Dipole Moment and Charge Reorganization in Photoredox Catalysts

Abstract We report evidence of excited-state ion pair reorganisation in a cationic iridium (III) photoredox catalyst in 1,4-dioxane. Microwave-frequency dielectric-loss measurements combined with accurate calculations of dipolar relaxation time allow us to assign both ground and excited-state molecular dipole moments in solution and determine the polarizability volume in the excited-state. These measurements show significant changes in ground-state dipole moment between [Ir[dF(CF3)ppy]2(dtbpy)]PF6 (10.74 Debye) and [Ir[dF(CF3)ppy]2(dtbpy)]BArF4 (4.86 Debye). Photoexcitation of each complex results in population of highly mixed ligand centered and metal-to-ligand charge transfer states with enormous polarizability. Relaxation to the lowest lying excited-state leads to a negative change in the dipole moment for [Ir[dF(CF3)ppy]2(dtbpy)]PF6, and a positive change in dipole moment for [Ir[dF(CF3)ppy]2(dtbpy)]BArF4. These observations are consistent with a sub-nanosecond reorganization with the PF6- counter-ion, which cancels the dipole moment of the lowest lying excited-state, a process which is absent for the BArF-4 counter-ion. Taken together, these observations suggest contact-ion pair formation between the cationic metal complex and the PF6- anion and, at most, solvent-separated pairing with BArF-4. The dynamic ion pair reorganisation we observe with the PF6- counter-ion may substantially modify both the thermodynamic potential available for electron transfer and kinetically inhibit oxidative catalysis, as the anion moves to cover the positively charged end of the molecule, providing a possible mechanistic explanation for recently observed trends in the catalytic activity of these complexes as a function of anion identity in low-polarity solvents. These tunable ion-pair dynamics could prove to be a valuable tool for tailoring the reactivity of both new and extant photocatalysts.