Changes in UV-Vis spectra in host-guest complexes I'm doing some background reading before conducting an experiment, but am getting a little confused.  What serves as evidence for the formation of a host-guest complex when conducting analysis through UV-visible absorption?

From the sources that I have read and have personally observed in the lab, formation of host-guest complexes can push equilibrium to favour a specific species, like an acidic form or a basic form of the guest molecule due to stabilizing interactions within the host (think cyclodextrins + phenolphthalein).  This is reflected in absorption spectra as an increase/decrease in absorbance of the peaks to reflect the increasing/decreasing proportion of acidic/basic species of the guest molecule. 

Another way of telling if the host-guest complex is formed is if the there is an increase in the molar absorption coefficient or a shift in the absorption peaks of the guest molecule.  Another conflicting source tells me that when a guest molecule is complexed, absorption of the guest molecule should decrease.  Can anybody clarify which of these changes serves as evidence for formation of a host-guest complex?
Can anybody clarify which serves as evidence for formation of a host-guest complex?


There's no single, general answer to this.

As you aptly described in the body of your post, there is a wide variety of changes to the UV-Vis absorption of a host-guest system on binding of the host and guest.  The changes to the recorded spectrum could fall into any of these categories, depending on the particular structural features of host and guest, and the exact manner that they bind to one another.

AFAIK, the only way to know how the spectrum changes when host and guest bind, is to perform careful experimentation and observe the changes that occur.