Helical electronic transitions of spiroconjugated molecules

The two pi-systems of allene can combine into helical molecular orbitals (MOs), yet the helicity is lost in the pi-pi* transitions due to excited state mixing. In spiroconjugated molecules the relative orientation of the two pi-systems is different and consequently only half the pi-MOs become helical. We show that the helicity of the electronic transitions of methyl-substituted spiropentadiene is symmetry protected. As a result, helical pi-conjugation can manifest in observable electronic and chiroptical properties.