Interaction of octopus-like cobalt(II) phthalocyaninate with fullerene C70 studied by ESR spectroscopy Текст научной статьи по специальности «Химические науки»

Фталоцианины

Phthalocyanines

iVJaKporaTepoLii/JKj-JbJ

Статья

Paper

http://macroheterocycles.isuct.ru

DOI: 10.6060/mhc181113m

Interaction of Octopus-like Cobalt(II) Phthalocyaninate with Fullerene C70 Studied by ESR Spectroscopy

Alexander G. Martynov,a@ Irina V. Nefedova,b Nikolay N. Efimov,b Elena A. Ugolkova,b Vadim V. Minin,b Yulia G. Gorbunova,ab and Aslan Yu. Tsivadzeab

aA.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 119071 Moscow, Russia bN.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia @Corresponding author E-mail: Martynov.Alexandre@gmail.com

This work reports on synthesis and characterizations of cobalt(II) complex 1Co with octopus-like phthalocyanine ligand bearing eight peripheral O-benzyl-diethyleneglycol substituents. Concentration-dependent investigation of UV-Vis spectra of 1Co in chloroform evidenced of its aggregation at high concentrations (0.3 mM) with the formation of dimers with dimerization constant (4.8±0.2)103 M-1. Supramolecular assembling of 1Co with fullerene C70 was studied by UV-Vis titration and by ESR spectroscopy. Both methods suggest the absence of notable interactions between electronic systems of 1Co and C70 in ground state, the formation of the assembly 1Co*C70 occurs via noncovalent interactions.

Keywords: Phthalocyanine, cobalt, fullerene, electronic spin resonance, supramolecular assembling.

Исследование взаимодействия осьминогоподобного фталоцианината кобальта(П) с фуллереном О70 методом спектроскопии ЭПР

А. Г. Мартынов,а@ И. В. Нефедова,ь Н. Н. Ефимовь Е. А. Уголкова,ь В. В. Минин,ь Ю. Г. Горбунова,'а,ь А. Ю. ЦивадзеаЬ

аИнститут физической химии и электрохимии им. А.Н. Фрумкина РАН, 119071 Москва, Россия ъИнститут общей и неорганической химии им. Н.С. Курнакова РАН, 119991 Москва, Россия @Е-таИ: Martynov.Alexandre@gmail.com

Синтезирован фталоцианинат кобалъта(П) 1Со, содержащий восемь фрагментов О-бензилдиэтиленгликоля. Методом ЭСП изучена агрегация комплекса в концентрированных растворах (до 0.3 мМ в хлороформе). С использованием регрессионного анализа установлено, что в концентрированных растворах происходит димеризация молекул 1Со, константа димеризации составляет (4.8±0.2)103 М-1. Взаимодействие 1Со с фуллереном С70 исследовано с использованием методов спектрофотометрического титрования и спектроскопии ЭПР. Показано, что образование супрамолекулярного ансамбля 1Со*С70 происходит за счет нековалентных взаимодействий и не сопровождается заметным взаимодействием между электронными системами 1Со и С70.

Ключевые слова: Фталоцианин, кобальт, фуллерен, ЭПР, супрамолекулярная сборка.

Introduction

Experimental

Donor-acceptor assemblies based on tetrapyrrolic macrocycles and nanocarbon materials - fullerenes, gra-phenes, nanotubes, etc., are promising conductive materials and photoactive components of photovoltaic devices.[1-7] Such hybrid materials can be formed either by covalent bonding of tetrapyrroles to nanocarbons, or by their supra-molecular assembling. The former method results in transformation of sp2-carbon atoms in intact nanocarbons into sp3-C, which cannot participate in electronic conjugation. Supramolecular approach does not have this disadvantage, it is widely used to form co-crystals of fullerenes with porphyrins and phthalocyanines,[8-11] however assembling of fullerenes with tetrapyrroles in solution requires accurate design of receptor groups providing efficient and controllable binding and selectivity.[12-15]

Previously, we have synthesized new fullerene-binding receptors - zinc and magnesium phthalocyaninates bearing eight peripheral O-benzyldiethyleneglycol substituents starting from [2'-(2''-benzylethoxy)ethoxy]phthalonitrile[16] (Figure 1). These molecules resemble octopuses, which capture fullerenes with their eight "limbs" via nonco-valent hydrophobic and n-n interactions. Using UV-Vis and fluorescence spectroscopy we demonstrated high affinity of receptors for C60 and C70, with selectivity to C70: binding constants for C70 were almost two times higher than for C60. This result was rationalized using semi-empirical calculations at PM6-DH2 level.

