Electron Transfer and Exchange Interactions in Mixed-Valence Radical Anions, Radical Cations, and Neutral Diradicals...

Investigations on how molecular bridge structure influences electronic communication between functional units provide key insights into structure-property relationships and are essential for designing materials with tailored electronic properties. Here, we examine how the bridge electron density affects magnetic exchange (2J) across a series of diradicals and charge transfer (CT) coupling in their corresponding mixed-valence (MV) anions and cations. Four diradicals composed of polychlorinated pyridyldiphenylmethyl (PyBTM) units linked by substituted 2,7-fluorene bridges of varying electron density were synthesized and electrochemically converted into isostructural MV species, granting access to magnetic and CT coupling across three redox states within an identical atomic framework. For the diradicals, combined experimental and quantum chemical studies showed stronger antiferromagnetic coupling with higher bridge electron density. Analysis of direct and kinetic exchange contributions revealed dynamic spin polarization as the main factor governing magnetic coupling with electron-rich bridges yielding larger |2J| values. Complementary analyses of the MV anions and cations within the generalized Mulliken-Hush framework demonstrated effective CT coupling in both redox states. In each case, an energetically elevated CT state with bridge-centered charge localization strongly influenced the coupling. Frontier orbital analysis of hole and electron transfer clarified how donor- and acceptor-substitution modulates CT coupling. These results establish bridge electron density as a key parameter for tuning magnetic exchange and CT coupling, providing a basis for designing molecular architectures with controllable charge transport and magnetic properties.