Induction and structural control of chiral nematic phases by the use of photoresponsive tris(β-diketonato) Co(III) and Ru(III) complexes

Two kinds of tris(beta-diketonato) metal(III) complexes denoted as [M(acac)(2)(LC12)] and [M(acac)(2)(L-azo)] (M = Ru(III) and Co(III); acac = acetylacetonato; LC12 = 1,3-didodecyloxyphenyl-1,3-propanedionato; L-azo = 3-[4'-(4 ''-(butoxy)phenylazo)phenyl]-pentane-2,4-dionato) were synthesized and optically resolved into A, A isomers. Here, LC12 and L-azo were designed to be elongated perpendicular to and in parallel with the molecular C-2 axis, respectively. The metal complexes were doped into three kinds of nematic liquid crystals (MBBA, EBBA, and ZLI-1132). Both dopants induced chiral nematic phases with relatively high helical twisting powers. With a purpose to clarify the mechanisms of helical induction, order parameters (S) were determined by means of polarized UV-vis measurements on nematic samples doped with racemic Co(III) complexes. As a result, the long axes of the elongated ligands in both [Co(acac)(2)(LC12)] and [Co(acac)(2)(trans-L-azo)] were found to align in the direction of the director vector with S = 0.50 +/- 0.05 and 0.60 +/- 0.05, respectively. Based on the results, the photoresponsive behavior of Delta- or Lambda-[Ru(acac)(2)(L-azo)] was interpreted in terms of the variation of S accompanied with the cis-trans isomerization of the azobenzene moiety in L-azo.