Synthesis and absorption properties of zinc-phthalocyanineswith photoresponsive azobenzene groups

Article abstract of DOI:10.3987/COM-04-10230

The phthalocyanines (1) and (2) with eight photoisomerizableazobenzene groups have been synthesized and spectroscopically characterized. These macrocycles exhibit partial trans-cis isomerzation properties upon irradiation of UV/VIS light.

Full text of DOI:10.3987/COM-04-10230

HETEROCYCLES, Vol. 63, No. 12, 2004, pp. 2869 - 2874
Received, 15th September, 2004, Accepted, 21st October, 2004, Published online, 26th October, 2004
SYNTHESIS
AND
ABSORPTION
PROPERTIES
OF
ZINC-
PHTHALOCYANINES WITH PHOTORESPONSIVE AZOBENZENE
GROUPS
Ki-Jong Han and Kwang-Yol Kay*
Department of Molecular Science and Technology, Ajou University, Suwon
442-749, Korea. E-mail: kykay@ajou.ac.kr
Abstract – The phthalocyanines (1) and (2) with eight photoisomerizable
azobenzene groups have been synthesized and spectroscopically characterized.
These macrocycles exhibit partial trans-cis isomerzation properties upon
irradiation of UV/VIS light.
INTRODUCTION
The physical and chemical properties of phthalocyanines (Pc) have long attracted an interest owing to
their intriguing electrical, optical, photochemical and catalytic properties.1 A particularly attractive
feature of Pc is the dependence of the properties of the molecule on the nature of the peripheral functional
groups, as well as the electronic properties of the central metal cations in the phthalocyanine ring.2
Unsubstituted phthalocyanines are not very soluble and tend to aggregate in solution; however, the
addition of peripheral chains can increase solubility, processibility, and facilitate the formation of discotic
mesophase.3 Recently, many new phthalocyanines with long alkyl,4 alkoxy,5 alkylthio6 and
alkoxyphenoxy7 groups have been prepared to study on their diverse functionality such as liquid
crystalline properties2,3,9 as well as highly ordered thin film properties.8 In line with these aspects, we
report herein the first synthesis of zinc-phthalocyanines (1) and (2) with eight photoisomerizable
azobenzene groups at the periphery. These macrocycles incorporated with eight long-chain azobenzene
substituents exhibit trans-cis isomerization properties upon irradiation of UV/VIS light.

RESULTS AND DISCUSSION
The synthetic routes to octasubstituted zinc phthalocyanines (1) and (2) are shown in Scheme 1. From
commercially available 4-hydroxyanilline (3) azobenzene derivatives (9) and (10) were obtained via
several steps, and then converted to the respective dinitriles (11) and (12). Cyclization of dicyanobenzene
derivatives (11) and (12) with zinc acetate and 1.8-diazabicyclo[5.4.0]undec-7-ene (DBU) in 1-pentanol
led to the formation of 1 and 2, respectively, in moderate yields. All of these macrocycles possess good
solubility in common organic solvents and were purified mainly by column chromatography with hexane
mixed with THF as eluent. The latter was employed in order to disrupt the molecular aggregation
1
associated with the large π-system. The H NMR spectra of 1 and 2 in CDCl3, showed only broad
resonances due to the aliphatic side chains; the aromatic protons appeared as broad signals deshielded in
the regions δ 7.45 ~ 8.20 and 6.70 ~ 7.11 assignable to the ring protons and azobenzene protons. Eight
methylene protons (O-CH2-) resonanced in the region δ 3.95 as a broad band.
c
d
a
b
RO
NH2
N
N
OR
HO
RO
NHAc
HO
NHAc
HO
NH2
5 R = C9H19
6 R = C18H37
9 R = C9H19
10 R = C18H37
7 R = C9H19
8 R = C18H37
3
4
OR
RO
N
N
N
N
OR
RO
O
O
N
N
N
N
N
N
N
N
O
O
O
O
N N
OR
O
f
NC
NC
Zn N
N
e
N
N
N
O
N
OR
N
N
N
N
11R = C9H19
12R = C18H37
RO
O
O
OR
N
N
N
N
1 R= C9H19
2 R= C18H37
RO
OR
Scheme 1
a) Ac2O, THF b) RBr, K2CO3, MeCN c) 6N HCl, EtOH d) phenol, NaNO2, 35% HCl, H2O/THF
e) 4,5-dichlorophthalonitrile, K2CO3, THF f) Zn(OAc)2, DBU, n-pentanol

