Synthesis and supramolecular self-assembly of phenothiazine functionalized by carboxyphenyl fragments

Article abstract of DOI:10.1007/s11172-020-2765-z

Dispersions of non-toxic compounds characterized by an intense absorbance in the near-IR range find use in photodynamic therapy as effcient antibacterial and anticancer agents. An approach to the preparation of aqueous dispersions of nanoparticles with controlled morphology is described. 3,7-Bis(arylamino)phenothiazin-5-ium derivatives containing the ester and carboxylic groups were synthesized for the first time by the oxidative addition of methyl anthranilate to phenothiazin-5-ium tetraiodide with subsequent hydrolysis of the ester groups. The association with 3-phenylimino-7-phenylaminophenothiazine was studied for the synthesized bis(carboxyl)phenothiazinium derivative. The obtained 1: 1 associate is characterized by the bathochromic shift. The nanoprecipitation of the obtained nanoassociate in a methanol-water system was studied, and the morphology of the prepared dispersions was characterized: 3-phenylimino-7-phenylaminophenothiazine forms nanofiber structures, bis(carboxyl) derivative forms spherical particles, and the two-component associate forms facet-shaped particles

Full text of DOI:10.1007/s11172-020-2765-z

Russian Chemical Bulletin, International Edition, Vol. 69, No. 2, pp. 333—338, February, 2020
333
Synthesis and supramolecular self-assembly of phenothiazine
functionalized by carboxyphenyl fragments*
A. I. Khadieva, V. V. Gorbachuk, and I. I. Stoikov
Kazan (Volga Region) Federal University, Alexander Butlerov Institute of Chemistry,
18 ul. Kremlevskaya, 420008 Kazan, Russian Federation.
Fax: +7 (843) 233 7416. E-mail: Ivan.Stoikov@mail.ru
Dispersions of non-toxic compounds characterized by an intense absorbance in the near-IR
range find use in photodynamic therapy as efficient antibacterial and anticancer agents.
An approach to the preparation of aqueous dispersions of nanoparticles with controlled mor-
phology is described. 3,7-Bis(arylamino)phenothiazin-5-ium derivatives containing the es-
ter and carboxylic groups were synthesized for the first time by the oxidative addition of
methyl anthranilate to phenothiazin-5-ium tetraiodide with subsequent hydrolysis of the ester
groups. The association with 3-phenylimino-7-phenylaminophenothiazine was studied for the
synthesized bis(carboxyl)phenothiazinium derivative. The obtained 1 : 1 associate is character-
ized by the bathochromic shift. The nanoprecipitation of the obtained nanoassociate in
a methanol—water system was studied, and the morphology of the prepared dispersions was
characterized: 3-phenylimino-7-phenylaminophenothiazine forms nanofiber structures,
bis(carboxyl) derivative forms spherical particles, and the two-component associate forms
facet-shaped particles.
Key words: phenothiazine, synthesis, methyl anthranilate, self-assembly, supramolecular
associates, nanoprecipitation.
The vigorous publication activity observed in the recent
years in the field of synthesis and study of new organic
oligo- and polyaromatic compounds is caused by the whole
series of practically important materials based on these
substances. For example, the electric and photoelectric
properties of the organic semiconductors1 are attractive
for electronics, since they allow one to produce materials
characterized by a low weight, flexibility, resistance to
deformations,2 and intense absorption in the near-IR
range,3 which is important for manufacturing flexible
electronics and semitransparent solar cells. Polyaromatic
materials and compounds intensely absorbing in the long-
wavelength range, which results in the development of
photochemical and photothermal processes, are especially
interesting for the application in medical practice. The
radiation of the near-IR range is characterized by the high
penetrating ability toward biological tissues without their
damage.4 At present, the photoactive antimicrobial,5
antibacterial,6 and anticancer7 drugs based on organic
oligo- and polyaromatic compounds attract increased
attention of the researchers, since this trend provides
prospects of surmounting resistance to antiobiotics pro-
duced by bacteria. In addition, photoactivity makes it
possible to use the indicated materials directly in the area
of incision during surgery5 and for localized therapy of
cancer tumors.7,8
Nowadays porphyrins are the most frequently used
photodynamic agents.8 The phenothiazine derivatives
should be distinguished among other compound-compet-
itors applied in this area as the most synthetically avail-
able.5 The dialkylaminophenothiazine derivatives (struc-
tural analogs of Methylene Blue) are mainly presented in
the literature as new antimicrobial agents and remedies
for DNA footprinting.9,10 The introduction of arylamino
groups into the phenothiazine structure instead of di-
alkylamino groups significantly decreases the solubility of
the synthesized derivatives in aqueous media but increases
the absorption intensity in the near-IR range.11 The in-
creased absorption in the near-IR range characteristic of
these derivatives is explained by the steric influence of the
relatively bulky aromatic fragments on packing of the
phenothiazine derivatives in aqueous media. For example,
it is shown for the phenothiazine derivative containing
N-methylphenylamine groups that the presence of the
aromatic fragments decreases the aggregation at the con-
centrations lower than 10–3 mol L–1. It should be men-
tioned that the aggregation decreases the yield of singlet
oxygen and, therefore, decreases the efficiency of photo-
dynamic therapy.12 The photoactive nanoparticles based
on porphyrins13,14 and some phenothiazine derivatives15
have already been described in several publications,
* Based on the materials of the Markovnikov Congress on Organic
Chemistry (June 21—28, 2019, Moscow—Kazan, Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 0333—0338, February, 2020.
1066-5285/20/6902-0333 © 2020 Springer Science+Business Media LLC

