Synthesis, electrochemical and photochemical properties of unsymmetrically substituted zinc phthalocyanine complexes

Article abstract of DOI:10.1016/S0277-5387(02)01226-3

The synthesis of symmetrically and unsymmetrically substituted zinc phthalocyanine (ZnPc) derivatives are reported. The ZnPc derivatives are synthesized by ring expansion of subphthalocyanine complexes. Ring substitution is effected with tert-butyl phenol

Full text of DOI:10.1016/S0277-5387(02)01226-3

Polyhedron 21 (2002) 2463ꢁ2472
/
www.elsevier.com/locate/poly
Synthesis, electrochemical and photochemical properties of
unsymmetrically substituted zinc phthalocyanine complexes
Pulane Matlaba, Tebello Nyokong *
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
Received 26 April 2002; accepted 30 August 2002
Abstract
The synthesis of symmetrically and unsymmetrically substituted zinc phthalocyanine (ZnPc) derivatives are reported. The ZnPc
derivatives are synthesized by ring expansion of subphthalocyanine complexes. Ring substitution is effected with tert-butyl phenol,
naphthol and hydroxybenzoic acid. Comparison of the redox potentials for complexes substituted with varying numbers of tert-
butyl phenol: 1 (complex 5), 2 (complex 6), 3 (complex 7), 6 (complex 8) and 8 (complex 9) shows that the complexes with the highest
number of substituents are more difficult to oxidize and easier to reduce. All the reported complexes are relatively photostable, with
photobleaching quantum yields of the order of 10ꢀ5. Singlet oxygen quantum yields for the complexes ranged from 0.22 to 0.68.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Zinc phthalocyanine; Sensitizer; Singlet oxygen; Photobleaching
1. Introduction
The major limitation of phthalocyanines for use in
PDT is the lack of selectivity towards tumor cells.
Unsymmetrically substituted phthalocyanines offer a
possibility of improving selectivity since groups with
different properties may be attached to the ring. We
report in this work on the synthesis of unsymmetrically
substituted zinc phthalocyanine complexes. Substitu-
ents, which enhance the solubility of the phthalocya-
nines, are employed. The most efficient method of
synthesis of unsymmetrically substituted phthalocya-
nines is through synthesis of a symmetrically or un-
symmetrically substituted sub-phthalocyanine (Sub-Pc),
followed by ring enlargement using a substituted
Phthalocyanines are an interesting class of com-
pounds with increasingly diverse industrial and biome-
dical applications including photosensitization [1], linear
optics [2], catalysis [3], liquid crystals [4], and gas sensing
[5]. Specificity in the applications of phthalocyanines
can be introduced by modification of the phthalocya-
nine ring or changes in the central metal. Remarkable
progress has been made in recent years in the use of
phthalocyanine derivatives as sensitizers for photody-
namic therapy (PDT) of cancer [6,7]. Zinc phthalocya-
nine complexes have good photosensitizing properties
phthalonitrile [15ꢁ18].
/
for PDT [8ꢁ
cyanine complexes are water-soluble but form aggre-
gates in solution [11ꢁ13]. Aggregation diminishes the
/
10]. Sulfonated or carboxy zinc phthalo-
It is widely believed that during PDT, the photo-
sensitizer is excited to its triplet state, and then transfers
the energy to the ground-state triplet oxygen O2(3Sg)
forming the excited-state singlet oxygen, O2 (1Dg),
through the so called Type II mechanism. The O2
(1Dg) species is believed to be the chief agent in PDT.
Type I mechanism involves electron transfer from the
excited photosensitizer to the triplet state oxygen or to
the biological substrate. Both Type I and Type II
mechanisms may be involved during PDT [19,20]. It is
important to determine singlet oxygen quantum yields
of the photosensitizers. Studies on the photodegradation
/
photosensitizing ability of metallophthalocyanine (MPc)
complexes. Amphiphilic unsymmetrically substituted
phthalocyanines have been shown to have cell penetrat-
ing powers [14].
* Corresponding author. Tel.: ꢂ
5109
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
/
27-46-603-8260; fax: ꢂ27-46-622-
/
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 2 2 6 - 3

2464
P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ2472
/
(photobleaching) of the phthalocyanines are also im-
portant for determining the stability of the MPc
complexes during PDT action. We report on the singlet
oxygen quantum yields and the photobleaching studies
of a series of unsymmetrically and symmetrically sub-
stituted zinc phthalocyanines (Scheme 1).
2.2. Synthesis
2.2.1. Substituted dicyanobenzene (2 and 4)
4,5-Dichlorodicyanobenzene (1) was synthesized from
dichlorophthalic anhydride according to a reported
procedure [21]. The disubstituted dicyanobenzenes
(R1ꢃ
/
R2 (for 2) or R3ꢃR4 for 4) were then synthesized
/
from 4,5-dichlorodicyanobenzene using the general
procedure reported before [21] as follows: dry potassium
carbonate (0.64 g, 0.0046 mol) was suspended in DMSO
under nitrogen. To this suspension, 4,5-dichlorodicya-
nobenzene and the appropriate amount of the respective
phenoxy substituent (2-naphthol, 4-tert-butylphenol or
4-hydroxybenzoic acid) were added. See Table 1 for the
amounts of reagents used. Further aliquots of potassium
carbonate were added after 4 h, and 24 h of stirring at
room temperature (r.t.), giving total amounts shown in
Table 1. The mixture was further stirred at r.t. and
under nitrogen atmosphere for the total time shown in
Table 1. The product was purified following literature
2. Experimental
2.1. Materials
N,N?-dimethylformamide (DMF) used for electro-
chemical studies was freshly distilled. Dimethylsulfoxide
(DMSO) was dried over alumina before use. Thionyl
chloride, potassium carbonate, nitrophthalimide, di-
chlorophthalic anhydride, dicarboxynaphthalene, 2-
naphthol, 4-tert-butylphenol, 4-hydroxybenzoic acid,
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), diazabicy-
methods [21]. Monosubstituted dicyanobenzenes (R2ꢃ
/
clooctane
(DABCO),
1,3-diphenylisobenzofuran
H (for 2) or R3ꢃH (for 4)) were synthesized by the
/
(DPBF) and chlorobenzene (ClB) were purchased either
from Aldrich or Merck and were used as received.
