Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(CN), which enhances ΔFe via stabilizing t2g orbitals. The inflexible geometry of the macrocycle results in short Fe–N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid–base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an FeII macrocyclic species.

Due to their small size and unique properties, single-molecule electronics have long seen research interest from experimentalists and theoreticians alike. From a theoretical standpoint, modeling these systems using electronic structure theory can be difficult due to the importance of electron correlation in the determination of molecular properties, and this electron correlation can be computationally expensive to consider, particularly multiconfigurational correlation energy. In this work, we develop a new approach for the study of single-molecule electronic systems, denoted NEGF-MCPDFT, which combines multiconfiguration pair-density functional theory (MCPDFT) with the non-equilibrium Green’s function formalism (NEGF). The use of MCPDFT with NEGF allows for the efficient inclusion of both static and dynamic electron correlations in the description of the junction’s electronic structure. Complete active space self-consistent field wave functions are used as references in the MCPDFT calculation, and as with any active space method, effort must be made to determine the proper orbital character to include in the active space. We perform conductance and transmission calculations on a series of alkanes (predominantly single-configurational character) and benzyne (multiconfigurational character), exploring the role that active space selection has on the computed results. For the alkane junctions explored (where dynamic electron correlation dominates), the MCPDFT-NEGF results agree well with the DFT-NEGF results. For the benzyne junction (which has a significant static correlation), we see clear differences in the MCPDFT-NEGF and DFT-NEGF results and evidence that NEGF-MCPDFT is capturing additional electron correlation effects beyond those provided by the Perdew–Burke–Ernzerhof functional.

Electronic switches built from single molecules such as biphenyl dithiol are promising replacements for traditional electronic devices. To support experimental investigations of molecular switches, charge transport values are typically predicted using non‐equilibrium Green's functions constructed using density functional theory (NEGF‐DFT). Previous studies of biphenyl dithiol, however, have usually overestimated experimental conductance values and have not reproduced a drop in conductance at low torsional angles. In this paper, we employ a methodology that constructs the non‐equilibrium Green's functions from 2‐electron reduced density matrix theory (NEGF‐RDM). This approach has previously predicted lower conductance values than NEGF‐DFT for systems where NEGF‐DFT is known to overestimate transport. In the first direct comparison of the NEGF‐RDM method and experimental results, we examine the biphenyl dithiol at a range of torsional angles. We evaluate both the quantitative accuracy of multiple NEGF methods and provide insights into the experimentally observed drop in conductance at small torsional angles.

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong -donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) with a 1.25 ns metal-to-ligand charge transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties examined in a variety of solvents. The HMTI ligand is highly pi-acidic due to low lying pi*(C=N), which enhances DeltaFe via stabilizing t2g orbitals. The inflexible geometry of the macrocy-cle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an Fe(II) macrocyclic species.

Replication of the long lifetimes of 4d transition metal complexes in their 3d counterparts is desirable for both cost reduction and environmental concerns. Fe II (rac-HMTI)(CN) 2 is an Fe II complex with a remarkable nanosecond lifetime metal-to-ligand charge transfer (MLCT) state in low polarity solvents. Architecturally, the Fe II center is ligated to axiallyoriented strong field cyano ligands, and equatorially to a rigid [14]-tetracene-N4 macrocycle. This rigidity enforces poor vibrational overlap of excited states, significantly raising the barrier of vibrational relaxation and extending their lifetimes. Fe(HMTI)(CN) 2 is studied by optical transient absorption, and spectroelectrochemical studies reproduce the features of the OTA at potentials consistent with metal oxidation and ligand reduction, confirming the attribution of MLCT character to the transition. DFT, TDDFT and CASSCF computational methods are also used to create a theoretical potential energy manifold, to better describe the deactivation mechanism of the excited state. Further understanding of this molecule's photophysics will allow for more targeted development of longer-lived Fe II complexes.