Discotic liquid crystals of transition metal complexes 40: unique double-clearing behavior of a series of novel discotic metallomesogens based on bis(phthalocyaninato)europium(III) complexes

Abstract

It has long been believed that the phase transition behavior and clearing points (c.p.s) of phthalocyanine-based metallomesogens scarcely change regardless of the kind of central metal. In this work, we have synthesized a series of novel sandwich-type, europium complexes, bis[2,3,9,10,16,17,23,24-octakis(3,4-dialkoxyphenoxy)phthalocyaninato]europium(III) (abbreviated as {[( C n O )2 PhO ]8 Pc } 2 Eu (n = 8-16) (2a–2i)). Their mesomorphic properties were investigated by polarization microscope, differential scanning calorimeter and temperature dependent X-ray diffraction techniques to make a comparison with the phase transition behavior and c.p.s of the previously reported lutetium homologs ({[( C n O )2 PhO ]8 Pc }2 Lu (n = 8-16) (1). Very interestingly, the c.p.s of {[( C n O )2 PhO ]8 Pc }2 Eu complexes (2) are higher by 35–61°C than those of {[( C n O )2 PhO ]8 Pc }2 Lu complexes (1) having the same chain length (n). Furthermore, these Eu complexes (2) exhibited unique double-clearing behavior, whereas the Lu complexes (1) did not. For instance, the Eu complex (2i: n = 16) exhibited the following phase transition sequence: crystalline phase (K) 49°C Colh 133°C [Formula: see text] 158°C [Formula: see text] 216°C I.L.; the I.L. very slowly transformed into a Coltet mesophase; on further heating, Coltet cleared into I.L. again at 240°C. On the other hand, the corresponding Lu complex (1i: n = 16) exhibited a phase transition sequence of K 50°C Colh 124°C [Formula: see text] 148°C Coltet 205°C I.L., and did not exhibit the doubleclearing behavior. Thus, it was revealed for the first time, in the phthalocyanine-based metallomesogens, that the central metal had a large influence on their c.p.s and their phase transition behavior.