[Feldtharen]

How will the formation of a pyrrole dimer affect the ionization potential and the electron affinity, compared to a single pyrrole?
When looking at the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in pyrrole and pyrrole dimer one knows that the difference in energy between the HOMO and LUMO (ΔE) is smaller in the pyrrole dimer (ΔE(Pyrrole)>ΔE(Pyrrole dimer)). And also known is that the HOMO in the pyrrole dimer is higher in energy than the HOMO for a single pyrrole. Also that the LUMO is lower in energy in pyrrole dimer than the LUMO in pyrrole. How does this affect the ionization potential? Is it easier to ionize pyrrole dimer or pyrrole? How does this affect the electron affinity? Does it release (or needs to spend) more energy when adding an electron to a pyrrole dimer than pyrrole?

[Corribus]

To ionize something, you are removing an electron from the HOMO (although see note below), and the ionization potential is a measure of the amount of energy it takes to remove an electron. If the HOMO of compound A is destabilized compared to compound B (i.e., the HOMO of A is higher energy than in compound B), it takes less energy to remove an electron located in that orbital. Electron affinity would have similar considerations with the LUMO, because the LUMO is usually the electron accepter.

(note below) Note that orbital energies are not the same as state energies. When you remove an electron you not only have to consider the orbital energy, which is usually determined from model 1 electron systems. In real chemical systems you also have to consider the energy of interaction between other electrons, which can be substantial when electron correlation is strong. For qualitative comparisons of "similar compounds", the difference in orbital energies is usually sufficient to predict trends, but if you are trying to quantitatively calculate the ionization potentials, electron-electron interactions cannot be neglected. You may well read about Koopman's Theorem. 

[Feldtharen]

Thank you so much for your answer! Could you please expand on your thought about the electron affinity considering LUMO?