Concept of colour change due to the change in energy difference of d-orbitals splitting pattern when heat is supplied

At low temperatures, bis(diethylammonium)tetrachloronickelate(II) is a light yellow-brown colour, and when heated to temperatures over 72-73 degrees, it changes to a brilliant bluish colour.

In the low temperature arrangement, neighboring molecules share bridging chlorines (shown in the figure above) in a two-dimensional sheet such that each compound shares two of its chlorines with nearby molecules, and has two chlorines shared with it. At low temperatures, the compound has an octahedral geometry. As temperature increases, the higher molecular motion breaks the bridging bonds, and the compound loses two chlorine atoms to form a four coordinate transition molecule compound in a tetrahedral structure.

The property of a substance that has the ability to change colour on a change in temperature is known as thermochromism. It is observed in both organic and inorganic compounds. There are many mechanisms by which thermochromism occurs, and a change in coordination geometry (as seen in bis(diethylammonium)tetrachloronickelate(II) above) is just one of them.

In an octahedral structure, the energy difference between the d-orbitals are larger than in tetrahedral because there are less effective overlaps between the p-orbitals of Cl and the d-orbitals of the metal cation.

When exposed to visible light, the electron in the lower energy orbital will absorb the energy of the light that is equal to the energy difference between the d-orbitals to be promoted to the higher energy d orbital. Since the energy difference between the d-orbitals in the octahedral structure is higher, the wavelength of the light absorbed will be smaller than the one absorbed by the electron in the lower energy orbital in the tetrahedral structure (E=hc/λ, h is the Plank constant and c is the speed of light). The light reflected will be the complement of the light absorbed by the electron (which can be determined from the colour wheel).

Since the tetrahedral compound gives rise to a brilliant blue colour, it implies that a colour close to orange (longer wavelength) is absorbed. In contrast, for the octahedral compound, the colour was a light yellow-brown, which implies that blue/purple (shorter wavelength) is absorbed.

Application in self-tinted windows: Self-tinting suntuitive glass by Pleotint (PPG IdeaScapes)

The octahedral complex undergoes a reversible reaction to form a more highly coloured tetrahedral complex. Pleotint’s ligand exchange systems are extruded into plastic to give films of variable tints. These films are then laminated between glass. The lightly tinted thermochromic film warms up in the presence of direct sunlight and darkens to absorb a significant portion of the sunlight shining directly on the window. This helps to reduce the build-up of solar heat within the room and keeps the room cool.

It takes 20-30 minutes to change back to its original colour.

Its colour-changing range is 25-65 degrees.

Taken from: http://www.suntuitive.com/gallery.html