The synthesis and fluorescence behaviour of phthalocyanines unsymmetrically substituted with naphthol and carboxy groups

Article abstract of DOI:10.1016/j.dyepig.2009.11.010

Unsymmetrically substituted phthalocyanines 8,15,22-tris-(naphtho)-2-(carboxy)phthalocyanine, [8,15,22-tris-(naphtho)-2-(carboxy)phthalocyanato]zinc(II), 8,15,22-tris-(naphtho)-4,5-(3-carboxy-1,2-dioxyphenyl)phthalocyanine and [8,15,22-tris-(naphtho)-4,5-(3-carboxy-1,2-dioxyphenoxy)phthalocyanato]zinc(II) were prepared using the mixed phthalonitrile cyclotetramerization of 3-(1-naphthoxy) phthalonitrile with a carboxylic acid phthalonitrile. The phthalocyanines were separated using column chromatography employing a mixture of THF, ammonia and water. The novel compounds were characterized using UV-Vis, IR, ¹H NMR and mass spectrometry as well as elemental analysis. Fluorescence quantum yields were found to range from 0.05 to 0.16.

Full text of DOI:10.1016/j.dyepig.2009.11.010

Dyes and Pigments 86 (2010) 68e73
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and fluorescence behaviour of phthalocyanines unsymmetrically
substituted with naphthol and carboxy groups
*
Nolwazi Nombona, Edith Antunes, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Unsymmetrically substituted phthalocyanines 8,15,22-tris-(naphtho)-2-(carboxy)phthalocyanine,
[8,15,22-tris-(naphtho)-2-(carboxy)phthalocyanato]zinc(II), 8,15,22-tris-(naphtho)-4,5-(3-carboxy-1,2-
dioxyphenyl)phthalocyanine and [8,15,22-tris-(naphtho)-4,5-(3-carboxy-1,2-dioxyphenoxy)phthalocya-
nato]zinc(II) were prepared using the mixed phthalonitrile cyclotetramerization of 3-(1-naphthoxy)
phthalonitrile with a carboxylic acid phthalonitrile. The phthalocyanines were separated using column
chromatography employing a mixture of THF, ammonia and water. The novel compounds were char-
acterized using UVeVis, IR, 1H NMR and mass spectrometry as well as elemental analysis. Fluorescence
quantum yields were found to range from 0.05 to 0.16.
Received 29 September 2009
Received in revised form
13 November 2009
Accepted 17 November 2009
Available online 16 December 2009
Keywords:
Zinc phthalocyanine
Carboxy
Ó 2009 Elsevier Ltd. All rights reserved.
Naphthol
Fluorescence
1. Introduction
condensation method that is based on the reaction of the two differ-
ently substituted phthalonitriles to afford a mixture of six compounds,
Although phthalocyanines (Pcs) enjoy widespread use as
conventional dyes and pigments, they have also found importance
in a number of applications including photodynamic therapy,
owing to their good singlet oxygen generation ability [1e6].
Interest in improving the photo-physical properties of the Pc ring
system continues, resulting in interest in securing novel Pcs whose
structures promote good photo-physical behavior [7e12].
This paper concerns the synthesis and fluorescence behaviour of
unsymmetrically substituted metal-free and zinc Pcs which contain
three naphthol units and one carboxylic acid functional group. The
molecules may be attached to biological molecules as the COOH
group can interact with the amino group of the former. The
naphthol group was used to enhance solubility and reduce aggre-
gation owing to the well-known effect of such bulky substituents
on steric hindrance [7].
with the A3B type Pc being of highest yield [7,16e18]. MPc molecules
which are symmetrically tetra (or octa) substituted with naphthol [19]
or carboxy [20,21] groups have been reported. This work reports the
synthesis of unsymmetrically substituted zinc phthalocyanines
(ZnPcs) containing both naphthol and carboxyl groups.
2. Experimental
2.1. Materials
Tetrahydrofuran (THF), 25% aq ammonium hydroxide solution,
dimethylformamide (DMF), dichloromethane (DCM), dime-
thylsulfoxide (DMSO), chloroform (CHCl3), glacial acetic acid and
pentanol were purchased from Saarchem. Potassium carbonate,
zinc acetate, dihydroxy carboxylic acid (5) and lithium metal were
purchased form SigmaeAldrich. Silica gel for column chromatog-
raphy was purchased from Merck. 3-Nitrophthalonitrile (1) and
4,5-dichlorophthalonitrile (4) were synthesized as reported in
literature [22,23].
The possibility of synthesizing unsymmetrical Pcs with
substituents situated at specific positions enables fine-tuning of
physical properties, thereby enhancing the technological applica-
tions of the phthalocyanines [7,13e15]. The presence of different
functional groups in the same molecule may provide additional
features such as increased solubility and reactivity.
2.2. Equipment
Pcs that comprise three identical (A) and one different (B) isoindole
units (A3B type) have been prepared using a non-selective statistical
UVeVis absorption spectra were obtained using the Varian Cary
500 UVeVis/NIR. Fluorescence excitation and emission spectra
were recorded with Varian Eclipse spectrophotometer. IR data (KBr
pellets) was obtained using the PerkineElmer spectrum 2000 FTIR
* Corresponding author. Tel.: þ27 46 6038260; fax: þ27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.11.010

