Routes to synthesis of porphyrins covalently bound to poly(carbazole)s and poly(fluorene)s: Structural and computational studies on oligomers

Article abstract of DOI:10.1016/j.molstruc.2012.06.056

We report the synthesis, spectroscopic and computational characterization of carbazole and fluorene oligomers containing pendant porphyrins prepared through Suzuki type coupling reactions. Molecular modeling, combining ab initio and semiempirical PM6 meth

Full text of DOI:10.1016/j.molstruc.2012.06.056

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Current Organic Chemistry, 2013, 17, 1103-1107
1103
Synthesis of a Rigid Fused Porphyrin-Phthalocyanine Hetero-Dyad with Two
Different Metals
Mário J.F. Calvete*, João P.C. Tomé and José A.S. Cavaleiro
Department of Chemistry/QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal
Abstract: The synthesis and characterization of the first rigid fused porphyrin-phthalocyanine hetero-dyad with two dif-
ferent metals is reported; was based on porphyrin’s dienophile features in Diels–Alder reactions. This synthetic procedure
represents a new approach for the synthesis of extended systems based on tetrapyrrolic macrocycles.
Keywords: Bimetallic systems, Diels-Alder reactions, Extended heterocyclic systems, Porphyrins, Phthalocyanines.
INTRODUCTION
studied the reaction of several meso-tetraarylporphyrins with a
pyrazine o-quinodimethane derivative, whose reactions gave rise
not to the expected chlorins or bacteriochlorins but instead to novel
ꢀ-extended porphyrins [46]. Following our previous work, here we
report the preparation of a fused porphyrin-phthalocyanine dyad, as
a new approach to highly functional dyes.
Phthalocyanines and porphyrins are two classes of stable, ro-
bust and functional tetrapyrrolic derivatives that are well-known
macrocyclic compounds with highly delocalized large ꢀ-electron
systems. Phthalocyanines have always received considerable atten-
tion as active materials for organic electronics [1], sensors [2, 3],
photovoltaic cells [4], electrochromic devices [5] and nonlinear
optics [6-9], among other applications [10-13]. Also the uniqueness
of porphyrins endows them distinctive properties useful in diverse
fields such as materials for supramolecular chemistry [14-16],
biomimetic models for photosynthesis [17], catalysis [18], photo-
voltaic cells [19,20], non linear optics [21], including optical limit-
ing [6].
MATERIALS AND METHODS
General
Chemicals were bought from Sigma-Aldrich and normally used
as received, unless otherwise stated. Solvents were dried and/or
1
distilled prior to use. Solution H and 13C NMR spectra were re-
corded with a Bruker AMX 300 spectrometer at 300 and 75 MHz,
respectively. Deuterated chloroform (CDCl3) was used as solvent
(except when indicated) and tetramethylsilane as internal reference;
the chemical shifts (ꢁ) are expressed in ppm. MALDI-TOF mass
spectra were determined with a Bruker Reflex III spectrometer
using dithranol as matrix. The UV/Vis spectra were recorded with a
Uvikon spectrophotometer. Elemental analyses were performed
with a Hekatech CHNS analysator Euro EA 300. Column chroma-
tography was carried out with silica gel (Merck, 230–400 mesh).
Analytical TLC was carried out on precoated sheets with silica gel
(0.2 mm thick, Merck).
From a synthetic point of view, the assembly of stable, smaller
units in the final step is advantageous for the construction of a large
system. This is due to the intrinsic instability towards oxygen and
the low solubility in common solvents that results from the highly
planar nature of this type of compounds. The porphyrin ꢀ system is
sufficiently large and stable to be used as such a basic unit for the
preparation of a larger ꢀ system. Therefore, examples of successful
preparative methods for ꢀ-system fused porphyrin dimers have been
reported by several groups [22-26], as well as planar or nearly pla-
nar binuclear Pcs [27-31], together with hetero-arrays of porphyrin-
phthalocyanine systems [32-34]. Few examples of porphyrin–
phthalocyanine (Por-Pc) dyads have been described, usually linked
by specific different types of spacers connecting the two macrocyc-
lic subunits [35-38], to which our group also contributed [33-39].
