Exchange Spin Coupling in Optically Excited States

10 March 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


In optically excited states in molecules and materials, coupling between local electron spins plays an important role for their photoemission properties and is interesting for potential applications in quantum information processing. Recently, it was experimentally demonstrated that the photogenerated local spins in donor–acceptor metal complexes can interact with the spin of an attached radical, resulting in a spin-coupling dependent mixing of excited doublet states, which controls the local spin density distributions on donor, acceptor, and radical subunits in optically excited states. In this work, we propose an energy-difference scheme to evaluate spin coupling in optically excited states, using unrestricted and spin-flip simplified time-dependent density functional theory (sTDDFT). We apply it to three platinum complexes which have been studied experimentally to validate our methodology. We find that all computed coupling constants are in excellent agreement with the experimental data. In addition, we show that the spin coupling between donor and acceptor in the optically excited state can be fine-tuned by replacing platinum with palladium and zinc in the structure. Besides the two previously discussed excited doublet states (one bright and one dark), our calculations reveal a third, bright excited doublet state which was not considered previously. This third state possesses the inverse spin polarization on donor and acceptor with respect to the previously studied bright doublet state and is by an order of magnitude brighter, which might be interesting for optically controlling local spin polarizations with potential applications in spin-only information transfer and manipulation of connected qubits.


Exchange spin coupling
Excited states
Local spins

Supplementary materials

Supporting Information
This document contains the derivation of the symmetric orthogonalization scheme according to Löwdin, a discussion of the derivation of the mixing strength parameter λ, a discussion of the equations for local spins within the HDvV Hamiltonian, additional information on the molecular orbitals of all systems under investigation, the most important contributions from configurations in the sTDDFT calculations, the oscillator strengths and excited-state energies, the derivation of the expressions for the coupling constants.


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