In this review, we discuss the rich chemistry of helicenes and helicenoids containing maingroup elements. Enantioenriched helicenic derivatives containing main-group elements B, Si, N and P, either incorporated within the helical backbone or grafted to it, will be thoroughly presented. We will describe their synthesis, resolution, and asymmetric synthesis, their structural features, electronic and chiroptical properties, emission, together with other photochemical properties and applications. OUTLINE 14 The absorption spectrum of 19c recorded in CH 2 Cl 2 solutions exhibited a maximum at 376 nm which is significantly red shifted compared with that of double [5]helicene 19a (310 nm) due to the more extended π-conjugation in 19c. According to TD-DFT calculations, the low-energy absorption band from 400 to 500 nm is assignable to the HOMO→LUMO transition corresponding to a π−π* transition of the fully conjugated bis-helical system. DFT calculations revealed a very high theoretical isomerization barrier of 45.1 kcal/mol for the (P,P)/(M,M)-19c stereoisomer, i.e. 4.4 kcal/mol higher than the (P,M) stereoisomer (not observed experimentally). The high configurational stability of 19c enabled to obtain the pure (P,P)and (M,M)-19c by chiral HPLC (Daicel Chiralcel IE column; EtOAc/MeOH = 9:1 as eluent) and to study their chiroptical properties. The ECD spectrum of (P,P)-19c in CH 2 Cl 2 displayed two strong positive bands at 275 and 320 nm, a strong negative one at 375 nm and a broad and less intense one between 425-475 nm. 52 Note that the isomerization barrier of the OBO-fused double [7]helicene is much higher than for oxabora[6]helicene 9 (vide supra) and higher than carbo[7]helicene (42.0 kcal/ mol), indicating the advantage of double helicenes over monohelicenes in terms of conformational stability. Indeed no racemization was observed upon heating enantiomer (P,P) and (M,M)-19c for 24 h up to 200 °C. 52 Note that in 2017 a longer double OBO-helicene analogue was deposited on a Au(111) under UHV conditions and its surface-assisted cyclodehydrogenation into a planar OBO-perihexacene was observed by STM. 54 Similarly to 6 and 9, the strong bond alternation found in the BOC4 rings of 19a 51 may account for the small NICS value of 1.3 while the surrounding C6 rings, including the distorted central benzene ring, show large negative NICS values (Scheme 3c). For comparison, in the all-carbon analogue tetrabenzo-[a,f,j,o]perylene shown in Scheme 3c, the central C6 ring shows a positive NICS value, indicating substantial antiaromatic character. Notably, molecular orbital calculations of 19a indicate that the HOMO and LUMO are spread over the C6 rings rather than on the boron and oxygen atoms, accounting for the substantial stability of 19a (Scheme 3c). Borahelicenes are efficient blue fluorophores. Indeed, azabora[6]helicene 6 displays blue fluorescence at 447 nm (Table 2), while smaller achiral [4]helicenic systems were successfully used as host materials in phosphorescent OLEDs with efficiencies better than the classical 4,4'...
