The use, in electron paramagnetic resonance (EPR) magnetometry, of tetracyanoquinodimethane derivatives corresponding to the formula: …<IMAGE>… in which AH<+> is a cation derived from a heterocyclic aromatic base containing at least three aromatic rings and a single NH<+> group included in at least one of these rings. …<??>AH<+> is for example acridinium or benzoquinolinium. 
Use of tetracyanoquinodimethane derivatives in electron paramagnetic resonance magnetometry