 min. The pH was then adjusted to 6 by addition of sodium ac-
etate anhydrous. The presence of excess sodium nitrite was indi-
cated using starch/KI paper. Before the coupling, a few drops of
aqueous urea solution were added to destroy excess nitrous acid.
The reaction mixture was filtrated, and washed with water to

248
Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
for X-ray analysis were obtained by slow evaporation at 0e5 ꢀC of
solutions of the pigment in tetrahydrofuran. IR (KBr,
/cmꢁ1): 3442
n
(amide I, NeH bond), 1657 (ketone, C]O stretch); ES-MS: calcd for
C
26H24N6O4 Mꢁ 483.1775, found 483.1766; 1H NMR (400 MHz,
CDCl3)
d
14.78 (s, 2H), 11.53 (s, 2H), 7.68 (s, 4H), 7.45 (d, J ¼ 4.2 Hz,
8H), 7.22 (dt, J ¼ 8.6, 4.3 Hz, 2H), 2.63 (s, 6H).
Compound 2-b was obtained as a brownish yellow powder
(9.7 g, 93.2%) synthesized according to Section 2.2.2. Solvates 2-b
suitable for X-ray analysis were obtained from slow evaporation
of a dichloromethane solution of the complex at 0e5 ꢀC. IR (KBr,
n/
cmꢁ1): 3438 (amide I, NeH bond), 1660 (ketone, C]O stretch); ES-
MS: calcd for C26H22N6O4F2 Mꢁ 519.1587, found 519.1581; 1H NMR
(400 MHz, CDCl3)
d 14.78 (s, 2H), 11.53 (s, 2H), 7.68 (s, 4H), 7.45 (d,
J ¼ 4.2 Hz, 8H), 7.22 (dt, J ¼ 8.6, 4.3 Hz, 2H), 2.63 (s, 6H); 19F NMR
Fig. 2. Molecular structure of 1-a with 30% probability displacement ellipsoids
(intramolecular hydrogen bonds indicated).
(377 MHz, CDCl3)
d 116.74 (s).
2.3. Crystal data
The X-ray diffraction data for crystals were collected with MoK
a
radiation (0.71073 Å) at 113(2) K, using a Rigaku XtaLAB P200
diffractometer. The structures were solved by direct methods and
refined by full-matrix least-squares on F2 using the SHELX-97
program package. The non-hydrogen atoms were refined aniso-
tropically. H atoms bound to N or O atoms were located in the
difference Fourier map, their coordinates freely refined and their
displacement parameters were treated as riding on the bound
atom.
The crystallographic data (excluding structure factors) have
been deposited with the Cambridge Crystallographic Data Centre as
the supplementary publication CCDC 1405080, 1403624, 1437541
and 1406682. These data can be obtained free of charge via https://
summary.ccdc.cam.ac.uk/structure-summary-form.
Fig. 3. Molecular structure of 1-b with 30% probability displacement ellipsoids
(intramolecular hydrogen bonds indicated).
Compound 1-a: C16H15N3O2, Mr ¼ 281.31; yellow rod-like
crystals 0.20 ꢂ 0.18 ꢂ 0.12 mm3; monoclinic, space group P21/n;
nitrite was indicated using starch/KI paper. Before the coupling, a
few drops of aqueous urea solution were added to destroy excess
nitrous acid. The reaction mixture was filtrated and washed with
water to obtain a yellow diazon solution. The diazon solution was
added to the suspension of N,N0-(1,4-phenylene)bis-(acetoaceta-
mide) (5.80 g, 0.021 mol) in methanol with mechanical agitation
over 30 min and kept for 1 h at room temperature. The terminal
was detected by H-acid test. The slurry was filtered, then washed
with water, methanol and ethyl acetate until the colatuie was col-
ourless. The presscake was dried under vacuum at 60 ꢀC for 12 h to
provide a greenish yellow solid (8.9 g, 91.8%). Solvates 2-a suitable
a ¼ 14.751(3) Å, b ¼ 4.4229(7) Å, c ¼ 21.480(4) Å,
a
¼ 90ꢀ,
b
¼
104.786(3)ꢀ,
g
l
¼
90ꢀ;
V
¼
1355.0(4) Å3;
radiation);
(I), goodness-of-fit on
Z
¼
4;
Dc ¼ 1.379 g cmꢁ3
;
¼ 0.71073 Å (MoK
a
m
¼ 0.094;
F ¼ 592; R1 ¼ 0.0362, wR2 ¼ 0.0979 for I > 2
s
F2 ¼ 1.074.
