Chimerical Pyrene‐Based [7]Helicenes as Twisted Polycondensed Aromatics

Abstract

Chimerical pyrene-based dibenzo[7]helicene rac-1 and 2H-pyran[7]helicene (M,R,R)-(-)-2, in which two pyrene subunits are fused to the [7]helicene/[7]heterohelicene scaffold, were synthesised by means of Ni(0) - or Co(I) -mediated [2+2+2] cycloisomerisation of dipyrenyl-acetylene-derived triynes. Pyrene-based dibenzo[7]helicene 1 was obtained in enantioenriched form by enantioselective cycloisomerisation under Ni(0) /QUINAP catalysis (57 % ee) or in enantiopure form by racemate resolution by liquid chromatography on a chiral column. 1,3-Allylic-type strain-controlled diastereoselective cycloisomerisation was employed in the synthesis of enantiopure (M,R,R)-(-)-2. Physicochemical properties of 1 and 2 encompassing the helicity assignment, stability to racemisation, X-ray crystal structure, UV/Vis, experimental/calculated electronic circular dichroism and fluorescence spectra were studied. Accordingly, comparison of the X-ray crystal structure of (M,R,R)-(-)-2 with calculated structures (DFT: B3LYP/cc-pVDZ, B97D/cc-pVDZ) indicated that its helical backbone is slightly over-flattened owing to intramolecular dispersion forces between tert-butylated pyrene subunits. Both 1 and 2 are fluorescent (with quantum yields in dichloromethane of ΦF =0.10 and 0.17, respectively) and are suggested to form intramolecular excimer states upon excitation, which are remarkably stabilised and exhibit large Stokes shifts (296 and 203 nm, respectively).

Synthetic Control of the Excited‐State Dynamics and Circularly Polarized Luminescence of Fluorescent “Push–Pull” Tetrathia[9]helicenes

“…Namely, the push–pull character was insufficient for increased Φ FL values. However, more interestingly, the Φ FL of Me 2 N‐QTTH is 0.39 (0.43 in benzene), which is one of the highest Φ FL values reported for helicene skeletons without disrupted π‐conjugation . The Φ FL value is approximately 20 times larger than that of TTH ( Φ FL =0.02).…”

Theoretical study on the electronic structure and second-order nonlinear optical properties of benzannulated or selenophene-annulated expanded helicenes

“…20 The pyrenebased dibenzo [7]helicene and 2H-pyran [7]helicene lateral derivatives with the uorescent quantum yields in dichloromethane of FF ¼ 0.10 and 0.17 have been reported. 21 They form remarkably stabilised intramolecular excimer states upon excitation and exhibit large Stokes shis respectively. Nuckolls et al 22 demonstrated that perylenediimide (PDI)-based pextended helicenes results in the collision of p-electron clouds between two PDI subunits, which enhance the delocalization of electrons.…”

π-Expanded Axially Chiral Biaryls and Their Emissions: Molecular Design, Syntheses, Optical Resolution, Absolute Configuration, and Circularly Polarized Luminescence of 1,1′-Bipyrene-2,2′-diols

“…7 The issues to be addressed were 1) the solubility improvement of 2 and 2) optical resolution. PAHs are notorious for their poor solubilities and our targets 2, dimers of pyrene, ought to be sparingly soluble materials.…”