As an example, recently our group has synthesized a Por-Pc dyad in
which the two chromophoric units were directly connected, there-
fore dispensing any linker unit [40].
Synthesis
1,2-dibromo-4,5-dimethylbenzene
[47],
1,2-dicyano-4,5-
dimethylbenzene [48] and 4,5-bis-(bromomethyl)phthalonitrile 2
[49] were prepared according to slight modification of the corre-
sponding literature proceedings.
Preparation of Porphyrinic Substituted Phthalonitrile 3
Over the past years, we have reported that porphyrin’s periph-
eral double bonds can take part in Diels–Alder reactions [41, 42],
1,3-dipolar cycloadditions [43, 44] and other electrocyclic reactions
[45] yielding novel chlorins, bacteriochlorins and isobacteriochlo-
rins. Some of these compounds show excellent spectroscopic prop-
erties to be used in medicinal applications related with photody-
namic therapy of cancer cells, among other possible applications.
Based on our previous results on Diels–Alder reaction of meso-
tetraarylporphyrins with o-benzoquinodimethane [41], we also have
Meso–tetraphenylporphyrin 1 (100 mg, 0.163 mmol) was
placed in
a 50 mL round flask and dissolved in 1,2,4-
trichlorobenzene (6 mL). Then, 100 mg (0.32 mmol, 2 equiv.) of
3,4-bis(bromomethyl)phthalonitrile 2 was added, followed by so-
dium iodide (290 mg, 3.26 mmol, 20 equiv). The reaction mixture
was then refluxed (215 ºC) for 10 hours. After cooling to room
temperature the reaction mixture was diluted with 150 mL of chlo-
roform and a saturated aqueous solution of sodium thiosulfate was
added (50 mL). The heterogeneous mixture was then stirred for half
an hour to destroy the remaining iodine. After drying with sodium
sulfate, the crude solid was then subjected to column chromatogra-
*Address correspondence to this author at the Department of Chemistry, University of
Coimbra, Rua Larga, 3004-535 Coimbra Portugal; Tel: +00351966174744;
Fax: +351239827703; E-mail: mcalvete@qui.uc.pt
1875-5348/13 $58.00+.00
© 2013 Bentham Science Publishers

1104 Current Organic Chemistry, 2013, Vol. 17, No. 10
Calvete et al.
phy on silica-gel using a gradient of solvents as eluent, starting
from a mixture 1:1 light petroleum/dichloromethane to collect the
first fraction, identified as porphyrinic starting material. The second
fraction (eluted with mixture 1:5 light petroleum/dichloromethane)
was then collected and the solvent evaporated. The solid was then
dissolved in a 3:1 mixture of chloroform/methanol (200 mL) and
100 mg of zinc (II) acetate dissolved in methanol was added. After
refluxing the mixture for half hour the solvent was then evaporated.
The remaining solid was subjected to a column chromatography.
By elution with a mixture 1:5 light petroleum/dichloromethane, 3
was separated from its non metallated corresponding porphyrin, as
a minor fraction. The solvent was then evaporated and the solid
recrystallized from chloroform/methanol. Yield: 60 mg (44%), dark
reactions, which probably occurred prior to aromatization of the
system [46]. Pyrazines are ꢆ deficient heterocycles and so these
diazines are more easily attacked by nucleophiles than benzene
derivatives, and also are prone to intramolecular electrocyclizations,
if there are electron withdrawing groups in the system (such as
nitrile). Introduction of a benzene derivative instead of a pyrazine
derivative proved therefore to limit the tendency for such side reac-
tivity. Thus, we prepared the 1,2-dicyano-3,4(bisbromomethyl)
benzene compound 2 [49], aiming to prepare a suitable phthalo-
cyanine-like precursor, which was obtained as the major product of
the Diels-Alder reaction between compound
2 and meso-
tetraphenylporphyrin 1 [50] to obtain porphyrin fused phthalonitrile
3, in 44% yield (Scheme 1).
