Synthesis, characterization and nonlinear optical properties of nonaggregating hexadeca-substituted phthalocyanines

Article abstract of DOI:10.1016/j.tetlet.2008.10.102

The microwave-assisted synthesis, characterization and photophysical properties of hexadeca-substituted phthalocyanine (Pc) complexes with bulky phenoxy substituents are reported. NMR and UV-Vis analyses confirm the ability of bulky groups to induce steri

Full text of DOI:10.1016/j.tetlet.2008.10.102

Tetrahedron Letters 50 (2009) 165–168
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, characterization and nonlinear optical properties
of nonaggregating hexadeca-substituted phthalocyanines
a
a
b
Saad Makhseed a, , Moyyad Al-Sawah , Jacob Samuel , Hacene Manaa
*
a Chemistry Department, Kuwait University, PO Box 5969, Safat 13060, Kuwait
b Physics Department, Kuwait University, PO Box 5969, Safat 13060, Kuwait
a r t i c l e i n f o
a b s t r a c t
Article history:
The microwave-assisted synthesis, characterization and photophysical properties of hexadeca-substi-
tuted phthalocyanine (Pc) complexes with bulky phenoxy substituents are reported. NMR and UV–Vis
analyses confirm the ability of bulky groups to induce steric isolation of Pc cores even in the solid state.
The Z-scan measurement indicates that the Zn-containing derivative has promising nonlinear optical
properties (NLOs).
Received 6 August 2008
Revised 13 October 2008
Accepted 22 October 2008
Available online 25 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Phthalocyanine (Pc) and its derivatives are a class of organic
functional materials that display interesting catalytic, electronic,
and optical properties in addition to their widespread use as blue
and green colorants.1 Due to their fascinating properties, Pc-based
materials find applications in nonlinear optics (including optical
limitation).2 optical data storage,3 photodynamic cancer therapy,4
sensors,5 catalysis,6 and solar energy conversion.7 However, the
use of these materials is limited by their low solubility in common
solvents, which results in intermolecular aggregation in both solu-
tion and solid state, thus causing a drastic decay of their optical
properties.8 In particular, the aggregation behavior of Pcs can
greatly affect the photodynamic activity and nonlinear optical
properties through reducing the active absorbing excited-state life-
time.9,10 Therefore, our current research has focused on modifying
the phthalocyanine itself to tune both molecular and bulk proper-
ties for specific applications in the fields of optical limiting and
photodynamic therapy (PDT).
The large architectural flexibility in the structure of phthalocy-
anines gives such materials attractive advantages to enhance their
optical properties (color, aggregation, optical absorption, etc.) tai-
lored to the required applications. Thus, many strategies have been
investigated to prevent self-association of phthalocyanines in solu-
tion and in the condensed state.11 Recently, we reported that bulky
phenoxy substituents placed on the peripheral position of the
phthalocyanine core perfectly prohibit close self-association of
the macrocycle even within solid thin films and can result in the
formation of remarkable cubic crystals containing massive sol-
vent-filled voids.12 In addition to the benefits arising from control
over the molecular self-association (in modifying the photophysi-
cal and opto-electronic properties of phthalocyanine macrocycles),
tuning the ground-state absorption (i.e., the position of the princi-
ple Q-band absorption and the molar absorption coefficient) is crit-
ical to the application of Pcs as photosensitizers and optical
limitation systems. For example, the Q-band bathochromic shift
in the visible spectrum into the near-IR region gives a wider trans-
parent window in the spectrum. This is required for possible mate-
rial applications of Pcs, which make them potentially suitable for
PDT as photosensitizers and in nonlinear optics (e.g., human eye
protector). Placing phenoxy groups at the nonperipheral positions
(1, 4, 8, 11, 15, 18, 22 and 25) of the phthalocyanine ring results in
a shift of the Q-band in the visible spectrum into the near-IR region
so that the Pc does not display its characteristic blue or green col-
or.13 The aim of our work is to synthesize 1,2,3,4,8,9,10,11,15,16,
17,18,22,23,24,25-hexadecakis(2,6-dimethylphenoxy)phthalocya-
nine (Pc; M = 2H, Zn, Co, Ni), which contains sterically hindered
phenoxy substituents at each of the 16 peripheral and nonperiph-
eral sites of the macrocycle. To the best of our knowledge, the syn-
thesis, structural characterization and material properties of such
interesting Pc-based materials are described for the first time.
