A theoretical study on magnetic properties of bis-TEMPO diradicals with possible application

Three different pairs of isomeric trans and corresponding cis forms of fluoroprotein chromophore (FPC) fragments coupled bis-2,2′,6,6′-tetramethylpiperidinyloxy (TEMPO) based diradicals have been designed, and subsequently the magnetic properties of these diradicals have been investigated. We have used blue shifted variants of green fluorescent protein (cyan fluorescent protein or CFP), orange variant of Discosoma coral (DsRed) and yellow mutant of Zoanthus sp. (zFP) chromophore fragments as spin couplers. Along with the gas phase magnetic exchange coupling constant (J), the zero field splitting parameters (ZFSs) of all the diradicals in water and also in blood plasma medium have been calculated. We have found the all molecules to be ferromagnetic in nature with trans diradicals having lower spin–spin ZFS parameter (|D|) and the higher magnetic exchange coupling constants values than their respective fluorescent cis isomers. This has also been noted that due to the presence of the fluoroprotein chromophores, the resulting diradicals can switch back and forth between the isomeric trans and cis forms upon irradiation of the proper wavelength of light. The time dependent density functional (TDDFT) technique is used to measure the actual wavelength of this conversion for these diradicals. A comparison between the evaluated ZFS values with other nitroxide based synthesized molecular systems which are in use as magnetic resonance imaging (MRI) contrast agents (MRICAs) indicates that if these diradicals are synthesized, they can be useful as MRICAs. A relation between the distance and the spin–spin ZFS parameter is also made. To support this fact, we have computed the solvent accessible surface area, molecular volume, the surface to volume ratio, and molecular diameter of each diradical species to address their potentiality to act as contrast agents in MRI. Finally the ability of these diradicals to act as MRICAs is further justified with the help of molecular electrostatic potential surfaces.