On the parity in helical twisting power of Ru(III) 1,3-diketonates of C2 symmetry in nematic liquid crystals

It is demonstrated that the sign of helical twisting power (HTP) of an enantiomeric Ru(III) complex of type [Ru(acac)(2)L] can be switched by choosing L from either L-per or L-para, which is elongated either perpendicular or parallel to the C-2 symmetry axis, and four states become available in combination with Delta Lambda-chirality of the metal center. Complexes 1-n, in which 4,4'-dialkoxylated dibenzoylmethanate ligands are used as L-per, and 2 having L-para = 3-(4'-decyloxyphenyl)pentane-2,4-dionate ligand were prepared for this purpose. They were optically resolved into the enantiomers by means of a clay column chromatography, and their performance as chiral dopants was evaluated in nematic liquid crystals including a room-temperature system, N-methoxybenzylidene-4-n-butylaniline (MBBA), which allowed facile measurements of the helical pitch lengths and CID spectra in the induced chiral nematic states. The induced CID signals have provided a clear evidence for the helical inversion between the two structure types, 1 and 2, of the same chirality. The twisting power of these six-coordinate metal complexes and their structure versus twist sense correlations are interpreted by the shape model. Intrinsically high HTP of Delta-[Ru(acac)(2)L-per] has also allowed for observation of the pitch band due to the selective reflection in the visible wavelength range at the doping level of 2 mol % in MBBA.