Non-peripherally alkylamino-substituted phthalocyanines: Synthesis, spectral, photophysical and acid-base properties

Article abstract of DOI:10.1142/S108842461950024X

Non-peripherally substituted metal-free and zinc phthalocyanines (Pcs) bearing four diethylamino groups and four Br atoms were prepared. Optimal conditions for synthesis of corresponding precursor (i.e. 3-bromo-6-(diethylamino)phthalonitrile) either by nucleophilic substitution or by Buchwald-Hartwig coupling were studied. Noteworthy, 3,6-bis(diethylamino)phthalonitrile was also formed, nevertheless only at low yield (typically below 1%) and all attempts for its cyclotetramerization failed. Q bands of prepared Pcs were strongly red shifted up to the near-IR region (769 and 800 nm in THF for zinc and metal-free Pc, respectively). Unusually large hypsochromic shifts of the Q bands, 130 and 80 nm for metal-free and zinc Pc, respectively, were observed upon treating these Pcs with trifluoroacetic acid, which was attributed to the protonation of non-peripheral amines. Treatment with sulfuric acid led to subsequent protonation on the azomethine nitrogens as well. Photophysical study revealed low fluorescence emission of both derivatives (φF <0.03, in THF) and efficient singlet oxygen production only for zinc Pc (φΔ = 0.77 in THF and 0.60 in DMF).

Full text of DOI:10.1142/S108842461950024X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2019; 23: 1–10
DOI: 10.1142/S108842461950024X
Published at http://www.worldscinet.com/jpp/
Non-peripherally alkylamino-substituted phthalocyanines:
Synthesis, spectral, photophysical and acid-base properties
Lucia Kociscakova, Merve Ipek Senipek, Petr Zimcik*◊ and Veronika Novakova*◊
Faculty of Pharmacy in Hradec Kralove, Charles University, Ak. Heyrovskeho 1203, Hradec Kralove,
50005, Czech Republic
This paper is part of the 2019 Women in Porphyrin Science special issue.
Received 13 December 2018
Accepted 5 February 2019
ABSTRACT: Non-peripherally substituted metal-free and zinc phthalocyanines (Pcs) bearing four
diethylamino groups and four Br atoms were prepared. Optimal conditions for synthesis of corresponding
precursor (i.e. 3-bromo-6-(diethylamino)phthalonitrile) either by nucleophilic substitution or by
Buchwald–Hartwig coupling were studied. Noteworthy, 3,6-bis(diethylamino)phthalonitrile was also
formed, nevertheless only at low yield (typically below 1%) and all attempts for its cyclotetramerization
failed. Q bands of prepared Pcs were strongly red shifted up to the near-IR region (769 and 800 nm in
THF for zinc and metal-free Pc, respectively). Unusually large hypsochromic shifts of the Q bands,
130 and 80 nm for metal-free and zinc Pc, respectively, were observed upon treating these Pcs with
trifluoroacetic acid, which was attributed to the protonation of non-peripheral amines. Treatment with
sulfuric acid led to subsequent protonation on the azomethine nitrogens as well. Photophysical study
revealed low fluorescence emission of both derivatives (FF < 0.03, in THF) and efficient singlet oxygen
production only for zinc Pc (FD = 0.77 in THF and 0.60 in DMF).
KEYWORDS: phthalocyanine, non-peripheral, Buchwald–Hartwig coupling, fluorescence, singlet
oxygen, protonation.
the near-IR region is highly appreciated because such
INTRODUCTION
light penetrates deeper into tissues, is less scattered and,
Since 1907, when an unidentified blue product was
as a low-energy light, it causes only limited damage to
described for the first time [1], phthalocyanines (Pcs)
tissues.
have become an important point of interest of many
Hence, aim of this study was to synthesize and study
researchers. Macrocyclic cores of Pcs consist of 18
delocalized p electrons that afford the molecule useful
spectral, photophysical and biological properties and
enable their applications e.g. as dyes and pigments
[2], catalysts [3, 4], in organic solar cells [5] or as
new Pcs with red-shifted absorption bands (Q bands)
up to the near-IR region. Many structural factors which
have positive effect on the red shift of the Q band
have been described in the literature. Among them,
shifting substituents to non-peripheral positions [8, 9],
photosensitizers in photodynamic therapy [6, 7].
condensation of additional aromatic rings [10, 11],
Especially for the last application, strong absorption in
giving rise to higher analogues of Pcs, coordination of
phosphorus to the center of the macrocycle [12] and
◊ SPP full member in good standing
protonation of the macrocycle [13, 14] are the most
important. Because higher analogues of Pcs usually
suffer from increased aggregation and protonation is
pH dependent, we focused on the non-peripherally
substituted Pcs. Non-peripheral substituents attached
through heteroatoms have substantial effects on the
*Correspondenceto:VeronikaNovakova,tel.:+420495-067-394,
fax: +420 495-067-167, email: veronika.novakova@faf.cuni.cz,
web: http://portal.faf.cuni.cz/Groups/Azaphthalocyaninegroup/;
Petr Zimcik, tel.: +420 495-067-257, email: petr.zimcik@faf.
cuni.cz.
Copyright © 2019 World Scientific Publishing Company

2
L. KOCISCAKOVA ET AL.
position of the main absorption Q band. For example,
the presence of oxygen, sulfur, selenium or tellurium in
this position shifts the Q band to the near infrared area
[8, 15]. The strongest effect is observed for alkylamino
substituents, as recently discovered for a series of non-
peripherally substituted tetrapyridoporphyrazines [16].
Introduction of amino substituents to the non-peripheral
positions of Pcs is, however, a challenging task and to
the best of our knowledge, only a limited number of such
derivatives have been prepared so far [17–19].
reaction mixture was left to warm to room temperature.
Afterwards, 3,6-dihydroxyphthalonitrile (5.0 g, 0.0313
mol) was added and the reaction was heated for 30 min
at 70°C. Acetonitrile was evaporated under reduced
pressure and the residue was stirred at 230°C for 1 h and
then at 330°C in an open flask until the gaseous HBr was
removed. The product was dissolved in CH2Cl2, washed
with NaHCO3 (5% w/w) and dried over anhydrous
Na2SO4. Recrystallization from THF gave a solid (2.74 g)
and a mother liquor. The mother liquor was evaporated to
dryness and purified by column chromatography on silica
with toluene/CHCl3 3:1 as a mobile phase. It was then
combined with the solid from crystallization, and column
chromatography was repeated with the same mobile
phase. Yield 3.53 g of white needle-like crystals (40%).
