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This work studies the impact of the electrostatic interaction between analyte molecules and silver nanoparticles (Ag NPs) on the intensity of surface-enhanced Raman scattering (SERS). For this, we fabricated nanostructured plasmonic films by immobilization of Ag NPs on glass plates and functionalized them by a set of differently charged hydrophilic thiols (sodium 2-mercaptoethyl sulfonate, mercaptopropionic acid, 2-mercaptoethanol, 2-(dimethylamino)ethanethiol hydrochloride, and thiocholine) to vary the surface charge of the SERS substrate. We used two oppositely charged porphyrins, cationic copper(II) tetrakis(4-N-methylpyridyl) porphine (CuTMpyP4) and anionic copper(II) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (CuTSPP4), with equal charge value and similar structure as model analytes to probe the SERS signal. Our results indicate that the SERS spectrum intensity strongly, up to complete signal disappearance, correlates with the surface charge of the substrate, which tends to be negative. Using the data obtained and our model SERS system, we analyzed the modification of the Ag surface by different reagents (lithium chloride, polyethylenimine, polyhexamethylene guanidine, and multicharged metal ions). Finally, all those surface modifications were tested using a negatively charged oligonucleotide labeled with Black Hole Quencher dye. Only the addition of copper ions into the analyte solution yielded a good SERS signal. Considering the strong interaction of copper ions with the oligonucleotide molecules, we suppose that inversion of the analyte charge played a key role in this case, instead of a change of charge of the substrate surface. Changing the charge of analytes could be a promising way to get clear SERS spectra of negatively charged molecules on Ag SERS-active supports.

Resonance Raman scattering (RRS) spectra of the complexes of 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin with Co(II) and Cu(II) (CoIITMpyP4 and CuTMpyP4) in various solvents and in complexes with DNA are studied. Under nanosecond pulsed excitation, additional lines have been found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity and in a complex with DNA. At the same time, such lines are absent in the spectra of CoIITMpyP4 in pure DMF, dimethyl sulfoxide, water, and alcohols under the same excitation conditions. To interpret the experimental data, we have calculated the structure and vibrations of the solvate complexes of CoIITMpyP4 and CuTMpyP4 with water, methanol, and formic acid in the ground and in excited states. Based on the obtained data, additional lines in the Raman spectra are assigned to the dd excited state corresponding to the d(z2) → d(x2 – y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in rate constant kic of the internal conversion due to an increase in energy gap ΔE between the ground and dd states of CoIITMpyP4. A decrease in the kic value in the case of binding of CoIITMpyP4 and CuTMpyP4 to DNA is caused by the additional interaction of the extra-ligand water molecule with one of its bases directly or via several water molecules immobilized on DNA surface. This complicates the conformational rearrangement of the water molecule in the dd excited state, which gives rise to an increase in the ΔE value.

The study of the excited states and photophysical characteristics of β-nitro-tetraphenylporphyrin (TPP–NO2) has been carried out using resonance Raman scattering (RRS) spectroscopy and methods of the density functional theory. The appearance of new lines, the intensity of which depends on the composition of the matrix and excitation wavelength, has been found in the TPP–NO2 RRS spectra in the low-temperature matrix. The calculation of the vibrational states of TPP–NO2 allowed the linking of the additional lines with the asymmetric vibrations of the nitro group and valence C–C vibrations of the phenyl ring (Ph1) that was nearest to it. The activation of these modes is related to the specific features of the TPP–NO2 geometry in the charge transfer (CT) state from Ph1 to the porphyrin macrocycle. It has been concluded on the basis of the analysis of the data of the study of the RRS spectra and the results of calculations that use the СAM-B3LYP and wB97XD functionals that the CT states do not play a significant role in the TPP–NO2 fluorescence quenching, as previously assumed. The fluorescence quenching owes to strengthening channels of internal and inter-conversion by reducing the energy gaps ΔE(S1 – T1) and ΔE(S1 – S0) as well as increasing the spin-orbit coupling between the S1 and T1 states. It has been shown that TPP–NO2 is characterized by conformational heterogeneity both in the ground and in the excited states, which explains the absence of the monoexponentiality of fluorescence decay kinetics.

