Polyynes are structurally similar to even-carbon cumulenes but are more stable due to the single-triple bond topology, and is thus an oft-used motif for molecular linkers. As in cumulenes, when the end-groups are rotated polyynes become optically active. Some π-orbitals become helical as the symmetry is reduced and the π-systems mix. The mixing is much less significant than in the cumulenes, and at small torsion angles the mixing is not visually clear (Figure ). Although the HOMO and LUMO only achieve weak helicity, there are lower-lying π-MOs that become more helical. The HOMO-LUMO transition of tolane is electric-dipole allowed and becomes optically active with torsion. Just as there is no visually clear helicity in the MOs, there is no significant helicity in the transition density (Figure ). While there is charge rotating around the polyynic wire in both transitions at high torsion angles, it seems that it is primarily the components on the terminal phenyl groups that gives a non-zero ⃗ m. This charge rotation between the phenyl groups increases with higher torsion angle. The rotatory strengths therefore increase until the phenyl groups are almost in perpendicular orientation, as plotted in Figure .