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16Фталоцианины
Phthalocyanines
iVJaKporaTepoLii/JKj-JbJ
Статья
Paper
http://macroheterocycles.isuct.ru
DOI: 10.6060/mhc160646k
Effect of pH on Acid-Base and Photophysicochemical Properties of 2,3,9,10,16,17,23,24-Octacarboxyphthalocyanines in Aqueous Media
Oleg L. Kaliya,ab Nina A. Kuznetsova,b@ Roman A. Bulgakov,b Ludmila I. Solovyova,b Ekaterina N. Shevchenko,b Ludmila K. Slivka,b and Evgeny A. Lukyanetsb
Moscow Technological University, Institute of Fine Chemical Technology, 119571 Moscow, Russia hOrganic Intermediates and Dyes Institute, 123001 Moscow, Russia @Corresponding author E-mail: lab32@niopik.ru
The results of study of pH effect on the aggregation, acid-base and photophysicochemical properties of 2,3,9,10,16,17,23,24-octacarboxy substituted Zn, Al, Co, Pt and Pd phthalocyanines (MPcoc) in aqueous media are presented. The ionization of peripheral carboxyl groups and axial ligation of central metal atom are the principal factors, governing aggregation. The multi-step ionization of MPcocs, induced by change of pH, was considered as pseudo two-step process affecting four carboxyl groups on each step. The photophysicochemical properties of three differently ionized forms of ZnPcoc (unionized, tetra- and octacarboxylated) were studied.
Keywords: Octacarboxyphthalocyanine, ionization, aggregation, fluorescence, singlet oxygen, photostability.
Влияние рН на кислотно—основные, фото- и физико-химические свойства 2,3,9,10,16,17,23,24—октакарбоксифталоцианинов в водной среде
О. Л. Калия,^ Н. А. Кузнецоваь@ Р. А. Булгаков^ Л. И. Соловьеваь Е. Н. Шевченко,ь Л. К. Сливка,ь Е. А. Лукьянец
Московский технологический университет, Институт тонкой химической технологии, 119571 Москва, Россия ъФедералъное государственное унитарное предприятие «Научно-исследовательский институт органических полупродуктов и красителей» (ФГУП ГНЦ «НИОПИК»), 123001 Москва, Россия @Е-таИ: lab32@niopik.ru
Исследовано влияние рН на агрегацию, кислотно-основные, фото- и физико-химические свойства 2,3,9,10,16,17,23,24-октакарбоксизамещенных фталоцианинов 2п, А1, Со, Pt и Pd (МРсос) в водной среде. Ионизация карбоксильных групп на периферии и наличие аксиальных лигандов у центрального атома металла являются ключевыми факторами, определяющими степень агрегации МРсос. Многоступенчатая ионизация МРсос при изменении рН была аппроксимирована двухступенчатым процессом с ионизацией четырех карбоксильных групп на каждой ступени. Исследованы фото- и физико-химические свойства трех различно ионизованных форм ZnPcoc (кислотной, тетра- и окта-ионизованной).
Ключевые слова: Октакарбоксифталоцианин, ионизация, агрегация, флуоресценция, синглетный кислород, фото стабильно сть.
Introduction
Methods
Phthalocyanines (Pc) substituted with carboxylic groups are the object of wide investigations.11"31 They are interesting as photosensitizers for photodynamic therapy,[4] dye-sensitized solar cells,[56] catalysts of different reactions, including so-called catalytic therapy of cancer,[78] and so on. The most known, to date, in this series are highly symmetric 2,3,9,10,16,17,23,24-octacarboxy metallophthalocyanines (MPcoc). Methods of synthesis and some properties of MPcoc were reviewed recently.[9,10] In form of their alkali salts MPcoc are highly soluble in water, that is important for practical applications. The corresponding acids have tendency for self-association (aggregation) in solutions[31112] due mainly to the stack p-p interactions of adjacent Pc macrocycles. In the case of MPcoc acids the interaction via hydrogen bonding should be also taken into consideration. The aggregation of Pcs influences their properties, including photochemical and photocatalytic efficiency,[1314] the latter drastically decreases due to an enhance of radiationless deactivation of excited singlet state. In order to rationalize catalytic, photochemical and photobiological effects it is important to characterize more in detail the aggregation and properties of MPcoc differently ionized forms.
