Iron Alkynyl Helicenes: Redox‐Triggered Chiroptical Tuning in the IR and Near‐IR Spectral Regions and Suitable for Telecommunications Applications

Abstract

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 mm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged.

Redox‐Active Chiroptical Switching in Mono‐ and Bis‐Iron Ethynylcarbo[6]helicenes Studied by Electronic and Vibrational Circular Dichroism and Resonance Raman Optical Activity

“…Helicene–ethynyl derivatives 2 a and 2 b , grafted with one or two electroactive Fe( κ 2 ‐dppe)( η 5 ‐C 5 Me 5 ) fragments, respectively (dppe: 1,2‐bis(diphenylphosphino)ethane, C 5 Me 5 : pentamethylcyclopentadienyl), were prepared from the organic precursors 2‐ethynyl‐carbo[6]helicene 1 a and 2,15‐diethynylcarbo[6]helicene 1 b by a recently described method (Scheme ) . Introducing one or two electroactive “Fe( κ 2 ‐dppe)( η 5 ‐C 5 Me 5 )” units onto a helicene–alkynyl core significantly modifies its (chir)optical and redox properties.…”

“…However, an equivalent impact on the vibrational optical activity, namely, vibrational circular dichroism (VCD) and Raman optical activity (ROA), has only recently been demonstrated. Not long ago, we reported the synthesis of new organometallic helicenes based on iron alkynylcarbo[6]helicenes that displayed unprecedented redox‐triggered chiroptical switching activity (Scheme ) . Upon oxidation, strong changes in vibrational local or extended coupled modes, were observed by VCD and ROA.…”

“… Organometallic mono‐ and bis‐iron ethynylcarbo[6]helicene derivatives 2 a , b obtained from mono‐ and bis‐ethynylcarbo[6]helicenes 1 a , b , and their redox‐switching processes. i ) Fe( κ 2 ‐dppe)( η 5 ‐C 5 Me 5 )Cl, MeOH, THF, NaPF 6 ; ii ) THF, t BuOK …”

“…Herein, we present a complete account of our experimental and theoretical studies, including a detailed analysis of ECD and VCD spectra, with an emphasis on monometallic system 2 a because it was barely discussed in our initial report . Furthermore, for the first time, the ROA spectrum of an organometallic helicene species, namely, 2 a , is calculated under resonance conditions, by using a recently developed method, and analyzed with respect to experiment.…”

Sulfoxide‐Induced Homochiral Folding of ortho‐Phenylene Ethynylenes (o‐OPEs) by Silver(I) Templating: Structure and Chiroptical Properties

“…The switching event could be followed by NMR spectroscopy, electronic circular dichroism (ECD), fluorescence, less common CPL spectroscopy, and by X‐ray crystallography. Remarkably, the switching process could also be observed in the vibrational wavelength range, by means of VCD …”