Excited State Magnetic Exchange Interactions Enable Large Spin Polarization Effects

Abstract

Excited state processes involving multiple electron spin centers are crucial elements for both spintronics and quantum information processing. Herein, we describe an addressable excited state mechanism for precise control of electron spin polarization. This mechanism derives from excited state magnetic exchange couplings that occur between the electron spins of a photogenerated electron-hole pair and that of an organic radical. The process is initiated by absorption of a photon followed by ultrafast relaxation within the excited state spin manifold. This leads to dramatic changes in spin polarization between excited states of the same multiplicity. Moreover, this photoinitiated spin polarization process can be "read" spectroscopically using a magnetooptical technique that is sensitive to the excited state electron spin polarizations and allows for the evaluation of wave functions that give rise to these polarizations. This system is unique in that it requires neither intersystem crossing nor magnetic resonance techniques to create dynamic spin-polarization effects in molecules.

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…The impact of the π‐radical spin on the charge‐separated excited‐state dynamics was also investigated in metal complexes containing π‐radical ligands . As shown in Figure a, Shultz, Kirk, and co‐workers synthesized radical complexes of the ( t Bu 2 bpy)Pt(Cat‐R) type, where Cat, bpy, and R stand for 3‐ tert ‐butyl‐ ortho ‐catecholate, bipyridine, and nitronyl‐nitroxide radical (NN), respectively, and investigated their photoexcited states by magnetic circular dichroism (MCD) spectroscopy.…”

“…The impact of the p-radical spin on the charge-separated excited-state dynamics was also investigated in metal complexes containing p-radical ligands. [108][109] As shown in Figure 11 a, Shultz,K irk, and co-workerss ynthesized radical complexes of the (tBu 2 bpy)Pt(Cat-R) type, where Cat, bpy,a nd Rs tand for 3tert-butyl-ortho-catecholate,b ipyridine, and nitronyl-nitroxide radical( NN), respectively,a nd investigated their photoexcited states by magnetic circular dichroism (MCD) spectroscopy.I n these complexes, upon excitation of the ligand-to-ligand charget ransfer (LLCT) band, ap hotoinduced charget ransfer occurs from Cat to bpy,l eadingt oa( bpyC)Pt(SQCNNC)t hree-spin system.F or three-spin systems, the spin wavefunctions of the sing-doublet (j S 1 ,1/2 >)a nd trip-doublet (j T 1 ,1/2 >)u ndergo mixing depending on the l,whichisg iven by Equation(2)…”

Density functional theory and ab initio studies on hyperfine coupling constants of phosphinyl radicals

“…Among them are electronic (UV/Vis), vibrational (infrared and Raman), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopy. The latter—also called electron spin resonance (ESR) spectroscopy—has proved to be indispensable in studies of radical species, which attract the attention of researchers due to their important roles in physics, chemistry, biochemistry, environmental, and material science …”

Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

“…The triplet state of the (bpy)M(CAT) parent chromophore is otherwise inaccessible due to non-competitive intersystem crossing (ISC) rates compared to efficient non-radiative relaxation back to the singlet ground state. 13,15 Chromophores with appended radicals have been used to provide high-resolution information regarding the nature of their low-energy excited states, [11][12][16][17] illustrate how multiple, pairwise, excited state magnetic exchange interactions influence photophysical processes, 11 and highlight how the excited state chromophore triplet -radical magnetic exchange interaction and associated excited state dynamics can affect electron spin polarization (ESP). 11, 16-17, 22-23, 25-34 In most systems of this type, ESP is a result of the spin selectivity of the transitions between nearly degenerate 2 T 1 and 4 T 1 states, and ESP is observed when these states are relatively long lived.…”