Scheme 1 shows the synthetic route for the new phthalocya-
nines substituted with bulky dimethylphenoxy groups. The pht-
halonitrile precursor 2,3,4,5-tetrakis(2,6-dimethylphenoxy)phtha-
lonitrile 1 was prepared in good yield (75%) from the aromatic
nucleophilic substitution reaction between the anion of 2,6-
dimethylphenol and commercially available tetrafluorophthalonit-
rile.14 Spectroscopic characterizations of 1 were consistent with its
expected structure, and the IR spectrum shows an intense band at
2220 cmÀ1 corresponding to the CN groups. Initial attempts at for-
mation of Pc 2 using a mixture of lithium pentoxide/pentanol, the
most common procedure, resulted in a very low yield and isolation
of the pure product from other Pc by-products could not be
achieved using various purification techniques (e.g., chromatogra-
phy). This was expected due to the steric bulk exerted by the four
substituents placed on the phthalonitrile ring 1. Other common
synthetic procedures were also used, but the outcome was the
* Corresponding author. Tel.: +965 4985538; fax: +965 4816482.
E-mail address: smakhseed@kuc01.kuniv.edu.kw (S. Makhseed).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.102

166
S. Makhseed et al. / Tetrahedron Letters 50 (2009) 165–168
RO
RO
OR
OR
F
F
F
F
CN
CN
OR
OR
OR
N
OR
OR
N
N
M
N
RO
RO
CN
CN
OR
OR
RO
RO
ii
i
N
N
+
OH
N
N
OR
1
RO
OR
R = 2,6-dimethylphenol
2: 3:
M = 2H, M = Zn
4: M = Co, 5: M = Ni
RO
OR
Scheme 1. Microwave-assisted synthesis of hexadeca-substituted phthalocynaine derivatives. Reagents and conditions: (i) anhydrous K2CO3, DMF, 120 °C, 72 h; (ii)
microwave irradiation, 350 W, hydroquinone, appropriate metal salt, hexanol/DBU, 160 °C, 10 min.
same as in the first trial. Furthermore, metal ion template reactions
in hexanol using zinc acetate, nickel acetate, or cobalt(II) acetate
gave the same unpurifiable low yield products. Consequently,
microwave-assisted synthesis was applied as an alternative ap-
proach in preparing the target phthalocyanines in good yield and
high purity. The synthesis of metal-free phthalocyanine 2 was best
achieved by the cyclotetramerisation of a phthalonitrile precursor
1 in the presence of hydroquinone in hexanol using a microwave-
assisted method at 350 W for 10 min.15 Following the same proce-
dure, metal-containing Pcs (M = Zn, Ni, and Co) were prepared suc-
cessfully in good yields using the appropriate metal acetate.16–18
All the prepared phthalocyanines were purified easily by re-crys-
tallization from ethanol/DCM (3:1) showing that the reaction pro-
ceeded efficiently with fewer by-products. As a result of placing
bulky substituents around the Pc ring, all the prepared complexes
are highly soluble in common organic solvents (e.g., THF, CHCl3,
DCM, and toluene) thereby facilitating spectroscopic characteriza-
tion and also renders these compounds suitable for making high
quality spin-coated films. Spectroscopic analyses of 2–5 were con-
sistent with their expected structure. For example, fast atom bom-
bardment (FAB) mass spectroscopy exhibited the parent ion for Pc-
2 with a cluster of peaks centered at 2433.8 corresponding to that
calculated from the molecular formula (Fig. 1). The purity of each
material was confirmed by elemental analysis and thin layer
chromatography.
Figure 2. Aliphatic region (methyl groups) of the 1H NMR spectra of (a) 3,4,5,6-
tetrakis(2,6-dimethylphenoxy)phthalonitrile 1; (b) Pc-3, in CDCl3.
It is well known that disc-like macromolecules such as phthal-
ocyanines tend to form aggregates, which can have different char-
acteristics (e.g., optical properties) from the corresponding
monomer. UV–Vis and NMR spectroscopic techniques were, there-
fore, used to investigate the aggregation behavior of the prepared
complexes. The 1H NMR spectrum of 1 displays two singlets for
the methyl groups as there are two types of phenoxy groups (ortho
and meta to each nitrile). In comparison to 1, the Pc complexes
surprisingly show four singlets related to the methyl hydrogens
(Fig. 2).
This difference can be attributed to greater steric hindrance and
the ring current effect induced by Pc ring formation. This indicates
that the phenoxy substituents are forced out of the plane of the
macrocycle, where they adopt a configuration giving four different
methyl hydrogen environments.19 Due to the deshielding effect
caused by the strong ring current of the Pc, the two upfield peaks
(1.24 and 1.30 ppm) belong to the methyl aryloxy groups attached
to the nonperipheral positions. In contrast, the downfield peaks
(1.92 and 2.00 ppm) belong to the peripheral-positioned methyls.