The effects of pH on aggregation of zinc, copper, nickel and metal-free 2,3,9,10,16,17,23,24-octacarboxyphthalo-cyanines in aqueous solutions have been studied in wide pH interval using UV-vis absorption and fluorescence emission spectroscopy.[311121516] Unlike to pH < 5, where only the aggregates have been observed, at pH > 8.0 exclusive presence of monomers was found. Such strong influence of pH on the aggregation of MPcoc is caused by dissociation of carboxyl groups, resulting in accumulation of negative charge on the molecules, electrostatic repulsion of which prevents their aggregation. The pH value of monomer-associate transition depends on the central metal, increasing in the order: Zn = Cu < Ni.[15]
In this paper, we present the effect of nature of central metal atom and, especially, its ability to axial coordination on pH-dependent aggregation of MPcoc in aqueous solution. The MPcoc (M=Al, Zn, Pt, Pd, Co) were studied, where Zn, Al and Co complexes are capable of axial coordination, unlike to PtPcoc and PdPcoc. We also report on pH dependence of spectral-fluorescent properties of ZnPcoc, monomerized in Cremophor EL solution.
Electronic absorption spectra were recorded with Hewlett Packard 8453 spectrophotometer, fluorescence spectra - with Cary Eclipse spectrofluorimeter. The monochromatic light intensity was measured with a calibrated Si photodiode (Thorlabs).
The fluorescence quantum yields (Ffl) were determined by the comparative method according to equation: Ffl=Fflref(F/Fref) (Aref/A)(h2/h2ref), in which F, A and h are the measured fluorescence (area under the emission peak), the absorbance at the excitation wavelength and the refractive index of the solvent, respectively. ZnPcoc octaanion was used as the reference (Fflref = 0.18 at pH 10).[21] The ratio Aref/A was equal to 1 (the optical densities at a wavelength of excitation were the same in our experiments and equal to 0.1-0.2).
The singlet oxygen quantum yields (FA) were estimated by the chemical-trapping method in water - Cremophor EL (4 %, v/v) solutions by using ZnPcoc octaanion as a reference (F™f = 0.54 in PBS with pH 7.4)[22] and DPBF as the singlet oxygen trap. DPBF is insoluble in water, but it is readily solubilized in aqueous solution of Cremophor EL. The light source for the singlet oxygen measurements was a red light from a 150 W xenon lamp, passed through a water tank for cooling, a yellow glass cut-off filter and an interference filter (684 nm). Eq. (1) was employed for the calculations of F.:
WTref
F =Fref-^
D WrefIh,
(1)
where FAref is the singlet oxygen quantum yield for reference; W and Wf are the trap photobleaching rates in the presence of sensitizer and reference, respectively, and I, and I, re-f are the intensities of
' r J 7 abs abs
their light absorption. The initial trap concentrations are kept the same for reference and the samples. The accuracy of FA estimation was about 10 %.
The photodegradation of ZnPcoc ionized forms was investigated under emitting diode IS-2-693 (ZAO "Technoeksan", Russia) irradiation, light flux of 688-698 nm and 1.1T017 photons-s^cm-2 at the sample surface. For the photodegradation quantum yields (Fp) determination, a 3 mL portion of air-saturated 5 |iM ZnPcoc aqueous solution, containing 4 % (v/v) of Cremophor EL, was adjusted to pH 7.4, 5 or 2.8 and irradiated in the Q band region. The decrease of the ZnPcoc absorbance (A) was measured in the maximum of the red absorption band as a function of irradiation time. The quantum yields Fp were calculated from the initial part of the A(t) dependence curves according to equation (2):
F,
V dA
Iabsle dt '
(2)
Experimental
The Al, Zn, Pt, Pd and Co 2,3,9,10,16,17,23,24-octacarboxy-phthalocyanines were synthesized according to papers.[4,17-19] Cremophor EL and 1,3-diphenylisobenzofuran (DPBF) were purchased from Fluka. DPBF in Cremophor EL aqueous solution, 4 % (v/v), was solubilized according to the methodology described earlier,[20] that is DPBF (50 mg) and Cremophor EL (400 mg) were first dissolved in 20 mL of toluene, then toluene was evaporated, residue was dried at temperature about 40-50 °C and sonicated for 10 min with water (10 mL) in ultrasonic water bath. After sonication, fine solution with insignificant light scattering due to the presence of Cremophor EL micelles was obtained. Solutions were prepared in deionized water, pH was adjusted by addition of HCl or NaOH.
