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Macrocyclic molecules play key roles in basic processes in living organisms. Free bases and the metal complexes of porphyrins exhibit a wide range of important optical properties. In these systems the position of the most intense absorption band depends on the peripheral substituents of the macrocycle. Sol-gel methods have generally allowed the successful trapping of porphyrins into inorganic networks. The materials obtained are strong and transparent monolithic gels, but in the majority of cases the red fluorescence of the porphyrins disappears with ageing. We have evaluated the effect of the type and spatial disposition of the substituents in the porphyrin macrocycle periphery on key optical properties, with particular emphasis on the conservation of red fluorescence when porphyrins are simply trapped or covalently bonded to the inorganic matrix. Here, we report the use of the sol-gel procedures to obtain monolithic gels with the hydroxyl- or amino-substituted α, β, γ, δ-tetraphenylporphyrins, (H2T(S)PP), simply trapped or covalently bonded to the SiO2 matrix. [ABSTRACT FROM AUTHOR]

Particle track techniques, which enable estimation of the uranium and thorium contents in films where energy self-absorption is negligible, are presented. These techniques allow calibration of uranium and thorium thin films with adequate precision to be used in neutron fluence determinations for fission-track dating. Calibration via a particles is relatively simple for the case of uranium films, whereas for thorium films it is necessary to use the spectrometric characteristics of the employed track detector. Besides those based on a particles, calibration procedures via fission fragments are presented both for uranium as well as for thorium films. [ABSTRACT FROM AUTHOR]

A value of the decay constant for spontaneous fission of 238U, lf, determined by particle-track detector techniques, is presented. The main source of systematic error in fission-track measurements of lf, the so called “neutron dosimetry', has been avoided. Instead of calibrating the neutron source and the usual mica detector through tracks of 235U fission induced by thermal neutrons, spontaneous-fission tracks and alpha-particle tracks of 238U and 242Pu were used. A value of lf = (8.66±0.38).10-17 y-1 has been obtained. [ABSTRACT FROM AUTHOR]

In the last recommendation of the International Union of Pure and Applied Chemistry (I.U.P.A.C.) on spontaneous fission half-lives for ground-state nuclides, a number of measurements of 238U based on fission-track techniques were discarded. The arguments given by the authors are not clear. In this work a more detailed discussion of these determinations is given, considering the possible systematical errors inherent in fission-track approaches. [ABSTRACT FROM AUTHOR]