Symmetric, twinned, and double-decker phthalocyanines substituted by trialkylated pentaerythritol

Article abstract of DOI:10.3906/kim-1202-64

Trialkylated pentaerythritol was chosen as a bulky substituent for a set of 3 free-base phthalocyanines: a symmetric, a twinned, and a double-decker derivative. The bulkiness of this substituent lowered the aggregation of the phthalocyanines. The electronic absorptions were comparatively investigated: the twinned phthalocyanine exhibited a significant near-infrared shifted absorption. The neutral radical Eu(III) complex was oxidized by bromine. These 3 phthalocyanines are promising for use as molecular materials. TUeBITAK.

Full text of DOI:10.3906/kim-1202-64

Turk J Chem
36 (2012) , 493 – 502.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1202-64
Symmetric, twinned, and double-decker phthalocyanines
substituted by trialkylated pentaerythritol
∗
∗
¨
¨
˙
Muzaffer KOC¸ , Ay¸se Gu¨l GUREK, Fabienne DUMOULIN , Vefa AHSEN
Department of Chemistry, Gebze Institute of Technology, P.O. Box 141,
41400 Gebze, Kocaeli-TURKEY
e-mails: fdumoulin@gyte.edu.tr, ahsen@gyte.edu.tr
Received: 27.02.2012
Trialkylated pentaerythritol was chosen as a bulky substituent for a set of 3 free-base phthalocyanines:
a symmetric, a twinned, and a double-decker derivative. The bulkiness of this substituent lowered the
aggregation of the phthalocyanines. The electronic absorptions were comparatively investigated: the twinned
phthalocyanine exhibited a significant near-infrared shifted absorption. The neutral radical Eu(III) complex
was oxidized by bromine. These 3 phthalocyanines are promising for use as molecular materials.
Key Words: Phthalocyanine, twinned, alkylated pentaerythritol, europium, double-decker, electronic near-
infrared delocalization
Introduction
Phthalocyanines, also named tetrabenzotetraazaporphyrins, are tetrapyrrolic derivatives with a delocalized 18
π electronic structure, thus exhibiting original spectroscopic, photophysical, photochemical, and electronic
properties, including a near-infrared absorption centered generally at 680 nm but extendable up to 1000 nm
by extending the electronic delocalization.1 The unique structure of phthalocyanines offers several possibilities
for such modulations: the substitution pattern (nature, number, and position of the substituents) and the
metallation pattern (with possible axial substitution depending on the nature of the metal).
These properties are suitable in molecular materials for various applications emerging in recent years:2 ph-
thalocyanines are extensively used as sensors,3−5 photosensitizers for photodynamic therapy6 and photocatalysis,7
oxidation catalysts,8,9 or materials exhibiting non-linear properties.10,11 Many of the required properties can
be enhanced and tailored by an extension of electronic delocalization. Several twinned phthalocyanines have
∗Corresponding author
493

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Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
been reported to date, joined by one or more benzene ring.1,12−20 The effect of the electronic delocalization
on a near-infrared (NIR) shifted maximum absorption was demonstrated by a comparison with a twinned
phthalocyanine in which the joining unit does not allow the conjugation over the 2 phthalocyanine rings.21
Naphthalocyanines22 and related tetraimidazophthalocyanines23 exhibit similar NIR shifted absorption. The
formation of bisphthalocyanines, or double-deckers, is another way to extend the range of possible applications.
One of the main drawbacks of phthalocyanines is their aggregation tendencies. Substituting phthalocya-
nines with bulky substituents is a preeminent strategy developed to limit aggregation. tert-Butylphthalonitrile
is even a commercial compound. tert-Butyl substitution has some limits in the case of tert-butylsulfanyl substi-
tution, as it has a tendency to stick anyway to columns and limits purification possibilities, especially in the case
of asymmetrically substituted derivatives.24−26 The size of substituents is likely to have an effect even on the
metal oxidation state of N-bridged phthalocyanines.27 The position of substituents is another important factor:
non-peripheral substituents, as they prevent macrocycles coming to close to each other, inhibit aggregation as
well.28 This is particularly important for photosensitizers at avoiding the quenching of the electronic events
leading to the formation of singlet oxygen.28−30 The addition of dissociating agents such as Triton-X100 is
another option.
Trialkylated pentaerythritol substituent appeared to be particularly adequate to avoid or lower aggrega-
tion. The presence of 3 alkyl chains on a tertiary carbon is likely to enhance the solubility of the phthalocyanines
and to reduce even more their aggregation.
We therefore decided to substitute 3 phthalocyanine derivatives with trialkylated pentaerythritol and
designed the symmetrically substituted monomeric derivative (1), the twinned phthalocyanine (2) having 2
phthalocyanine macrocycles united by a single benzene ring, and the europium (III) double-decker (3). The
structures of these 3 targeted phthalocyanines are represented in Figure 1.
Results and discussion
Syntheses and characterizations
Phthalonitrile (7) is the key precursor of the synthesis of the 3 targeted phthalocyanines. Its synthesis is
represented in Scheme 1.
Tri-O-hexyl pentaerythritol (5) was prepared from pentaerythritol in phase transfer catalysis conditions
as previously described by us (4).31 It was then used as a nucleophile in the nitro substitution achieved on
4-nitrophthalonitrile (6), a widely used reaction. The substitution was performed in standard conditions: 60 ◦ C
in DMF, in the presence of potassium carbonate. Two equivalents of 1 were employed: the steric hindrance of
tri-O-hexyl pentaerythritol (5) slows the reaction compared to the case of a less hindered alcohol. The structure
of phthalonitrile (7) was fully confirmed by the analyses performed. The ESI spectrum shows the molecular
adduct of Na as well as the 2M+Na peak, both fitting the theoretical pattern (Figure 2). 1 H- and 13 C-NMR
and IR are in accordance with the proposed structure.
Phthalonitrile (7) was cyclotetramerized in lithium templating conditions. This was followed by the
lithium removal and led to phthalocyanine 1 in excellent yield (58%). Twinned phthalocyanine (2) was prepared
by a mixed condensation of bisdiiminoisoindoline (8)17 with a 15-fold of phthalonitrile (7), also in lithium
494