Compound 1-b: C16H14FN3O2, Mr ¼ 299.30; yellow needle
crystals 0.20
a ¼ 7.6813(13) Å, b ¼ 9.5520(13) Å, c ¼ 11.392(2) Å,
ꢂ
0.18
ꢂ
0.12; triclinic, space group P1;
a
¼ 99.62(5)ꢀ,
b
¼
108.04(3)ꢀ,
g
¼
110.07(3)ꢀ;
V
¼
710.7(3) Å3;
Z
¼
2;
Dc ¼ 1.399 g cmꢁ3
;
l
¼ 0.71073 Å (MoK
a
radiation);
m
¼ 0.104;
Table 1
Selected geometric parameters for compounds 1-a, 1-b, 2-a and 2-b (Å, ꢀ).
1-a
1.3060(13)
1.4101(14)
1.3253(14)
1.2421(14)
177.85(11)
ꢁ177.30(10)
2.61(18)
1-b
1.303(3)
1.405(3)
1.321(3)
1.243(2)
ꢁ178.9(2)
166.2(2)
0.6(4)
2-a
2-b
N(2)eN(3)
N(3)eC(11)
N(2)eC(8)
C(7)eO(1)
C(11)eN(3)eN(2)eC(8)
N(2)eN(3)eC(11)eC(16)
N(2)eC(8)eC(7)eO(1)
C(7)eN(1)eC(1)eC(6)
N(1)eN(2)
N(1)eC(1)
N(2)eC(7)
C(10)eN(3)
N(2)eN(1)eC(1)eC(6)
N(2)eC(7)eC(10)eO(2)
N(2)eC(7)eC(10)eN(3)
C(7)eC(10)eN(3)eC(11)
1.304(6)
1.410(6)
1.319(1)
1.351(1)
166.33(10)
1.57(15)
1.305(4)
1.400(5)
1.317(4)
1.342(5)
ꢁ176.7(3)
3.7(5)
ꢁ177.94(9)
ꢁ175.9(3)
178.6(3)
ꢁ168.98(11)
179.9(2)
179.08(9)
Table 2
Intramolecular hydrogen bonds in compounds 1-a and 1-b.
DeH/A
d (DeH)
d (H/A)
d (D/A)
<(DHA)
1-a
1-b
1-a
1-b
1-a
1-b
1-a
1-b
N(1)eH(1)/O(2)
N(3)eH(3)/O(1)
0.915(14)
1.063(18)
0.83(3)
0.92(3)
1.880(14)
1.657(17)
1.93(2)
1.84(3)
2.6728(14)
2.5424(13)
2.658(2)
2.562(2)
143.7(11)
137.3(14)
145(2)
134(3)

Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
249
Table 3
Intramolecular interaction in compounds 2-a and 2-b.
DeH/A
d (D-H)
d (H/A)
d (D/A)
<(DHA)
2-a
2-b
2-a
2-b
2-a
2-b
2-a
2-b
N(1)eH(1)/O(2)
N(3)eH(3)/O(1)
N(4)eH(4)/O(4)
N(6)eH(6)/O(3)
0.898(15)
0.898(11)
0.898(10)
0.895(10)
0.897(10)
0.898(10)
1.872(15)
1.890(15)
1.83(3)
1.93(3)
1.90(3)
1.83(3)
2.5791
2.6704
2.556(4)
2.668(4)
2.653(4)
2.566(4)
134.1
144.2
137(3)
139(3)
140(3)
137(3)
F ¼ 312; R1 ¼ 0.0615, wR2 ¼ 0.1605 for I > 2
s
(I), goodness-of-fit on
3. Results and discussion
3.1. The geometric configuration and molecular packing
F2 ¼ 0.997.