Tetrabutylammonium perchlorate (TBAP) was recrys-
tallized from ethanol and used as an electrolyte for
electrochemical studies.
procedure outlined above for the disubstituted deriva-
tives using nitrophthalonitrile instead of dichlorodicya-
nobenzene. The amounts for the synthesis are shown in
Table 1. 4-Nitrophthalonitrile was synthesized from 4-
nitrophthalimide using reported procedures [22].
Scheme 1.

P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ
/2472
2465
Scheme 1 (continued).
Dicyanonaphthalonitrile was synthesized from dicar-
boxynaphthalene following reported procedures [21].
[16], and characterized by UVꢁ
NMR spectroscopies.
/
Vis, infrared and 1H
2.2.3. Sub-naphthalocyanine (Sub-NPc, 3b)
2.2.2. Sub-phthalocyanines(3a)
Sub-naphthalocyanine was synthesized from dicyano-
naphthalonitrile following the procedure outlined above
for Sub-Pc, by replacing 2 with dicyanonaphthalonitrile.
The unsubstituted, tri- or hexa-substituted subphtha-
locyanines were synthesized by following reported
procedures [15,16] as follows. The respective dichloro-
cyanobenzene (1), di- or mono-substituted dicyanoben-
zene (2, 0.4 g, 0.003 mol), was suspended in 1-
chloronaphthalene (6 ml), stirred at r.t. in an atmo-
sphere of nitrogen and in the presence of 0.001 mol (0.12
g) BCl3. The reaction mixture was heated to 220 8C for
0.5 h. The products were purified according to reference
2.2.4. Symmetrically (9) and unsymmetrically (5ꢁ
/
8,
10ꢁ14) substituted zincphthalocyanines (Scheme 1)
/
These complexes were synthesized according to lit-
erature methods [23] by dissolving (0.25 g, 0.00092 mol)
the unsubstituted or substituted dicyanobenzene (4) to
Table 1
Amounts of reagents used for synthesis of substituted dicyanobenzenes
Substituent
Amount of substituent added
(g, mol)
4,5-Dichloro-dicyanobenzene
(g, mol)
Total K2CO3 added
(g, mol)
Total stirring time
Di-substituted
4-Hydroxy benzoic acid
4-tert-Butylphenol
2-Naphthol
0.21, 0.00021
2.2, 0.015
8.6, 0.06
0.36, 0.0021
1.0, 0.005
2.0, 0.0095
1.92, 0.0138
4.1, 0.12
8.2, 0.24
28 h
28 h
28 h
Mono-substituted
4-Dihydroxy benzoic acid
4-tert-Butylphenol
2-Naphthol
0.43, 0.0031
4.4, 0.03
1.3, 0.009
0.36, 0.0021
3.4, 0.02
1.0, 0.0059
1.92, 0.0138
16.5, 0.12
5.4, 0.039
5 days
48 h
5 days

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P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ2472
/
be used for ring expansion in a mixture of freshly
distilled DMSOꢁ1-chloronaphthalane (4/2 ml) in the
presence of DBU (0.1 g, 0.00065 mol). The reaction
mixture was heated to 130 8C, then a suspension of the
appropriate unsubstituted or substituted Sub-Pc (3a) or
the sub-naphthacyanine (3b) (0.2 g, 0.00046 mol) and
zinc(II) acetate dihydrate (0.11 g, 0.0005 mol) in the
2.2.8. 2,3,9,10,16,17-Hexa(tert-butylphenoxy)
phthalocyanato zinc (8)
/
Yield 30%. IR (KBr), n (cmꢀ1): 2946 (CÃ
/
H); 1699,
1486, 1409, 1332, 1273 (CÃ
/
OÃ/C); 1091, 889, 774, 752,
1
467. H NMR (d-DMSO, 400 Hz): 9.80 (s, 8H, 1,4-Pc);
8.75 (s, 2H, 2,3-Pc); 8.00 (d, 12H, 1?,5?-H); 7.70 (d, 12H,
2?,4?-H); 1.3 (s, 54H, CH3). lmax (nm) (log o) in DMF:
670 (4.75), 605 (3.91), 342 (4.09).
DMSOꢁ1-chloronaphthalene solvent mixture was
/
added drop-wise to the heated mixture over a period
of 1 h. After cooling, glacial acetic acid (0.04 g, 0.65
mol) was added and the solvents evaporated. For
complex 9, there was no need for ring expansion of
Sub-Pc, hence the complex was synthesized by reacting
dicyanobenzene disubstituted with tert-butylphenol di-
rectly with zinc (II) acetate dihydrate. For all complexes,
methanol was added and the product was separated by
centrifugation. The product was further purified by thin
layer chromatography (TLC), using ethyl acetate and
toluene for elution.
2.2.9. 2,3,9,10,16,17,23,24-Octa(tert-butylphenoxy)
phthalocyanato zinc (9)
Yield 38%. IR (KBr), n (cmꢀ1): 3056, 2959 (CÃ
/
H);
2622, 2550, 2433, 1954, 1719, 1607, 1506, 1484, 1456,
C); 1244, 1209, 1164, 1114,
1407, 1331, 1283 (CÃ
/
OÃ
/
1092, 1059, 950, 886, 829, 794, 774, 751, 728, 634, 570,
1
499, 435. H NMR (d-DMSO, 400 Hz): 9.5 (s, 8H, 1,4-
Pc); 7.5 (d, 16H, 1?,5?-H); 7.0 (d, 16H, 2?,4?-H); 1.2 (s,
72H, CH3). lmax (nm) (log o) in DMF: 670 (4.73), 605
(4.20), 335 (4.48).
