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2-NO 2 -5,10,15,20-tetraphenylporphyrin (H 2 TPP-NO 2 ) 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 Q X00 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 Q X state energy and an increase of its dipole moment µ with a decrease of the angle θ between the plane of the NO 2 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 Q X 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 S 1 f S n absorption show that the fluorescent Q X state of H 2 TPP-NO 2 has mainly 1 (π,π*) character with a relatively small charge-transfer admixture.
The spectral-luminescence properties of a tetraazachlorin derivative with a norbornene fragment annelated to a reduced pyrrole ring and its complexes with zinc and palladium have been studied at 293 and 77 K. For the norbornene-substituted free base, differences in fluorescence from unsubstituted tetraazachlroin and its dibenzobarrelene-substituted analog are found. The fluorescence lifetime is observed to rise by ~7 times for the free base and by ~1.6 ties for the Zn complex on going from 293 to 77 K. An essential dependence of the photophysical parameters on the nature of the solvent is noted. The fluorescence polarization spectrum of the norbornene-substituted tetraazachlorin reveals in the Soret band region at least four electronic transitions. For the Pd complex, weak phosphorescence in the near IR region has been detected; the 0-0 band maximum is at 990 nm and the singlet-triplet interval amounts to 5800 cm -1 , which is larger by 400 cm -1 than for Pd tetraazaporphine. The quantum yields of the photosensitized formation of singlet oxygen have been determined using a relative luminescence method.
UDC 535.37 Palladium and platinum complexes of benzo-fused hydroporphyrazines, analogs of phthalocyanine in whose molecules one or two isoindole fragments are substituted by hydrogenated pyrrole rings giving macrocycles of the chlorin, bacteriochlorin, and isobacteriochlorin types, have been synthesized. Phosphorescence in the near IR region has been detected at 291 and 77 K for complexes of tribenzotetraazachlorin and dibenzotetraazaisobacteriochlorin. The energy of the lowest triplet level T 1 has been determined from the positions of 0-0-bands in phosphorescence spectra. As compared to complexes of phthalocyanine, hydrogenation of a pyrrole ring has a stronger influence on the energy of the T 1 level (lowering) whereas hydrogenation of two adjacent pyrrole rings results in a larger S 1 -T 1 gap. The phosphorescence quantum yield and lifetime have been measured. Based on the data obtained, the rate constants of the radiative and non-radiative de-excitation of the T 1 level have been calculated. The highest phosphorescence quantum yield is obtained for Pt dibenzotetraazaisobacteriochlorin (1.7% at 77 K). The influence of the nature of the metal atom on the electronic absorption spectra of metal complexes of benzohydroporphyrazines is analyzed taking into account literature data for bacteriochlorophyll a, metal complexes of bacteriopheophytin a, and chlorophyll-like molecules having less complex structures.Keywords: palladium and platinum complexes, benzohydroporphyrazines, absorption spectrum, phosphorescence spectrum, phosphorescence quantum yield and lifetime, singlet oxygen formation.Introduction. The energy of the lowest triplet state E T 1 is an important physical property of molecules that influences the course of photophysical and photochemical processes. The value of E T 1 can be determined directly by measuring the phosphorescence spectrum. However, the phosphorescence quantum yield is in many instances small. This hinders the measurements, especially if uncontrolled impurities are present.The magnitude of the singlet-triplet gap E S 1 -E T 1 = ΔE ST , where E S 1 is the energy of the lowest singlet state, is approximately transferable in a series of related compounds so that E T 1 can be estimated for the whole series from the values of E S 1 and ΔE ST . For metal complexes of tetrapyrroles (loosely speaking, metalloporphyrins), the search for phosphorescence is best carried out for palladium and platinum complexes, which have higher phosphorescence yields than complexes of other metals.Two of us (E.A.L. and E.A.M.) prepared new compounds of the hydroporphyrazine subclass (porphyrazine is a synonym for meso-tetraazaporphine, H 2 PA H 2 TAP) with benzene and naphthalene aromatic rings fused to the nonhydrogenated pyrrole rings as the nickel and vanadium complexes and several free bases [1][2][3][4].Herein the Pd-and Pt-complexes of benzohydroporphyrazines are prepared. Phosphorescence in the near-IR spectral region is sought and is one of the goals of the synthesis. It is noteworthy that the spectra...
