Helicene-Helicenoid Hybrid Systems by Triptycene Annulation of Helical Coumarins: Improved Chiroptical Properties

The excited-state properties of helicenes-such as emission quantum yields and singlet lifetimes-generally decline as the number of ortho-fused aromatic rings increases, due to state-mixing and greater skeletal flexibility. Building on our finding that rigid, 3D triptycene units can significantly alter these properties, we designed and synthesized a series of tetra- and pentahelical coumarins annulated with triptycene in proximate and distal orientations (THC-prox, THC-dist, PHC-prox, and PHC-dist). These hybrid systems structurally integrate features of both helicenes and helicenoids. Comprehensive photophysical studies reveal that triptycene annulation enhances radiative decay in all systems and improves solid-state emission despite their loose molecular packing. The pentahelicene-helicenoid PHC-prox displays particularly remarkable behavior. Owing to its high enantiomerization barrier, PHC-prox can be resolved into configurationally stable isomers. It exhibits high fluorescence quantum yields in solution and solid states, prolonged singlet lifetime, and the largest Stokes shift among the molecules investigated. Its enantiopure forms show a gLum of 5.2 × 10^-3 (B_CPL ≈ 1.0), a value among the highest reported for [5]- and [6]-helicenes. These results establish helicene-helicenoid hybrids as a promising new class of chiroptical materials with exceptional photophysical and configurational properties.