Melting point 245.9–247.1°C; 1H NMR (500 MHz,
DMSO-d6) d (ppm) 8.1 (s, 2H, ArH); 13C NMR (126
MHz, DMSO-d6) d (ppm) 138.7, 125.3, 120.0, 114.7; IR
(ATR): νmax (cm-1) 3105, 3063, 2234 (CN), 1959, 1556,
1445, 1394, 1204.
EXPERIMENTAL
General
All of the organic solvents used in the synthesis
were of analytical grade. All of the other chemicals for
the syntheses were purchased from certified suppliers
(i.e. Sigma–Aldrich, TCI Europe, Acros, and Merck)
and used as received. TLC was performed on Merck
aluminum sheets coated with silica gel 60 F254. Merck
Kieselgel 60 (0.040−0.063 mm) was used for column
chromatography. The melting points were measured on
an Electrothermal IA9200-series digital melting-point
apparatus (Electrothermal Engineering, Southend-on-
Sea, Essex, Great Britain). The infrared spectra were
measured on a Nicolet 6700 spectrometer in ATR
General method for synthesis of compounds 4
and 5 by nucleophilic substitution
3,6-dibromophthalonitrile (1), 2,3-dicyano-1,4-pheny-
lene bis(trifluoromethanesulfonate) (2) or 2,3-dicyano-1,
4-phenylene bis(4-methylbenzenesulfonate) (3) (1 equiv)
and a base (3 equiv) were dissolved under argon
atmosphere in anhydrous DMSO in oven dried glass.
Diethylamine (10 equiv) was added and the reaction was
heated at 80°C for 24 h or 7 days. The amount of the
arisen product was quantified by the method described
below. The product of a scale-up reaction was purified
as follows: water was added and product was extracted
with ethyl acetate, then with chloroform (2×). Organic
layers were collected and dried over anhydrous Na2CO3.
3-Bromo-6-(diethylamino)phthalonitrile (4) and 3,6-
bis(diethylamino)phthalonitrile (5) were isolated using
column chromatography on silica with hexane/ethyl
acetate 3:1 as a mobile phase (Rf (4) = 0.42, Rf (5) = 0.29).
Each product was then purified separately by column
chromatography on silica with chloroform/toluene 1:1
and hexane/ethyl acetate 3:1 for compounds 4 and 5,
respectively.Yields are summarized in Table 1.Analytical
data for 3-bromo-6-(diethylamino)phthalonitrile (4):
Yellow solid; Melting point 114.0–115.5°C (MeOH); 1H
NMR (500 MHz, CDCl3): d (ppm) 7.56 (d, J = 9.4 Hz,
1H, ArH), 6.97 (d, J = 9.4 Hz, 1H, ArH), 3.51 (q, J =
7.1 Hz, 4H, NCH2), 1.24 (t, J = 7.1 Hz, 6H, CH3); 13C
NMR (126 MHz, CDCl3): d (ppm) 151.9, 136.7, 122.4,
120.3, 116.0, 115.1, 113.0, 102.0, 46.1, 12.8; IR (ATR):
νmax (cm-1) 3101, 2980, 2934, 2231 (CN), 2211 (CN),
1585, 1534, 1467, 1414, 1383, 1357, 1317, 1289, 1210;
HRMS (ESI) calcd. for C12H12BrN3+H+ 278.0287,
found 278.0294 [M + H]+, 100%. Analytical data for
3,6-bis(diethylamino)phthalonitrile (5): Yellow oil that
1
mode. The H and 13C NMR spectra were recorded on
a VNMR S500 NMR spectrometer. The chemical shifts
are reported relative to Si(CH3)4 and were referenced to
the signal of the solvent. High-resolution mass spectra
(HR MS) were measured using a UHPLC system
Acquity UPLC I-class (Waters, Millford, USA) coupled
to a high resolution Synapt G2Si mass spectrometer
(Waters, Manchester, UK) based on Q-TOF. The
chromatography for this HR MS measurement was
performed using an Acquity UPLC BEH300 C4 (2.1
× 50 mm, 1.7 mm) column with a gradient elution
consisting of acetonitrile and 0.1% formic acid with
-1
.
a flow rate of 0.4 mL min . Electrospray ionization
was operated in both positive and negative modes. ESI
spectra were recorded in the range 50–2000 m/z using
leucine-enkefaline as a lock mass reference and sodium
iodide for calibration. 2,3-dicyano-1,4-phenylene bis(tri-
fluoromethanesulfonate) (2) [20] and 2,3-dicyano-1,4-
phenylene bis(4-methylbenzenesulfonate) (3) [21] were
prepared according to the published procedures.
Synthesis of precursors
Preparation of 3,6-dibromophthalonitrile (1). Com-
pound 1 was synthesized based on a procedure reported
in the literature [22] with few modifications. Triphenyl-
phosphine (22.5 g, 0.858 mol) was mixed with acetonitrile
(100 mL), sonicated for 10 min at room temperature and
then cooled down to 0°C by ice bath. Bromine (14 g,
4.49 mL, 0.0876 mol) was added dropwise at 0°C and the
1
slowly solidified; Melting point 94.6–95.7°C; H NMR
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 2–10

NON-PERIPHERALLY ALKYLAMINO-SUBSTITUTED PHTHALOCYANINES
3
(500 MHz, CDCl3) d (ppm) 7.12 (s, 2H, ArH), 3.30 (q,
126 mg of dark solid (mixture of isomers, 16%). 1H NMR
(500 MHz, CDCl3/pyridine-d5 3:1) d (ppm) 8.13–8.32 (m,
4H, ArH), 7.49–7.61 (m, 4H, ArH), 3.98–4.40 (m, 16H,
NCH2), 0.63–1.53 (m, 24H, CH3), central hydrogens
were not detected (Fig. S1); 13C NMR (126 MHz,
CDCl3/pyridine-d5 3:1): d (ppm) 148.3, 148.0, 147.5,
130.9, 129.8, 122.7, 122.6, 122.3, 122.2, 108.6, 108.4,
48.4, 48.2, 12.4 (some of the signals were not detected)
(Fig. S2); IR (ATR): νmax (cm-1) 2968, 2930, 2867, 1574,
1563, 1558, 1488, 1446, 1372, 1299, 1255, 1230, 1207,
1180, 1148, 1129, 1095, 1041, 1018, 953, 868; HRMS
(ESI) calcd. for C48H50Br4N12+H+ 1111.1086, found
1111.1082 [M + H]+, 100% (Fig. S3); UV-vis (THF): λabs,
nm 800 (107 200 M-1.cm-1), 715sh, 342 (55 480).
