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. 2024 Mar 11;63(11):e202319318.
doi: 10.1002/anie.202319318. Epub 2024 Jan 29.

Helitwistacenes-Combining Lateral and Longitudinal Helicity Results in Solvent-Induced Inversion of Circularly Polarized Light

Israa Shioukhi  1 , Harikrishna Batchu  1 , Gal Schwartz  2 , Louis Minion  3 , Yinon Deree  1 , Benny Bogoslavsky  1 , Linda J W Shimon  4 , Jessica Wade  3   5 , Roy Hoffman  1 , Matthew J Fuchter  3 , Gil Markovich  2 , Ori Gidron  1
Affiliations

Helitwistacenes-Combining Lateral and Longitudinal Helicity Results in Solvent-Induced Inversion of Circularly Polarized Light

Israa Shioukhi et al. Angew Chem Int Ed Engl. .

Abstract

Helicity is expressed differently in ortho- and para-fused acenes-helicenes and twistacenes, respectively. While the extent of helicity is constant in helicenes, it can be tuned in twistacenes, and the handedness of flexible twistacenes is often determined by more rigid helicenes. Here, we combine helicenes with rigid twistacenes consisting of a tunable degree of twisting, forming helitwistacenes. While the X-ray structures reveal that the connection does not affect the helicity of each moiety, their electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra are strongly affected by the helicity of the twistacene unit, resulting in solvent-induced sign inversion. ROESY NMR and TD-DFT calculations support this observation, which is explained by differences in the relative orientation of the helicene and twistacene moieties.

Keywords: Chirality; Circular Dichroism; Circularly Polarized Luminescence; Helicenes; Twistacenes.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Top: Schematic representation of [6]helicene and twistacene, showing the C2 axes (orange) and stereogenic axes (red). Bottom: Combining helicenes and tethered twistacene with alkyne spacers.
Scheme 1
Scheme 1
Top: Synthetic pathway for the preparation of helicene‐twistacenes (TnH) and helicene‐twistacene‐helicenes (HTnH). Et3N, triethylamine; PPh3, triphenylphosphine; THF, tetrahydrofuran. Middle and bottom: calculated (B3LYP/6‐31G(d)) structures of T4H (MM; 13 %, PP; 26, PM; 13 %), HT4H (MMM; 13 %, PPP; 14 %, MPM; 17 %), T8H (MM; 13 %, PP; 16 %) and HT8H (MMM; 11 %, PPP; 17 %, MPM; 15 %). Maroon/red: P handedness, blue/green: M handedness.
Figure 2
Figure 2
X‐ray structures. (a) side view and top view of MPM‐HT4H. (b) Side view of MP‐T4H. Hydrogen atoms are omitted for clarity. (c) Packing of MPM‐HT4H (spacefill).
Figure 3
Figure 3
(a) Normalized UV/Vis absorption spectra of twistacene (T4 , blue line), helicene (H, green line) and PPP‐HT4H (red line) and the calculated spectrum (pink). (b) The natural transition orbitals involved with labelled transitions (yellow→blue represents the difference in electron density for S0→Sn). (c) ECD spectra of helitwistacenes. All experimental spectra were measured in tetrahydrofuran.
Figure 4
Figure 4
(a) ECD of PPP‐HT4H and MMM‐HT4H and CPL of PPP‐HT4H in tetrahydrofuran (red), acetonitrile (blue) and toluene (green). The smoothed CPL spectrum is displayed in bold and the original data are displayed by thin lines. (b) The maximal intensity of the S0→S1 transition vs. solvent polarity (dielectric constant) of MMM‐HT4H at 493 nm.
Figure 5
Figure 5
(a) Calculated (B3LYP‐GD3/6‐31G(d)) torsional energy of PPP‐HT4H (blue) and PPP‐HT8H (red). Calculated (TD‐DFT/CAM‐B3LYP/6‐31G(d)) ECD for PPP‐HT4H (b) and PPP‐HT8H (c) at their local and global minima. Magnetic and electric transition dipole moment (MTDM and ETDM) vectors are represented by purple and orange lines, respectively, for each conformer with their relative angles.
Figure 6
Figure 6
ROESY NMR spectra recorded in CD3NO2:C6D6 9 : 1 (blue trace) and C6D6 (red trace) of PPP‐HT4H.
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