Influence of onium substituent in phthaloyl ring on thermodynamic parameters of equilibrium between dimer and free radicals of 2-(4-dimethylaminophenyl)-1,3-indandiones

Abstract

2,2′-Bis[2-(4-dimethylaminophenyl)-5-trimethylammonio-1,3-indandione] diperchlorate and 2,2′-bis[2-(4-dimethylaminophenyl)-5-(2,4,6-triphenylpyridinio)-1,3-indandione] diperchlorate have very definite thermochromic and mechanochromic properties, owing to reversible dissociation into free radicals. The rupture energy of the central bond C²-C^2′ in 5-substituted 2,2′'-bis[2-(4-dimethylaminophenyl)-1,3-indandiones] in acetonitrile is 63–75 kJ/mole; in the dimer with the bulky 5-(2,4,6-triphenylpyridinium) substituent, the rupture energy is 51 kJ/mole. The equilibrium constants follow the Hammett equation (p=1.53, acetonitrile, 293 K). Radicals with onium substituents are thermodynamically the most stable of the arylindandionyl radicals that are thus far known, owing to the significant contribution of electrostatic interactions.