Exciton reactions in ultrathin molecular wires, filaments and pores: A case study of kinetics and self-ordering in low dimensions

Non-Classical Reaction Kinetics: Experiments

It has been well established by theory and simulations that the reaction rates of diffusion-limited reactions can be affected by the spatial dimension in which they occur. The types of reactions A + B → C, A + A → A, and A + C→, C have been shown, theoretically and/or by simulation, to exhibit non-classical reaction kinetics in low and fractal dimensions. We present here experimental results from several 1D and fractal systems.

An A+B → C type reaction was experimentally investigated in a long, thin capillary tube in which the reactants, A and B, are initially segregated. This initial segregation of reactants means that the net diffusion is along the length of the capillary only, making the system effectively 1D and allowing some of the properties of the resulting reaction front to be studied. The reaction rate of excitonic fusion, A + A →A, as well as trapping, A + C→ C, reactions were observed via phosphorescence(P) and delayed fluorescence(F) of naphthalene within the channels of Vycor glass, in isotopically mixed naphthalene crystals and in the isolated chains of dilute polymer blends. In these experiments, the non-classical kinetics are measured in terms of the heterogeneity exponent, h, from the equation: Rate ∼ F =; kt-hpn,, which gives the time dependence of the rate coefficient. Classically h =; 0, while h =; 1/2 in 1D, as well as in the fractal dimensions discussed here, for A + A → A as well as A + C →C type reactions.

Dynamics of the depletion zone at a finite-sized imperfect trap in two dimensions: photobleaching experiments and simulations

The kinetics of the growth of depletion zones around a static trap in an effective two-dimensional geometry were studied experimentally with photobleaching of fluorescein dye by a focused laser beam. The phototrap served as an imperfect trap with a finite size. The growth of the depletion zone was monitored by the theta distance, defined as the distance from the trap to the point where the concentration of the reactants reaches a given arbitrary fraction theta (0<theta<1) of its initial value, which could be directly measured experimentally. At the asymptotic limit, the results confirm the theoretical nonuniversal t(theta/2) scaling behavior for the theta distance. We also find an effect of fast expansion at an early time of the depletion zone inside an imperfect trap. Both the imperfect trapping strength and the finite trap size are found to control the early-time behavior, while the trap shape does not much affect the dynamics of the theta distance. A dimensional crossover was found for a perfect trap with a finite radius, when the theta distance was measured from the trap surface. The actual trapping efficiency was determined for different laser powers of the phototrap. Results are supported by analytical equations, exact enumerations, and Monte Carlo simulations.

Synthesis and electronic properties of low-dimensional bis(benzene) vanadium reduced mesoporous niobium oxide composites

The first bis(benzene) vanadium mesoporous niobium oxide composite was synthesized and characterized. The XRD pattern of this material shows a peak (100) centered at 45 A, identical to that of starting material. The nitrogen adsorption and desorption analyses of this material exhibit a slight decrease of BET surface area from 580 m(2) g(-1) to 467 m(2) g(-1) and a concomitant decrease in pore size and volume from 28 A and 0.500 cm(3) g(-1) to 25 A and 0.363 cm(3) g(-1), respectively. The powder EPR spectrum shows eight lines which can be assigned to (51)V from bis(benzene) vanadium as well as other resonances that can be assigned to the corresponding cation. The presence of two or more organometallic vanadium species was further confirmed by UV, (1)H-MAS NMR, and XPS methods. Broadened and shifted Nb 3/2, 5/2 peaks were also observed, providing further evidence that the mesoporous transition metal oxide framework was reduced by the organometallic. SQUID magnetometer measurements on this material show paramagnetic behavior with a small contribution of spin glass behavior. The conductivity of this material was 10(-4) ohm(-1) cm(-1), significantly greater than that measured for the analogous bis(benzene) chromium composite previously studied. Since alkali-metal reduced mesoporous niobium oxide materials are insulating, this conductivity was attributed to the low-dimensional bis-arene vanadium phase in the pores and rationalized according to the balance between the Hubbard potential and the bandwidth estimated for the relevant organometallic species. A bis-1,3,5-tri-tert-butylbenzene yttrium composite was also synthesized; however, complete loss of ligand upon reduction of the mesostructure was observed, indicating that this dopant behaves more like an alkali naphthalene reagent in its reactions with mesoporous niobium oxide than other bis(arene) complexes investigated by our group.

Correlation functions for diffusion-limited annihilation, A+A-->0

The full hierarchy of multiple-point correlation functions for diffusion-limited annihilation, A+A-->0, is obtained analytically and explicitly, following the method of intervals. In the long-time asymptotic limit, the correlation functions of annihilation are identical to those of coalescence, A+A-->A, despite differences between the two models in other statistical measures, such as the interparticle distribution function.

Conducting Polyaniline Filaments in a Mesoporous Channel Host

Conducting filaments of polyaniline have been prepared in the 3-nanometer-wide hexagonal channel system of the aluminosilicate MCM-41. Adsorption of aniline vapor into the dehydrated host, followed by reaction with peroxydisulfate, leads to encapsulated polyaniline filaments. Spectroscopic data show that the filaments are in the protonated emeraldine salt form, and chromatography indicates chain lengths of several hundred aniline rings. The filaments have significant conductivity while encapsulated in the channels, as measured by microwave absorption at 2.6 gigahertz. This demonstration of conjugated polymers with mobile charge carriers in nanometer channels represents a step toward the design of nanometer electronic devices.