This work aims at the impact of the electrostatic interaction between analyte molecules and silver nanoparticles (Ag NPs) on the surface enhanced Raman scattering (SERS) performance. For this, we fabricated nanostructured plasmonic films by immobilization of Ag NPs on glass plates and functionalized them by a set of differently charged hydrophilic thiols (sodium 2-mercaptoethyl sulfonate, mercaptopropionic acid, 2-mercaptoethanol, 2-(dimethylamino) ethanethiol hydrochloride and thiocholine) to vary the surface charge of the SERS-substrate. We used two oppositely charged porphyrins, cationic Cu(II)-tetrakis(4-N-methylpyridyl) porphine (CuTMpyP4) and anionic Cu(II)-5,10,15,20-tetrakis(4-sulphonatophenyl) porphine (CuTSPP4), with equal charge value and similar structure as model analytes to probe SERS signal. Our results indicate that the SERS spectrum intensity strongly, up to complete signal disappearance, correlates with the substrate’s surface charge that tends to be negative. Using the data obtained and our model SERS-system, we analyzed modification of Ag surface by different reagents (lithium chloride, polyethyleneimine, polyhexamethylene guanidine and multicharged metal ions). Finally, all those surface modifications were tested using a negatively charged oligonucleotide labeled with Black Hole Quencher (BHQ1) dye. Only addition of copper ions into the analyte solution allowed to get a good SERS signal. Considering strong interaction of copper ions with the DNA molecule, we suppose that the analyte charge inversion played the key role in that case, instead of the recharging of the substrate surface. Analyte recharging could be a promising way to get intensive SERS spectra of negatively charged molecules on Ag SERS-active supports.

Absorption and resonant light-scattering spectra of nanoparticles (NPs) of silver, and their complexes with water-soluble Cu(II)-5,10,15,20-tetrakis(4-N-methylpyridinium)-porphyrin (CuTMpyP4) and Fe(II)-5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (FeTSPP) have been compared. It is shown that in the presence of cationic CuTMpyP4, the band of surface plasmon resonance in the absorption and resonant scattering spectra of silver NPs is shifted to the long-wavelength region that is associated with the agglomeration of the particles caused by the Coulomb attraction between the silver particles and the porphyrin molecules. Addition of anionic FeTSPP to the silver NP solution does not lead to any spectral changes. The observed effect of silver-NP association induced by the cationic porphyrin can be used to develop an optical method for the detection of nanoparticles in solutions.

The review reports on the results of our research work on nanostructured metal films onto porous silicon. Principal steps of the techniques allowing fabrication of metal films completely inheriting morphological pattern of different types of porous silicon are presented. It is shown, that giving of the nanostructured pattern to metal films by means of porous silicon template opens their new structural, optical, mechanical and electrical properties, which can be successfully applied in nanoelectronics and biomedicine, particularly including devices based on superconductivity effect, SERS analysis with picomolar sensitivity and transdermal drug delivery by electroporation.

Preparation of the Au/Ag composite in the 1 : 1 water–ethanol mixture in the presence of poly-N-vinylpyrrolidone has been optimized; the so prepared composite is applicable in the absorption spectroscopy based on the effect of plasmon resonance energy transfer. Of the studied cationic, anionic, and non-ionic polymer, only the 340 kDa poly-N-vinylpyrrolidone induced the formation of a monodisperse Au/Ag sol with the average hydrodynamic diameter of the particles of ≈73 nm. The composite structure built of the polymer core covered with the Au/Ag nanoparticles has likely formed. The prepared composite binds both anionic and cationic metal porphyrins preserving the efficient transfer of the plasmon resonance energy.

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.

Nanostructured silver films with different thicknesses were prepared by vapor deposition onto the surface of the anodic aluminum oxide (AAO) template to be used as surface-enhanced Raman scattering (SERS) active substrates. Both the peak position of the surface plasmon resonance (SPR) band and SERS enhancement of silvered AAO samples displayed non-monotonous dependence on Ag layer thickness. Using 441.6 nm excitation and a water-soluble cationic porphyrin, Cu(ii)-tetrakis(4-N-methylpyridyl) (CuTMPyP4), as a SERS-reporting analyte, two maxima of the SERS enhancement were obtained for Ag layers of 15 and 120 nm thickness. Thickness dependencies have been analyzed taking into account the type of SPR modes identified by means of quasicrystalline approximation (QCA) of statistical theory of multiple scattering of waves and multi-Lorentzian deconvolution. The analysis revealed that SERS enhancement is related to the absolute magnitude of the distance between excitation wavelength and spectral position of collective SPR mode. It was shown that matching of excitation wavelength and the most intensive SPR modes with non-radiative decay, generated mainly by coherent interaction of higher-order plasmon resonant modes (quadrupole and octupole), plays a dominate role in SERS performance. Besides, it has been observed that more intense SERS signal can be obtained when the analyte deposited on the Ag/AAO substrate was excited through the AAO template rather than from the silvered side. Our results demonstrate that appropriate excitation geometry and fine-tuning of the optical properties of the Ag/AAO substrate by adjusting the thickness of the Ag layer with respect to particular excitation wavelength can contribute to more effective SERS enhancement.