1
red powder. H NMR (300 MHz, CDCl3): ꢀ, ppm 7.53-7.80 (m,
The next step was to promote the cyclotetramerization of this
porphyrin based phthalonitrile, which was accomplished by tem-
plate reaction with large excess of commercially available 4-
tertbutylphthalonitrile, using a high boiling alcohol as solvent with
magnesium as template aid, to obtain compound 4 in 11% yield, a
linear fused porphyrin-phthalocyanine dyad, through the ꢀ-positions
of the porphyrinic subsystem, having a benzo unit as spacer be-
tween the two macrocycles (Scheme 1). Compound 4 appears as a
mixture of isomers, as consequence of the tertbutylphthalonitrile’s
nature. This substituents were used in attempt to avoid aggregation,
which is usually an issue regarding extended porphyrin or phthalo-
cyanine materials, which can form ꢆ-ꢆ stacked aggregates, due to
their large aromatic systems. The UV-Vis normalized spectra of
dilute solutions from 3 and 4 are shown in (Fig. 1), recorded in
tetrahydrofurane. Compound 3 presents an absorption spectrum in
which the disruption of symmetry is very clear, as a consequence of
an increase in the conjugation for the molecule. The B band split is
demonstrative of that effect, with peaks at 420 and 448 nm. In the
case of compound 4, aggregation emerges as a problem, even using
THF as solvent, as it can be seen by the UV-Vis spectra in (Fig. 1).
Therefore a broadened spectrum is the result, with consequent di-
minishing of extinction absorption coefficient (ꢅ = 6.2x104 for ꢄ =
686.0 nm), when compared with reference magnesium
(tBu)4phthalocyanine (ꢅ = 2.0x105 for ꢄ = 678.0 nm). The Soret
band from compound 4 is also blue-shifted, when comparing to the
reference. A small red shift (8 nm), is observed when comparing
both spectra, regarding the Q-band positions, which can be ac-
counted to ꢆ-electron delocalization, which can be corroborated by
the capability to draw mesomeric forms including both rings.
12H, 5,10,15,20-Ar-m- and p-H), 8.22-8.25 (m, 8H, 5,10,15,20-Ar-
o-H), 8.56 (s, 2H, external naphthalene unit-H),8.87-8.93 (m, 6H,
ꢁ-H); 9.02 (s, 2H, internal naphthalene unit-H). 13C NMR (75 MHz,
CDCl3): ꢀ, ppm 109.6, 111.2, 113.1, 115.6, 118.2, 118.5, 120.1,
120.4, 120.5, 126.7, 127.8, 129.9, 131.1, 133.5, 133.9, 134.4, 134.6,
136.4, 139.9, 142.0, 154.5, 156.8. MS (MALDI-TOF) (m/z):
827.180 [M]+. IR (KBr): ꢂ˜ = 2959, 2926, 2230 (CꢃN), 1605, 1481,
1462, 1391, 1358, 1275, 1263, 1234, 1105, 1061, 802, 696 cm–1.
UV/Vis ꢄ (nm), (ꢅ) (mol-1 dm3 cm-1): 420.0 (9.16x104), 448.0
(1.38x105), 528.0 (1.2x104), 604.0 (4x103).
Preparation of Porphyrin-Phthalocyanine Dyad 4
Magnesium filings (15 mg) were suspended in pentan-1-ol (ca.