8H, J = 7.2 Hz, NCH2), 1.14 (t, 12H, J = 7.0 Hz, CH3); IR
(ATR): νmax (cm-1) 2975, 2933, 2870, 2212 (CN), 1604,
1504, 1468, 1446, 1417, 1383, 1358, 1333, 1319, 1272,
1245, 1215, 1187, 1170, 1148, 1098, 1082, 1027, 986;
HRMS (ESI) calcd. for C16H22N4+H+ 271.1917, found
271.1920 [M + H]+, 100%.
General procedure for synthesis of compounds 4
and 5 by Buchwald–Hartwig coupling
3,6-dibromophthalonitrile (1), 2,3-dicyano-1,4-pheny-
lene bis(trifluoromethanesulfonate) (2) or 2,3-dicyano-
1,4-phenylene bis(4-methylbenzenesulfonate) (3)(1equiv),
a base (5 equiv), Pd catalyst (0.02 equiv) and co-catalyst
(0.05 equiv) were put into a thoroughly oven-dried flask
and the flask was filled with argon atmosphere. Solvent
(toluene/tBuOH 5:1 or DMSO) and diethylamine
(6 equiv) were added by a syringe and the reaction
was heated (the particular temperature is mentioned in
Table 2) for 24 h. The amount of arisen product was
quantified by the method described below.
Synthesis of 1,8(11),15(18),22(25)-tetrabromo-4,-
11(8),18(15),25(22)-tetrakis(diethylamino)phthalo-
cyaninato zinc (II) (6Zn). Pc 6H (30 mg, 26.9 mmol)
and anhydrous zinc acetate (49.4 mg, 269 mmol) in
pyridine (5 mL) were heated at reflux for 1 h. The solvent
was evaporated, the solid was washed with water and the
product was purified by column chromatography on silica
using chloroform/THF 1:1 as a mobile phase. Finally,
the product was washed with methanol and hexane and
the solid was collected by filtration. Yield 30 mg of dark
Quantification of products 4 and 5
1
The reaction mixture was quantitatively transferred
into a flask and all solvents were evaporated to dryness.
THF (2.0 mL) was added and 5 mL of the solution was
spotted to the corner of a TLC plate (5 × 5 cm). Products
4 and 5 were isolated from a two-dimensional TLC using
hexane/ethyl acetate 3:1 and chloroform/toluene 1:1 as
eluents. Appropriate spots were quantitatively scraped
fromTLC, extracted byTHF (2.0 mL) and filtered directly
to a cuvette. The amount of the product was determined
from a calibration curve on the basis of fluorescence
intensity at 472 nm and 505 nm for compounds 4 and 5,
respectively (λexc = 400 nm). Measurement was done in
duplicate and the results presented in the Tables 1 and 2
represent the mean of the two experiments.
solid (mixture of isomers, 95%). H NMR (500 MHz,
CDCl3/pyridine-d5 3:1) d (ppm) 8.18–8.27 (m, 4H, ArH),
7.49–7.62 (m, 4H, ArH), 4.12–4.44 (m, 16H, NCH2),
0.89–1.48 (m, 24H, CH3) (Fig. S4); 13C NMR (126 MHz,
CDCl3/pyridine-d5 3:1): d (ppm) 152.41, 152.39, 152.14,
152.11, 151.9, 151.8, 151.7, 148.3, 148.1, 147.9, 147.81,
147.78, 147.02, 146.98, 146.8, 137.9, 137.80, 137.77,
137.62, 137.58, 135.1, 135.0, 134.9, 134.6, 130.1,
130.0, 129.9, 129.8, 129.6, 122.0, 121.8, 121.5, 108.6,
108.5, 108.4, 108.3, 108.2, 48.83, 48.76, 48.7, 48.42,
48.37, 48.2 12.38, 12.36, 12.34, 12.29, 12.26 (some of
the signals were not detected) (Fig. S5); IR (ATR): νmax
(cm-1) 2968, 2929, 2869, 1569, 1482, 1446, 1373, 1299,
1254, 1217, 1180, 1148, 1117, 1092, 1063, 1021, 963;
HRMS (ESI) calcd for C48H48Br4N12Zn+H+ 1179.0168
(as the most intense peak of the cluster), found 1179.0168
Synthesis of macrocycles
[M + H]+, 100% (Fig. S6). UV-vis (THF): λabs, nm 769
-1
(107 470 M-1 cm ), 693sh, 349 nm (46 760).
.
Synthesis of 1,8(11),15(18),22(25)-tetrabromo-4,-
11(8),18(15),25(22)-tetrakis(diethylamino)
phthalocyanine (6H). Magnesium turnings (0.478 g,
19.6 mmol) and a crystal of iodine in anhydrous butanol
(4 mL) were heated at reflux for 3 h until magnesium
butoxide was formed. Compound 4 (0.793 g, 2.85 mmol)
was added and the reaction was refluxed overnight. A
mixture of water/MeOH/acetic acid 10:10:1.5 (20 mL)
was added to remove excess magnesium butoxide. The
product dissolved in this mixture, yielding blue-green
solution. The product was extracted with chloroform
and purified by column chromatography on silica
with chloroform/THF 5:1 as a mobile phase. Column
chromatography was repeated using chloroform/THF 8:1
as a mobile phase. Finally, the product was washed with
methanol and the solid was collected by filtration. Yield
Determination of fluorescence quantum yields
Thefluorescencequantumyields(FF)weredetermined
in THF by a comparative method using unsubstituted zinc
phthalocyanine (ZnPc, Sigma-Aldrich) as a reference
(FF = 0.32 in THF [23]). The absorbance at the excitation
wavelength was held below 0.01, and the absorbance at
the Q-band maximum was held below 0.05 to limit an
inner filter effect. The value of FF was calculated using
Equation 1:
2
−A

S  
 
R   S 
n
F
FR
1−10
1−10
ΦSF = ΦFR
(1)

S  

nR
−A

 
 

Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 3–10

4
L. KOCISCAKOVA ET AL.
where F is the integrated area under the emission
spectrum, A is the absorbance at the excitation
wavelength and n is the refractive index of the solvent.
