I know this isn’t strictly on-topic for #FluorescenceFriday (the one on the right is #Fluorescent, see below), but I had to share.

This was taken in 2016 by Ally Formanuik ( @millsgroupchem alumni).

From left to right it shows:
Th(III), Th(IV), U(III), U(IV), U(V) and U(VI).

The electric blue of Th(III) is its most striking feature, and caused by the 5d1 configuration. Th(IV) has an empty valence shell and is colorless.

From U(III) to U(VI) it goes f3 to f0. The striking colors are a combination of LMCT + sharp (formally forbidden) f-f transitions.

For those keeping up, yes U(VI), f0, can’t have f-f transitions. Instead the (yellow here) color is mostly due to intense LMCT of axially coordinated O atoms in [UO2(Cl)2(THF)2] here. The yellow color can be modulate by equatorial ligands from yellow, to green, orange, and red.

For real #FluorescenceFriday, some {UO2}2+ from @ToriaStafford‘s PhD ( @Team_Natrajan).

Ln emission is sharp and line-like due to essentially ionic bonding.

Conversely, Uranyl emission is vibronically resolved. Spacing caused by U=O bonds and is sensitive to the coord sphere.