ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2009, Vol. 83, No. 6, pp. 1044–1050. © Pleiades Publishing, Ltd., 2009. Original Russian Text © D.A. Makarov, N.A. Kuznetsova, O.A. Yuzhakova, L.P. Savvina, O.L. Kaliya, E.A. Lukyanets, V.M. Negrimovskii, M.G. Strakhovskaya, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 6, pp. 1183–1190.
PHOTOCHEMISTRY AND MAGNETOCHEMISTRY
Effects of the Degree of Substitution on the Physicochemical Properties and Photodynamic Activity of Zinc and Aluminum Phthalocyanine Polycations D. A. Makarova, N. A. Kuznetsovaa, O. A. Yuzhakovaa, L. P. Savvinaa, O. L. Kaliyaa, E. A. Lukyanetsa, V. M. Negrimovskiia, and M. G. Strakhovskayab a Research
Institute of Organic Intermediates and Dyes, Moscow, Russia of Biology, Moscow State University, Moscow, Russia e-mail:
[email protected];
[email protected]
b Faculty
Received June 5, 2008
Abstract—A series of zinc and aluminum phthalocyanines containing 3–8 pyridiniomethyl or cholinyl substituents on average were synthesized. As the number of cation substituents increased, in aqueous solutions, the aggregation ability of phthalocyanines decreased, while the quantum yields of fluorescence and singlet oxygen generation increased. The photodynamic inactivation of coliform bacteria sensitized by zinc and aluminum phthalocyanine polycations with an increase in the substitution degree became more effective. DOI: 10.1134/S0036024409060326
INTRODUCTION Phthalocyanines possessing unique physicochemical properties have found use not only as dyes [1], but also as modern functional materials for nonlinear optics [2], gas sensors [3], catalysis [4, 5], etc. One of the most actively developed applications of phthalocyanines and their metal complexes (MPc) is photodynamic therapy (PDT) of oncologic and other diseases [6, 7]. In PDT, a photosensitizer is accumulated in pathological tissue and, after irradiation with visible light, activates the oxygen present in the medium, and reactive oxygen species oxidize cell biomolecules and lead to the loss of the vital functions of the cell [8]. Zinc and aluminum phthalocyanines are very popular sensitizers for PDT. These are type II sensitizers, which form cytotoxic singlet oxygen (1é2) [8] according to the scheme MPc
hν
3MPc* 1O 2
1MPc*
+ 3O2
+ biomolecule
isc
3MPc*,
MPc + 1O2, oxidation products,
where 1MPc* is phthalocyanine in the singlet excited state, 3MPc* is phthalocyanine in the triplet excited state, and isc is interconversion from the singlet to triplet state. Zinc and aluminum phthalocyanines in the monomer form have fairly high quantum yields of the generation of both triplet excited states and singlet oxygen, which is responsible for their high phototoxic effect [8]. In aqueous media, however, phthalocyanines can form aggregates that are inactive in photochemical pro-
cesses. In aggregates, the excited states are quickly deactivated in nonradiative processes, and the quantum yield of the generation of singlet oxygen decreases. Since water is a biological medium, it is important to seek sensitizers based on phthalocyanines that remain monomeric in aqueous solutions. These compounds are also of interest for the photodynamic inactivation of pathogenic microorganisms for therapeutic and water sterilization purposes [9–11], especially in view of the emergence of polyresistant strains. With bacteria resistant to antibiotics, the photodynamic method is no less effective [12]. Using electrostatic repulsion forces between likely charged molecules is one of the methods to reduce the aggregation of phthalocyanines in aqueous media. This is achieved by introducing ionogen substituents in the macroring. The introduction of eight anion [13, 14] or eight cation [15, 16] substituents led to complete monomerization of zinc phthalocyanines, which were more liable to aggregation than aluminum phthalocyanines, in water. At the same time, four charged substituents [14, 17–19] suppressed aggregation only partially. Studies of phthalocyanines with different solubilizing substituents showed that only zinc and aluminum phthalocyanines having positively charged substituents in the macroring could effectively sensitize inactivation of various pathogenic microorganisms [16, 19, 20]. Antimicrobial sensitizers from this class show high activity against not only gram-positive but also gramnegative bacteria, which are generally stable against noncation sensitizers [19, 21]. Previously [15], we studied several aspects of the effect of the nature of positively charged substituents
