No abstract

Materials presenting a stable and reversible switch of physical properties in the solid state are of major interest either for fundamental interests or potential industrial applications. In this context, the design of metal complexes showing a light-induced crossover from one spin state to another, leading to a major change of magnetic and optical properties, is probably one of the most appealing challenges. The so-denoted spin-crossover materials undergo, in some cases, a reversible photoswitch between two magnetic states, but, unfortunately, lifetimes of the photomagnetic states for compounds known so far are long enough only at low temperatures; this prohibits any applications. We have measured and collected the temperatures above which the photomagnetic effect disappears for more than sixty spin-crossover compounds. On the basis of this large data base, a correlation between the nature of the coordination sphere of the metal and the photomagnetic lifetime can be drawn. Such correlation allows us to propose here a general guideline for the rational design of materials with long-lived photomagnetic lifetimes. This result clearly opens the way towards room-temperature photonic materials, based on the spin-crossover phenomenon, which will be of great interest for future communication devices.

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.