2.2.5. 2-Mono(tert-butylphenoxy) phthalocyanato zinc
(5)
Yield 35%. Anal. Calc. for C42H28N8O1Zn×
6(C4H8O2): C, 63.16; H, 6.06; N, 8.93. Found: C,
63.99; H, 6.24; N, 8.14%.
IR (KBr), n (cmꢀ1). 3962, 3056, 1715, 1602, 1558,
2.2.10. 2,3-Bis(naphthoxy) phthalocyanato zinc(II)
(10)
/
Yield 20%. Anal. Calc. for C42H28N8O1Zn×
C, 69.60; H, 3.57; N, 12.50. Found: C, 68.78; H, 2.70; N,
14.17%. IR (KBr), n (cmꢀ1): 3053, 2930 (CÃ
H); 2859,
2625, 2547, 1957, 1713, 1600, 1484, 1408, 1334, 1285
(CÃOÃC); 1210, 1165, 1059, 962, 913, 887, 809, 771,
752, 721, 634, 571, 500, 471, 433. H NMR (d-DMSO,
400 Hz): 9.25 (s, 8H, 1,4-Pc); 8.25 (s, 6H, 2,3-Pc); 8.8ꢁ
/2(H2O):
/
1507 (CÃ
/
H); 1471, 1394, 1363, 1336, 1262 (CÃ
/
OÃ
/
C);
/
/
1
1
1235, 1174, 1091, 1046, 946, 803. H NMR (d-DMSO,
400 Hz): 8.90 (s, 8H, 1,4-Pc); 8.85 (s, 7H, 2,3-Pc); 8.10
(d, 2H, 1?,5?-H); 7.40 (d, 2H, 2?,4?-H); 1.30 (s, 9H, CH3).
lmax (nm) (log o) in DMF: 675 (5.09), 609 (3.71), 348
(3.96).
/
7.5 (m, 14H, naphthoxy). lmax (nm) (log o) in DMF: 670
(4.97), 605(4.24), 335 (4.69).
2.2.11. 2-Mono-nitro phthalocyanato zinc(II) (11)
2.2.6. 2,3-Bis(tert-butylphenoxy) phthalocyanato zinc
(6)
Yield 41%. Anal. Calc. for C32H15N9O2Zn×
/
2(C4H8O2): C, 60.15; H, 3.88; N, 15.79. Found: C,
59.67; H, 3.18; N, 16.28%. IR (KBr), n (cmꢀ1): 3931,
3332, 1950, 1702, 1652, 1458, 1322 (NO2); 1162, 1055,
Yield 40%. Anal. Calc. for C52H40N8O2Zn×
63.58; H, 5.30; N, 11.41. Found: C, 61.54; H, 3.54; N,
13.41%. IR (KBr), n (cmꢀ1): 3403, 2964 (CÃ
H); 1601,
OÃC); 831,
/
6H2O: C,
1
/
885, 724, 632, 567, 495, 433. H NMR (d-DMSO, 400
Hz): 9.70 (s, 8H, 1,4-Pc); 8.60 (s, 6H, 2,3-H). lmax (nm)
(log o) in DMF: 670 (4.00), 605 (3.50), 343 (4.10).
1508 (CÃ
/
H); 1392, 1247, 1106, 994, 895 (CÃ
/
/
745, 726, 477. 1H NMR (d-DMSO, 400 Hz): 9.20 (s, 6H,
1,4-Pc); 8.50 (s, 2H, 1,4-Pc); 8.17 (s, 6H, 2,3-Pc); 8.00 (d,
4H, 1?,5?-H); 7.93 (d, 4H, 2?,4?-H); 1.45 (s, 18H, CH3).
lmax (nm) (log o) in DMF: 678 (4.47), 611 (3.74), 359
(4.00).
2.2.12. 2,3-Bis(carboxyphenoxy) phthalocyanato
zinc(II) (12)
Yield 48%. Anal. Calc. for C46H24N8O6Zn×
/5H2O: C,
57.68; H, 3.76; N, 11.70. Found: C, 59.76; H, 3.58; N,
11.85%. IR (KBr), n (cmꢀ1): 3437, 3050 (OH); 1602,
2.2.7. 2,9,16-Tri(tert-butylphenoxy) phthalocyanato
zinc (7)
1502, 1403, 1271 (CÃ
858, 782, 745, 699, 618, 495, 433. H NMR (d-DMSO,
400 Hz): 9.4ꢁ9.6 (s, 8H, 1,4-Pc); 8.35 (d, 6H, 2,3-Pc);
/
OÃ
/
C); 1213, 1165, 1013, 951, 889,
1
Yield 43%. IR (KBr), n (cmꢀ1): 3385, 2931 (CÃ
/H);
1709, 1625, 1485, 1331, 1250 (CÃ
/
OÃ
/
C); 1185, 1092, 887,
/
1
776, 751, 725, 467. H NMR (d-DMSO, 400 Hz): 9.48
(s, 8H, 1,4-Pc); 8.54 (m, 2H, 2,3-Pc); 8.10 (m, 3H, 2,3-
Pc); 7.60 (d, 6H, 1?, 5?-H); 7.30 (d, 6H, 2?, 4?-H); 1.3 (s,
27H, CH3). lmax (nm) (log o) in DMF: 669 (4.57), 604
(3.82), 335 (4.24).
7.95 (d, 4H, 1?,5?-H); 7.50 (d, 4H, 2?,4?-H); 2H from
COOH are not seen as their peak is too weak compared
with that of the Pc ring and the phenoxy ring. lmax (nm)
(log o) in DMF: 671 (6.05), 606 (4.32), 357 (3.71), 284
(4.57).