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on the physicochemical and photosensitizing properties of zinc and aluminum phthalocyanine octa- and hexadecacations. Zinc octa(pyridiniomethyl)- and octa(cholinyl)phthalocyanines proved most effective and promising from the viewpoint of practical applications among the compounds studied. As a continuation of our work in this direction, we studied the influence of the number of positively charged groups in the macroring of zinc and aluminum phthalocyanines on the state of these complexes in aqueous solution and their spectral-luminescent properties, efficiency of singlet oxygen generation, and photoinduced antimicrobial activity. For this purpose, we synthesized several phthalocyanine cations, which were mixtures of complexes with different number of substituents and had the average degree of substitution from 3 to 8,
A solution of phthalocyanine (5 × 10–6 mol/l) containing 3 × 10–5 mol/l of BPAA was irradiated in a standard 1 cm path length cell. A DKSSh-150 lamp was used as a source of light. Phthalocyanine was excited into the long-wavelength absorption band Q through a ZhS-18 glass light filter and an interference filter with transmission 680 ± 25 nm. The intensity of light fluxes was determined using a Spectra Physics 404 power meter. The fraction of light absorbed by the sample was calculated by integrating the overlapping of the transmission spectra of light filters and the absorption spectrum of phthalocyanine. Photosensitized oxidation of BPAA was monitored spectrophotometrically by measuring a decrease in absorption at a 393 nm maximum. To calculate Φ∆, we used the equation Φ ∆ = Φ ∆ W I abs /W I abs , ref
N M
N
N
N
R=
R=
+
N
N +
OH
Cln–
N
(CH2R)n
N N
(MPcPymn),
(MPcCholn),
M = Zn, AlCl.
EXPERIMENTAL The electronic absorption spectra were measured on a Hewlett-Packard 8453 spectrophotometer. The fluorescence spectra were recorded on a Hitachi-850 spectrofluorimeter with a xenon lamp as a source of excitation. The quantum yields of fluorescence (Φfl) of phthalocyanine polycations in aqueous solutions were measured relative to a solution of unsubstituted zinc phthalocyanine in DMSO used as a reference (Φfl = 0.20 [22]) with the same excitation intensity at λexc. This gave the spectra normalized to the fluorescence intensity of the reference. The quantum yields of singlet oxygen (Φ∆) were determined relative to zinc octa(cholinyl)phthalocyanine (ZnPcChol8) as a reference, whose Φ∆ in aqueous solution is 0.65 [15]. An acceptor of singlet oxygen was 9,10-bis-(4-(trimethylammonio)phenyl)anthracene dihydrochloride (BPAA) obtained as described in [23]. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
ref
ref
where Φ ∆ is the quantum yield of singlet oxygen generation by the reference; W and Wref are the rates of BPAA consumption during sensitization by phthalocyref anine and the reference; and Iabs and I abs are the numbers of light quanta absorbed by the sensitizer and reference, respectively. The initial concentration of BPAA was constant. To determine the efficiency of the photodynamic inactivation of gram-negative bacteria in the presence of phthalocyanine cations, we used a bioluminescent bacterial test system based on the gene-engineered strain Escherichia coli pXen7 [24]. The intensity of bioluminescence was measured on a Biotox-6 luminometer (Moscow). Phthalocyanine solutions (20 µl) in distilled water were added to a rehydrated sample of bacteria (1 ml). For samples with phthalocyanines, the incubation time was 5 min. The samples were irradiated at 20–22°ë using an EKOMP source of cold white light. The intensity of radiation (λ ≥ 400 nm) at the sample level was 17 mW/cm2. Each experimental value was obtained by averaging over five experiments. The error of the method was up to 10%. The photobactericidal activity of phthalocyanines was studied on coliform bacteria of water from the Moskva river, which was the model of sewage. The test phthalocyanine polycations with a concentration of 1.5 × 10–6 mol/l were introduced in water and incubated for 1 h, where after the solution was irradiated in a glass reactor with a water cooling system. A 500 W OSRAM halogen lamp was the source of light placed at a distance of 15 cm from the reactor in a projector. The intensity of incident light was 40 mW/cm2. The sample was irradiated for 30 min while continuously bubbling air through the solution. The content of the total coliform bacteria (TCB) measured as the number of colony-forming units (CFUs) per 100 ml of water was determined before and after treatment by membrane filtration and growing the inoculation on an Endo medium; after that, the number of colonies was calculated [25]. ref
N
1045
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2 3 4 5
0.4
0 550
600
650
700
750 λ, nm
Fig. 1. Electronic absorption spectra of ZnPcPymn in water for n = (1) 8, (2) 6.5, (3) 5.5, (4) 5, and (5) 4; concentration 5 × 10–6 mol/l.
