Branching ratio and L2+L3 intensities of 3d-transition metals in phthalocyanines and the amine complexes

doi:10.1016/S0968-4328(99)00116-X

Micron

Volume 31, Issue 4, August 2000, Pages 373-380

https://doi.org/10.1016/S0968-4328(99)00116-X Get rights and content

Abstract

L_2,3 inner-shell excitation spectra were obtained by electron energy-loss spectroscopy (EELS) for the divalent first transition series metals in phthalocyanine complexes (MPc) such as titanium oxide phthalocyanine (TiOPc), fluoro-chromium phthalocyanine (CrFPc), manganese phthalocyanine (MnPc), iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc) and copper phthalocyanine (CuPc). It was found that the value of normalized total intensity of I(L₂+L₃) was nearly proportional to the formal electron vacancies of each 3d-state, and the values of the branching ratio, I(L₃)/I((L₂+L₃), represented a high-spin-state rather than low-spin-state for MnPc, FePc and NiPc. EELS was also applied to charge-transfer complexes of FePc with an amine such as pyridine or γ-picoline. It was concluded that their I(L₂+L₃) intensity of Fe showed the decrease in vacancies of 3d-states on the formation of the charge-transfer complex with these amines, which suggests some electron transfer from the amine to Fe in phthalocyanine. The EELS study provides beneficial information for investigating the electronic states of the specific metal sites in organic materials.

Introduction

For the 3d-transition metal atoms with open shell structures, characteristics of such metal compounds predominately depend on their d-electronic states, which contribute to the chemical bonding and also are affected easily by surrounding atoms. Electron energy-loss spectroscopy (EELS) can be used to get information on the d-states even from a small region of a specimen. Like other optical analytical methods, dipole selection rule Δl=±1 is adopted and the L_2,3 spectra which can be obtained as a result of an excitation, for example, from the initial states 2p⁶3dⁿ to the final states 2p⁵3dⁿ⁺¹ will be available to investigate d-occupancy and other d-electronic states.

In these recent years, a lot of researches about 3d-transition metals have been performed by studying L-edge spectra. First of all, the correspondence between peak energies in EELS and X-ray absorption spectroscopy (XAS) has been established by Colliex and Jouffrey (1972). After that, systematic studies on EELS spectra have been carried out on metal oxides (Rask et al., 1987, Paterson and Krivanek, 1990, Kurata and Colliex, 1993). They have shown that the peak position and fine structures of oxygen K-edge and the intensity ratio between metal L₂- and L₃-edges, I(L₃)/I(L₂), are sensitive to the oxidation state of metal ions. Starace and Horsley have pointed out that the total intensity of L₂+L₃ is proportional to the number of valence holes (Starace, 1972, Horsley, 1982), and then Pearson et al. have used this correlation to measure the local density of 3d-states for metal alloys by EELS (Pearson et al., 1988). From the theoretical side, Thole and Laan have revealed the linear relation between X-ray absorption branching ratio, the intensities ratio between L₃ absorption and the total absorption of L₂+L₃(I(L₃)/I(L₂+L₃)), and the angular component of valence band spin–orbit expectation value (Thole and Laan, 1988a). They have, therefore, indicated that the branching ratio is a measure of the angular part of the spin–orbit operator. They have calculated the branching ratios of a series of 3d-transition metal ions assuming both various initial and final transition states (Thole and Laan, 1988b), in which they have described that the branching ratio is changing because of an electrostatic interaction between d-electron and valence hole, and also because of a spin–orbit correlation of d-electrons. Further, they also have considered the effect of crystal field on the branching ratio.

Various transition metal oxides, especially with octahedral symmetry, have been investigated in detail by the authors described above. However, for organic materials with transition metals, L-edge XAS of some metal phthalocyanines (MPc) like manganese phthalocyanine (MnPc), iron phthalocyanine (FePc), nickel phthalocyanine (NiPc) and copper phthalocyanine (CuPc) is the only single study ever reported which concerned L-edge of organic 3d-transition metal complexes (Koch et al., 1985).

In the present study, we examined the relation between 3d-occupancy and L-edge spectra of phthalocyanines (Pc) with various 3d-metals (Fig. 1): titanium oxide phthalocyanine (TiOPc); fluoro-chromium phthalocyanine (CrFPc); MnPc; FePc; cobalt phthalocyanine (CoPc); NiPc; and CuPc. We also examined the spectral changes in FePc due to complex formation with gas phase amines, where the amines are expected to act as electron donors. This experiment would give us further information on charge transfer in organic materials, which provides a possibility to fabricate a gas sensor with the charge transfer between donor and acceptor in organic metal complexes. The branching ratio I(L₃)/I(L₂+L₃), which depends on both the valence band spin–orbit coupling and the electrostatic interactions between core-hole and valence-electron, was also obtained in each complex and compared to the calculated values on metal oxides by Thole and Laan (1988b). As a result, the present study exhibited good correspondence to the case of high-spin-state of 3d-metals series from d⁴ to d⁷. As demonstrated in the study, the EELS method on organic materials is available to obtain information on electronic states.

