Effective Electron Transfer Directionality on Coupled Multidimensional Potential Energy Surfaces

<jats:p>Electron transfer (ET) is microscopically reversible under a closed, finite, Hermitian Hamiltonian. Yet molecular and interfacial ET processes can exhibit pronounced directionality, with forward transfer occurring readily while back-transfer is strongly suppressed. Traditional theories account for this behavior through thermodynamic driving forces, free-energy surfaces, or coupling to environmental degrees of freedom. Here, I ask whether purely mechanical quantum dynamics on coupled, multidimensional potential energy surfaces can already produce effective ET directionality in the absence of an explicit dissipative environment. I construct a minimal two-state, two-coordinate model consisting of harmonic diabatic wells coupled through tunable electronic interactions, with one ET-active coordinate and one orthogonal relaxation coordinate. Using stationary-point analysis and quantum wavepacket propagation via a split-operator method, I show that population transfer is followed by nuclear motion away from the crossing region and into a product basin. This multidimensional motion geometrically constrains return to the crossing region and suppresses back-transfer over the simulated timescale. Although the underlying Hamiltonian remains fully reversible, the resulting dynamics exhibit effective ET directionality. These results demonstrate how multidimensional potential-energy-surface geometry and internal nuclear motion can contribute to directional ET dynamics without an explicit thermodynamic bath or environment-induced dissipation.</jats:p>