Thus, the resulting conformational arrangement adopted by the
substituents ensures that cofacial self-association of the Pc cores
is prohibited. This is clearly deduced from the 1H NMR spectra,
which show no broadening or resonance shifts over a broad con-
centration range as evidence of no aggregation.
The aggregation behavior was further assessed by UV–Vis spec-
troscopy in both solution and solid state as spin-coated films. Usu-
ally, aggregation of Pcs is indicated by a hypsochromic shift of the
Q-band at 700 nm and its gradual broadening in a hypochromic
fashion. As anticipated, the UV–Vis absorption spectra of metal-
free Pc 2 along with the metal ion-containing derivatives 3–5 show
no evidence of aggregation as shown by the position and the
appearance of the intense Q-band. For example, the absorption
spectrum of 3 showed a single sharp Q-band at kmax 747 nm, which
is typical of nonaggregated species (Fig. 3). In comparison to the
Figure 1. FAB mass spectrum of Pc-2 with a cluster of peaks centered at 2433.8.

S. Makhseed et al. / Tetrahedron Letters 50 (2009) 165–168
167
continuous wave (cw) pumping at 632.8 nm confirm that 3 shows
optical limiting properties, which are attributed mainly to nonlin-
ear refraction. The large nonlinearity of this molecule probably re-
sults due to the strong linear absorption of the Q-band combined
with a high thermo-optic coefficient.21 The estimated values of
n2, b, and v(3) are among the highest reported values in the litera-
ture for nonlinear material with cw excitation.22 Furthermore, the
low threshold optical limiting (OL) at the two selected wavelengths
(Table 1) clearly suggests that this robust complex 3 is character-
ized by large nonlinearity, and could be a potential candidate for
optical limiting applications in the low cw region.
To conclude, it is possible to prepare Pc derivatives derived from
severely crowded phthalonitrile with four bulky phenoxyl susbtit-
uents by microwave-mediated reaction. NMR and UV–Vis studies
clearly show that the position adopted by the substituents relative
to the Pc ring is the most important factor influencing the molec-
ular packing of Pcs in the solid state and leading to materials with
intrinsically true-solid solution properties. The nonaggregation
behavior, high solubility and red-shifted Q-band make such new
chromophore materials attractive candidates in applications such
as PDT as photosensitizers, optoelectronic and near-IR devices. Fur-
thermore, the large optical limiting parameter of ZnPc-3 may lead
to a nonlinear media for several optical applications, and further
investigation is in progress to explore this phenomenon.
1.1
0.9
0.7
0.5
0.3
0.1
-0.1
300
400
500
600
700
800
Wavelength (nm)
Figure 3. Absorption spectra of Pc-3 at different concentrations in THF.
reported peripheral octaaryloxy-substituted phthalocyanine,12
placing additional aryloxy substituents at the nonperipheral sites
clearly produces a bathochromic shift of the Q-band absorption
maxima of about 65 nm as in the case of 3. In this study, the aggre-
gation behavior of the complexes 2–5 was examined at different
concentrations in THF. As depicted in Figure 3, the appearance of
the Q-band absorption maxima remained unchanged as the con-
centration increases, and its apparent molar extinction coefficient
remained almost constant indicating a purely monomeric form,
which obeyed the Beer–Lambert Law in the studied concentration
range. Thin films of Pcs 2–5 were prepared to further investigate
the molecular packing of the Pc compounds in the solid state.
Spin-coated films were deposited onto an untreated glass slide
from chloroform solution. The UV–Vis spectra of spin-coated films
of Pcs 2–5 were identical to those obtained from dilute solution
indicating that the Pc chromophore is not perturbed by intermo-
lecular exciton effects even in the solid state. Therefore, consistent
with previous reports,11 severe steric crowding at the Pc core is the
most effective factor in reducing cofacial interactions between the
Pc cores as was revealed by both UV–Vis and NMR spectroscopy.
