The two perpendicularly oriented π-systems of allene mix into helical molecular orbitals (MOs) when the symmetry of the molecule is reduced. However, the π-π∗ transitions of allenes are linear combinations of two excitations that always consist of both helicities; consequently, the electronic transitions are not helical. Here, we examine the electronic structure of spiroconjugated molecules, which have the same parent symmetry as allene but with different relative orientation of the two π-systems. We show how the π-mixing in spiropentadiene is analogous to the helical π-mixing in allene. However, in spiroconjugated systems only half the π-MOs become helical. Due to this difference, the π-π∗ transitions in substituted spiropentadiene come in near-degenerate pairs where the helicity is symmetry protected, and consequently there is no significant mixing between excitations involving MOs of opposite helicity. This inherent helicity of the π-π* transitions is verified by computation of the change of electron density. These transitions have big rotatory strengths where the sign correlates with the helicity of the transition. The electronic helicity of spiroconjugated molecules thus manifests itself in observable electronic and optical properties.
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