¨
Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
OC6H13
OC6H13
C6H13O
O
OC6H13
OC6H13
C6H13
O
OC6H13
OC6H13
N
N
C6H13
O
N
H
O
O
N
N
OC6H13
OC6H13
OC6H13
OC6H13
H
N
N
N
C6H13O
C6H13
O
O
O
OC6H13
OC6H13
C6H13
O
OC6H13
O
C6H13
O
C6H13
O
C6H13O
OC6H13
N
N
N
N
N
N
N
H
N
H
O
C6H13
O
O
1
N
N
N
H
N
H
N
N
N
N
O
O
OC6H13
OC6H13
OC6H13
OC6H13
OC6H13
C6H13
O
C6H13
O
C6H13
O
C6H13
O
OC6H13
O
C6H13
O
C6H13O
2
O
N
N
N
N
C6H13
O
N
N
N
N
O
OC6H13
OC6H13
O
C6H13
O
OC6H13
C6H13
O
Eu
OC6H13
OC6H13
OC6H13
C6H13
N
O
OC6H13
C6H13
O
O
O
N
N
N
C6H13
O
N
N
N
O
OC6H13
OC6H13
N
O
C6H13
O
OC6H13
C6H13
O
3
Figure 1. Structures of trialkylated pentaerythritol substituted monomeric phthalocyanine (1), of twinned phthalocya-
nine (2), and of europium (III) double-decker (3). Only one of the possible isomers is represented.
templating conditions (Scheme 2). The strong excess of phthalonitrile (7) aims to avoid the self-condensation
of the bisdiiminoisoindoline leading to undesired polymeric by-products. Only 2 major compounds are indeed
obtained: the symmetric derivative 1 and the desired phthalocyanine 2 (8%). This yield is quite good as the
495

¨
Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
n-C6H13Br
HO
HO
OH
OH
C6H13O
C6H13O
OH
NaOH 50
%
n-Bu4NBr
OC6H13
4
5
C6H13O
C6H13O
C6H13O
K2CO3
DMF
C6H13O
C6H13O
OH
OC6H13
O2N
CN
CN
O
CN
CN
+
5
7
6
Scheme 1. Synthesis of the precursor phthalonitrile 7.
Figure 2. Theoretical and experimental ESI spectra of phthalonitrile 7.
496

