Mixed-valence molecules as platforms for ultrafast cooling via sudden electric-field switching

In this article, we propose the concept of ultrafast cooling under sudden electric field quenching based on the use of binuclear mixed valence (MV) molecules. The formalism earlier developed in the study of molecular quantum cellular automata is used by considering the scenario in which one polarized MV dimer (“driver-cell”) creates a dipole electric field acting on the adjacent dimer (“working cell”). To describe such dimers, the conventional vibronic theory of MV compounds adapted to the present case is applied. The considered process includes two distinct stages: (1) sudden decrease in the driver-cell polarization leading to a violation of the thermal equilibrium, and (2) subsequent dielectric relaxation restoring the thermal equilibrium, accompanied by the heat exchange between the dimers and the phonon bath. In search of a switching regime under which such a two-step process is accompanied by heat absorption (cooling), we face the need to suppress as much as possible the transitions between the ground and excited vibronic states. This is shown to be possible if the fast partial quenching of the driver-cell polarization occurs without changing its sign and also provided that the polarization does not approach too close to the avoided crossing point of two low-lying vibronic levels vs field. For this switching regime, the heat absorption arises from decreasing the energy gap between the two low-lying levels, which creates underpopulation of the excited level, resulting in heat transfer from the phonon bath to the quantum subsystem, i.e., cooling of the environment. It is shown that for systems with a strong vibronic coupling and/or weak electron transfer, this type of cooling is possible for a wider range of the driver-cell polarization variation, which makes MV systems with weak delocalization promising candidates for performing ultrafast cooling.