Vibronic coupling shapes circularly polarized luminescence and quantum yields of carbo [6]helicene and bora [6]helicene

Theoretical guidance for the development of high-performance fluorophores based on accurate predictions of spectral properties and fluorescence quantum yields would be highly beneficial for the purposeful design of new fluorophores. This work presents a theoretical investigation into bora[6]helicene and its designed carbo[6]helicene counterpart to understand the impact of substituting a carbon substituent for a boron substituent on optical properties and quantum efficiencies. The differences in electronic characteristics between bora[6]helicene and carbo[6]helicene, as well as the effects of carbon substitution on spectral properties, were analyzed. Here, we applied the time-dependent correlation function approaches to calculate the transition nature, circularly polarized luminescence (CPL), internal conversion (IC), and intersystem crossing (ISC) rates of these two molecules. The results indicate that bora[6]helicene and carbo[6]helicene exhibit entirely distinct electronic properties with different HOMO-LUMO gaps. Theoretical vibronic spectroscopy results further confirm that carbon substitution significantly affects chiroptical properties, including spectral shape, width, intensity, and wavelength. To further characterize the excited states, radiative and non-radiative rates were calculated to predict fluorescence quantum yields (QYs). And we analyze the effect of different broadening functions (Gaussian, Lorentzian, or Voigt) on QYs. The calculated Voigt broadening function QYs of bora[6]helicene is consistent with the experimental value, confirming the reliability of our model. Compared with the theoretically predicted carbo[6]helicene, the QYs of bora[6]helicene is significantly larger. This work proves that compared with a carbon atom, the [6]helicene doped with a boron atom significantly improves the fluorescence quantum yield.