A facile, one-step, and environmentally friendly fabrication of anisotropic gold nanostructures and size-controlled spherical silver nanoparticles (NP) using biopolymer pectin is reported. The reduction of Au and Ag ions was carried out at room temperature using an increasing concentration of pectin, which acts as the single source of reducing and stabilizing agent. The as-formed NPs were studied by UV-vis, infrared Fourier transform and surface-enhanced Raman spectroscopies, as well as transmission electron microscopy and energy dispersive X-ray spectroscopy. A high yield of anisotropic gold nanostructures was observed at low concentrations of pectin, while its increase results in the formation of smaller sharp edged perfect triangles with a considerable number of quasi-spherically shaped gold NP. On the other hand, the size of spherical silver NP decreased as the biopolymer concentration in the solution increased. The surface-enhanced Raman scattering enhancement of different NPs was evaluated using a Cu-complex of cationic tetrakis(4-N-methylpyridyl)porphyrin as a probe molecule at 441.6 and 532 nm excitation. Great enhancement of Raman signal was obtained with a pectin–silver NP and for most of them their levels were higher than that for the routinely synthesized citrate silver NP.

A bimetallic (Au)Ag nanocomposite with a core/shell structure was synthesized in aqueous solution and a H2O/EtOH mixture (1:1) containing polymers (carboxymethylcellulose, sodium polystyrenesulfonate, polyvinylpyrrolidone, dextran). Its structure and optical properties were characterized. The shape and position of scattering bands of colloidal noble-metal nanoparticles with optical density

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.

Since its discovery surface enhanced Raman scattering (SERS) has been attracting a constantly increasing attention of the scientific community as this method demonstrates an extremely high sensitivity and allows detection of the lowest possible concentration of substances including biological specimens up to single molecule [1]. The enormous increase of the Raman signal is observed for the molecules adsorbed on the surfaces consisted of metallic nanoparticles (NPs), especially of the noble metals. The electromagnetic mechanism plays the most valuable role in such enhancement and is caused by plasmon resonance in the certain places on the metallic surfaces. Recently, use of ordered arrays of metallic nanovoids has been found to significantly improve reproducibility and enhancement of SERS signal due to specificity of the plasmon modes within metallic cavities which geometry is well controlled at the fabrication process [2, 3]. However, engineering of such arrays is often connected with an alternation of a number of liquid and dry operations which is desirable to avoid for technology simplification [4]. An urgent problem as well is still a development of the technology that provides an integration of SERS-active substrate with other elements on the single Si substrate. In the present work we propose fabrication of SERS-active metallic/macro porous silicon (PS) nanovoid arrays consisted of only electrochemical and electroless liquid steps. As an initial template we used PS formed by anodizing of p-Si (100) wafer in solution of HF:DMSO=10:46 at 8 mA/cm for 7 min. The depth and diameter of pores varied in the ranges 700-1200 nm and 500-1500 nm, respectively. It is in good agreement with the dimensions of nanovoids usually applied for SERS [24]. Two types of metallized nanovoids (single and hybrid) were fabricated by liquid deposition of Ag and Ni on PS. To form first type of substrate based on single metal (Ag), PS was immersed into the 3mM AgNO3+0.5M HF water solution for 40-200 min. As reaction of Ag reduction has positive standard potential, Ag ions attract electrons from Si atoms. In that way in water-based solutions Si is oxidized to SiO2 while Ag is reduced. The addition of HF provides removing of SiO2 for continuous supplying of Ag ions with electrons. To form hybrid metallization (Ag/Ni), Ni was electrochemically deposited on PS from the solution of 213 g/l NiSO4—7H2O+5 g/l NiCl2—6H2O+25 g/l H3BO3+3 g/l saccharin at 10 mA/cm 2 for 2.5 min. Such regime resulted in covering PS surface with continuous Ni film of 70 nm thickness. After that Ag was deposited by electroless method from the previously described solution for 5-30 min. The morphology and structure of the samples were studied with SEM Hitachi S-4800. SERS activity of substrates was tested using water-soluble CuTMpyP4 as an analyte compound. Raman spectra were registered with the spectrometer Solar TII DM160MS3504I, equipped with CCDdetector. The source of continuous excitation was the HeCd laser Liconix (λ=441.6 nm). Figure 1 shows typical spectra of CuTMpyP4 on the initial PS (a), Ag/PS (b) and Ag/Ni/PS (c). Both of metallized substrates demonstrated SERS-activity. However, hybrid metallization resulted in six orders of magnitude increase of the signal intensity in comparison to pure PS while Ag/PS provided enhancement of five orders of magnitude.

