these compounds are prepared by generally known preparative reactions for example compounds of formula 10 wherein b is a 1 5 substituted 2 4 dinitrobenzene or diphenylene sulfone can be prepared by reacting a 1 5 dihalo 2 4 dinitro benzene of the formula str9 or 4 4 39 dihalo 3 3 39 dinitrophenylsulfone of the formula str10 in which hal is fluorine chlorine or bromine with a spin label of the formula str11 to obtain a product of the formula str12 and reacting the last named product of formula 15 with a dicarboxylic acid mercapto or hydroxy acid of the formula str13 and isolating the corresponding compound of formula 10 dianionic spin labels containing a phenolic group and a carboxylic group are prepared by reacting a spin label containing a carboxylic and an amino mercapto or hydroxy group of the formula str14 with a 1 5 dihalo 2 4 dinitrobenzene of the formula 12 or a 4 4 39 dihalo 3 3 39 dinitro phenylsulfone of the formula 13 and treating the resulting compound of the formula str15 with base to obtain the corresponding phenolic carboxylic acid as the phenate and carboxylate salt of formula str16 in each of the above compunds a product containing a nitro phenylene residue can be reduced to the corresponding phenylene compound by conventional procedures e g reduction of the nitro groups to amine followed by diazotization and reduction with hypophosphorous acid one preferred dianionic aromatic spin label is 4 5 n aspartic acid 2 4 dinitroanilino 2 2 6 6 tetramethyl 1 piperidinooxyl or 1 n 2 2 6 6 tetramethyl 1 oxyl 4 aminopiperidinyl 5 n 1 aspartic acid 2.