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Статья Paper
NEXAFS Study of Zinc Porphyrins Intercalated into V2O5 Xerogel
Victor N. Sivkov,a@ Galina S. Zabrodina,b Sergei V. Nekipelov,a Olga V. Petrova,' Elena A. Shchupak,b Denis V. Vyalikh,c and Sergei L. Molodtsovc
aKomi Science Center Ural Division RAS, 167982 Syktyvkar, Russia
bG.A. Razuvaev Institute of Organometallic Chemistry RAS, 603950 Nizhniy Novgorod, Russia cTechnische Universitat Dresden, D-01062, Dresden, Germany @Corresponding author E-mail: svn@dm.komisc.ru
For the first time the NEXAFS N1s- and C1s- spectra of composites on the basis of macrocyclic molecules - zinc meso-tetrakis(4-pyridyl)porphyrin, zinc meso-tetrakis(N-methylpyridinium-4-yl)porphyrin tetraiodide and V2O5nH2O xerogel were studied by a total electron yield method with using synchrotron radiation of Russian-Germany beamline at BESSY-II. These intercalation complexes are insoluble in water, stable and exhibit good electrochemical properties. The host-guest interaction leads to the great changes of NEXAFS N1s- and C1s-spectra of the synthesized composites as compared with pristine compounds spectra. The studies show the 2.1 eV blue shift energy position and great increase oscillator strength of the n-resonance in the NEXAFS N1s- spectra ofpyridine cycle when ZnPor(Py)4 intercalated into V2O5 nH2O xerogel.
Keywords: NEXAFS, zinc porphyrin complexes, intercalation, vanadium pentoxide xerogel.
Introduction
The unique properties of the 3d-transition metal porphyrins and related macrocycles have widely applications in various fields in science and technology.[1] One of the such important applications is incorporation of metal porphyrin into porous or layered materials. Intercalated compounds are a special family of materials formed by inclusion of guest species into the layered host lattice. The intercalation reactions involving layered host lattices and different kinds of guest materials have been most extensively studied, because of the structural flexibility and ability to adapt to the geometry of the intercalated guest species by free adjustment of the interlayer separation and wide applications of these compounds.[2]
The important examples of this class of materials are the intercalation compounds on the basis of porphyrins and their analogues due to catalytical, photocatalytical, photochemical, photophysical properties of the macrocyclic ring. The intercalation complexes possess better catalytic activity compared to the starting porphyrins due to the molecular distribution of porphyrins in solid matrix.
Vanadium pentoxide xerogel has been widely studied as the host matrix for intercalation compounds prepara-tion.[3]
The materials based on the porphyrin derivatives intercalated into V2O5-nH2O xerogel possess the high application potential as catalysts, sensors and modified electrodes. Physical and chemical properties of these systems should be dependent on orientation of porphyrin ring inside V2O5-nH2O xerogel lattice and host-guest interaction. The synthesis and properties of intercalation compounds obtained by introduction of cationic forms of pyridyl substituted porphyrins from aqueous solutions
into V2O5-nH2O xerogel have been previously described.[4] These intercalation complexes are insoluble in water (unlike the starting reagents), stable, and exhibit good catalytical and electrochemical properties. The X-ray diffraction data of these intercalation compounds show the increase of the interlamella distances of xerogel. The stacking of the lamellar structure is dependent on the orientation of metalloporphyrin between the V2O5 layers. UV and IR-spectra indicate that the porphyrin derivatives in the composite material may be in the protonated form.
The aim of this work is to study the intercalation compounds based on the macrocyclic molecules of zinc meso-tetrakis(4-pyridyl)porphyrin (ZnTPyP), zinc meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (ZnTMPyP), and V2O5-nH2O xerogel (VPX) by the method of NEXAFS spectroscopy. The protonation of complexes should lead to the great changes of the electron density distribution in the porphyrin during intercalation and the great changes of near edge X-ray fine structure (NEXAFS) N1s - and C1s - spectra of the synthesized ZnTPyP-VPX, ZnTMPyP-VPX composites as compared with pristine compounds spectra. The NEXAFS spectroscopy is the available tool for the investigation both of the changes in porphyrin macromolecules and the TMPyP-xerogel interaction, because the NEXAFS-spectroscopy methods are characterized by atomic selectivity, dipole selection rules, fast response atomic composition and spatial conformation.[5]
Experimental
ZnTPyP-VPX and ZnTMPyP-VPX composite samples were produced by inclusion of cationic form of substituted porphyrins from aqueous solutions into V2O5-nH2O xerogel. The gel was supplied onto glass plates with a brush in a regular layer and
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NEXAFS Study of Zinc Porphyrins Intercalated into V2O5 Xerogel
dried in air or in a vacuum.141 The samples for the investigation by TEY (Total Electron Yield) method were prepared by the pressing compounds on the Cu metal surface. Additional Ti-films of the 160210 nm thickness mounted onto Au-grid with small mesh were used for the effective suppression and the measurement of the second-order light and the VUV-stray radiation.[6] The incident photon flux (E = 280-450 eV) was alternately measured using a TEY detector recording electron yield from the clean Au-photocathodes. Experiments were performed at BESSY-II using radiation from the Russian-German beamline. This dipole beamline was proven to be suitable for the measuring of absorption cross section in NEXAFS range of C1s- and N1s-edges.[7] The NEXAFS C1s- and N1s-spectra were acquired in a total electron yield mode. The resolution for the NEXAFS-measurements was varied from 80 to 100 meV.