1 mL) in a 25 mL flask equipped with an water condenser. This
suspension was heated to 185 ºC (reflux) and maintained at this
temperature until slurry was formed (ca. 1 h). Octan-1-ol (0.5 mL)
was added to this slurry, followed by a mixture of compound 3 (83
mg, 1 mmol) and 4-tert-butylphthalonitrile (1.1 g, 6 mmol). The
reaction was heated till 185 ºC and kept at this temperature. After 8
hours, following TLC checking, not all starting porphyrinic mate-
rial was consumed, and more 4-tert-butylphthalonitrile (1.1 g, 6
mmol) was added. The reaction mixture was left at 185 ºC for an-
other 8-9 hours and then cooled to room temperature and poured
into 100 mL of a 1:10 water/methanol mixture. The obtained solid
was collected and subjected to column chromatography on silica
gel. The first collected fraction (eluted with chloroform) was identi-
fied as magnesium tetra-tert-butylphthalocyanine (AAAA-
compound). Changing solvent to chloroform with 2 % tetrahydro-
furane the title compound was then eluted, collected and dried in a
1
For compound 3, its 1H-NMR spectrum presents the resonances
belonging to the naphthalene ring protons around ꢀ 8.56 ppm, shift
corresponding to the hydrogens closer to the porphyrin ring and
downfield at ꢀ 9.02 ppm shift, belonging to the ones closer to the
cyano groups, due to their electron withdrawing character. The ꢀ-
pyrrole protons appear broadened around 8.9 ppm, since there is a
loss of symmetry in the molecule, when compared to the starting
5,10,15,20-tetraphenylporphyrin. The resonances belonging to the
phenyl protons are observed at ꢀ 7.53-7.80 and 8.22-8.25 ppm as
broad peaks, integrating for 12 and 8 protons respectively. In case
vacuum oven. Yield: 40 mg (11%), green powder; H NMR (300
MHz, CDCl3): ꢀ, ppm 1.83 (m, 27 H, C(CH3)3), 7.43-7.60 (m, 12H,
5,10,15,20-Ar-m- and p-H), 7.70-7.79 (2 brs, 4H, naphthalene unit-
H), 8.22-8.25 (m, 8H, 5,10,15,20-Ar-o-H), 8.30 (m, 3H, Pc-H),
8.71-8.83 (m, 6H, Por-ꢁ-H); 9.10-9.30(m, 6H, Pc-H). 13C NMR (75
MHz, CDCl3): ꢀ, ppm 31.1, 31.3, 35.3, 110.3, 114.3, 114.5, 120.4,
120.6, 121.2, 126.7, 127.8, 130.2, 134.6, 135.8, 136.5, 140.8, 142.0,
153.3, 157.8. Mass Spectrum (MALDI-TOF) (m/z): 1405.474 [M]+.
E. A. Anal. Calc. for C90H66MgN12Zn: C, 76.92; H, 4.73; N,
11.96%. Found: C, 76.44; H, 5.10; N, 11.64 %. UV/Vis ꢄ (nm), ꢅ
(mol-1 dm3 cm-1): 348.0 (5.8x104), 452.0 (6.4x104), 686.0 (6.4x104).
1
of compound 4, its H-NMR spectrum presents the resonances be-
longing to the naphthalene ring protons between ꢀ 7.70 and 7.79
ppm, while Pc-ring protons appear at ꢀ 8.30 (external) and 9.10-
9.30 ppm (internal). The ꢀ-pyrrole protons appear broadened
around ꢀ 8.83 ppm and the resonances belonging to the phenyl pro-
tons are observed at ꢀ 7.70-7.79 and 8.22-8.25 ppm as broad peaks,
integrating for 12 and 8 protons, respectively.
RESULTS AND DISCUSSION
Following the concept of new ꢆ-extended porphyrins [46], we
planned to use a benzene derivative instead of a pyrazine derivative.