P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ
/2472
2467
2.2.13. 2,9,16-Tri(tert-butylphenoxy)-23-
mono(carboxyphenoxy) phthalocyanato zinc(II) (13)
l
ꢀ1) was introduced to a 1 cm pathlength UVꢁ
/
Vis cell
and photolyzed in the Q band region with a General
Electric Quartz line lamp (300 W). For singlet oxygen
quantum yield studies, photolysis was performed in the
presence of DPBF as a singlet oxygen quencher. To
avoid chain reactions induced by DPBF in the presence
of singlet oxygen [26] the concentration of DPBF was
Yield 20%. Anal. Calc. for C42H28N8O1Zn×
C, 66.19; H, 4.20; N, 14.71. Found: C, 66.57; H, 4.21; N,
16.36%. IR (KBr), n (cmꢀ1): 3056, 2946 (CÃ
H); 1716,
C); 1218, 1162,
/2(H2O):
/
1658, 1597, 1499, 1453, 1270 (CÃ
/
OÃ
/
1107, 1015, 883, 759, 690, 619, 486. 1H NMR (d-DMSO,
400 Hz): 9.40 (s, 8H, 1,4-Pc); 9.00 (s, 4H, 2,3-Pc); 7.55
(d, 8H, 2?,4?-H); 7.42 (d, 8H, 1?,5?,-H); 1.28 (t, 27H,
CH3). lmax (nm) (log o) in DMF: 682 (4.97), 614 (4.24),
346 (4.69).
lowered to ꢀ
/
3ꢄ
10ꢀ5 mol lꢀ1. The DPBF decay at 417
/
nm was monitored. A 600 nm glass cut off filter (Schott)
and a water filter were used to filter off ultraviolet and
far infrared radiation. And an interference filter (Intor,
670 nm with a bandwidth of 20 nm) was placed in the
light path before the sample. The light intensity at 670
nm was measured with a power meter (Lasermate) and
2.2.14. 2,3-Tribenzo phthalocyanato zinc(II) (14)
Yield 38%. Anal. Calc. for C44H22N8Zn×
60.45; H, 4.35; N, 12.82. Found: C, 60.77; H, 5.67; N,
H);
/8H2O: C,
was found to be 5ꢄ .
1016 photons sꢀ1 cmꢀ2
/
14.71%. IR (KBr), n (cmꢀ1): 3389, 3180, 3056 (CÃ
/
Quantum yields of singlet oxygen (FD) photogenera-
tion were determined using the relative method with
unsubstituted zinc phthalocyanine (ZnPc) as reference,
using the following relationship Eq. (1).
1720, 1440, 1255, 1009, 797, 752, 615, 468. 1H NMR (d-
DMSO, 400 Hz): 9.45 (d, 8H, 1,4-Pc); 8.30 (d, 6H, 6,7-
H); 7.90 (d, 6H, 5,8-H). lmax (nm) (log o) in DMF: 672
(4.76), 607 (4.07), 344 (4.69).
PcZn
W ꢄ I
abs
WPcZn ꢄ Iabs
FD ꢃFPDcZn
(1)
2.3. Electrochemical methods
Electrochemical data were collected with the BioA-
nalytical Systems (BAS) model 100 B/W electrochemical
workstation. For cyclic and square wave voltammetries,
a platinum disk (3.0 mm diameter) was used as a
working electrode. Platinum wire auxiliary and Ag
wire pseudo-reference electrodes were employed for all
where, FDPcZn is the singlet oxygen quantum yield in the
presence of unsubstituted ZnPc as a standard. For ZnPc
standard, FDPcZn in DMSO is 0.67 [27]. W and WZnPc
are the DPBF photobleaching rates in the presence of
substituted ZnPc derivatives under investigation and
unsubstituted ZnPc standard, respectively. Iabs and
IaZbsnPc are the rates of light absorption by the substituted
ZnPc derivatives and ZnPc standard, respectively. The
initial DPBF concentrations were kept the same for
both the ZnPc reference and the samples. No significant
changes in the absorbance of the substituted ZnPc
derivatives was observed during the singlet oxygen
studies. All the experiments were carried out at r.t.
The quantum yields for photobleaching were deter-
mined using Eq. (2).
electrochemical experiments. DMF containing ꢀ0.5
/
mol dmꢀ3 TBAP was employed as an electrolyte.
Nitrogen gas was bubbled through the solutions prior
to recording cyclic voltammograms, and a nitrogen
atmosphere was maintained throughout the cyclic
voltammogram scans. The ferrociniumꢁ
/
ferrocene
Fc) couple was employed as an internal standard,
(Fcꢂ ꢁ
and potentials were referenced versus saturated calomel
electrode (SCE) using the reported [24] potential of 0.46
/
V (vs. SCE) for the Fcꢂ ꢁ
/
Fc couple in DMF. The
cathodic to anodic peak separation was found to be 80
mV for the Fcꢂ ꢁ
Fc couple. Prior to cyclic voltammetry
(CV) scans, the platinum working electrode was po-
lished with alumina (B10 mm) on a Buehler felt pad,
(Ct ꢀ C0) ꢄ V ꢄ NA
/
Fꢃꢀ
(2)
Iabs ꢄ S ꢄ t
/
followed by washing with deionized water and rinsing
with methanol and DMF. For cyclic and square wave
voltammetry studies concentrations were of the order of
where Ct and C0 in mole lꢀ1 are the concentrations of
the substituted ZnPc derivatives after and prior to
irradiation, respectively, the changes in concentration
of the dyes was about 15% for these studies. Values of Ct
and C0 were obtained in DMF using the Q band
extinction coefficient given in the Section 2. V is the
reaction volume, S the irradiated area of the cell, t the
irradiation time, NA is the Avogadro’s number and Iabs
the overlap integral of the radiation source light
intensity and the absorption of the substituted ZnPc
derivatives (the action spectrum) in the region of the
interference filter transmittance and is defined by
Eq. (3).
