Ruthenium‐Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching

Abstract

The properties of mono- and bis-Ru-vinyl[6]helicene complexes (2 a and 2 b, respectively), recently synthesized by using molecular engineering of helicenes based on the grafting of lateral organometallic substituents on the π-helical backbone through a vinyl bridge, are presented. These helicene derivatives are thoroughly characterized, with special attention given to their chiroptical properties and redox switching activity. The UV/Vis and electronic circular dichroism (ECD) spectra of P and M enantiopure species, both in the neutral and oxidized states ([2 a](·+), [2 b](·+), and [2 b](2+)), are analyzed with the aid of quantum-chemical calculations. The extended π-conjugation facilitated by the vinyl moiety, clearly visible in the electronic structures of 2 a,b, introduces new active bands in the ECD spectra that consequently lead to a significant increase in optical rotation of Ru-vinylhelicenes compared with the organic precursors. The vibrational circular dichroism (VCD) spectra were measured and calculated for both the organic and organometallic species and constitute the first examples of VCD for metal-based helicene derivatives. Finally, the redox-triggered chiroptical switching activity of 2 a,b is examined in detail by using ECD spectroscopy. The modifications of the ECD spectra in the UV/Vis and NIR region are well reproduced and rationalized by calculations.

Conformational changes and chiroptical switching of enantiopure bis-helicenic terpyridine upon Zn2+ binding

“…10 Indeed ( M , M )- and ( P , P )- 1 enantiomers demonstrated mirror-imaged CPL spectra (Figure S20) with opposite g lum values (( P , P )- 1 : +8.6 × 10 -3 and ( M , M )- 1 : -8.4 × 10 -3 ) around the emission maximum (∼ 420 nm). These values are slightly stronger than for related molecules such as helicene-bipyridine derivatives ( g lum ∼ ±10 -3 ) 5d or other CPL-active organic molecule. 11 …”

“…In this regard, several transition-metal complexes formed from helical π-conjugated ligands have recently been shown to display unprecedented chiroptical and electronic properties that are switchable by redox or acid/base stimuli. 5 While in those examples the helical topology and integrity of the chemical backbone was maintained upon switching, constructing a scaffold in which movements can take place through geometrical and conformational changes is particularly appealing. Such conformational changes in chiral systems may lead to molecular motors with effective unidirectional motion.…”

“…The emission properties are in line with previously described nitrogen-containing [6]helicene derivatives. 5d …”

“…Additionally, the Zn(II) complexation has a strong effect on the LUMO and LUMO+1 of 1 (both π*), leading to a concentration of their density around the zinc ion in the terpyridine part of the ligand (see SI for the MO analysis). 5d The HOMO and other high lying occupied π-orbitals remain helicene-centered and therefore the complexation strongly increases the character of charge-transfer from the helicene to the terpyridine part of the ligand in the low-energy part of the spectrum. 5d Note that no Zn orbitals were found to contribute to the π-conjugated system of the helicene ligand, as expected for closed-shell d 10 metal centers.…”

“…5d The HOMO and other high lying occupied π-orbitals remain helicene-centered and therefore the complexation strongly increases the character of charge-transfer from the helicene to the terpyridine part of the ligand in the low-energy part of the spectrum. 5d Note that no Zn orbitals were found to contribute to the π-conjugated system of the helicene ligand, as expected for closed-shell d 10 metal centers. The primary effect of the Zn moiety is therefore to activate the structural transformation of the aza[6]helicene fragments, rigidify the system, and induce charge-transfer excitations within the ligand.…”