Previously we have observed side reactivity when using a bis-
bromomethylpyrazine derivative, namely by oxidative coupling

Synthesis of a Rigid Fused Porphyrin-Phthalocyanine Hetero-Dyad
Current Organic Chemistry, 2013, Vol. 17, No. 10 1105
Br
CN
Br
CN
CN
iii)
i)
ii)
CN
Br
Br
1
1
+
N
Zn
N
N
N
iv)
v)
CN
N
N
NH
HN
CN
3
2
vi)
N
N
N
N
N
Zn
N
N
N
Mg
N
N
N
N
4
Scheme 1. Preparation of dyad 4. i) Br2, 0-5 ºC (1.5 h), then 20 ºC, 12 hours; CuCN, DMF, 145 ºC, 8 h; NBS, AIBN, 12 h; iv) NaI, 1,2,4-trichlorobenzene,
215 ºC, 10 h; v) Zn(OAc)2, CHCl3/MeOH, 60 ºC, 0.5 h; vi) tBu-phthalonitrile, Mg/pentan-1-ol, octan-1-ol, 185 ºC, 16 h.
CONCLUSION
porphyrin fused phthalonitrile in reasonable yield, as starting mate-
rials for the preparation of the fused dyad by cyclotetramerization
with tBu-phthalonitrile having metallic Mg as template. Regarding
the use of the chosen metals for coordinating in the macrocycles
cavities, they were chosen for the simple reason that these metals
(zinc as central metal for porphyrin moiety and magnesium as cen-
tral metal for the phthalocyanine counterpart) are considered to be
easily removable in both cases, by use of acidic conditions. Cur-
rently, research on this issue and its application in on several
chemical, physical or materials fields, depending upon the choice of
metals, is in progress.
In conclusion, by taking advantage from porphyrin’s peripheral
double bonds ability to take part in Diels–Alder reactions, we pro-
moted the cyclotetramerization of a porphyrin based phthalonitrile,
with large excess of 4-tertbutylphthalonitrile, in order to obtain the
corresponding porphyrin-phthalocyanine dyad. The porphyrinic
phthalonitrile was idealized from previous knowledge on the capa-
bility of ꢀ,ꢀ’bis-bromomethyl benzenes to generate suitable Diels-
Alder dienes, like o-quinodimethanes. The electron withdrawing
effect of the cyano groups presence allowed the preparation of the

1106 Current Organic Chemistry, 2013, Vol. 17, No. 10
Calvete et al.
Fig. (1). UV-Vis spectra depiction from compound 3 (dashed line), compound 4 (dotted line) and reference Mg(tBu)4phthalocyanine (full line), taken in tetra-
hydrofurane.
on the photophysics and optical limiting properties. J. Phys. Chem. A 2008,
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CONFLICT OF INTEREST
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Smith K.M.; Guilard R.; eds. The Porphyrin Handbook, Amsterdam: Elsevier
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The author(s) confirm that this article content has no conflict of
interest.
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ACKNOWLEDGEMENTS
Pandey, R.K.; Zheng, G. in: Porphyrins as photosensitizers in photodynamic
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Chou, J.H.; Kosal, M.E.; Nalwa, H.S.; Rakow, N.A.; Suslick, K.S. in: Appli-
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Thanks are due to Fundação para a Ciência e a Tecnologia
(FCT), (COMPETE – Programa Operacional Factores de Competi-
tividade), QREN/FEDER for funding through Projects
PPTDC/DG/QUI/82011/2006 and PTDC/QUI/74150/2006. M.J.F.
[12]
[13]
Calvete
also
thanks FCT
for
his post-doc
grant
(SFRH/BPD/26775/2006).