¨
Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
formation of a twinned phthalocyanine can be considered as the double formation of AB3-type derivative, a
reaction that we usually perform in 15%-20% yield.32−34 The MALDI spectrum of the clean mixture of these 2
phthalocyanines is represented in Figure 3. This mixture is then separated by gel-permeation chromatography
(Biobeads). Analyses are in accordance with the proposed structure.
Europium double-decker complex 3 (Figure 1) was prepared from 1, using Eu(OAc)3 as the metal salt,
in 1-chloronaphthalene and DBU. Due to the steric hindrance of the substituents likely to prevent the required
proximity of 2 macrocycles to form the double-decker complex, the obtained 32% yield is quite satisfactory.
Electronic absorption properties
The maximum absorption wavelengths and related extinction coefficient of 1 and 2 are summarized in the
Table. Related spectra are presented in Figure 4. The spectrum of 1 is typical of a free-base phthalocyanine:
the absence of a coordinating metal at the center of the phthalocyanine induces a loss of symmetry, which
results in a characteristic split Q band. A NIR shift of roughly 140 nm compared to 1 is induced by the
electronic delocalization over the 2 phthalocyanine macrocycles. Remaining aggregation enlarges the Q band
of the twinned phthalocyanine 2, but the splitting of the Q band is visible both on the monomeric and on the
aggregated form of the phthalocyanine.
The UV-Vis spectra of the double-decker europium complex 3 under its neutral radical form and of
the oxidized form are presented in Figure 5. The neutral complex (3) is oxidized using a solution of Br2 in
CHCl3. The characteristic band corresponding to the radical absorption centered at 1750 nm disappears with
the addition of bromine and the subsequent oxidation of the complex. This is consistent with our previous
observations.35,3
Table. UV-Vis data of compounds 1 and 2.
Compound Solvent λmax nm (log ε)
704 (5.25)
THF
666 (5.17)
344 (4.96)
707 (4.76)
670 (4.68)
344 (4.48)
845 (4.05)
780 (3.28)
690 (4.60)
344 (4.58)
845 (4.27)
780 (4.32)
690 (467)
344 (4.84)
1
2
CHCl3
THF
CHCl3
497

¨
Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
Scheme 2. Preparation of twinned phthalocyanine 2.
Figure 3. MALDI spectrum (matrix: DHB) of the crude mixture of phthalocyanines 1 and 2.
498

¨
Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
Experimental
Materials and methods
All reaction solvents were dried and purified as described by Perrin and Armarego.37 Optical spectra in the UV-
visible region were recorded with a Shimadzu 2001 UV spectrophotometer using a 1 cm path length cuvette at
room temperature. The mass spectra were recorded on a LCQ-ion trap (Thermo Finnigan, San Jose, CA, USA),
equipped with an electrospray source and matrix assisted laser desorption ionization BRUKER Microflex LT
using 2,5-dihydroxybenzoic acid (DHB) as matrix. 1 H- and 13 C-NMR spectra were recorded in CDCl3 solutions
on a Varian 500 MHz spectrometer. IR spectrum was recorded between 4000 and 650 cm−1 using a Perkin
Elmer Spectrum 100 FT-IR spectrometer with an attenuated total reflection (ATR) accessory featuring a zinc
selenide (ZnSe) crystal.
1.4
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.2
1
0.8
0.6
0.4
0.2
0
300
400
500
600
700
800
900
350
550
750
950
1150 1350 1550 1750 1950
Figure 4. UV-Vis spectra of 1 (discontinued line) and 2
(continued line) in chloroform. Concentration 10 μM.
Figure 5. UV-Vis spectra of 3: neutral radical form
(continued line) and oxidized form (discontinued line) in
chloroform. Concentration 20 μM.
Syntheses
Phthalonitrile (7). Trialkykated pentaerythritol (5)31 (2.5 g, 6.44 mmol), 4-nitrophthalonitrile (6) (560
mg, 3.22 mmol) and potassium carbonate (9 g, 65 mmol) were stirred for 3 days in DMF (20 mL) at 60 ◦ C.
The cooled reaction mixture was diluted with dichloromethane (200 mL), washed with water (3 × 200 mL),
dried on sodium sulfate, and concentrated. The expected phthalonitrile was then purified by silica gel column
chromatography, yielding a transparent oil. 2.1 g, 60%. C31 H50 N2 O4 . MW 514.74. 1 H-NMR (500 MHz,
CDCl3 , δ, ppm): 7.31, 7.22, 7.20 (3H, aromatics), 4.08 (s, 2H, CAr OCH2 C), 3.46 (s, 6H, CCH2 OC6 H13),
3.37 (t, 6H, -OCH2 C5 H11), 1.52 (m, 6H, -CH2 C4 H9), 1.28 (m, 18H, - (CH2)3 CH3), 0.89 (s, 9H, -CH3).
13 C-NMR (126 MHz, CDCl3, δ, ppm): 162.88, 135.30, 120.00, 119.65, 117.46, 115.99, 115.45, 107.20, 77.50,
77.25, 76.99, 71.96, 69.19, 69.01, 45.44, 31.99, 29.74, 25.95, 22.99, 14.39. FT-IR (cm−1) 3084, 2931, 2861, 2605,
2363, 2232, 1599, 1564, 1490, 1464, 1428, 1376, 1322, 1253, 1205, 1170, 1105, 1023, 924, 881, 835, 727, 522. ESI-
MS: isotopic cluster peaking at m/z537.48 [M+Na]+ , 1051.92 [2M+Na]+ . Anal. Calcd for C31 H50 N2 O4 : C,
72.33; H, 9.79; N, 5.44%. Found C, 72.38; H, 9.75; N, 5.40%.
499

