Article pubs.acs.org/JPCC
Dynamic Characteristic of Molecular Structure of Poly(orthomethoxyaniline) with Magnetic Probes Vladimir A. Shapovalov,*,† Vladimir V. Shapovalov,‡ Miriam H. Rafailovich,§ Stanislaw Piechota,∥ Alexandr F. Dmitruk,⊥ Elena I. Aksimentyeva,¶ and Anton S. Mazur† †
Donetsk Institute for Physics and Engineering named after O. O. Galkin, National Academy of Sciences of Ukraine, R. Luxemburg 72, Donetsk 83114, Ukraine ‡ The Garcia Center for Polymers at Engineered Interfaces, Department of Physics, Queens College of CUNY, New York 11367, United States § Department of Materials Science and Engineering, Stony Brook, New York 11794-3366, United States ∥ Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32, Warsaw 02-668, Poland ⊥ The L. M. Litvinenko Institute of Physical-Organic and Coal Chemistry, National Academy of Sciences of Ukraine, R. Luxemburg 70, Donetsk 83114, Ukraine ¶ Ivan Franko National University of Lviv, Universitetskaya str. 1, Lviv 79000, Ukraine ABSTRACT: The modern progress of bionanotechnologies and molecular electronics requires development of new organic materials with specific characteristics for formation of ultrathin films with specified magnetic or conductive properties. Here we need new experimental and computing methods creation. In this work a new effect is experimentally discovered in temperature behavior of electronic properties of the complex with a magnetic probe (iron ion) in poly(ortho-methoxyaniline) (PoMA). Using quantum chemical calculation we determine the structure of the magnetic probe’s immediate environment. The electron structure of the polymer specifies peculiarities of its doping and charge transport. The iron magnetic probe allows measuring of the complex’s electron structure dynamic characteristics in the polymer using the method of electron spin resonance for temperature range T = 4.2−295 K. The height of the barrier between the minima of potential of the intramolecular electric field of the complex is established.
1. INTRODUCTION The Nobel Prize in Chemistry 20001 demonstrated the importance of polymers with controlled electron characteristics. Amino compounds are of special interest.2,3 On the basis of such polymers, new organic materials are being developed for bionanotechnologies and molecular electronics. Such developments require new experimental and quantum chemical methods of computing.4,5 These methods are necessary for modeling of a specific ion’s interaction with molecular environment as well as the environment structure determination. Such specific complexes inside a molecule influence upon the feasibility and rate of chemical reactions with compounds, their pharmaceutical properties, as well as those of self-organization and nanostructures formation. We have conducted research in ion interaction with the immediate molecular environment in compounds of different structure symmetry.6−12 These articles are studying the spectra of electron spin resonance (ESR) in a number of model compounds with magnetic centers. The temperature transformation of the ESR spectra experiments have revealed a new effect in the temperature dependence of the electronic states of the complexes with magnetic probes. ESR spectroscopy is one © XXXX American Chemical Society
of the most informative physical methods for analysis compounds containing paramagnetic ions of the iron group. Paramagnetic ion embedded in the tested compound is a kind of a magnetic probe, providing information on the spectroscopic and structural characteristics of the ion’s environment.
2. EXPERIMENTAL AND THEORETICAL METHODS Electron paramagnetic resonance (EPR) spectra of poly(orthomethoxyaniline) samples (PoMA) with iron probes were studied using an X-band ESR spectrometer in the temperature range (4.2−295 K). The quantum chemical calculation of molecular structure of poly(ortho-methoxyaniline) with iron magnetic probes is done using the methods described in other papers.4,5 The method of chemical synthesis under the influence of ferric(III) chloride and ammonium persulfate as an oxidant13−15 is used to produce samples of of poly(orthomethoxyaniline) with iron probes. An iron magnetic probe is Received: November 20, 2012 Revised: March 15, 2013
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introduced to PoMA molecular structure on the synthesis stage. Samples’ element analysis on iron content demonstrated that the content of iron in samples is 0.1 mass percent. The analysis was conducted using potentiometric titration.
