The helical MO transitions belong to the b 1 irreducible representation and mix by linear combinations into non-degenerate transitions. The resulting transition dipoles into the two B 1 excited states are shown in the right column of Figure ; either ⃗ µ or ⃗ m z cancels out, and the two transitions are optically inactive. The transitions involving MOs of differing helicity also mix in similar fashion (bottom rows of Figure ); those two are totalsymmetric and are electric and magnetic dipole forbidden. We thus see why the allene transitions are optically inactive. It is the linear combination of the two helical MO transitions (P ± M, Figure ) that limits the optical activity of allenic molecules. The D 2d -structure represents a case where the orbital helicity is perfectly cancelled. To increase the optical activity we must split the degenerate helical MOs, so they will not contribute equally to the excited states. This is the principle of electrohelicity-driven optical activity in allene, and we will explore how this is applied in terms of molecular design.