Effect of peripheral substitution on the electronic absorption and magnetic circular dichroism (MCD) spectra of metal-free azo-coupled bisphthalocyanine

Article abstract of DOI:10.1016/j.tetlet.2009.09.108

A new azo-coupled bisphthalocyanine is synthesized from the corresponding quinoxaline oxime which can be obtained by the reaction of s-trans-chloroethanedial with N{double bond, long}N conjugated metal-free phthalocyanine. The phthalocyanine is synthesize

Full text of DOI:10.1016/j.tetlet.2009.09.108

Tetrahedron Letters 50 (2009) 6775–6778
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effect of peripheral substitution on the electronic absorption and magnetic
circular dichroism (MCD) spectra of metal-free azo-coupled bisphthalocyanine
c,
Ümit Salan a, Nagao Kobayashi b, , Özer Bekarog˘lu
*
*
a
_
Department of Chemistry, Marmara University, Göztepe-Istanbul 34722, Turkey
b Department of Chemistry, Graduate School of Science Tohoku University, Sendai 980-857, Japan
c
_
Department of Chemistry, Technical University of Istanbul, Maslak, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2009
Revised 7 September 2009
Accepted 18 September 2009
Available online 24 September 2009
A new azo-coupled bisphthalocyanine is synthesized from the corresponding quinoxaline oxime which
can be obtained by the reaction of s-trans-chloroethanedial with N@N conjugated metal-free phthalocy-
anine. The phthalocyanine is synthesized by the reaction of 4-nitro-o-phenylenediamine with 2-nitro-
9,10,16,17,23,24-hexa(hexylthio)phthalocyanine. Novel compounds are characterized by elemental anal-
ysis, UV/vis, IR and 1H NMR, and MALDI-TOF spectroscopy. The effect of the azo units on the position and
intensity of the electronic absorption and magnetic circular dichroism (MCD) spectra of the bisphthalo-
cyanine are examined for the N@N conjugated metal-free phthalocyanine.
Ó 2009 Elsevier Ltd. All rights reserved.
Introduction
in several ways. On annulation of additional benzene rings to the
phthalocyanine moiety, the p-system is extended such that strong
Phthalocyanines are a class of macrocyclic compound possess-
red shifts of the Q band are observed for naphthalocyanines (Nc) at
ing a conjugated system of 18
p-electrons. They exhibit a number
k ꢀ740–810 nm, and anthracyanines (Ac) at k ꢀ830–860 nm.15–17
of unique properties, which make them of great interest in various
scientific and technological areas ranging from nanotechnology to
medicine. Phthalocyanines, which were first developed as dyes and
pigments have been explored in numerous technological applica-
tions, such as photovoltaic solar cells,1 electrophotography,2
molecular electronics,3 Langmuir–Blodgett films,4,5 electrochro-
mism in display devices,6 gas sensors,7,8 liquid crystals,9–11 nonlin-
ear optics,12 and medical applications.13
Another method for extending the p-electron system and thus
shifting the Q band into the near infrared (NIR) region is the dimer-
ization of phthalocyanines and related macrocycles via common
annelated benzene rings or other fully conjugated linkages.17
While numerous syntheses and applications of azo dyes have
been reported,18 the relationship between molecular structure
and absorption wavelengths has also received significant inter-
est.19 However, publications on phthalocyanines containing an
azo chromophore are limited.20 Herein, we report the synthesis,
characterization, and electronic absorption and magnetic circular
dichroism (MCD) spectra of a novel metal-free bisphthalocyanine
containing three azo groups in a linking unit.
