S. N. Terekhov

Electronic structure of metalloporphyrin π-anions

Abstract The transformation of metalloporphyrin molecules into their mono- and di-π-anions during contact of the metalloporphyrin solution with the Na and K metals has been examined by electronic absorption spectroscopy, ESR, 1 H and 13 C NMR, IR and Raman spectroscopy. The charge distribution is calculated by the CNDO/2 method. It is shown that the generation of anions is accompanied by characteristic changes in all the spectra and in the charge distribution. Hyperfine splitting in the ESR spectra of anion radicals of zinc complexes of monoaza- and diazaetioporphyrins is detected. ESR spectra are simulated on the basis of McConnel and Chesnuts relation and unpaired electron distribution. In 1 H NMR spectra of dianions large upfield shifts of proton signals under the influence of paramagnetic ring current are observed. The 13 C chemical shifts of neutral porphyrins correlate with the charge distribution, while those of the corresponding dianions are determined by changes in the mean average excitation energy, paramagnetic ring current, and electron density. The majority of the bands of IR and Raman spectra undergo a low-frequency shift on passing to anions. A comprehensive analysis reveals a qualitative picture of changes in specific bonds of a molecule and the electronic structure as a whole. These data are confirmed by quantum-chemical calculation of the electron density in dianions.

Binding of the cationic 5-coordinate Zn(II)-5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin to DNA and model polynucleotides: Ionic-strength dependent intercalation in [poly(dG-dC)]2

Interactions of water-soluble cationic porphyrins and their metal- derivatives with DNA attract interest of investigators as for fundamental studies (for example, the use of photoexcited porphyrins as probes of DNA local structure and dynamics [1]) and for applied biomedical investigations as well (for example, the photodestruction of viral nucleic acids in blood and blood components by using visible light [2]). In spite of fairly well elaborated formal classification of the types of porphyrin-DNA interactions, the relative selectivity of GC vs AT binding, the nature of the charge interactions between the porphyrins and the polymers, the exact geometries of the porphyrin-polymer complexes and at last the porphyrin-induced DNA structural distortions are not yet fully understood and are subjects to very active discussions [3].

Nanostructures formed by displacement of porous silicon with copper: from nanoparticles to porous membranes

The application of porous silicon as a template for the fabrication of nanosized copper objects is reported. Three different types of nanostructures were formed by displacement deposition of copper on porous silicon from hydrofluoric acid-based solutions of copper sulphate: (1) copper nanoparticles, (2) quasi-continuous copper films, and (3) free porous copper membranes. Managing the parameters of porous silicon (pore sizes, porosity), deposition time, and wettability of the copper sulphate solution has allowed to achieve such variety of the copper structures. Elemental and structural analyses of the obtained structures are presented. Young modulus measurements of the porous copper membrane have been carried out and its modest activity in surface enhanced Raman spectroscopy is declared.

Raman spectra of tetrapyrrole photosensitizer chlorin e6 and their interpretation

The resonance and surface enhanced Raman scattering (SERS) spectra of chlorin e 6 trisodium salt in aqueous solutions, solid films, and adsorbed on the silver-coated surface of porous silicon are presented. Using the quantum-mechanical density functional method, the geometric structure and vibrational frequencies of the chlorin e6 molecule are calculated and the Raman spectrum of this molecule is interpreted for the first time. The geometry of sorption of chlorin e6 on the surface of a nanostructured silver film is considered based on a comparative analysis of the resonance Raman and SERS Raman spectra in the approximation of a short-range mechanism of Raman scattering enhancement.

Determination of antimony by surface-enhanced Raman spectroscopy.

A highly sensitive method for the detection and quantitative evaluation of antimony(III) using the surface-enhanced Raman scattering (SERS) technique is demonstrated. The method is based on the analysis of SERS spectra intensity of antimony bound to phenylfluorone (Sb-PhF). Phenylfluorone is widely used as an organic reagent for the spectrophotometric determination of some heavy metals. For the SERS experiment a Sb-PhF complex was adsorbed onto the silvered porous silicon substrate. The significant degradation of the SERS signal was observed during measurements in the air. The time evolution of SERS spectra at ambient and degassed conditions was investigated to find an optimal regime for SERS measurements. The limit of Sb detection in degassed samples was determined to be near 1 ng/mL, which is one order of magnitude less than that attainable by the photometric approach. The linear range of the method to Sb(III) was found to a mass concentration range of 1–10 ng/mL. This approach permits an absolute quantity of Sb(III) to be detected at the picogram level (∼50 pg). It is remarkable that a very small sample volume (50 μL) is required for SERS analysis. Moreover this technique offers high selectivity owing to the distinctive vibrational features for the metallorganic complex and to the resonance character of Raman spectra. The proposed SERS-based detection of Sb is a fast and highly sensitive method for use in environmental and industrial waste monitoring as well as for forensic science to determine gunshot residue. We expect that the approach reported herein can be further extended to develop new detection techniques for other heavy metals.