Khadieva et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 2, February, 2020
whereas the studies in the field of preparation of similar
nanoparticles from structural analogs of Methylene Blue
are absent. In this work, we studied the formation of dis-
persions of the water-insoluble phenothiazine derivatives
during nanoprecipitation and proposed a new approach
to their stabilization. The formation of binary associates,
in which the phenothiazine derivative containing carboxyl
groups acts as an acid and the deprotonated phenothiazine
derivative acts as a base, was demonstrated. This approach
made it possible to enhance the stability of the dispersions
formed upon nanoprecipitation.
sensors.18 The fused aromatic systems bearing pheno-
thiazine fragments find use in organoelectronics.19 In spite
of significant advances in the modern chemistry, the problem
of functionalizing the phenothiazin-5-ium platform by
proton-donor/proton-acceptor fragments for targeted
self-assembly of related nanomaterials is far from solution.
Compound 3 containing the ester fragment was syn-
thesized by the oxidative addition of methyl anthranilate
to phenothiazin-5-ium (2) in methanol (Scheme 1). The
hydrolysis of derivative 3 in a THF—water mixture by
lithium hydroxide gave compound 5. 3-Phenylamino-7-
phenyliminophenothiazine (6) was synthesized using
a known procedure by the oxidative addition of aniline to
phenothiazin-5-ium tetraiodide to give salt 4 and subse-
quent deprotonation and removal of the iodide anion with
a mixture of ammonia and sodium thiosulfate.20,21
The structures and compositions of compounds 3 and
5 were confirmed by the data of 1Н and 13С NMR spectro-
scopy, IR spectroscopy, mass spectrometry, and elemen-
tal analysis. It should be mentioned that the electronic
and spatial structures of the carboxyl (5) and ester (3)
derivatives are close, which is observed in the similar
chemical shifts of the 13С{H} NMR spectra. The 1H and
13C NMR spectra of compound 5 exhibit no signal cor-
responding to the methyl group. The chemical shifts of
Results and Discussion
Synthesis of the phenothiazine derivatives containing
ester and carboxyl groups. The present structural variety
of the Methylene Blue analogs remains restricted from the
viewpoint of supramolecular chemistry, and their applica-
tion as complementary blocks of supramolecular associates
is a nontrivial task. The synthesis of the arylaminopheno-
thiazine derivatives is described in several publications,
which also show the possibility of preparing mono- and
bis(arylamino) derivatives11 and their use in supramo-
lecular ensembles with cyclophanes16 and electroconduct-
ing polymers17 and as components of the electrochemical
Scheme 1

