Circular Dichroism of (Di)methyl- and Diaza[6]helicenes. A Combined Theoretical and Experimental Study

Circular dichroism (CD) and relevant chiroptical properties of (di)­methyl- and diaza[6]­helicenes were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations, results of which were compared with the corresponding experimental data obtained for newly synthesized enantiopure helicenes. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level accurately reproduced the experimental CD spectra in both excitation energy and rotational strength. The electric and magnetic transition dipole moment vectors for the helical sense-responsive ¹B_b and the substitution-sensitive ¹L_b bands were compared with those for parent carbo[6]­helicene, from which the effects of methyl and nitrogen introduced at different positions upon the experimental CD spectra were discussed to separately evaluate the electronic and steric consequences of the substitution to the chiroptical properties. The electronic effects of substitution on CD spectra were further investigated theoretically by employing a series of 3,3-disubstituted [6]­helicenes. This first systematic investigation allows us not only to accurately reproduce the experimental CD spectra of known substituted helicenes but also to directly envisage the chiroptical properties of unknown helicenes

Theoretical and Experimental Studies on Circular Dichroism of Carbo[<i>n</i>]helicenes

The chiroptical properties of a series of carbo­[<i>n</i>]­helicenes (<i>n</i> = 4–10) were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level nicely reproduced the experimental CD spectra in both excitation energy and rotational strength without any shift or scaling. These calculations afforded the electric and the magnetic transition dipole moment vectors in [<i>n</i>]­helicenes, allowing us to discuss the observed rotational strengths as a function of the number of benzene rings. Although the observed CD intensity was not immediately correlated to any of the calculated parameters, the anisotropy (<i>g</i>) factor of the ¹B_b band and the specific rotation were found inversely proportional to <i>n</i> and nicely correlated with the helical pitch, but discontinuous at <i>n</i> = 6, where the aromatic rings start to overlap. In contrast, the <i>g</i> factor at the ¹B_a band was rather insensitive to <i>n</i>. It was also revealed that the excitation energies of the ¹B_b and ¹B_a bands are inversely proportional to <i>n</i> over the entire range of <i>n</i> examined. The theoretical predictions also enabled us to rectify the erroneous experimental CD spectra of [5]- and [6]­helicenes reported earlier, by using the enantiopure samples resolved by chiral HPLC

Theoretical and Experimental Studies of Circular Dichroism of Mono- and Diazonia[6]helicenes

Combined experimental and theoretical studies revealed the characteristic circular dichroism (CD) spectral profiles of mono- and diazonia[6]­helicenes, which were distinctly different from those reported for parent [6]­helicene and neutral (di)­aza-analogues. Aza[6]­helicenes and [6]­helicene showed bisignate Cotton effects (CEs) at the ¹B_a and ¹B_b bands, along with a weak CE at the ¹L_b band, where the signs of the former bands are responsible for the helical chirality of the helicenes while the sign of the latter is susceptive to the various factors such as electronic and steric effects. Protonation to monoaza[6]­helicenes produces azonia[6]­helicenes, showing dramatic changes in the CE pattern from the two bisignate to a three positive, two negative CE extremum series of comparable magnitudes, while dual protonation to diaza[6]­helicenes forming diazonia[6]­helicenes led to only nominal changes (slightly different rotational strength and excitation energy) in the CE pattern. Such rather complicated and contrasting CE behaviors of mono- versus diazoniahelicenes are derived mostly from the electronic effects of (unsymmetrical) protonation because the structures of neutral, mono-, and dicationic species are essentially identical to each other. Compared with those of neutral (di)­aza[6]­helicenes, the experimental CD spectra of (di)­azonia[6]­helicenes were less satisfactorily reproduced by the theoretical calculations at the state-of-the-art RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level, most probably due to the inadequate incorporation of the effects of solvation. Nevertheless, the bytheoretical predictions were reasonably accurate and highly valuable in assigning the observed CE and elucidating the origin of the elaborate CD spectral behaviors upon protonation through inspection of the molecular orbital configuration of each transition, encouraging the extended use of the present protocol for analyzing the CD spectral behavior of aza- and other heteroatom-incorporated helicenes upon protonation. The CD spectral behavior upon metal ligation will also be explained through further theoretical and experimental studies

Inherently Chiral Azonia[6]helicene-Modified β‑Cyclodextrin: Synthesis, Characterization, and Chirality Sensing of Underivatized Amino Acids in Water

The (<i>P</i>)- and (<i>M</i>)-3-azonia­[6]­helicenyl β-cyclodextrins exhibit l/d selectivities of up to 12.4 and <i>P</i>/<i>M</i> preferences of up to 28.2 upon complexation with underivatized proteinogenic amino acids in aqueous solution at pH 7.3