In the present work we have synthesized cobalt(II) complex 1Co with octopus-like phthalocyanine ligand, investigated aggregation of 1Co in chloroform and studied its interaction with C70 by spectrophotometric titration. Due to the presence of one unpaired electron in Co2+ ion with ^'-configuration this interaction could be studied by ESR spectroscopy. It let us draw a conclusion about assembling of 1Co and C70 molecules via noncovalent dipole-dipole interactions.

[2'-(2''-Benzylethoxy)ethoxy]phthalonitrile 2 was synthesized according to previously reported procedures.[1718] Cobalt acetate tetrahydrate (Aldrich) was dried at 90 °C in vacuum to obtain anhydrous salt. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU, Merck) was distilled over CaH2 in vacuum and stored under argon. Isoamyl alcohol (Sigma-Aldrich) was distilled over sodium and stored under argon. Chloroform was distilled over NaHCO3 to remove acidic impurities. Neutral alumina (Macherey Nagel) was used for chromatography. Other reagents and solvents were purchased from commercial suppliers and were used without additional purification.

UV-Vis spectra were recorded at room temperature on Cary-100 and Thermo Evolution 210 spectrophotometers in 0.1-1 cm pathlength cuvettes. MALDI TOF mass-spectra were measured on Ultraflex spectrometer (Bruker Daltonics) with 2,5-dihydroxy-benzoic acid (DHB), used as a matrix.

X-Band ESR spectra were measured at 9.8 GHz microwave frequency on Bruker Elexsys E-680X radiospectrometer in the temperature range of 300-100 K. Since low-spin cobalt(II) complexes tend to form adducts with molecular oxygen, we paid particular attention to sample preparation. Samples of 1Co were dissolved under vacuum in solvents, which were previously deoxy-genated by repeated freeze-pump-thaw cycles. Resulting solutions were transferred into cells for ESR measurements. The spectra were measured below the melting points of solvents.

Resulting ESR spectra were described using rhombic spin Hamiltonian with S=1/2 spin, Zeeman and hyperfine interactions:

Hglala = g$HzSz + g$HxSx + gfiHSy + AIzSz + BixSx + CiySy (1)

Here gz, gx, gy - z, x, y - components of g-tensor, A, B, C - z, x, y -components of HFS tensor, Sz, Sx, Sy - projections of spin operator onto coordination axes, 5=1/2, Iz, Ix, I - projections of nuclear spin operator onto coordination axes, 7=3.5.

The parameters of ESR spectra were found using best approximation method, which minimizes the error function

F = Z(YT - YEf /N

(2)

Here YE - the array of the observed intensities of ESR signals at different values of magnetic field, Yf - theoretical values of intensities at the same values of magnetic field, N - number of points. Theoretical spectra were plotted according to the previously reported procedure.[19] Line shapes were described using Gaussian and Lorentzian functions.[20] The line widths were parameterized using relaxation theory:[21]

--at +pkmf +ykm]

(3)

Figure 1. Octopus-like fullerene receptors 1M. M=Zn and Mg (Ref.[16]), Co (this work).

Here mj - projection of nuclear spin on the magnetic field direction, k=x, y, z, ak Pk, Yk - broadening parameters in corresponding orientations. Minimization of error function (Eq. 2) implied variation of g-factors, HFS constants, line widths and shapes.