Self-assembly of phenothiazine derivatives
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 2, February, 2020
335
Scheme 2
the carbon atoms of the carboxyl (δ 167.2, compound 5)
and methoxycarbonyl (δ 165.8, compound 3) groups dif-
fer insignificantly. The IR spectra contain the character-
istic sets of absorption bands corresponding to vibrations
of the phenothiazinium fragments (compound 3: 1581,
1489, and 1380 cm–1; compound 5: 1578, 1496, and
1377 cm–1). The positions and shapes of the absorption
bands of the C(O)—O fragment in the IR spectra of com-
pounds 3 and 5 are different being 1687 cm–1 for com-
pound 5 and 1680 cm–1 for compound 3.
Study of the association and nanoprecipitation of com-
pounds 5 and 6. Since we pioneered in synthesizing the
structural analog of Methylene Blue containing carboxyl
groups, it seemed of interest to study the interaction of
derivative 5 with deprotonated derivative 6. Similar systems
were not described earlier, and it is necessary to study them
for the application of supramolecular chemistry approaches
for the development of related associates and dispersions.
Based on the fact of the presence of two carboxyl groups
in compound 5 and taking into account that compound 6
is protonated with the equimolar amount of the acid, the
formation of associates with different stoichiometries with
the molar ratios of compounds 5 and 6 of 1 : 1 or 1 : 2 can
be assumed. The method of isomolar series in methanol
was used for the determination of the stoichiometries of
the formed associates: the intensity of the absorption band
of the binary associate in methanol at 660 nm corresponded
to the 1 : 1 stoichiometry (Fig. 1, Scheme 2). This conclu-
sion is confirmed by 1Н NMR spectroscopy data: the
According to the dynamic light scattering data (Fig. 2),
true solutions of compounds 5 and 6 and a mixture of
compounds 5 and 6 are formed in methanol at a concen-
tration of 10–4 mol L–1. It is known that nanoprecipitation
is a convenient approach for the preparation of dispersions
of nanoparticles. The nanoprecipitation of compounds 5
and 6 and their associate was conducted from solutions in
methanol into water under ultrasonication, and the con-
centration of the resulting dispersions was 10–5 mol L–1
.
The dispersions of submicronic particles were formed, and
the data on the hydrodynamic diameter and polydispersity
of the obtained particles are presented in Table 1.
The obtained dispersions were studied by scanning
electron microscopy (SEM) (Fig. 3). According to the
I (%)
a
1
largest changes in the Н NMR spectra compared to the
1Н NMR spectra of the starting compounds were observed
at the equimolar ratio of compounds 5 and 6 for the total
concentration of compounds 5 and 6 in DMSO-d6 equal
to 1 mmol L–1. Thus, the NMR spectroscopic data also
confirm the formation of the 1 : 1 complex.
15
10
5
1•100
1•101
1•102
1•103
d/nm
The ratio 5 : 6 = 1 : 1 was chosen for the nanopre-
cipitation of the compounds based on the data obtained.
I (%)
b
20
15
10
5
C6(I – I0)•106/mol L–1
50
40
30
20
10
1•100
1•101
1•102
1•103
d/nm
I (%)
20
c
15
10
5
1.00
0.75
0.50
0.25 C6/(C5 + C6)
1•100
1•101
1•102
1•103
d/nm
Fig. 1. Isomolar series plot for compounds 5 and 6 in methanol
Fig. 2. Size distributions of the particles obtained by the nano-
precipitation (methanol—water) of compounds 5 (a), 6 (b), and
a 1 : 1 mixture of compounds 5 and 6 (с) (DLS data).
according to the UV spectroscopy data. The total concentration
of compounds 5 and 6 in the mixtures is 10–5 mol L–1
.

Khadieva et al.
336
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 2, February, 2020
a
b
2 μm
300 nm
c
d
300 nm
300 nm
Fig. 3. SEM images of the particles obtained by the nanoprecipitation of compounds 6 (a, b), 5 (c), and a 1 : 1 mixture of compounds
5 and 6 (d).
SEM images, nanoprecipitation of 3-phenylimino-7-
phenylaminophenothiazine (6) gives filament structures,
whereas spherical nanoparticles are formed upon the
nanoprecipitation of carboxyl derivative 5.
According to the SEM images, the combination of
two compounds 5 and 6 into a binary associate results
in changes in the morphology of the particles obtained
by nanoprecipitation. Unlike the nanoparticles ob-
tained by the nanoprecipitation of one-component solu-
tions of compounds 5 and 6, the nanoprecipitation of
their mixture gave faceted (close to the cubic shape)
nanoparticles.
3,7-Bis(arylamino)phenothiazin-5-ium derivatives
containing ester and carboxyl groups were synthesized for
the first time and characterized. The association of the
synthesized bis(carboxyl) derivative in water in the pres-
ence of 7-phenylamino-3-phenyliminophenothiazine was
studied, and the formation of 1 : 1 associates was shown.
The formation of two-component acid-base associates of
the structural analogs of Methylene Blue makes it possible
to change the morphology of particles formed by nano-
precipitation from a solution in methanol to water. The
nanoprecipitation of 7-phenylamino-3-phenylimino-
phenothiazine affords nanofibers, whereas spherical par-
ticles are formed by the nanoprecipitation of the pheno-
thiazine derivative containing antranilic acid fragments.
The combination of these compounds in the binary as-
sociate allows one to obtain faceted nanoparticles.
Thus, the possibility of synthesizing stable aqueous
dispersions based on the bis(carbonyl)phenothiazine de-
rivatives was shown. The developed nontrivial approach
to the stabilization of water-insoluble arylaminopheno-
Table 1. Dynamic light scattering data for disper-
sions of compounds 5 and 6 and their equimolar
mixture (C = 10–5 mol L–1
)
Compound
d/nm
PDI
5
6
140
260
1
3
1
0.07 0.02
0.16 0.02
0.06 0.02
Mixture of 5 and 6 237
Note: d is the average hydrodynamic diameter, and
PDI is the polydispersity index.

