Saturated substituents that hyperconjugate into the helical πsystem break the degeneracies of the frontier π-MOs. Breaking the symmetry of allene with simple substituents such as in 1,3-dimethylallene makes the molecule optically active. However, the rotatory strength of the allowed S 0 → S 4 transition is still modest (Table ) because the helical frontier MOs remain near-degenerate. If ethyl substituents are used instead, the rotatory strength comes close to the limit of the pure MO transitions (156•10 -40 erg•esu•cm/G, Figure ). While the substituents themselves may also contribute to increasing the rotatory strength, this enhancement is primarily driven by the perturbation of the allene π-system. As a control, we can place one or two ethyl substituents at the 1-position instead of at the 1,3-positions (ESI Figure ). Despite all molecules having similar low symmetry (Table ), the increase in the rotatory strength is much smaller because the substituent pattern in ethylallene and 1,1-diethylallene does not fully retain the helical frontier MOs and break their degeneracy. This is also evident for 1,3-diethylallene as its optical activity will depend on the conformation of the substituents. We described this mechanism in detail in a previous communication. Table Rotatory strength of S 0 → S 4 transition of substituted allenes