Tuning Transition Electric and Magnetic Dipole Moments: [7]Helicenes Showing Intense Circularly Polarized Luminescence.

Helicenes are promising candidates for chiral optoelectronic materials because of their helically twisted π-conjugated system. However, the emission intensity of unsubstituted helicenes is very weak (Φf < 0.05) due to a small oscillator strength for the S1 → S0 transition. In this work, we investigated the substitution position of the [7]helicene framework so that the S1 → S0 transition has a large transition magnetic dipole moment (TMDM) and is partially symmetry-allowed. A [7]helicene derivative thus designed showed a large fluorescence emission rate (kf = 0.02 ns-1) and a large TMDM for the S1 → S0 transition (|m| = 2.37 × 10-20 erg·Gauss-1), which are more than 10 times greater than those of unsubstituted [7]helicene (kf = 0.001 ns-1, |m| = 0.045 × 10-20 erg·Gauss-1). As a result, we achieved the [7]helicene derivative whose dissymmetry factor of CPL and fluorescence quantum yield were both high (|gCPL| = 1.3 × 10-2, Φf = 0.17) in the solution phase.

Circularly Polarized Luminescence Designed from Molecular Orbitals: A Figure-Eight-Shaped [5]Helicene Dimer with D2 Symmetry.

Although many chiral molecules exhibiting circularly polarized luminescence (CPL) have been reported recently, few molecular design guidelines are known for increasing the dissymmetry factor for CPL (glum). We demonstrate that a figure-eight-shaped molecule with D2 symmetry has excellent chiroptical properties (Φf = 0.08, |glum| = 1.5 × 10-2), which are attributed to the parallel arrangement of µ and m as well as a good |µ|/|m| ratio.

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).