Cobalt(77) 2,3,9,10,16,17,23,24-octakis[2'-(2''-benzylethoxy) ethoxy]phthalocyaninate (1Co). The mixture of phthalonitrile 2 (118 mg, 0.23 mmol), Co(OAc)2 (21 mg, 0.12 mmol) and DBU (35 mg, 0.23 mmol) in 3 mL of isoamyl alcohol was refluxed under argon for 24 h. After cooling to room temperature the dark-green reaction mixture was diluted with hexane, the formed precipitate was filtered, the filtrate was discarded and the solid was washed off the filter with chloroform. After column chromatography on neutral alumina (elution with CHCl3+(0-5)vol.% MeOH) followed by size-exclusion chromatography (Bio-Beads S-X1, elution with CHCl3+2.5vol.% MeOH) the complex 1Co was isolated as a dark-green sticky solid (33 mg, yield 27 %). m/z (MALDI-TOF) found 2124.8 [M]+, calculated for C^H^CoNO 2125.0. UV-Vis

L 1 120 128 8 24

(CHCl3) X (lg e) nm: 297 (4.87), 606 (4.39), 670 (5.04).

a

Octopus-Like Cobalt(II) Phthalocyaninate Results and Discussion

Synthesis and Spectral Characterization of 1Co

Synthesis of complex 1Co was performed starting from the previously reported phthalonitrile 2 using its DBU-promoted template condensation in the presence of anhydrous Co(OAc)2 in refluxing isoamyl alcohol (Scheme 1).

CN

О

0 XjC

■o O-^^CN

2

M(OAC)2, DBU

iAmOH,

150 °C, Ar yield - 27%

Scheme 1.

The synthesized complex was characterized by MALDI-TOF mass-spectrometry and UV-Vis spectroscopy. The presence of paramagnetic Co2+ ion in the molecule precluded application of NMR spectroscopy for characterization because of strong broadening and shifting of resonance signals in spectra.

The investigation of concentration-dependent UV-Vis spectra of 1Co in CHCl3 evidenced that the increase of the complex concentration resulted in decrease of the efficient extinction coefficient (eeff) with simultaneous broadening of the g-band and its vibrational satellite. For example, lgseff changed from 5.04 to 4.87 upon increase of C1Co from 440-6 M to 340-4 M (Figures 2, 3). This spectral behavior evidenced of phthalocyanine aggregating in solution.[22]

Analysis of nonlinear ranges of plots in Figure 3 was performed using approach, proposed by Mataga,[23] which was widely used for the analysis of phthalocyanine aggregation.[24-26] In accordance with this approach, the solution with total concentration of dissolved chromo-phore C0 is characterized by efficient extinction coefficient e =Al'1Cn which is different from extinction of monomeric

eff 0

compound eM in the case if the molecules of chromophor are aggregating. Putting n as an aggregation number in the equilibrium nM^M and K as a constant of this

300 400 500 600 Wavelength, nm

700

800

Figure 2. UV-Vis spectra of 1Co in CHCl3 at different concentrations.

12-1

11-

E

S? 104

10

10

10*

. M

1Co'

Figure 3. Dependence of efficient extinction coefficient

e =Al'1C'1 at 670 nm on concentration for 1Co in CHCL.

eff 0 3

equilibrium, the following relation between all these values was proposed:

lg

Co •

^ e ^

1 _ zeL

V eM

= lg ( n • K ) + n • lg

C

fe S

"eff

(4)

ТЪш, ptottmg lg[C0(1 - £/£M1)] vs. МС0-£у£м1] should give a straight line with the slope, equal to n. Indeed, plotting the corresponding values for 1Co gave linear dependency with the slope n=1.7 (Figure 4), suggesting that mainly dimerization of phthalocyanine molecules occurs upon aggregation.

-3,5 и -4,0-4,5-"E -5,0? -5,5-

ï -a,o;

-6,5-7,0-

-5,6

-5,2

-4,8 -4,4

"OlWJ

-4,0

-3,6

Figure 4. Graphical analysis of eeff values for determination of aggregation number n in accordance with Eq. (4), obtained for g-band of 1Co at 670 nm («=1.70±0.05, R2=0.993).