The Synthesis of Poly(chloromethyl)-Substituted Phthalocyanines As in [15], triethylamine (4 ml) and symm-dichlorodimethyl ether (6 ml) were added by drops to anhydrous aluminum chloride (10 g), while the temperature was kept below 80°ë. The zinc [26] or chloroaluminum [27] phthalocyanine (0.5 g) was added to the reaction mixture. The mixture was stirred for 0.5–3 h at 90– 95°ë. During the reaction, samples were periodically taken for preliminary evaluation of the degree of substitution. For this purpose, a sample was treated with water, washed with methanol (zinc complexes) or aqueous methanol (aluminum complexes), and dried. The long-wavelength absorption maximum was determined in dimethylformamide. The introduction of chloromethyl groups led to a substantial bathochromic shift of the Q band. The shift was proportional to the number of substituents, ~1.8 nm for zinc phthalocyanines and ~2.5 nm for aluminum phthalocyanines per group. When the expected degree of substitution was achieved, the hot reaction mixture was poured into hydrochloric acid with ice. The residue was filtered off, washed with cooled water and methanol (for zinc complexes) or aqueous methanol (for aluminum complexes), and dried. This procedure gave poly(chloromethyl) substituted phthalocyanines in 90–95% yields. The degree of substitution was ultimately assessed from the content of chlorine determined by elemental analysis. By varying the duration of chloromethylation, we were able to obtain a series of substituted products, in which the average number of chloromethyl and cationic (after interactions with amines) substituents varied from 3 to 8. For zinc complexes with the degrees of substitution 0, 4, 5, 5.5, 6.5, 7, and 8, the long-wavelength absorption maxima in DMF were at 670, 676, 678, 679, 681, 682, and 683 nm, respectively. For chloroaluminum complexes with the degrees of substitution 0, 3, 4.5, 6.5, and 8, the long-wavelength absorption
maxima in DMF were at 672, 678, 681, 687, and 690 nm, respectively. Synthesis of Poly(pyridiniomethyl) Substituted Zinc Phthalocyanines Poly(chloromethyl) substituted zinc phthalocyanine (0.10 g) was heated at 90–100°ë in excess pyridine for 1 h. The precipitate was filtered off, reprecipitated from methanol with acetone, washed with hot acetone, and dried at 85–90°ë in a vacuum. The product was obtained in 80–90% yields. Synthesis of Poly(cholinyl) Substituted Phthalocyanines Poly(chloromethyl) substituted zinc or chloroaluminum phthalocyanine (0.50 g) was heated in a mixture of dimethylformamide (1 ml) and 2-dimethylaminoethanol (1 ml) for 1–2 h at 100°ë. The precipitate was purified as described above for pyridiniomethyl substituted complexes to obtain the product in 75–85% yields. RESULTS AND DISCUSSION Electronic Absorption Spectra In the region 550–750 nm (Q band), the electronic absorption spectra of the ZnPcPymn, ZnPcCholn, and ClAlPcCholn complexes (Figs. 1–3, respectively) in water have bands with maxima at ~680 and 630– 640 nm. We see that, for all groups of complexes, an increase in the degree of substitution is accompanied by a sequential decrease in the intensity of the short-wavelength band and an increase in the intensity of the longwavelength band. For each series of compounds, an isobestic point (at ~655 nm) in the spectra of isomolar solutions is indicative of the presence of two forms of molecules, whose ratio depends on the number of substituents. The
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1
0.8
2
1047
3 4 0.4
0 550
600
650
700
750 λ, nm
Fig. 2. Electronic absorption spectra of ZnPcëholn in water for n = (1) 8 and 7, (2) 5.5, (3) 5, and (4) 4; concentration 5 × 10−6 mol/l.
