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ISSN 1998-9539

XPS Study of Asymmetrical Tetraarylporphyrins and Their Ytterbium Complexes

Rais N. Mozhchil,a,b@ Aleksey P. Menushenkov,a Andrey M. Ionov,b Andrey F. Mironov,c and Valentina D. Rumyantsevac,d
aNational Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409 Moscow, Russia
bInstitute of Solid State Physics, Russian Academy of Sciences, 142432 Moscow, Russia
cM.V. Lomonosov Moscow State University of Fine Chemical Technologies, 119571 Moscow, Russia
dKotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, 141120 Fryazino, Moscow Region, Russia
@Corresponding author E-mail:
Organic molecular films of RE porphyrins have attracted attention over the past years in view of potential applications as templates for nanomaterials synthesis, biology, and medicine. As shown for a wide class of porphyrins one can vary electronic state by ligands and metal doping. Additional interest in ytterbium porphyrins complexes is determined by a possibility of their use as effective markers in luminescent diagnostics of malignant tumors since porphyrins are capable to accumulate in various types of cancer cells and tumors microvessels. Such substances can be used at present for the cancer photodynamic therapy. Unlike the free bases of porphyrins ytterbium complexes under the light irradiation do not create toxic concentration of singlet oxygen 1О2 retaining a high affinity to malignant tumors at the same time. For understanding and tailoring their properties knowledge of electronic structure and bonding in these compounds are required. The occupied and empty states of porphyrins which are involved in chemical bonding are essentially responsible for promising properties and should be studied by optic photoelectron spectroscopy.
In present work electronic structure and chemical bonding in tetrakis-porphyrins and precursors were studied experimentally by X-ray (XPS) photoemission spectroscopy. To obtain this information high resolution of N1s, C1s, valence band spectra were measured at Kratos AXIS Ultra DLD spectrometer (the photon energy 1486.69 eV, Al Kα mono) (total resolution Ag 3d5/2 was about 0.48 eV). Spectra were calibrated using Ag3d5/2 and In3d5/2 lines. Samples for X-ray photoemission spectroscopy were prepared by chemical deposition (drop coating solution of the compounds) self-assembling techniques ex situ from the CHCl3 or CCl4 solution onto Ag and/or the sample was pressed in an indium substrate ex situ. The surfaces of ex situ prepared samples were additionally cleaned in situ by resistive heating up to ~ 400 K in ultra high vacuum (UHV) and/or cleaned by ion gun. The base pressure during measurements was in the range 5∙10-10–2∙10-9 torr. All elements of pristine TPP and metalloporphyrins were found in XPS spectra (photon energy 1486.69 eV (Al Kα) after moderate annealing in UHV. The X-ray photoemission data show these different atomic constituents in accordance with its states in the molecules which can be related to the peaks of N1s, C1s, O1s, Yb4d appearing in the electronic spectra. Different peaks were seen in the C1s spectra related to the chemically unequivalent C atoms in the molecules. Wide peak in the range 282 and 290 eV were seen in the wide C1s spectra that are related to the unequivalent C atoms in the molecules (aromatic at 284.5eV, 286.5 eV и 288.5 eV for metoxycarbonil and acetilacetone group respectively). There are two peaks at the spectrum of the core levels of the N1s level in the pristine TPP. These N1s levels with binding energies of 399.8 and 397.8 eV were assigned to sp3 and sp2 nitrogen respectively (pyrrol- and aza-states associated with the protonated and not protonated nitrogen). The central atom of Yb, replacing the two hydrogen atoms has bound to the all equivalent the nitrogen atoms. In symmetric Yb metalloporphyrin charge distribution is more uniform for N1s spectra, thus wide peak of N1s states shows small difference between pyrrole- and aza-nitrogen in metalloporphyrins, in consequence of that formed a single N1s state. In N1s spectra of asymmetric Yb pyridyl-phenyl complexes two peaks of nitrogen structure were observed at 399.6 and 398.5 eV related with pyridyl group and central Yb-N group features. Note that there are imposing the peaks of N1s and Yb4p, whose parameters are derived from the decomposition peak at Yb4p1/2 in Yb oxide. In the O1s core spectra of asymmetric triscomplexes the intensity of peak at 533eV (related with methoxycarbonil group) is smaller compare with tetracomplexes in accordance with structural formula. The analysis of Yb4d electronic states shows that spectra do not consist of the usual spin-orbit split doublet, but instead they are composed of asymmetric peak with multiplet splitting. For Yb two cases are possible. Divalent Yb has a filled 4f shell, i.e. a 4f14 configuration, and the 4d spectra show the usual doublet with a 3:2 ratio, while for trivalent Yb, 4f13, the 4d peaks consist of a multiplet. Using the parameters of metal Yb analysis and Yb sesquioxide we can conclude that the multiplet 4d spectrum of Yb is clearly indicative of trivalent state of Yb atom in symmetric.Yb(acac)-5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrin, and assymmetric.Yb(acac)-5-(4-pyridyl)-10,15,20-tris(4-methoxycarbonylphenyl)porphyrin, Yb(acac)-5(3-pyridyl)-10,15,20-tris(4-methoxycarbonylphenyl) porphyrin.



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