The quest to model carbyne has been largely tackled through increasingly longer polyynes. [n]Cumulenes are an alternative class of molecules to model carbyne. The major difference between polyynes and cumulenes is that polyynes maintain strong bond length alternation (BLA) while cumulenes do not. The development of longer [n]cumulenes is challenged by decreased stability relative to oligoynes. This study compares the persistence of tetraaryl[n]cumulenes based on ortho‐substituent(s) of aryl endgroups. A trend emerges in which bulkier ortho‐substituents offer more persistent [n]cumulenes. The enhanced stability is consistent with a combination of steric shielding and disrupted electronic communication between the endgroups and cumulenic framework. Kinetically stabilized [n]cumulenes [n]otBu (n = 5, 7, 9, 11, 13) are successfully synthesized. The longest derivatives, [11]otBu and [13]otBu, are studied in solution, and the electronic absorption properties are compared experimentally and computationally. The terminal alkylidene groups of [n]cumulenes have significant effects on the optical properties and result in an additional optical state, S1, relative to oligoynes. The S1 state gives a lower fundamental optical energy gap (Eg) in [n]cumulenes relative to oligoynes and decreases as the molecules transition from D2 to D∞h symmetry. The highest energy state (S4) is allowed by symmetry at any length and shows remarkable similarity to the analogous absorptions of polyynes.

When a small electric bias is applied to a single-molecule junction, current will flow through the molecule via a tunneling mechanism. In molecules with a cyclic or helical structure there may be circular currents, giving rise to a uni-directional magnetic field. Here, we implement the Biot Savart law and calculate the magnetic field resulting from the ballistic current density for a selection of molecules. We find that three prerequisites are important for achieving a substantial magnetic field in a single-molecule junction. (1) The current must be high, (2) the ring current must be unidirectional within the bias window, and (3) the diameter of the ring current must be small. We identify both cyclic and linear molecules that potentially fulfill these requirements. In cyclic annulenes with bond-length alternation the current-induced magnetic field can approach the mT-range whereas archetypical cyclic molecules, such as benzene, are not suitable candidates for the generation of a substantial magnetic field. In linear carbon chains with circular currents due to their helical π-systems, the magnetic field is in the mT-range. When the bias window is gated closer to resonance, we show that the magnetic field can potentially reach the sub-Tesla range. Our results provide proof-of-concept for achieving experimentally relevant current-induced magnetic fields in molecular wires at low bias.

Molecules where the lowest excited singlet state is lower in energy than the lowest triplet are highly promising for a number of organic materials applications as efficiency limitations stemming from spin statistics are overcome. All molecules known to possess such singlet-triplet inversions exhibit a pattern of spatially alternating but nonoverlapping HOMO and LUMO orbitals, meaning the lowest excited states are of a local character. Here, we demonstrate that derivatives of the bicyclic hydrocarbon calicene exhibit Hund's rule violations in charge-transfer (CT) states between its rings. These CT states can be tuned with substituents, so that the first excited singlet and triplet state are energetically inverted. This provides a conceptual connection between the emerging fields of inverted gap molecules and existing molecular design rules for state-of-the-art thermally activated delayed fluorescence materials.

Inverted singlet-triplet gaps may lead to novel molecular emitters if a rational design approach can be achieved. We uncover a substituent strategy that enables tuning of the gap and succeed in inducing inversion in near-gapless molecules such as azupyrene. Based on known inverted-gap emitters, such as heptazine, we design substituted analogs with even more negative singlet-triplet gaps than in the parent systems. The inversion is lost if the reverse substituent-strategy is used. We thus demonstrate a definite set of conceptual design rules for inverted gap molecules.

Molecules with Hund's rule violations between low-lying singlet and triplet states may enable a new generation of fluorescent emitters. However, only a few classes of molecules are known with this property at the current time. Here, we use a high-throughput screening algorithm of the FORMED database to uncover a class of compounds where the first excited state violates Hund's rule. We examine this class of bridged [10]- and [14]annulene perimeters with saturated bridges, and relate them to known conjugated polycyclic systems with Hund's rule violations. Despite the structural similarities with the related class of nonalternant polycyclic hydrocarbons, the mechanism is different in these bridged annulene perimeters. Here, two molecular orbital configurations contribute to each excited state. Consequently, a Hund's rule violation can only be unambiguously assigned based on the symmetry of the lowest excited singlet and triplet states. With several examples of synthetically realistic molecules, the class of bridged [10]- and [14]annulenes thus provides a structural link between the known nonalternant and alternant (azaphenalene) classes of molecules violating Hund's rule. These design principles may open avenues for the identification of new types of molecules where the order of the photophysically relevant first excited singlet and triplet states are inverted.

