On validity and limits of deducing the degree of charge transfer from shifts of cyano vibrations

Understanding the p-doping of organic semiconductors often relies on spectroscopic fingerprints of cyano vibrations, which strongly depend on the charge state of the dopant molecule following intermolecular charge transfer. Interpreting these vibrations can be difficult as a number of other factors can impact them. Here, we formalize the assumptions behind the determination of molecular charge from cyano vibrations and we use computational modeling to demonstrate key obfuscating factors in this process. We notably demonstrate that cyano vibrations do not necessarily shift linearly with the molecular charge and investigate which molecular parameters can explain that. Finally, we provide guidelines for the study of charge transfers involving new molecular dopants based on their cyano vibrations.