Compound 2-a: C26H24N6O4$C4H8O, Mr ¼ 556.62; yellow rod-
like crystals 0.20 ꢂ 0.18 ꢂ 0.12; monoclinic, space group C2/c;
a ¼ 20.397(3) Åꢀ, b ¼ 6.7384(7) Å, c ¼ 21.441(3) Å,
a
¼ 90.00ꢀ,
Figs. 2 and 3 show the molecular structures and indicate the
atomic labelling schemes for compound 1-a and 1-b. There exists
significant difference in molecular configurations in view of the
similarities between the molecular structures. The single crystal X-
ray crystallographics reveal that they adopt ketohydrazone
tautomeric form, which is in accord with the previous studies [11].
The N(2)eN(3), N(2)eC(8) and C(7)eO(1) bond lengths (Table 1),
similar in magnitude to the previously reported acetoacetanilide
pigments [12], can be used to confirm these configurations. Both
1-a and 1-b deviate from planarity, however, the mean deviation
from the plane defined by all non-hydrogen atoms is 0.1118 Å and
0.0938 Å respectively, which indicate better planarity of molecules
upon fluorination. The benzene rings are inclined at an angle of
17.9ꢀ with respect to the other in 1-a while the same torsion angle
is 9.9ꢀ in 1-b. A noticeable observation is the torsion angle of
N(2)eN(3)eC(11)eC(16) and C(7)eN(1)eC(1)eC(6) (Table 1),
which is ꢁ177.30(10)ꢀ, ꢁ168.98(11)ꢀ for 1-a and 166.2(2)ꢀ,
179.9(2)ꢀ for 1-b, which indicate an opposite torsion direction by
fluorine substitution. The shortest (strongest) contacts in 1-a and
1-b is NeH/O intramolecular hydrogen bonding involving N(3)/
O(1) and N(1)/O(2). The bond length of NeH/O in 1-a and 1-b is
almost identical, as shown in Table 2. However, the deviation of
O(2) from the molecular plane causes a weakening of N(1)eH(1)/
O(2) hydrogen bonding. Compared to phenyl ring CeC bond
lengths, there exists significant differences (Supporting materials),
which may result in the instability of phenyl rings and affect the
stability of pigments.
b
¼
113.159(3) ,
g
¼
90.00ꢀ;
¼ 0.71073 Å (MoK
V
¼
2709.4(6) Å3;
a
Z
¼
4;
Dc ¼ 1.365 g cmꢁ3
;
l
radiation);
s(I), goodness-of-fit on
m
¼ 0.095;
F ¼ 1176; R1 ¼0.0348, wR2 ¼ 0.0949 for I > 2
F2 ¼ 1.043.
Compound 2-b: C26H22F2N6O4$CH2Cl2, Mr ¼ 605.42; orange
needle crystals 0.20 ꢂ 0.18 ꢂ 0.12; monoclinic, space group P21/c;
a ¼ 8.240(5) Å, b ¼ 35.420(16) Å, c ¼ 9.482 (5) Å,
a
¼ 90.00ꢀ,
b
¼
101.761(13)ꢀ,
g
l
¼
90.00ꢀ;
V
¼
2709(2) Å3;
radiation);
(I), goodness-of-fit on
Z
¼
4;
Dc ¼ 1.484 g cmꢁ3
;
¼ 0.71073 Å (MoK
a
m
¼ 0.300;
F ¼ 1248; R1 ¼ 0.0680, wR2 ¼ 0.1767 for I > 2
s
F2 ¼ 1.050.