In this study, we aimed to synthesize a new trans-azo-coupled
bisphthalocyanine starting from the corresponding quinoxaline
oxime by making use of the template effect of cobalt(II) ions. The
synthesis of azo-coupled metal-free bisphthalocyanine 6 is shown
in Scheme 1. When a solution of the quinoxaline oxime and cobal-
t(II) chloride in absolute DMF was refluxed for 6 h, compound 6
was isolated as a green precipitate. The mechanism for the cou-
pling process from 5 to 6 is not explained as yet.21 The quinoxaline
oxime was synthesized in one step by reaction of s-trans-chloroe-
thanedial dioxime with compound 4 and elimination of NH2–
OH.21–23 Compound 4 was synthesized by the reaction of 4-nitro-
o-phenylenediamine with 2-nitro-9,10,16,17,23,24-hexa(hexyl-
thio)phthalocyanine 3, which was obtained from 4,5-bis(hexyl-
thio)phthalonitrile 1 and 4-nitrophthalonitrile 2 (Scheme 1). The
coupling reaction between 3 and 4-nitro-o-phenylenediamine
Phthalocyanines have received significant attention particularly
because of their use in photodynamic cancer therapy. For this
*
application, the wavelength of the major
p?p transitions in the
UV/vis region is of critical importance. The phthalocyanines show
typical electronic spectra with two strong absorption regions,
one in the UV region at about 300–350 nm (the B band) and the
other in the visible region at about 600–700 nm (the Q band).
The characteristic Q-band transition of metal-free phthalocyanines
with D2h symmetry is observed as a split band of high intensity in
the visible region. Monomolecular phthalocyanines show Q band
absorptions at k ꢀ680 nm with high extinction coefficients
e
>105 MÀ1 cmÀ1. Substituted phthalocyanines, especially those con-
taining electron-donating substituents, exhibit a bathochromic
shift of their Q band up to k ꢀ760 nm.14 The position and band-
width of the absorption bands of phthalocyanines can be adjusted
* Corresponding authors. Tel.: +90 216 3590130; fax: +90 216 3860824 (Ö.B.).
E-mail addresses: nagaok@mail.edu.tains.tohoku.ac.jp (N. Kobayashi), obek@
itu.edu.tr (Ö. Bekarog˘lu).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.108

6776
Ü. Salan et al. / Tetrahedron Letters 50 (2009) 6775–6778
H13C6S
SC6H13
N
N
NC
NC
SC6H13
SC6H13
NC
NC
NO2
H
13C6S
N
NO2
LiCl, pentanol
+
HN
NH
N
160-180 oC, 8 h
N
H13C6S
1
2
N
SC6H13
H13C6S
3
(11%)
4-nitro-o-phenylenediamine,
Zn, NaOH, THF;
reflux, overnight
H13C6S
H13C6S
SC6H13
SC6H13
H
N
NH2
NH2
N
OH
N
N
N
N
N
H13C6S
H13C6S
s-trans-chloroethanedial dioxime
N
N
N
N
N
N
ethanol
HN
HN
NH
N
N
NH
N
N
H
13C6S
H
13C6S
rt, overnight
N
N
SC6H13
SC6H13
H13C6S
H13C6S
5
4
(30%)
CoCl2.6H2O
reflux, 6 h
H13C6S
SC6H13
N
N
N
SC6H13
SC6H13
HN
NH
N
N
N
N
N
N
N
SC6H13
H13C6S
N
N
N
N
SC6H13
H13C6S
N
N
H13C6S
N
N
N
HN
NH
N
N
H13C6S
N
SC6H13
H13C6S
6
(35%)
Scheme 1.
was regioselective resulting in the trans-form product. The regiose-
lectivity arises from the fact that the resonance energy of the trans-
form is approximately 3.5 kcal/mol lower than the cis-form. It was
shown to be trans-2,2-azoquinoxaline unequivocally by X-ray sin-
gle cystal analysis.22 The spectroscopic data confirmed the pro-
posed structures of all of the new compounds.