Zinc phthalocyanine complexes have attracted much interest
due to their high triplet quantum yield and long lifetimes, and
hence display attractive properties in the field of nonlinear optical
applications.20 Hence, the nonlinear optical properties of ZnPc-3
were studied using the Z-Scan technique with a continuous low
energy laser regime. The preliminary study showed that the
mechanism or the nature of the nonlinear absorption in 3 is depen-
dent on the applied wavelength of the excitation beam. It was
found that the complex 3 exhibits reverse saturable excited-state
absorption (RSA) at 532 nm. Conversely, the nonlinear behavior
changes to saturable absorption (SA) at 632.8 nm, which is close
to the Q-band absorption at 747 nm and shows nonlinearity based
on the refraction mechanism. The experimental results are shown
in Table 1. As evaluated by open aperture Z-scan at 532 nm, the Zn-
centered complex 3 exhibited a reduction in the transmission
about the focus lens. This is typical of an induced positive nonlin-
ear absorption of the incident laser beam, which is attributed to
RSA. However, the negative nonlinear refraction sign from a closed
aperture and SA type behavior from an open aperture by using
Acknowledgment
The authors acknowledge the support of this work by Kuwait
University (Grant Nos. SC04/07, GS01/01 and GS01/03).
References and notes
1. McKeown, N. B. Phthalocyanine Materials: Synthesis, Structure and Function;
CUP: Cambridge, 1998.
2. de la Torre, G.; Vazquez, P.; Agulló-López, F.; Torres, T. J. Mater. Chem. 1998, 8,
1671.
3. Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem., Int. Ed. Engl. 1989,
28, 1445.
4. Rosenthal, I. Photochem. Photobiol. 1991, 53, 859.
5. Guillaud, G.; Simon, J.; Germain, J. P. Coord. Chem. Rev. 1998, 180, 1433.
6. Meunier, B.; Sorokin, A. Acc. Chem. Res. 1997, 30, 470.
7. Nazeeruddin, M. K.; Humphry-Baker, R.; Gratzel, M.; Murrer, B. A. Chem.
Commun. 1998, 719.
8. Vacus, J.; Simon, J. Adv. Mater. 1995, 7, 797.
9. Maya, E. M.; Snow, A. W.; Shirk, J. S.; Pong, R. G. S.; Flom, S. R.; Roberts, G. L. J.
Mater. Chem. 2003, 13, 1603.
10. Henderson, B. W.; Dougherty, T. J.; Rosenthal, I. Photochem. Photobiol. 1992, 55,
145.
11. (a) Brewis, M.; Clarkson, G. J.; Helliwell, M.; Holder, A. M.; McKeown, N. B.
Chem. Eur. J. 2000, 6, 4630; (b) Brewis, M.; Helliwell, M.; McKeown, N. B.;
Reynolds, S.; Shawcross, A. Tetrahedron Lett. 2001, 42, 813; (c) Makhseed, S.;
Ibrahim, F.; Bezzu, C. G.; McKeown, N. B. Tetrahedron Lett. 2007, 48, 7358–7361.
12. McKeown, N. B.; Makhseed, S.; Msayib, K. J.; Ooi, L.-L.; Helliwell, M.; Warren, J.
E. Angew. Chem., Int. Ed. 2005, 44, 7546.
13. Bhardwaj, N.; Andraos, J.; Leznoff, C. C. Can. J. Chem. 2002, 80, 141.
14. 3,4,5,6-Tetrakis(2,6-dimethylphenoxy)phthalonitrile (1): To a stirred solution of
3,4,5,6-tetrafluorophthalonitrile (1.0 g, 5 mmol) and 2,6-dimethylphenol
(3.05 g, 25 mmol) in dry DMF (50 ml), anhydrous potassium carbonate
(5.17 g, 37.4 mmol) was added. The mixture was heated for 72 h at 120 °C
under a nitrogen atmosphere. After cooling, the mixture was poured into
acidified water (200 ml). The resulting precipitate was collected and
recrystallized from methanol to yield the product as yellow crystals (50%);
mp 215 °C. 1H NMR (400 MHz, CDCl3): d 2.01 (s, 12H, CH3); 2.02 (s, 12H, CH3);
7.01 (d, 8H, J = 7.2 Hz); 7.07 (t, 4H, J = 6 Hz). MS (EI): m/z 608 [M+]. Anal. Calcd
for C40H36N2O4: C, 78.92; H, 5.96; N, 4.60. Found: C, 78.50; H, 5.83; N, 4.54.