A
1
0.8
2 3
0.4
0 550
600
650
700
750 λ, nm
Fig. 3. Electronic absorption spectra of ClAlPcCholn in water for n = (1) 6.5 and 8, (2) 4.5, and (3) 3; concentration 5 × 10−6 mol/l.
absorption band of substituted phthalocyanines in water at 630–640 nm corresponds to contact dimers [18, 28]. In the spectra of ZnPcPym8, ZnPcCholn (n = 7 and 8), and ClAlPcCholn (n = 6.5 and 8), there is no additional absorption at 630–640 nm; the narrow and intense Q band leads us to suggest that these complexes exist as monomers in solution. When the degree of substitution n decreases, the equilibrium shifts toward the formation of dimers. The data shown in Fig. 1 indicate that, in the series ZnPcPymn, the average number of charged substituents n = 6.5 is not sufficient for complete monomerization. In the series ZnPcCholn, the presence of dimers in aqueous solutions is distinguished in the spectra for compounds with n ≤ 5.5. The absorption spectra of ClAlPcCholn complexes with n = 6.5 and 8 are similar, but an aqueous solution of ClAlPcChol4.5 already contains a certain amount of dimers (Fig. 3), which increases and becomes substantial for ClAlPcChol3. Thus, over the range from n = 3 to 8, the degree of dimerization of phthalocyanines depends substantially on the number of cation substituents in the molecule, which determine the efficiency of electrostatic repulsion. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Table 1 lists the absorption maxima of the monomers and the corresponding molar extinction coefficients εmax for aqueous solutions of the compounds. It can be seen that the coefficients εmax depend strongly on the degree of substitution n. For zinc complexes, the Q band is not shifted when the average number n increases from 4 to 8 (Table 1). For ClAlPcCholn, however, the maximum of this band shows a tendency toward a slight (~4 nm) bathochromic shift when n increases from 3 to 8. For solutions containing only monomers and dimers, ε λ = αε λ + ( 1 – α )ε λ .
(1)
α = ( ε λ – ε λ )/ ( ε λ – ε λ ),
(2)
d
m
It follows that m
m
d
where ελ, ε λ , and ε λ are the molar extinction coefficients at a wavelength λ for substituted phthalocyanine and its monomer and dimer forms (per phthalocyanine molecule in the latter case), respectively; and α is the degree of dimerization (the fraction of dye molecules in dimers).
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Table 1. Luminescence spectral characteristics, degree of dimerization (α), and quantum yields (Φ∆) of singlet oxygen for polycationic zinc and aluminum phthalocyanines in water (c = 5 × 10–6 mol/l) No.
Compound
λmax
εmax
α
λ max
fl
Φfl
Φ∆
1 2 3 4 5 6 7 8 9 10 11 12 13 14
ZnPcPym4 ZnPcPym5 ZnPcPym5.5 ZnPcPym6.5 ZnPcPym8 ZnPcChol4 ZnPcChol5 ZnPcChol5.5 ZnPcChol7 ZnPcChol8 ClAlPcChol3 ClAlPcChol4.5 ClAlPcChol6.5 ClAlPcChol8
677 677 677 677 677 677 680 680 680 680 680 681 682 684
70000 90000 110000 170000 180000 90000 130000 140000 190000 190000 80000 150000 185000 190000
0.76 0.66 0.47 0.13 0 0.60 0.35 0.27 0 0 0.70 0.20 0 0
688 688 689 690 692 690 690 692 693 694 688 689 691 692
0.05 0.07 0.09 0.12 0.12 0.08 0.08 0.09 0.10 0.11 0.17 0.12 0.17 0.18
0.10 0.13 0.19 0.38 0.45 [15] 0.25 0.38 0.46 0.60 0.65 [15] 0.15 0.23 0.29 0.38 [15]
Using (2) for complexes at a concentration of 5 × mol/l, we evaluated the degree of their dimerizad tion α in water. The ε λ coefficient at a maximum λmax of the monomer was set equal to 30000, taking into account the data of [16]. For calculations, we used the optical densities at the maximum of the monomer, at which the error in the extinction of the dimer is minimized. The resulting α values are listed in Table 1; the dependence of the degree of dimerization on the number of positively charged substituents is shown in Fig. 4.