543.422+621.357Luminescent 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.Introduction. One of the main areas of progress in nanotechnology is the fabrication and formation of nanostructures with regularly distributed nanoclusters of a certain size and shape. This is due to a large extent by the fact that such geometric properties and the position of the clusters have a critical influence on many important properties of nanomaterials such as optical, electronic, magnetic, etc. Anodic alumina (AA) fabricated by electrochemical oxidation of aluminum in acidic electrolytes has much potential in this sense [1]. The grown AA substrates consist of an ordered periodic structure of nanopores that are perpendicular to the substrate surfaces and have diameters (10-300 nm) and distances between them (20-900 nm) that are regulated by the process conditions. Recently AA membranes have been widely used to fabricate nanostructures for application in solar energy, quantum electronics, and optics [2]. Detailed information on their properties, including optical, is needed in order to develop new nanocomposites based on substrates and films of porous alumina and to predict the properties of the fabricated materials.Questions about the photoluminescence of AA films have been discussed for 30 years. Even in the first research reports on AA films prepared under various fabrication conditions and annealing temperatures it was demonstrated that the photoluminescence spectra and intensity were independent of the purity of the starting aluminum and the impurities in it [3,4]. The strongest luminescence was observed if a solution of oxalic acid was used as the electrolyte. The hypothesis was formulated [3, 4] that the photoluminescence was associated with adsorption of H 2 O at active centers (defects) on the surface of the oxide film. Subsequent research on AA films fabricated in various electrolytes and with variable anodizing regimes [5] showed that the emission intensity was much greater if solutions of organic acids were used. The conclusion was drawn that carboxylic groups incorporated into the oxide films during the anodizing process were the luminescence centers. A similar viewpoint was expressed later [6, 7] that considered the photoluminescence due to traces of oxalic acid present in the AA as an impurity. Based on the obtained temperature dependences of photoluminescence and absorption spectra in addition to electron paramagnetic resonance [8,9], it was proposed that the photoluminescence and optical absorption in the wavelength range 200-600 nm was due to oxygen vacancies (F + -centers) in alumina membranes. Finally, the conclusion was made [10-12] that photoluminescence
UDC 535.37We have investigated the spectral characteristics, quantum yield ϕ F , lifetime τ F , and polarization of fluorescence of NH-isomers of dibenzobarrelene disubstituted tetraazaisobacteriochlorin at 293 and 77 K. The following information has been obtained from low-temperature measurements: 1) the weak shoulder on the edge of the long-wavelength 0-0 absorption band disappears at 77 K and, therefore, can be assigned to the cis-NH-isomer [the principal component of the tautomeric mixture is the trans-isomer a (b)]; 2) the low value of ϕ F at room temperature is due to temperature-viscosity quenching whereas ϕ F at 77 K is close to 1 and τ F = 5.3 nsec; 3) the energy of the weak electronic transition G → Q 2 and the interval ∆E Q 2 Q 1 ≈ 2750 cm -1 have been determined. Quantum-chemical calculations found that replacing the methine bridges by nitrogen stabilizes the non-aromatic trans-isomer a (b) whereas the non-planarity of the macrocycle and the alternation of bond lengths increase for the cis-isomers. The calculated electronic absorption spectra of the a isomer reproduce rather well the experimental data. It is shown that the four-orbital model is applicable for the lower excited states Q 1 and Q 2 for all NH-isomers.
The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tet rapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl substituted tetraazachlorins. The objects of this study include unsubstituted tet raazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl substituted tetraazachlorins, fluorescence buildup occurs as the tem perature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The sin glet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethyl hexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the low est excited electronic states Q x (S 1 ) and Q y (S 2 ).
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