Results and Discussion
Figures 1 and 2 show NEXAFS Cls- and Nls-spectra of the composite ZnTPyP-VPX, pristine ZnTPyP, ZnTMPyP-VPX, H2P, ZnTPP studied in this work, and C6H6 and C5NH5 molecules, studied earlier.[8] At present NEXAFS C1s- and Nls-spectra of H2P and ZnTPP molecules are well known both experimentally and theoretically.[9-11] NEXAFS 1s-spectra obtained in present work well agree with these researches. The C1s-spectra fine structure elements of studied compounds consist of three groups: low peaks A1 and A2 (except molecules), high peaks B1 and B2 (except C6H6) and wide bands C-E. In the spectra of benzene and pyridine there are no peaks A1 and A2, assigned to C1s^n* transitions in carbonic atoms ofthe flatporphyrin macrocycle,which consists of four pyrrole subunits bridged by four meso-carbon atoms. High peaks B1 (energy position at 285.0 eV in the benzene spectrum and 285.1 eV in spectra of the other compounds) and B2 (energy position at 285.6 eV in the composite spectra and protonated TMPyP and 285.4 eV in spectra of the other compounds) are assigned to C1s^n* transitions in carbonic atoms of the aromatic rings of pyridine and benzene. The second absorption peak B2 reflects the electronic transitions in carbonic atoms neighbouring nitrogen atom. Wide bands C-E are assigned to C1s^c* transitions. Figure 1 shows that the intensities of picks A1 and A2 as against B1 and B2 are decreased going from the spectra of H2P and ZnTPP to the spectra of TMPyP without changing energy positions, what is, probably, connected with increasing number of carbon atoms in these compounds. The ZnTPP and TMPyP C1s-spectra look like a simple superposition of the other two, confirming that there is almost no interaction between the n states of the macrocycle and those of the phenyl and pyridyl groups, respectively. Observed differences in C1s-spectra of TMPyP and ZnTPyP-VPX composite cover the peak B2 intensity and the energy position variations are, probably, connected with the redistribution electronic change differs of nitrogen atom and neighbouring carbonic atoms in the pyridine cycle, which must occur in the protonated forms. From this point of view, it is interesting to look at the NEXAFS N1s-spectra. In Figure 2 the X-ray absorption spectrum at the nitrogen 1s-edge of studied compounds are presented. Taking into account the NEXAFS N1s-spectra characteristic features we can divide these compounds into three groups: (1) H2P; (2) ZnTPP, ZnTPyP and C5NH5 molecule; (3) ZnTMPyP and composite ZnTPyP-VPX. The spectrum of H2P consists of two peaks - A (398.8 eV) and A (400.05 eV) connected
Figure 1. NEXAFS C1s-spectra of the synthesized composite ZnTPyP-VPX, initial compounds ZnTPyP, ZnTMPyP, H2P, ZnTPP and molecules of C6H6 and C5H5N.[8]
6 6 5 5
Figure 2. NEXAFS N1s-spectra of the synthesized composite ZnTPyP-VPX, initial compounds ZnTPyP, ZnTMPyP, H2P,
ZnTPP and molecule of C5H5N.t8]
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Energy, eV
Figure 3. NEXAFS N1s-spectra of the synthesized composites ZnTPyP-VPX, ZnTMPyP-VPX and initial compounds ZnTPyP, ZnTMPyP.
with the electronic transitions from the N1s core level to the lowest unoccupied molecular orbital of the macrocycle n* system in the nitrogen atoms (Mac N) and the protonated nitrogen atoms (Mac N-H), respectively. The chemical shift of 2.25 eV between these atoms ensues of the protonated nitrogen.1111 The NEXAFS Nls-spectrum of ZnTPP consists of peak A (398.4 eV) and doublet band C-D assigned to the transitions to the n* states of the porphyrin macrocycle. In the pyridine spectrum the intensity peak B (398.6 eV), assigned to transitions to the n* state of the pyridine ring, dominates. The shoulder A (398.3 eV) and high peak B (398.7 eV) in the ZnTPyP spectrum correspond to the peaks A and B in spectra of ZnTPP and pyridine, respectively. The presence of methyl group in ZnP(Py+Me)4 and the result in the NEXAFS Nls-spectra great changes are expressed in the blue shift energy positions of peak B. This effect is observed in the Nls-spectra of ZnP(Py)4 intercalated into V2O5^nH2O xerogel, that makes it possible to suppose an existence of the protonated nitrogen atoms in the ZnTMPyP intercalated into xerogel. The fact, that the blue shift value (2.1 eV) in the composite N1s-spectrum well correlates with the shift in the H2P N1s-spectrum (2.25 eV) due to nitrogen protonation. Figure 3 shows, that NEXAFS N1s-spectrum of ZnTMPyP
is identical to the NEXAFS Nls-spectra of the composite ZnTPyP-VPX. Thus, intercalation of these compounds into VPX does not reduce the additional protonation of nitrogen atom in pyridine groups.
Conclusions
Using synchrotron radiation by NEXAFS spectroscopy methods we have investigated the Cls- and Nls-spectra of the composites ZnTPyP-VPX and ZnTMPyP-VPX. For the first time we have identified the NEXAFS Nls-absorption spectra modification (2.1 eV blue shift of the pyridine cycle n*-resonance) when ZnTPyP molecule is intercalated into V2O5^nH2O xerogel. The investigation results show wide possibilities of NEXAFS spectroscopy methods for the protonated process studies of the macrocycle compounds and composites on its basis.
Acknowledgements. This work was supportedby the Bilateral Program of the Russian-German Laboratory at BESSY II. We would like to thank Olga N. Suvorova, Aleksei I. Kirillov (G.A. Razuvaev Institute of Organometallic Chemistry RAS, Nizhniy Novgorod) and D. Woehrle (Bremen University) for the given standard samples and for helpful discussion.
References
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Received 20.05.2011 Accepted 20.06.2011
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