Self-assembly of phenothiazine derivatives
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 2, February, 2020
337
obtained intensively colored solution was filtered, and the pre-
cipitate was collected and washed with methanol. The yield of
compound 3 (m.p. 198 °С) was 106 mg (40%). Found (%):
C, 53.94; H, 3.56; I, 20.35; N, 6.74; S, 5.14. Calculated (%):
C, 53.98; H, 3.57; I, 20.31; N, 6.72; S, 5.17. 1Н NMR, δ: 10.97
(s, 2 H, NH); 8.13 (d, 2 H, H(1), H(9), J = 9.2 Hz); 8.03 (d, 2 H,
H(3´), J = 7.9 Hz); 7.78 (dd, 2 H, H(4´), J = 7.6 Hz, J = 7.9 Hz);
7.67 (d, 2 H, H(6´), J = 7.8 Hz); 7.57 (d, 2 H, H(2), H(8),
J = 9.2 Hz); 7.52 (dd, 2 H, H(5´), J = 6.2 Hz, J = 7.9 Hz); 7.49
(s, 2 H, H(4), H(6)), 3.80 (s, 6 H, COOMe). 13C NMR, δ:
165.8 (С(O)O), 152.4 (C(2´)), 139.0, 137.3, 137.1, 135.5, 134.2,
131.7, 127.4, 126.1, 124.6, 122.5, 107.3, 52.6 (CH3). IR (ATR),
ν/cm–1: 1687 (C(O)—O), 1581, 1489, 1380 (phenothiazinium
fragment), 1132 (C—N). MS (ESI), m/z (Irel (%)): 496.3 (100)
[M + H]+.
3,7-Bis((2-(carboxyl)phenyl)amino)phenothiazin-5-ium chlo-
ride (5). Lithium hydroxide monohydrate (0.24 g, 10 mmol) and
THF (20 mL) were added to compound 3 (0.62 g, 1 mmol), and
then water (2 mL) was added with vigorous stirring. The mixture
was refluxed for 8 h. The solvent was evaporated on a rotary
evaporator, concentrated hydrochloric acid was added to the
residue, and the obtained mixture was vigorously stirred at room
temperature for 10 h. The precipitate formed was filtered off and
washed with 2 М HCl. The yield of compound 5 (m.p 250 °С)
was 353 mg (70%). Found (%): C, 61.97; H, 3.60; Cl, 7.03;
N, 8.34; S, 6.36. Calculated (%): C, 62.01; H, 3.61; Cl, 7.01;
N, 8.31; S, 6.31. 1Н NMR, δ: 8.10 (d, 2 H, H(1), H(9), J = 8.1 Hz);
8.04 (d, 2 H, H(3´), J = 8.1 Hz); 7.73 (dd, 2 H, H(4´), J = 8.1 Hz,
J = 7.3 Hz); 7.63 (d, 2 H, H(2), H(8), J = 8.1 Hz); 7.57 (d, 2 H,
H(6´), J = 8.1 Hz); 7.51 (s, 2 H, H(4), H(6)); 7.46 (dd, 2 H,
H(5´), J = 8.1 Hz, J = 7.8 Hz). 13C NMR, δ: 167.2 (С(O)O),
152.1 (С(2)), 138.9, 137.6, 137.1, 135.3, 133.9, 132.0, 127.1,
125.4, 125.1, 122.6, 107.5. IR (ATR), ν/cm–1: 1680 (C(O)O),
1578, 1496, 1377 (phenothiazinium fragment), 1127 (C—N).
MS (ESI), m/z (Irel (%)): 468.2 (100) [M + H]+.
thiazine derivatives can be applied for the preparation of
agents for photothermal therapy, which intensively absorb
in the near-IR range.
Experimental
Phenothiazine (99%, Alfa-Aesar), I2 (reagent grade), chloro-
form (reagent grade), methanol (reagent grade), methyl anthra-
nilate (99%, Acros Organics), and sodium thiosulfate (anhydrous,
98%, Sigma-Aldrich) were used. 1Н and 13С NMR spectra were
recorded on a Bruker Avance 400 spectrometer at the working
frequencies 400.0 and 100.0 MHz, respectively. Chemical shifts
were determined relative to the signals of residual protons of the
deuterated solvent (DMSO-d5). The concentration of the exam-
ined solutions was 10 mmol L–1. The proposed assignments of
the signals in the NMR spectra were made by the simulation of
the spin-spin systems. The SEM images of the particles were
obtained using a Carl Zeiss Auriga Cross Beam microscope on
the silicon support surface. Dispersions of colloidal particles with
a concentration of 10–5 g mL–1 were deposited onto the support
surface and dried for 1 h in a vacuum desiccator.
IR attenuated total reflectance (ATR) spectra were recorded
in the wave number range 400—4000 cm–1 on a Spectrum 400
FT-IR spectrometer (Perkin Elmer): the resolution was 1 cm–1
,
64 scans were acquired, and the acquisition time was 16 s. Mass