The elemental analysis results for compounds 3, 4, and 6 were
in good agreement with the calculated values. A diagnostic feature
of phthalocyanine formation from compounds 1 and 2 is the disap-
pearance of the sharp intense CN vibrational bands in the IR spec-
tra. Other IR bands of compound 3 include C–H stretching bands at
2854, 2925, 2954, and 3060 cmÀ1 and the bending bands of the ni-
tro group at 1342 and 1523 cmÀ1. The band at around 3417 cmÀ1
can be attributed to the N–H stretching frequency of the inner core
of the metal-free phthalocyanine 3. The chemical shifts in the 1H
NMR spectra of compounds 3, 4, and 6 were found to be in agree-
ment with the values reported for similar compounds.22,23 In the IR
spectrum of compound 3, the intense absorption bands at 1520
and 1340 cmÀ1 corresponding to the NO2 bending vibrations disap-
peared after its conversion into compound 4. The IR spectrum of
compound
4 included NH2 stretching bands at 3387 and
3514 cmÀ1. Comparison of the IR spectral data of compounds 4
and 6 clearly indicated the conversion of the NH2 groups into
N@N moieties by the disappearance of the NH2 bands and the

Ü. Salan et al. / Tetrahedron Letters 50 (2009) 6775–6778
6777
appearance of new imine bands at 1473 and 1490 cmÀ1 in 6. MAL-
DI-TOF samples were prepared by mixing the appropriate complex
reaction mixture was cooled to room temperature and precipitated
by adding water. After filtration, the product was washed with cold
ethanol and diethyl ether. The residue was fractionated on a silica
gel column, eluting with CHCl3, and a gradient of CHCl3–THF up to
100% THF. Yield: 172 mg (11%). This compound is soluble in CHCl3
(2 mg/mL in chloroform) with the matrix,
namic acid (10 mg/mL in chloroform) solution (1:20 v/v) in a
0.5 mL EppendorfÒ micro-tube. Finally 1
L of this mixture was
a-cyano-4-hydroxycin-
l
deposited on the sample plate, dried at room temperature, and
then analyzed. The molecular ion peaks of 3, 4, and 6 were ob-
served at 1255, 1344, and 2758 Da, respectively. Apart from the
molecular ion peaks, H2O and 2H2O adducts were also present.
Figure 1 shows the electronic absorption and magnetic circular
dichroism (MCD) spectra of compound 6 in CHCl3. The Q and Soret
bands appeared at 702 and 356 nm, respectively. The bands are at
slightly longer wavelength than those of phthalocyanines (Pcs)
without peripheral substituents. One notable feature is that the Q
band is not split, although metal-free Pcs generally show split
peaks which can be assigned to Qx and Qy transitions. This is quite
a rare phenomenon in metal-free Pcs, although metal-free Pcs hav-
ing a Q band at a wavelength longer than ca. 780 nm are known to
have unsplit Q bands.24 As a summation of the effect of the periph-
eral azo and alkylthio groups, the Qx and Qy splitting is thought to
decrease, to produce an apparently single Q band which is charac-
teristic of metal Pc species. The MCD spectrum is also similar to
that of metal Pcs, such that dispersion-type pseudo Faraday A terms
were recorded corresponding to the Q and Soret absorption peaks.
The intramolecular interaction of the two Pc units appears small,
since both the Q and Soret bands are similar in shape to those of
typical monomeric Pc species. Although the relative intensity of
the Soret band compared to the Q band is larger than in normal
Pcs, this may be due to the superimposition of the absorption
due to the connecting chromophore units (quinoxaline and azo24).
and CH2Cl2. Mp >350 °C. IR (KBr) m
max=cmÀ1 730, 820, 840, 1020,
1070, 1110, 1140, 1342, 1381, 1458, 1523, 1595, 1610, 1640,
2854, 2925, 2954, 3060, 3417. Anal Calcd for C68H89N9O2S6: C,
65.01; H, 7.10; N, 10.04; S, 15.30. Found: C, 64.93; H, 7.22; N,
10.15; S, 15.20. 1H NMR (400 MHz, CDCl3) d 1.20 (t, J = 6 Hz,
18H), 1.25–1.70 (m, 48H), 2.80 (t, J = 6 Hz, 12H), 6.80–7.90 (m,
9H). (MALDI-TOF): m/z 1255 (M+).
