Fractal Structure in Ionization Dynamics

Chaotic dynamics in open systems produces fractals. This can be seen, for example, in the ionization of an electron from a hydrogen atom in applied parallel electric and magnetic fields. We summarize some of our recent work on the order that can be found in these fractals.     

Electronic interactions in tertiary oligophenylureas

The syntheses, structures, and spectroscopy of a series of oligomeric tertiary oligophenylureas possessing one to five phenyl rings are reported. A convergent synthetic method employing tertiary monoamine and diamine building blocks is employed. NMR and molecular modeling are indicative of folded structures for all of the oligophenylureas in which adjacent phenyl rings have a splayed face-to-face geometry. NMR chemical shifts, absorption and emission maxima, and electrochemical oxidation potentials are all dependent upon the number of phenyl rings. The addition of a first inner phenyl has a pronounced effect on the chemical shifts, while a second and third inner phenyl have diminished effects. The oxidation potentials of the oligophenylureas display an abrupt decrease upon the addition of the second inner phenyl. The absorption and emission spectra are relatively insensitive to the addition of one to three inner phenyl rings. The electronic structures of the oligophenylureas possessing one to eight rings have been analyzed using ZINDO calculations. The frontier orbitals of the ureas with one to three phenyl rings are localized on a single phenyl ring (the inner ring for the three-ring urea), whereas the frontier orbitals of the higher oligomers are delocalized over two phenyl rings. In all cases, urea-localized n,pi* transitions are lower in energy than the phenyl-localized pi,pi* transitions. The changes in properties with added phenyl rings parallel those previously observed for multilayered cyclophanes; however, they are less pronounced because of weaker coupling between the phenyl rings of the oligophenylureas.     

Preparation and thermal decomposition of certain. Substituted bis-(thenoyl) peroxides

The effect of various substituents on the rates of thermal decomposition of substituted bis-(thenoyl) peroxides has been investigated. Nine unreported peroxides were prepared, including derivatives of 2- and 3-thenoic acids. The thermal decomposition rates of these peroxides were examined, in the presence of styrene as a free radical scavenger. First order kinetics were observed in all cases studied. In general it was found that electron releasing substituents increase, while electron withdrawing groups decrease, the decomposition rate; the only exception being bis-(5-nitro-3-thenoyl) peroxide. Entropies and energies of activation were determined and found to be linearly related for the peroxides studied.     

Convergent synthesis of nonsymmetric pi-stacked protophanes assembled with urea linkers

A convergent approach has been developed for the preparation of nonsymmetric tertiary arylureas possessing two or three urea linkages. This approach has been used for the preparation of ureas possessing 1-naphthylenyl and 4-nitrophenyl end groups separated by either one or two phenylene diamine bridging units. These ureas were constructed as prototypes for donor-bridge-acceptor systems based on tertiary arylurea architecture. AM1 calculations indicate a preference of these arylureas for folded, protophane structures in which the aryl groups are loosely pi-stacked. Analysis of the (1)H NMR chemical shifts supports the assignment of folded structures in solution. The absorption and luminescence spectra of tertiary ureas possessing 1-naphthylenyl and/or 4-nitrophenyl are reported. The absence of fluorescence and appearance of structured phosphorescence at 77 K are attributed to nitrophenyl-localized lowest energy singlet and triplet states. Localization of excitation on the acceptor chromophore precludes investigation of charge transfer in these systems.     

Structural Factors and Mechanisms Underlying the Improved Photodynamic Cell Killing with Silicon Phthalocyanine Photosensitizers Directed to Lysosomes Versus Mitochondria

The phthalocyanine photosensitizer Pc 4 has been shown to bind preferentially to mitochondrial and endoplasmic reticulum membranes. Upon photoirradiation of Pc 4-loaded cells, membrane components, especially Bcl-2, are photodamaged and apoptosis, as indicated by activation of caspase-3 and cleavage of poly(ADP-ribose) polymerase, is triggered. A series of analogs of Pc 4 were synthesized, and the results demonstrate that Pcs with the aminopropylsiloxy ligand of Pc 4 or a similar one on one side of the Pc ring and a second large axial ligand on the other side of the ring have unexpected properties, including enhanced cell uptake, greater monomerization resulting in greater intracellular fluorescence and three-fold higher affinity constants for liposomes. The hydroxyl-bearing axial ligands tend to reduce aggregation of the Pc and direct it to lysosomes, resulting in four to six times more killing of cells, as defined by loss of clonogenicity, than with Pc 4. Whereas Pc 4-PDT photodamages Bcl-2 and Bcl-xL, Pc 181-PDT causes much less photodamage to Bcl-2 over the same dose–response range relative to cell killing, with earlier cleavage of Bid and slower caspase-3-dependent apoptosis. Therefore, within this series of photosensitizers, these hydroxyl-bearing axial ligands are less aggregated than is Pc 4, tend to localize to lysosomes and are more effective in overall cell killing than is Pc 4, but induce apoptosis more slowly and by a modified pathway.     

Dynamic Renner effect in collisions of C+ with H2

The dynamic Renner effect is shown to permit formation of CH⁺₂ in its first excited (²B₁) state from low energy collisions of C⁺ + H₂. The consequences for C⁺ + H₂. The consequences for C⁺ + H₂ radiative association are discussed.