10ꢀ4 mol dmꢀ3
.
2.4. Photodegradation and determination of singlet
oxygen studies
Photodegradation and singlet oxygen quantum yield
determinations were carried out using experimental set-
up which has been described in detail somewhere else
[19,25]. Experiments were performed in air. Typically a 2
ml DMSO solution of the complex (about 1ꢄ
10ꢀ5 mol
/

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P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ2472
/
Iabs ꢃ (1ꢀ10Al )Ildl
(3)
hydroxybenzoic acid (12). Complexes 7, and 13 were
synthesized using Sub-Pc substituted with three tert-
butylphenol groups and ring expanding with unsubsti-
tuted dicyanobenzene (7) or hydroxybenzoic acid (13).
Complex 8 was synthesized using Sub-Pc substituted
with six tert-butylphenol groups and ring expanding
with unsubstituted dicyanobenzene (8). Complex 9 was
sythesized from dicyanobenzene disubstituted with tert-
butylphenol. Complex 14 was synthesized from sub-
naphthalocyanine (Sub-NPc, 3b, Scheme 2) and ring
expanding with dicyanobenzene (4). In all cases an
excess of the dicyanobenzene (over the Sub-Pc or Sub-
NPc) was employed. The complexes gave satisfactory
spectroscopic analyses, confirming the absence of chlori-
nated derivatives. The formation of the ZnPc derivatives
g
where Il is the intensity of light and Al is the
absorbance of the ZnPc derivative at wavelength, l.
Experiments were performed also whereby the solutions
were deaerated with N2 gas or saturated with O2 in
order to study the role of oxygen in the photodegrada-
tion process.
Triplet state life times were determined for complexes
5, 7 and 8 using a flash photolysis system. The excitation
pulse was provided by a Nd-Yag (in frequency-doubled
mode, providing 160 mJ, 8 ns pulses of laser light at 0ꢁ
/
10 Hz), pumping a Lambda-Physik FL2002 dye laser.
Single laser pulse energies used ranged from 0.001 to 2
mJ. A 75 W xenon arc lamp provided the analyzing
light. Triplet state quantum yields (fT) were calculated
by a comparative method using zinc phthalocyanine as a
was monitored using UVꢁVis spectroscopy, Fig. 1. An
/
expected decrease in the absorption band of the Sub-Pc
in the 570 nm region and the simultaneous formation of
the Q and B bands of the phthalocyanine was observed.
standard (fT ZnPcꢃ
UVꢁVis spectra were recorded on a Varian 500 UVꢁ
VisꢁNIR spectrophotometer. IR spectra (KBr pellets)
Elmer Spectrum 2000 FTIR
/
0.25 [28]).
/
/
The reaction was complete within 1ꢁ
the complex. Complex 5 required only 1 h, while the rest
of the complexes needed 2ꢁ4 h. Reaction times much
/4 h depending on
/
were recorded on a Perkinꢁ
/
spectrometer. 1H nuclear magnetic resonance (NMR,
400 MHz) spectra were obtained in CDCl3 and d6-
DMSO using the Bruker EMX 400 NMR spectrometer.
/
longer than these have been reported for these types of
reactions [29]. The lack of clear isosbestic points in Fig.
1, could be due to the presence of differently ring
substituted isomers.
Yields of the ZnPc derivatives were comparable or
higher than those reported in the literature [29] for
synthesis of Pcs from Sub-Pcs. The complexes were
soluble in DMF or DMSO to a varying degree.
Complexes containing tert-butylphenol substituents or
hydroxybenzoic acid were found to be more soluble.
3. Results and discussion
3.1. Synthesis and characterization
Synthesis of unsubstituted and substituted Sub-Pcs
has been reported by several researchers [15,16,29ꢁ
/31].
Complexes 1ꢁ9, 12 and 13 were soluble in chloroform.
/
The Sub-Pc complexes have been employed as starting
materials for the formation of unsymetrically substi-
tuted MPc complexes by ring expansion using substi-
tuted or unsubstituted phthalonitriles. The use of Sub-
Pcs as intermediates for the synthesis of unsymmtrically
substituted phthalocyanines has gained popularity over
using statistical condensation of two differently sub-
stituted phthalonitrile derivatives. The later results in a
complex mixture of products. However, using the
reaction mixture containing phthalonitriles and boron
trihalide, gives ring-halogenated Sub-Pc, in addition to
the desired products. The halogenated Sub-Pcs is
difficult to separate. The Sub-Pc complexes were used
to prepare the unsymmetrically substituted zinc phtha-
locyanines according to Schemes 1 and 2. The product
mixture will inevitably contain halogenated Pc deriva-
tives and the desired products were separated using
preparative TLC.
Elemental analysis was not obtained for some of the
complexes because of the difficulty in obtaining reliable
results due to the presence of differently ring substituted
isomers.
In general the spectra of the complexes was typical of
non-aggregated species at concentrations lower than
1ꢄ
10ꢀ5 mol dmꢀ3, as judged the fact that Beer’s law
/
was obeyed, Fig. 2. The Q band wavelengths of
complexes under discussion ranged from 669 (complex
7) to 682 nm (complex 13) in DMF, Table 2. The Q
band maximum for complex 13 is shifted to longer
wavelengths compared with the rest of the complexes.
This complex contains two different type of substituents:
tert-butylphenol and hydroxybenzoic acid on the phtha-
locyanine ring, while the rest of the complexes contain
one type of substituent. It is well known [30] that the Q
band shifts to longer wavelengths with enlargement of p-
conjugated system and that the shifts became less, the
The ZnPc derivatives (5, 6, 10ꢁ/12) were synthesized
larger the molecules. On going from complex 5ꢁ9, the
/
using unsubstituted Sub-Pc and ring expanding using
dicyanobenzene which was mono-substituted with tert-
butylphenol (5) or nitro (11); and dicyanobenzene di-
substituted with tert-butylphenol (6), naphthol (10) or
bulkiness of the molecule increases as the number of
tert-butylphenol groups is increased from 1 (5) to 8 (for
9). For these complexes, the wavelength of the Q band
increases as follows: 6 (678)ꢁ
/
5 (675)ꢁ
/
8 (670)ꢃ9
/

P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ
/2472
2469
Scheme 2.
