A. Yu. Panarin

Relaxation processes with participation of excited S1 and T1 states of spatially distorted meso-phenyl-substituted octamethylporphyrins

We have performed comparative analysis of the spectral properties and photophysical parameters of spatially distorted octamethylporphyrins (OMPs) upon varying the number (from one to four) and position of bulky meso-phenyl substituents in liquid solutions at 293 K and in solid media at 77 K. It has been substantiated that, for the series of studied compounds, which are characterized by a high degree of nonplanar distortions, considerable changes in the spectral-kinetic properties of singlet and triplet states at 293 K in liquid solutions are determined by the dynamic relaxation of the tetrapyrrole macrocycle that is caused by steric interactions of meso-phenyls with neighboring bulky groups β-CH3. The decay of T1 states of mono- and di-meso-phenyl-substituted (trans) OMP molecules in liquid solutions has a biexponential character, which is indicative of conformational rearrangements of the macrocycle in the T1 state. We are the first to measure the phosphorescence of spatially distorted metal-free OMP molecules in solid solutions at 77 K. It has been sub-stantiated that the decrease in the phosphorescence lifetime and quantum yield observed in the series of studied compounds with increasing number of meso-phenyl substituents at 77 K is related to a gradual increase in the probability of the nonradiative intersystem crossing, caused by the lowering of the energy of the T1 level. We have revealed that spatially distorted tetrapyrrole macrocycles in rigid media at 77 K in the ground state are represented by two conformations. It has been found that the oxygen quenching rate constants of excited S1 and T1 electronic states of planar and spatially distorted porphyrin molecules are close to each other and depend weakly on the character distortion of the macrocycle. We have shown that the singlet oxygen generation quantum yield (γδ) of studied compounds depends on the number of meso-phenyl substituents in the OMP molecule and is determined by the character of the conformational dynamics of the tetrapyrrole macrocycle at 293 K.

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.

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.

Photophysical properties of porphyrins with sterically distorted and partially screened macrocycles

We have experimentally studied the spectral-luminescent and photophysical properties, as well as interaction processes with molecular oxygen, of eclipsed β-alkyl-substituted porphyrins para-methoxyphenyl rings of which in opposite meso-positions of the macrocycle are linked with each other in the ortho-position with the (-OCH2)-Ph-(CH2O-) bridge. Using methods of the density functional theory, we have calculated the structures of these porphyrins in the ground and lowest triplet states, electronic transitions and vibrational states, and matrix elements of the direct spin-orbit interaction. It has been found that the lifetime of the T1 state of the investigated compounds is noticeably shortened due to the enhancement of the internal conversion. This enhancement is caused by the conformational dynamics, which promotes increasing Franck-Condon factors and the spin-orbit interaction between the T1 and S0 states. Nevertheless, this does not lead to a considerable decrease in the singlet-oxygen formation quantum yield. We have calculated the structures of a porphin dimer and possible dimeric forms of examined porphyrins. It has been shown that distortion of the porphyrin macrocycle according to type of ruffling weakly affects the binding energy of the porphyrin macrocycles in the dimer, this effect being unable to prevent aggregation. At the same time, the screening of the macrocycle by the phenyl ring lowers the probability of formation of oligomeric structures, which, in view of the determined photophysical properties of investigated compounds, may be used in photochemical applications that require elevated concentrations of porphyrins.

Spectral manifestation of surface plasmon resonance of silver nanoparticles upon interaction with water-soluble metalloporphyrins

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.

Formation of a gold/silver-containing composite

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.