Cross-conjugated molecules are known to exhibit destructive quantum interference, a property that has recently received considerable attention in single-molecule electronics. Destructive quantum interference can be understood as an antiresonance in the elastic transmission near the Fermi energy and leading to suppressed levels of elastic current. In most theoretical studies, only the elastic contributions to the current are taken into account. In this paper, we study the inelastic contributions to the current in cross-conjugated molecules and find that while the inelastic contribution to the current is larger than for molecules without interference, the overall behavior of the molecule is still dominated by the quantum interference feature. Second, an ongoing challenge for single molecule electronics is understanding and controlling the local geometry at the molecule-surface interface. With this in mind, we investigate a spectroscopic method capable of providing insight into these junctions for cross-conjugated molecules: inelastic electron tunneling spectroscopy (IETS). IETS has the advantage that the molecule interface is probed directly by the tunneling current. Previously, it has been thought that overtones are not observable in IETS. Here, overtones are predicted to be strong and, in some cases, the dominant spectroscopic features. We study the origin of the overtones and find that the interference features in these molecules are the key ingredient. The interference feature is a property of the transmission channels of the π system only, and consequently, in the vicinity of the interference feature, the transmission channels of the σ system and the π system become equally transmissive. This allows for scattering between the different transmission channels, which serves as a pathway to bypass the interference feature. A simple model calculation is able to reproduce the results obtained from atomistic calculations, and we use this to interpret these findings.

The 2,6,10-tris(dialkylamino)trioxatriangulenium dyes (ATOTA(+)) are highly stabilised cationic chromophores with D(3h) symmetry. The symmetry gives rise to a degeneracy of the main electronic transition. In low polarity solvents significant splitting of this degenerate transition is observed and assigned to ion pair formation. Ion pairing of the 2,6,10-tris(dioctylamino)trioxatriangulenium ion with Cl(-), BF(4)(-), PF(6)(-) and TRISPHAT anions was studied using absorption spectroscopy. A clear correlation is found between the size of the anion and the splitting of the ATOTA(+) transitions. In benzene the Cl(-) salt displays a splitting of 1955 cm(-1), while the salt of the much larger TRISPHAT ion has a splitting of 1543 cm(-1). TD-DFT calculations confirm the splitting of the states and provide a detailed insight into the electronic structure of the ion pairs. The different degree of splitting in different ion pairs is found to correlate with the magnitude of the electric field generated in each ion pair, thus leading to the conclusion that the effect seen is an internal Stark effect. By insertion of an amphiphilic derivative of the ATOTA(+) chromophore in an oriented lamellar liquid crystal, it was possible to resolve the two bands of the double peak spectrum and show their perpendicular orientation in the molecular framework, as predicted by the calculations.

Destructive quantum interference in single molecule electronics is an intriguing phenomenon; however, distinguishing quantum interference effects from generically low transmission is not trivial. In this paper, we discuss how quantum interference effects in the transmission lead to either low current or a particular line shape in current-voltage curves, depending on the position of the interference feature. Second, we consider how inelastic electron tunneling spectroscopy can be used to probe the presence of an interference feature by identifying vibrational modes that are selectively suppressed when quantum interference effects dominate. That is, we expand the understanding of propensity rules in inelastic electron tunneling spectroscopy to molecules with destructive quantum interference.

Triarylmethylium cations with the three rings linked by two bridging groups constitute a special class of [4]heterohelicenium dyes that combine high configurational stability with the optical properties of classic dye compounds. The racemization barriers and electronic circular dichroism of seven [4]heterohelicenium analogues are investigated with density functional theory to consider different design strategies. The racemization barriers are examined with the B3LYP functional utilizing the basis set 6-31+G* with respect to bridging heteroatoms in the helicenium motif and with different bulky substituents in the helix pitch. The racemization barriers of the [4]heterohelicenium are found to be highly dependent on the relative size of the bridging atom. Nucleophilic attack at the carbenium ion leads to formation of center adducts for which the racemization barriers are found to be lowered by 8-12 kJ/mol. This finding of a nucleophilic racemization catalysis may be rationalized by the loss of conjugation upon formation of an sp(3)-hybridized carbon in the center adducts. The effect of the heteroatom substitution and center adduct formation is reflected in the electronic properties of the compound calculated with the Coulomb-attenuated method CAM-B3LYP with the basis set 6-311++G**. The excitation energies are found to be highly dependent on the twisting of the helicenium framework and only weakly influenced by the electronic nature of the bridging substituent. The electronic circular dichroism is evaluated, as the rotational strength is found to be highly dependent on both the overall molecular structure and substitution pattern but with no simple correlation between structure and circular dichroism (CD) response. The calculations reveal that the magnitude of rotational strength in most cases is dominated by the angle between the electronic and magnetic transition dipole moments. Finally, it is found that computational screening of many different structures and substituents might be needed to find target structures with maximized rotational strength.