How to determine the efficiency of intermolecular charge transfer?
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.