Fig. 1. Changes in UVꢁVis spectra during the formation of ZnPc
/
derivatives from Sub-Pc. Spectra at the start (a), and at the end (b), of
the reaction. Solvent, DMF.
Fig. 2. Beer’s law dependence of the ZnPc derivatives in DMF.
due mainly to the destabilization of the HOMO. Lack of
symmetry in phthalocyanines results in the splitting in
the Q band [29]. Fig. 3 shows a representative spectra of
the complexes, and no split in the Q band was observed
showing that the complexes still have some symmetry.
Infrared spectra observed for the phthalocyanine
complexes are all very similar. All the compounds with
(670)ꢀ7 (669 nm). Thus the Q band wavelength is
/
shorter for the more bulky molecules (complexes 7, 8
and 9) and longer for less bulky molecules (complexes 5
and 6), in agreement with the literature [30]. The shift to
longer wavelength could be due to either the destabiliza-
tion of the highest occupied molecular orbital (HOMO)
or the stabilization of the lowest unoccupied molecular
orbital (LUMO). It has been observed that the shifting
of the Q band observed on increasing the p-system, is
the exception of 11 and 14, show a band at ꢀ
cmꢀ1 (CÃ
OÃC vibration) which is characteristic of a
phenyl ether functionality.
/
1200
/
/

2470
P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ2472
/
Table 2
Quantum yields for singlet oxygen and photobleaching for the ZnPc
derivatives in DMSO
3.2. Voltammetric studies
The redox properties of the ZnPc derivatives were
studied using both CV and Osteryoung square wave
voltammetry (OSWV) in DMF containing TBAP. Figs. 4
and 5 show the cyclic voltammogram and square wave
voltammograms for complexes 8 and 5, respectively. Two
reduction couples labeled II and III was observed, and
oxidation couple(s) (labeled as I or Ia and Ib) were also
observed. The cyclic voltammograms for the first scan
were similar to those of the second and subsequent scans,
suggesting that no complicated reactions occurs follow-
ing oxidation or reduction. For all couples, the cathodic
to anodic peak currents (ipc/ipa) were near unity, and
cathodic to anodic peak separation ranged from 80 to 150
mV, suggesting reversible to quasi-reversible behavior.
The cathodic to anodic peak separation was found to be
a
Complex
lQ band (nm)
FD
105 Fphotobleaching
5
6
675
678
669
670
670
670
670
671
682
672
0.68 (114)
0.49
3.9
4.1
5.8
6.5
3.3
4.1
3.8
3.7
3.3
3.0
7
0.57 (240)
0.51 (129)
0.52
8
9
10
11
12
13
14
0.43
0.49
0.40
0.22
0.32
80 mV for the Fcꢂ ꢁ
Fc couple. The peak currents
/
increased linearly with the square root of scan rates,
for scan rates ranging from 5 to 800 mV sꢀ1, indicating
that the electrode reactions are purely diffusion-con-
trolled. The broadness of some of the redox couples could
be a result of the presence of isomers.
All processes are ring based since the central Zn metal
is electroinactive [32]. Using controlled potential colou-
metry, couples II and III are assigned to ring reduction
in ZnPc derivatives and the formation of Pcꢀ3ꢁ
/
Pcꢀ2
and Pcꢀ4ꢁ
/
Pcꢀ3, respectively. The separation between
Fig. 3. Electronic absorption spectra of selected complexes (5ꢁ
/
7) in
10ꢀ6 mol dmꢀ3
.
the first and second ring reductions was found to range
between 0.4 and 0.5 V in agreement with the separation
reported [32] for ring based reductions in non-transition
metal phthalocyanines. For complex 5, Fig. 5, two
closely spaced couples (Ia and Ib) were observed for
the oxidation process. For complex 8, the splitting was
more easily observed in the SWV, Fig. 4. The splitting
DMF. Concentrationꢃ5ꢄ
/
/
1H NMR spectra of the complexes was recorded in d-
DMSO. Characteristic resonances due to the peripheral
(1,4 positions, see Schemes 1 and 2 for numbering)
protons of the phthalocyanes were observed as mainly
singlets between 8.5 and 9.8 ppm, integrating for a total
of eight protons. A doublet was observed for these
protons for complex 14. In addition, 2,3 protons for the
phthalocyanine ring were observed at resonances ran-
ging from 8.0 to 9.0. For mono-substituted ZnPc
derivatives (5 and 11) a singlet integrating for a total
of seven protons (2,3 protons) was observed for each
complex. For di-substituted ZnPc derivatives (6, 10 and
12), singlets or doublets integrating for six protons was
observed for each complex. Tri-substituted complex (7)
gave a multiplets integrating for a total of five protons,
and for the hexasubstituted complex (8), a singlet
integrating for two protons was observed as expected.
Tetrasubstituted complex 13 gave a singlet for the 2,3
protons of the phthalocyanine ring, integrating for a
total of four protons, Complex 9 is octasubstituted
hence contained no 2,3 protons.
The substituents on the phthalocyanine ring showed
resonances due to the aromatic rings and these inte-
grated correctly. In the Section 2, the ring protons of the
substituents are denoted as 2?,4?-H or 1?,5?-H, and the
numbering is shown for complex 5 in Scheme 1.
Fig. 4. Osteryoung square wave voltammogram (a) and cyclic
10ꢀ4 mol dmꢀ3 of complex 8 in DMF
voltammogram (b) of 1ꢄ
/
containing 0.1 mol dmꢀ3 TBAP. Scan rateꢃ
/
100 mV sꢀ1
.

