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 DeltaLambda-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 CD spectra in the induced chiral nematic states. The induced CD 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.