Observing Kinetic Selectivity in Anthracene Photodimerization through Selective Quenching by Excited States of Proximate Rare Earth Cations

Towards development of new separations strategies for rare earth elements (REs; La-Lu, Y, Sc), the discovery of divergent reactivity for RE complexes is an essential first step. We report the synthesis, photophysical characterization, and photodimerization reactivity of a series of kinetically inert rare earth coordination compounds encapsulated by a 1,4,7,10 tetraazacyclododecane-1,4,7-triacetic acid (DO3A) ligand appended with a triazole-bridged anthracene arm, RE(DO3A-anthracene), 2-RE (RE = La-Pr, Sm-Lu, Y). Ultraviolet (UV) light irradiation of all 2-RE compounds produces the respective photodimers, 3-RE. The reaction kinetics were studied using UV-Vis spectroscopy and found to have no correlation with size of the RE cation, a rare outcome in rare earths chemistry. A dichotomy was found between diamagnetic, fast-reacting congeners and paramagnetic, slow-reacting congeners, with 2-Ce, 2-Gd, and 2-Tb as notable paramagnetic, fast reacting exceptions. It is hypothesized that photodimerization can occur for this system by a singlet and/or a triplet pathway, the latter of which can be suppressed by addition of a triplet quencher (cyclononatetraene, COT), which promotes the singlet-only pathway. The reaction rates of the singlet-only pathway were found to correlate well with the fluorescence lifetimes of 2-RE. The findings presented here are discussed in the context of a previously reported solid-state photodimerization of a series of anthracene-based rare earth compounds, wherein differences are attributed to the intermediacy of an excimer in the solid-state reaction and no such intermediacy in the present solution studies. This study lays a key groundwork for a selective rare earth separation system based on anthracene photodimerization where kinetic selectivity is achieved through excited state quenching.