Radical Spin Polarization and Magnetosensitivity from Reversible Energy Transfer

Molecular spins provide potential building units for future quantum information science and spintronic technologies. In particular, doublet (S = 1/2) and triplet (S = 1) molecular spin states have the potential for excellent optical and spin properties for these applications if useful photon-spin mechanisms at room temperature can be devised. Here we explore the potential of exploiting reversible energy transfer between triplet and doublet states to establish magnetosensitive luminescence and spin polarization. We investigate the dependence of the photon-spin mechanism on the magnitude and sign of the exchange interaction between the doublet and triplet spin components in amorphous and crystalline model systems. The design of a magnetic field inclination sensor is proposed from understanding the required “structure” (spin interactions) to “function” (magnetosensitivity).

This results in |   (0) ⟩ as the overall spin states for the radical-triplet system, i.e. |  ±1/2 ⟩, We can use time-independent perturbation theory to estimate the radical-triplet eigenstates near to anticrossings with the addition of the ZFS interaction |   (1) ⟩ .
To first order with non-degenerate |   (0) ⟩ : The width of MFE features for luminescence/spin polarization depends on the rate that doublet character is hybridized as an anticrossing is approached.The MFE width is affected both by the convergence rate of radical-triplet eigenstates Pj at the anticrossing (i.e.relative magnetic field quantum numbers of states k and j) and the magnitude of ⟨  (0) | ̂ |  (0) ⟩.
For a ZFS tensor with principal axis at an angle θ to the applied magnetic field, ⟨  (0) | ̂ |  (0) ⟩ can be calculated for all possible state intersections: For both  = 3||/2 and  = 3, the width of MFE features is seen to increase with ||.At  = 3||/2, the width of the MFE feature varies with sin 2 θ, whilst at  = 3|| the width of both intersections varies with sin 2θ.

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Figure S1.Magnetic response for a system of strongly coupled radical-triplet pairs (|| = 20||, i.e. || ≫ ||) with all triplet states at a specified orientation to an applied magnetic field.MFEs for doublet photoluminescence are shown for (a) ferromagnetic ( > 0) and (b) antiferromagnetic ( < 0) radical-triplet exchange coupling with varying triplet ZFS tensor orientation to an applied magnetic field.MFEs for the excited doublet state spin polarization of the radical are similarly shown for (c) ferromagnetic and (b) antiferromagnetic radicaltriplet exchange coupling with varying triplet ZFS orientation to an applied magnetic field.

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Figure S2.Magnetic response for a system of weakly coupled radical-triplet pairs (|| = 0.02||, i.e. || ≪ ||) with all triplet states at a specified orientation to an applied magnetic field.MFEs for doublet photoluminescence are shown for (a) ferromagnetic ( > 0) and (b) antiferromagnetic ( < 0) radical-triplet exchange coupling with varying triplet ZFS tensor orientation to an applied magnetic field.Inset: MFEs extending up to gµBB = 10||.MFEs for the excited doublet state spin polarization of the radical are similarly shown for (c) ferromagnetic and (b) antiferromagnetic radical-triplet exchange coupling with varying triplet ZFS orientation to an applied magnetic field.Inset: MFEs extending up to gµBB =10||.