[blackcat]

For organic molecules, what are the possible reasons when excitation and absorption spectra have different shapes???

[Corribus]

There are lots of reasons. Is this an academic question or something particular?

[blackcat]

There are lots of reasons. Is this an academic question or something particular?

Well, maybe can you offer me the most important/common reasons?

I know it can be due to impurity.

[Corribus]

Basically, if there are any electronic states that don't deactivate via fluorescence this will cause the excitation spectra to differ from the absorption spectra. E.g., absorption into a higher lying electronic state that undergoes photochemistry that is competitive with internal conversion to the lower-lying, emissive state. Nonlinear absorption effects or polarization effects can also lead to this phenomenon, and it can also be observed with fluorophores that explore a large conformational space, if the excitation band is sufficiently narrow in bandwidth. These are the first things that come to mind, but certainly there are others.