Synthesis of metal-azo dyes and their optical and thermal properties as recording materials for DVD-R

Article abstract of DOI:10.1246/bcsj.75.2067

Four kinds of azo dyes and their metal-azo dyes were synthesized in order to obtain a high reflective index and better thermal stability as a recording material for DVD-R. Several kinds of novel azo dyes based on benzoic acid, 6-methoxybenzothiazole, benzothiazole-6-carboxylic acid, and benzimidazole have been synthesized. These azo dyes reacted with metal ions, such as Ni²⁺, Cu²⁺, Zn²⁺, and Co²⁺, as ligands. All structures of the compounds were confirmed by ¹H-NMR and FT-IR analyses. The optical and thermal properties of azo dyes and their metal-azo dyes were characterized by UV-vis and TGA analyses. In the UV-vis spectra, the absorption maxima of metal-azo dyes show at longer wavelength than those of azo dyes. These spectral shifts are affected by substituent groups, electron-donating or -withdrawing groups, of azo dyes and metal ions. The metal-azo dyes were thermally stable up to 300°C, which were confirmed by TGA analyses. Particularly, in the case of an azo dye complexed with Zn based on 6-methoxybenzothiazole, good absorption at 635 nm was observed, and its thermal stability was maintained up to 300°C, with high refractive index of about 2.08.

Full text of DOI:10.1246/bcsj.75.2067

c. Jpn.
e Chemical © 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 2067–2070 (2002) 2067
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Synthesis of Metal-Azo Dyes and Their Optical and Thermal Properties
as Recording Materials for DVD-R
Synthesis of Metal-Azo Dyes and Their Properties
H. Park et al.
,
†
Hyeyoung Park, Eung-Ryul Kim, Dong Jin Kim,* and Haiwon Lee*
02
6
7
7
0
Department of Chemistry, Hanyang University, Seoul 133-791, Korea
†
Medicinal Chemistry Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea
SJA8
0
9-2673
(
Received January 8, 2002)
0
0
0
06
Four kinds of azo dyes and their metal-azo dyes were synthesized in order to obtain a high reflective index and bet-
2
2
ter thermal stability as a recording material for DVD-R. Several kinds of novel azo dyes based on benzoic acid, 6-meth-
oxybenzothiazole, benzothiazole-6-carboxylic acid, and benzimidazole have been synthesized. These azo dyes reacted
2
+
2+
2+
2+
1
with metal ions, such as Ni , Cu , Zn , and Co , as ligands. All structures of the compounds were confirmed by H-
NMR and FT-IR analyses. The optical and thermal properties of azo dyes and their metal-azo dyes were characterized by
UV-vis and TGA analyses. In the UV-vis spectra, the absorption maxima of metal-azo dyes show at longer wavelength
than those of azo dyes. These spectral shifts are affected by substituent groups, electron-donating or -withdrawing
groups, of azo dyes and metal ions. The metal-azo dyes were thermally stable up to 300 °C, which were confirmed by
TGA analyses. Particularly, in the case of an azo dye complexed with Zn based on 6-methoxybenzothiazole, good ab-
sorption at 635 nm was observed, and its thermal stability was maintained up to 300 °C, with high refractive index of
about 2.08.
.
.
2
3
Optical data storage has become a core technology in the
multimedia environment for both the entertainment and com-
puting industries. Rapidly increased information solicits a
groups of azo dyes and metal ions.
Experimental
1–2
more convenient and efficient storage method. The structure
of a DVD-R (Digital Versatile Disc-Recordable) consists of a
polycarbonate substrate, a recording layer, a metal reflective
Samples. 2-Aminobenzoic acid (1), 2-amino-benzothiazole-
-carboxylic acid (2), 2-amino-6-methoxy benzothiazole (3), 2-
6
aminobenzimidazole (4), 3-(diethylamino)phenol (5), sodium hy-
drogen carbonate, sulfuric acid, nitrosyl hydrogensulfate, nick-
el(Ⅱ) acetate (a), cupper(Ⅱ) acetate (b), zinc nitrate (c), and co-
balt(Ⅱ) nitrate (d) were obtained from Aldrich Chemical Co. and
were used without further purification.
3
layer, and a protective layer as a kind of optical data storage.
The recording layer is easily decomposed by laser light with
4–6
6
35 and 650 nm wavelengths.
The recording layer has a
high refractive index, so it can effectively absorb light and gen-
erate heat when the laser beam is focused on the film. The heat
is transferred to both the PC substrate and the recording layer.
In order to achieve a better storage medium, the following fac-
tors should be considered for synthesizing new recording ma-
terials: (1) a high refractive index at 635 nm with the most gen-
erally used in a recent DVD player, (2) thermal stability at high
temperature up to 250 °C, and (3) good solubility for a spin
coating process onto a polycarbonate substrate. In general, cy-
anine dyes, phthalocyanine dyes, and metal-azo complexes
Preparation of Azo and Metal Azo Dyes.
Figure 1 shows
synthetic schemes of azo dyes and metal-azo dyes. To the amine
compounds (1–4), (1 mmol) in sulfuric acid (17 mL), 35% ni-
trosyl hydrogensulfate (3.8 g) was slowly added at −20 °C. After
stirring for 2 h at the same temperature, 3-(diethylamino)phenol
(5) (1 mmol) in methanol (20 mL) was added into the resulting di-
azonium solution at −20 °C. After completion of diazo-coupling,
the reaction mixture was neutralized with a saturated sodium hy-
drogencarbonate solution to pH 6. The product was obtained by
filtering and recrystallized with methanol. To the azo dyes (6–9),
7
–10
dyes are used for DVD-R.
stable against light and not durable for repeated reading
use.
As for cyanine dyes, they are un-
(
1 mmol) in methanol (10 mL), metal(Ⅱ) acetate solution (0.5
mmol) in methanol were added. After completion of complex-
ation, the reaction mixture was diluted with water (50 mL). The
precipitated metal azo dyes were filtered, washed with hot water,
and dried.
11–13
Phthalocyanine dyes also have demerits, such as a
worse solubility and higher cost than cyanine dyes. On the
other hand, metal-azo complexes are more stable than cyanine
dyes against light, provide easier control of the wavelength ac-
cording to the substituted groups, and have good thermal sta-
bility with a metal complex. Based upon the consideration of
the above requirements, we designed and synthesized several
azo dyes and their metal-azo dyes. Herein, we report on the
syntheses of the compounds. Their optical and thermal proper-
ties for DVD-R are discussed while considering substituted
Analytical Data for Final Compounds (8, 8a, and 8c).
1
Compound 8: H NMR (CDCl
2
3
) δ = 1.17 (t, 3H), 3.35–3.59 (m,
H), 6.15 (s, 1H), 6.42 (d, 1H), 7.1 (d, 1H), 7.5 (d, 1H), 7.82 (d,
1H), and 8.28 (s, 1H); Anal. Calcd for C H N O S: C, 60.65; H,
1
8
20
4
2
5.66; N, 15.72; S, 9.00%. Found: C, 60.63; H, 5.56; N, 15.66; S,
9.02%; IR (KBr pallet) 3428 (OH) and 1630 cm− (NwN), Com-
1
44 8 4 2
pound 8a: Anal. Calcd for C38H N NiO S : C, 57.08; H, 5.55; N,