P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ
/2472
2471
3.3. Photobleaching and singlet oxygen studies
Photobleaching of complexes is characterised by
complete disappearance of the spectra due to the
degradation of the phthalocyanine ring. Photobleaching
studies were undertaken in order to determine the effects
of the different substituents on the stability of the ZnPc
derivatives in the presence of light.
Photobleaching studies were performed in DMSO and
the quantum yields for photobleaching are listed in Table
2. Photobleaching quantum yields were of the order of
10ꢀ5 for all the complexes. Photodecomposition of
phthalocyanine complexes usually occurs through oxi-
dative cleavage [33]. Comparing complexes which con-
tain an increasing number of the tert-butyl phenol
substituents (5ꢁ9), we observe a low photobleaching
/
quantum yield (hence higher stability) for complex 9
which was shown to be more difficult to oxidize by
electrochemical methods, hence confirming that oxida-
tive degradation occurs for these complexes. Complex 5
was easier to oxidize electrochemically, but is relatively
stable, suggesting that it is not only the oxidative
mechanism which plays a part in the degradation of
the ring in these ZnPc derivatives. It is normally accepted
[33] that during photodegradation of MPc complexes O2
(1Dg) is generated which attacks the phthalocyanine ring.
However, the attack of O2 (1Dg) on the Pc ring may also
be affected by the shielding effects of the ring substituents
[34]. It has been shown [33,34] that the singlet oxygen
interacts with the phthalocyanine, resulting in the
macrocycle destruction and formation of phthalimide
as the product of photodegradation.
Quantum yields of photobleaching increased when
oxygen was bubbled to solutions, confirming that
oxidative degradation occurs. Inhibition of the photo-
bleaching process in the presence of the singlet oxygen
quencher, DABCO, Fig. 6 is in agreement with the
involvement of singlet oxygen in the degradation of the
phthalocyanine macrocycle.
Fig. 5. Osteryoung square wave voltammogram (a) and cyclic
voltammogram (b) of 1ꢄ
10ꢀ4 mol dmꢀ3 of complex 5 in DMF
containing 0.1 mol dmꢀ3 TBAP. Scan rateꢃ100 mV sꢀ1
/
/
.
was observed to varying extents for the ZnPc deriva-
tives. The observation of two closely spaced oxidation
peaks has been attributed to the presence of both the
aggregated and unaggregated substituted ZnPc com-
plexes in solution [32]. At the concentrations employed
for voltammetric studies in this work (ꢀ
10ꢀ4 mol
/
dmꢀ3), aggregation of the ZnPc derivatives is expected
and was confirmed by the broadening of the spectra at
these concentrations. The observation of two closely
spaced oxidation peaks could also be a result of the
presence of isomers. Thus the two couples observed for
the oxidation process in Fig. 5, are attributed to the ring
oxidation of the aggregated and non-aggregated species.
Table 3 compares the half-wave potentials (E1/2) for
complexes 5, 7, 8 and 9 containing increasing number of
tert-butyl phenol substituents. The Pc ring becomes
easier to reduce and more difficult to oxidize as the
number of tert-butyl phenol substituents increase from 1
(complex 5) to 8 (complex 9), Table 3. The presence of
electron donating methyl groups is expected to make the
ring more difficult to reduce and easier to oxidize. Thus
it is expected that as the number of tert-butyl phenol
substituents increases, the ring should be more difficult
to reduce and easier to oxidize. The reverse is observed
in Table 3.
Quantum yields of singlet oxygen photogeneration
(FD) of compounds 5ꢁ14 are listed in Table 2. These
/
values range from 0.22 to 0.68. No photobleaching of
the complexes was observed during the determination of
singlet oxygen quantum yields, since these processes
Table 3
Comparison of redox potentials for complexes 5, 7, 8 and 9 substituted with varying numbers of tert-butyl phenol substituents
Complex
Number of tert-butyl phenol substituents
E1/2 (V vs. SCE)
I Pcꢀ1 ꢁPcꢀ2
/
II Pcꢀ2/Pcꢀ3
III Pcꢀ3 ꢁPcꢀ4
/
5
7
8
9
1
3
6
8
0.35, 0.55
0.75
0.70
ꢀ1.08
ꢀ0.82
ꢀ0.84
ꢀ0.68
ꢀ1.56
ꢀ1.32
ꢀ1.32
ꢀ1.12
0.97
Solvent, DMF containing TBAP. Pc(ꢀ2), phthalocyanine dianion.

2472
P. Matlaba, T. Nyokong / Polyhedron 21 (2002) 2463ꢁ2472
/
[2] S. Nalwa, J.S. Shirk, in: C.C. Leznoff, A.B.P. Lever (Eds.),
Phthalocyanines: Properties and Applications, vol. 4 (Chapter 4),
VCH, New York, 1996.
[3] K. Hanabusa, H. Shirai, in: C.C. Leznoff, A.B.P. Lever (Eds.),
Phthalocyanines: Properties and Applications, vol. 2 (Chapter 4),
VCH, New York, 1993.
[4] J. Simon, P. Bassoul, in: C.C. Leznoff, A.B.P. Lever (Eds.),
Phthalocyanines: Properties and Applications, vol. 2 (Chapter 5),
VCH, New York, 1993.
[5] N.B. McKeown, Phthalocyanine Materials: Synthesis, Structure
and Function, Cambridge University Press, Cambridge, 1998.
[6] E. Lukyanets, J. Porphyrins Phthalocyanines 3 (1999) 424.
[7] R. Bonnett, Chem. Soc. Rev. 24 (1995) 19.
Fig. 6. Kinetic curves for the photobleaching of complex 5 in DMSO
saturated with oxygen and nitrogen, and containing DABCO (2ꢄ
/
10ꢀ3 mol dmꢀ3).
[8] I. Rosenthal, Photochem. Photobiol. 53 (1991) 859.