N. Nombona et al. / Dyes and Pigments 86 (2010) 68e73
69
spectrometer. 1H NMR spectra were recorded using a Bruker
AMX 400 MHz spectrometer. Elemental analysis was done using
a Vario-Elementar Microcube ELIII. MALDI-TOF mass spectrometry
was carried out at the University of Stellenbosch using ABI voyager
DE-STR MALDI-TOF instrument.
cmꢀ1]: 2958e2869 (carboxylic acid OH), 1157 (CeO), 1096 (NeH).
1H NMR (THF-d8):
, ppm 8.80 (2H, s, PceH), 7.22 (3H, broad s,
d
PceH), 6.32e5.10 (28H, m, AreH, PceH), ꢀ1.02 (2H, s, H2Pc).
MALDI-TOF-MS m/z Calc: 984.28; Found (Mꢀ): 985.0.
2.3.3.2. [8,15,22-Tris-(naptho)-4,5-(3-carboxy-1,2-dioxyphenyl)phth-
2.3. Synthesis
alocyanine] (8a). Yield: <5% UVeVis [(THF/lmax/nm (log 3))] 710
(4.51), 680 (4.47), 646 (4.05), 617 (3.93), 354 (4.29). C76H68N8O5: C,
75.72: H, 5.69: N, 9.30%; found C, 75.27: H, 5.62; N, 10.08%. IR [(KBr)
nmax/cmꢀ1]: 3053e2853 (carboxylic acid OH), 1246 (CeO), 1082
2.3.1. 3-(1-Naphthoxy) phthalonitrile (3)
1-Naphthol (2, 2.11 g, 14.6 mmol) was dissolved in dry DMSO
(13 mL) and 3-nitrophthalonitrile (1, 2.5 g, 14.4 mmol) was added
under inert atmosphere. To this reaction mixture finely ground
anhydrous potassium carbonate (4.15 g, 30 mmol) was added. After
4 h of stirring at room temperature further potassium carbonate
(1.06 g, 7.66 mmol) was added and this same amount was added
again after 24 h of stirring. After 48 h of stirring, the reaction
mixture was poured into water (50 mL) resulting in the formation
of brown precipitates. Thin layer chromatography (TLC) was per-
formed to determine the consumption of the starting materials and
complete conversion of compound 1 was observed after 48 h. The
brown product was centrifuged and recrystallised from methanol.
Yield: 3.7 g (67%). IR [(KBr) nmax/cmꢀ1]: 2228 (CN), 1273
(NeH). 1H NMR (THF-d8):
d, ppm 9.00 (2H, s, PceH), 7.15 (3H, m,
AreH), 6.40 (4H, m, PceH), 5.93 (5H, d, PceH), 5.80e5.25 (21H, m,
AreH), ꢀ2.00 (2H, s, H2Pc). MALDI-TOF-MS m/z Calc: 1090.29;
Found (Mꢀ): 1090.9.
2.3.4. Synthesis of zinc phthalocyanines (7b, 8b)
The unmetallated phthalocyanines (7a or 8a) were refluxed for
1 h in the presence of excess zinc (II) acetate in pentanol (20 mL). To
the resulting product, methanol (30 mL) was added to precipitate
out the dark green crude products. For purification, the procedure
of 7a and 8a was followed.
(CeOeC). 1H NMR (DMSO-d6):
d
, ppm 8.21 (1H, d, Ar0eH), 7.95 (1H,
2.3.4.1. [8,15,22-Tris-(naptho)-2-(carboxy)phthalocyanato]zinc(II)
(7b). Yield: <5% UVeVis [(THF/lmax/nm (log 3))] 680 (4.15), 651
t, Ar0eH), 7.85 (1H, d, Ar0eH), 7.75 (1H, t, AreH), 7.7e7.55 (4H, m,
Ar0eH), 7.35 (1H, d, AreH), 7.15 (1H, d, AreH).
(3.40), 614 (3.44), 357 (3.95). C70H64N8O5Zn: C, 72.19: H, 5.71: N,
9.62%; found C, 71.44: H, 5.87; N, 8.07%. IR [(KBr) nmax/cmꢀ1]:
2.3.2. 4,5-(3-Carboxy-1,2-dioxyphenyl) phthalonitrile (6)
2955e2854 (carboxylic acid OH), 1244 (CeO). 1H NMR (THF-d8):
d,
A mixture of 4,5-dichlorophthalonitrile (4, 6 g, 30.5 mmol), 3,4
dihydroxy carboxylic acid (5, 4.72 g, 30.5 mmol) and dry DMSO
(23 mL) was stirred at 90 ꢁC, whilst finely ground anhydrous
potassium carbonate (2.12 g, 15.3 mmol) was added every five
minutes until eight portions were added. The reaction was stirred
for 60 min and cooled to room temperature. Water (50 mL) was
added to the reaction mixture and the pH of this solution was
dropped to one, with stirring, using a 10% HCl solution. The product
was extracted with ethyl acetate and the organic extract was
washed with water and dried under MgSO4. The product was
obtained as a brown solid.
ppm 10.82 (2H, s, PceH), 9.10 (3H, broad s, PceH), 8.22e6.91 (28H, m,
AreH, PceH). MALDI-TOF-MS m/z Calc: 1046.19; Found (Mþ): 1045.7.
2.3.4.2. [8,15,22-Tris-(naptho)-4,5-(3-carboxy-1,2-dioxyphenoxy)ph-
thalocyanato]zinc(II) (8b). Yield: <5% UVeVis [(THF/lmax/nm
(log
H, 5.39: N, 10.82%; found C, 69.94: H, 6.12; N, 10.14%. IR [(K Br) nmax
cmꢀ1]: 3052e2849 (carboxylic acid OH),1237 (CeO). 1H NMR (THF-
d8): , 9.00 (1H, br s, OH), 8.25 (3H, br d, PceH), 7.78 (6H, m, PceH),
3))] 685 (3.98), 654 (3.21), 355 (3.60). C76H66N8O5Zn: C, 70.83:
/
d
7.60e7.19 (21H, m, AreH), 7.15 (2H, d, PceH), 6.80 (3H, d, AreH).
MALDI-TOF-MS m/z Calc: 1152.20; Found (Mꢀ): 1153.2.
Yield: 4.3 g (71%). IR [(KBr) nmax/cmꢀ1]: 3050(OH), 2235 (CN),1718
and 1337 (CeO).1H NMR (DMSO-d6):
d, ppm 7.80e7.84 (2H, d, AreH),
2.4. Photo-physical properties
7.64e7.58 (1H, d, Ar0eH), 7.32 (1H, s, Ar0eH), 7.10e7.14 (1H, d, Ar0eH).
2.4.1. Flourescence quantum yields
2.3.3. Synthesis of unmetallated phthalocyanines (7a, 8a)
The comparative method was used to determine the fluores-
cence quantum yields (FF) of the phthalocyanine complexes using
equation (1) [24].
In 25 mL of dry pentanol, one equivalent of 5 (for 7a) or 6 (for 8a)
and three equivalents of 3 were stirred under reflux in an argon
atmosphere at 140 ꢁC for 10 min. 100 mg of lithium was then added
and the reaction mixture was refluxed for a further 2 h. Thereafter
the reaction mixture was left to cool to room temperature. Glacial
acetic acid (40 mL) was added to the mixture resulting in unme-
tallated phthalocyanines. The resulting precipitates were centri-
fuged and washed with water. The product was purified by repeated
re-precipitation from THF into methanol followed by column
chromatography on silica gel as substrate using THF:NH4OH:H2O in
a 1:1:1 ratio as the eluting solvent mixture. Under these conditions,
Pcs with polar side chains (COOH groups) are expected to be easily
retained by the silica and hence are slower to elute through the
column. Essentially the first fraction eluted is expected to be the four
naphthoxy-substituted Pc followed by the desired mono-function-
alized Pc. Three fractions were observed on the column, the third
fraction remained at the top of the column.
F$ AStd$n2
FF
¼
FFðStdÞ
(1)
F
Std$A$n2Std
where F and Fstd are the areas under the fluorescence curves of
the complexes and the standard respectively. A and Astd are the
respective absorbances of the sample and the standard at the
excitation wavelength and n and nstd are the refractive indices of
the solvents used for the sample and standard respectively. ZnPc
was used as a standard in DMSO where FF ¼ 0.20 [25].
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the dinitrile precursors; 3-nitrophthalonitrile
(1)and4,5-dichlorophthalonitrile(4)was carriedoutasaccording to
literature [22,23]. Scheme 1 shows the synthetic procedures used to
obtain 3-(1-naphthaenyloxy) phthalonitrile (3) and 4,5-(3-carboxy-
1,2-dioxyphenyl) phthalonitrile (6). The electron-withdrawing
2.3.3.1. [8,15,22-Tris-(naptho)-2-(carboxy)phthalocyanine]
Yield: <5% UVeVis [(THF/lmax/nm (log ))] 714 (4.96), 684 (4.93),
653 (4.46), 621 (4.33), 351 (4.59). Calcd. for: C70H66N8O5: C, 76.48:
H, 6.05: N, 10.19%; found C, 75.14: H, 5.91; N, 9.23%. IR [(KBr)nmax
(7a).
3
/

