ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2018, Vol. 54, No. 2, pp. 170–173. © Pleiades Publishing, Ltd., 2018. Original Russian Text © G.A. Gromova, A.V. Lobanov, Yu.G. Gorbunova, A.Yu. Tsivadze, 2018, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2018, Vol. 54, No. 2, pp. 129–132.

MOLECULAR AND SUPRAMOLECULAR STRUCTURES AT THE INTERFACES

The First Example of Electron Phototransfer with the Participation of Two-Decker Lanthanide Phthalocyaninate G. A. Gromovaa, A. V. Lobanovb, c, *, Yu. G. Gorbunovad, e, and A. Yu. Tsivadzed, e a

Moscow Technological University, Institute of Fine Chemical Technologies, Moscow, 119571 Russia bSemenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia c Plekhanov Russian University of Economics, Moscow, 117997 Russia dFrumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, 119071 Russia eKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia *e-mail: [email protected] Received August 9, 2017

Abstract⎯The photochemical activity of the anionic form of two-decker ytterbium and lutetium pthalocyaninates (LnPc2, Ln = Yb, Lu) is observed in the process of electron transfer to 2-methyl-1,4-naphthoquinone (MNQ). Under illumination of solutions of LnPc2 (1 × 10–5 mol/L) and MNQ (5 × 10–5 mol/L) in dimethyl formamide by light with wavelength λph > 630 nm, the anionic form of the two-decker phthalocyaninate [(Pc2–)Ln3+(Pc2–)]– passes into the neutral monoradical form [(Pc2–)Ln3+(Pc–)]0•. The photochemical redox process is accompanied by accumulation of the reduced form of MNQ. The observed effect is the first example of electron phototransfer with the participation of two-decker lanthanide phthalocyaninates. Keywords: phthalocyanines, lanthanides, two-decker complexes, 2-methyl-1,4-naphthoquinone, redox transitions, electron phototransfer DOI: 10.1134/S2070205118020065

INTRODUCTION Phthalocyanines (Pcs) are characterized by an enormous set of important properties, which is why their areas of use are constantly expanding [1–5]. Pc metal complexes are widely used as dyes, catalysts of chemical reactions, semiconductor materials, thermally stable polymers, laser dyes, and optical filters. Lately, it has been suggested to use Pcs in devices for information storage and display, in liquid-crystal composites, and in nonlinear optics. The presence of oxidizing properties of Pc metal complexes in the ground electron state and reducing properties in excited singlet and triplet states cause valuable photochemical properties that are of interest for photocatalysis, photobiology, and photomedicine [6–11]. The physical chemistry of sandwich two- and three-decker Pcs has been intensely worked out in recent decades [12–18]. Two-decker phthalocyanine complexes of lanthanides (LnPc2) are characterized by high stability and intensive absorption in the red spectral range. The most important feature of LnPc2 is their ability to participate in intramolecular electron transfer and, therefore, the possibility of existing in several redox forms, of which those of greatest interest are the anionic [(Pc2–)Ln3+(Pc2–)]– and neutral radi-

cal [(Pc2–)Ln3+(Pc–)]0• forms, which have different spectra. Studies of photochemical properties of lanthanide complexes of phthalocyanines have shown that their redox process can occur both on the phthalocyanine macroring itself and in the central metal ion as dependent on a number of factors: the nature of substituents in the phthalocyanine ring, degree of oxidation of the central metal ion, and solvent nature [5, 7, 8, 19–21]. The microenvironment is known to affect the redox transition between the two stable forms of LnPc2 (Ln = Ho, Er, Yb, Lu) in their supramolecular complexes with polymer macromolecules, micelles, and proteins [22]. The efficiency of the redox transition increases at a decrease in the ionic radius of Ln3+. The specific electronic structure of two-decker phthalocyanine metal complexes characterized by the presence of intramolecular interaction between πelectron systems of the neighboring macrocycles results in annihilation of excited triplet states. This regularity does not allow two-decker phthalocyanine complexes to act as photosensitizers capable of triplet–triplet energy transfer to the substrate molecules. At the same time, the possibility of photochemical stimulation of redox transitions in LnPc2 molecules remains an open issue.

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D 1.0 0.8

620 nm

670 nm

0.6 0.4

t, min 0 0.5 1.0 1.5 2.0 2.5 3.5 5.0 7.5

0.2 0 500

600

λ, nm

700

800

Fig. 1. Variation of the electronic-absorption spectrum of the LuPc2 solution in DMFA in the presence of MNQ under irradiation.

The aim of this work was to establish the photochemical redox transitions of two-decker ytterbium and lutetium phthalocyaninates in the presence of electron acceptors using spectral methods. EXPERIMENTAL The objects of research were two-decker ytterbium and lutetium phthalocyaninates synthesized and purified according to the techniques described earlier [23– 25]. LnPc2 solutions were prepared using distilled DMFA. The electron acceptor was 2-methyl-1,4naphthoquinone (MNQ) (Sigma). Two milliliters of a solution of individual LnPc2 (1 × 10–5 mol/L) in DMFA or a solution containing LnPc2 (1 × 10–5 mol/L) and MNQ (5 × 10–5 mol/L) were placed into a quartz cuvette with a width of 1 cm in order to perform the photochemical experiment. Photolysis of the solutions was carried out using light from a halogen lamp with a power of 150 W with a system of lenses, a capacitor and a KS-13 color filter cutting off the radiation with λph < 630 nm. The luminous power was 10 mW/cm2. The moment of switching the light on was assumed to be the start of the kinetic experiment. Electronic-absorption spectra of LnPc2 were periodically registered using a HACH DR-4000V spectrophotometer (United States) in the same cuvettes. Absorption spectra were plotted and mathematically processed using the Origin 8.0 software. RESULTS AND DISCUSSION Electron-absorption spectra (EASs) of the solutions of anionic forms of two-decker ytterbium and lutetium phthalocyaninates [(Pc2–)Ln3+(Pc2–)]– are

