Influence of central versus peripheral coordination of Pt and Pd atoms on physical-chemical properties of octa-4,5-carboxyphthalocyanines Текст научной статьи по специальности «Химические науки»

Phthalocyanines Фталоцианины

Макрогэтэроцмклы

http://macroheterocycles.isuct.ru

Paper Статья

DOI: 10.6060/mhc150353k

Influence of Central versus Peripheral Coordination of Pt and Pd Atoms on Physical-Chemical Properties of Octa-4r5-carboxyphthalocyanines

Nina A. Kuznetsova,@ Roman A. Bulgakov, Ludmila I. Solovyova, Ekaterina N. Shevchenko, Oleg L. Kaliya, and Evgeny A. Lukyanets

Organic Intermediates and Dyes Institute, 123995 Moscow, Russia Corresponding author E-mail: [email protected]

New Pt11 and Pd11 complexes of2,3,9,10,16,17,23,24-octacarboxyphthalocyanine as well as covalent tetraconjugates of diaqua palladium(II) and octacarboxy substituted zinc phthalocyanine were synthesized and characterized by elemental analysis, UV-Vis and IR spectroscopy. Their properties (solubility, absorption and luminescence, singlet oxygen quantum yields and photostability) are herein reported. It was found that introduction of Pd or Pt atom to the phthalocyanine core has a profound effect, while modification of the periphery with four fragments of diaqua platinum or diaqua palladium has only a limited effect on photochemical and photophysical properties of metallophthalocyanines. These data demonstrate the higher sensitivity of the phthalocyanine electronic structure, their singlet and triplet excited states and the probability of transitions between them to modification of the core than of the periphery.

Keywords: Octacarboxyphthalocyanine, platinum, palladium, complex, covalent conjugate, photophysics, photochemistry.

Сравнение влияния центральной и периферической координации атомов Pt и Pd на физико-химические свойства окта-4,5-карбоксифталоцианинов

Н. А. Кузнецова,@ Р. А. Булгаков, Л. И. Соловьева, Е. Н. Шевченко, О. Л. Калия, Е. А. Лукьянец

Федеральное государственное унитарное предприятие «Научно-исследовательский институт органических полупродуктов и красителей» (ФГУП ГНЦ «НИОПИК»), 123995 Москва, Россия @Е-таИ: [email protected]

Впервые синтезированы и охарактеризованы данными элементного анализа, электронной, ИК-спектроскопии октакарбоксифталоцианины платины и палладия, а также тетраконъюгат октакарбоксифталоцианина цинка с диаквапалладием. Изучены их спектрально-люминесцентные и фотохимические свойства. Внедрение одного атома Pd или Рг в центр оказывает значительно большее влияние на электронные спектры поглощения, флуоресценцию, генерацию синглетного кислорода и фотостабильность фталоцианина по сравнению с внедрением четырех таких атомов по периферии в виде ковалентно присоединенных фрагментов диакваплатины и диаквапалладия. Полученные результаты демонстрируют большую чувствительность электронной структуры макроцикла, нижних возбужденных синглетного и триплетного состояний металлофталоцианинов и вероятности переходов между ними к модификации центра, чем периферии.

Ключевые слова: Октакарбоксифталоцианин, платина, палладий, комплекс, ковалентный конъюгат, фотофизика, фотохимия.

Central and Peripheral Coordination of Pt and Pd Atoms by Carboxyphthalocyanines Introduction Experimental

Phthalocyanines (Pc) are knownto have wide application in various areas - as pigments and dyes,[1] non-linear optical materials,121 gas sensors,[3] catalysts and photocatalysts[4] and as photosensitizers for photodynamic therapy.[56] The structure of phthalocyanines can be modified in several ways to adjust their valuable properties. Various substituents can be introduced into the benzene rings (electrophilic substitution, aza-substitution, condensation with the formation of additional rings), while the core can have the central atom or its axial ligands replaced by other ones. The most important of the forementioned structural factors influencing the catalytic and photocatalytic properties is the nature of the complexed metal atom. Notably, some recently developed novel catalysts and photocatalysts are based on bimetallic porphyrins, Pc and their analogs that include a metal ion not only in their core but in the periphery, which is achieved through coordination with the substituents. [7-11] In particular, we had earlier obtained[1213] a number of bimetallic covalent conjugates of Pt11 with phthalocyanines that present interest for the development of novel anticancer double action agents that combine photodynamic and catalytic[12] properties of phthalocyanines with the cytotoxicity of platinum.