Plasmonic nanostructures demonstrating an activity in the surface-enhanced Raman scattering (SERS) spectroscopy have been fabricated by an immersion deposition of silver nanoparticles from silver salt solution on mesoporous silicon (meso-PS). The SERS signal intensity has been found to follow the periodical repacking of the silver nanoparticles, which grow according to the Volmer-Weber mechanism. The ratio of silver salt concentration and immersion time substantially manages the SERS intensity. It has been established that optimal conditions of nanostructured silver layers formation for a maximal Raman enhancement can be chosen taking into account a special parameter called effective time: a product of the silver salt concentration on the immersion deposition time. The detection limit for porphyrin molecules CuTMPyP4 adsorbed on the silvered PS has been evaluated as 10(-11) M.

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.

A study of the water-soluble cationic CoII-tetrakis(N-methyl-4-pyridiniumyl)porphyrin, CoIITMpyP(4), complexed with calf thymus DNA and synthetic double-stranded and single-stranded adenine and thymine containing polynucleotides, was carried out by resonance Raman (RR) and transient absorption spectroscopies. Under high intensity nanosecond pulsed excitation, transient ν4* and ν2* bands appear in the RR spectra of CoIITMpyP(4) in the presence of DNA, poly(dA-dT)2 and poly(dT). On the basis of picosecond transient absorption measurements the lifetime of the corresponding transient species, ascribed to CoIITMpyP(4) electronic excited states of charge transfer (CT) origin (π, dz2), was found to be 10 ± 4 ns, i.e. three orders of magnitude longer than the excited state lifetime of free CoII porphyrin in aqueous solution. Such a considerable lengthening of the CT state is interpreted in terms of electron transfer from the pyridyl groups to the core ring, owing to the rotation of these groups and subsequent molecular flattening necessary to accommodate the porphyrin within the minor groove nearby T and AT sites.

Results are reported from a study of the optical properties of porous alumina films obtained by anodizing in a water solution of sulfuric acid and modified by thermal annealing in air at T ≥ 850°C. A comparative analysis of the data shows that the near-UV and visible photoluminescence of alumina anodized in a sulfuric acid solution is caused primarily by oxygen divacancies (F2, F $$ _2^{+} $$ , and F $$ _2^{2 + } $$ centers), while sulfate ions have little effect on the luminescence properties of anodic alumina in this spectral range.

A method for fabricating surface-enhanced Raman scattering (SERS)-active substrates by immersion deposition of silver on a macroporous silicon (macro-PS) template with pore diameters and depth ranging from 500-1000 nm is developed. The procedure for the formation of nanostructured silver films in the layers of macro-PS was optimized. Silver particles of dimensions in the nano- and submicron-scale were formed on the external surface of the macro-PS immersed in the water-ethanol solution of AgNO3, while the inner pore walls were covered by smaller, 10-30 nm diameter, silver nanoparticles. Upon introducing the hydrofluoric acid to the reaction mixture, the size of nanoparticles grown on the pore walls increased up to 100-150 nm. Such nanostructures were found to yield SERS-signal intensities from CuTMpyP4 analyte molecules of the same order to those obtained from silvered mesoporous silicon reported previously. The tested storage stability for the silvered macro-PS-based samples reached up to 8 months. However, degradation of the SERS intensity under illumination by the laser beam during spectral measurements was observed. To improve the stability of the SERS-signal a hybrid structure involving graphene oxide deposited on the top of analyte molecules adsorbed on the Ag/macro-PS was formed. A systematic observation of the time evolution of the characteristic peak at 1365 cm-1 showed that the addition of the oxidized graphene layer over the analyte results in ∼2 times slower decay of the Raman intensity, indicating that the graphene coating can be used to enhance the stability of the SERS-signal from the CuTMpyP4 molecules.

We have optimized the procedure for preparation of nanostructured silver films on the surface of mesoporous silicon (PSi) to use them as active substrates in surface-enhanced Raman scattering (SERS) spectroscopy. The greatest enhancement of the SERS signal was observed for samples obtained when the silver was deposited on PSi from an aqueous AgNO3 solution with concentration 1⋅10–2 M over a 10–15 minute period. The detection limit for rhodamine 6G on SERS-active substrates prepared by the optimized procedure was 1⋅10–10 M. The enhancement factor for the SERS signal on these surfaces was estimated as ≈2⋅108. We have shown that SERS-active substrates based on mesoporous silicon are promising for detection and study of complex organic compounds, in particular tetrapyrrole molecules.

Luminescent properties of anodic alumina fabricated in anodizing solutions containing oxalic, sulfuric, and phosphoric acids in addition to those modified by thermal annealing are investigated. Comparison of the obtained data shows that F+-centers are responsible for the luminescence band at 390 nm. The intense photoluminescence band of porous anodic alumina substrates at 450–500 nm is associated with oxalate anions.

