Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

Martin L Kirk; David A Shultz; Ju Chen; Patrick Hewitt; David Daley; Sangita Paudel; Art van der Est

doi:10.1021/jacs.1c04149

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

J Am Chem Soc. 2021 Jul 21;143(28):10519-10523. doi: 10.1021/jacs.1c04149. Epub 2021 Jul 12.

Authors

Martin L Kirk^{1

2}, David A Shultz³, Ju Chen¹, Patrick Hewitt³, David Daley³, Sangita Paudel¹, Art van der Est⁴

Affiliations

¹ Department of Chemistry and Chemical Biology, The University of New Mexico, Albuquerque, New Mexico 87131-0001, United States.
² The Center for High Technology Materials, The University of New Mexico, Albuquerque, New Mexico 87106, United States.
³ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
⁴ Department of Chemistry, Brock University, St. Catharines, Ontario, Canada L2S 3A1.

PMID: 34251803
DOI: 10.1021/jacs.1c04149

Abstract

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (²S₀/D₀) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a m-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, M = Pd^II (1-Pd) or Pt^II (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy^•-, CAT^•+ = semiquinone (SQ), and NN^•), charge-separated doublet ²S₁ (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a ²T₁ (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric ²T₁ and ⁴T₁ states (1-Pd and 1-Pt) and spin-orbit-induced transitions between ²T₁ and NN-based quartet states (1-Pt) account for the differences in polarization.