Toward Achieving the Theoretical Limit of Electron Spin Polarization in Covalently Linked Radical-Chromophore Dyads

Electron spin systems with non-Boltzmann spin distributions are commonly observed in photophysical and photochemical processes involving free radicals. Understanding the origin of electron spin polarization (ESP) reveals detailed insight into the spin-dependent interactions occurring in these processes. The ability to transfer ESP to the nuclear spins of the solvent to produce large nuclear polarization is itself an active area of research in the field called dynamic nuclear polarization (DNP). Harnessing ESP in such fields demands a large magnitude of ESP and importantly on free radicals that are stable. In this work, we explored various factors that could play a prominent role in generation of ESP on the stable nitroxyl radical. By exploiting the dependence of ESP on the zero-field splitting parameter D_T of the chromophore triplets and a careful choice of the chromophores for efficient quenching of their excited states by a free radical, we present here our strategies for designing covalently linked chromophore-free radical dyads, which could generate high magnitudes of ESP. Syntheses and EPR studies on a series of such dyads to demonstrate successful realization of our strategy are reported here. Our work reveals that, along with the D_T value and the triplet quenching efficiency, the relative orientational dynamics of the radical and chromophore moiety and an efficient intersystem crossing of the chromophore primarily govern the ESP of the chromophore-radical dyads. One such molecule, an anthraquinone moiety linked to a TEMPO free radical that optimally satisfies the above criteria, produces a very large value of ESP-about 300 times its Boltzmann value-reaching almost half of the theoretical maximum of the radical-triplet pair mechanism of ESP. Coupled with its excellent photostability in benzene and the use of ambient conditions, this molecule should prove to be highly desirable in photoinduced DNP studies and other applications such as highly sensitive magnetometers, which require generation of large nuclear spin polarization in the solvent by transferring the electron spin polarization.