where V is the reaction volume, Iats is the intensity of light absorption by the sample (einsteins per second), l is the optical path length, e - decimal molar absorption coefficient at the maximum of the absorption band (Table 2). The term dA/dt was determined graphically. The quantum yields of photodegradation were estimated with about 15 % accuracy.
All the experiments were carried out at ambient temperature.
Results and Discussion
Behavior of AlPcoc, ZnPcoc, PtPcoc, PdPcoc and CoPcoc in aqueous solutions was studied by UV-vis spectral and fluorescent methods, as a rule at l abs and l fl (Table 1).
' max max v '
Figure 1 represents the influence of pH on absorbance of MPcoc aqueous solutions at Q-band maxima. Generally,
Table 1. Spectral-fluorescent data and pH of monomer-aggregate transition for MPcoc.
MPcoc pH of monomer-aggregate transition Monomer (pH 8-9) l abs, nm l fl, nm max ' max ' Aggregate (pH 3), l abs max
AlPcoc 4.05 690 695 638
ZnPcoc 6.1 690 700 638-639
PdPcoc 6.6 680 - 603
PtPcoc 6.5 669 - 603
CoPcoc 5.3 678 - 628
MPcoc within the pH range from 7 to 10 at Q band maxima have high absorption, which does not change significantly. Electronic absorption spectra at pH > 7 have the intensive Q0-0 bands, typical for monomeric species. Titration of these solutions by HCl, at pH, specific for each complex, results in the decrease of absorbance due to replacement of the narrow and intensive Q0-0 bands by the broad bands of aggregates
(influence of pH on spectra of MPcoc are exemplified with PtPcoc on Figure 2).
In water, ionized MPcoc owing to the electrostatic repulsion between molecules and better solubility of a carboxylate form exist as monomers, whereas unionized MPcoc have limited solubility and demonstrate the aggregation behavior. According to literature on the aggregation of Pc, two main aggregate species have been identified as J- and H-aggregates with J-aggregates marked by a red-shift in the monomer peak due to head-to-tail aggregation, while H-aggregates are characterized hy a hlue shift caused hy face-to-face aggregation.[1920] All aggregates of MPcoc under investigation in the Q region have broadened bands, blue shifted (Figure 2, Table 1) in relation to the Q band of the monomer, hence associated with the H-aggregates. As it was found in paper,[11] ZnPcoc at low concentrations in slightly acidic (pH 4-5) aqueous solutions forms dimers. At lower pH values and higher concentrations the multimeric forms were present almost exclusively. In this paper we did not study the association degree of MPcoc, however, for CoPcoc
A
max
0,7 n 0,6 0,5 0,4H 0,3 0,20,1 -0,0
A
r
-•1
-Л-А 2
—▲ ▲-A—
//
I
S
1-1-1-1-1-1-1-1-1-1-1-1-1-1-Г"
234567
8 9 10 PH
A
max
0,8
0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0
T-'-1-1-1-1-1-'-1-1-1-'-1-1-г
2345678
9 10 PH
Figure 1. Influence of pH on absorbance of MPcoc solutions at Q-band maximum, MPcoc (1maxabs, nm), A: 1 - PtPcoc (669), 2 - PdPcoc (680); B: 1 - AlPcoc (690), 2 - CoPcoc (678), 3 - ZnPcoc (698).
pH = 10
540
560
580
600
620
640
660
Wavelength, nm
Figure 2. Absorption spectral changes in the course of titration of PtPcoc in water with HCl in pH range 5.3 - 10.
3
two waves on the titration curve, basing on spectral data, may be tentatively associated with dimerization at pH 5.3 and multimerization at pH 3.4. The Pt, Pd and Al MPcoc complexes directly gave aggregates with strongly broadened Q bands, typical for mixture of multimers.
