The synthesis of novel asymmetrical benzoins and bis(dithiobenzil) metal complex dyes containing ethoxyethoxy groups and their absorption characteristics

Article abstract of DOI:10.1080/15421406.2012.691676

Asymmetrical bis(dithiobenzil) metal complex (BMC), as a NIR(Near Infra-Red) absorbing dye, was synthesized via preparation of asymmetric benzoin intermediates. Four synthesized dyes were characterized by elemental analyses and GC-Mass, ¹H-NMR(nuclear magnetic resonance) analysis. The absorption characteristics of the synthesized dyes in the NIR region were measured using UV-VIS spectrophotometer. Maximum absorbing wavelength was shifted depending on the character and position of substituents in the bis(dithobenzil) metal complex.

Full text of DOI:10.1080/15421406.2012.691676

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The Synthesis of Novel Asymmetrical
Benzoins and Bis(dithiobenzil)
Metal Complex Dyes Containing
Ethoxyethoxy Groups and Their
Absorption Characteristics
Na-Rim Kim a & Jae-Hong Choi a
a Department of Textile System Engineering , Kyungpook National
University , Daegu , 702–701 , Korea
Published online: 20 Aug 2012.
To cite this article: Na-Rim Kim & Jae-Hong Choi (2012) The Synthesis of Novel Asymmetrical
Benzoins and Bis(dithiobenzil) Metal Complex Dyes Containing Ethoxyethoxy Groups and
Their Absorption Characteristics, Molecular Crystals and Liquid Crystals, 564:1, 50-55, DOI:
10.1080/15421406.2012.691676
To link to this article: http://dx.doi.org/10.1080/15421406.2012.691676
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Mol. Cryst. Liq. Cryst., Vol. 564: pp. 50–55, 2012
Copyright © Kyungpook National University
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.691676
The Synthesis of Novel Asymmetrical Benzoins
and Bis(dithiobenzil) Metal Complex Dyes
Containing Ethoxyethoxy Groups and Their
Absorption Characteristics
NA-RIM KIM AND JAE-HONG CHOI∗
Department of Textile System Engineering, Kyungpook National University,
Daegu 702–701, Korea
Asymmetrical bis(dithiobenzil) metal complex (BMC), as a NIR(Near Infra-Red) ab-
sorbing dye, was synthesized via preparation of asymmetric benzoin intermediates.
1
Four synthesized dyes were characterized by elemental analyses and GC-Mass, H-
NMR(nuclear magnetic resonance) analysis. The absorption characteristics of the syn-
thesized dyes in the NIR region were measured using UV-VIS spectrophotometer. Max-
imum absorbing wavelength was shifted depending on the character and position of
substituents in the bis(dithobenzil) metal complex.
Keywords Bis(dithiobenzil) metal complex; NIR absorbing dye; asymmetrical ben-
zoin; absorption maximum
Introduction
Bis(dithiobenzil) metal complex (Figure 1) is known to be effective in inhibiting the laser-
induced fading of colored thin layers on recording disks. It has also been applied to the
optical filter on PDP (Plasma Display Panel) panel for NIR absorbing [1,2]. NIR radiations
from the PDP affects the electronic equipment located around the PDP, such as a cordless
television or a video deck making use of a near infrared light remote controller, and causes
malfunctions. Therefore, it is necessary to use NIR dyes which absorb maximally in the
range of 850 ∼ 950nm for the optical filter of PDP. However, most symmetric nickel
complexes containing the same four substituents in the phenyl ring or unsubstituted phenyl
groups show the absorption maximum to be shorter than 850 nm which are not suitable for
the optical filter [3].
In this work, some different substituents have been introduced into the phenyl rings to
make the nickel complex dyes to be bathochromically shifted in comparison with that of
unsubstituted bis(dithiobenzil) metal complex. As shown in Figure 2, ethoxyethoxy groups
were introduced to phenyl ring of intermediate 1 by O-alkylation reaction. Then, electron
withdrawing group(chloro) or electron donating group(methoxy) was further introduced
into the phenyl groups of bis(dithiobenzil) metal complexes.
∗Address correspondence to Prof. Jae hong Choi, Department of Textile System Engineer-
ing, Kyungpook National University, Sangyuk-dong, Buk-gu, Daegu 702–701, Korea (ROK). Tel:
(+82)53-950-5644; Fax: (+82)53-950-6617. E-mail: jaehong@knu.ac.kr
[322]/50