The equilibrium between the monomer M and the dimer D is described by the dimerization constant Kd (Eq. 5) and the material balance (Eq. 6):

[ D

Kd =

[ M ]2

Co =[ M ] + 2 [ D ]

(5)

(6)

These equations can be used to evaluate the equilibrium concentration of the monomer:

[ M ] =

V1 + 8C0K -1

4K

(7)

The optical density at certain wavelength is an additive value, which is determined by absorptions of both monomer and dimer depending on their equilibrium concentrations and characteristic extinction coefficients. Taking into account the equation of material balance (Eq. 6), it can be expressed in the following way:

A = s m •[ M ] + E fl •[ D ] = (E mj-[ M ] +

C -s

(8)

Combination of Eqs. (7) and (8) gives nonlinear equation (9), which expresses optical density as a function of concentration of dissolved compound. Other terms of this equation, namely equilibrium constant Kd as well as extinction coefficients of monomer and dimer (sM and sD, respectively) are unknown, but they can be found by nonlinear regression analysis, therefore, we can reveal the UV-Vis spectra of monomer and dimer. Since changes in UV-Vis spectra caused by aggregation depend on mutual arrangement of chromophores within the dimer, its architecture can be proposed.

A = s,,-^-

A/1 + 8 Cq Kd -1 + C0 s

4 K

2

(9)

To perform this analysis, we have used the 0-band region. The array of UV-Vis data for 35 wavelengths (from 550 to 720 nm with 5 nm step) was chosen. The data was taken for 11 solutions with concentrations in the range of 4-10-6 ^ 3-10-4 M (altogether 35x11=385 data points). The values of s,, for 670 nm was fixed at the value of 1.10105 L-M-1-cm-1 and all other parameters were varied until convergence was achieved. The value of found equilibrium constant Kd was (4.8±0.2>103 M-1. The calculated UV-Vis spectra of both monomer and dimer are given in Figure 5. In accordance with the excitonic model of chromophore interaction, the blue-shifted maximum of the aggregate 0-band suggests significant overlap of stacked phthalocyanine n-systems (H-dimer).[27]

12-

9-

6-

3-

550

600 650

Wavelength, nm

700

Figure 5. Calculated UV-Vis spectra of monomelic and dimeric forms of 1Co in CHCL.

Studies of Interaction between 1Co and C70

The interaction of C70 with the 1Co was studied by UV-Vis titration of its solution in CHCl3 with the solution of fullerene in toluene. This titration did not result in any notable shift of the Pcs 0-band (Figure 6) suggesting the absence of significant ground state interaction between molecular orbitals of 1Co and C,„.

0,0

-I 331| -Hi 3791 6701

I V 461 |

300

400

500 600 Wavelength, nm

700

800

Figure 6. Spectrophotometry titration of solution of 1Co in CHCl3 (C1Co=1.5-10-5 M) with the solution of C70 in toluene (CC7(i=8.9-10-4 M). Each step corresponds to addition of 0.2 equivalents of fullerene.

The interaction of 1Co with C70 could be studied by ESR due to the presence of one unpaired electron in low-spin Co2+ ion. Figure 7 shows the spectrum of 1Co solution in CHCl3 deoxygenated by repeated freeze-pump-thaw cycles. The complex was ESR silent at room temperature; therefore, its spectrum was measured at 100 K.

The components of hyperfine splitting are superimposed with the broad band, which cannot be compensated by the correction of the baseline. We suppose that this line originates from the aggregation of 1Co in the solution at the applied concentration (~10-3 M), which is needed for acquisition of informative spectrum. Moreover, we expected that the spectrum would have axial symmetry; however, good match between the experimental and calculated spectra could be obtained only in rhombic approximation. This might be due to some deformation of the macrocycle because of the flexibility of bulky substituents.[28] The ESR spectrum of 1Co reveals well resolved hyperfine structure containing eight lines in parallel orientation of g-tensor and poorly resolved structure in perpendicular orientation, which arises from magnetic interaction between unpaired electron spin and nuclear spin (I=7/2) of Co2+ ion. The spin Hamiltonian parameters are listed in Table 1.

