Distinct Helical Molecular Orbitals through Conformational Lock**

Abstract

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.

Quantification of the Helicality of Helical Molecular Orbitals

“…Polyynes are linear chains of sp-hybridized carbon that can exhibit helical MOs visually similar to those seen in ↵,!-substituted even [n]cumulenes. [10][11][12]19,20,25,26 Similarly, odd [n]cumulenes can also exhibit helical MOs when the end-groups are rotated away from a co-planar orientation as we show at the top of Figure 4. There is almost free rotation for the end-groups of polyynes with the ground-state orientation determined by the type of end-groups.…”

“…Given the interest in helical MOs as a means to understand related properties and the range of molecular systems in which these MOs appear, [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] it is problematic that the assessment of helical MOs relies on visual inspection alone. It is thus altogether binary; if an MO is shaped like a coil we may deem it to be helical.…”

Helical electronic transitions of spiroconjugated molecules

“…Still, relations to molecular properties have been proposed. 12,[18][19][20][21][22][23] Given that the through-bond electron delocalization in spiroconjugated molecules is closely related to that in allene (Figure 1), an interesting question is whether different types of p-systems are also limited by the dual-helicity of the MOs?…”

Helical Electronic Transitions of Spiroconjugated Molecules

“…Still, relations to molecular properties have been proposed. 12,[18][19][20][21][22][23] Given that the through-bond electron delocalization in spiroconjugated molecules is closely related to that in allene (Figure 1), an interesting question is whether different types of p-systems are also limited by the dual-helicity of the MOs?…”