3. RESULTS AND DISCUSSION In this paper, we research properties of PoMA. Electronic and magnetic properties of PoMA doped with magnetic ions are not well studied as opposed to its analog, polyaniline.13−15 The subject under study of this work is PoMA. This polymer is quantitatively soluble in acids and organic solvents like chloroform, acetonitrile, dimethyl formamide, and so forth. This allows PoMA ultrathin films production on various surfaces.16,17 Such a polymer’s property is due to the presence of (−OCH3) electron-donor methoxyl substitute of a benzene ring in ortho-position to amides. It is due to these unique features that PoMA is an exceptional candidate for utilization in organic displays and sensors18,19 as well as in organic bioelectronics.3 The (−OCH3) substitute creates local peculiarities of the PoMa molecular structure. These peculiarities are explained by the conjugation length reduction of a polymer chain. This results in lower conductivity of substituted polyaminoarenes as compared to polyaniline.20 The protonation degree of the reaction center (cation-radical) gets changed. As a consequence, the polymer’s electronic structure changes.21,22 It conditions characteristics of the polymer doping and the charge transport. To determine parameters of those characteristics we introduced an iron magnetic probe into the PoMA molecular structure on the synthesis stage. For better understanding of special features of the equipotential surface shape in the vicinity of magnetic probes locations one should build a molecular structure model for the poly(o-methoxyaniline) chain fragment. For the purpose the molecular structure quantum chemical calculation4,5 was done. A description of similar structures can be found in monograph.23 In the calculation algorithm, the geometry optimization was conducted with structures calculation on all independent variables. The energy minimum location was determined based on quantum chemical calculation results for oscillatory spectra. The calculation demonstrated that of all the possible environments of the iron ion in the organic molecule only those environments that are energetically feasible are formed using double bonds (π-electrons) and with heteroatoms having undivided electron pairs. Quantum chemical calculation of the molecular structure of poly(ortho-methoxyaniline) coordination polymer demonstrated the presence of two structurally nonequivalent positions of magnetic probes Fe(1) and Fe(2) (Figure 1). The presence of such structurally nonequivalent positions of magnetic probes results in some specific features of the polymer’s chemical and physical properties. The Fe(1) and Fe(2) positions of magnetic probes in the PoMA matrix are marked with arrows directed toward 17 and 26 nitrogen atoms (Figure 1). Figure 2 demonstrates coordination complexes formed with those atoms. The resulting structure of the energy states is a sextet with a coordination number equal to 6. It agrees with the calculated charge distribution in the structures obtained and ionization potentials of the polymer and the iron ion. Quantum-chemical calculation allowed distance values determination from the iron ion’s equilibrium position to atoms of the first coordination sphere. Complexes with iron probes in PoMA are of the
Figure 1. Molecular structure of the chain fragment of poly(orthomethoxyaniline). Structurally nonequivalent positions of magnetic probes Fe(1) and Fe(2) are marked with arrows.
Figure 2. Molecular structure of the coordination complex with the iron ion in poly(ortho-methoxyaniline). The iron ion under investigation is surrounded by the following atoms: nitrogen, two carbon C atoms, oxygen, and two chlorine atoms.
molecular structure represented in Figure 2. The iron ion under investigation is surrounded by the following atoms: nitrogen N at the distance of 1.91 Å, two carbon C atoms at the distances of 2.16 and 2.11 Å, oxygen O at the distance of 2.46 Å, and two chlorine Cl atoms at the distances of 1.91 and 1.93 Å. Studies of ESR spectra of iron ions in poly(orthomethoxyaniline) were conducted using an X-band EPR spectrometer within temperature range (4.2−295 K). ESR spectrum of iron ions consists of two lines (Figure 3). Values of g-factors of lines 1 and 2 are as follows: g1 = 4.1 ± 0.1, g-factor of line 2 equals to g2 = 2.15 ± 0.1 at the temperature T = 4.2 K. Intensity of the resonant line 1 is getting lower with temperature growth while that of the line 2 is getting higher.
