Precise Synthesis of Donor–Acceptor [17]Helicenes Exhibiting Circularly Polarized Emission

Here we present a five-step synthesis of two circular polarized light emitting donor-acceptor (D-A) [17]helicenes. The concise yet high-yielding synthetic strategy was achieved by dedicated precursor design, with bond-order control and substitution effects, for photocyclization. The structures and chiroptical properties of D-A [17]helicenes were comprehensively characterized by NMR, HRMS, UV-vis, CD, fluorescence and CPL spectroscopies, as well as single-crystal X-ray diffraction. By introducing D-A system into helicene scaffold, we demonstrate a nearly 10-fold increase in fluorescence quantum yield compared to the unsubstituted [14]helicene, due to an enhanced fluorescence emission rate (k_f). The emission wavelength can be tuned by introducing different functional groups. D-A [17]helicenes emitting circularly polarized light with a dissymmetry factor (g_lum) of up to 1 × 10^-2. DFT calculations reveal that D-A functionalization increases the oscillator strengths (f) thereby increasing k_f. This highly efficient synthetic strategy not only facilitates the synthesis of the longest chiral helicene reported to date but also establishes design principles for developing chiral long helicenes with circularly polarized light emission.