I want to understand how the ground state absorption of a molecule shifts if an electron is added or removed. I found these two publications 1 and 2 that observe shifts, but both times a redshift, be it anion or cation. I am specifically asking about the HOMO/LUMO gap (although then it should probably be called HOMO/SOMO or SOMO/LUMO for anion and cation, respectively) and not additional excitations.
Is there a general rule? If not how could the observation made by the publications be explained. (I am not aware that they explained it..) I see this argument: "An extended π-system increases the absorption wavelength to obtain colored materials with lower energetic gaps" but I think it applies to modification of the molecule by adding atoms. I dont see how this works for anion/cation.
I'd also be happy if the answer was: There is no general rule, because... Or this question is unprecise, since your picture of electronic structure is too simple, you need to take into account that...
