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J Phys Chem A. 2013 Jun 20;117(24):5082-92. doi: 10.1021/jp403426w. Epub 2013 Jun 7.

Theoretical and experimental studies of circular dichroism of mono- and diazonia[6]helicenes.

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Department of Applied Chemistry, Graduate School of Engineering, Osaka University , 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.


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 (1)Ba and (1)Bb bands, along with a weak CE at the (1)Lb 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.

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