On the Spin-State Dependence of Redox Potentials of Spin Crossover Complexes

Reversible Switching of Strong Light-Matter Coupling Using Spin-Crossover Molecular Materials

The formation of hybrid light-matter states through the resonant interaction of confined electromagnetic fields with matter excitations has emerged as a fascinating tool for controlling quantum-mechanical states and then manipulating the functionalities and chemical reactivity landscape of molecular materials. Here we report the first observation of switchable strong light-matter coupling involving bistable spin-crossover molecules. Spectroscopic measurements, supported by transfer-matrix and coupled-oscillator simulations, reveal Rabi splitting values of up to 550 meV, which at 15% of the molecular excitation energy enter the regime of ultrastrong coupling. We find that the thermally induced switching of molecules between their low-spin and high-spin states allows fine control of the light-matter hybridization strength, offering the appealing possibility of reversible switching between the ultrastrong- and weak-coupling regimes within a single photonic structure. Through this work, we show that spin-crossover molecular compounds constitute a promising class of active nanomaterials in the burgeoning context of tunable polaritonic devices and polaritonic chemistry.

Dimerization of [FeIII(bpy)3]3+ in Aqueous Solutions: Elucidating a Mechanism Based on Historical Proposals, Electrochemical Data, and Computational Free Energy Analysis

Iron(II) tris-bipyridine, [Fe^II(bpy)₃]²⁺, is a historically significant organometallic coordination complex with attractive redox and photophysical properties. With respect to energy storage, it is a low-cost, high-redox potential complex and thus attractive for use as a catholyte in aqueous redox flow batteries. Despite these favorable characteristics, its oxidized Fe(III) form undergoes dimerization to form μ-O-[Fe^III(bpy)₂(H₂O)]₂⁴⁺, leading to a dramatic ∼0.7 V decrease during battery discharge. To date, the energetics and complete mechanism of this slow, sequential electrochemical-chemical (EC) process, which includes electron transfer, nucleophilic attack, ligand cleavage, μ-oxo bond formation, and spin state transition, have not been elucidated. Using cyclic voltammetry, redox flow battery data, and density functional theory calculations guided by previously proposed mechanisms, we modeled more than 100 complexes and performed more than 50 geometry scans to resolve the key steps dictating these complex chemical processes. Quantitative free energy surfaces are developed to model the mechanism of dimerization accounting for the spins and identities of any possible Fe(II), Fe(III), or Fe(IV) intermediates. Electrochemical reduction of the dimer regenerates [Fe^II(bpy)₃]²⁺ in an overall reversible process. Computational electrochemistry interrogates the influence of spin state, coordination environment, and molecular conformation at the electrode-electrolyte interface through a proposed stepwise dimer reduction process. Experimentally, we show that the considerable overpotential associated with this event can be catalytically mitigated with disparate materials, including platinum, copper hexacyanoferrate, and activated carbon. The findings are of fundamental and applied significance and could elevate [Fe^II(bpy)₃]²⁺ and its derivatives to play a vital role in the burgeoning renewable energy economy.

Discovery of Kinetic Effect in a Valence Tautomeric Cobalt-Dioxolene Complex

Two isostructural valence tautomeric (VT) complexes with different critical temperatures were prepared and fully investigated through a series of magnetic, structural, spectral, and differential scanning calorimetry evidence. The kinetic effect in the VT complex was observed for the first time through scan-rate-dependent studies and further validated by annealing tests.

Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

Novel cobalt and zinc complexes with the tetradentate ppq (8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) ligand have been synthesized and fully characterized. Electrochemical measurements have shown that the formal monovalent complex [Co(ppq)(PPh₃)]⁺ (2) undergoes two stepwise ligand-based electroreductions in DMF, affording a [Co(ppq)DMF]^-1 species. Theoretical calculations have described the electronic structure of [Co(ppq)DMF]^-1 as a low-spin Co(II) center coupling with a triple-reduced ppq radical ligand. In the presence of triethylammonium as the proton donor, the cobalt complex efficiently drives electrocatalytic hydrogen evolution with a maximum turnover frequency of thousands per second. A mechanistic investigation proposes an EECC H₂-evolving pathway, where the second ligand-based redox process (E), generating the [Co(ppq)DMF]^-1 intermediate, initiates proton reduction, and the second proton transfer process (C) is the rate-determining step. This work provides a unique example for understanding the role of redox-active ligands in electrocatalytic H₂ evolution by transition metal sites.

How the Donor/Acceptor Spin States Affect the Electronic Couplings in Molecular Charge-Transfer Processes?

The electronic coupling matrix element H_AB is an essential ingredient of most electron-transfer theories. H_AB depends on the overlap between donor and acceptor wave functions and is affected by the involved states’ spin. We classify the spin-state effects into three categories: orbital occupation, spin-dependent electron density, and density delocalization. The orbital occupancy reflects the diverse chemical nature and reactivity of the spin states of interest. The effect of spin-dependent density is related to a more compact electron density cloud at lower spin states due to decreased exchange interactions between electrons. Density delocalization is strongly connected with the covalency concept that increases the spatial extent of the diabatic state’s electron density in specific directions. We illustrate these effects with high-level ab initio calculations on model direct donor–acceptor systems relevant to metal oxide materials and biological electron transfer. Obtained results can be used to benchmark existing methods for H_AB calculations in complicated cases such as spin-crossover materials or antiferromagnetically coupled systems.