Control over the Emission Properties of [5]Helicenes Based on the Symmetry and Energy Levels of Their Molecular Orbitals.

The effect of different substituents on the fluorescence properties of [5]helicene derivatives was investigated in terms of molecular orbital symmetry. Unsubstituted [5]helicene is nonemissive due to the symmetry-forbidden S1 → S0 transition. However, the fluorescence emission rate constant (kf ) of [5]helicenes is efficiently increased by removing the orbital degeneracy involved in the S1 → S0 transition. As a result, we achieved a [5]helicene derivative exhibiting a high fluorescence quantum yield (Φf = 0.23) and short emission lifetime (⟨τf ⟩ = 1.5 ns), which is in marked contrast to unsubstituted [5]helicene (Φf = 0.04 and ⟨τf ⟩ = 26 ns).