Effects of intramolecular spin polarization on the physical and photophysical properties of exchange-coupled systems

Heisenberg spin exchange takes place when two or more unpaired spins in close proximity interact, so that their relative orientations are no longer independent of one another. Previous studies on dimeric complexes of [MM’(tren)2(CAn-)]m+ (M, M’ = CrIII or GaIII, CA) is the bisdentate chloranilate ligand, tren is tris(2-aminoethyl)amine) provided experimental evidence that the presence of spin exchange can affect optical and magnetic properties. The bridging ligand is redox active, which can be changed to yield various redox states via redox reactions, e.g. semiquinone, catecholate, or quinone. These simple systems provide a convenient platform for the studies of both spectroscopic and magnetic behaviors. In semiquinone form, the unpaired electrons on CrIII can interact with the unpaired electron on the bridging ligand. The different forms of ligand can turn “on” or “off” the exchange coupling interactions within the dimeric complex.The Heisenberg model of these systems predicts that the introduction of spin exchange results in a net thermodynamic stabilization of the system. The results seen in cyclic voltammetry (CV) experiments suggest that the larger potential peak separation (ΔEechem) of [Cr2(tren)2(CA)]m+ may be a thermodynamic consequence of spin exchange compared to [Ga2(tren)2(CA)]m+. This may be the first time that the thermodynamic stabilization energy of spin exchange is possibly quantified by other physical measurements.iiiIn order to examine the effects of the thermodynamic stabilization by the spin exchange interaction and establish the thermodynamic correlation seen in both electrochemical and magnetic behaviors, CrIII and GaIII analogues with various substituents on the tetraoxolene bridge are synthesized and characterized. R= H, F, Cl, Br, I, cyano, phenyl, and piperidino make excellent choices of substituents. Additionally, horizontally-elongated bridging ligands, anthracene and naphthalene tetraoxo-derivatives, were synthesized and coordinated to CrIII for the studies, because the electron mobility within these conjugated ligands can delocalize the spin density further away from the metal ions. In addition, the effects of intraligand electron delocalization can be examined by incorporating N,N-dimethylaminophenyl and cyanophenyl substituents on an anilate-bridging ligand to change the directionality of spin polarization. Density Functional Theory (DFT) calculations were employed to validate synthetic viability and provide some in-depth insight of the experimental data.The cyclic voltammetry and variable-temperature magnetic susceptibility data of these [Cr2(tren)2(L)]n+ and [Ga2(tren)2(L)]n+ systems were collected and compared. Among all, the thermodynamic stabilization observed in the [Cr2(tren)2(Me2-AnT)]n+ complex is the weakest. This result matches the prediction of both our hypothesis and our DFT calculations. Both electrochemical and magnetic measurements are employed to provide experimental evidence, which will become advantageous tools for the further study of spin polarization, photophysics and photoelectronic properties of spin-coupled systems.