Electronic Structure of Helicenes, C₂S Helicenes, and Thiaheterohelicenes

Abstract

The structures of linearly and ortho-fused helical polyaromatic hydrocarbon oligomers and polymers and their isoelectronic thiophene variants are studied using density functional theory (DFT). Structural and optical absorption data are compared with experiments where possible and excellent agreement is obtained. The results are interpreted with reference to orbital interaction diagrams. Infinite helicene tends to adopt a symmetry close to 61. C2S helicene is predicted to have an approximately 263 symmetry leading to an interdigitated S···S network parallel to the helical axis. Thiaheterohelicene has an approximately 72 helical structure. Periodic boundary condition (PBC) calculation and highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap extrapolation at the B3LYP/6-31G* level indicate a smaller band gap for helicene compared to phenacene. This difference is mainly due to the gap reducing effects of the transannular π−π interactions across the helical pitch in helicene. The calculated band gap is much smaller for linear thienoacene than that for isomeric C2S helicene due to the lack of effective conjugation pathway for the latter system. While thiaheterohelicene is structurally between the two large gap systems, helicene and C2S helicene, its band gap is significantly lower than either of the two.

Diazadithia[7]helicenes: Synthetic Exploration, Solid‐State Structure, and Properties

“…From the redox potentials of 6 c , a HOMO–LUMO gap of (2.96±0.05) eV can be estimated under the stated experimental conditions. This gap is only slightly larger than the optical band gap of 2.85 eV determined from the intersection of the normalized absorption and emission spectra, and therefore these values are more similar than is typically observed 19a,b. Assuming in the simplest case that these energy levels are indeed responsible for the observed oxidation and reduction,19c using the electrochemically determined HOMO–LUMO gap and the empirical relationship E HOMO =(−1.4 E onset (ox1)−4.6) eV,19d we can also estimate the HOMO and LUMO energies as −6.0 and −3.2 eV, respectively, relative to the vacuum level.…”

Synthesis and Characterization of a Helicene‐Based Imidazolium Salt and Its Application in Organic Molecular Electronics

“…Kertesz et al determined the [6]helicene HOMO-LUMO gap value to be 3.99 eV. [10] When comparing the HOMO orbitals of 2 to the 2-methyl[6]helicene, the electronic density slightly decreases along the aromatic system toward the imidazolium ring ( Figure 3B). This trend follows the distribution of the p-surface in the y axis of the helicenes with decreasing HOMO (from À5.53 eV for the 2-methyl [6]helicene to À6.15 eV for 2) and LUMO levels (from À1.56 eV for the 2-methyl [6]helicene to À2.23 eV for 2).…”

Extraordinary deformation capacity of smallest carbohelicene springs

“…Each carbohelicene has been marked as NX(Y,Z) where X means the number of atoms along the internal atomic chain per helicene spiral (see Fig. For example, the perfect carbohelicene is denoted N6 (6). The two numbers in the brackets separated by comma show that the carbohelicene spiral was constructed by alternation of two different rings (e.g.…”