Circularly polarized luminescence of Hetero[n]helicenes with 2,1,3-thiadiazole rings at both ends: Design of magnetically-allowed electronic transitions via heteroatom embedding

Design of excellent circularly polarized luminescence (CPL) emitters requires chiral molecules with large transition magnetic dipole moments (TMDM, |m|). In this work [5]-, [7]-, and [9]helicene derivatives with 2,1,3-thiadiazole rings at both ends, TD[5]H, TD[7]H, and TD[9]H, were designed and synthesized. We revealed that TD[9]H showed an excellent CPL with a large glum of +0.04 at 520 nm in toluene, which was significantly larger than that of unsubstituted carbo[7]helicene (glum = +0.008). Density functional theory (DFT) calculations suggested that the remarkable glum of TD[9]H was attributed to the magnetically allowed transition with a large TMDM (|m| = 2.3 × 10−20 erg G−1) for the S1→S0 transition.