The superscripts R and S correspond to the reference and
sample, respectively. All experiments were performed in
triplicate with the data representing the mean (estimated
error 15%).
Protonation
A total of 2.5 mL of a dye solution (1 μM) in
chloroform was transferred to a 10 × 10 mm quartz
cell, and the absorption spectrum was recorded. Defined
amounts (typically 5–50 μL) of triturated TFA or pure
TFA were subsequently added and the absorption spectra
were measured after each addition. The absorption
spectra were corrected for dilution.
Determination of quantum yields of singlet
oxygen production
RESULTS AND DISCUSSION
The quantum yields of the singlet oxygen production
(FD) were determined in THF according to a previously
published procedure [24] using the decomposition of
a chemical trap 1,3-diphenylisobenzofuran (DPBF)
with ZnPc as a reference (FD = 0.53 in THF [25]). The
detailed procedure was as follows: 2.5 mL of a DPBF
stock solution in THF (5 × 10-5 M) was transferred into a
10 mm × 10 mm quartz optical cell and bubbled with
oxygen for 1 min. A stock solution of a sample in
THF was then added (typically 20 mL) to achieve an
absorbance of the final solution in the Q-band maximum
of approximately 0.1. The solution was stirred and
irradiated using a xenon lamp (100 W, ozone-free XE
DC short-arc lamp, Newport). The incident light was
filtered through a water filter (6 cm) and an OG530
cut-off filter (Newport) to remove the heat and the
light below 523 nm, respectively. Decrease of DPBF in
solution with irradiation time was monitored at 414 nm.
All experiments were performed in triplicate, and the
data presented herein represent the mean of the three
experiments (estimated error: 15%).
Synthesis
Generally, synthesis of non-peripherally substituted
Pcs is based on a cyclotetramerization of appropriately
3,6-disubstituted phthalonitriles. As the synthesis of
3,6-dialkylamino-substituted phthalonitriles has been
shown to be challenging, we decided to compare two
basic methods that may provide such derivatives, i.e.
nucleophilic substitution and Buchwald–Hartwig
cross coupling reaction (Scheme 1). The latter has
recently been successfully used for synthesis of 3- and
3,6-bis(diarylamino)phthalonitriles [18]. The results
are summarized in Tables 1 and 2. Yields of mono-
(compound 4) and disubstituted (compound 5) products
have been determined by measuring the fluorescence
intensity of a sample obtained from two-dimensional
TLC (see Experimental part).
Starting material for nucleophilic substitution (com-
pound 1) was prepared similarly to an earlier published
method [22], however, repeated column chromatography
Scheme 1. Synthesis of precursors and target Pcs
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NON-PERIPHERALLY ALKYLAMINO-SUBSTITUTED PHTHALOCYANINES
5
was necessary to obtain the pure product. Nucleophilic
as starting materials. All attempts with compounds 2
and 3 resulted in decomposition of the starting material
without the formation of any products. Only in the case
of phthalonitrile 1, we observed two spots on the TLC
corresponding to the desired products 4 and 5. Hence,
compound 1 was chosen as the best starting material
for the following experiments to study in detail optimal
conditions for Buchwald–Hartwig cross coupling
reactions. Obviously, a base may change the ratio of
formation of products 4 and 5 (compare entries 3–5
in Table 2). Using K2CO3 as a base slightly improved
the yield of 4 but not 5, while Cs2CO3 and K3PO4 gave
higher yields of 5. Because disubstituted 5 was also a
target precursor, the following experiments were done
employing Cs2CO3 and K3PO4 as the bases. Lowering the
reaction temperature (compare entry 3 and 6, entry 5 and
7 in Table 2) led to a decrease of the yield of 5, which
was more pronounced when using Cs2CO3 as a base.
Importantly, the solvent was found to be a key parameter.
Once the reaction was performed in DMSO, yields
of monosubstituted 4 significantly increased and the
amount of 5 also slightly improved in both cases. At this
moment, it cannot be excluded that reactions in DMSO
substitution of compound 1 with diethylamine and
K2CO3 or Cs2CO3 as a base in DMSO under the air led
to the formation of monosubstituted 4 only with rather
low yield. Strictly anhydrous conditions were found
to be necessary to obtain at least a small amount of
3,6-dialkylamino phthalonitrile 5 and slightly improved
yields of both 4 and 5 were achieved using K2CO3 as
a base instead of Cs2CO3. A reaction time of 7 days
instead of 24 h resulted in increased yields of both 4
and 5. Attempts to use phthalonitriles 2 and 3 (structures
shown in Scheme 1) as starting materials for nucleophilic
substitution did not provide even traces did not provide
even traces of 4 or 5 and the main product was always
3,6-dihydroxyphthalonitrile most likely due to amidolysis
of the ester bond.
The Buchwald–Hartwig cross coupling reaction is
a modern palladium-catalyzed method for synthesis
of aryl amines from primary or secondary amines
and arylhalides (i.e. compound 1) or aryls bearing a
good leaving group (i.e. compound 2 or 3). In pilot
experiments, we have tried the reaction conditions shown
in Table 2 at entries 1, 2, 3 using compounds 1, 2 or 3
Table 1. Reaction conditions for synthesis of 4 and 5 by nucleophilic substitution using compound 1 as a starting
material
Amine
Base
Solvent
Reaction time
Reaction temperature, °C Yield of 4, %
Yield of 5, %
DEA
DEA
DEA
K2CO3
K2CO3
Cs2CO3
DMSO
DMSO
DMSO
24 h
80
80
80
64
73
71
0.95
1.3
7 days
7 days
0.86
DEA — diethylamine.
