The influence of heteroatoms on the circularly polarized luminescence performance of [7]helicene derivatives: aromatic vs. non-aromatic five-membered rings

Helicenes are promising candidates for circularly polarized luminescence (CPL) materials, although the performance is poor due to the unsatisfactory dissymmetric factor (g(lum)) and fluorescence quantum efficiency (Phi(F)). Herein, the influence of heteroatoms (C, Si, Ge, O, S and Se) on the electronic structures and chiroptical properties of [7]helicene derivatives is systematically investigated using density functional theory (DFT) and time-dependent DFT calculations combined with the thermal vibration correlation function theory. The results reveal that the non-radiative energy consumption processes for helicene systems are closely related to the variation of bond length upon electronic excitation. Moreover, by introducing five-membered rings and heteroatoms, the dipole-forbidden S-1 -> S-0 emission of [7]helicene changes to dipole-allowed transition due to the rearrangement of occupied orbitals and lifting of the nearly degenerate orbitals, resulting in an enhancement of Phi(F). As the heteroatomic radius increases, Phi(F) decreases while the g(lum) increases. Compared with the derivatives containing aromatic five-membered rings ([7]H-O, [7]H-S, and [7]H-Se), the non-aromatic counterparts ([7]H-C, [7]H-Si, and [7]H-Ge) exhibit a balance in Phi(F) and g(lum) values. The present study helps to clarify the relationship between structures and chiroptical properties and offers a feasible strategy for the future design of helicene-based CPL materials.