Romain Gautier (CR)
Noncentrosymmetric compounds are interesting for different properties such as second harmonic generation, piezoelectricity or optical rotation. In our group, we recently focus on racemates and the properties related to noncentrosymmetry. Racemates have been assumed to be optically inactive for the last 160 years. The cancelation of opposite optical rotations from left- and right-handed enantiomers (racemates) was discussed by Pasteur for tartaric acids and has never been dismissed. In our recent work, we showed, using single-crystal circular dichroism measurements, that specific arrangements of racemic units can lead to optical activity and that as many as one in twenty racemic compounds are potential optically active materials. Indeed, the enantiomers of opposite handedness can arrange into the 21 non-enantiomorphous point groups while optical activity can be described in the eleven enaniomorphous point-groups and four non-enantiomorphous point-groups (Figure). Thus, the “optical activity” and “Racemic compound” groups are not independent. The optical activity from racemic materials has been confirmed by the measurement of single-crystal circular dichroism on two compounds crystallizing in the point-group mm2 (Figure). In these crystal structures, the arrangement of these units is a key to breaking the centrosymmetry and leads to the presence of optical activity.
a) The 32 crystallographic point groups and optical activity, Centrosymmetric = Black, Polar/Non chiral = Red, Chiral/Non polar = Blue, Chiral/Polar = Purple, Noncentrosymmetric/Non chiral/ Non polar = Green). Circular dichroism spectrum of (b) [Zn(bpy)3](CrO4)0.5NO36.5H2O (Ccc2), and (c) [Cu(H2O)(bpy)2]2[HfF6]2.3H2O (Pna21).
Romain Gautier (CR)
“Optical activity from racemates”
R. Gautier*, J. M. Klingsporn, R.P. Van Duyne, K. R. Poeppelmeier*, Nature Materials, 15, 591-592 (2016)
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"Un racémique peut-il être optiquement actif?"
R. Gautier*, Actualités Chimiques, Accepted (2016)
- Northwestern University, USA
- Nara Institute of Science and Technology, Japan