spectra were obtained on an AmazonX mass spectrometer (Bruker
Daltonik GmbH, Germany) with an ionic trap (electrospray
ionization mode). The measurements were performed in the
positive ion detection mode in the m/z range from 70 to 3000.
The voltage on the capillary was −3500 V. Nitrogen at 250 °С
served as a nebulizer gas with a flow rate of 10 L min–1
.
A methanol—water (7 : 3, vol/vol) system with a flow rate of
0.2 mL min–1 was used as an eluent (Agilent 1260 chromatograph,
USA). The analyzed sample was dissolved in methanol to a con-
centration of 10–6 g L–1. A sample was injected through
a Rheodyne 7725 injector (Rheodyne, USA). The sample volume
was 20 μL. The TrapControl 7.0 software (Bruker Daltonik
GmbH, Germany) was used for mass spectrometer control-
ling and data collecting. The data were processed using the
DataAnalysis 4.0 SP4 program (Bruker Daltonik GmbH,
Germany). The particle sizes in the dispersion were determined
by dynamic light scattering (DLS) on a Zetasizer Nano ZS in-
strument (Malvern) equipped with a He—Ne laser (4 mV,
wavelength 633 nm, scattered light detection angle 173°) with the
automatic determination of the position of measuring inside cells.
The prolonged treatment with ultrasound was conducted using
an Elmasonics S30H ultrasonic bath. The samples were dispersed
with ultrasound on a Sonics Vibracell VCX 500 instrument using
a stepped microtype (diameter 3 mm) immersed into a mixture
of the solvent and compound insoluble in this solvent. Dionized
ultrapure water (specific resistance >18.0 MOhm cm at 25 °С)
was obtained from distilled water on a Millipore-Q system.
Phenothiazin-5-ium tetraiodide (2)20 and compounds 421
and 622 were synthesized by known procedures.
Nanoprecipitation of compounds 5 and 6 and their binary as-
sociate. Solutions of compounds 5 and 6 in methanol with
concentrations of 10–3 mol L–1 were prepared for nanoprecipi-
tation. Each obtained solution (100 μL) was added to water
(10 mL) during ultrasonication to a residual volume of 10 mL.
Study of the stoichiometry of an associate of compounds 5 and
6 by spectrophotometry. Solutions of compounds 5 and 6 with
concentrations of 10–5 mol L–1 were mixed in volume ratios of
4 : 1, 3 : 1, 2 : 1, 3 : 2, 1 : 1, 2 : 3, 1 : 2, 1 : 3, and 1 : 4. Electronic
absorption spectra were recorded in quartz cells, and the solution
volume for measurements was 3 mL. The plot of the isomolar
series was constructed by the absorption band at 660 nm.
Study of the stoichiometry of the association of compounds 5
and 6 by 1Н NMR spectroscopy. Solutions were prepared by the
dissolution of compounds 5 and 6 in the NMR tubes in the
molar ratios of 3 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 3. The amounts
corresponded to the total concentrations of 5 and 6 equal to
10–3 mol L–1
.
This work was financially supported by the Russian
Science Foundation (Project No. 18-73-00293). The UV
spectroscopy studies were subsidized in the framework of
the Kazan (Volga Region) Federal University Competitive
Growth Program among World Class Academic Centers
and Universities.
3,7-Bis((2-(methoxycarbonyl)phenyl)amino)phenothiazin-5-
ium iodide (3). A solution of methyl anthranilate (0.584 g,
3.86 mmol) in methanol (20 mL) was added to a suspension of
phenothiazin-5-ium tetraiodide (2) (0.3 g, 0.425 mmol), and the
mixture was vigorously stirred at room temperature for 24 h. The

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Received September 30, 2019;
accepted December 2, 2019