Another avenue for enhancement of the optical activity of lowsymmetry allenes is through its covalent dimers, bis-allenes. A number of such molecules have been synthesized with different orientations of the allenic units. The frontier MOs of the allene units mix into combinations of the helical MOs, which are thus partially retained (ESI Figure ). The electronic transitions are composed similarly to those of allene. We examine an allene dimer where the allenic units are placed in parallel and rotated relative to each other, and two bis-allenes where the allene moeities have similar orientation (Figure ). The allene dimer is shown at 45 o . The hypothetical Bis-allene1 has an angle of 45 o , while silicon-based Bis-allene2 at 56.7 o was synthesized by Lin et al. All three have D 2 symmetry and we look at the 2B 3 excited state where the large off-axis component in ⃗ m of allene (cf. Figure ) enables high optical activity. The two MO excitations that contribute to the 2B 3 transition of the 45 o allene dimer are shown in Figure . The MOs are in-phase (HOMO-1 and LUMO) and out-of-phase combinations (HOMO and LUMO+1) of helical MOs. As in the optically active transitions of allene, the MO helicites match for the occupied and unoccupied MOs. The direct MO products thus look like those of allene with a clear helical movement of charge in each allenic unit. As the allenic units are rotated 45 o relative to each other, the electric and magnetic dipole moment point out of the screen, towards the viewer. The same transition is shown for Bis-allene1 in Figure . Again, it is visually clear that the helical components of the transition contribute significantly to the optical activity of the transition. The dihedral angle dependence of the rotatory strength of the dimer and the two bis-allenes is plotted in Figure . The rotatory strength of the allene dimer maxes at 642 • 10 -40 erg•esu•cm/G at 56 o orientation of the dimer. The two bis-allenes do not reach this level, but nonetheless the optical activity is significantly improved from monomer allene. The design of optically active allene-based macromolecules may benefit from considering the helical electronic structure of allene. We aim to explore such design rules in a future study.