The current study describes and compares the influence of Pt11 and Pd11 coordination with the Pc core and periphery on the physico-chemical and photochemical properties of the corresponding complexes. In order to achieve the goal of the study, novel compounds were synthesized - platinum and palladium octacarboxyphthalocy-anines (PtPcoc and PdPcoc), their properties were compared to the ones of the primary compound ZnPcoc, which exerts photodynamic properties, and to covalent peripheral conjugates of ZnPcoc with diaqua platinum and diaqua palladium (ZnPcocPt4(H2O)8 and ZnPcocPd4(H2O)8). The compound ZnPcocPd4(H2O)8 was synthesized in the course of the current study as novel.

The studied phthalocyanine complexes are presented in Figure 1.

Syntheses

In the current work we used dimethyl sulfoxide (chem.pur., from Spectr-Khim LLC), 1,3-diphenylisobenzofuran and 1,4-diazabicyclo[2.2.2]octane (DABCO) from Fluka, and unsubstituted zinc phthalocyanine from Organic Intermediates and Dyes Institute.

Octasodium salt of zinc 2,3,9,10,16,17,23,24-octacarboxy-phthalocyanine (ZnPcocNa8) and the covalent tetraconjugate of zinc phthalocyanine with diaqua platinum (ZnPcocPt4(H2O)8) were synthesized by known procedures.[1314]

Covalent tetraconjugate of zinc phthalocyanine with diaqua palladium (ZnPcocPd4(H2O)^. The compound was synthesized in an analogous manner to the published procedure[13] for the diaqua platinum conjugate. To a solution of ZnPcocNa8 (30 mg, 0.02714 mmol) in 3 ml of H20 was added K2PdCl4, which had been obtained immediately before the synthesis from 19.26 mg (0.1086 mmol) PdCl2 and 16.19 mg (0.272 mmol) KCl in 3 ml of H20. The reaction mixture was left for 3 hrs at a temperature of about 55 °C. The product was precipitated with a mixture of ethanol and methanol (1:2). The formed precipitate was filtered, washed with a mixture of H2O:MeOH:EtOH = 1:1:2 and dried in vacuo at 60 °C. Yield - 27.5 mg (68 %). Calculated for C40H24N8O24Pd4Zn, %: C 32.21; H 1.62; N 7.51. Found, %: C 33.05; H 2.43; N 7.65. UV-Vis (DMSO) I

v ' max

nm: 351, 689. IR (KBr) v cm-1: 1358, 1447, 1565, 1588, 1620,

max

1693.

Platinum 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine (PtPcoc). A mixture of 0.300 g of 4,5-bis(ethoxycarbonyl) phthalonitrile[14] and 0.090 g of platinum dichloride was heated at 175-180 °C for 1.5 h, the melt was then cooled, treated with 40 ml of ethanol, filtered and then treated with ethanol once again. The crude product was air-dried on the filter, dissolved in a mixture of chloroform with ethanol (10:1 v/v) and passed through a bed of silica gel. The solvent was removed in vacuo. The residue (0.21 g) was introduced into a 10 % aqueous sodium hydroxide solution (5 ml) and stirred for 5 hrs under a constant flow of argon. The reaction mixture was then cooled, the precipitate filtered and treated with 25 % hydrochloric acid. The suspension was filtered, washed with 15 % hydrochloric acid and distilled water to pH 3-4 (the phthalocyanine complex was slightly soluble in the washings), pressed and dried. The crude product was washed with ethanol to remove the residual acid and dried. The phthalocyanine complex