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.

Using the methods of absorption spectroscopy and resonance Raman scattering, we have made a comparative study of water-soluble cationic Co(II)- and Co(III)-5,10,15,20-tetrakis(4-N-methyl-pyridiniumyl)porphyrins(CoII(TMpy-P4) and CoIII(TMpy-P4) with DNA and synthetic single- and double-stranded polynucleotides. It has been found that CoII(TMpy-P4), which in aqueous solutions containing no nucleic acids is readily oxidized to CoIII(TMpy-P4) by the molecular oxygen dissolved there, markedly changes its oxidation ability when bound to nucleic acid. Under interaction with guanine-containing polynucleotides, the rate of change in the oxidation state of cobalt from Co(II) to Co(III) increases as compared to such for the solution of porphyrin in a buffer. But in complexes with DNA and adenine- and thymine-containing polynucleotides, CoII(TMpy-P4), on the contrary, becomes stable. The observed effects of inhibition/promotion of CoII(TMpy-P4) oxidation when it is bound to nucleic acids are assumed to be due to the weakening/strengthening of the electron-donor properties of the extraligand on cobalt, which leads to a change in cobalt affinity to molecular oxygen. The role of immobilized water molecules in the change in the properties of cobalt extraligands has been considered. On the basis of the analysis of the spectral data obtained, modes for incorporating CoII(TMpy-P4) into DNA and model polynucleotides are proposed.

A comparative study of Co(II) and Co(III) complexes of the water-soluble cationic tetrakis(N-methyl-4-pyridiniumyl)porphyrin [CoII- and CoIIITMpyP(4)] with calf thymus DNA (CT DNA) and synthetic double-stranded (ds) and single-stranded (ss) polynucleotides, namely, poly(dA-dT)2, poly(dG-dC)2, [poly(dA)·poly(dT)], poly(A), poly(dT), poly(G) and poly(C) was carried out by resonance Raman (RR) as well as stationary absorption spectroscopy. It is shown that free CoIITMpyP(4) in aqueous solution is easily oxidized to CoIIITMpyP(4) by dissolved molecular oxygen, but Co(II) to Co(III) oxidation ability fundamentally changes when CoIITMpyP(4) is bound to nucleic acids (NA). In double helical DNA and AT-containing polynucleotides, where the porphyrin is assumed to be outside bound mainly in the minor groove near AT sites, Co(II) is stable for weeks and even months. It is suggested that Co(II) axial ligation by immobilized water (i.e. water molecule that is hydrogen bound to O or N atoms from DNA bases) results in such CoIITMpyP(4) stabilization. In contrast, CoIITMpyP(4) binding with ds and ss guanine-containing polynucleotides results in instantaneous Co(II) to Co(III) transformation. This can be explained by Co ion ligation by the guanine N7 atoms, which are known to be strong electron-donating ligands. It is assumed that partial intercalation occurs when CoIITMpyP(4) is bound with poly(dG-dC)2, in particular, at low ionic strength values, where cobalt ion is Co(III) in character. Copyright © 2005 John Wiley & Sons, Ltd.

By the resonance Raman scattering spectroscopy (RRS) method, photoinduced processes involving cationic Ni-5,10,15,20-tetrakis(4-N-methylapyridiniumyl)porphyrin (Ni(TMpy-P4)) in a phosphate water buffer have been investigated. Excitation into different regions of the Soret band at wavelengths of 397, 423, 441.6, and 457.9 nm permitted separate investigation of the behavior of the 4- and 6-coordinated forms of Ni(TMpy-P4). It has been found that Ni(TMpy-P4) is characterized by two photoinduced processes: the deactivation of initially 4-coordinated Ni(TMpy-P4) is followed by the addition to it, as axial ligands, of two water molecules from the environment to form a nonequilibrium 6-coordinated form Ni(TMpy-P4)(H2O)2 in the ground state, whereas in the process of deactivation of the excited 6-coordinated complex there occurs a dissociation of the axial water ligands to form a nonequilibrium 4-coordinated complex Ni(TMpy-P4) in the ground state. Some of the 4-coordinated molecules of Ni(TMpy-P4) are deactivated through the excited B 1g *-state without undergoing extracoordination. Quantitative analysis of the RRS spectra using the global fitting method has shown that in the course of deactivation ∼33% of photoexcited 6-coordinated molecules of Ni(TMpy-P4)(H2O)2 retain their axial ligands, relaxing through the “dark” long-lived intermediate state of the 6-coordinated form.