Chemical reduction of a [4]cumulene with cesium metal was explored, and the structural changes stemming from electron acquisition are detailed using X-ray crystallography. It is found that the [4]cumulene undergoes dramatic geometric changes upon stepwise reduction, including bending of the cumulenic core and twisting of the endgroups from orthogonal to planar. The structural deformation is consistent with early theoretical reports that suggest that the twisting should occur upon reduction of both even and odd [n]cumulenes. The current results, on the other hand, are inconsistent with a previous experimental study of a [3]cumulene in which the predicted twisting is not observed upon reduction. DFT calculations reveal that the barrier to deformation is an order of magnitude lower in a [3]cumulene than a [4]cumulene, allowing the barrier to be overcome in the solid-state. To bend or not to bend? The chemical reduction of a [4]cumulene using cesium gives the mono- and doubly-reduced cumulenes. Crystallographic analysis reveals dramatic geometric changes upon stepwise reduction, including bending of the cumulenic core and twisting of the endgroups from orthogonal to planar.+ image

Electrohelicity arises in molecules such as allene and spiropentadiene when their symmetry is reduced and helical frontier molecular orbitals (MOs) appear. Such molecules are optically active and electrohelicity has been suggested as a possible design principle for increasing the chiroptical response. Here we examine the fundamental link between electrohelicity and optical activity by studying the origin of the electric and magnetic transition dipole moments of the π-π* transitions. We show that the helical character of the MOs drives the optical activity in allene, and we use this knowledge to design allenic molecules with increased chiroptical response. We further examine longer carbyne-like molecules. While the MO helicity also contributes to the optical activity in non-planar butatriene, the simplest cumulene, we show there is no relation between the chiroptical response and the helical π-MOs of tolane, a simple polyyne. Finally, we demonstrate that the optical activity of spiropentadiene is inherently linked to mixing of its two π-systems rather than the helical shape of its occupied π-MOs. We thus find that the fundamental connection between electrohelicity and optical activity is very molecule dependent. Although electrohelicity is not the underlying principle, we show that the chiroptical response can be enhanced through insight into the helical nature of electronic transitions.

Molecules where the first excited singlet state is lower energy than the first excited triplet state have the potential to revolutionize OLEDs. This inverted singlet-triplet gap violates Hund’s rule and currently there are only a few molecules which are known to have this property. Here, we screen the complete set of non-alternant hydrocarbons consisting of 5-, 6-, 7-membered rings fused into two-, three- and four-ring polycyclic systems. We identify several molecules where the symmetry of the ground-state structure is broken due to bond-length alternation. Through symmetry-constrained optimizations we identify several molecular cores where the singlet-triplet gap is inverted when the structure is in a higher symmetry, pentalene being a known example. We uncover a strategy to stabilize the molecular cores into their higher-symmetry structures with electron donors or acceptors. We design several substituted pentalenes, s-indacenes, and indeno[1,2,3-ef]heptalenes with inverted gaps, among which there are several synthetically known examples. In contrast to known inverted gap emitters, we identify the double-bond delocalized structure of their conjugated cores as the necessary condition to achieve the inverted gap. This strategy enables chemical tuning and paves the way for the rational design of polycyclic hydrocarbons with inverted singlet-triplet gaps. These molecules are prospective emitters if their properties can be optimized for use in OLEDs.

Molecules with inversion of the singlet and triplet excited-state energies are highly promising for the development of organic light-emitting diodes (OLEDs). To date, azaphenalenes are the only class of molecules where these inversions have been identified. Here, we screen a curated database of organic crystal structures to identify existing compounds for violations of Hund's rule in the lowest excited states. We identify two further classes with this behavior. The first, a class of zwitterions, has limited relevance to molecular emitters as the singlet-triplet inversions occur in the third excited singlet state. The second class consists of two D2h -symmetry non-alternant hydrocarbons, a fused azulene dimer and a bicalicene, whose lowest excited singlet states violate Hund's rule. Due to the connectivity of the polycyclic structure, they achieve this symmetry through aromatic stabilization. These hydrocarbons show promise as the next generation of building blocks for OLED emitters.

