Here we provide compelling evidence that Stark spectroscopy is a sensitive probe of spin changes that occur during light absorption through heavy atom spin-orbit coupling. The Stark spectra of [M(bpy)3]2+, where bpy is 2,2’-bipyrdine and M = Fe (Z =26), Ru (Z=44), and Os (Z=76), complexes that are rigorously diamagnetic in their ground states reveal an enhanced sensitivity to low energy transitions that correspond to the direct population of triplet excited states. Importantly, the Stark spectra of [Fe(bpy)3]2+ provides compelling evidence for the direct population of a high spin excited state with red light that is not resolved in the absorption spectrum. The normally unresolved ground state absorbance of these formally spin forbidden transitions is amplified by three major factors: low-temperature, significant charge transfer distances (Δμ), and required changes in orbital angular momentum (Δα). Through Stark spectroscopy, direct excitation to high-spin excited states is more readily identified and investigated in earth abundant first row transition metals, a finding relevant to emerging applications in energy up-conversion, dye-sensitization, spintronics, photo-redox catalysis, and organic light emitting diodes (OLEDs).

Stark spectroscopic characterization of metal-to-ligand charge transfers: Evidence for spin change in low-energy transitions of transition metal bipyridyl complexes

ACS Fall 2020 Virtual Meeting

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