Several theoretical studies have proposed strategies to reach helical molecular orbitals (Hel-MOs) in [n]cumulenes. While chiral even-[n] cumulenes feature Hel-MOs, odd-[n] cumulenes may also present them if the terminal groups lie on different planes. However, the hitherto proposed systems have been either experimentally unfeasible or resulted in opposite pseudodegenerated Hel-MOs, impeding their use in real applicatons. To overcome this challenge, we hereby demonstrate the introduction of a remarkable energy difference between helical orbitals of opposite twist by fixing the torsion angle between the terminal groups in butadiyne fragments. In order to experimentally lock the conformation of the terminal groups, we designed cyclic architectures by combining acetylenes with chiral spirobifluorenes. A straightforward synthetic strategy along with the high stability allowed the isolation and full characterization of systems presenting distinct helical orbitals. Finally, a thorough computational analysis revealed that the most significant optical responses of these systems originate mainly from the exciton coupling between the featured diphenylbutadiyne fragments. This novel strategy opens now access to the development of systems with distinct helical molecular orbitals suitable for their implementation into chiroptical and optoelectronic applications Scheme 1. General representation of acetylene (top left), [2]cumulene (top center), [3]cumulene (top right) and schematic representation of two possible paths for the formation of helical orbitals in [2]cumulenes (bottom left) and acetylenes (bottom right). Black spheres represent functional groups that can be the same or different and grey lobes stand for p atomic orbitals.

The concept of transmission channel plays a key role in the theory of molecular conductors. They are usually constructed from one-electron functions (molecular spin orbitals) in the extensively employed LCAO-MO method. Thus, the electric conductance can be obtained from the transmission matrix, whose diagonalization leads to a set of transmission eigenchannels. In this work, it is shown how using the time-energy uncertainty relation approach, the electron transport can be also analysed in terms of coupled transmission channels. Here, the transmission channels are constructed in terms of electron deformation orbitals (EDOs), which in turn arise from linear combinations of occupied and unoccupied one-electron functions. These transmission channels allow linking the electric conduction in molecular junctions with the traditional interpretation in mesoscopic systems, where the electron transport occurs by promotion of electrons from the valence to the conduction bands. Moreover, they lead to an expression of the electric conductance within the Landauer formalism, with the conductance given by the product of the quantum conductance and a transmission function. Transmission channels of junctions formed by polycyclic aromatic chains attached to gold atoms are analysed in detail. Symmetry rules for the MOs involved in these channels are established, giving rise to a straightforward method for the analysis of the transport ability in molecular systems.electron deformation orbitals, molecular conductance, molecular electronics, transmission channel 1 | I N TR ODU C TI ON The field of molecular electronics aims to revolutionize the current electronic components by introducing materials constructed at atomic scale. [1][2][3][4][5] Fabrication of electronic circuits using single molecular structures is fascinating and situates miniaturization of electronic devices in the limits of current nanotechnology. Nowadays, it is possible to form molecular junctions where a single molecular structure is linked, even covalently, to metallic electrodes. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Therefore, an electronic current induced across the junction by an external voltage can be measured. However, from the current stage to the development of efficient molecular circuits integrated in complex electronic devices there is still a long way to go. During this transition, theoretical methods, which go several steps ahead with respect to experimental developments, will play a crucial role. These methods will be all-important in an efficient design of molecular electronic devices that may execute specific functions. It is in this aspect that there is plenty of room for new theoretical approaches to emerge.In designing molecular electronic devices, the effects of structural changes on the electric conduction, including conformational modifications, isomerization processes, or chemical substitutions, must be perfectly controlled and understood. [44][45][46] Then, the study of the electron transport in molecular junction...

Feasibility of anti-ohmic transport in the ground state of molecular wires, intrinsically related to superexchange, is called into question.

No abstract

An extraordinary new family of molecular junctions, inaccurately referred to as "anti-Ohmic" wires in the recent literature, has been proposed based on theoretical predictions. The unusual electron transport observed for these systems, characterized by a reversed exponential decay of their electrical conductance, might revolutionize the design of molecular electronic devices. This behavior, which has been associated with intrinsic diradical nature, is reexamined in this work. Since the diradical character arises from a near-degeneracy of the frontier orbitals, the employment of a multireference approach is mandatory. CASSCF calculations on a set of nanowires based on polycyclic aromatic hydrocarbons (PAHs) demonstrate that, in the frame of an appropriate multireference treatment, the ground state of these systems shows the expected exponential decay of the conductance. Interestingly, these calculations do evidence a reversed exponential decay of the conductance, although now in several excited states. Similar results have been obtained for other recently proposed candidates to "anti-Ohmic" wires. These findings open new horizons for possible applications in molecular electronics of these promising systems.