Photoreduction of ferric-tetraphenylporphyrin in oxygen-containing solvents revealed by resonance Raman and absorption spectroscopy

Abstract Photoreduction is observed for Fe III (TPP)Cl (TPP is tetraphenylporphyrin) in some oxygen (O)-containing solvents under anaerobic conditions using resonance Raman (RR) and absorption spectroscopy. This process is found to be initiated by visible light in the 390–450 nm region. The coincidence of RR and absorption spectra of photoinduced species and of chemically reduced ones reveals that the final product of photoreduction is the high-spin Fe II (TPP)L complex, where L = THF (tetrahydrofuran), DMF (dimethyl formamide) or 1,4-dioxane. Such a photoreduction is not observed under anaerobic conditions for the Fe III (TPP)Cl complex in benzene, pyridine or CH 2 Cl 2 , nor in the abovementioned solvents under aerobic conditions. No photoreduction is observed for Fe III (OEP)Cl (OEP = octaethylporphyrin). A mechanism for the photoinduced phenomenon observed for Fe III (TPP)Cl in O-containing solvents under anaerobic conditions is proposed.

Silver-coated nanoporous silicon as SERS-active substrate for investigation of tetrapyrrolic molecules

A fabrication of the surface-enhanced Raman scattering (SERS) active substrates has been developed by immersion plating of the silver onto porous silicon (Ag-por-Si). Effects of the porous silicon morphology on the structure of the Ag films have been analyzed by scanning electronic microscope. Activity of the Ag-coated por-Si toward SERS was estimated using water-soluble porphyrins as analyte molecules. The Ag-por-Si samples were appeared to be sensitive and stable SERS-active substrates. It has been found that the variation of SERS signal scanned over the sample surface is less than 9%. Additionally, plates of Ag-por-Si displayed high sensitivity being stored within 1 month in the air. It was shown that even several minutes incubation of Ag-por-Si in solution is enough to obtain well-defined SERS signal. SERS spectra of the water-soluble cationic tetrakis(N-methylpyridyl)porphyrin (H2TMPyP4) and anionic tetrakis(sulfonatophenyl)porphyrin (H2TPPS) were compared with their resonance Raman (RR) spectra. For the cationic H2TMPyP4 the partial metallation has been detected during impregnation of the porphyrin molecules on the Ag-por-Si surface. In case of the anionic H2TPPS there was no evidence of the silver complex formation.

Surface-enhanced Raman spectra of a complex of antimony with phenylfluorone and their interpretation

We present the resonance Raman scattering and surface enhanced Raman scattering (SERS) spectra of a complex of antimony with phenylfluorone (Sb-PF). Using the quantum-mechanical density functional method, we have calculated the geometric structure and frequencies of vibrations of the Sb-PF molecule and proposed for the first time an interpretation of its Raman spectra. Based on analysis of spectral changes that occur upon formation of a complex of phenylfluorone with Sb, we have found lines—markers—that can be used for detection and quantitative determination of the content of antimony by the SERS spectroscopy method.

Progress in the Development of SERS-Active Substrates Based on Metal-Coated Porous Silicon

The present work gives an overview of the developments in surface-enhanced Raman scattering (SERS) with metal-coated porous silicon used as an active substrate. We focused this review on the research referenced to SERS-active materials based on porous silicon, beginning from the patent application in 2002 and enclosing the studies of this year. Porous silicon and metal deposition technologies are discussed. Since the earliest studies, a number of fundamentally different plasmonic nanostructures including metallic dendrites, quasi-ordered arrays of metallic nanoparticles (NPs), and metallic nanovoids have been grown on porous silicon, defined by the morphology of this host material. SERS-active substrates based on porous silicon have been found to combine a high and well-reproducible signal level, storage stability, cost-effective technology and handy use. They make it possible to identify and study many compounds including biomolecules with a detection limit varying from milli- to femtomolar concentrations. The progress reviewed here demonstrates the great prospects for the extensive use of the metal-coated porous silicon for bioanalysis by SERS-spectroscopy.

Resonance Raman and absorption properties of Zn(II) and Ni(II) polynitroporphyrins and their chemical reduction products

Electron-deficient metallocomplexes of dodeca- and octanitroporphyrins produced by the introduction of 8 β-nitro substituents or 8 β-nitro and a meta-nitro substituent on each meso-aryl ring of Zn(II) and Ni(II) [meso-tetra-(2,6-dichlorophenyl)porphyrin] (Zn8, Ni8 and Zn12, Ni12, respectively) and their air-stable reduced species have been characterized by steady-state absorption and Soret-excited resonance Raman spectroscopies. One-electron reduced species of the metallocomplexes were produced by three different procedures (in deprotonated tetrahydrofuran in air ambient, in tetralhydrofuran with the addition of piperidine in air ambient, and in contact with a sodium mirror under vacuum) and demonstrated similar absorption and RR spectra. It is concluded on the basis of the RR spectra that the one-electron reduction products of the studied polynitrosubstituted metalloporphyrins are π–anion radicals in character.