[9] K. Tabata, K. Fukushima, I. Okura, J. Porphyrins Phthalocya-
nines 4 (2000) 278.
occur much slower than singlet oxygen production. It is
well known that aggregation of the phthalocyanine
complexes results in decreased photochemical activity
due to enhanced probability of radiationless decay of
excited states. The concentration employed for FD and
[10] L.R. Milgrom, Colours of Life, Oxford University Press, Oxford,
1977.
[11] V. Iliev, V. Alexiev, L. Bilyarska, J. Mol. Catal. A Chem. 137
(1999) 15.
photobleaching studies was 1ꢄ
10ꢀ6 mol dmꢀ3, hence
no aggregation is expected according to Fig. 2. Compar-
ing complexes which, contain an increasing number of
/
[12] P.C. Martin, M. Gouterman, B.V. Pepich, G.E. Renzeoni, D.C.
Schindele, Inorg. Chem. 30 (1991) 3305.
[13] K. Lang, D.M. Wagnerova, J. Brodilova, J. Photochem. Photo-
biol. A: Chem 72 (1993) 9.
the tert-butyl phenol substituents (5ꢁ9), complex 5 with
/
[14] R.W. Boyle, D. Dolphin, Photochem. Photobiol. 64 (1996) 469.
[15] M. Geyer, F. Plenzig, J. Rausechnabel, M. Hanach, B. del Rey, A.
Sastre, T. Torres, Synthesis (1996) 1139.
one substituent gave the largest value for FD. Triplet life
times (tT) were determined for complexes 5, 7 and 8 and
are listed in Table 3. Of the three complexes, the largest
tT value was observed for complex 7 and the lowest for 5
even though the later has the highest FD value. A direct
correlation between FD, and tT or FD was also not
evident for substituted MPc complexes reported in the
literature [3]. The lowest FD value was observed for
complex 14 containing naphthalene units.
In conclusion voltammetric behavior of the substi-
tuted ZnPc derivatives under discussion show that as the
number of tert-butyl substituents increase, the com-
plexes become more difficult to oxidize and easier to
reduce. The complexes are relatively stable to photo-
degradation with quantum yields of photobleaching of
the order of 10ꢀ5. The majority of the complexes have
singlet oxygen quantum yields in the 0.4 and 0.5 region,
hence show promise as candidates for PDT.
[16] H. Ali, S.K. Sim, J.E. van Lier, J. Chem. Res. (S) (1999) 496.
[17] N. Kobayashi, R. Kondo, S.-I. Nakajima, T. Osa, J. Am. Chem.
Soc. 112 (1990) 9640.
[18] C.G. Claessens, T. Torres, Chem. Eur. J. 6 (2000) 2168.
[19] I. Seotsanyana-Mokhosi, N. Kuznetsova, T. Nyokong, J. Photo-
chem. Photobiol. A: Chem. 140 (2001) 215.
[20] S.B. Brown, T.G. Truscott, Chem. Br. (1993) 955.
[21] D. Wohrle, M. Eskes, K. Shigehara, A. Yamada, Synthesis (1993)
¨
194.
[22] H. Kliesch, A. Weitemeyer, S. Muller, D. Wohrle, D. Liebig. Ann.
¨
¨
(1995) 1272.
[23] A. Weitermeyer, H. Kliesch, D. Wohrle, J. Org. Chem. 60 (1995)
¨
4900.
[24] M. Sekota, T. Nyokong, Polyhedron 15 (1996) 2901.
[25] D.M. Maree, N. Kuznetsova, T. Nyokong, J. Photochem.
Photobiol. A: Chem. 140 (2001) 117.
[26] W. Spiller, H. Kliesch, D. Wohrle, S. Hackbarth, B. Roder, G.
¨
Schnurpfeil, J. Porphyrins Phthalocyanines 2 (1998) 145.
[27] N. Kuznetsova, E. Makarova, S. Dashkevich, N. Gretsova, V.
Negrimovsky, O. Kaliya, E. Luk’yanets, Zh. Obshch. Khim. 70
(2000) 140.
[28] P. Jacques, A.M. Braun, Helv. Chim. Acta 64 (1981) 1800.
[29] N. Kobayashi, T. Ishizaki, K. Ishii, H. Konami, J. Am. Chem.
Soc. 121 (1999) 9096.
Acknowledgements
[30] N. Kobayashi, H. Konami, in: C.C. Leznoff, A.B.P. Lever (Eds.),
Phthalocyanines: Properties and Applications, vol. 4 (Chapter 9),
VCH, New York, 1999.
This work is supported by Rhodes University and by
the Foundation of Research Development in South
Africa. We wish Imperial College (Professor David
Phillips) for use of Photolysis equipment.
[31] G. Schmid, M. Sommerauer, M. Hanack, in: C.C. Leznoff, A.B.P.
Lever (Eds.), Phthalocyanines: Properties and Applications, vol. 4
(Chapter 1), VCH, New York, 1996.
[32] A.B.P. Lever, E.R. Milaeva, G. Speier, in: C.C. Leznoff, A.B.P.
Lever (Eds.), Phthalocyanines: Properties and Applications, vol. 3
(Chapter 1), VCH, New York, 1993.
References
[33] G. Winter, H. Heckmann, P. Haisch, W. Eberhardt, M. Hanack,
L. Luer, H.J. Egelhaaf, D. Oelkrug, J. Am. Chem. Soc. 120 (1998)
¨
[1] I. Rosenthal, E. Ben-Hur, in: C.C. Leznoff, A.B.P. Lever (Eds.),
Phthalocyanines: Properties and Applications, vol. 1 (Chapter 6),
VCH, New York, 1989.
11663.
[34] G. Schnurpfeil, A. Sobbi, W. Spiller, H. Kliesch, D. Wohrle, J.
¨
Porphyrins Phthalocyanines 1 (1997) 159.