1.0
0.8
0.6
0.4
0.2
0.0
-0.2
1
2
200
300
400
500
600
700
800
Wavelength (nm)
Figure 1. Absorption spectrum of 1 and 2
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
Wavelength (nm)
Figure 2. Absorption spectrum of 2 under irradiation
conditions(365 nm:0, 30, 60, 120, 360 and 420s)
The UV/VIS spectra of the phthalocyanines (1) and (2) in chloroform showed the typical pattern, mainly
*
the π- π transition of the heteroaromatic 18 - πelectron system (Figure 1) : A large intense Q band in the
visible region of around 681 nm was accompanied by more or less resolved weak satellite bands. In the
ultraviolet region of around 360 nm the characteristic Soret or B band was also observed. Peripheral
substitution of phthalocyanines by alkoxyazobenzene chains gave bathochromic shifts of both the Q band
(681 nm for 1 and 683 nm for 2) and Soret band (358 nm for 1 and 360 nm for 2) compared to the
unsubstituted Zn-phthalocyanine (Soret band 340 nm, Q band 666 nm in THF).10 However, absorption
*
bands of azogroups (around 350 nm, π- π transition, trans form)11 were overlapped with the Soret bands
of 1 and 2 : Irradiation of 365 nm light proved this overlapping and exhibited the expected

photoresponsive behavior of 1 and 2. For example, dark incubation of a chloroform solution(10-5M) of 2
served to maximize the absorption at 360 nm (εmax = 79730) corresponding to the trans-azobenzene
chromophore. Irradiation of this solution with 365 nm light resulted in partial photoisomerization to
cis-azobenzene, as evidenced by a decrease in the absorbance at 360 nm and an increase in absorbance at
*
around 450 nm (n -π transition, cis form)11 (Figure 2). A photostationary state was reached within
approximately 420 seconds. At this state we could estimate the yield of the isomerized azobenzene groups
to be about 30% on the basis of the contribution ratio of the Soret band and azobenzene groups for the
molar absorptivity at 360 nm. In most of Zn-phthalocyanines12, 13, 14 the general ratio of Soret band and Q
band was found to be about 2:5. From this ratio we derived the molar absorptivity contribution of
azobenzene groups in 2. This value thus obtained (around 42,000) is bigger than the molar absorptivity of
10 (around 30,000), but it seems to be reasonable because compound (2) has eight azobenzene groups.
Finally we could deduce the yield of the isomerized azobenzene groups from the change value (around
13,000) of molar absorptivity after irradiation . Thermal reversion to the original dark-incubated spectrum
was observed over the course of approximately 17 h at 293 K in the dark. However, exposure to bright
sunlight for a period of only several seconds also effect almost complete reversions to the trans isomer.
The compound (1) also showed the same trends in trans-cis reversible isomerization phenomena. This
partial isomerization phenomena of 1 and 2 can be ascribed to not enough space for eight azobenzene
groups in 1 and 2 to undergo trans-cis isomerization.
The mesophase behaviors of 1 and 2 were not observed by differential scanning calorimetry (DSC) and
polarized optical microscopy : The DSC thermograms of 1 and 2 showed only a single endothermic peak
within the range of 25 ~ 400℃, but no well-defined transition was observed, showing gradual
decomposition with decreasing volume above 380℃. Polarized optical microscopy showed neither melt
nor mesophase for 1 and 2 upon heating or cooling between 30 and 320℃.
EXPERIMENTAL
All chemicals and regents were purchased from Aldrich and used as received. Analytical thin layer
chromatography was performed on pre-coated Merck silica gel 60 F254 TLC plate. Purification was
1
13
performed by flash column chromatography by using Merck 230-400 mesh silica gel. H NMR and C
NMR spectral data were obtained on a Jeol JNM-ECP 400 MHz NMR spectrometer. IR spectra were
recorded using a Jasco FT/IR-410 spectrophotometer with internal calibration
General procedure for the synthesis of 1 and 2: A mixture of 4-(4-nonyloxyphenylazo)phenol (9, 751
mg, 2.2 mmol), K2CO3 (552 mg, 4.0 mmol), 4,5-dichlorophthalonitrile (197 mg, 1.0 mmol) and
18-crown-6 (80 mg, 0.3 mmol) in acetone (50 mL) was heated at reflux for 10 h. Upon cooling, the

