In order to gain a deeper understanding of the impact of BN/CC replacement on the electronic properties of these helicenes, DFT and TD-DFT calculations were performed. Taking 1-Mes and H (H = a mesityl functionalised all carbon-[5]helicene) as examples, the lowest energy transition of 1-Mes is dominated by the HOMO→LUMO transition with the oscillator strength (f) being 0.3296 (Fig. ). Notably, the incorporation of the BN unit alters the symmetry of the frontier orbitals (relative to the CC analogue H) with a significantly different orbital distribution throughout the helical backbone (note there is no contribution to the frontier orbitals from the mesityl group for any of these helicenes). Analogous to previous reports on all carbon- [5]helicenes, for H, the S0 to S1 transitions contains two major as previously discussed for carbon [5]helicenes. The replacement of a CC unit with a BN unit breaks the C2 symmetric distribution of the HOMO and LUMO across the helical backbone, and results in unsymmetric key frontier orbitals. Consequently, the S0-S1 transition for 1-Mes has a higher oscillator strength, thus stronger absorptions for the lowest energy transition and improved ΦPL are observed for Attempts were made to separate the enantiomers of 6-Mes and 7-Mes using chiral HPLC. In our hands, while 6-Mes could be resolved, 7-Mes could not, despite utilising a wide range of solvent and column conditions. As expected, 6-Mes reveals significantly higher tolerance to racemisation than its shorter homologue 1-Mes. Thus, the optically pure BN- [6]helicene can be handled without any special precautions as no deterioration of enantiomeric excess could be observed either at room temperature or somewhat elevated temperatures. To get more insight into its configurational stability, the sample of (M)-6-Mes in 1,2-dichlorobenzene was heated at 130-150 °C for 2-64 h followed by HPLC analysis of enantiomeric ratios. A barely visible shoulder started to arise after heating the sample at 150 °C for 16 h. To produce ca. 10% of the second enantiomer, the sample had to be kept at this temperature for over 2.5 days. The P-M interconversion process at 140 °C was significantly slower. To compare the stability of 6-Mes and carbo [6]helicene, which possesses the inversion barrier of ca. 36 kcal mol -1 , the sample composition of carbo [6]helicene under given conditions was calculated based on the available thermodynamic parameters. This analysis indicates that this BN-isostere shows similar inversion barrier to that of all-carbon derivative.