Molecules with inversion of the singlet and triplet excited state energies are highly promising for the development of a new generation of OLED emitters. Currently, only one class of molecules with these inversions have been identified, in the form of azaphenalenes. Here, we screen a curated database of reported organic crystal structures to identify existing compounds for violations of Hund’s rule in the lowest excited states and identify two new classes of molecules with this behavior. The first is a class of zwitterions where the singlet-triplet inversions occur in the third excited singlet state, which limits their relevance to molecular emitters. The second class consists of two high-symmetry odd-membered polycyclic aromatic hydrocarbons, a fused azulene dimer and a bicalicene, where the lowest excited singlet states violate Hund’s rule. The high D2h symmetry of the two molecules is stabilized by the connectivity of the polycyclic structure, which we rationalize by studying the aromaticity of the ring components. This class of aromaticity-stabilized high-symmetry hydrocarbons shows promise as the next generation of building blocks for organic light-emitting diode emitters.

The allene radical cation can be stabilized both by Jahn-Teller distortion of the bond lengths and by torsion of the end-groups. However, only the latter happens and the allene radical cation relaxes into a twisted D2 symmetry structure with equal double-bond lengths. Here we revisit the Jahn-Teller distortion of allene and spiropentadiene by assessing the possible implications of their helical π-systems in the radical cations. We describe a general relation between the structure and the number of π-electrons in spiroconjugated and linearly conjugated systems. Through constrained optimizations we compare the stabilization achieved by bond-length alternation and axial torsion in the radical cations, which we explain with a simple frontier molecular orbital (MO) picture. While structurally different, allene and spiropentadiene have similar helical frontier MOs. Both cations relax through torsion because the stabilization of their helical frontier MOs is bigger than that which can be achieved by linear π-conjugation. Electrohelicity thus manifests in molecular systems with partial occupation as a helical π-conjugation effect, which evidently provides more stabilization than its linear counterpart in terms of the Jahn-Teller distortion. This mechanism may be a driving factor for the relaxation in a range of spiroconjugated and linearly conjugated cationic systems.

The single-molecule conductance of saturated molecules can potentially be fully suppressed by destructive quantum interference in their σ-system. However, only few molecules with σ-interference have been identified and the structure-property relationship remains to be elucidated. Here, we explore the role of substituents in modulating the electronic transmission of saturated molecules. In functionalized bicyclo[2.2.2]octanes, the transmission is suppressed by σ- interference when fluorine substituents are applied. For bicyclo[2.2.2]octasilane and - octagermanes the transmission is suppressed when carbon-based substituents are used, and such molecules are likely to be highly insulating. For the carbon-based substituents we find a strong correlation between the appropriate Hammett constants and the transmission. The substituent effect enables systematic optimization of the insulating properties of saturated molecular cores.

Designing highly insulating sub-nanometer molecules is difficult because tunneling conductance increases exponentially with decreasing molecular length. This challenge is further enhanced by the fact that most molecules cannot achieve full conductance suppression with destructive quantum interference. Here, we present results for a series of small saturated heterocyclic alkanes where we show that conductance is suppressed due to destructive interference. Using the STM-BJ technique and density functional theory calculations, we confirm that their single-molecule junction conductance is lower than analogous alkanes of similar length. We rationalize the suppression of conductance in the junctions through analysis of the computed ballistic current density. We find there are highly symmetric ring currents, which reverse direction at the antiresonance in the Landauer transmission near the Fermi energy. This pattern has not been seen in earlier studies of larger bicyclic systems exhibiting interference effects and constitutes clear-cut evidence of destructive σ-interference. The finding of heterocyclic alkanes with destructive quantum interference charts a pathway for chemical design of short molecular insulators using organic molecules.

Probing structural changes of a molecule induced by charge transfer is important for understanding the physicochemical properties of molecules and developing new electronic devices. Here, we interrogate the structural changes of a single diketopyrrolopyrrole (DPP) molecule induced by charge transport at a high bias using scanning tunneling microscope break junction (STM-BJ) techniques. Specifically, we demonstrate that application of a high bias increases the average nonresonant conductance of single Au-DPP-Au junctions. We infer from the increased conductance that resonant charge transport induces planarization of the molecular backbone. We further show that this conformational planarization is assisted by thermally activated junction reorganization. The planarization only occurs under specific electronic conditions, which we rationalize by ab initio calculations. These results emphasize the need for a comprehensive view of single-molecule junctions which includes both the electronic properties and structure of the molecules and the electrodes when designing electrically driven single-molecule motors.