solution was filtered and the filtrate was evaporated. The crude solids were purified by chromatography
(silica gel, 30% THF/hexane) to give 11 ( 425 mg, 52.7%) as a bright orange solid.
A mixture of 11 (280 mg, 0.35 mmol), Zn(OAc)2 (24 mg, 0.13 mmol) and 1.8-diazabicyclo[5.4.0]-
undec-7-ene (0.2 mL) in n-pentanol (5 mL) was heated to reflux for 12 h. After the solution was cooled it
was diluted with MeOH and the black precipitates were filtered. The crude solids were purified by
chromatography(silica gel, 50% THF/hexane) to give 1 (140 mg, 49%) as a green solid. The
octadecyloxy group substituted phthalocyanine (2) was synthesized by similar procedure like as 1.
The characterizations of 11, 12, 1 and 2.
Compound (11) : 52.7 % yield; Rf = 0.4 (THF : Hexane = 1 : 3); mp 145-146 ℃(recrystallized from
1
ethanol); FT-IR (KBr) : ν = 2921, 2852, 2233, 1581, 1488, 1241, 1145 cm-1; H-NMR (CDCl3, 400
MHz) : δ = 8.00 ~ 7.90 (m, 10H), 7.25 ~ 7.14 (m, 4H), 7.06 ~ 6.85 (m, 4H), 4.08 (t, J=6.4 Hz, 4H), 1.80
(m, 4H), 1.40 ~ 1.20 (m, 24H), 0.90 (t, J=7.1 Hz, 6H); Anal. Calcd for C50H56N6O4 : C 74.60, H 7.01, N
10.44. Found C 74.53, H 7.03, N 10.47.
Compound (12) : 60.5 % yield; Rf = 0.5 (THF : Hexane = 1 : 5); mp 133-134 ℃(recrystallized from
1
ethanol); FT-IR (KBr) : ν = 2917, 2850, 2233, 1581, 1490, 1245, 1145 cm-1; H-NMR (CDCl3, 400
MHz) : δ = 8.10 ~ 7.90 (m, 10H), 7.21 ~ 7.15 (m, 4H), 7.03 ~ 6.99 (m, 4H), 4.08 ~ 4.03 (t, J=6.3 Hz, 4H),
1.85~1.80 (m, 4H), 1.59 ~ 1.13 (m, 60H), 0.90 ~ 0.88 (t, J=7.2 Hz, 6H); Anal. Calcd for C68H92N6O4 : C
77.23, H 8.77, N 7.95. Found C 77.16, H 8.80, N 7.99.
Compound (1) : 49 % yield; Rf = 0.5 (THF : Hexane = 1 : 3); mp >368 ℃(decomp, recrystallized from
ethanol); FT-IR (KBr) : ν = 2923, 2852, 1598, 1492, 1430, 1392, 1245, 1145 cm-1; 1H-NMR (CDCl3, 400
MHz) : δ = 7.86 ~ 7.46 (s , 40H), 7.11 ~ 6.72 (s, 32H), 3.95 (s, 16H), 1.79(s, 16H) 1.56~1.20 (m, 96H),
0.92 (t, J=7.1 Hz 24H); UV/VIS (CHCl3, 10-5): λmax (nm) [ε] = 249[37910], 286[35030], 358[79850],
614[18620], 681[100990]; MALDI–TOF MS : m/z 2136, 2068, 1820, 1815, 574; Anal. Calcd for
C200H224N24O16Zn : C 73.11, H 6.87, N 10.23. Found C 73.20, H 6.79, N 10.19.
Compound (2) : 56 % yield; Rf = 0.5 (THF : Hexane = 1 : 2); mp >379 ℃(decomp, recrystallized from
1
ethanol); FT-IR (KBr) : ν = 2921, 2850, 1598, 1492, 1392, 1247, 1145 cm-1; H-NMR (CDCl3, 400
MHz) : δ = 8.20 ~ 7.45 (s , 40H), 7.10 ~ 6.70 (s, 32H), 3.94 (s, 16H), 1.78(s, 16H) 1.60 ~ 0.98 (m, 240H),
0.87 (t, J=7.2 Hz, 24H); UV/VIS (CHCl3, 10-5): λmax (nm) [ε] = 251[34040], 285[23030], 360[79730],
615[10420], 683[89300]; MALDI–TOF MS : m/z 3914, 3438, 2960, 2583, 2220, 574. Anal. Calcd for
C272H368N24O16Zn : C 76.06, H 8.64, N 7.83. Found C 76.17, H 8.57, N 7.91.
ACKNOWLEDGEMENTS
The authors thank the Ministry of Education of Korea for financial support (BK 21 program).

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