10–6
The data of Fig. 4 suggest that the degree of dimerization changes over a narrow range of n from 5 to 7 for α
ZnPcPymn, from 4 to 6 for ZnPcCholn, and from 4 to 3 for ClAlPcCholn, while α approaches 1 for n < 5, 4, and 3 for the series of compounds, respectively. Moreover, from the data of Table 1 and Fig. 4, it is evident that zinc phthalocyanines from the ZnPcPymn series are more aggregated than the corresponding ZnPcCholn with the same degree of substitution. Thus, when n decreased from its maximum value (n = 8), α increased much more rapidly for ZnPcPymn than for ZnPcCholn. The oxyalkyl substituents at the quaternary nitrogen atom are evidently more hydrophilic, create greater steric hindrances than pyridiniomethyl groups, and thus hinder aggregation. Aluminum phthalocyanines ClAlPcCholn are less aggregated than zinc complexes in aqueous solution; this was attributed to the presence of an axial ligand at the aluminum atom, which hinders the sandwich orientation of molecules during aggregation [18].
0.9
Fluorescence Spectra
1 2
0.6
3
0.3
0
2
4
6
8 n
Fig. 4. Dependences of the degree of dimerization α of (1) ZnPcPymn, (2) ZnPcCholn, and (3) ClAlPcCholn in water on n; concentration 5 × 10–6 mol/l.
As is known, zinc and aluminum phthalocyanines possess fluorescence [29, 30]. We found that their polycation derivatives also fluoresced in aqueous solution. fl The quantum yields (Φfl) and maxima ( λ max ) of fluorescence are presented in Table 1. As the degree of substitution increased in all series, the fluorescence spectra experienced an insignificant (~4 nm) bathochromic shift (Table 1), while the spectrum shape did not markedly change. The Stokes shift was ~10 nm, which is typical of phthalocyanines [31]. In the absence of pronounced dimerization for cationic zinc complexes, Φfl was 0.10–0.12, which corresponded to the known values for other substituted zinc phthalocyanines [29, 30]. The difference between Φfl = 0.12 for ZnPcPym8 and
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Φfl = 0.10 for ZnPcChol8 did not exceed measurement errors. For both series, the efficiency of fluorescence in water decreased as the degree of substitution lowered because of the shift of equilibrium toward dimer formation (Table 1). Under the conditions of our experiments, the dependence of Φfl on dimerization was more pronounced for ZnPcPymn. All cationic aluminum phthalocyanines in aqueous solution had Φfl ≅ 0.17–0.22 irrespective of the degree of dimerization (Table 1). Thus, the dependence of the efficiency of fluorescence on the degree of substitution is most pronounced for ZnPcPymn, less pronounced for ZnPcCholn, and was not observed for ClAlPcCholn, which correlated with the degree of their dimerization in solution.
1/D37, Òm2/J 5
4
3 ZnPcChol4 ZnPcChol8 2 0
1
2
3 c × 106, mol/l
Fig. 5. Dependences of the efficiency of photoinduced quenching of bioluminescence of E. coli pXen7 on dye concentration in an incubation medium.
Efficiency of the Generation of Singlet Oxygen The quantum yields of the generation of singlet oxygen Φ∆ quantitatively characterize the photodynamic activity of sensitizers by the type II mechanism. For the compounds under study, Φ∆ were determined in aqueous media (Table 1). Of all metal complexes studied, the ZnPcChol8 and ZnPcChol7 monomers in aqueous solution have the largest values of Φ∆, which differ within the experimental error (0.65 and 0.60, respectively). As expected because dimers cannot generate singlet oxygen, Φ∆ decreased as the degree of substitution in ZnPcCholn lowered and the degree of dimerization grew (Table 1). According to the data of Table 1, similar dependences were observed for ZnPcPymn and ClAlPcCholn. The quantum yield of ZnPcPymn was slightly lower than that of ZnPcCholn with the same degree of substitution in agreement with the data of [15] for the corresponding octasubstituted derivatives. The lower quantum yields Φ∆ of aluminum phthalocyanines compared with those of zinc phthalocyanines (Table 1) also agree with the known dependence of Φ∆ of phthalocyanines on the nature of the central metal atom in the macroring [15, 32, 33]. Antimicrobial Photodynamic Activity The photodynamic efficiency of sensitizers with respect to microorganisms of different types depends on many factors, including their ability to interact with bacterial shell, penetrate through it, and generate