Figure 3. Energy barrier diagram for Fe(1) and Fe(2) probes of poly(ortho-methoxyaniline). The x-axis represents the location of potential wells with Fe(1) and Fe(2) magnetic probes in the molecular structure of a chain fragment of poly(ortho-methoxyaniline) in relative units. B
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total number of lines (lines 1b−5b) in the course of measurements of angular dependencies remains constant. In contrast with single crystals, the ESR spectrum of the Fe3+ ion complexes in polymers consist of only two resonance lines instead of five (Figure 5, lines 1 and 2).9,29
Intensity of the line 1 is maximal for low temperatures while intensity of the line 2 is maximal for high temperatures. The ESR spectrum temperature transformation observed is due to peculiarities (multiminimum character) of molecular electric field potential surface shape. The similar behavior is displayed with manifestation of Jahn−Teller effect6,24−28 for Cu2+ ions. The observable temperature transformation of ESR spectrum with iron7−12 is explained by the existence of peculiarities (multiminimum character) of molecular electric field (Figure 3) potential surface shape in PoMA structure due to structurally nonequivalent positions of iron ions (Figure 1). Figure 3 shows energy barrier diagram for Fe(1) and Fe(2) probes of poly(ortho-methoxyaniline). The x-axis represents the location of potential wells with Fe(1) and Fe(2) magnetic probes in the molecular structure of a chain fragment of poly(ortho-methoxyaniline) in relative units. In order to explain ESR spectra of iron ions in poly(orthomethoxyaniline) we will compare ESR spectra of iron ions in compounds with the long-range order of symmetry (crystals) with the spectra in structures with the short-range order of symmetry (polymers). Interpretation of ESR spectra for iron ions in structures of compounds with the long-range order of symmetry (crystals) are provided in the monograph by Abragam and Bleaney.26 ESR spectra of iron ions in compounds with the long-range order of symmetry (crystals) are shown in Figure 4. Figure 4
Figure 5. ESR spectra of iron ions in poly(ortho-methoxyaniline) at temperatures T = 4.2, 8.5, 49, and 295 K, produced on X-band EPR spectrometer. Redistribution of intensities of lines (1 and 2) occurs with change of temperature. Figure 4. Structure of energy levels of S = 5/2 spin multiplet and the form of the ESR spectrum for Fe3+. The ESR spectrum has five lines (lines 1b−5b) in compounds of high symmetry structure (single crystals). The central line 3b corresponds to transition −1/2 ←→ 1/2. Lines 1b − 2b correspond to transitions 5/2 ←→ 3/2 and −3/2 ←→ −1/2. Lines 4b−5b correspond to transitions 1/2 ←→ 3/2 and 3/2 ←→ 5/2. In contrast, the ESR spectrum of Fe3+ ions in poly(orthomethoxyaniline) consists of only two resonance lines (lines 1a−2a).
ESR spectra of iron ions in structures with the long-range order of symmetry (single crystals) and in structures with the short-range order of symmetry (polymers) have different number of lines due to the following reason. Complexes with magnetic ions of iron in single crystal are structurally equivalent. ESR spectra from iron ions superpose each other. The total spectrum consists of 5 lines (Figure 4, lines 1b, 2b, 3b, 4b, and 5b). Line 3b is isotropic with g-factor g ≅ 2. Lines 1b, 2b, 4b, and 5b are anisotropic. Position of these lines in magnetic field in single crystals gets changed with the study of angular dependencies of ESR spectrum of iron ions. However the total number of lines (lines 1b−5b) remains constant. Complexes with iron ions are structurally nonequivalent in polymer (Figure 1). ESR spectrum of each complex with iron has one isotropic line. Such isotropic lines of ESR spectra from all complexes with iron superpose each other. The resulting isotropic line of EPR spectrum is line 2 with g-factor g ≅ 2 (Figure 5). Anisotropic lines of ESR spectra of iron ions in polymer from all complexes with iron superpose each other. The resulting line is formed by the anisotropic lines of EPR spectra and is line 1 with g-factor g ≅ 4 (Figure 5). The reason is in mixing of energy states of iron ions shown in Figure 4. Such mixing of
also represents the structure of energy levels of the spin multiplet S = 5/2 of Fe3+ ion in a crystal. The ESR spectrum has five lines (lines 1b−5b) in compounds with high symmetry structure (single crystals). The central line 3b corresponds to transition 1/2 ←→ −1/2. Lines 1b−2b correspond to transition 5/2 ←→ 3/2 and −3/2 ←→ −1/2. Lines 4b−5b correspond to transition 5/2 ←→ 3/2 and 3/2 ←→ 1/2.26 The spectrum consists of an isotropic line and four anisotropic lines. The line 3b in Figure 4 is isotropic and has a g-factor g ≅ 2. Position of this isotropic line in magnetic field is not changed in the course of measurements of angular dependencies of ESR spectra of iron ions in single crystals. Lines 1b, 2b, 4b, and 5b are anisotropic. Positions of these anisotropic lines in magnetic field get changed in the course of measurements of angular dependencies of ESR spectra of iron ions in single crystals. The C
