Covalently Bound Nitroxyl Radicals in an Organic Framework.

A series of covalent organic framework (COF) structures is synthesized that possesses a tunable density of covalently bound nitroxyl radicals within the COF pores. The highest density of organic radicals produces an electron paramagnetic resonance (EPR) signal that suggests the majority of radicals strongly interact with other radicals, whereas for smaller loadings the EPR signals indicate the radicals are primarily isolated but with restricted motion. The dielectric loss as determined from microwave absorption of the framework structures compared with an amorphous control suggests that free motion of the radicals is inhibited when more than 25% of available sites are occupied. The ability to tune the mode of radical interactions and the subsequent effect on redox, electrical, and optical characteristics in a porous framework may lead to a class of structures with properties ideal for photoelectrochemistry or energy storage.

Close Packing of Nitroxide Radicals in Stable Organic Radical Polymeric Materials.

The relationship between the polymer network and electronic transport properties for stable radical polymeric materials has come under investigation owing to their potential application in electronic devices. For the radical polymer poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate), it is unclear whether the radical packing is optimal for charge transport partially because the relationship between radical packing and molecular structure is not well-understood. Using the paramagnetic nitroxide radical as a probe of the polymer and synthetic techniques to control the radical concentration on the methyl methacrylate backbone, we investigate the dependence of radical concentration on molecular structure. The electron paramagnetic resonance data indicate that radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups. Below 60% coverage, the polymer rearranges to accommodate larger radical-radical spacing. These findings are consistent with theoretical calculations and help explain some experimentally determined electron-transport properties.

Impurity and free-carrier effects on the far-infrared dispersion spectrum of silicon.

We investigate a generalized Cauchy power-series expansion for the index of refraction of an n-type elemental semiconductor in the region of IR transparency. A plot of index versus photon energy squared should be very nearly linear if all absorptions lie above the transparent region. However, free carriers produce far-IR absorption, and the dispersive signature of this is a deviation from linearity in the mid- to far-IR. By retaining terms with negative exponents in the index expansion, we find a substantially improved fit to index measurements. Moreover, the free-carrier density may be determined from the coefficients in the regression fit. The method has been used to evaluate several extensive index measurements for doped silicon found in the literature.