I have measured UV/Vis-absorbance spectra of an organic molecule (IT-4F, n-type acceptor material for organic photovoltaics) in different solvents (chlorobenzene, chloroform, toluene and o-dichlorobenzene).
Why do I see a different shift in the absorption maximum for these solvents? Which solvent properties influence this shift?
The main absorption peak is at wavelengths of ~670 nm for toluene, ~680 nm for chlorobenzene, ~685 nm for dichlorobenzene and ~690 nmfor chloroform.
If possible I would also be grateful for papers explaining this behavior.
Chloroform most probably does not fit, but the other three follow, as it seems, Kundt's rule. You find some details about this rule and an explanation in Article Beyond Beer's Law: Revisiting the Lorentz-Lorenz Equation
For more general reading- search Solvatochromism
https://en.wikipedia.org/wiki/Solvatochromism
I admit I was telling only half the truth, but so does the wikipedia article: "With various solvents there is a different effect on the electronic ground state and excited state of the solute, so that the size of energy gap between them changes as the solvent changes".
In the paper referenced above it is assumed that this gap stays the same corresponding to the idealization that the solute molecules enter empty spaces within the solvent's molecules without (much) interaction. Even then electromagnetic coupling due local field effects can redshift band positions, the more the higher the combined oscillator strength of the solvent is and the higher is its index of refraction and its dispersion, which is essentially Kundt's rule. This rule seems to apply to roughly half of all solvent-solute systems. Certainly, effects through direct interactions which lead in the extreme case to dissociation, association etc. come on top, change the size of the energy gap and can change a redshift into a blueshift...
Hi Thomas Mayerhöfer - I was just pointing to the Wikipedia entry as a starting point, introducing some of the terminology. As you know, this is an interesting area, with a rich scientific history, which Daniel Maximilian Steger should explore.
Thank you a lot for your answers Thomas Mayerhöfer and Hugh J Byrne. I will have a deeper look into this topic.
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