Resonance Raman (RR) spectra of Co(II) and Co(III) complexes of the water-soluble cationic tetrakis(N-methyl-4-pyridinyl)porphyrin [CoIITMpyP(4), CoIIITMpyP(4)] were obtained in aqueous and non-aqueous media, using both B- and Q-band excitation wavelengths. RR spectra of CoIITMpyP(4) are observed for the first time, and they are compared with those of CoIIITMpyP(4). Wavenumbers of the ν2, ν4, ν8 and ν6 oxidation state-sensitive bands depend on pH in aqueous solution and on the nature of the solvent (methanol, ethanol, DMF, DMSO): they lie in distinct spectral ranges for Co(II) vs Co(III) porphyrins. In addition, wavenumber shifts induced by axial ligation cover regions twofold broader for CoIIITMpyP(4) than for CoIITMpyP(4). This shows that the electronic influence of axial ligands on the π-system of the macrocycle is more effective when cobalt is in a higher oxidation state. This can be due to (i) smaller axial bond lengths of Co(III) porphyrin and, hence, more efficient cobalt–axial ligand interaction as compared with Co(II) complexes, and (ii) CoIIITMpyP(4) being involved in bis-adducts whereas CoIITMpyP(4) is mainly bound with only one axial ligand. The response of the porphyrin ring to the binding of axial ligands having various electron-donating–withdrawing properties was monitored by the behaviour of these oxidation-state sensitive RR bands: their shifts observed in a series of solvents were compared with Gutmann donor and acceptor numbers (DN, AN) of the solvents. A linear positive correlation was observed between Gutmann AN and ν2, ν4, ν8 and ν6 band wavenumbers for CoIIITMpyP(4), whereas a linear negative correlation was observed for the same bands of CoIITMpyP(4) when DN increases. This means that enhanced electron-accepting properties of axial ligand result in a scarcity of cobalt dπ-orbitals, which in turn leads to a decrease in π back-donation from cobalt to the porphyrin macrocycle. Copyright © 2003 John Wiley & Sons, Ltd.

Resonance Raman spectra (RRS) of Co(II)‐ and Co(III)‐5,10,15,20‐tetrakis(4‐N‐methylpyridinium)porphyrin ((CoII(Tmpy‐P4), and CoIII(TMPy‐P4)) in aqueous solutions at different pH as well as in organic solvents (methanol, ethanol, DMSO, DMF) are obtained. The increased sensitivity of the oscillation frequencies ν2, ν4, ν8, and ν6 — the “markers” of the oxidation state of a metal — to the nature of an axial ligand has been revealed. For CoIII(Tmpy‐P4), the shifts of the indicated frequencies in extracoordination have turned out to be twofold larger than those for CoII(Tmpy‐P4). The spectral effects observed are related to different electron influence of the extraligands on the π‐system of the porphyrin ring. In the case of Co(III)‐porphyrin, interaction of the dπ‐orbitals of the metal and the eg*‐orbitals of the macrocycle is more efficient since its ionic radius is smaller than for the Co(II)‐complex. For CoIII(Tmpy‐P4), a linear correlation between the oscillation frequencies ν2, ν4, ν8, and ν6 and the experimental Gutmann parameters characterizing the electron‐acceptor properties of solvents is found.

The resonance Raman scattering (RS) spectra of a series of nitro-substituted derivatives of the free base of tetraphenyl porphyrin that were obtained on continuous excitation near the Soret band have been investigated. For the molecules in which the NO2 group is located in the β-position of pyrrole rings or in the para-position of meso-phenyl substituents, an intense line of the mode within the range 1340–1355 cm−1 is discovered. Based on the obtained resonance RS spectra of the diprotonated derivatives of nitro-tetraarylporphyrins, it is assigned to symmetrical vibration of a nitro group (ν s NO2). Activation of the ν s NO2 vibration is indicative of the existence of the π-delocalization effect for a number of nitro-substituted porphyrins at which interaction between the molecular orbitals of the nitro-substituents and the porphyrin macrocycle occurs. Moreover, the π-delocalization is typical of both direct addition of a nitro-substituent to the porphyrin ring and of its binding via a meso-phenyl group. The results obtained demonstrate the informative value of the resonance RS spectroscopy for creating and studying model donor–acceptor systems with the participation of porphyrins.

We present a brief review of the results of an investigation into the electronic structure of π- and M-anions of tetrapyrrole compounds by the methods of molecular spectroscopy, namely, of electronic absorption and picosecond spectroscopy and also of IR, RRS-, EPR-, NMR1N- and NMR13S-spectroscopy. Experimental data are compared with the results of quantum-chemical calculations.

