Influence of the atmosphere on the phosphorescence of a brominated diphenylphosphine oxide-ethyl naphthaleneimide dyad.

The molecular dyad diphenylphosphine-ethyl bromine naphthaleneimide (Br-DPPENI) emits strong fluorescence and phosphorescence. We studied the intensity and lifetime of the phosphorescence of Br-DPPENI embedded in PMMA films in atmospheres of air, He, Ar, N2, and O2 as a function of pressure between vacuum and ambient pressure (1 bar). The experiment was performed in the frequency domain with a two-channel lock-in amplifier, and the data were analyzed with the polar plot or phasor technique. Reversible shortening of the lifetime due to triplet-triplet annihilation was found in the presence of atmospheric or pure oxygen. With small modulation frequencies, an additional slow component of the phosphorescence dynamics is observed, which is ascribed to the diffusion of oxygen into the sample films.

Linear and nonlinear photoexcitation dynamics in pi-conjugated polymers.

Linear and nonlinear recombination kinetics with various lifetime distributions were identified for long-lived photoexcitations in a series of pi-conjugated polymer films using modulation frequency and excitation intensity dependencies of the photoinduced absorption. This includes monomolecular, bimolecular, and defect-limited recombination processes that lead to saturation. Using generalized kinetics parameters, we found characteristic plots for all recombination processes. Specifically, the bimolecular recombination process shows superlinear intensity dependence away from the steady state; on the contrary, dispersive bimolecular recombination leads to sublinear dependence.

Redox metal chelation ameliorates radiation-induced bone marrow toxicity in a mouse model.

Since zinc desferrioxamine (Zn-DFO) has been shown to be a very potent protector against injuries induced by redox-active metal ions, we examined its protective effect against radiation-induced toxicity. We found that treatment with Zn-DFO given before TBI increased the survival of mice irradiated with 7.5 and 8.5 Gy. Zn-DFO also protected against radiation-induced myelosuppression and body weight loss, while soluble Il6 levels in serum were normalized in mice pretreated with Zn-DFO. We concluded that administration of Zn-DFO prior to TBI protected BALB/c mice from radiation-induced toxicity, increasing survival rates by up to 75%. The biological effect of Zn-DFO is known to result from its effect on the production of intracellular hydroxyl free radicals mediated by redox-active metal ions, and both metal chelation and zinc delivery appear to be equally likely mechanisms for this outcome. We suggest that radiation-induced toxicity is caused by the deleterious effect of redox-active metal ions, and that compounds which modulate this redox activity may act as radioprotectors.

Site effects in controlling the chemical reactivity in crystals: solid-state photochromism of N-(n-propyl)nitrospiropyrane.

The dynamics of decay of the photoproducts of the two non equivalent molecules of N-(n-propyl) nitrospiropyrane in the crystalline state is significantly different due to the effect of the specific site where each of the molecules is located in the crystal latice.

DNA-templated assembly and electrode attachment of a conducting silver wire.

Recent research in the field of nanometre-scale electronics has focused on two fundamental issues: the operating principles of small-scale devices, and schemes that lead to their realization and eventual integration into useful circuits. Experimental studies on molecular to submicrometre quantum dots and on the electrical transport in carbon nanotubes have confirmed theoretical predictions of an increasing role for charging effects as the device size diminishes. Nevertheless, the construction of nanometre-scale circuits from such devices remains problematic, largely owing to the difficulties of achieving inter-element wiring and electrical interfacing to macroscopic electrodes. The use of molecular recognition processes and the self-assembly of molecules into supramolecular structures might help overcome these difficulties. In this context, DNA has the appropriate molecular-recognition and mechanical properties, but poor electrical characteristics prevent its direct use in electrical circuits. Here we describe a two-step procedure that may allow the application of DNA to the construction of functional circuits. In our scheme, hybridization of the DNA molecule with surface-bound oligonucleotides is first used to stretch it between two gold electrodes; the DNA molecule is then used as a template for the vectorial growth of a 12 microm long, 100 nm wide conductive silver wire. The experiment confirms that the recognition capabilities of DNA can be exploited for the targeted attachment of functional wires.