2
068 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Synthesis of Metal-Azo Dyes and Their Properties
Fig. 1. Synthesis of azo dyes and metal-azo dyes.
1
KBr pallet) 1597 cm− (NwN), Compound 8c: Anal. Calcd for
Zn: C, 56.60; H, 5.50; N, 13.90; S, 7.95%. Found:
4.01; S, 8.02%. Found: C, 52.25; H, 4.62; N, 13.44; S, 8.37%; IR
1
(
38 44 8 4 2
C H N O S
−
1
C, 57.60; H, 4.27; N, 14.16;S, 9.26%; IR (KBr pallet) 1597 cm
NwN).
Instrumentation. FT-IR spectra were recorded as KBr pel-
(
−
1
lets over the range 4000–200 cm using a Perkin-Elmer system.
1
The H NMR spectra were obtained on a Varian INOVA (400
MHz) spectrometer. Elemental analyses were performed with an
elemental analyzer (EA1110). UV spectra were recorded as PC
films coated by 2.0 wt% over the range 300–800 nm using a Scin-
co spectrophotometer. The refractive indexes were measured by a
Rudolph Technologies ellipsometer. Thermal properties were an-
alyzed with a Rheometric Scientific System (TGA1000, DSC
PLUS) at a heating rate of 10 °C/min under a nitrogen atmo-
sphere.
Fig. 2. UV spectra of MBT-A-X dyes: (a) 8; (b) 8a; (c) 8c.
516 (8), and 489 nm (9), respectively. These spectral shifts are
affected by the substituent groups of the azo dye and metal
1
6
ions. The longer double bond in the azo dye caused longer
max wavelength from the shorter double bond in the normal-
Results and Discussion
λ
The diazonium salt was easily obtained by using sodium ni-
trite. Azo dyes were synthesized by azo coupling. These prod-
ucts were confirmed by the H NMR and FT-IR spectra. FT-IR
ized UV-vis. spectrum. Also a λmax of 7 was observed to be al-
most identical to that of 8. The π–π* transitions of an azo con-
taining benzothiazole moiety were not affected by the substitu-
1
spectra indicate the presence of coordination sites of metal-azo
3
ent group (–COOH, –OCH ). However, a λmax of 7 was ob-
−
1
dyes. A strong band at around 3310 cm corresponding to
OH in the azo ligand was very weak, and a sharp single band
served at a longer wavelength than that of 9 due to a sulfur at-
om. In the normalized UV-vis Spectra, the λmax were observed
at 476 (6a), 537 (7a), 541 (8a), and 522 nm (9a), respectively.
6a and 9a showed a slight absorption shoulder near 635 nm.
In contrast, 8a showed a very broad absorption band at 635
nm; 7a showed a sharp absorption band at 635 nm.
−
1
around 1630 cm corresponding to NwN in the azo ligand
−
1
was shifted to 1597 cm in the spectra of the metal com-
1
4–16
plex.
identical. Therefore, strong coordinations between metal ions
The peak of carbonyl and other peaks were almost
1
4
and hydroxy and azo groups were formed.
Thermal characterizations of the products were accom-
plished by TGA and DSC analyses (Figs. 3 and 4). The onset
temperatures of weight loss were 240 (6), 220 (7), and 243 °C
(8), respectively. The decomposition temperatures were 300
(8a), 310 (8b), 300 (8c), and 280 °C (8d), respectively. Metal-
azo dyes were more thermally stable at about 50 °C higher
than that of the azo dyes. Also, the metal-azo dyes showed
crystallization temperatures as shown in Fig. 4. The melting
temperatures were 179 (8), 187 (8a), and 177 (8c), respective-
ly.
Optical characterization of the products was accomplished
using a UV-vis. spectra. Figure 2 shows the optical properties
of azo and their metal-azo dyes. The recording material should
be decomposed by 635 nm laser absorption; for recording it
should absorb a 635 nm wavelength. Also, for reading the disc
recorded by 635 nm, the same laser source is used. This means
that a material should reflect a little light at 635 nm. Therefore
the value of the absorbance for the recording material is good
at about 0.4–0.5 in the normalized UV-vis. spectrum. The π–
1
8
π* transitions were observed in the spectra of azo dyes.
The recording dye must be soluble in specific organic sol-
vents, such as 2,2,3,3-tetrafluoro-1-propanol (TFP) or diace-
tone alcohol (DAA), 4-hydroxy-4-methyl-2-pentane, because
Their π–π* transitions were exhibited by the absorption with
the maximum absorbances (λmax) at around 427 (6), 512 (7),