Section snippets

Materials and methods

The MPc films were prepared by vapor deposition in a vacuum of $1×10^{−5} Pa$ on a freshly cleaved thin substrate of muscovite mica, on which the b-axes of needle-like phthalocyanine crystals were grown parallel to the substrate surface, except for the cases of TiOPc and MnPc. Before evaporation, the substrate was pre-heated at 573 K for 1 or 2 h and kept at 423 or 523 K during the deposition of MPc. The averaged thickness of the films was controlled with a quartz crystal microbalance to be about 10–20

Metal phthalocyanines

The films of FePc, CoPc, NiPc and CuPc exhibit epitaxial growth keeping their b-axes (molecular stacking axis) parallel to the substrate surfaces as shown in an electron diffraction and a low-magnified TEM image in Fig. 3 for the case of CuPc. This kind of epitaxy has been reported already by the previous studies of MPc (Uyeda et al., 1965, Ashida, 1966). The CrFPc film shows also epitaxial growth as shown in Fig. 4 and the diffraction pattern exhibits the film structure isomorphic with CuPc,

Concluding remarks

L_2,3 inner-shell spectra were obtained by EELS for the divalent first transition series metals in phthalocyanine complexes. It was found that the value of normalized total intensity of I(L₂+L₃) is nearly proportional to the formal electron vacancies of each 3d-state, and the values of the branching ratio, I(L₃)/I((L₂+L₃), represent a high-spin-state rather than low-spin-state for MnPc, FePc and NiPc.

EELS was also applied to charge-transfer complexes of FePc with amine. It is concluded that

Acknowledgements

This work was partly supported by a Grant-in-Aid for Science Research on Priority Areas from the Ministry of Education, Science, Sports and Culture, Japan.

References (19)

E.E. Koch et al.
High-resolution soft X-ray excitation spectra of 3d-metal phthalocyanines
Chem. Phys. Lett.
(1985)
H. Kurata et al.
EELS study of radiation damage in chlorinated Cu-phthalocyanine and poly GeO-phthalocyanine
Ultramicroscopy
(1992)
J.H. Paterson et al.
ELNES of 3d transition-metal oxides. II. variation with oxidation state and crystal structure
Ultramicroscopy
(1990)
J.H. Rask et al.
Determination of manganese oxidation states in solids by electron energy-loss spectroscopy
Ultramicroscopy
(1987)
B.T. Thole et al.
Spin-mixed ground state of Fe phthalocyanine and the temperature-dependent branching ratio in X-ray absorption spectroscopy
Chem. Phys. Lett.
(1988)
M. Ashida
The orientation overgrowth of metal-phthalocyanines on the surface of single crystals. I. vacuum-condensed films on muscovite
Bull. Chem. Soc. Jpn
(1966)
C. Colliex et al.
Diffusion inelastique des electrons dans un solide par excitation de niveaux atomiques profonds. I. spectres de pertes d'energie
Philos. Mag.
(1972)
R.F. Egerton
Electron Energy-loss Spectroscopy in the Electron Microscopy
(1996)
J.A. Horsley
Relationship between the area of L_2,3 X-ray absorption edge resonances and the d orbital occupancy in compounds of platinum and iridium
J. Chem. Phys.
(1982)

There are more references available in the full text version of this article.

Cited by (14)