active forms of oxygen. We studied the efficiency of polycationic zinc and aluminum phthalocyanines in photoinactivation of bacteria on the gene-engineered bioluminescent E. coli pXen7 strain, which was used as a basis of a bacterial test system, and on wild strains of coliform bacteria (TCB factor), which are classic indicator microorganisms in evaluation of bacterial contamination of water. Figure 5 shows the characteristic dependences of the efficiency of photoinduced quenching of bioluminescence of the E. coli pXen7 strain on dye concentration in an incubation medium for zinc tetra- and octa(choliRUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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nyl)phthalocyanines. As shown in [24], the quenching of bioluminescence correlates with a decrease in the quantity of colony-forming units (CFUs) in a test culture. The parameter 1/D37 (the reciprocal of the dose of light that causes 37% quenching) reflects the efficiency of the photodynamic inactivation of bacteria or the photobactericidal activity. It follows from the results of our study that ZnPcChol8, which has a 2.6 times higher quantum yield of singlet oxygen generation Φ∆ than ZnPcChol4, has a higher photobactericidal activity at concentrations higher than 1 × 10–7 mol/l. At 1 × 10–7 mol/l, the difference is not reliable. Similar data were obtained for the other series of dyes. According to Fig. 5, the concentration dependences of the photobactericidal activity of polycation phthalocyanines have a maximum at 1.5 × 10–6 mol/l. At higher concentrations, photobactericidal activity decreased probably because of the adverse dye aggregation effects on the surface of bacterial cells. Table 2 lists the CFU factors of sewage before and after photodynamic treatment with polycation zinc and aluminum phthalocyanines at an optimum concentration of 1.5 × 10−6 mol/l. The photodisinfecting effect decreased as the degree of substitution lowered in all series of compounds. Thus, at high degrees of substitution (n = 6.5 and 8 for ZnPcPymn, n = 5.5–8 for ZnPcCholn, and n = 6.5 for ClAlPcCholn), coliform bacteria were completely killed in model basin water. At lower degrees of substitution, the effect was also significant, but water still contained viable coliform bacteria. This behavior is primarily attributed to a decrease in the quantum yields Φ∆ of generation of cytotoxic singlet oxygen by aggregated sensitizers with low degrees of substitution. To summarize, we synthesized series of polycationic zinc and aluminum phthalocyanines containing three to eight pyridiniomethyl or cholinyl substituents in the macroring. In aqueous solution, a decrease in the degree of substitution led to dimerization (aggregation)
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Table 2. Contents of the total coliform bacteria (TCB, CFU/100 ml) in sewage (I) before and (II) after photodynamic treatment with polycationic zinc and aluminum phthalocyanines Compound
I
II
1 2 3 4 5 6 7
625 625 625 625 625 725 725
8 10 3 0 0 28 15
Compound
I
II
8 9 10 11 12 13
725 725 725 1500 1500 1500
0 0 0 40 18 0
of phthalocyanines and generally to a decrease in the efficiency of fluorescence and quantum yields of singlet oxygen generation. The photodynamic inactivation of coliform bacteria by polycationic zinc and aluminum phthalocyanines in sewage decreased with the degree of substitution as a consequence of a decrease in the efficiency of cytotoxic singlet oxygen generation by the aggregated sensitizer. ACKNOWLEDGMENTS This work was financially supported by the Government of Moscow and the Russian Foundation for Basic Research (project no. 07-03-00191). REFERENCES 1. The Phthalocyanines, Ed. by F. H. Moser (CRC Press, Thomas, Boca Raton, Fla, 1983), Vols. 1, 2. 2. G. Torre, P. Vazquez, and F. Agullo-Lopez, Chem. Rev. 104, 3723 (2004). 3. S. Dogo, J. P. Germain, C. Maleysson, et al., Thin Solid Films 219, 251 (1992). 4. O. L. Kaliya, E. A. Lukyanets, and G. N. Vorozhtsov, J. Porphyrins Phthalocyanines 3, 592 (1999). 5. D. Wohrle, O. Suvorova, R. Gerdes, et al., J. Porphyrins Phthalocyanines 8, 1020 (2004). 6. E. A. Lukyanets, Ros. Khim. Zh., No. 5, 9 (1998). 7. R. Ackroyd, C. Kelty, and N. Brown, Photochem. Photobiol. 74, 656 (2001). 8. N. A. Kuznetsova and O. L. Kaliya, Ros. Khim. Zh., No. 5, 36 (1998).
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