Interaction of cationic 5,10,15,20-tetrakis(4-N-methyl pyridyl) porphyrin with mono-and polynucleotides: A study by picosecond fluorescence spectroscopy

The interaction of water-soluble cationic 5,10,15,20-tetrakis(4-N-methyl pyridyl) porphyrin (H₂TMPyP4) with some mono-and polynucleotides is studied by time-resolved and steady-state fluorescence spectroscopy, as well as by steady-state absorption spectroscopy. The fluorescence decay kinetics are analyzed by reconstructing the decay time distributions, which made it possible to describe in more detail than previously the complexes formed due to the interaction. The main effect of binding of H₂TMPyP4 adenosine 5′-monophosphate and to poly(dA-dT)₂ is shown to be an increase in the fluorescence lifetime from 4.6 ns in the solution to 8.3 and 12.3 ns, respectively. This effect is explained by a less polar (in comparison with water) environment of porphyrin in complexes, which leads to a decrease in the quenching action of the intramolecular charge transfer state between the porphyrin macrocycle and methyl pyridyl groups. In the case of complex formation with guanine-containing nucleotides (guanosine 5′-monophosphate and poly(dG-dC)₂), the effect of decrease in the quenching action of the intramolecular charge transfer state caused by a decrease in the medium polarity is superimposed by a stronger effect of decrease in the fluorescence lifetime of porphyrin as a result of intermolecular electron transfer from guanine to excited porphyrin. A high sensitivity of this intermolecular quenching to the mutual arrangement of the electron donor and the electron acceptor makes it possible to reveal four types of complexes between H₂TMPyP4 and guanosine 5′-monophosphate, which differ in the positions of four broad peaks in the porphyrin fluorescence decay time distribution (0.1, 0.7, 2.4, and 6.1 ns). For the complex with poly(dG-dC)₂, a narrow peak at 2.8 ns prevails in the fluorescence decay time distribution, with the contributions from two additional narrow peaks at 1.0 and 6.2 ns being small.     

Photoinduced axial ligation and deligation dynamics of nonplanar nickel dodecaarylporphyrins

The ground- and excited-state metal-ligand dynamics of nonplanar nickel(II) 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (NiDPP) and two fluorinated analogues (NiF(20)DPP and NiF(28)DPP) have been investigated using static and time-resolved absorption spectroscopy in toluene and in ligating media that differ in basicity, aromaticity, and steric encumbrance. Because of the electronic and steric consequences of nonplanarity, NiDPP does not bind axial ligands in the ground state, but metal coordination does occur after photoexcitation with multistep dynamics that depend on the properties of the ligand. Following the structural relaxations that occur in all nickel porphyrins within approximately 10 ps, ligand binding to photoexcited NiDPP is progressively longer in pyridine, piperidine, and 3,5-lutidine (25-100 ps) but does not occur at all in 2,6-lutidine in which the ligating nitrogen is sterically encumbered. The transient intermediate that is formed, which nominally could be either a five- or six-coordinate species, also has a ligand-dependent lifetime (200-550 ps). Decay of this intermediate occurs partially via ligand release to re-form the uncoordinated species, in competition with binding of the second axial ligand and/or conformational/electronic relaxations (of a six-coordinate intermediate) to give the ground state of the bis-ligated photoproduct. The finding that the photoproduct channel principally depends on ligand characteristics along with the time-evolving spectra suggests that the transient intermediate may involve a five-coordinate species. In contrast to NiDPP, the fluorinated analogues NiF(20)DPP and NiF(28)DPP do coordinate axial ligands in the ground state but eject them after photoexcitation. Collectively, these results demonstrate the sensitivity with which the electronic and structural characteristics of the macrocycle, substituents, and solvent (ligands) can govern the photophysical and photochemical properties of nonplanar porphyrins and open new avenues for exploring photoinduced ligand association and dissociation behavior.     

Formation of SERS-active silver structures on the surface of mesoporous silicon

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. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 298–306, March–April, 2009.     