Chemical interaction and electronic structure in a compositionally complex alloy: A case study by means of X-ray absorption and X-ray photoelectron spectroscopy
2021, Journal of Alloys and Compounds
Citation Excerpt :
The BR parameter is known to be very sensitive to the ground state symmetry [55,56]. Many different experimental studies have shown that the BR in TM compounds [45,57–60] and pure metals [45,51,61,62] strongly deviates from the statistical ratio of 2/3, which can be obtained for a free atom in the absence of electrostatic core-valence hole interactions in the final state 2p53dn+1 (n: d-electrons number in the atom initial state) of the dipole L2,3 transition and spin-orbit coupling in the initial 3dn state, that is, in the jj coupling limit [55]. A non-statistical BR of NEXAFS L2,3 spectra of TMs is a result of interplay between F2pd, G1pd, G3pd Slater integrals (2p – d electrostatic interaction) in the final state, spin-orbit coupling ξ2p of the core-hole and spin-orbit coupling ξd in the initial state.
Chemical interaction and changes in local electronic structure of Cr, Fe, Co, Ni and Cu transition metals (TMs) upon formation of an Al₈Co₁₇Cr₁₇Cu₈Fe₁₇Ni₃₃ compositionally complex alloy (CCA) have been studied by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. It was found that upon CCA formation, occupancy of the Cr, Co and Ni 3d states changes and the maximum of the occupied and empty Ni 3d states density shifts away from Fermi level (E_f) by 0.5 and 0.6 eV, respectively, whereas the Cr 3d empty states maximum shifts towards E_f by 0.3 eV, compared to the corresponding pure metals. The absence of significant charge transfer between the elements was established, pointing to the balancing of the 3d states occupancy change by involvement of delocalized 4s and 4p states into the charge redistribution. Despite the expected formation of strong Al–TMs covalent bonds, the Al role in the transformation of the TMs 3d electronic states is negligible. The work demonstrates a decisive role of Cr in the Ni local electronic structure transformation and suggests formation of directional Ni–Cr bonds with covalent character. These findings can be helpful for tuning deformation properties and phase stability of the CCA.
First-principles multi-electron calculations for L<inf>2,3</inf> ELNES/XANES of 3d transition metal monoxides
2006, Ultramicroscopy
A computational method having quantitatively predictive performance of L_2,3 ELNES/XANES of 3d transition metal (TM) compounds without any empirical parameter is given. Calculations are made on three TM monoxides with rock salt structures, MnO, FeO and CoO using model clusters composed of a TM ion with six coordinating oxide ions. Multi-electron wavefuctions are expressed by a linear combination of Slater determinants made by fully relativistic molecular orbitals for TM-2p, 3d and O-2p. Relative intensity and positions of subpeaks are in excellent agreement to experimental spectra. Our analyses of multi-electron eigenstates found that these subpeaks originate not from different one electron configurations but entirely from the multiplet effects. The dependence of the branching ratio on the formal number of 3d electrons agrees with that of an empirical atomic multiplet theory reported in literature.
Use of analytical microscopy to analyze the speciation of copper and chromium ions onto a low-cost biomaterial
2004, Journal of Colloid and Interface Science
Citation Excerpt :
EDXS has been used to identify and determine the extent of metal sorbed in different microstructures but is insensitive to its oxidation state. On the contrary, EELS, which directly measures the energy loss of incident electrons inelastically scattered from atoms, is a direct probe of the electron configuration around atoms and can determine the oxidation state of 3d and 4d transition metals with a high spatial resolution [9–11]. TEM–EELS techniques have been used for many years in Materials Sciences and have recently found applications in biology and environmental sciences [11–15].
In this paper, we present our study of the speciation of copper and hexavalent chromium sorbed onto a lignocellulosic substrate, using analytical microscopy. The lignocellulosic substrate constitutes a low-cost biomaterial that can be used in wastewater treatment. Transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDXS) was used to determine the speciation of the two metal ions on the constitutive moieties of the lignocellulosic substrate. The use of a staining agent sensitive to carbon unsaturation allowed us to differentiate between the microstructures rich in lignin entities and those rich in cellulose entities. The EDX analysis showed that metal ions are preferentially sorbed onto microstructures rich in lignin moieties. The energy electron loss spectroscopy (EELS) was used to determine the oxidation states of chromium in association with lignocellulosic moieties. We showed that the sorption process of hexavalent chromium requires the reduction of Cr(VI) into Cr(III) and the probable oxidation of lignin moieties.
Structural characterization of Fe(1 1 0) islands grown on α-Al<inf>2</inf>O<inf>3</inf>(0 0 0 1)
2003, Thin Solid Films
The morphology, size distribution, epitaxial relationships and lattice distortion in nanometer sized Fe(1 1 0) oriented islands grown on Al₂O₃(0 0 0 1) are studied and discussed as a function of the amount of deposited Fe. Combined scanning electron microscopy and transmission electron microscopy (TEM) as well as atomic force microscopy measurements show the dependence of these parameters with island size. For the smallest island sizes, an iron lattice distortion with respect to bulk values is found at their edges. Electron energy loss spectroscopy experiments demonstrate the absence of oxidation of the Fe islands either due to the high temperature deposition on Al₂O₃ or to the polishing procedure prior to TEM observations. From the analysis of the relative intensity for the L₂ and L₃ Fe peaks, a high spin ground state for the Fe atoms is deduced, probably correlated with the highly distorted regions at the edges of the islands.
All-electron CI calculations of 3d transition-metal L<inf>2,3</inf> XANES using zeroth-order regular approximation for relativistic effects
2009, Journal of Physics Condensed Matter
A theoretical study of the occupied and unoccupied electronic structure of high-and intermediate-spin transition metal phthalocyaninato (Pc) complexes: VPc, CrPc, MnPc, and FePc
2021, Nanomaterials

View all citing articles on Scopus

View full text

Branching ratio and L2+L3 intensities of 3d-transition metals in phthalocyanines and the amine complexes

Abstract

Introduction

Section snippets

Materials and methods

Metal phthalocyanines

Concluding remarks

Acknowledgements

Chem. Phys. Lett.

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

Chem. Phys. Lett.

The orientation overgrowth of metal-phthalocyanines on the surface of single crystals. I. vacuum-condensed films on muscovite

Bull. Chem. Soc. Jpn

Diffusion inelastique des electrons dans un solide par excitation de niveaux atomiques profonds. I. spectres de pertes d'energie

Philos. Mag.

Electron Energy-loss Spectroscopy in the Electron Microscopy

Relationship between the area of L2,3 X-ray absorption edge resonances and the d orbital occupancy in compounds of platinum and iridium

J. Chem. Phys.

Branching ratio and L₂+L₃ intensities of 3d-transition metals in phthalocyanines and the amine complexes

Relationship between the area of L_2,3 X-ray absorption edge resonances and the d orbital occupancy in compounds of platinum and iridium