Synthesis of Novel Asymmetrical Benzoins and BMC
[323]/51
Figure 1. Chemical structure of unsubstitued bis(dithiobenzil) metal complex (BMC).
The structures of complex dyes prepared were confirmed using an Elemental Analyzer
and GC-Mass, 1H-NMR and their optical properties were measured by means of UV-VIS
Spectroscopy.
Experimental
Synthesis
3,4-Bis(2-ethoxyethoxy)benzaldehyde (compound 1). 3,4-dihydroxybenzaldehyde (30.0 g,
0.22 mol), sodium carbonate (46.04 g, 0.43 mol), potassium iodide (7.21 g, 0.04 mol) and 1-
chloro-2-ethoxyethane (58.95 g, 0.54 mol) were dissolved in DMF (Dimethylformamide,
150 g) and stirred for four hours at 13◦C When the reaction completed and cooled to
room temperature, methylene chloride (150 g) and distilled water (150 g) were added.
The methylene chloride layer was separated from the aqueous layer, followed by drying
with MgSO4, anh., and filtered. Evaporation of the methylene chloride gave a 3,4-bis(2-
ethoxyethoxy)benzaldehyde (compound 1). Yield: 84%. C15H22O5 Found C:62.68 H:7.03
O:27.65, Calculated C:63.81 H:7.85 O:28.33, MS 282.33(M+)
Asymmetrical benzoins (3a–e) were obtained by condensation reaction of compound
1 and corresponding aromatic aldehydes (2a–e). Under the presence of potassium cyanide
(6.83 g, 0.10 mol) in 65% ethanol (30 g), compound 1 (20.0 g, 0.07 mol) and ben-
zaldehyde (2a) (7.5 g, 0.07 mol) were stirred for four hours at room temperature. 2-(3,4-
Bis(2-ethoxyethoxy)phenyl)-2-hydroxy-1-phenylethanone (3a) was extracted with methy-
lene chloride (30 g) and obtained after drying (MgSO4, anh.) and evaporation under reduced
pressure of the methylene chloride solution. Other asymmetrical benzoins (3b–e) were ob-
tained using the same procedure.
Figure 2. The synthetic method for compound 1.

52/[324]
N.-R. Kim and J.-H. Choi
Figure 3. Synthetic scheme and the structures of the dyes 4a–e.
3a Yield: 62%. C22H28O6 Found C:67.93 H:7.13 O:24.11, Calculated C:68.02 H:7.27
O:24.71, MS 388.45(M+)
3b Yield: 71%. C22H27ClO6 Found C:61.78 H:6.12 O:22.08, Calculated C:62.48 H:6.44
O:22.70, MS 422.90(M+)
3c Yield: 36%. C23H30O7 Found C:67.08 H:7.14 O:25.14, Calculated C:66.01 H:7.23
O:26.76, MS 418.48(M+)
3d Yield: 40%. C23H30O7 Found C:66.12 H:6.99 O:26.38, Calculated C:66.01 H:7.23
O:26.76, MS 418.48(M+)
3e Yield: 61%. C23H30O7 Found C:65.13 H:7.22 O:26.58, Calculated C:66.01 H:7.23
O:26.76, MS 418.48(M+)
2-(3,4-Bis(2-ethoxyethoxy)phenyl)-2-hydroxy-1-phenylethanone (3a) (20.0g, 0.05
mol) and P4S10 (7.32g, 0.02mol) were dissolved in dioxane (200g) and stirred for two
hours at 60◦C. Then, NiCl2·6H2O (8.31g, 0.04mol) was added uniformly at temperature
of 110◦C for three hours. Finally, bis(dithiobenzil) metal complex 4a was obtained. Other
bis(dithiobenzil) metal complexes (4b–e) were obtained via the same procedure. Prepared
dyes were purified with a column chromatography with methylene chloride : methanol
(25 : 1) as eluent.
4a Yield: 45%. C44H52NiO6S4 Found C:58.48 H:5.45 O:14.03, Calculated C:58.99 H:5.85
O:14.29, MS 895.83(M+), 1H-NMR (CDCl3) δ(ppm) 1.10(s, 3H), 3.50(s, 2H),
3.79(s, 2H), 4.31(s, 2H), 6.94(m, 1H), 6.95(m, 1H), 6.99(m, 1H), 7.30(m, 1H),
7.45(m, 1H), 7.50(m, 1H)

