Absorption and fluorescence of 8-azasteroids in the gas phase

Extensive studies of the spectral-luminescent characteristics of four 8-azasteroids and a model compound 2-(3,3-dimethyl-1,2,3,4-tetrahydro-1-isoquinolidene)-5,5-dimethyl-1,3-cyclohexanedion in the gas phase have been made. From the analysis of the dependences of the absorption spectra on the vapor pressure (T_low) and the fluorescence spectra on T_low and the exciting radiation wavelength (_exc) a conclusion on the presence in the vapors of the investigated 8-azasteroids of three absorption and fluorescence centers (S-, M-, and L-centers) has been drawn. The absorption spectra of these centers strongly overlap. Their long-wave absorption boundaries have been determined. The dependence of the fluorescence spectra of all three centers on _exc, which is inherent in rarefied gases of individual organic molecules, is observed. The S-centers are the molecules of the initial steroids, and the M- and L-centers are the molecules of thermo- and phototransformations of the initial steroids. The model compound in the gas phase is characterized by the same dependences of the fluorescence spectra on T_low and _exc as those inherent in 8-azasteroids. Taking into account the additional data obtained as a result of investigation of the absorption and fluorescence spectra of solutions of the substances extracted from vacuum cells after the investigation of 8-azasteroids and the model compound in the gas phase, conclusions on the nature of the M- and L-centers have been drawn.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 48–57, January–February, 2005.     

Synthesis and spectral-luminescence properties of the conjugate of 24-epibrassinolide with porphyrin

The synthesis of a previously unknown conjugate of 24-epibrassinolide with porphyrin is described. Four molecules of 24-epibrassinolide are bonded to a molecule of porphyrin by selective formation of cyclical boric ethers in diol groups of the side chain. Electronic spectra of the synthesized conjugate of 24-epibrassinolide with porphyrin are measured and analyzed. The spectral and luminescence characteristics of the initial porphyrin and the conjugate are found to be similar. The quantum yield and fluorescence lifetime of the conjugate in tetrahydrofuran are 0.06 and 10.6 ns, respectively. The extinction coefficient of the conjugate in tetrahydrofuran is 385,000 (M∙cm)^–1 at a wavelength of 418 nm.     

Specific Fluorescence Properties and Picosecond Transient Absorption of 8-Azasteroids

The specific fluorescence properties as well as picosecond transient absorption features have been studied for two 8-azasteroids. It is shown that at various excitation wavelengths the essentially different final excited electronic states are realized. Because of the multicenter character of 8-azasteroids the spectroscopic data obtained may be analyzed on a basis of the mesomeric tautomerism model taking into account the dynamic combination of cis- and trans-configurations. The dependence of fluorescence spectral characteristics on the solvent nature is a manifestation of intermolecular H-bond interactions.     

Electronic absorption spectra of antiviral aminophenol derivatives

Electronic absorption spectra of aminophenol derivatives in solutions have been studied. A general property of the absorption spectra of these compounds, the dependence of the maximum of a long-wavelength absorption band on the solvent polarity, is revealed. As a rule, the absorption band maximum of compounds possessing pharmacological properties shows a greater shift to short wavelength with an increase in the medium polarity than that of inactive compounds. Absorption measurements of solutions of aminophenol derivatives can be used for a tentative estimation of their antiviral activity.     

Delayed Fluorescence and Phosphorescence of 8-Aza-d-Homogonane in the Gas and Condensed Phases

The delayed fluorescence spectrum for 8-aza-d-homogonane in the gas phase consists of a band of E-type delayed fluorescence of M-centers and a band of P-type delayed fluorescence of L-centers being the products of photo- and thermotransformations of a basic steroid. Triplet-triplet energy transfer from the M-centers to the L-centers is established and its efficiency is determined. For 8-aza-d-homogonane in frozen hexane solutions at T=77 K only the phosphorescence of the M- and L-centers is revealed. The phosphorescence of the basic steroid itself ( nm) as well as that of the M- and L-centers ( and 532 nm, respectively) is seen in a mixture of frozen tetrahydrofuran and toluene solutions. This is evident of the fact that the basic steroid has the products of its transformations, whose amount grows due to irradiation and heating. The M- and L-centers are stable molecules.     

Near Infrared Phosphorescent,Non‐oxidizable Palladium and Platinum Perfluoro‐phthalocyanines

New Pd^II and Pt^II complexes with a highly electron‐deficient ligand (H₂PcF₆₄) were conveniently prepared in a three‐step synthesis. This is the first time that the phosphorescence of phthalocyanines with a H₂PcF₆₄ framework has been measured. Based on these measurements, the triplet‐state energies (E_T) were directly determined. Transient absorption experiments revealed broad T₁→T_n absorption spanning from ca. 350 to ca. 1000 nm and allowed determination of the triplet‐state lifetimes. Removal of the Pd or Pt from the perfluoro‐phthalocyanine resulted in a significant increase of the triplet lifetime for H₂PcF₆₄. The very efficient intersystem crossing observed for both PdPcF₆₄ and PtPcF₆₄ leads to residual fluorescence and suppresses the fluorescence lifetimes to less than 50 ps. The absence of Pd and Pt in the perfluoro‐phthalocyanine ligand, viz. H₂PcF₆₄, led to a recovery of fluorescence. Cyclic voltamperometry studies pointed to complete resistance of PdPcF₆₄ and PtPcF₆₄ to oxidation and very strong electron affinity, which rendered these materials very good electron acceptors (n‐type materials). The presence of d‐orbital metals such as Pd^II and Pt^II in the phthalocyanine ring stabilizes their reduced forms, as indicated by the spectroelectrochemical experiments. PdPcF₆₄ and PtPcF₆₄ easily sensitize singlet oxygen production with very high quantum yields. Both phthalocyanines presented resistance to photodegradation in the solid state under aerobic conditions and under intense irradiation.