To show the sensitivity of ESR spectra of 1Co to rearrangement of spin density, we have also measured ESR spectrum of 1Co in aerobic conditions. To enhance interaction of 1Co with molecular oxygen pyridine was added to the sample which coordinates to cobalt ion in transposition to O2 molecule.[29,30] The appearance of spectrum is given in Figure 8, and the spin Hamiltonian parameters of the resulting spectrum are given in the Table 1. These

Octopus-Like Cobalt(II) Phthalocyaninate

Table 1. Parameters of spin Hamiltonian for the cobalt phthalocyaninates 1Co and its adducts with fullerene C70 and molecular oxygen.

gz gx gy A, x103 cm-1 B, x103 cm-1 C, x103 cm-1

1Co 1.991 2.119 2.390 11.41 4.10 6.58

1Co-O2 1.991 2.065 2.008 0.76 1.61 1.02

1Co<70 1.986 2.118 2.388 11.34 4.15 6.60

parameters evidence that in the formed adduct of 1Co with molecular oxygen almost 80 % of spin density is localized on the dioxygen group and it can be formally described as PcCo(III)O2". This experiment evidences of high sensitivity of low-spin cobalt complex with one electron at dz2 orbital to addition of molecules with uncompensated spin density.

Finally, Figure 9 shows ESR spectrum of 1Co in the presence of fullerene C70 measured under anaerobic conditions. The parameters of spin Hamiltonian of 1Co^C70 are similar to those of starting 1Co (Table 1), however, the HFS lines in parallel orientation of g-tensor almost vanish. The significant broadening of lines in parallel orientation is characterized by a, p and у values, which are found by computational modeling. They are assigned to relaxation behavior of systems - rotations, vibrations, etc. In the case of 1Co these values are equal to az=25.12 G, Pz=2.18 G, Yz=3.11 G, while in the case of 1Co^C70 these values are equal to az=46.18 G, Pz=5.01 G, Yz=3 82 G. Upon formation of the complex with fullerene, the most notable change is observed in the case of a term, which characterizes the rate of molecular rotation. Therefore, it can be concluded that 1Co interacts with C70 via dipole-dipole interactions significant additional delocalization of spin density, which is in line with the previous studies,[31] although further theoretical work is required for complete description of CoPc-C70 interaction which might also include ст-donor and n-acceptor interactions between the fullerene and the CoPc unit. Such type of interaction was proposed previously to describe properties of the covalent CoPor-C60 diad.[32]

1Co, studied its aggregation in solution by concentration-dependent UV-Vis spectroscopy and investigated interaction between 1Co with C70 by ESR spectroscopy.

We have shown the absence of notable coupling between electronic systems of Co(II) complex and fullerene, their interaction occurs via noncovalent dipole-dipole interactions, which manifests in changes of line shapes in ESR spectra without significant alteration of spin Hamiltonian parameters.

Previous theoretical studies of complexes of CoPc with fullerene suggest that such assemblies can act as bidirectional switches with tuneable direction of electron transport which can be achieved by oxidation of reduction of CoPc molecules.[33] Our study proposes the approach to con-

- Experimental 1Co*0, Calculated

3250 3300 3350 3400 3450 3500 3550 H, mT

Conclusions

In the present work, we have synthesized and investigated novel octopus-like cobalt(II) phthalocyaninate

Experimental 1Co Calculated

2500

3000 3500 H, mT

4000

4500

Figure 7. Experimental (solid line) and calculated (dashed line) ESR spectra of 1Co in deoxygenated CHCl3 at 100 K.

Figure 8. Experimental (solid line) and calculated (dashed line) ESR spectra of 1Co in the mixture of CHCl3 and pyridine (1:1 vol.) in the presence of air at 100 K.

Experimental 1Co«C70 Calculated

2500

3000 3500 H, mT

4000

4500

Figure 9. Experimental (solid line) and calculated (dashed line) ESR spectra of 1Co in deoxygenated CHCl3 after addition

of excess of C70 in toluene at 100 K.

struction of such assemblies suggesting the perspectives of further development of octopus-like phthalocyanines and studies of their receptor properties.

Acknowledgements. This work was supported by Russian Foundation for Basic Research (grant 18-03-01003). Vadim V. Minin and Nikolay N. Efimov thank the Presidium of the Russian Academy of Sciences (Program I.36) for financial support of EPR measurements. The physical-chemical measurements were performed using equipment of Shared Facility Centres of the A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, RAS, and the N.S. Kurnakov Institute of General and Inorganic Chemistry, RAS.

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Received 18.11.2018 Accepted 24.12.2018