70
N. Nombona et al. / Dyes and Pigments 86 (2010) 68e73
CN
CN
CN
Dry DMSO
K2CO3
CN
+
O
RT
48 hrs
OH
NO2
2
1
3
O
O
Cl
Cl
CN
Dry DMSO
OH
OH
O
O
NC
NC
HO
OH
+
K2CO3
CN
90 oC
3 hrs
5
4
6
Scheme 1. Synthesis of carboxylated phthalonitriles 3 and 6.
capabilityof the dinitrile functionalities makes 1 and 4 susceptible to
nucleophilic attack. The base catalyzed nucleophilic substitution of
the nitro and chloride groups was performed in dry DMSO at room
temperature under inert nitrogen atmosphere. Good yields were
obtained for both reactions. The synthetic procedure outlined in
Schemes 2 and 3 show the statistical condensation approach used
for the synthesis of the A3B type Pcs. This method is based on the
reaction of two differently substituted phthalonitriles in a ratio of
3:1. Chromatographic techniques were used for the isolation of the
desired product and the A3B type Pcs were isolated with very low
(<5%) percentage yields. The asymmetric unmetallated phthalocy-
anines (7a and 8a) were soluble in THF and only partially soluble in
otherorganic solvents includingethanol, chloroform, DMF, DCM and
DMSO. The metallation of Pcs 7a and 8a in dry pentanol using zinc
(II) acetate gave Pcs 7b and 8b with low percentage yields after
purification. The compounds were characterized by UVeVis, IR,
NMR and mass spectral data, as well as elemental analysis. The
disappearance of the C^N stretch at 2228 cmꢀ1 and 2235 cmꢀ1 in
the IR spectra of phthalonitriles 3 and 6 confirmed the conversion
into the corresponding phthalocyanine analogues (7a and 8a). IR
data also confirmed the conversion of 7a and 8a to the respective Zn
phthalocyanines by the disappearance of the NeH stretches at 1096
and 1082 cmꢀ1. The disappearance of the inner cavity protons in the
1H NMR spectra also confirmed metallation.
The 1H NMR spectra of all complexes in THF-d8 were compli-
cated and gave broadened peaks due to possible intermolecular
aggregation of the Pc as well as the presence of isomers. The 1H
NMR spectra was used to confirm the number of protons expected
for the complexes. For complexes 7a and 7b, 33 aromatic protons
were obtained. In similar manner complexes 8a and 8b gave 35
aromatic protons respectively. In addition, ring cavity protons (2)
for 7a and 8a were observed as a weak peaks at ꢀ1.02 and
ꢀ2.00 ppm as anticipated. The non-peripheral ring protons were
observed as two signals at 8.80 and 7.22 ppm for 7a and at 7.15 and
6.40 for 7b. The 1H NMR spectra data for 7a and 8a were similar to
that of respective complexes 7b and 8b, except that the proton
signals for the latter (ZnPc derivatives) were deshielded, this may
be caused by the electron-withdrawing effect caused by the
O
CN
CN
O
N
N
N
N
H
CN
CN
+
HO
Li, dry pentanol
CH3COOH
O
COOH
N
H
N
O
N
N
O
5
3
7a
O
Dry pentanol
Zn(CH3COO)2
N
N
N
Zn
COOH
N
N
O
N
N
N
O
7b
Scheme 2. Synthesis of 8, 15, 22-tris-(naptho)-2-(carboxy)phthalocyanine analogues.