H. Park et al.
Bull. Chem. Soc. Jpn., 75, No. 9 (2002) 2069
Table 1. Summary of Physical and Optical Properties for Azo and Metal-Azo Dyes
a)
c)
c)
d)
Product
Solubility
Refractive index
λ
max /nm
Abs.
d
T /°C
b)
number
at 632 nm
at 635 nm
0.0
0.19
0.21
0.02
0.02
0.11
0.20
0.05
6
7
8
9
O
O
O
O
X
O
O
O
O
—
—
1.66
—
—
—
1.87
—
2.08
427
512
516
489
476
537
541
522
517
240
220
243
—
290
250
292
220
284
6
7
8
9
8
a
a
a
a
c
0.54
a) in TFP solvent (O: good, X: bad), b) on Si-substrate,
c) on PC substrate with TFP solvent, d) 10 °C/min under N
2
gas
16–18
high-modulated amplitude of the recording signal.
To im-
prove the recording sensitivity with high refraction and a high
signal amplitude, the recording materials should have high n
values. The n values of the products were measured by an el-
lipsometer with the optimized thickness and optical constant
(k) at 632 nm. The k value means the absorbance at 632 nm, it
was obtained from the UV-vis spectrum. The n values of met-
al-azo dyes are higher than those of azo dyes because the n val-
ue is enhanced by metal. Table 1 gives a summary of the phys-
ical and optical properties for the azo and metal-azo dyes.
Conclusion
In summary, we synthesized four kinds of azo dyes and their
metal-azo dyes in order to obtain a high reflective index and
better thermal stability as a recording material. Almost all of
the metal-azo dyes had good thermal stability up to 300 °C.
From the UV-vis spectrum, it could be seen that the azo dye
complexed with Ni based on 2-amino-benzothiazole-6-carbox-
ylic acid (7a) had good optical property, but its solubility was
very poor. The azo dye complexed with Zn based on 2-amino-
Fig. 3. TGA curves of MBT- A-X dyes: (a) 8; (b) 8a; (c) 8c.
6
-methoxy benzothiazole (8c) showed good optical, thermal,
and solubility properties compared to other metal-azo dyes.
The absorption maximum of 8c was observed at 517 nm and
the decomposition temperature was 280 °C. The UV-vis spec-
trum of 8c included a steep and weak absorption band at 635
nm. Also, the refractive index of 8c was 2.08 at 632 nm wave-
length. Because of the optical and thermal characteristics of
8
c were nearly satisfied, it will be suitable as a recording dye
for DVD-R.
This work was supported by a program of National Re-
search Laboratory, the Ministry of Science and Technology
Fig. 4. DSC curves of MBT-A-X dyes: (a) 8; (b) 8a; (c) 8c.
(
Grant Number: 99-N-NL-01-C-103) and by the National Pro-
gram for Tera-level Nanodevices of the Ministry of Science
and Technology.
the recording layer is applied on a polycarbonate substrate by
the spin-coating method. In general, more than 2.0 wt% solu-
bility is necessary. All of the synthesized materials were solu-
ble in TFP solvent except for 6a. Although the optical proper-
ty of 6a was good, disc fabrication will be hard due to poor sol-
ubility. DVD-R dye has an absorption maximum at 590–610
nm and an absorption edge at the recording or reading wave-
length (635–650 nm), the refractive index (n) value of the dye
is maximized at 650 nm. This high refractive index results in a
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