Helical Threads: Enantiomerically Pure Carbo[6]Helicene Oligomers

Abstract

We report the synthesis of enantiomerically pure carbo[6]helicene oligomers with buta-1,3-diyne-1,4-diyl bridges between the helicene nuclei. The synthesis of monomeric (±)-2,15-bis[(triisopropylsilyl)ethynyl]carbo[6]helicene was achieved in 25 % yield over six steps. Pure (+)-(P)- and (-)-(M)-enantiomers were obtained by HPLC on a chiral stationary phase. The dimeric (+)-(P) - and (-)-(M) -configured and the tetrameric (+)-(P) - and (-)-(M) -configured oligomers were obtained by sequential oxidative acetylenic coupling. The ECD spectra of the tetrameric oligomers displayed large Cotton effect intensities of Δϵ=-851 m cm at λ=370 nm ((M) -enantiomer). We transformed the buta-1,3-diyne-1,4-diyl bridge in the dimeric (P) and (M) oligomer by heteroaromatization into a thiene-2,5-diyl linker. Although the resulting chromophore showed reduced ECD intensities, it exhibited a remarkably strong fluorescence emission at 450-500 nm, with an absolute quantum yield of 25 %.

Helicene Monomers and Dimers: Chiral Chromophores Featuring Strong Circularly Polarized Luminescence

“…Dimer ( P ) 2 ‐ 3 is linked by pyrrole‐2,5‐diyl and capped with triisopropylsilyl (TIPS) groups. Dimers ( P ) 2 ‐ 1 and ( P ) 2 ‐ 2 , and monomers ( P )‐ 8 and ( P )‐ 9 (shown only in abbreviated form with ‐[6]‐ being the carbo[6]helicene with the substitution pattern shown in the dimers; for full structures, see the Supporting Information) were reported in our earlier communication …”

“…The UV/Vis and ECD spectra of enantiomerically pure ( P ) 2 ‐ and ( M ) 2 ‐ 2 – 7 are shown in Figure and in Section S4 of the Supporting Information, and the major absorption bands are reported in Table . We had previously noted a red‐shift in the highest‐wavelength absorbance of thiene‐2,5‐diyl linked ( P ) 2 ‐ 2 relative to buta‐1,3‐diyne‐1,4‐diyl linked ( P ) 2 ‐ 1 , but this was not observed for the pyrrole‐2,5‐diyl linked ( P ) 2 ‐ 3 (Figure top). Similarly, the UV/Vis spectra of the doubly arylalkynylated compounds, such as ( P ) 2 ‐ 7 (Figure , bottom), revealed only minor changes, differing only in spectral shapes (see also Figures S1 and S2 in the Supporting Information) from ( P ) 2 ‐ 2 , but showing similar maximal absorbance at ≈450 nm.…”

“…Recently, we reported a novel class of enantiomerically pure, buta‐1,3‐diyne‐1,4‐diyl linked carbo[6]helicene oligomers . Dimeric (+)‐( P ) 2 ‐ and (−)‐( M ) 2 ‐ 1 displayed moderate fluorescence quantum yields ( Φ f =0.11).…”

“…Dimeric (+)‐( P ) 2 ‐ and (−)‐( M ) 2 ‐ 1 displayed moderate fluorescence quantum yields ( Φ f =0.11). Starting from dimeric ( P ) 2 ‐ 1 , we obtained by heteroaromatization thiene‐2,5‐diyl linked (+)‐( P ) 2 ‐ 2 , which displayed an increased Φ f value of 0.25, one of the highest Φ f values reported to date for a carbohelicene. We proposed that the fluorescence of ( P ) 2 ‐ 2 originated from the push‐pull substitution on the helicene cores, with the thienyl ring as electron donor and the alkynes as acceptors and thus set out to further validate this hypothesis.…”

“…Similarly, the UV/Vis spectra of the doubly arylalkynylated compounds, such as ( P ) 2 ‐ 7 (Figure , bottom), revealed only minor changes, differing only in spectral shapes (see also Figures S1 and S2 in the Supporting Information) from ( P ) 2 ‐ 2 , but showing similar maximal absorbance at ≈450 nm. In general, the dimeric carbo[6]helicenes all feature very strong UV/Vis absorptions and ECD bands, non‐linearly enhanced with respect to the corresponding monomers ( P )‐ 8 or ( P )‐ 9 , a remarkable feat that is still difficult to achieve for helicenes reported to date …”