The two pi-systems of allene can mix into helical molecular orbitals (MOs), yet the helicity is lost in the pi-pi∗ transitions. In spiroconjugated molecules the relative orientation of the two π- systems is different as only half the pi-MOs become helical. Consequently, the helicity of the electronic transitions is symmetry protected and thus helical pi-conjugation can manifest in observable electronic and chiroptical properties.

The frontier molecular orbital (MO) topology of linear carbon molecules, such as polyynes, can be visually identified as helices. However, there is no clear way to quantify the helical curvature of these π-MOs and it is thus challenging to quantify correlations between the helical curvature and molecular properties. In this paper, we develop a method that enables us to compute the helical curvature of MOs based on their nodal planes. Using this method, we define a robust way of quantifying the helical nature of MOs (helicality) by their deviation from a perfect helix. We explore several limiting cases, including polyynes, metallacumulenes, cyclic allenes, and spiroconjugated sys- tems where the change in helical curvature is subtle, yet clearly highlighted with this method. For example, we show that strain only has a minor effect on the helicality of the frontier orbitals of cycloallenes and that the MOs of spiroconjugated systems are close to perfect helices around the spiro-carbon. Our work provides a well-defined method for assessing orbital helicality beyond visual inspection of MO isosurfaces, thus paving the way for future studies of how the helicality of π-MOs affects molecular properties.

Correction for ‘Helical orbitals and circular currents in linear carbon wires’ by Marc H. Garner et al., Chem. Sci., 2019, 10, 4598–4608.

Helical frontier molecular orbitals (MOs) appear in disubstituted allenes and even-n cumulenes. Chiral molecules are optically active, but while these molecules are single-handed chiral, π-orbitals of both helicities are present. Here we computationally examine whether the optical activity of chiral cumulenes is controlled by the axial chirality or the helicity of the electronic structure. We exploit hyperconjugation with alkyl, silaalkyl, and germaalkyl substituents to adjust the MO helicity without altering the axial chirality. For the same axial chirality, we observe an inversion of the helical MOs contribution to the electronic transitions and a change of sign in the electronic circular dichroism and optical rotation dispersion spectra. While the magnitude of the chiroptical response also increases, it is similar to that of chiral cumulenes without helical π-orbitals. Overall, helical π-orbitals correlate with the big chiroptical response in cumulenes, but are not a prerequisite for it.

Molecular dielectric materials require ostensibly conflicting requirements of high polarizability and low conductivity. As previous efforts towards molecular insulators focused on saturated molecules, where the polarizability will be generally less than conjugated molecules, it remains an open question whether electron transport can be comprehensively suppressed in conjugated systems. Here, we demonstrate that the s-transmission in conjugated oligophenylsystems can be suppressed by destructive sigma-interference. Using density functional theory, we study the Landauer transmission and ballistic current density, which allow us to determine how the transmission is affected by structural changes in the molecule. In para-linked phenyl rings, the sigma- transmission can be suppressed by changing the remaining hydrogens to methyl substituents due to the inherent gauche-like structure of a benzene ring. When two neighboring phenyl rings are orthogonal to each other, the transmission through both pi- and sigma-systems can be effectively suppressed, making methyl-substituted oligophenyls promising candidates for conjugated insulators.

Molecular dielectric materials require ostensibly conflicting requirements of high polarizability and low conductivity. As previous efforts towards molecular insulators focused on saturated molecules, it remains an open question whether pi- and sigma-transport can be simultaneously suppressed in conjugated systems. Here, we demonstrate that there are conjugated molecules where the sigma-transmission is suppressed by destructive sigma-interference, while the pi-transmission can be suppressed by a localized disruption of conjugation. Using density functional theory, we study the Landauer transmission and ballistic current density, which allow us to determine how the transmission is affected by various structural changes in the molecule. We find that in para-linked oligophenyl rings the sigma-transmission can be suppressed by changing the remaining hydrogens to methyl groups due to the inherent gauche-like structure of the carbon backbone within a benzene ring, similar to what was previously seen in saturated systems. At the same time, the methyl groups fulfil a dual purpose as they modulate the twist angle between neighboring phenyl rings. When neighboring rings are orthogonal to each other, the transmission through both pi- and sigma-systems is effectively suppressed. Alternatively, breaking conjugation in a single phenyl ring by saturating two carbons atoms with two methyl substituents on each carbon, results in suppressed pi- and sigma-transport independent of dihedral angle. These two strategies demonstrate that methyl-substituted oligophenyls are promising candidates for the development of molecular dielectric materials.