From the periodic table to photochemical molecular devices and machines

It is well known that the position of an element in the periodic table is related to its electronic structure and that elements belonging to the same column exhibit similar properties because they have the same outer-shell electronic configurations. The similarity between two elements, as predicted by the periodic table, extends to the chemical properties of their metal complexes, but fades and may also completely vanish in going from chemistry to photochemistry, a new dimension of chemistry that follows its own rules. For example, [Ru(bpy)3]2+ and [Fe(bpy)3]2+ (bpy = 2,20-bipyridine) exhibit completely different photochemical behavior, with the first, but not the second one, being a undamental component of several molecular devices and machines. After a brief recall of the important ideas advanced one century ago by Giacomo Ciamician, the father of photochemistry and the prophet of solar energy utilization, this paper illustrates examples of light-powered molecular devices and machines: wires, switches, extension cables, antennas, and nanomotors.