HOOC HOOC

HOOC HOOC

.COOH COOH

COOH COOH

MPc0C (M = Zn, Pt, Pd)

-h2o

H2O-»-M_0 o

O O—l\jl -

¡ I

H20 H'2O

ZnPc0CM4(H20)8 (M = Pt, Pd).

r-H20

Figure 1. Structural formulae of the studied phthalocyanine complexes.

was purified by precipitation from dilute aqueous sodium hydroxide with 50 % acetic acid and then dried over phosphorus pentoxide in vacuo at ambient temperature. The product yielded 0.051 g of PtPcoc (17.5 % on the basis of 4,5-bis(ethoxycarbonyl)phthalonitrile). IR (KBr) v cm-1: 1708 (C=0). UV-Vis (0.1% NaOH) I nm: 661,

v ' max v ' v ' max

595, 291 (with the relative intensities 1.68:0.38:1); (DMSO) Xmax nm: 669, 605 (with the relative intensities 5.1:1). Found, %: C 43.44, 43.29; H 2.05, 1.91; N 10.19, 10.04. C^H, N O Pt.2H,O.

7 7 7 7 7 40 16 8 16 2

Calculated, %: C 43.84; H 1.84; N 10.23.

Palladium 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine (PdPcoc). A mixture of 1.10 g 4,5-bis(ethoxycarbonyl)phthalonitrile and 0.20 g palladium(II) dichloride was heated at 190-195 °C for 2.5 h, the melt was cooled and treated with ethanol and chloroform-ethanol mixture as described above for the platinum complex. After the removal of the solvents the residue was refluxed for 6 hrs in 7 % alcoholic solution of sodium hydroxide. The suspension was filtered and the precipitate washed with hot ethanol (5 portions of 10-15 ml), pressed and stirred with 100 ml of 50 % acetic acid, then the suspension was filtered. The precipitate was consequently washed with 50 % and 10 % acetic acid, pressed, dried on the filter, then continuously stirred in ethanol and filtered until the filtrate was colorless. Thus obtained crude product was purified in an analogous manner to the platinum complex by reprecipitation from sodium hydroxide solution. The product yielded 0.081 g PdPcoc (16.5 %). IR (KBr) v cm-1: 1705 (C=0). UV-Vis (DMSO) I nm: 680,

max max

612, (relative intensivity 5.4:1). Found, %: C 48.28, 48.37; H 1.91, 1.85; N 11.08, 11.15. C H NO Pd.2EO. Calculated, %: C 48.53;

40 16 8 16 2

H 1.84; N 11.33.

(ZnPc), the Oa of which in DMSO is known (®A=0.67), served as a standard. The complex solution (~5T0-6 mol/l), containing DPIBF (3T0"5 mol/l), was irradiated in the standard cuvette of 1 cm optical layer thickness by light of DKSSh-150 lamp through the glass light filter KS-10 and interference filter with transmittance of684±25 nm. Light intensity on the sample surface was 2.4T015 quantum-cm-2^"1. The percentage of absorbed light of the sample was estimated by integrating overlapping spectrum of the exciting light and the absorption spectrum of the complexes. Photosensitized oxidation of DFIBF was monitored spectrophotometrically by decrease of maximum absorption at a wavelength of 418 nm. DFIBF initial concentration in all experiments was constant.

To calculate ®A the following equation was used:

Fa =

&ref-

w ■ lass

Wref ■ I„,

where FDD - the quantum yield of singlet oxygen generation by the reference standard (zinc phthalocyanine); W and Wref - the rates of consumption of DPIBF for sensitization with the studied compound and the standard; Iabs and Iabs - the amount of light quanta absorbed by the sensitizer and the standard, correspondingly.

The experimental error for the determination of ®A was about

10 %.

Results and Discussion

Materials and Methods

The IR spectra were recorded in KBr pellets on a FSM-1201 FT instrument and the electronic absorption spectra on a Hewlett Packard 8453 spectrophotometer. The light beam intensity was measured with a Thorlabs silicon photodiode.