N. Nombona et al. / Dyes and Pigments 86 (2010) 68e73
71
CN
CN
O
N
N
O
N
O
N
Li, dry pentanol
CH3COOH
O
O
H
NC
O
O
O
OH
+
OH
N
N
NC
H
N
O
N
6
3
O
8a
O
Dry pentanol
Zn(CH3COO)2
N
N
O
N
O
O
OH
Zn
N
N
N
O
N
N
O
8b
Scheme 3. Synthesis of 8,15,22-tris-(naptho)-4,5-(3-carboxy-1,2-dioxyphenyl) phthalocyanine analogues.
insertion of the Zn metal. The carboxylic acid proton was only
observed for complex 8b as a broad peak at 9.00 ppm. The acid
proton is often difficult to locate due to its varying chemical shift,
also the presence of water and oxygen in the sample may lead to
the OH not being seen as it is easily exchangeable with deuterated
solvents and the exchange is too fast on the NMR timescale, hence
the peak is hardly observed. The integration of the peaks corre-
sponded suitably to the expected number of protons for all
complexes, further confirming the relative purity of the complexes.
The purified monofunctional phthalocyanines were further char-
acterized by mass spectra. The expected mass values corresponded
with the found values for all complexes. There was no indication of
other substituted derivatives on all mass spectra.
3.2. Ground state electronic absorption, fluorescence
spectra and quantum yields
The absorption spectra of the Pcs in THF show a characteristic
sharp Q bands, Fig. 1. These pep* transitions were observed at 710,
680 (7a), 715, 685 (8a), 680 (7b) and 685 (8b), Fig. 1. The Q band
maxima of all the complexes are summarized in Table 1. There is
a 5 nm difference between the Q-band positions of compounds 7
and 8. This implies that the electron density of compound 8 is
higher than that of compound 7 due to the dioxyphenyl substit-
uent. This results in the lowering of the HOMOeLOMO gap of
complexes 8a and 8b. The aggregation behavior of the Pcs were
investigated in THF (Fig. 2 using complex 7a as an example), no new
bands were observed for all complexes signifying no aggregation
behavior at these concentrations, probably due to the bulky nature
of the ring substituents. Beer's law was observed for all complexes
for concentrations less than 5 ꢂ 10ꢀ5 M. The complexes are
unsymmetrically substituted and it would be expected that there is
some Q band splitting due to loss of symmetry.
The excitation spectra for 7a and 8a were similar to the corre-
sponding absorption spectra in that there was no change in Q band
Table 1
UVeVis spectral parameters of H2Pc (7a, 8a) and ZnPc (7b, 8b) analogues in THF.
MPc
Qabs (nm)
Qems (nm)
Qexc (nm)
FF
7a
8a
7b
8b
710,680
715, 685
680
718
722
685
698
713, 678
716, 687
680
0.16
0.10
0.05
0.12
685
687
Fig. 1. Absorption spectra of (a) complex 7a, 8a and (b) 7b and 8b in THF.
Qabs ¼ Q band absorption maximum; Qems ¼ Q band emission maximum; Qexc ¼ Q
Concentrations w 1 ꢂ 10ꢀ6 mol Lꢀ1
.
band excitation maximum.