¨
Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
Phthalocyanine (1). To phthalonitrile 7 (51 mg, 0.1 mmol) stirred in pentanol at 60 ◦ C were added
granules of lithium. The formation of lithium pentanolate was observable with an emission of hydrogen gas,
followed immediately by the dark blue-green coloration of the reaction mixture. The reaction mixture was then
slowly heated up to 130 ◦ C and stirred overnight at this temperature. After cooling down and addition of
ethanol and water, the waxy dark-blue phthalocyanine was filtered on paper and recovered in dichloromethane
(100 mL). Water (100 mL) was added and the 2 phases were strongly stirred at room temperature overnight
to remove the lithium; then dichloromethane was evaporated. The phthalocyanine was filtered and purified by
silica gel column chromatography eluted by dichloromethane, yielding 30 mg of 1 (58%). C124 H202 N8 O16 , MW
2060.97. MS-MALDI-TOF (DHB) m/z2061.51 [M+H]+
.
1 H-NMR (500 MHz, CDCl3 , δ, ppm): 9.32, 8.89,
7.72 (12H, aromatics), 4.54 (s, 8H, CAr OCH2 C), 3.71 (bs, 24H, CCH2 OC6 H13), 3.50 (t, 24H, -OCH2 C5 H11),
1.23-1.59 (m, 96, (CH2)4 CH3), 0.79 (s, 36H, CH3), -0.5 (bs, 2H, NH). FT-IR (cm−1) 2927, 2856, 1611, 1483,
1457, 1428, 1376, 1343, 1322, 1237, 1169, 1095, 1051, 1005, 922, 851, 823, 749, 721, 700, 648.
Twinned phthalocyanine (2). A mixture of bisdiiminoisoindoline 817 (45 mg, 0.25 mmol) and
phthalonitrile 7 (15 equiv., 1.9 g, 3.7 mmol) was dissolved in dichloromethane, concentrated 3 times to ensure a
homogeneous mixture, and then dried under vacuum. Pentanol (20 mL) and lithium were added. The emission
of hydrogen gas was immediately followed by the dark blue-green coloration of the reaction mixture. The
stirring continued overnight at 130 ◦ C and the reaction mixture was cooled down and diluted with ethanol and
water until complete precipitation of the phthalocyanines. After filtration, the phthalocyanines were recovered
in dichloromethane (200 mL). Water (200 mL) was added and the 2 phases were strongly stirred at room
temperature overnight; then dichloromethane was evaporated. The phthalocyanine mixture was filtered and
cleaned by silica gel column chromatography eluted by dichloromethane. The clean mixture of phthalocyanines
1 and 2 was then separated by repeated loadings on an exclusion-diffusion chromatography, leading to 1 (512
mg) and 2 (65 mg, 8%). C196 H306 N16 O24 , MW 3270.62. MS-MALDI-TOF (DHB) m/z[M+H]+ 3271,83.
FT-IR (cm−1) 2927, 2856, 1611, 1483, 1458, 1375, 1344, 1313, 1282, 1234, 1169, 1097, 1019, 922, 823, 745, 727,
646.
Double-decker Eu(III) bisphthalocyanine (3). A mixture of dry europium acetate powder (8 mg,
0.025 mmol), DBU (0.2 mL), and phthalocyanine 1 (106 mg, 0.051 mmol) in 1-chloronapthalene (1 mL) was
refluxed for 5 h under nitrogen to give dark-green solution; then the solvent was removed under reduced
pressure. The crude product was dissolved in dichloromethane, filtered and loaded on a silica gel column
eluted by a mixture of CH2 Cl2 /MeOH (100:1). Further purification on preparative silica gel TLC (eluent
CH2 Cl2/n-hexane 100/1) led to a waxy dark green solid. Yield: 70 mg (32%). C248 H400 EuN16 O32 , MW
4269.88. MS-MALDI-TOF (DHB) m/z4270.85 [M+H]+ . FT-IR (cm−1) 3045, 2954-2857, 1604, 1486, 1457,
1319, 1275, 1230, 1101, 1065, 925, 826, 756.
Conclusion
The interest in trialkylated pentaerythritol as a phthalocyanine substitution pattern lowering their aggregation
has been evidenced by the preparation of the 3 phthalocyanines derivatives described in this paper: a sym-
metrically substituted monomeric, a twinned, and a double-decker derivative. We are now preparing metalated
derivatives to investigate their relevant properties.
500

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Symmetric, twinned, and double-decker phthalocyanines substituted by..., M. KOC¸, et al.
Acknowledgements
¨
˙
The Scientific and Technological Research Council of Turkey (TUBITAK) (PIA Turkey-France bilateral project
˙
¨
Bosphorus 109M356) is gratefully acknowledged. The authors wish to thank Ilker Un and Nuray Demircan for
the NMR experiments as well as Bu¨nyemin C¸o¸sut for MALDI and LC-ESI-MS analyses.
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