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characterized by a Q-band split into two components: an intense Q1 band in the range of 618–620 nm and a low-intensity Q2 band at 695 nm. Such splitting is due to exciton interaction of dianions of two phthalocyanine deckers. The EASs of solutions of neutral monoradical forms of [(Pc2–)Ln3+(Pc–)]0• is characterized by the presence of a single intense absorption Q-band in the range of 660–680 nm. Under illumination of LnPc2 solutions (1 × 10‒5 mol/L) and MNQ (5 × 10–5 mol/L) in dimethyl formamide by light with wavelength λph > 630 nm, the anionic form of the two-decker phthalocyaninate [(Pc2–)Ln3+(Pc2–)]– passes into the neutral monoradical form [(Pc2–)Ln3+(Pc–)]0•, as shown in Fig. 1 using the example of LuPc2. In the course of illumination, a decrease in the intensity of absorption bands of the anionic form of the phthalocyanine complex is observed, as are the appearance and growth of the absorption band of its neutral monoradical form. Expansion of absorption spectra into individual components corresponding to [(Pc2–)Ln3+(Pc2–)]– and [(Pc2–)Ln3+(Pc–)]0• allowed obtaining kinetic dependences of a photostimulated redox transition (Fig. 2). As can be seen in Fig. 2, kinetic curves in the initial approximation can be described by a (pseudo)first-order equation and ∼50% of the twodecker metal complex undergo oxidation in the time period of 3–5 min. It is essential to point out that, in the absence of the MNQ electron acceptor, there is no photochemical transition between the two redox forms both in the case of YbPc2 and in the case of LuPc2 (Fig. 2). There is also no redox transition (no spectral changes) when LnPc2 and MNQ are simultaneously present in the DMFA solution at the same concentration ratio in the absence of irradiation. The observed regularities and the circumstance that the photolyzing light with wavelength λph > 630 nm is absorbed by [(Pc2–)Ln3+(Pc2–)]–, but not by MNQ, allow concluding that irradiation of solutions of LnPc2 and MNQ in DMFA results in photochemical electron transfer from the anionic form of [(Pc2–)Ln3+(Pc2–)]– to MNQ molecules, which results in the course of photolysis in accumulation of the neutral radical form [(Pc2–)Ln3+(Pc–)]0• of the two-decker phthalocyaninate. This conclusion agrees with the result of comparison of redox potentials of [(Pc2–)Ln3+(Pc2–)]–, [(Pc2–)Ln3+(Pc–)]0• [26] and MNQ in the ground electron state and [(Pc2–)Ln3+(Pc2–)]–* in the first singlet excited state (Fig. 3). One can see from the energy scheme in Fig. 3 that electron transfer to the MNQ molecule is possible only with participation of the anionic form of a two-decker phthalocyanine in the excited state, but cannot be realized via interaction of the solution components in the ground state. Therefore, the necessary condition of electron transfer from the two-decker phthalocyaninate to quinine acceptors is irradiation.

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(a) 100

E, eV

4

3

–2

[(Pc2–)Ln3+(Pc2–)]–* –1.6 O

mol %

80

CH3

1 60

–0.59 hν

O

40 20

+0.35 [(Pc2–)Ln3+(Pc2–)]–

2

+0.83 [(Pc2–)Ln3+(Pc–)]0•

0

5

10

3

100

25

30 2

4 Fig. 3. Energy scheme of electron phototransfer from LnPc2 to MNQ.

80 mol %

15 20 t, min (b)

1

considered in this work can act as a physico-chemical model of the photobiological process.

60 40

ACKNOWLEDGMENTS

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0

This work was financially supported by the Russian Foundation for Basic Research, project no. 18-0300539.

2

5

10

15 t, min

20

25

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Fig. 2. Kinetic curves of (1) accumulation of [(Pc2‒)Ln3+(Pc–)]0• under irradiation of solutions of LnPc2 and MNQ in DMFA and (2) consumption of [(Pc2–)Ln3+(Pc2–)]– under irradiation of solutions of LnPc2 in DMFA in the presence of MNQ and (3) in the absence of MNQ and under interaction of LnPc2 and MNQ in DMFA under darkroom conditions, where Ln = Lu (a) and Yb (b).

Thus, the work shows for the first time the photochemical activity of LuPc2 and YbPc2 in the course of electron transfer with participation of their anionic forms. The observed effect has a number of important consequences. The complication of the LnPc2–acceptor system by addition of an electron donor will probably allow realizing the photocatalytic process, in which LnPc2 is a reversible electron carrier. Besides, the ability of LnPc2 to act as an electron donor is of interest for development of biomedical applications, in which LnPc2 can provide photodynamical processes of the first type. One must also note that naphthoquinone acceptors are electron carriers in photosynthesis and, therefore, the LnPc2–MNQ system

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Translated by M. Ehrenburg

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