Table 2. Reaction conditions for synthesis of 4 and 5 by Buchwald–Hartwig coupling using compound 1 as a starting material a
Entry Amine
Catalyst
Co-catalyst Base
X-Phos Cs2CO3 Toluene, tBuOH
KOtB THF
Solvent
Reaction temperature Yield of 4, % Yield of 5, %
1
2
3
4
5
6
7
DEA
Pd(OAc)2
110°C
70°C
traces
traces
1.6
traces
traces
0.99
0.49
0.95
0.60
0.91
DBuA RuPhos-PdG3 Ru-Phos
DEA
DEA
DEA
DEA
DEA
RuPhos-PdG3 Ru-Phos
RuPhos-PdG3 Ru-Phos
RuPhos-PdG3 Ru-Phos
RuPhos-PdG3 Ru-Phos
RuPhos-PdG3 Ru-Phos
Cs2CO3 Toluene, tBuOH
K2CO3 Toluene, tBuOH
K3PO4 Toluene, tBuOH
Cs2CO3 Toluene, tBuOH
K3PO4 Toluene, tBuOH
110°C
110°C
110°C
80°C
8.2
5.4
1.9
80°C
4.9
8
9
DEA
DEA
RuPhos-PdG3 Ru-Phos
RuPhos-PdG3 Ru-Phos
Cs2CO3 DMSO
K3PO4 DMSO
80°C
80°C
58
79
0.71
1.4
a DEA — diethylamine, DBuA — dibutylamine, tBuOH — tert-butanol, KOtB — potassium tert-butoxide, THF — tetrahydrofurane, DMSO —
dimethylsulfoxide; btraces of product were parent on TLC in the case of compound 1 as a starting material.
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L. KOCISCAKOVA ET AL.
are accomplished by the mechanism of nucleophilic
substitution and not by Buchwald–Hartwig cross
coupling because the yields are very close to that obtained
from nucleophilic substitution in DMSO (Table 1).
As a consequence of similar yields, the precursors 4
and 5 were prepared in large scale using conditions of
nucleophilic substitution instead of cross coupling. Both
compounds were characterized by NMR, IR and HR
MS data. Interestingly, two vibration peaks appeared in
the IR spectrum of 4 for CN groups in at 2231 cm-1 and
2211 cm-1 that may be a consequence of the dissimilar
electronic properties of these two carbonitrile groups.
All attempts to cyclotetramerize phthalonitrile 5 by the
Linstead method using lithium or magnesium butoxide
in butanol failed. Increasing the reaction temperature
by employing octanol as a solvent did not help. Similar
negative results were observed for the reported synthesis
of 3,6-bis(diarylamino)phthalonitriles [18] and most
likely can be attributed to a very strong electron-donating
effect of amino substituents. Cyclotetramerization of
phthalonitrile 4 by the template method also failed, and
the corresponding Pc was formed only under Linstead
conditions. Lithium butoxide as an initiator of the reaction
gave the desired 6H, however, many side products were
observed on the TLC. Therefore, magnesium butoxide (a
milder initiator of cyclotetramerization) was chosen and
corresponding magnesium Pc was expected as a product
of cyclotetramerization. According to the position of the
Q band in the absorption spectrum (at 768 nm, in THF)
of a sample taken directly from the reaction, magnesium
Pc was probably formed (see Fig. S7). However, removal
of the excess of magnesium butoxide by water/MeOH/
acetic acid induced partial demetalation of the macrocycle
as revealed by TLC. Therefore, the mixture of magnesium
and metal-free Pcs was treated with p-toluenesulfonic acid
to achieve complete demetalation and directly acquire
metal-free 6H in 16% yield. Attempts to coordinate
magnesium back into a metal-free 6H failed. On the other
hand, heating of 6H with zinc acetate in pyridine provided
6Zn in a good yield of 95%. The structure of target Pcs
was confirmed by NMR analysis and HR MS. For 6Zn,
the signal of the most intense peak of the cluster in HR MS
had to be used instead of the first peak. The peak of the
exact mass was not observed due to the very low abundance
of these isotopes. However, the shape of the cluster and
the position of the most intense peak corresponded well
to the calculated cluster and, therefore, it may be used
for the structure confirmation (Fig. S6). Noteworthy, Pcs
6H and 6Zn were obtained as a mixture of isomers due
to asymmetry of precursor 4, as was obvious from the
splitting of signals in the NMR spectra (Figs S1, S2, S4
and S5) and overlapping spots on TLC. Interestingly,
the use of more bulky substituents (e.g. dibutylamino or
diarylamino) led preferentially to the formation of the
least sterically stressed C4h isomer indicating that the
diethylamino substituent in 4 is not sufficiently sterically
demanding to induce formation of just one isomer [17, 18].
Spectral and photophysical properties
First, spectral properties of 6H and 6Zn were studied
in THF. The absorption spectra had a characteristic shape
with low-energy Q bands at 800 and 769 nm, respectively,
and high-energy B bands at 342 and 349 nm, respectively
(Table 3). Despite the unsymmetrical character of 6H, its
Qbandwasnotsplit, whichisconsistentwiththeliterature
reports on metal-free Pcs and related compounds that
absorb in near-IR [14, 26, 27]. Wavelength-dependent
splitting of the Q band was studied in detail by Kobayashi
et al. [8] using absorption and MCD spectroscopy. They
found that splitting generally decreases with increasing
Q-band wavelengths and that metal-free Pcs absorbing
at around 800 nm do not show apparent splitting.
Attachment of diethylamino groups at non-peripheral
positions in 6H and 6Zn resulted in Q bands in the
near-IR area. By comparing the position of the Q band
with non-peripheral 1,8,15,22-tetrakis(dibutylamino)
phthalocyanines (770 and 746 nm in chloroform for
metal-free and zinc derivatives, respectively [17]), we
may conclude that Br contributed substantially to the
p-electronic system because 6H and 6Zn have their
Q bands in chloroform at 810 and 759 nm, respectively.
Red shifting of absorption spectra is highly desirable for
use in photodynamic therapy. Using an excitation light of
longer wavelengths results in a limited interaction with
endogenous chromophores and such light penetrates
deeper into tissues.