Resonance Raman (RR) spectra of meso-monosubstituted derivatives of metallo-octaalkylporphyrins were measured in solution and in solid form and compared with those of the analogous unsubstituted complexes. For planar complexes: (2,5,8,12,18-pentamethyl-3,7,13,17-tetraethyl-porphyrinato)zinc(II), 1 ; (2,8,12,18-tetramethyl-3,7,13,17-tetraethyl-5-phenyl-porphyrinato)zinc(II), 2 ; (2,3,7,8,12,13,17,18-octaethyl-5-methyl-porphyrinato)copper(II), 3 , appreciably large downshifts (9–13 cm −1 ) of ν 2 mode involving mainly C β C β stretching vibrations were detected. This fact was explained by steric interactions of meso-substituents with adjacent β -ethyl groups resulting in the increase of β -ethyl substituents hyperconjugation with the a 1u orbital antibonding for the C β C β bonds. RR studies also indicate the absence of any significant electronic and kinematic effects of meso-substitution. In the case of (2,3,7,8,12,13,17,18-octaethyl-5-methyl-porphyrinato)Ni(II) ( 4 ), (2,5,8,12,18-pentamethyl-3,7,13,17-tetraethyl-porphyrinato)Ni(II) ( 5 ), and (3,5,7,12,17-pentamethyl-2,8,12,17-tetraethyl-porphyrinato)Ni(II) ( 6 ) complexes the substitution with a single methyl group at one of the methine-bridge carbon leads to the nonplanar distortion with a ruffled type structure. The degree of nonplanarity is modulated by the kind and ordering of alkyl groups on the pyrrole rings and increases in the series of porphyrins 6→5→4 . The RR spectra of all meso-methylsubstituted Ni-porphyrins in solution show the evidence of the conformational heterogeneity because of the presence of conformers with various but close degree of the macrocycle distortion.

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].

2-NO2-5,10,15,20-tetraphenylporphyrin (H2TPP−NO2) is shown to exist in solution as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluorescence spectra of the tautomers contain two well-resolved narrow bands that are slightly (∼300 cm-1) Stokes-shifted with respect to the corresponding QX00 absorption bands, with the spectrum of the more stable tautomer being ∼500 cm-1 red-shifted as compared to the spectrum of the less stable tautomer. At room temperature, the tautomers show almost identical broad and structureless fluorescence spectra, markedly red-shifted relative to the longest wavelength absorption band. The Stokes shift increases with an increase of solvent polarity, being as large as ∼2000 cm-1 in N,N-dimethylformamide. Fluorescence lifetimes of the tautomers are found to be markedly (about a factor of 2) different, with both decreasing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical quantum chemical calculations. Charge-transfer (CT) states are found to be located between the porphyrinic Q and B states for both tautomers, and a different energy of the CT states gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the QX state energy and an increase of its dipole moment μ with a decrease of the angle θ between the plane of the NO2 group and the porphyrin plane (relative to the ground-state equilibrium geometry with θ = 80°). We suggest that, in solution, such an increase in the μ value should result in an increase of the stabilization interaction between the polar porphyrin and solvent molecules and, consequently, in flattening of the QX state energy curve as a function of θ and a shift of the minimum of this curve to θ values less than 80°. Both quantum chemical calculations and picosecond transient absorption measurements of the S1 → Sn absorption show that the fluorescent QX state of H2TPP−NO2 has mainly 1(π,π*) character with a relatively small charge-transfer admixture.

It is found that in absorption spectra of porphyrin dimers linked by a flexible bridge, the shape of the Soret band strongly depends on the solvent, which is explained by differences in the conformation composition of the dimers studied. The conformation composition was analyzed with the deconvolution method, which made it possible to resolve and sharpen all components of the complex contour, of the Soret band. Using different solvents, we studied changes in the behavior of every elementary band with a model of exciton (dipole-dipole) interaction and found that in all the solutions this dimer occurred in the form of face-to-face and side-to-side conformers. The relative content of face-to-face conformation markedly increased with the polarity of the solvent.

SERS-active arrays of bimetallic nanovoids have been fabricated by Ni electrochemical plating followed by Ag immersion deposition on macroporous silicon (PS). Presence of HF in the solution for Ag deposition has been found to result in fabrication of the SERS-active substrates with 20 times enhanced Raman intensity in comparison with those formed in HF-free solution. The detection limit of R6G adsorbed on the bimetallic substrates has been determined to reach 10 M.