Fluorescence of the <Emphasis Type="Italic">cis</Emphasis>-dimers of porphyrins containing ethylene bridges

The absorption and fluorescence spectra of solutions of the porphyrin dimers containing ethylene bridges, binding two Zn(II)octaethylporphyrin molecules or one Zn(II)octaethylporphyrin and one octaethylporphyrin molecule into cis conformation, were studied. For both studied cis-dimers, structureless absorption spectra were observed, only in general resembling the spectra of initial molecules, as well as weak fluorescence with a wide bell-shaped spectrum with a maximum at 680–700 nm and a quantum yield of about Φ_f≈3×10^?5, which is three orders of magnitude lower as compared to the fluorescence of initial monomeric porphyrins. A strong dependence of the fluorescence yield on a medium viscosity was revealed. In addition, low-yield irreversible cis-trans photoisomerization of cis-dimers was detected. A mechanism responsible for the strong intramolecular quenching of fluorescence of the porphyrin dimers containing ethylene bridges is suggested to be a conformation transformation in the region of the ethylene bridge, leading the molecular system to the region of “conic crossing” of the energy surfaces of excited and ground electron states.     

Effect of Nanocrystal Size on Anti-Stokes Photoluminescence in Lead Halide Perovskites

JETP Letters - We report size-dependent anti-Stokes photoluminescence (ASPL) of methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) under resonant laser excitation. The ASPL efficiency...     

Simulation of surface-modified porous silicon photonic crystals for biosensing applications

In this work realistic biosensing structures based on the integration of porous silicon photonic crystals with polymer coating technology are presented. Microcavities and rugate filters are chosen as the photonic crystal configuration. The deposition of a polymer layer on the pore walls of these structures is proposed to improve the selectivity and sensitivity of the sensing function. A complete effective refractive index model including the polymer layer, the target and external effects like silicon oxidation has been developed in order to accurately simulate the structures. It is expected that the proposed structures could be used as low cost, highly integrated and highly sensitive biological sensors.     

Influence of preparation and storage conditions on photoluminescence of porous silicon powder with embedded Si nanocrystals

The time changes of photoluminescence (PL) characteristics of porous silicon (porSi) powder during storing in different ambients have been reported. A porous silicon material with embedded Si nanocrystals of size of few nanometers was prepared by an electrochemical method from 10 to 20 Ωcm p-type Si wafers, and both constant and pulse current anodization regimes were used. A powder with a submicron average particle size was obtained by simple mechanical lift-off of the porous layer followed by additional manual milling. The air, hexane, and water as storage media were used, and modification by a nonionic surfactant (undecylenic acid) of the porSi surface was applied in the latter case. Dependence of PL characteristics on preparation and storage conditions was then studied. A remarkable blue shift of a position of PL maximum was observed in time for porSi powders in each storage media. In water suspension a many-fold build-up (10–30) of PL intensity in a time scale of few days was accompanied by an observed blue shift. Photoluminescence time behavior of porSi powders was described by a known mechanism of the change of porSi PL from free exciton emission of Si nanocrystals to luminescence of localized oxidized states on the Si nanocrystal surface.     

Inhibition and Promotion of the Oxidation Processes of Water-Soluble Cationic Co(II)-Porphyrin Under Its Interaction with Nucleic Acids

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(Co^II(TMpy-P4) and Co^III(TMpy-P4) with DNA and synthetic single- and double-stranded polynucleotides. It has been found that Co^II(TMpy-P4), which in aqueous solutions containing no nucleic acids is readily oxidized to Co^III(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 Co^II(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 Co^II(TMpy-P4) into DNA and model polynucleotides are proposed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 538–545, July–August 2005.     

PICOSECOND ABSORPTION STUDIES OF Zn-OCTAETHYLPORPHIN π-MONOANIONS

Abstract— Using the example of Zn-octaethylporphin π-monoanion solution in tetrahydrofuran, we studied the dynamics of ultrafast photoprocesses in porphyrin π-anions using a tunable picosecond absorption spectrometer. The deactivation kinetics of the lowest excited electronic (doublet) state of Zn-octaethylporphin iT-monoanion has been determined and was found to be followed by a single exponential function with the characteristic time 135 ps. It is shown that upon picosecond excitation to the long wavelength absorption band (λ_max= 830 nm) the process of two-quantum photoionization of the involved monoanion is easily realized, with the results that the observable kinetics of spectral changes exhibit a strong dependence on the excitation intensity.