Covalent Radical Pairs as Spin Qubits: Influence of Rapid Electron Motion between Two Equivalent Sites on Spin Coherence

Ultrafast
photodriven electron transfer reactions starting from
an excited singlet state in an organic donor–acceptor molecule
generate a radical pair (RP) in which the two spins are initially
entangled and, in principle, can serve as coupled spin qubits in quantum
information science (QIS) applications, provided that spin coherence
lifetimes in these RPs are long. Here we investigate the effects of
electron transfer between two equivalent sites comprising the reduced
acceptor of the RP. A covalent electron donor–acceptor molecule
(D–C–A24+) including a p-methoxyaniline donor
(D), a 4-aminonaphthalene-1,8-imide chromophoric primary
acceptor (C), and a m-xylene bridged
cyclophane having two equivalent phenyl-extended viologens (A24+) as a
secondary acceptor was synthesized along with the analogous molecule
having one phenyl-extended viologen acceptor and a second, more difficult
to reduce 2,5-dimethoxyphenyl-extended viologen in a very similar
cyclophane structure (D–C–A4+). Photoexcitation of C within each molecule results
in subnanosecond formation of D+•–C–A23+• and D+•–C–A3+•. The spin dynamics of these RPs were characterized by time-resolved
EPR spectroscopy and magnetic field effects on the RP yield in both
CH3CN and CD3CN. The data show that rapid electron
hopping within A23+• promotes spin decoherence in D+•–C–A23+• relative to D+•–C–A3+• having a monomeric acceptor, while the interaction of the
RP electron spins with the nuclear spins of the solvent have little
or no effect on the spin dynamics. These observations provide important
information for designing and understanding novel molecular assemblies
of spin qubits with long coherence times for QIS applications.