Synthesis of Novel Asymmetrical Benzoins and BMC
[325]/53
4b Yield: 51%. C44H50Cl2NiO8S4 Found C:53.98 H:5.12 O:13.98, Calculated C:54.78
H:5.22 O:13.27, MS 964.72(M+), 1H-NMR (CDCl3) δ(ppm) 1.10(s, 3H), 3.50(s, 2H),
3.79(s, 2H), 4.31(s, 2H), 6.94(m, 1H), 6.95(m, 1H), 6.99(m, 1H), 7.49(m, 1H), 7.89(m,
1H)
4c Yield: 42%. C46H56NiO10S4 Found C:56.98 H:6.32 O:17.03, Calculated C:57.80 H:5.90
O:16.74, MS 955.88(M+), 1H-NMR (CDCl3) δ(ppm) 1.10(s, 3H), 3.50(s, 2H), 3.79(s,
2H), 3.83(s, 3H), 4.31(s, 2H), 6.94(m, 1H), 6.95(m, 1H), 6.99(m, 1H), 7.01(m, 1H),
7.19(m, 1H), 7.39(m, 1H)
4d Yield: 44%. C22H27ClO6 Found C:57.75 H:4.95 O:16.57, Calculated C:57.80 H:5.90
O:16.74, MS 955.88(M+), 1H-NMR (CDCl3) δ(ppm) 1.10(s, 3H), 3.50(s, 2H), 3.79(s,
2H), 3.83(s, 3H), 4.31(s, 2H), 6.84(m, 1H), 6.94(m, 1H), 6.95(m, 1H), 7.05(m, 1H),
7.06(m, 1H), 7.64(m, 1H)
4e Yield: 49%. C46H56NiO10S4 Found C:57.16 H:5.95 O:17.02, Calculated C:57.80 H:5.90
O:16.74, MS 955.88(M+), 1H-NMR (CDCl3) δ(ppm) 1.10(s, 3H), 3.50(s, 2H), 3.79(s,
2H), 3.83(s, 3H), 4.31(s, 2H), 6.94(m, 1H), 6.95(m, 1H), 6.99(m, 1H), 7.39(m, 1H)
Structural Analysis
Structural analysis of synthesized dyes were measured using an EA 1108 (E.A.) and an
HP 6890 & Agilent 5973N MSD (GC-Mass). 1H-NMR spectra were recorded at 25◦C on
a NMR 400-MHz spectrometer (Bruker, Germany) with CDCl3 as a solvent.
UV-visible absorption spectra were obtained from a Shimadzu UV-2100.
Results and Discussion
The absorption maxima (λmax) in the near infrared region of bis(dithiobenzil) metal com-
plexes prepared are shown in Table 1. The absorption characteristic of these bis(dithiolene)
metal complexes involves the interaction between the lowest unoccupied π molecular or-
bitals of the ligand and the occupied orbitals of the central metal. The light absorption in the
near-infrared region is assigned to the first allowed π-π∗ transition and involves molecular
orbitals extending over the whole molecule [4]. The intensity of the absorption band in the
near-infrared region of bis(dithiobenzil) metal complex, shown in Figure 1, was sensitive
to substituents in the para, ortho, meta-position of phenyl group and is assigned to a π-π∗
transition of ligands [5].
Table 1. Absorption maxima of the synthesized dyes 4a–e and BMC
Dye number
λmax (nm)
Unsubstituted BMC
855
925
906
888
906
938
4a
4b
4c
4d
4e
∗Determined in CH2Cl2.