The kinetics of photodestruction. The kinetic experiments for the phthalocyanine platinum and palladium complexes' photo-bleaching were conducted using an IS-2-693 photodiode (Tehnoek-san JSC) as the light source, which produced a luminous power of 1.1T017 quant-cm^-s"1 in the range of 688-698 nm. A solution of the complex in DMSO was transferred to a standard 1 cm cuvette (optical density for the maximum of the longwave absorption band - 0.7-0.8, C ~5T0"6 mol/l) and irradiated while monitoring the photodestruction kinetics by spectrophotometry, recording the changes in the optical density for the Q band maximum.

Quantum yields of singlet oxygen (®A) were determined by relative method using chemical quencher of 102 - 1,3-diphenyl-isobenzofuran (DPIBF). The unsubstituted zinc phthalocyanine

The synthesis of the ZnPcocPd4(H2O)8 tetraconjugate was conducted similarly to an earlier developed procedure1131 for the diaqua platinum conjugate (Scheme 1).

The structure of ZnPcocPd4(H2O)8 was confirmed by elemental analysis and IR spectroscopy. Comparison of the IR spectrum of ZnPcocPd4(H2O)8 with that of the starting ZnPcoc shows significant alterations in the 15001700 cm-1 range, while the IR spectra of ZnPcocPd4(H2O)8 and the previously studied[13] ZnPcocPt4(H2O)8 demonstrate a high degree of similarity (Figure 2). On the basis of IR spectra of palladium acetate[16] and analogous platinum conjugates,[1113] the bands at 1447, 1620 and 1693 cm-1 in the spectrum of ZnPcocPd4(H2O)8 can be identified as carboxyl group vibrations.

The synthesis of PtPcoc and PdPcoc was performed by fusing 4,5-bis(ethoxycarbonyl)phthalonitrile[14] with platinum or palladium chloride correspondingly with

Scheme 1. Synthesis of covalent tetraconjugate of zinc octacarboxyphthalocyanine with diaqua palladium (ZnPcocPd4(H2O)8). Макрогетероциmbl /Macroheterocycles 2015 8(2) 143-149

subsequent saponification of the reaction product containing octaethyl ester of the respective metal 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine and the products of its partial hydrolysis, followed by acidification of the formed PtPcoc or PdPcoc sodium salts (Scheme 2). The obtained complexes are soluble in water, phosphate buffer, dilute aqueous alkali, ethanol in the presence of organic bases (triethylamine) and in aprotic polar solvents (dimethyl sulfoxide, dimethyl formamide).

The maxima of the longwave absorption Q-bands of the studied metal complexes lie in the range of 669-698 nm (Table 1). It can be seen (Figure 3A) that in the MPcoc series the transition from Zn to Pt results in a substantial (30 nm) hypsochromic shift of the longwave Q-band in the electronic absorption spectrum. At the same time the transition from ZnPcoc to its peripheral tetraconjugate ZnPcocM4(H2O)8, regardless of the significant structural changes caused by the introduction of four diaqua platinum or diaqua palladium fragments, does not influence the Q-band position. However, as it is demonstrated in Figure 3B, its vibrational satellite (Q0-1) has a considerable but identical for both of the complexes shortwave shift, which probably is caused by the alterations in their skeletal vibrations.

The quantum yields of fluorescence (®fl) and singlet oxygen generation (®A) ofthe studied compounds demonstrate that an internal heavy atom effect exists, which depends on the position of Pt and Pt (in the core or in the periphery of the macrocycle). This effect is evident in enhancement of spin forbidden intersystem transitions, resulting in fluorescence quenching, increasing the yield and decreasing the lifetime of triplet states. The spin-orbit perturbations introduced by heavy atoms allow the transitions between states of different spin multiplicities. It should be noted that internal heavy

HCL*0 O^.OH

M = Pt, Pd

Scheme 2. Synthesis of platinum and palladium octacarboxyphthalocyanines (MPcoc, M = Pt, Pd).