2.4. Preparation of industrial stoving paints and assessment of
pigment properties
The industrial stoving paints was prepared according to the
previous reported procedures [10].
2.4.1. Overspray fastness
Drawdowns of the cured thermosetting acrylic melamine paint
were oversprayed with the white paint and cured at 100 ꢀC for
10 min. The bleed into the white overspray patterns were assessed
as the colour difference (DE) comparison with the corresponding
white finish.
2.4.2. Light fastness
Crystals 1-a and 1-b adopt different packing arrangements
(Figs. 4e7). In crystals 1-a, molecules form inclined stacks of inter-
leaved molecules at the angle of about 80ꢀ by C(15)eH(15)/O(2)
interaction. In crystals 1-b, there stacks in columns parallel to the
(010) plane with adjacent molecules antiparallel. There is no
Light fastness was measured by exposing samples of paint films
on a Solar Simulator PL-X500C (Xe-lamp, 400e1100 nm) at the
intensity of 75 mW/cm2 for 4, 8, 12, 16, 20, 24, 28 h. The light
fastness was assessed as the colour difference (
exposed and unexposed samples.
DE) between the
Fig. 4. Part of packing of crystals 1-a viewed down the a-axis.

250
Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
Fig. 5. Part of packing of crystals 1-a viewed down the b-axis.
Fig. 6. Part of packing of crystals 1-b viewed down the a-axis (intermolecular hydrogen bonds indicated).
intermolecular hydrogen bonds in 1-a crystals but it's not the case
for 1-b, there is C(15)eH(15)/O(1) intermolecular hydrogen
bonding between the two adjacent molecules in the same layer. The
centroid to centroid distances of adjacent molecules in 1-a crystals is
4.4229(12) Å while the same distance in 1-b crystals is 4.1325(16) Å,
which indicate more effective packing of the molecules.
As to compounds 2-a and 2-b, there has an obvious difference
in the molecular structure (Figs. 8 and 9). Both pigments exist

Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
251
Fig. 7. Part of packing of crystals 1-b viewed down the c-axis (intermolecular hydrogen bonds indicated).
Fig. 8. Molecular structure of 2-a with 30% probability displacement ellipsoids (solvent molecules. omitted for clarity and intramolecular hydrogen bonds indicated).
Fig. 9. Molecular structure of 2-b with 30% probability displacement ellipsoids (solvent molecules omitted for clarity and intramolecular hydrogen bonds indicated).
bisketohydrazone tautomeric form, which is consistent with the
previous research [13]. Solvates 2-a crystallize in the C2/c space
group with one tetrahydrofuran molecule while Solvates 2-b
crystallize in the P21/c space group with one dichloromethane
molecule. The shorter N/O distance involving N(1)/O(2) and
N(3)/O(1) indicate stronger intramolecular interactions (Table 3),
which could rationalize the better planarity of 2-b compared with
that of 2-a. Solvates 2-a and 2-b adopt different molecular packing
modes. The molecules of solvates 2-a lie in layers parallel to the a-
axis (Fig. 10). One half of each molecule lies sandwiched between
adjacent layers and the molecules are parallel to each other. The
molecules of solvates 2-b pack as columns composed of two stacks
in a herringbone arrangement and the two stacks meet where F(2)
is 2.67 Å from H(3A) in the neighbouring stack viewed down the c-
axis (Fig. 11). O(3) involving solvent molecule makes a CeH/O
hydrogen bond with H(9A) (2.32 Å) within the layers in 2-a sol-
vates. Nevertheless, there are two short CeH/O intermolecular
hydrogen bonding (less than 2.8 Å) involving C(25)/O(4) and
C(5)/O(1) (less than 3.6 Å) in 2-b solvates. Furthermore, within
the stacks of 2-b solvates the shortest CeX/p interaction of
3.295(4) Å is between C(17)eO(3) and the ring [C(1)eC(6)]
centroid at 1 ꢁ x, 2 ꢁ y, 1 ꢁ z.