In addition to absorption at longer wavelengths,
a monomeric character is another requirement for a
successful photosensitizer. Absorption spectra of 6H
and 6Zn in common organic solvents (DMF, pyridine
and THF) (Fig. S8) showed steep Q bands indicating
monomeric character of the studied Pcs in these solvents.
Broadening of Q bands with formation of an additional
red-shifted band was observed only at 6Zn in the non-
coordinating solvent chloroform and more in toluene.
Addition of pyridine to 6Zn in toluene led to complete
monomerization. Weak aggregation of 6Zn in toluene
and chloroform may probably be assigned to formation of
so-called J-dimers with supramolecular assembly based
on a connection between a non-peripheral amine and a
central zinc(II), similarly to what has been published
before for aza-analogues of Pcs [28]. Monomeric
character of both 6H and 6Zn in THF was further
demonstrated by overlapping the fluorescence excitation
spectra with corresponding absorption spectra (Fig. 1).
The fluorescence emission spectra were of typical shape
for Pcs and mirrored the Q band absorption. Stokes shifts
of 6H and 6Zn were 36 and 33 nm, respectively, which
are slightly higher values compared to the majority of
Pcs [26]. Increased Stokes shifts may indicate noticeable
changes in the Pcs upon excitation.
Good photophysical properties, especially efficient
singlet oxygen production, belong to the third factor
important for photodynamic therapy. Photophysical data
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NON-PERIPHERALLY ALKYLAMINO-SUBSTITUTED PHTHALOCYANINES
7
example, data for non-peripherally tetra(diethylamino)-
substituted tetra(3,4-pyrido)porphyrazines (TPyPz) were
published as follows: metal-free TPyPz was characterized
by absorption at 806 nm, FF = 0.02, FD = 0.01, and zinc
TPyPz had Q band at 772 nm, FF = 0.06, FD = 0.41 (all
in THF [16]). Determination of singlet oxygen quantum
yield of 6Zn in DMF, which may be considered as more
biocompatible solvent, gave FD = 0.60. This value is lower
than reported for zinc 1,8,15,22-tetrakis(dibutylamino)
phthalocyanine (FD = 0.67 in DMF [17]), which is
inconsistent with the expected heavy-atom effect of
bromo substituents and may be a result of measurement
under different experimental conditions.
The values of FF and FD for 6Zn in THF are compar-
able with non-peripheral alkylsulfanyl zinc Pcs (FD =
0.74–0.76 in THF [9]) indicating no or only negligible
intramolecular charge transfer (ICT) in this solvent
from the lone electron pairs of peripheral amines to a
macrocyclic core of 6Zn. However, a lower FD = 0.60 of
6Zn in DMF suggests that ICT may be more pronounced
in this highly polar solvent. Effective ICT that is strongly
dependent on the solvent polarity was described for
other amino-substituted Pcs and in particular for their
aza-analogues [14, 29, 30]. The possibility of quenching
of excited states of 6Zn by ICT was further studied by
protonation of donor amine (see below). Strong decreases
of FF and FD of metal-free derivatives close to zero
values have been systematically reported in the literature
for metal-free Pcs and related macrocycles, not limited to
alkylamino-substituted ones [14, 23]. This quenching of
Fig. 1. Normalized absorption (black, dotted), emission (red)
and excitation (green) spectra of 6H (a) and 6Zn (b) in THF
excited states was attributed to other relaxation pathways
that have not yet been completely elucidated.
of 6H and 6Zn measured in THF are summarized in
Table 3. THF was used as a solvent because it ensures the
monomeric character of the studied compounds without
formation of proton transfer complex, which has been
described for metal-free Pcs dissolved in coordinating
solvents like DMF and DMSO [14]. It is evident that
these Pcs are only weakly fluorescent (FF = 0.014
and 0.027 in THF for 6H and 6Zn, respectively), and
sufficient singlet oxygen production (i.e. FD = 0.77) was
determined for 6Zn only, while the metal-free analogue
showed a low FD of 0.017. These results are consistent
with the literature data for similar macrocycles. As an
Protonation
Interestingly, a different shape of absorption spectra
was observed for 6Zn in chloroform upon dilution
(Fig. S9). At lower concentrations of 6Zn, a new band
at 696 nm was more pronounced compared to the band
at 759 nm, which was attributed to monomeric 6Zn in
chloroform. This led us to an idea that non-peripheral
amines have been easily protonated by traces of acids in
chloroform, which is of course much more manifested at
diluted solutions where the amount of acid molecules per
Pc molecule is higher. As a result, protonated nitrogen
Table 3. Spectral and photophysical data of Pcs in THFa
Pc
λ
abs, nm (log ε)
800 (5.03)
λF, nm
836
FF
τF, ns
FD
0.017
0.77
6H
6Zn
0.014
0.60 (99%), 3.08 (1%)
769 (5.03)
802
0.027
0.68 (99%), 6.58 (1%)
a The absorption Q-band maximum (λabs), the fluorescence band maximum (λF), fluorescence
lifetime (τF) the quantum yield of singlet oxygen formation (FD), the quantum yield of
fluorescence (FF).
Copyright © 2019 World Scientific Publishing Company
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8
L. KOCISCAKOVA ET AL.
Fig. 2. Absorption spectra of 6Zn (a, c) and 6H (b, d) in chloroform upon addition of TFA (a, b) and in chloroform, trifluoroacetic
acid (TFA) and sulfuric acid (SA) (c, d)
cannot contribute to the conjugated system of the
macrocycle and Q band is blue shifted. This presumption
was confirmed by the addition of tetrabutylammonium
hydroxide to a diluted solution of 6Zn that resulted in
the disappearance of the band at 696 nm and restoration
of the band at 759 nm (Fig. S10). Easiness of acceptance
of a proton by non-peripheral nitrogens was the reason to
investigate the protonation of target Pcs more in detail.
Addition of trifluoroacetic acid (TFA) to a chloroform
solution of 6Zn gave rise to a significant hypsochromic
shift of the Q band (Fig. 2a). Several distinguishable
steps were evident during the titration. Up to 0.3% TFA
(v/v), a band with a maximum at 700 nm was increasing,
while the band at 759 nm was decreasing. Higher
concentration of TFA caused further shifts of the Q band
up to 684 nm at 16% TFA (v/v). As suggested above, this
hypsochromic shift probably occurs due to protonation
of non-peripheral nitrogens in diethylamino groups.