According to the shape of titration curves, compounds under investigation may be divided into two groups: first one consists of PtPcoc and PdPcoc - with sharp change in the absorbance at pH about 6.5 (Figure 1A), and the second one -of AlPcoc, CoPcoc and ZnPcoc - with titration curves mildly sloping at lower values of pH (Figure 1B), evidencing the higher solubility of these MPcoc and their ionized forms. It is noteworthy that the first group consists of complexes with metals, incapable of axial coordination, while complexes of the second group can coordinate axially,[23] in aqueous solutions - OH- and H2O species particularly. It is generally known also that presence of an axial ligand at the central metal atom hinders the sandwich orientation of phthalocyanine molecules during aggregation. Thus, observed differences of pH-aggregation behavior for two groups of MPcoc may be caused by their different solubility - under stepwise ionization conditions the less number of ionized groups on macrocycle periphery is needed for monomerization of coordinating AlPcoc, CoPcoc and ZnPcoc than in the case of PtPcoc and PdPcoc.
The effect of number of ionized carboxyl groups on macrocycle periphery upon aggregation of MPcoc in water has not been studied. However, it is known that for cationic aluminum and zinc phthalocyanines, correspondingly, about four and six charged groups per molecule are needed for monomerization.[24] Increase of pH of MPcoc solutions results in progressive dissociation of carboxyl groups and accumulation of negative charge on the periphery of molecule and, when electrostatic repulsion becomes sufficient, dye monomerization. Since in MPcoc the adjacent carboxyl groups exert the most efficient acceptor impact at each other, and by analogy with o-phthalic acid (pKa are 2.95 and 5.41),[25] it is reasonable to simplify multi-step ionization of carboxyl groups in MPcoc to two-step process, with pKa of first four carboxyl groups (one on each of four benzene rings of macrocycle) ionization standing far apart from those of other four ones. In this regard the differences in curves on Figure 1 can be tentatively explained as follows. Neutralization of the first four carboxylate groups in MPcoc octaanion evidently occurs at pH about 6. For Pt and Pd complexes this process is accompanied by abrupt aggregation
(Figure 1A). Compounds ZnPcoc and CoPcoc, being more soluble, have monomer-aggregate transition more sloping and shifted to lower pH. AlPcoc remains soluble till the other four carboxylates neutralization at pH 3.5-4, the apparent first step is visible as a wave in pH interval from 5 to 6 (Figure 1B). It should be mentioned that for this compound the titration curve may also be affected by deprotonation of the axial hydroxyl group.[26] Thus, it may be concluded that at pH below 3.5, unionized MPc(COOH)8, and at pH above 7 octacarboxylated MPc(COONa)8 form predominate. Intermediate tetraanion MPc(COOH)4(COONa)4 evidently prevails at pH about 5. In 3.5-4.5 and 5.5-6.5 pH intervals transitions MPc(COOH)8 - MPc(COOH)4(COONa)4 and MPc(COOH)4(COONa)4 - MPc(COONa)8 occur, correspondingly (Scheme 1).
To prove this statement, with ZnPcoc as an example we studied the effect of pH on its absorbance and fluorescence in the presence of surfactant (4 % Cremophor EL) for dis-aggregation. As it follows from solubility and monomerism of all differently ionized forms, the use of Cremophor EL solutions is advantageous. Figure 3 demonstrates changes of the absorbance at Q band maximum and fluorescence intensity during titration of ZnPcoc alkaline solution, containing Cremophor EL, by HCl. Within the pH range of 7-10, the absorption (1maxabs 690 nm) and fluorescence intensity (l fl 700 nm) of octaanion were nearly the same. Reducing
0,4 л
0,3-
<D о с го
-Q
О (Я
_Q <
0,2-
0,1 -
0,0
10
0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0
pH
Figure 3. Influence of pH on the absorbance (at 690 nm, ▼) and fluorescence intensity (at 700 nm, •) of ZnPcoc aqueous solutions, containing 4 % Cremophor EL.