Synthesis of [(RS)6PcN2PhA] 4
A mixure of metal-free phthalocyanine 3 (300 mg, 0.24 mmol)
and 4-nitro-o-phenylenediamine (37 mg, 0.08 mmol) was dis-
solved in THF. NaOH [0.2 mL (19.5 g NaOH in 45 mL)] and activated
Zn (0.48 mmol) were added to this solution and the reaction mix-
ture was refluxed overnight. The resultant product was filtered off
and acidified using HCl (10%). The residue was evaporated in va-
cuo. Yield: 96 mg (30%). Mp >350 °C. IR (KBr)
m
max=cmÀ1 730,
820, 840, 1020, 1070, 1110, 1140, 1340, 1380, 1460, 1610, 1640,
2850, 2920, 2960, 3060, 3387, 3514. Anal Calcd for metal-free
phthalocyanine C74H96N12S6: C, 66.07; H, 7.14; N, 12.50; S, 14.29.
Found: C, 65.95; H, 7.25; N, 12.60; S, 14.20. 1H NMR (400 MHz,
DMF-d7) d 0.90 (br s, 2H), 0.96 (t, 18H), 1.32 (br s, 2H), 1.42–1.70
(m, 48H), 2.90 (t, 12H), 6.80–7.80 (m, 12H). MS (MALDI-TOF): m/
z 1344 (M+).
Synthesis of [(RS)6Pc]2N6AQ 6
Synthesis of [(RS)6PcNO2] 3
Compound 4 (80 mg, 0.06 mmol) was dissolved in DMF (2 mL)
and added to absolute ethanol (18 mL). This solution, was simulta-
neously added dropwise over 30 min to a solution of s-trans-chlo-
roethanedial dioxime (8 mg, 0.06 mmol in 2 mL of ethanol) at room
temperature under an argon atmosphere, and stirring was contin-
ued overnight. The reaction mixture containing 5 was evaporated
and the residue stirred in dry DMF (3 mL) at 90 °C under an argon
atmosphere. CoCl2Á6H2O (0.07 mmol, 0.016 g) in dry DMF (1 mL)
was added to the stirred solution and the mixture was refluxed
for 6 h under argon. The resulting dark-green precipitate was fil-
tered off and washed with absolute ethanol. The crude product 6
was purified by column chromatography over silica gel, eluting
with CHCl3. Yield: 57 mg (35%). This compound was soluble in
A powdered mixture of compound 1 (1600 mg, 4.44 mmol), 2
(192 mg, 1.10 mmol), and LiCl (46 mg, 1.10 mmol) was dissolved
in pentanol under an argon atmosphere. The mixture was heated
at 160–180 °C for 8 h to give unsymmetrically substituted metal-
free phthalocyanines consisting of various ratios of 1 and 2. The
CHCl3 and CH2Cl2. Mp >350 °C. IR (KBr) m
max=cmÀ1 756, 840, 928,
1090, 1136, 1473, 1490, 1591, 1610, 2870, 2920, 3053, 3283. Anal
Calcd for metal-free bisphthalocyanine C152H186N26S12: C, 66.13; H,
6.75; N, 13.20; S, 13.92. Found: C, 65.80; H, 6.85; N, 13.55; S, 14.10.
1H NMR (400 MHz, DMF-d7) d 0.96 (t, J = 6 Hz, 36H), 1.26–1.70 (m,
96H), 2.90 (t, J = 6 Hz, 24H), 6.80–7.80 (m, 24H), 9.60 (s, 2H). MS
(MALDI-TOF): m/z 2758 (M+).
Acknowledgments
We would like to thank The Research Foundation of Marmara
University and the Commission of Scientific Research Project (BAP-
KO) [FEN-D-030408-0083] and the Turkish Academy of Sciences
(TUBA).
