ABSTRACT
Performing cyclic voltammetry at scan rates into the megavolt per second range allows the exploration of the nanosecond time scale as well as the creation of nanometric diffusion layers adjacent to the electrode surface. This latter property is used here to adjust precisely the diffusion layer width within the outer shell of a fourth-generation dendrimer molecule decorated by 64 [Ru(II)(tpy)2] redox centers (tpy = terpyridine). Thus the shape of the dendrimer molecule adsorbed onto the ultramicroelectrode surface can be explored voltammetrically in a way reminiscent of an analysis with a nanometric microtome. The quantitative analysis developed here applied to the experimental voltammograms demonstrates that in agreement with previous scanning tunneling microscopy (STM) studies the adsorbed dendrimer molecules are no more spherical as they are in solution but resemble more closely hemispheres resting onto the electrode surface on their diametrical planes. The same quantitative analysis gives access to the apparent diffusion coefficient featuring electron hopping between the [Ru(II)/ Ru(III)(tpy)2] redox centers distributed on the dendrimer surface. Based on the electron hopping rate constant thus measured and on a Smoluchowski-type model developed here to take into account viscosity effects during the displacement of the [Ru(II)/Ru(III)(tpy)2] redox centers around their equilibrium positions, it is shown that the [Ru(II)/Ru(III)(tpy)2] redox centers are extremely labile in their potential wells so that they may cross-talk considerably more easily than they would do in solution at an equivalent concentration.
ABSTRACT
Megavolt-per-second cyclic voltammetry is used to control the expansion with time of a diffusion layer created within the redox outer shell of a fourth generation dendrimer molecule adsorbed onto an electrode; one quarter of the dendrimer is shown in the picture. This allows the measurement of the degree of communication between the ruthenium(II/III) bis(terpyridyl) ("LRuL") redox centers borne at the extremity of the dendrimer tethers, as well as the characterization of the shape of the adsorbed dendrimer molecule.
ABSTRACT
Assemblies of trans-bis(N-methylpyridinium-4-yl)diphenylporphine ions on the surface of calf thymus DNA have been studied using several spectroscopic techniques: absorbance, circular dichroism, and resonance light scattering. The aggregation equilibrium can be treated as a two-state system-monomer and assembly-each bound to the nucleic acid template. The aggregate absorption spectrum in the Soret region is resolved into two bands of Lorentzian line shape, while the DNA-bound monomer spectrum in this region is composed of two Gaussian bands. The Beer-Lambert law is obeyed by both porphyrin forms. The assembly is also characterized by an extremely large, bisignate induced circular dichroism (CD) profile and by enhanced resonance light scattering (RLS). Both the CD and RLS intensities depend linearly on aggregate concentration. The RLS result is consistent with a model for the aggregates as being either of a characteristic size or of a fixed distribution of sizes, independent of total porphyrin concentration or ionic strength. Above threshold values of concentration and ionic strength, the mass action expression for the equilibrium has a particularly simple form: K' = cac-1; where cac is defined as the "critical assembly concentration."offe dependence of the cac upon temperature and ionic strength (NaCl) has been investigated at a fixed DNA concentration. The value of the cac scales as the inverse square of the sodium chloride concentration and, from temperature dependence studies, the aggregation process is shown to be exothermic.
