Many uranyl reactions, particularly those with organic acids, proceed, however, mainly or exclusively by light absorption in uranyl-anion complexes, which may absorb considerably more strongly than the free ions (cf. Chap. 2). Remarkably enough, it seems that in several cases excitation of a complex UO^'^A" is insxifficient to cause a reaction of UO^"*" with A" without a further encounter, [UO^''"A~]* + A". 1. PHOTOCHEMICAL REACTIONS OF URANYL IONS WITH INORGANIC COMPOUNDS Uranyl ions can serve in light either as oxidants, or as sensitizers for oxidation by other oxidants, particularly molecular oxygen (' autoxidation"). The results of photochemical experiments in which air was not rigorously excluded often are ambiguous because of the superimposition of these two phenomena. The best-known photochemical reactions of uranyl ions are those with organic compounds, such as formic acid, oxalic acid, and other fatty acids. Among reactions with inorganic reductants, only that with iodide has been investigated quantitatively, and even this reaction has been studied only with very crude techniques. 1.1 Oxidation of Iodide-Luther and Michie (1908) stated that uranyl salts "slowly precipitate iodine from potassium iodide solutions." This observation probably was made in the presence of light and air, and, most likely, refers to uranyl-sensitized photochemical autoxidation of iodide. Baur (1910), starting from a theory of the photogalvanic effect (Becquerel effect) in oxidationreduction systems, predicted that in the absence of air, light will cause a reversible shift of the oxidation-reduction equilibrium of the couples uranyl ion-uranous ion and iodine-iodide ion; he expected this shift to produce a strong photogalvanic effect. Trilimpler (1915) tried to detect the latter, but found only a very weak change of galvanic potential in light. He used a solution O.IM in UO2SO4, 0.02N in I2, 0,04N in KI and IN in H2SO4. It will be noted that it contained a considerable proportion of iodine, and we will see below that the photochemical reaction of uranyl ions with iodide ions stops with the formation of a small amount of free iodine. This may explain Trvimpler's negative results. That uranyl ions do react in light with iodide, even in the absence of air, was first observed, also in Baur's laboratory, by Hatt (1918). He noted that the liberation of iodine ceased after only a few per cent of the available iodide was oxidized. The final "photostationary" concentration of iodine depended on the intensity of illunnination, L, but increased much slower than proportionally with it. Let us assume that the reaction in light is a reversible oxidationreduction: "'^'' * "' dark (and'ught 7)"''^' ' '' '" The normal redox potential of the iodine-iodide couple is-0.535 volt and is independent of pH. The empirical uranyl-uranous potentials are variable and difficult to interpret because of complex formation, and probably also because of intermediate formation of U(V) ions (cf. Chap.), but from the thermodynamic data for the free ions U02"''' and ...

Reexamination of the theory of fluorescence time dependence owing to rotational diffusion of rigid macromolecules reveals deficiencies or hidden restrictions in each of the previous treatments. The correct master equation has five exponential decay terms, with preexponential factors that depend upon -the diffusion constants and, in a completely symmetrical fashion, upon the orientations of absorbing and emitting dipoles.If a dye molecule is bound in a definite way to a rigid macromolecule, and an isotropic solution of such molecules is excited by a short pulse of light polarized along a laboratory axis, the fluorescence will be polarized; the polarization anisotropy [r(t) ] will then evolve in time in a way determined by the characteristic reorientation rates of the macromolecule (1). Although the problem appears to be straightforward, previous treatments have not agreed upon the form of r(t). In a recent paper (2) (1, 4), and the results of a discontinuous jump model for reorientation (5) all allow for situations where the polarization ratio after the initial pulse is zero, but grows to a maximum and then decays back to zero in time; according to Tao (2), if r(0) = 0, then r(t) = 0 for all time. To resolve these discrepancies, we have carried through an independent solution to the problem and have obtained for r(t) a new expression, which is completely general and which contains as different special subcases corrected versions of the expressions given previously by Lombardi and Dafforn (3) and Tao (2). The formalism and results apply to a wide variety of experiments, not restricted to macromolecule fluorescence polarization decay; full details and discussion will be published later. The purpose of the present communication is to clarify the record by giving our' new expression and briefly comparing it with previous treatments.Our result for the problem posed by Tao (2) T= 1/(3D + 3Di).This expression can be derived by the procedure indicated by Lombardi and Dafforn (3). Our result, however, differs from theirs because we assume one absorption and one emission dipole vector fixed in general directions in the body, whereas special symmetry of either the absorption or emission probability is implicit in their treatment, and indeed was appropriate for the molecules that they considered. This special symmetry is that in which either the absorption or the emission probability (for a given orientation of polarizer or analyzer) is

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We studied the molecular dynamics of vanadyl-chelate complexes covalently attached to the surface of cascade polymers, dendrimers. The rotational correlation times of the ion-chelate complex were determined from computer simulations of their EPR spectra. The chelate 2-(4-isothiocyanatobenzyl)-6-methyldiethylenetriaminepentaacetic acid was covalently attached to ammonia core poly(amidoamine) (PAMAM) cascade polymers via a thiourea (TU) linkage, resulting in PAMAM-TU-DTPA cascade polymers. X-band EPR spectra of their vanadyl complexes were taken, and the A and g matrices were determined from the rigid limit spectra using the SIMPOW program. Spectra were fitted with modification of the slow-motional line-shape theory. Our results indicate that the rotational correlation times of the surface chelate increase with molecular weight and resemble those of "internal" segmental motions found in PAMAMs. For this macromolecular system, the rotational correlation times alone cannot account for differences in the relaxivity between high and moderate molecular weight species. These data are consistent with the hypothesis that the differences between linear-based and cascade polymer-based MRI contrast agents in the response of their relaxivity to molecular weight partially result from differing responses of their rotational correlation time to increases in molecular weight. A comparison of isotropic and anisotropic tumbling models indicates anisotropic tumbling of the ion-chelate complex at physiological temperatures, which is consistent with a model that incorporates segmental motions of the dendrimer side chains.