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Thus, the temperature redistribution of intensities of lines 1 and 2 of ESR spectrum of Fe3+ ions of poly(orthomethoxyaniline) occurs. Such temperature transformation of intensities of lines 1 and 2 of ESR spectrum of Fe3+ ions of poly(ortho-methoxyaniline) is determined by the height of the barrier of electric field potential E0 ≅ kT (Figure 3). The height of the barrier E0 depends upon molecular structure of the compound under investigation. The temperature change results in intensities (I1 and I2) redistribution of the two EPR spectrum lines. The sum of the specified intensities of line 1 and line 2 is a constant value equal to one for any temperature: I1 + I2 = 1. The temperature redistribution of the intensities depends on the barrier height E0 of the electric field potential in the expression of the ESR spectrum lines intensity (1). The height E0 of the electric field potential barrier between minima of the potential depends on parameters of the magnetic probe’s ligand environment. It is determined based on experimental data represented in Figure 6. Theoretical curves 1 and 2 are represented in the form of solid lines and are described by the formula
energy states is the result of structural nonequivalence of complexes with iron ions (Figure 1). It is described in the publications.9,29 The two ESR lines observed in the polymer change their intensities with temperature: redistribution of intensities of lines (1 and 2) occurs with change of temperature. Experimental results are represented on Figure 5 which shows the ESR spectra of iron ions in poly(ortho-methoxyaniline) at temperatures T = 4.2, 8.5, 49, and 295 K, produced on X-band ESR spectrometer. Transformation of intensities of lines of ESR spectra occurs in the both cases (single crystals as well as polymers) with temperature changes. The reasons for the redistribution of intensities are as follows. Lower energy levels in single crystals are predominantly populated at low temperatures T ≈ 4.2 K (Figure 4). These levels are situated near bottoms E4.2K of potential wells. Because of this the dominant ESR spectral line corresponds to 3/2 ←→ 5/2 transitions (line 5b) and the intensity of 5b line is higher than that of 1b line at low temperatures T ≈ 4.2 K. Therefore we observe that the intensity of line 1 is higher than that of line 2 (Figures 5 and 6) at low temperatures T ≈ 4.2 K.
⎛ −E ⎞ I = I0 exp⎜ 0 ⎟ ⎝ kT ⎠
(1)
where k is Boltzmann constant, T is temperature, and I0 is the temperature independent constant. The calculated height of potential barrier in molecular electric field is E0 = 2.2 cm−1 or 2.728 × 10−4 eV. Experimental curves are constructed using data obtained with ESR spectra measurements; theoretical curves 1 and 2 are calculated based on formula 1 and represented in the form of solid lines.
4. CONCLUSIONS In summary, complexes of poly(ortho-methoxyaniline) with magnetic probes are investigated. We discovered a new effect in temperature behavior of magnetic probes. This effect manifested in the case of structural nonequivalence of iron complexes (Figure 1). This structural nonequivalence results in orientational statistical averaging over the directions of symmetry axes of iron complexes. The wave functions of the multiplet S = 5/2 are mixed. The ESR spectrum of the Fe3+ ion consists of two resonance lines. With the change in temperature the line intensities vary. The total intensity of the two lines is always constant. We propose the method of measuring a new dynamic characteristic of electron states of poly(orthomethoxyaniline) with magnetic probes. The observed phenomenon provides the foundation for a new method of materials characterization.
Figure 6. Temperature transformation of intensities of lines 1 and 2 of the iron ions ESR spectrum in poly(ortho-methoxyaniline) for T = 4.2−295 K temperature range. Experimental curves are constructed using data obtained with ESR spectra measurements; theoretical curves 1 and 2 are calculated based on formula 1 and represented in the form of solid lines.
Higher energy levels in single crystals are predominantly populated at high temperatures T ≈ 295 K (Figure 4). These levels are situated in near-barrier and overbarrier states E295K of potential wells in polymers. The dominant spectral line corresponds to 5/2 ←→ 3/2 transitions (line 1b). Because of this, the intensity of 1b line is higher than that of 5b line at high temperatures T ≈ 295 K. Therefore, the intensity of line 2 is higher than that of line 1 (Figures 5 and 6) at high temperatures T ≈ 295 K. When such a temperature redistribution of intensities of lines of ESR spectra is observed in single crystals, it is used for determination of signs of constants for the spin Hamiltonian describing ESR spectra behavior.26 The signs of constants of the spin Hamiltonian can be both either positive or negative depending on the structure of energy levels (Figure 4). The arrangement of levels can be either as shown on Figure 4 or inverse.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +38050-292-56-14. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by Polish National Science Centre under research project for years 2010-2013 (Grant N 507 4924 38). D
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