Vibrational Raman optical activity of π‐conjugated helical systems: Hexahelicene and heterohelicenes

Abstract

Helicenes constitute a special class of molecules combining helical conformation with pi-electron delocalization. These confer to helicenes specific chirooptical properties. In this article, we investigate the vibrational signatures thanks to the simulation of vibrational Raman optical activity (VROA) spectra. For that, four representative helicenes: hexahelicene, tetrathia-[7]-helicene, and its pyrrole and furan analogs have been simulated and interpreted using a recently implemented analytical scheme. Helicenes show intense VROA peaks attributed to their pi-conjugated structure and associated with collective vibrational modes. In hexahelicene, the dominant VROA features are due to vibrational modes involving motions of the carbon skeleton and H-wagging, but the intensity finds its source almost exclusively in the former. In the case of the three heterohelicenes, the previous statement is also verified, and on changing the heteroatoms, similar modes presenting comparable atomic contribution patterns have been highlighted, though the vibrational and electronic properties are modified. Some fingerprints could therefore be associated with the helicity of the system. In particular, in forward spectra, most of the VROA bands are positive for left-handed helicenes. Nevertheless, the spectral patterns are quite complex, and no easy rule-of-thumb could distinguish between the different heterohelicenes. Then, considering the fact that most of the contributions originate from the C atoms (group coupling matrices decomposition), it can be concluded that the major role of the heteroatom is restricted to modifying the geometry and the normal modes. At last, the small impact of the gauge-origin on the calculated spectra using a relatively modest basis set (rDPS:3-21G) is demonstrated here in the case of the tetrathia-[7]-helicene molecule presenting a C(2) symmetry. This further demonstrates the adequacy of this basis set for VROA calculations.

Understanding the Signatures of Secondary‐Structure Elements in Proteins with Raman Optical Activity Spectroscopy

“…[44] For small molecules, this information can be used to gain insight into the generation of ROA signals. [45][46][47][48][49] Although all this information is in principle directly available from the calculation for larger molecules as well as its interpretation is more difficult. The vibrational spectra of polypeptides and proteins consist of many bands with a large number of close-lying normal modes contributing to each band.…”

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

“…[3] An interesting class of organometallic complexes are aryl-alkynyl-iron systems which display facile electron transfer [4] enabling the redox triggering of their optical properties (for example,n onlinear optical (NLO) activity, [4c,e,5a-c] luminescence, [5d,e] absorption, and color [5f] ). [7] We demonstrate for the first time how the chiroptical properties at wavelengths typical of vibration and suitable for telecommunications applications, by means of vibrational circular dichroism (VCD), [8] resonance Raman optical activity (RROA), [9] as well as the molar rotation (MR) and electronic circular dichroism (ECD) at 1.54 mm, are influenced upon oxidation to form radical cationic species. [7] We demonstrate for the first time how the chiroptical properties at wavelengths typical of vibration and suitable for telecommunications applications, by means of vibrational circular dichroism (VCD), [8] resonance Raman optical activity (RROA), [9] as well as the molar rotation (MR) and electronic circular dichroism (ECD) at 1.54 mm, are influenced upon oxidation to form radical cationic species.…”

A Vibrational Raman Optical Activity Study of 1,1′‐Binaphthyl Derivatives

“…[65][66][67][68][69] In particular, specific signatures of characteristic conformations, such as a helices and b sheets [36,37,48,49] and helical pitch, [65,[67][68][69] have been highlighted throughout all these studies. Therefore, the aim of this article is to search for VROA patterns associated with the angle between the two naphthalene rings and to see whether bands change sign on going from the cisoid to the transoid form, as observed for electronic CD and VCD spectroscopies.…”