[AoisPokemaster]

Hey guys, I am doing some research on solvatochromism, and I want to know what molecular features of the dye can affect the extent of solvatochromism - I understand how solvents with different polarities can affect the HOMO LUMO gap, but what are the structural features which affects the size of the bathochromic shift or the hypsochromic shifts in different solvents? What other factors do we have to consider when looking at the dye, which can tell us about the extent of their solvatochromic properties?

[Corribus]

It's just not an easy question to answer. To predict what solvents do to electronic transitions you have to know how the solvents interact with the ground and excited electronic states at the time of the transition. This can be particularly difficult for fluorescence in which, as remarked in the other thread, the relaxed excited state conformation can be quite a bit different than the ground state. Note it is the states that matter, not the orbitals. Of course the states are ultimately derived from electrons in orbitals, but deriving the former from the latter is not just a matter of a casual glance. And throwing solvent interactions on top of it... this is the stuff of computational chemistry.

Also note that other things can also interfere with what might otherwise be obvious trends. For instance, solubility and concentration can also be an issue. If the chromophore is really insoluble, or really concentrated, or both, it can aggregate, leading to excimer formation and etc. that can have enormous effects on the spectral properties.