The addition of tetrabutylammonium hydroxide led to a
restoration of the original spectrum of 6Zn in chloroform
(Fig. S11), indicating reversibility of the process. When
pure TFA was used as a solvent (Fig. 2c), the Q band was
located at 680 nm and a new bathochromically shifted
band (at 732 nm) was formed. This new band may be
attributed to subsequent protonation of azomethine
nitrogens that is typically manifested by red shifting of
the Q band. Dissolving 6Zn in sulfuric acid gave Pc with
azomethine nitrogens completely protonated with the
absorption maximum being at 732 nm. Behavior of 6H
upon titration with TFA was similar to 6Zn, but basicity
was significantly lower. In contrast to 6Zn, the absorption
spectrum of 6H in chloroform belongs predominantly
or almost exclusively to the non-protonated form. The
first shift of the Q band to 723 nm was observed up to
2.3% TFA (v/v). Isosbestic points clearly indicated that
one form is changing to another. Higher concentration of
TFA resulted in a split Q band with maxima at 670 and
707 nm. Protonation of azomethine nitrogens probably
does not take place in pure TFA (Fig. 2d), because only
steep split Q bands with maxima at 668 and 702 nm were
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 8–10

NON-PERIPHERALLY ALKYLAMINO-SUBSTITUTED PHTHALOCYANINES
9
observed (typical for D2h symmetry of the metal-free
explanation for this behavior at this moment, but changes
of electron density in the molecule upon protonation of
just one diethylamino group at low acid concentration
may account for this phenomenon. Increased amounts of
acid in the solution led to improvement of FF, which may
be explained by blocking of ICT upon full protonation
of peripheral diethylamines. A fluorescence maximum
at approximately 690 nm and accordance of excitation
spectra to blue-shifted absorption spectra indicate that
the fully protonated form of 6Zn is responsible for
fluorescence emission. The highest value of FF was
obtained in TFA (0.057), whereas in sulfuric acid FF
decreased up to 0.041 due to subsequent protonation
of azomethine nitrogens [32]. These results are in
accordance with the absorption spectra discussed above.
Pcs absorbing in this area). Azomethine nitrogens of 6H
were protonated in sulfuric acid, observed as a typical
band shifted to the red. However, the bands at 668 nm
and 702 nm still remained significant, indicating that the
protonation of azomethine nitrogens of 6H is not full
even in concentrated sulfuric acid.
Protonation caused by addition of TFA into a CDCl3
solution of 6Zn was further studied by 1H NMR
spectroscopy and compared with a non-protonated form
in CDCl3/pyridine-d5 (Fig. S12). With increasing degrees
of protonation, N–CH2 signals shifted downfield and
split significantly. At higher amount of TFA, a new broad
signal appeared at 14.5 ppm, which may be attributed
to the protonation of azomethine nitrogens, similarly as
published before in literature [31].
The effect of protonation on photophysical properties
of 6Zn was investigated by the means of fluorescence
spectroscopy and the data are summarized in Table 4.
Interestingly, weak protonation caused a decrease of
FF that was obvious from the values of FF = 0.0026,
0.0035 and 0.0078 determined in chloroform, chloroform
with 0.3% TFA (v/v), and DMF with 1.6% TFA (v/v),
respectively. Non-protonated 6Zn showed approximately
ten times higher FF as documented by FF = 0.027,
0.020 and 0.031 determined for 6Zn in THF, DMF
and chloroform neutralized by tetrabutylammonium
hydroxide, respectively. Unfortunately, we have no clear
CONCLUSIONS
In conclusion, we have prepared metal-free and
zinc Pcs bearing four diethylamino and four bromo
substituents in non-peripheral positions by Linstead
cyclotetramerization with magnesium butoxide as an
initiator of the reaction. All attempts to synthesize
octa(diethylamino)-substituted Pcs failed. Suitable
precursors were prepared by two different methods,
i.e. nucleophilic substitution and Buchwald–Hartwig
coupling. The yield of 3,6-dialkylamino-substituted
5 was always very low (only about 1%), while the
monosubstituted precursor 4 was obtained in good yields
of over 70% by both reaction types. However, we cannot
exclude the possibility that nucleophilic substitution
takes part even under the best conditions for Buchwald–
Hartwig coupling.
Table 4. Fluorescence properties of 6Zna
Solvent
FF
λF/nm
Target metal-free and zinc Pcs showed interesting
behavior under acidic conditions. Non-peripheral
nitrogens of diethylamino substituents readily accepted a
proton to form charged species. Interestingly, 6Zn was
protonated by even traces of acids present in chloroform.
Protonated amino substituents cannot contribute to
the conjugation system in Pcs by its lone pair, and as
a result, Q bands of 6Zn and 6H drastically shifted to
the red by 80 and 130 nm, respectively. Interestingly,
slight acidification of the solution led to a decrease in FF
of 6Zn, whereas higher amounts of acid resulted in an
improvement of FF probably due to blocking of ICT by
protonation of peripheral diethylamine. Protonation of
azomethine nitrogens in sulfuric acid caused a decrease
of FF, which is in accordance with the literature data.
Prepared zinc Pc absorbs strongly in the near-IR region (at
769 nm in THF), is in monomeric form in the most of the
organic solvents tried and possesses good quantum yield
of singlet oxygen production (FD = 0.77 in THF, 0.60 in
DMF) with weak fluorescence emission (FF = 0.027 in
THF, 0.020 in DMF). We believe that these results are a
good starting point for follow-up studies to develop an
optimal photosensitizer for photodynamic therapy.
THF
0.027
0.0026
0.031
0.0035
0.033
0.040
0.020
0.0078
0.021
0.026
0.057
0.041
802
CHCl3b
CHCl3 + TBAH
CHCl3 + 0.3% TFA
CHCl3 + 8% TFA
CHCl3 + 16% TFA
DMF
787
690
690
806
DMF + 1.6% TFA
DMF + 8% TFA
DMF + 25% TFA
trifluoroacetic acid
sulfuric acid
698
698
684
734
a The quantum yield of fluorescence (FF), b partially
protonated in chloroform (see discussion).