72
N. Nombona et al. / Dyes and Pigments 86 (2010) 68e73
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
y = 0.0944x - 0.0033
R² = 0.9995
2.00
4.00
6.00
8.00
10.00
Concentration (µM)
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 2. Absorption spectra of compounds 7a in THF at different concentrations. Inset a plot of Q-band absorbance versus concentration.
positions, but the relative intensities of the split pair were different,
suggesting a slight change in symmetry upon excitation, Fig. 3a
and b. Low symmetry Pc complexes, such as that of unmetallated
Pcs [26], fluoresce with only one main peak and is assigned as the
0e0 transition of the fluorescence, hence the observation of a single
main emission peak in Fig. 3a and b.
being mirror images of emission, Fig. 3c and d. The fluorescence
quantum yield (FF) values for the complexes were found to range
from 0.05 to 0.16, with complex 7a having the highest quantum
yield, Table 1. The FF values are in the range of MPc complexes [26].
Unmetallated complex 7a gave a larger FF values compared to 7b
containing the Zn central, this could be a result of intersystem
crossing to the triplet state in the presence of the heavy Zn central
metal, which then lowers the FF values. However for 8a and 8b,
The corresponding zinc phthalocyanines (7b and 8b) show
excitation spectra which are similar to absorption spectra with both
1.2
a
b
1.2
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
500
550
600
650
700
750
800
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
1.2
1
c
d
1.2
1
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
500
550
600
650
700
750
800
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Fig. 3. Absorption (solid line), excitation (red line) and emission (dashed line) spectra of (a) 7a, (b) 8a, (c) 7b and (d) 8b in THF. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)

N. Nombona et al. / Dyes and Pigments 86 (2010) 68e73
73
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for all compounds with complex 7a having the highest fluorescence
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Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF), South
Africa through DST/NRF South African Research Chairs Initiative for
Professor of Medicinal Chemistry and Nanotechnology as well as
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