Quasi-Free Electron States Responsible for Single-Molecule Conductance Enhancement in Stable Radical

Quantum Spin Transport Through Blatter's Diradicals and Triradicals

A single unpaired electron in an organic molecule residing in the singly occupied molecular orbital (SOMO) renders it an organic radical. It incorporates exchange splitting in the frontier occupied and unoccupied orbitals, separating the α- and β-orbitals. This fact enormously impacts the electron transport properties in organic radicals by promoting spin-polarized current and significantly enhancing conductance compared to their closed-shell counterparts. Exploring these phenomena, several monoradicals have been investigated through molecular spintronic experiments and theories. In this work, we addressed the impact of an increasing number of radical centers on the transport properties of multiradical molecular species by considering di- and triradicals based on a stable Blatter's radical. With an increasing number of radical centers, the number of SOMOs increases. Does the increased number of frontier SOMOs provide larger exchange splitting and better transport properties? Here, we observed that the spatial distributions of SOMOs and their coupling with electrodes play a decisive role compared with the presence of multiple unpaired electrons in the molecular systems.

Radical-induced single-molecule conductance tuning in 9,9'-bifluorenylidene derivatives

Single-molecule techniques provide new perspectives for understanding the relationship between spin delocalization of organic radicals and the intramolecular electronic structure. In this study, a series of 9,9'-bifluorenylidene (9,9'-BF) derivatives with four functionalization sites were synthesized, showcasing the orthogonalization of non-conducting (between radical sites) and conducting (between thiomethyl groups) paths. By precisely controlling the amount of radicals, or radical injection, ranging from mono-radical (Mono-PFPR) to diradicals, and among these, varying from medium (Di-PFPRy 0 = 0.66) to small (Di-PFNRy 0 = 0.11) to vanishing (Di-NFNRy 0 = 0) diradical characters (y 0 represents the diradical index), the influence of organic radical spin delocalization on the conducting path can be gradually modulated, transforming linear conjugated conducting channels into cross-conjugated channels and significantly reducing single-molecule conductance. This discovery provides an in-depth understanding of the complex relationship between radicals, spin delocalization, and molecular conductance, which is rather unique in the area of functional stable radical compounds. Ultimately, it provides forward-looking guidance for research on these materials in the field of organic electronic materials.

Gated off-site radical injection: Bidirectional conductance modulation in single-molecule junctions

Uncovering the effects of radical injection into responsive organic molecules is a long-sought goal, and the single-molecule junctions provide a unique way to investigate molecular conductance evolution during the radical injection. We can modulate the main channel conductance by using electronic injection from off-site neutral radicals acting as gating terminals. Two families of cyclopentadienone derivatives were synthesized, featuring the inter-pyridyl main conductance channel and the inter-radical paths that are linear (FCF) or cross conjugated (PCP). Using a scanning tunneling microscope break junction technique, we find that the injection of mono- and diradicals in the PCP system unexpectedly decreases the conductance regarding the closed-shell analog, while that of FCF systems increases. Through-bond and through-space conductance mechanisms are found in the FCF and PCP series, respectively, and jointly modulate the overall charge transmission. This off-site injection concept offers a promising approach for developing molecular devices by manipulating electrical conductance in single-molecule junctions.

Single-Molecule Conductance of Neutral Closed-Shell and Open-Shell Diradical Indenofluorenes

Organic diradicals are highly promising candidates as future components in molecular electronic and spintronic devices because of their low spin-orbit coupling. To advance toward final circuit realizations, a thorough knowledge of the behavior of diradicals within a single-molecule junction framework is imperative. In this work, we have measured for the first time the single-molecule conductance of a neutral open-shell diradical compound, a [2,1-b] isomer of indenofluorene (IF). Our results reveal that the conductance of the [2,1-b] isomer is about 1 order of magnitude higher than that of the corresponding closed-shell regioisomer [1,2-b] IF. This is significant, as it fundamentally demonstrates the possibility of forming stable single-molecule junctions using neutral diradical compounds which are also highly conducting. This opens up a new approach to the development of externally addressable spintronic devices operable at room temperature.

Zero-Bias Anti-Ohmic Behaviour in Diradicaloid Molecular Wires

Open-shell materials bearing multiple spin centres provide a key route to efficient charge transport in single-molecule electronic devices. They have narrow energy gaps, and their molecular orbitals align closely to the Fermi level of the metallic electrodes, thus allowing efficient electronic transport and higher conductance. Maintaining and stabilising multiple open-shell states-especially in contact with metallic electrodes-is however very challenging, generally requiring a continuous chemical or electrochemical potential to avoid self-immolation of the open-shell character. To overcome this issue, we designed, synthesised, and measured the conductance of a series of bis(indeno) fused acenes, where stability is imparted by a close-shell quinoidal conformation in resonance with the diradical electronic configuration. We show here that these compounds have anti-ohmic behaviour, with conductance increasing with increasing molecular length, at an unprecedented rate and across the entire bias window (± 1 . 3 V ${\pm 1.3\ V}$ ). Density Functional Theory (DFT) calculations support our findings, showing the rapidly narrowing HOMO-LUMO gap, unique to these diradicaloid structures, is responsible for the observed behaviour. Our results provide a framework for achieving efficient transport in neutral compounds and demonstrate the promise that diradicaloid materials have in single-molecule electronics, owing to their great stability and unique electronic structure.

Size Isn't Everything: Geometric Tuning in Polycyclic Aromatic Hydrocarbons and Its Implications for Carbon Nanodots

Recent developments in light-emitting carbon nanodots and molecular organic semiconductors have seen renewed interest in the properties of polycyclic aromatic hydrocarbons (PAHs) as a family. The networks of delocalized π electrons in sp2-hybridized carbon grant PAHs light-emissive properties right across the visible spectrum. However, the mechanistic understanding of their emission energy has been limited due to the ground state-focused methods of determination. This computational chemistry work, therefore, seeks to validate existing rules and elucidate new features and characteristics of PAHs that influence their emissions. Predictions based on (time-dependent) density functional theory account for the full 3-dimensional electronic structure of ground and excited states and reveal that twisting and near-degeneracies strongly influence emission spectra and may therefore be used to tune the color of PAHs and, hence, carbon nanodots. We particularly note that the influence of twisting goes beyond torsional destabilization of the ground-state and geometric relaxation of the excited state, with a third contribution associated with the electric transition dipole. Symmetries and peri-condensation may also have an effect, but this could not be statistically confirmed. In pursuing this goal, we demonstrate that with minimal changes to molecular size, the entire visible spectrum may be spanned by geometric modification alone; we have also provided a first estimate of emission energy for 35 molecules currently lacking published emission spectra as well as clear guidelines for when more sophisticated computational techniques are required to predict the properties of PAHs accurately.