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 9–10

10
L. KOCISCAKOVA ET AL.
Acknowledgments
14. Novakova V, Donzello MP, Ercolani C, Zimcik P
and Stuzhin PA. Coord. Chem. Rev. 2018; 361:
1–73.
15. Furuyama T, Satoh K, Kushiya T and Kobayashi N.
J. Am. Chem. Soc. 2014; 136: 765–776.
16. Vachova L, Machacek M, Kucera R, Demuth J,
Cermak P, Kopecky K, Miletin M, Jedlickova A,
Simunek T, Novakova V and Zimcik P. Org. Bio-
mol. Chem. 2015; 13: 5608–5612.
The work was financially supported by the project
EFSA-CDN (No. CZ.02.1.01/0.0/0.0/16_019/0000841)
co-funded by ERDF, by Charles University (SVV 260
401) and by Czech Science Foundation (17-19094S). The
authors wish to thank Jiří Kuneš for measuring NMR,
Radim Kučera for MS (ESI) measurements and Lucie
Nováková for HR MS measurements.
17. ChenY, Fang W, Wang K, Liu W and Jiang J. Inorg.
Chem. 2016; 55: 9289–9296.
Supporting information
Characterization and spectral proterties of target Pcs
and NMR spectroscopy of protonation of 6Zn (Figs S1–
S12) are given in the supplementary material. This material
is available free of charge via the Internet at http://www.
worldscinet.com/jpp/jpp.shtml.
18. Yamamoto S, Kuribayashi K, Murakami TN, Kwon
E, Stillman MJ, Kobayashi N, Miletin M, Jedickova
A, Simunek T, Novakova V and Zimcik, P. Chem. —
Eur. J. 2017; 23: 15446–15454.
19. Mikhalenko SA, Derkacheva VM and Lukyanets
EA. Zh. Obshch. Khim. 1981; 51: 1650–1657.
20. Al-Raqa SY. J. Porphyrins Phthalocyanines 2006;
10: 55–62.
REFERENCES
21. Mbambisa G, Tau P, Antunes E and Nyokong T.
Polyhedron 2007; 26: 5355–5364.
22. Wang R, Zhao Y, Zhu C and Huang X. J. Hetero-
cycl. Chem. 2015; 52: 1230–1233.
23. Zimcik P, Novakova V, Kopecky K, Miletin M, Uslu
Kobak RZ, Svandrlikova E, Váchová L and Lang,
K. Inorg. Chem. 2012; 51: 4215–4223.
24. Novakova V, Miletin M, Filandrová T, Lenčo J,
Růžička A and Zimcik P. J. Org. Chem. 2014; 79:
2082–2093.
25. Kaestner L, Cesson M, Kassab K, Christensen T,
Edminson PD, Cook MJ, Chambrier I and Jori G.
Photochem. Photobiol. Sci. 2003; 2: 660–667.
26. Kadish K, Smith KM and Guilard R. (Eds.) The
Porphyrin Handbook, Vols. 1–20. New York: Aca-
demic Press, 2003.
27. Donzello MP, Ercolani C, Gaberkorn AA, Kudrik
EV, Meneghetti M, Marcolongo G, Rizzoli C and
Stuzhin PA. Chem. —Eur. J. 2003; 9: 4009–4024.
28. Novakova V, Zimcik P, Kopecky K, Miletin M,
Kuneš J and Lang K. Eur. J. Org. Chem. 2008;
2008: 3260–2363.
29. Vachova L, NovakovaV, Kopecky K, Miletina M and
Zimcik P. Dalton Trans. 2012; 41: 11651–11656.
30. Novakova V, Zimcik P, Miletin M, Vachova L,
Kopecky K, Lang K, Chábera P and Polívka T.
Phys. Chem. Chem. Phys. 2010; 12: 2555–2563.
31. Safonova EA, Martynov AG, Nefedov SE, Kirako-
syan GA, Gorbunova YG and Tsivadze AY. Inorg.
Chem. 2016; 55: 2450–2459.
1. Braun A and Tcherniac J. Ber. Dtsch. Chem. Ges.
1907; 40: 2709–2714.
2. Wohrle D, Schnurpfeil G, Makarov SG, Kazarin
A and Suvorova ON. Macroheterocycles 2012; 5:
191–202.
3. Wohrle D, Suvorova O, Gerdes R, Bartels O, Lapok
L, Baziakina N, Makarov S and Slodek A. J. Por-
phyrins Phthalocyanines 2004; 8: 1020–1041.
4. Zagal JH, Griveau S, Silva JF, Nyokong T
and Bedioui F. Coord. Chem. Rev. 2010; 254:
2755–2791.
5. Ragoussi M-E, Ince M and Torres T. Eur. J. Org.
Chem. 2013; 2013: 6475–6489.
6. Allen CM, Sharman WM and Van Lier JE. J. Por-
phyrins Phthalocyanines 2001; 5: 161–169.
7. Ghazal B, Machacek M, Shalaby MA, Novakova V,
Zimcik P and Makhseed S. J. Med. Chem. 2017; 60:
6060–6076.
8. Kobayashi N, Ogata H, Nonaka N and Luk’yanets
EA. Chem. — Eur. J. 2003; 9: 5123–5134.
9. Cidlina A, Pausimova Z, Miletin M, Zimcik P and
Novakova V. J. Porphyrins Phthalocyanines 2015;
19: 1095–1106.
10. Novakova V, Zimcik P, Miletin M, Kopecky K and
Musil Z. Eur. J. Org. Chem. 2010; 2010: 732–739.
11. Cook MJ, Dunn AJ, Howe SD, Thomson AJ and
Harrison KJ. J. Chem. Soc., Perkin Trans. 1 1988:
2453–2458.
12. Kobayashi N, Furuyama T and Satoh K. J. Am.
Chem. Soc. 2011; 133: 19642–19645.
13. Petrik P, Zimcik P, Kopecky K, Musil Z, Miletin M
and Loukotova V. J. Porphyrins Phthalocyanines
2007; 11: 487–495.
32. Safonova EA, Meshkov IN, Polovkova MA,
Volostnykh MV, Tsivadze AY and Gorbunova YG.
Mendeleev Commun. 2018; 28: 275–277.
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 10–10