PH3-4 •
HOOC COO Na
Scheme 1. Ionization of "MPcot
^ ; К^соон
J—M—N |T HOOC^^ i K4^4 COONa
MPc(COOH)4(COO -)4 Na 4
Na "OOC COO" Na
T y^xAcooNa*
Na OOC
N^nT-N
yj
Na* OOC COO Na~
MPc(COO" )8 Na+
the pH from 7 to 5.5 caused the significant changes - decrease in the absorption and fluorescence intensities to minimum at pH 5-5.5 along with a 5 nm red shift in their spectra were observed. Then, with further pH decrease to about 3, the restoration, complete for fluorescence and partial for absorption, took place. In solutions of pH < 3 both absorption and fluorescence were nearly unchanged. These observations are in agreement with apparent two-step ionization of ZnPcoc: ranges of pH> 7, 5-5.5 and < 3 may be ascribed to predominant existence of ZnPcoc as octaanion, tetraanion and neutral acid, correspondingly. Spectral changes during transitions ZnPc(COOH)8 - ZnPc(COOH)4(COONa)4 and ZnPc(COOH)4(COONa)4 - ZnPc(COONa)8 are presented in Figure 4. Isosbestic points were observed for each process, indicating protonation-deprotonation without the presence of side reactions such as aggregation or decomposition.
We believe that both above-mentioned transitions are also multi-step, but close and overlapping processes, not resolvable spectroscopically.
Dependences of fluorescence intensity for AlPcoc and absorption for non-fluorescing CoPcoc vs. pH in the presence of Cremophor EL were similar to those of ZnPcoc in Figure 3, evidencing minimum at pH 5. Corresponding studies of PtPcoc and PdPcoc were unsuccessful because these complexes failed monomerization by surfactant at acidic conditions. However, for all MPcoc we suppose the similar two-step ionization with predominant existence of octaanion, tetraanion and neutral acid at pH > 7, about 5 and < 4, respectively.
For ZnPcoc we have compared the spectral-fluorescent, photochemical and photosensitizing properties of three differently ionized forms (Table 2). It should be noted that photophysical and photochemical properties of this complex
in aqueous medium and in the presence of hovine serum albumin were studied by Nyokong et al.[21,27] We found that presence of Cremophor EL made no difference in their absorption and fluorescence maxima. Unionized acidic and intermediate tetraanionic forms of ZnPcoc were studied for the first time in this paper. It was found that the Q-band absorption maxima of highly symmetric acid and octaanion are almost the same (689 and 690 nm, correspondingly), while that of tetraanion is 5 nm red shifted. The most striking effect ionization exerted upon molar extinction coefficient, which was 1.86 and 1.3 times higher for ZnPcoc octaanion compared to that of tetraanion and acid (Tahle 2).
Fluorescence emission peaks were observed at: 696 nm for ZnPc(COOH)8, 705 nm for ZnPc(COOH)4(COONa)4 and 700 nm for ZnPc(COONa)8 in water-4 % Cremophor EL (Table 2). Quantum yields of fluorescence (Ffl for ZnPc(COOH)8 and ZnPc(COOH)4(COONa)4 were estimated relatively to that of octaanion (F®=0.18 at pH 10,[21] similar value can he assumed for ZnPcoc octaanion in the presence of Cremophor EL). It was found that F for the three differently ionized forms of ZnPcoc were, however, not significantly (within experimental error of 10 %), affected by ionization (Table 2). Hence, the distinct minimum on plot of fluorescence intensity vs. pH (Figure 3) is caused not hy possihly lower F of tetraanion, but by decrease of incident light ahsorption.
Singlet oxygen quantum yields (FA) of differently ionized ZnPcoc forms are given in Tahle 2. The selective excitation of differently ionized ZnPcoc in the Q ahsorption band resulted in disappearance of DPBF (example for ZnPcoc acid is given in Figure 5). The DPBF absorption at 416 nm was monitored and the decrease in ahsorhance was
.Q
A
580 600 620 640 660 680 700 720
Wavelength, nm
I 1 1 1 I ■ ' 1 I 1 1 1 I ■ ' ' I
580 600 620 640 660 680 700 720
Wavelength, nm
Figure 4. Spectral changes during transitions ZnP^COOH^COONa), - ZnPc(COONa)8 (a) and ZnPc(COOH)8 - ZnPc(COOH)4(COONa)4 (b).