252
Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
Fig. 10. Part of packing of 2-a solvates viewed down the a-axis (intermolecular hydrogen bonds indicated).
Fig. 11. Part of packing of 2-b solvates viewed down the c-axis (intermolecular hydrogen bonds indicated).
Table 4
pep
interactionsa in 2-b solvates.
Cg / Cg
CgeCg distance
Symmetric code
Cg(1)
Cg(2)
Cg(3)
Cg(1) / Cg(3)
Cg(2) / Cg(2)
3.7827
3.6392
1 ꢁ x, ꢁy, 1 ꢁ z
2 ꢁ x, ꢁy, 1 ꢁ z
x, 1/2 ꢁ y, 1/2 þ z
a
Cg represents the centroid of the following rings: Cg1 ring (C1eC6), Cg2 ring (C11eC16), Cg3 ring (C21eC26).
The molecules in 2-b solvates overlap more closely than that of
2-a solvates with the closest centroid to centroid distance
3.639(3) Å (Cg2 / Cg2) at ꢁx, 2 ꢁ y, 1 ꢁ z (Table 4).
compared to that in solution. The observed smaller red shift of 1-b
may be due to weaker exciton coupling caused by the dihedral
angle (9.9ꢀ) between phenylhydrazone ring (C1eC6) and fluori-
nated benzene ring (C11eC16) in different molecules antiparallel.
However, the observed differences in film for 2-a and 2-b is mainly
attributed to CeH/O intermolecular hydrogen bondings involving
3.2. The technical performance
C(5)/O(1) and C(25)/O(4) and
Cg1 / Cg3) (Table 4).
pep interactions (Cg2 / Cg2 and
The UV/visible absorption spectra of compounds 1-a and 1-b in
chloroform solution are almost identical. Similarly, the UV/visible
absorption spectra for compounds 2-a and 2-b in chloroform so-
lution are also nearly the same. Thus, fluorination has little effect on
the electronic properties of the molecules in solution. However, it
was not the case for UV/visible absorption spectra in solid state, for
which both fluorinated pigments 1-b and 2-b showing a bath-
ochromic shift of the values of lmax compared with their non-
fluorinated analogues (Table 5). There has a bathochromic shift of
11 nm and 5 nm for compound 1-a and 1-b in film respectively
Table 5
Solution state and solid film UV/visible absorption spectra of the target compounds.
Compound
lmax/nm (solution)
lmax/nm (film)
Dlmax/nm
1-a
1-b
2-a
2-b
380
379
389
387
391
384
395
400
11
5
6
13

Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
253
Fig. 12. Scanning electron microscope images of pigments (a) 1-a; (b) 1-b; (c) 2-a; (d) 2-b.
Morphological differences between the fluorinated and non-
fluorinated pigments are shown in Fig. 12(a)e(d). Compound 1-a
Table 6
The mean diameter and polydispersity index of compounds 1-a, 1-b, 2-a and 2-b.
has a thick rod morphology given the expected high growth rate
Compound
Mean diameter (nm)
Poly-dispersity index
in the
p-p direction [14], whereas 1-b exists as irregular flakes in
1-a
1-b
2-a
2-b
1972.8
784.7
2960.6
637.5
0.499
0.261
0.234
0.240
different sizes. Compound 2-a exhibits prismatic crystals and packs
closely to bulks while compound 2-b forms a mixture of rods and
plates in different sizes. In both cases the fluorination give smaller
crystals, supported by the particle size measurement (Table 6),
especially for pigment 2-b which has two fluorine substitutes in its
molecules.
Table 7
The overspray fastness assessed as colour difference (
D
E).
