Gianluigi Albano

Stereochemical analysis of β-keto sulfoxides by circular dichroism

Three β-keto sulfoxides (1-3) were synthesized in enantiopure form and investigated by means of circular dichroism (CD) spectroscopy, both in electronic and vibrational range (ECD, VCD), in combination with quantum chemical calculations. For compound 2, the X-ray structure was available; thus, the ECD in the solid state was also considered to reveal the differences between the molecular species in both states. Despite the simplicity of all β-keto sulfoxides under investigation (29 atoms), reproducing even the major spectral VCD features failed for two compounds, making the use of VCD not ideal to assign their absolute configuration in a reliable way. We demonstrated, however, that the use of ECD spectroscopy, both in solution and solid state, can easily, unambiguously, and without any complication simulate all bands by applying the standard protocol for calculations. This study may stimulate the debate on the need of the use of two chiroptical methods simultaneously in the determination of absolute configurations.

Chiroptical response inversion upon sample flipping in thin films of a chiral benzo[1,2-b:4,5-b′]dithiophene-based oligothiophene

A new chiral benzo[1,2-b:4,5-b′]dithiophene-based oligothiophene has been synthesized and spectroscopically characterized in solution and in the solid state. The electronic circular dichroism (ECD) spectra of the thin films revealed the occurrence of an uncommon phenomenon, consisting of the inversion of the circular dichroism sign by sample flipping. To the best of our knowledge, this is unprecedented in samples where no preferential orientation of the chromophores has been induced, and must be due to spontaneous local anisotropies. It reveals peculiar features upon the usual solvent annealing procedure, but finally results in stable and very strong signals, which are perfectly reproduced in sign, and relative and even absolute magnitude. Although the recorded signal cannot be considered as a true ECD spectrum, it corresponds to a very large discrimination of light circular polarization, which is well explained by means of the Mueller matrix analysis and does not depend on instrumental defects. It is a genuine property of this material, which correlates with polarized optical microscopy (POM) images and only depends on the self-assembling properties of this molecule. This phenomenon may have relevant implications, on account of its strength and of the interest of oligothiophenes in current optoelectronic devices.