54/[326]
N.-R. Kim and J.-H. Choi
In the symmetrically substituted bis(dithiolene) metal complexes, the close linear
relationship exists between the λmax values and the Hammett modified substituent constants,
σp+, as shown in decreases of the absorption maxima in the order OCH3 < CH3 < Cl [6].
In asymmetrically substituted bis(dithiolene) metal complexes 4a–e, a non-linear re-
lationship exists between the λmax values and the Hammett substituent constants, σp.
However, when (σp(x)+ σp(H))/2 is used as a substituent constant for the asymmetric
complexes then a linear relationship exists between the ꢀλmax values and the σp values for
asymmetrically substituted bis(dithiolene) metal complexes.
As shown in Table 1, by comparison with unsubstituted BMC, the absorption maximum
of dye 4a shifted bathochromically (70 nm) which mainly due to the inductive effect exerted
by electron donating substituents (ethoxyethoxy groups) in the para- and meta-position of
the phenyl groups. In the case of dye 4b, a hypsochromic shift (19 nm) was observed in
comparison with that of dye 4a which can be best explained by the presence of electron
withdrawing group (4-chloro atom) at the para-position of phenyl ring leading to higher
energy level of excited state of dye 4b.
By the introduction of electron donating group (methoxy group) at ortho- or meta-
position in the phenyl ring, the absorption maxima of dyes 4c and 4d were also blue
shifted, 37 nm and 19 nm, respectively, which can be attributable to the steric hindrance
effect leading to the decrease of conjugation efficiency between the phenyl ring and the
chelate ring [7]. In contrast, the absorption maximum of 4e containing a para-methoxy
group was observed at 938nm which can be free from any steric hindrance. Therefore, it
can be concluded that the position of substituent in the phenyl ring affects the absorption
maximum due to its steric hindrance.
However, the bathochromic shift of 13 nm was found in dye 4e which contains methoxy
groups in the para-position of the phenyl ring, comparing to that of dye 4a, indicating more
efficient resonance effect arisen between the para-methoxy group and other π–electron
systems in the dye molecule without a steric hindrance.
Assuming these results, it can be concluded that the presence of an electron donating
group in the para-position of the phenyl ring can exert a bathochromic shift rather than
ortho- or meta-position. In this case the absorption maximum can be obtained at 938 nm
which seems to be very suitable for a NIR absorbing dye for optical filter of PDP.
Conclusions
In this work, five novel asymmetrical bis(dithiobenzil) metal complex dyes were prepared
via asymmetric benzoin intermediates. The final yield of bis(dithiolene) metal complex
dyes ranged in 42 ∼ 51%.
The absorption maximum of dye 4a shifted bathochromically (70 nm) which mainly
due to the inductive effect exerted by electron donating substituents (ethoxyethoxy groups)
in the para- and meta-position of the phenyl groups, whereas dye 4b showed a hypsochromic
shift (19 nm) in comparison with that of dye 4a which can be explained by the presence of
electron withdrawing group (4-chloro). However, steric hindrance on ortho-position of the
phenyl ring for dye 4c decreased the conjugation between the phenyl ring and the chelate
ring leading to further hypsochromic shift. This steric hindrance can be exempt by the
introduction of mehoxy group into the para-position of the phenyl ring for dye 4e that
absorbed maximally at 938 nm.

Synthesis of Novel Asymmetrical Benzoins and BMC
Acknowledgment
[327]/55
This work was supported by the National Research Foundation of Korea Grant funded by
the Korean Government [NRF-2012001638].
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