15. 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-Hexadecakis(2,6-dimethylphenoxy)phth-
alocyanine (2): 3,4,5,6-tetrakis(2,6-dimethylphenoxy)- phthalonitrile
1
(200 mg; 3.285 Â 10À4 mol), hydroquinone (4 mg), and dry 1-hexanol (1 ml)
Table 1
were placed in
a 10-ml microwave vessel and sealed. This mixture was
Nonlinear parameters of ZnPc-3 in toluene
irradiated at 350 W in a microwave (Single Mode Cavity Explorer Microwave
Synthesizer-CEM Corporation, NC, USA) for 10 min at 160 °C with stirring. After
cooling, the reaction mixture was precipitated from methanol. The crude
product was collected, recrystallized from ethanol/dichloromethane (3:1), and
dried under vacuum to yield 2 as a light green solid (9%); mp >300 °C. 1H NMR
(400 MHz, CDCl3): d 1.25 (s, 48H, CH3), 1.97 (s, 48H, CH3), 6.57 (d, 32H

168
S. Makhseed et al. / Tetrahedron Letters 50 (2009) 165–168
J = 4 Hz,), 6.64 (t, 16H J = 7.0). UV–Vis (THF): kmax
(
e
): 757
17. 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-Hexadecakis(2,6-dimethylphenoxy)phth-
alocyaninatocobalt(II) (4): Yield 18%; mp >300 °C. 1H NMR (400 MHz, CDCl3): d
0.95 (s, 48H, CH3), 1.34 (s, 48H, CH3), 7.55 (s, 32H), 7.73 (s, 16H). UV–Vis (THF):
(2.28 Â 105 MÀ1 cmÀ1
)
and 776.00 (2.59 Â 105 MÀ1 cmÀ1). MS (FAB) m/z:
2435. Anal. Calcd for C160H146N8O16: C, 78.88; H, 5.99; N, 4.60. Found: C,
78.50; H, 5.82; N, 4.76.
16. 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-Hexadecakis(2,6-dimethylphenoxy)phth-
alocyaninatozinc(II) (3): 3,4,5,6-tetrakis(2,6-dimethylphenoxy)phthalonitrile 1
(200 mg; 3.285 Â 10À4 mol) and anhydrous zinc(II) acetate (60 mg) were
placed in a 10-ml microwave vessel. After the addition of dry 1-hexanol
kmax
(e
): 738 (1.32 Â 105 MÀ1 cmÀ1) MS (FAB): m/z 2492 [M+]. Anal. Calcd for
C160H144N8O16Co: C, 77.07; H, 5.78; N, 4.49. Found: C, 77.22; H, 6.04; N, 4.59.
18. 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-Hexadecakis(2,6-dimethylphenoxy)phth-
alocyaninatonickel(II) (5): Yield 19 %; mp >300 °C. 1H NMR (400 MHz, CDCl3): d
1.23 (s, 48H, CH3), 1.94 (s, 48H, CH3), 6.54 (d, 32H, J = 7.0 Hz); 6.62 (t, 16H,
(1.0 mL) and DBU (10 drops), the vessel was sealed and irradiated in
a
J = 7.8 Hz). UV–Vis (THF): kmax
(e
): 749 (1.40 Â 105 MÀ1 cmÀ1) MS (FAB):m/z
microwave oven (350 W) for 10 min at 160 °C with stirring. After cooling, the
reaction mixture was precipitated from methanol. The crude product was
recrystallized from ethanol/dichloromethane (3:1) and dried under vacuum to
yield the product as a light green solid (24%); mp >300 °C. 1H NMR (400 MHz,
CDCl3): d 1.24 (s, 24H, CH3); 1.30 (s, 24H, CH3); 1.92 (s, 24H, CH3); 2.00 (s, 24H,
2490 [M+]. Anal. Calcd for C160H144N8O16Ni: C, 77.08; H, 5.78; N, 4.49. Found: C,
76.80; H, 5.76; N, 4.45.
19. (a) Makhseed, S.; Ibrahim, F.; Samuel, J.; Helliwell, M.; Warren, J. E.; Bezzu, C.
G.; McKeown, N. B. Chem. Eur. J. 2008, 14, 4810; (b) Makhseed, S.; Samuel, J.;
Ibrahim, F. Tetrahedron 2008, 64, 8871.
CH3); 6.54 (d, 32H, J = 7.2 Hz); 6.62 (t, 16H, J = 7.2 Hz). UV–Vis (THF): kmax
(e):
20. Swarts, J. C.; Langner, E. H. G.; Krokeide-Hove, N.; Cook, M. J. J. Mater. Chem.
2001, 11, 434.
21. Calvete, M.; Yang, G. Y.; Hanak, M. Synth. Met. 2004, 141, 231.
22. de la Torre, G.; Vazquez, P.; Agulló-López, F.; Torres, T. Chem. Rev. 2004, 104,
3723.
747 (3.06 Â 105 MÀ1 cmÀ1). MS (FAB) m/z: 2497 [M+]. Anal. Calcd for
C160H144N8O16Zn: C, 76.88; H, 5.76; N, 4.48. Found: C, 77.22; H, 6.05; N, 4.59.
The following Pcs (4 and 5) were prepared from 1 using a similar procedure to
that adopted for Pc 3.