It has been found that relatively small changes in conditions of crystallization in the synthesis process of μ-oxo dimers can lead to quite different products with dimeric or monomeric structures. Molecular structures of these products, discussed on the basis of Fe-octaethylporphyrin (FeOEP) complexes, have been studied using IR, NMR, EPR, resonance Raman (RR), mass and Mossbauer spectroscopies. The use of chloroform with admixture of alcohols (methanol, deuteromethanol and ethanol) in place of pure chloroform for crystallization of the eluate obtained during synthesis of μ-oxo dimers leads to the formation of alkoxy complexes instead of the μ-oxo dimers. It has been also established that dissolution of the μ-oxo dimers in chloroform-ethanol or chloroform-methanol mixtures leads to conversion of the μ-oxo dimers into the alkoxy complexes after evaporation of the solvents. Methoxy and ethoxy ligands in Fe(OEP)OCH 3 and Fe(OEP)OC 2 H 5 complexes can exchange the positions during addition of ethanol to the former and methanol to the latter complex in solution. The possibility of generation of doubly bridged structures of the type: OEP  Fe R R Fe  OEP where R is OCH 3 or OC 2 H 5 is discussed.

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.

Three types of Ag-coated arrays from porous anodic aluminum oxide (AAO) were prepared and studied as substrates for surface-enhanced Raman scattering (SERS). They were compared with Ag-coated porous silicon (PSi) samples. AAO-based substrates were prepared by the vapor deposition of silver directly onto the surface of porous AAO with different morphologies of the pores, whereas SERS-active island films on the PSi were prepared by immersion plating. The resulting metallic nanostructures were characterized by UV-vis absorption spectroscopy and scanning electron microscopy (SEM). Thermal evaporation leads to the formation of granular arrays of Ag nanoparticles on the surface of AAO. SERS activity of the substrates was tested using water-soluble cationic Zn(II)-tetrakis (4-N-methylpyridyl) porphyrin (ZnTMPyP4) as a probe molecule. The results indicate that all AAO-based substrates studied here exhibit some degree of SERS activity. Noteworthy, for excitation at 532 nm, signals from AAO-based substrates were comparable with those from the PSi-based ones, whereas for 441.6 nm excitation they were about twice higher. The strongest SERS-enhancement at 441.6 nm excitationwas provided by the AAO substrates with silver deposited on the monolith (originally nonporous) side of AAO. Preferential SERS-enhancement of the bands ascribed to the vibrations of the N-methylpyridinium group of ZnTMPyP4 when going to blue excitation was found. Copyright © 2010 John Wiley & Sons, Ltd.

Porous silicon (PS) prepared from n-type Si crystal is proposed as a new material for the fabrication of sensitive substrates for surface-enhanced Raman scattering (SERS). The formation procedure for nanostructured silver films on the surface of PS was optimized. Maximum of SERS enhancement for rhodamine 6G probing molecule is observed for samples obtained by the immersion plating from the water solution of AgNO 3 with the 10 mM concentration during 5 min. The dependence of morphological parameters of PS and corresponding silvered surfaces on the anodization current density has been studied. It is shown that the most SERS activities possess substrates produced from PS with lower porosity. The optimum of the PS layer thickness for high Raman signal is about 5 μm. The detection limit for rhodamine 6G adsorbed on Ag-coated PS from the 100 pM solution is established to be comparable with that for p-type PS-based substrates. Thus, the n-type porous silicon is suitable material for the preparation of sensitive SERS-active substrates.

Fourier transform infrared spectra of chlorin e(6) and its trisodium salt are presented. The geometrical structure and frequencies of normal vibrations of both compounds were calculated using density functional method with the PBE exchange-correlation functional. The frequencies were assigned and IR spectra of the studied molecules were interpreted for the first time. Spectral effects of degradation of chlorin e(6) and its trisodium salt upon their storage on the air are analyzed. The possible structures of degraded species are discussed.

5,15-Bis(4-N-methylpyridyl)-10,20-diphenylporphyrin (5,15-DiMPyP, also known as t-H2Pagg) is capable of forming long-range ordered DNA-templated aggregates in solution. The present work reports the study of fluorescent properties of 5,15-DiMPyP, both monomeric and aggregated in presence of DNA, performed by means of steady-state and time-resolved fluorescence spectroscopy. The S1 state of free monomeric 5,15-DiMPyP is affected by a low-lying intramolecular charge transfer state which is destabilized upon porphyrin monomer binding to DNA. The aggregates, DNA-templated and random, of 5,15-DiMPyP were found to be emissive in solution and had respective S1 state lifetimes of 1.8 ns and 5.2 ns.

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 (H 2 TMPyP4) and anionic tetrakis(sulfonatophenyl)porphyrin (H 2 TPPS) were compared with their resonance Raman (RR) spectra. For the cationic H 2 TMPyP4 the partial metallation has been detected during impregnation of the porphyrin molecules on the Ag- por -Si surface. In case of the anionic H 2 TPPS there was no evidence of the silver complex formation.