Question

As my last question (Semiconductors and their electronic bands) was badly structured, I decided to elaborate my questions a bit. As I now know, every solid/liquid forms a band structure, so all condensed matter. For simple elements this is easy enough to illustrate. But what happens when there are complicated molecules like organic molecules with many different bonds? Are those bands vastly different than when those molecules are in solution? For example, azo dyes come to mind whose color is the same in the solid state as in solution; also what about complexes for example Cu(II)-hexahydrate, whose color we usually interpret with CFT. Is there even crystal field splitting in a solid or does is form crystal field ‚bands‘? Somehow I am at a loss connecting the band concept to this.

Answer 1

Complex molecules in solid states show band structures with valence and conduction bands determined by molecular orbitals. Azo dyes retain color similar to solutions, while for compounds like Cu(II)-hexahydrate, crystal field theory is adapted to consider crystal field bands in the solid.

Step-by-step explanation:

Understanding the behavior of complex molecules like organic molecules with various bonds in solid states compared to solutions involves considering the electronic band structure. Molecular orbitals in solids form bands due to the close spacing of energy levels.

The valence band is lower in energy, and the conduction band is higher, with the band gap between them determining the solid's conductivity. In solid-state compounds like azo dyes, the color observed is a result of electron transitions between these energy bands, similar to how they behave in solutions, though the solid-state environment can lead to some differences due to intermolecular forces.

When looking at transition metal complexes like Cu(II)-hexahydrate, crystal field theory (CFT) is often employed to explain their colors and electronic structure. CFT considers the electrostatic interactions between metal ions and ligands. In solids, the concept of crystal field splitting transitions to crystal field bands, exhibiting similar properties to the discrete energy levels seen in isolated complexes but influenced by the extended structure and interactions in the solid crystal lattice.

As such, both CFT and band theory concepts are relevant for interpreting properties of transition metal complexes in various states of matter.