Perylene diimide-based stable chiral radical anions featuring strong NIR absorptions

Abstract

Air-stable helical radical anions represent a class of promising multifunctional molecules due to their intriguing magnetic and chiroptical properties. Twisting of the PDI core engendered by steric repulsion between the encumbered bay -substituents leads to the occurrence of atrop-enantiomers ( P- and M- enantiomers). However, despite significant distortion in these bay -substituted and ortho -fused PDI systems, a fast interconversion process between the P- and M- enantiomers prevents the isolation of enantiopure derivatives for further applications. As the activation parameters for conformational chiral systems are influenced by the size of the substituents, we speculated that annulation of four indole/benzothiophene rings to perylene skeleton with heteroatom at ortho -position should lead to conformationally stable double- [7] heterohelicene 1a (X = NH) and 1b (X = S) with overlapping terminal aromatic rings. The rigidity of the π-skeleton can be further enhanced via the hydrogen bonds and chalcogen bonds. The incorporation of heteroatoms in helicenes would offer the possibility to tune their bandgap. Based on our recent work, 1a and 1b were synthesized readily, and enantiomers could be completely separated by using chiral HPLC. Their opto-physical properties were studied by absorption and CD spectroscopy. The introduction of four fused rings induced a further bathochromic shift to NIR. More importantly, 1a and 1b still shows high g abs factor even above a wavelength of 700 nm, which enables its application in advanced chiroptical devices. In addition, Ortho -π-extended PDI double-[7] hetero-helicenes 1a and 1b could be transformed to the corresponding radical anions with addition of fluoride anion in tetrahydrofuran solution. These novel radical anions exhibited broad UV-vis absorption spectra up to 1300 nm and clear CD bands in the NIR region. EPR experiments further confirmed the formation of paramagnetic PDI.−. Interesting, the radial anions could survive in the solution in air for hours. This work can provide a new platform for indepth study on anion-π interactions, and therefore can facilitate further investigation on their applications.