Table 1 Spectral-fluorescent properties and quantum yields for singlet oxygen generation for MPcoc and covalent conjugates of ZnPcoc with diaqua platinum and diaqua palladium in DMSO.

Compound X abs, nm max ' X fl, nm max 5 ф Д

ZnPcoc 698 708[17] 0.24[17] 0.56[17]

PdPcoc 680 - - 0.78±0.08

PtPcoc 669 - - 0.81±0.08

ZnPcocPd4(H2O)8 698 714 0.12 0.60±0.05

ZnPcocPt4(H2O)8 698 713[13] 0.16[13] 0.51[13]

3 - ZnPcocPt4(H2O)8.

/8'

Figure 3. Electronic absorption spectra, А: ZnPcoc (-), PdPcoc (.......), PtPcoc (-----); В: ZnPcoc (-), ZnPcocPd4(H2O)8 (.......) and

ZnPcocPt4(H2O)8 (-----).

atom effect for phthalocyanines is currently described only by a few publications in contrast to this effect for porphyrins (e.g. the review by Solovyov and Borisevich[18]).

Compound ZnPcoc in DMSO when excited by the visible light exhibits fluorescence with Фа = 0.24 quantum yield.[17] The transition to PtPcoc and PdPcoc results in a complete quenching of the phthalocyanine fluorescence due to the heavy atom effect. At the same time, incorporation of four heavy atoms of Pt or Pd into the structure of ZnPcoc in the periphery of the macrocycle (transition to ZnPocPt4(H2O)8 and ZnPcocPd4(H2O)8) has a greatly less influence on Фа, which still decreases but not more than 2 times. The data in Table 1 demonstrate that effect of structural changes of the ZnPcoc periphery on the position of the fluorescence maximum is insignificant.

The value of Фд is an important parameter that defines the possibility of the use of the compounds as photocatalysts. All the studied phthalocyanine complexes were found to efficiently generate 1О2 in DMSO when excited with visible light. Figure 4 demonstrates the data from one of the experiments aimed at sensitization by the studied metal complexes of the photo-induced discoloration of the 1О2 scavenger (diphenylisobenzofuran, DPIBF) in DMSO. As it is seen, when PtPcoc was used as the photosensitizer for 1О2 formation and visible light with wavelengths close to the Q-band maximum was used, the absorption of DPIBF (X

max

= 418 nm) rapidly decreased. The Q-band of PtPcoc does not change, which testifies its considerable photostability. The singlet oxygen quencher - DABCO - decreased the rate of the photosensitized DPIBF oxidation, thereby confirming the participation of '02 in the current process.

The ®A values were measured as relative to the ZnPc standard. The results (Table 1) demonstrate that substitution of the central atom in ZnPcoc from Zn to the heavier Pt and Pd result in a significant increase of ®A. It is known that '02 is formed as the result of energy transfer from the sensitizer in triplet excited state to molecular oxygen, therefore increased ®A values testify the increased triplet states yield, i.e. the manifestation of the heavy atom effect. On the other hand, the change in ®A when the periphery of the macrocycle is modified with four fragments including the same heavy atoms, is insignificant and does not exceed the experimental error relative to the value of ®A = 0.56±0.05 for the parent ZnPcoc. It should be noted that in[10] incorporation of diaqua platinum fragments into silicon octacarboxyphthalocyanine resulted in a significant increase of ®A from 0.26 to 0.56. We did not observe such a notable effect in the case of ZnPcoc apparently due to the fact that for compound ZnPcoc already reaches 0.56.

