Exchange Spin Coupling in Optically Excited States

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. 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.