[14]
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Beletskaya, I.; Tyurin, V.S.; Tsivadze, A.Y.; Guilard, R.; Stern, C. Supramo-
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Preparation of 1,2-dibromo-4,5-dimethylbenzene
I2 (0.05 g) was added to o-xylene (26.5 g, 0.25 mol) in a 500 mL three-neck
round bottom flask equipped with condenser and connected to a sodium thio-
sulfate trap. This was followed by dropwise addition of Br2 (0.52 mol) over
1.5 h, ensuring the temperature between 0 and 5 ºC. The resultant solid cake
was left at room temperature overnight before being dissolved in diethylether
(200 mL), washed with 2 M solution of NaOH (2 X 100 mL), H2O (2 X 100
mL), dried over Na2S04, filtered, and concentrated in vacuum to afford a light
pink colored oil which crystallized upon standing. Recrystallization from
MeOH gave 53 g of a white crystalline solid (80% yield), which was charac-
terized as the product, in accordance to the literature
[31]
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[33]
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Fischer, A.; Henderson, G.N.; Iyer L.M.; Chag, C.J. ipso Nitration. XXVI:
Nitration of 1,2-dimethyl-4-nitrobenzene. Formation and reactions of ad-
ducts. Can. J. Chem. 1985, 63, 2390-2400.
[34]
[35]
Preparation of 1,2-dicyano-4,5-dimethylbenzene
0.1 mol (26.4 g) of 4,5-dibromoxylene was dissolved with 0.35 mol CuCN in
300 ml of dried DMF in a 250 mL three-neck round bottom flask equipped
with a condenser. The mixture stirred and kept under reflux (145°C), moni-
torized by TLC, for 8 hours. After cooling to room temperature, a solution of
600 ml ammonia was added to the reaction mixture, and aerated for approxi-
mately 12 hours. The precipitate was filtrated and washed with neutral water
until no ammonia could be found in the solution, and dried in an oven. The
solid was washed with acetone and chloroform, and the filtrate was evapo-
rated and subjected to silica gel column chromatography, with mixture 3:1 n-
hexane/dichloromethane as eluent. The title compound was collected and
evaporated to give 5.2 g (33% yield) of an off white solid, which was charac-
terized as the product, in accordance to the literature
[36]
[37]
[38]
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acterization of liquid crystalline tetra- and octa-substituted novel phthalo-
cyanines. Polyhedron 2007, 26, 1061-1069.
[39]
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[41]
Preparation of 4,5-bis-(bromomethyl) phthalonitrile 2
4,5-dimethyl phthalonitrile (6.4 g, 47 mmol), NBS (16.8 g, 95 mmol) and
AIBN (50 mg) were mixed in carbon tetrachloride (70 ml) and the mixture
was stirred vigorously, heated under reflux 16 h until all the NBS was con-
verted to the succinimide, which floated on the surface of the solution. The
solution was filtered while hot and evaporated to dryness. The yellow oily
residue was redissolved in methanol, cooled to 0 ºC. The white solid formed
was filtered and subjected to column chromatography using mixture 3:1 light
petroleum/dichloromethane. The first fraction was collected in form of white
solid, giving 6.0 g (40% yield), which was characterized as the product, in
accordance to the literature
Soares, A.R.M.; Martínez-Díaz, M.V.; Bruckner, A.; Pereira, A.M.V.M.;
Tomé, J.P.C.; Alonso, C.M.A.; Faustino, M.A.F.; Neves, M.G.P.M.S.; Tomé,
A.C.; Silva, A.M.S.; Cavaleiro, J.A.S.; Torres, T.; Guldi, D.M. Synthesis of
novel N-linked porphyrin-phthalocyanine dyads. Org. Lett. 2007, 9, 1557-
1560.
Tomé, A.C.; Lacerda, P.S.S.; Neves, M.G.P.M.S.; Cavaleiro, J.A.S. meso-
Arylporphyrins as dienophiles in Diels–Alder reactions: a novel approach to
the synthesis of chlorins, bacteriochlorins and naphthoporphyrins. Chem.
Commun. 1997, 1199-1200.
[50]
Gonsalves, A.M.A.R.; Varejão, J.M.T.B.; Pereira, M.M. Some new aspects
related to the synthesis of meso-substituted porphyrins. J. Heterocyclic Chem.
1991, 28, 635-640.
Received: November 12, 2012
Revised: January 25, 2013
Accepted: January 28, 2013