Table 2. Photophysicochemical properties of ZnPcoc differently ionized forms.
ZnPcoc l abs, nm max e l fl, nm max Фл Ф p
Acid (pH 2.8) 689 175000 696 0.20 0.55 0.5T0"6
Tetraanion (pH 5) 695 120000 705 0.19 0.59 1.110-5
Octaanion (pH 7.4) 690 224000 700 0.18 (pH 10)[23] 0.57 (PBS, pH 7.4)[24] 1.010-4
—i—1 450
—i—i—|—.—i—i—i—|
Wavelength, nm
350
400
500
550
600
650
Figure 5. The change of absorption spectrum upon irradiation time for air saturated solution of ZnPcoc acid (pH = 2.8) in the presence of DPBF and 4 % Cremophor EL. Excitation at Q band with filtered light of xenon lamp (150 W).
compared with that of standard under identical conditions. Since ®A value for ZnPcoc octaanion was reported previously (0.57),[22] this form was used as a standard for ®A of ZnPcoc acid and tetraanion determination. No photobleaching of the complexes was observed during determination of the singlet oxygen quantum yields, since these processes occur much slower than singlet oxygen production. The singlet oxygen quantum yields are found to be 0.55 and 0.59 for ZnPcoc acid and tetraanion, correspondingly. Hence, each of ZnPcoc three differently ionized forms generates 'O2 efficiently, with the same ®A considering experimental error.
Photobleaching of MPcoc is characterized by progressive disappearance of the spectra due to the degradation of the phthalocyanine ring. In order to determine the effect of the ionization on photostability photobleaching studies were undertaken for ZnPcoc in water in the presence of 4 % Cremophor EL at pH 7, 5 and 2.8. The largest quantum yield, Fp = 1.0T0-4, was observed for the octaanion ZnPc(COONa)8. It does not differ much from value 2.2T0-4, reported by us previously for this compound without Cremophor EL.[28] The photobleaching depends substantially on ionization degree of ZnPcoc - octaanion bleaches around hundred-fold faster as compared to neutral form, accordingly with our previous finding of electrophilic singlet oxygen contribution to ZnPcoc photodecomposition.[28]
Conclusions
induced by change of pH, was provided by study of ZnPcoc and AlPcoc in aqueous Cremophor EL solution. One from two carboxyl groups of each benzene ring of macrocycle is protonated-deprotonated within 5-6, and another - within 3.5-4 pH ranges. It was established that at pH below 3 insoluble unionized MPc(COOH)8, and at pH above 6.5 fully deprotonated MPc(COONa)8 forms predominate while intermediate tetraanion form MPc(COOH)4(COONa)4 prevails at pH about 5.
The photophysicochemical properties of three differently ionized forms of ZnPcoc were compared in Cremophor EL solution. The Q-band absorption maxima of highly symmetric acid and octaanion are almost the same (689 and 690 nm) while that of tetraanion is 5 nm red shifted. The most striking effect ionization exerted upon molar extinction coefficient, which was 1.85 and 1.3 times higher for ZnPcoc octaanion than those of tetraanion and acid. It was found that all differently ionized forms of ZnPcoc have nearly the same quantum yields of fluorescence (0.18-0.20) and singlet oxygen generation (0.55-0.59), while hundred-fold difference in photobleaching quantum yields was observed accordingly with electrophilic singlet oxygen contribution to its photodecomposition.
Acknowledgements. The reported study was funded by RFBR according to the research project Nos 16-03-00298 and 16-29-10674.
Effect of pH upon aggregation of MPcoc (where M=Al, Zn, Pt, Pd, Co) in aqueous media has been studied using absorption and fluorescence spectroscopy. The results show that the solubility and aggregation of MPcoc in aqueous solutions are affected by ionization of peripheral carboxyl groups but also by axial ligation of central metal atom. For Pt and Pd complexes pH decrease results in abrupt aggregation at pH about 6, while for capable to axial ligation ZnPcoc, AlPcoc and CoPcoc it is more sloping.
For the first time the evidence for the multi-step, notably pseudo two-step, process for MPcoc ionization,
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Received 21.05.2016 Accepted 22.06.2016