Thus, the data on fluorescence and the quantum yield of singlet oxygen collected in the present study show that introduction of Pt and Pd into the phthalocyanine core

A

Figure 4. Spectral changes in the course of DPIBF oxidation in DMSO, with sensitization by the complex PtPcoc (Xexcit = 684±25 nm): The curves 1-7 correspond to 0; 10; 20; 30; 40; 50; and 60 s of irradiation time. The Soret band of the complex (Xmax = 339 nm) does not interfere with kinetic measurements of photooxidation of DPIBF at its absorption maximum (418 nm).

is a much stronger factor for spin-orbital perturbations than introduction of the same metals to the periphery of the macrocycle. Similar behavior has been reported for porphyrins as well:[19,20] bonding of heavy halogen atoms to pyrrole rings of the conjugated system of the porphyrin increases the probability of intersystem transitions much more than peripheral substitution of phenyl rings for tetraphenylporphyrin. This is understandable because for both porphyrins and phthalocyanines the S1—>T1 intersystem crossing occurs with the involvement of states belonging to the n,n*-type, and when the heavy atom is positioned further from the core n-electron system to the periphery, the effect of n-orbits perturbation weakens.

The photostability of the studied complexes was evaluated by the kinetics of their photobleaching in DMSO saturated with air for excitation of their Q-bands (Figure 5). Introduction of heavy metals to the structure of phthalo-cyanines increases the stability of the complexes towards radiation and this effect for Pt and Pd being incorporated into the macrocyclic core is far greater than the one for the introduction of Pt and Pd to the periphery. It is notable that the heavier Pt atom inhibits the photobleaching of the macrocycle to a greater extent than Pd.

^radiation' ^

Figure 5. Kinetic curves for the photobleaching in air-saturated DMSO: 1 - PtPcoc, 2 - PdPcoc, 3 - ZnPcocPt4(H2O)8, 4 - ZnPcocPd4(H2O)8, 5 - ZnPcoc. Concentration ~5T0-6 mol/l, Xexc=688-698 nm (IS-2-693 photodiode). Data were collected for (Table 1).

max v '

Photodestruction of phthalocyanines is known to proceed via different competing mechanisms. The contribution of each to the overall process depends on the structure of a phthalocyanine and on the medium.[21] We have earlier demonstrated that the contribution of singlet oxygen to oxidation of the basic compound ZnPcoc is not high in an aqueous medium.[21] As the efficiency of photodestruction in the series of the studied compounds does not increase with increased ®A and is on the contrary decreased, it can be concluded that in the case of Pt and Pd complexes with MPcoc in DMSO the contribution of singlet oxygen to photooxidation is not significant and the mechanism is of the radical type. The formation of radicals is known to proceed generally via triplet states, the increased probability of formation of those in the presence of a heavy

metal seemingly presumes more intensive photodestruction. However, the primary photochemical processes that result in the formation of radicals are bimolecular[22] and the bimolecular reaction rate is defined not by the probability of formation of Т state but by its stationary concentration (the Т level population) which depends on the ratio of its formation and deactivation rates (intersystem transitions S1 —> Tj and Tj —> S0). Depending on the alteration of the rates of these processes, induced by the heavy atom, the population of the Tj level can either decrease or increase. It seems that when MPcoc form chelate complexes with heavy atoms of Pt and Pd the deactivation of Tj is more accelerated and the population of the highly reactive Tj state is lowered, which is the most probable cause of the increased photostability of the complexes.

Conclusions

Octacarboxyphthalocyanines PdPcoc and PtPcoc as well as ZnPcocPd4(H2O)8 tetraconjugate were synthesized as novel compounds. The compounds were characterized by elemental analysis, electronic and IR spectroscopy, their spectral, luminescent and photochemical properties were studied. The properties of PdPcoc and PtPcoc were compared to the ones for ZnPcoc peripheral diaqua platinum and diaqua palladium covalent conjugates. It was found that substitution of one atom (Zn) in the macrocyclic core for Pd or Pt has a much greater influence on electronic absorption spectra, fluorescence, generation of singlet oxygen and photostability of Pc compared to incorporation of four same atoms into the periphery in the form of diaqua platinum and diaqua palladium, which says for the greater sensitivity of the electronic structure of the central aromatic ring, the lowest excited singlet and triplet states of phthalocyanines and the probability of transitions between them to the modification of the core than to modification of the periphery.

Acknowledgements. The authors thank the Russian Foundation for Basic Research for the financial support (Projects № 13-03-00667 and 13-03-12094).

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Received 27.03.2015 Accepted 15.06.2015