It is very common that absorption features shift  from solution to solid form due to aggregation. This is due to a number of intermolecular interactions and their effects on energy states and therefore absorption features. The result can be hyperchromic, hypsochromic, bathochromic..... aggregation can be due to formation of J-aggregates, H-aggregates. Search any of these terms to find out more.

What is your solution: which solute (monomer, polymer, metallic compound, ...) in which solvent ? Without such elementary information, I doubt someone can help.

Alain

It is very common that absorption features shift  from solution to solid form due to aggregation. This is due to a number of intermolecular interactions and their effects on energy states and therefore absorption features. The result can be hyperchromic, hypsochromic, bathochromic..... aggregation can be due to formation of J-aggregates, H-aggregates. Search any of these terms to find out more.

I assume that you have recorded transmittance in both cases. Solution means, you used a cuvette and you have also recorded the transmittance of the solvent alone also in a cuvette. When you recorded the dropcasted sample, did you also do a blank measurement? Even if, in thin films effects due to interference occur that can not only lead to unexpected absorbance values (both the definition of absorbance as well as the Beer-Lambert Law are not compatible with Maxwell's equations) but also to peak shifts. Overall, the absorbance maximum can be considerably shifted from the oscillator frequency if the transition is strong even without any additional effects... this is not different from the IR spectral range since these are merely optical effects. These effects can certainly occur in addition to the structural effects Prof. Byrne was talking about.