References and notes
1. Wohrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129–138.
2. Loutfy, R. O.; Hor, A. M.; Hsiano, C. K.; Barayi, G.; Kazmaier, P. Pure Appl. Chem.
1989, 60, 1047–1054.
3. Simic-Glavaski, B.. In Phthalocyanines—Properties and Applications; Leznoff, C. C.,
Lever, A. B. P., Eds.; VCH: New York, 1993; Vol. 3, pp 119–166. Chapter 2.
Figure 1. Electronic absorption (bottom) and magnetic circular dichroism (MCD)
(top) spectra of compound 6.

6778
Ü. Salan et al. / Tetrahedron Letters 50 (2009) 6775–6778
4. Cook, M. J.; Dunn, A.; Daniel, M. F.; Hart, R. C. O.; Richardson, R. M.; Roser, S. J.
Thin Solid Films 1988, 159, 395–404.
14. Kobayashi, N.; Ogata, H.; Nonaka, N.; Luk’yanets, E. A. Chem. Eur. J. 2003, 9,
5123–5134.
5. Palacin, S.; Lesieur, P.; Stefanelli, I.; Barraud, A. Thin Solid Films 1988, 159, 83–
90.
15. Kobayashi, N.; Nakajima, S.; Ogata, H.; Fukuta, T. Chem. Eur. J. 2004, 10, 6294–
6312.
6. Nicholson, M. M.. In Phthalocyanines—Properties and Applications; Leznoff, C. C.,
Lever, A. B. P., Eds.; VCH: New York, 1993; Vol. 3, pp 71–117. Chapter 2.
7. Lefevre, D.; Porteu, F.; Balog, P.; Roulliay, M.; Zalczer, G.; Palacin, S. Langmuir
1993, 9, 150.
8. Shi, C.; Anson, F. C. Inorg. Chim. Acta 1994, 225, 215.
9. Van der Pol, J. F.; Neeleman, E.; Zwikker, J. W.; Nolte, R. J. M.; Drenth, W.;
Aerths, J.; Visser, R.; Picken, S. J. Liq. Cryst. 1989, 6, 577–592.
10. Simon, J.; Sirlin, C. Pure Appl. Chem. 1989, 61, 1625–1629.
11. Swarts, J. C.; Langner, E. H. G.; Krokeide-Hove, N.; Cook, M. J. J. Mater. Chem.
2001, 11, 434–443 and references cited therein.
16. Mack, J.; Stillman, M. J.; Kobayashi, N. Coord. Chem. Rev. 2007, 251, 429–453.
17. Kobayashi, N.; Nakai, K. Chem. Commun. 2007, 4077–4092.
18. Zollinger, H. Color Chemistry; VCH: Weinheim, 1987.
19. Herbst, W.; Hunger, K. Industrielle Organische Pigmente; VCH: Weinheim, 1987.
20. Yazici, A.; Ates, D.; Bekaroglu, O.; Kobayashi, N. J. Porphyrins Phthalocyanines
2006, 10, 1140–1144.
21. Ertasß, M.; Gürek, A.; Gül, A.; Belarog˘lu, Ö. J. Chem. Res. (S) 1988, 109, 1157–
1167.
22. Koçak, A.; Bekarog˘lu, Ö. Helv. Chim. Acta 1985, 68, 581.
23. Salan, Ü.; Altındal, A.; Bulut, M.; Bekarog˘lu, Ö. Tetrahedron Lett. 2005, 46, 6057–
6061.
12. De la Torre, G.; Torres, T.; Agullo-Lopez, F. Adv. Mater. 1997, 9, 265–269.
13. Ali, H.; Lier, J. E. Chem. Rev. 1999, 99, 2379–2450.
24. Kobayashi, N.; Osa, T. Carbohydr. Res. 1989, 192, 147–157.