How to determine the efficiency of intermolecular charge transfer? Lets say we have 2 molecules: A and B. Both of these molecules associate with another molecule C. Upon association two complexes form A-C and B-C. We can experimentally see that upon association a new charge transfer (CT) absorption band appears, additionally intensive CT emission appears. Additional evidence for CT can be obtained by DFT calculations (where HOMO is located on A or B and LUMO is located on C). By determining photo luminescence quantum yields (PLQY) it can be seen that A-C has 4 times higher PLQY than B-C.
Are there any ways to determine whether specifically the efficiency of the CT is responsible for this drop of PLQY?
Indeed, if both complex A-C and complex B-C experience CT emission, the efficiency of CT character present in the complexes may be responsible for the difference in PLQY. This however, will be the case if there is no reverse in CT polarity (for example if in both complexes C is the acceptor (and emitter), this may also the reason for porphyrin's comment). Hence if the criteria above are met and You should generally observe the same (or highly similar) emission from both of the complexes. That would also indicate that one species is responsible for the observed emission.




Calculating efficiency of CT. Although I have not seen extensive literature about CT efficiency, there are few papers which try to quantify the CT. The best example I could find is available here: https://onlinelibrary.wiley.com/doi/full/10.1002/jcc.24822. In that paper the authors are trying to quantify CT within a single molecule. Technically the same methodology should also apply for the intermolecular CT in A-C and B-C. Complaring the values obtained should provide some insight in the CT character between both species. Importantly, to do this calculation You need to know exactly how the complex looks. One way to obtain their geometry is to obtain X-Ray single crystal diffraction representations of A-C and B-C in the solid state (this however works only if A-C and B-C do not dissociate upon crystallization).



Godspeed.