Solvent resistance assessed as overpaint fastness is related to
the efficiency of molecular packing in crystals and to the strength of
intermolecular forces [15,16]. The poor resistance to solvent of
monazo (Hansa) yellow pigments may be interpreted by small
molecular size and intermolecular interactions involving only van
der Waals' forces, accounting for their limited application in in-
dustrial paints. There is CeH/O hydrogen bonding involving C(15)
and O(1) in 1-b crystals as well as more effective packing of mol-
ecules, which could rationalize a slight improved resistance to
solvent compared with that 1-a crystals (Table 7). For 2-a solvates
crystals, there is only C(9)eH(9A)/O(3) hydrogen bonding
involving solvent molecules while there are CeH/O hydrogen
Pigment
Overspray fastness
1-a
1-b
2-a
2-b
12.6
10.8
7.4
2.1
Table 8
Effect of light exposure on CIE L*a*b* DE value in full strength industrial paints.
Pigment
DE
bondings involving C(25)/O(4) and C(5)/O(1) as well as CeX/
p
4 h
1.59
6.86
6.97
8 h
6.64
9.03
7.08
12 h
16 h
20 h
24 h
28 h
interaction[C(17)eO(3)/Cg1] and interactions (Cg2 / Cg2
pep
1-a
1-b
2-a
2-b
6.93
10.68
7.58
7.07
11.86
8.48
7.49
12.44
9.56
7.68
12.95
10.08
36.97
8.43
14.38
11.26
37.46
and Cg1 / Cg3). It's not surprising that 2-b possess a significantly
better solvent resistance than 2-a.
27.69
29.97
31.84
33.0
35.81
The light fastness in full strength industrial paints and in 1:3
white reductions assessed by colour difference is illustrated in
Tables 8 and 9 and Fig. 13. The results demonstrate that in both
cases fluorination results in poorer light fastness. On the basis of
previous studies [17], the amide NeH group was the vulnerable
site for photochemical attack, increasing the strength of the
amide NeH bond would improve light fastness. For 1-a and 1-b,
the deviation of O(2) from molecular planarity causes a weak-
ening of the N(1)eH(1)/O(2) hydrogen bonding, which may be
a probable factor in explaining the inferior light fastness of 1-b.
In the same sense, as O(1) deviate from the molecular planarity,
it causes a weaker N(3)eH(3)/O(1) hydrogen bonding of 2-b
Table 9
Effect of light exposure on CIE L*a*b* DE value in 1:3 white reductions.
Pigment
DE
4 h
8 h
4.15
5.78
6.04
12 h
16 h
20 h
24 h
28 h
1-a
1-b
2-a
2-b
1.25
3.62
4.60
7.96
5.82
7.74
7.21
6.34
8.45
8.54
7.26
9.27
10.93
18.53
7.84
10.62
11.12
19.24
8.13
11.18
11.67
21.48
10.51
13.53
15.67

254
Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
Fig. 13. Effect of light exposure on CIE L*a*b* DE value for: (a) 1-a and 1-b (full strength); (b) 1-a and 1-b (1:3 white reduction); (c) 2-a and 2-b (full strength); (d) 2-a and 2-b (1:3
white reduction).
Fig. 14. The thermal gravimetric curves of compounds (a) 1-a and 1-b; (b) 2-a and 2-b.
which lead to poorer light fastness. Furthermore, light fastness is
also influenced by the particle size, according to particle size
measurement above, fluorination gives rise to smaller size and
thereby it has an adverse effect on light fastness. Generally, the
inferior light fastness upon fluorination may be attributed to
weaker hydrogen bonding as well as the smaller particle size.
The thermal stability assessed by decomposition temperature
shown in Fig. 14 is also essential for organic pigments. The results
reveal apparent influence on thermal behaviour upon fluorination.
The decomposition temperature is 7 ꢀC and 67 ꢀC lower in the case
of 1-b and 2-b than that of their nonfluorinated analogues
respectively, owing to the distortion of the benzene rings as the

Z. Li et al. / Dyes and Pigments 128 (2016) 246e255
255
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.01.027.
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