Columnar Metallomesogens Derived from 1,3,4-Oxadiazoles and X-ray Crystal Structure of Dichlorobis[2,5-bis(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole]palladium(II)

Abstract

Three series of palladium(II) complexes 1-3 derived from 1,3,4-oxadiazoles exhibiting mesogenic properties are prepared and characterized. It was found that the formation of mesophases was sensitive to the numbers of alkoxy side chains. The compounds 3 appended with twelve side chains exhibited columnar (Col) phases, however, all other compounds 1-3 appended with four or eight side chains formed crystalline phases. Some derivatives (n ) 8, 10, 12) of the series 3 were in fact mesogenic at room temperature. Powder XRD data indicated that the formation of columnar mesophase varied from rectangular (Col r ) to hexagonal arrangements (Col h ) was observed as carbon chains length increases. The crystal and molecular structure of palladium(II) complex of 2,5-bis(3,4,5-trimethoxy phenyl)-1,3,4oxadiazole (3; n ) 1) was determined by means of X-ray structural analysis. It crystallizes in the monoclinic space group p2 1 /n, with a ) 8.7736 (3) Å, b ) 19.1568 (6) Å, c ) 15.7834 (5) Å, β ) 100.106 (1)°, and Z ) 2. The geometry at palladium center is perfectly square planar, and two trans-chlorine atoms are almost perpendicularly located to the molecular plane determined by four 1,3,4-oxadiazole rings. The fluorescent properties of these compounds were also examined. All λ max peaks of the absorption and photoluminescence spectra of compounds 4 and 6 occurred at ca. 303-312 nm and 358-391 nm, respectively; whereas the quantum yields of some compounds were relatively low.

Synthesis and mesomorphic behaviour of hexacatenar di-hydrazine derivatives

“…As is reported, [35,36] T7-ben showed enantiotropic rectangular columnar mesophases, whereas T7-fuma and T7-mal with similar molecular shape could not form columnar mesophases, mainly because of the different central linking units. Compared with the T7-ben, the steric hindrance of T7-fuma with ethene group as central linking units decreased; thus the strength of intermolecular hydrogen bonding increased to result in the melting point increase (decomposition may be due to the ethene group unstable at high temperatures).…”

Thermotropic Liquid Crystals Based on Extended 2,5‐Disubstituted‐1,3,4‐Oxadiazoles: Structure–Property Relationships, Variable‐Temperature Powder X‐ray Diffraction, and Small‐Angle X‐ray Scattering Studies

“…For example, Lai et al. reported the liquid‐crystal properties of heterocyclic 2,5‐bis(3,4,5‐trialkoxyphenyl)‐1,3,4‐oxadiazoles11a (L) and the metallomesogens [L 2 PdCl 2 ] 11d. The formation of their columnar mesophases was strongly dependent on the size as well as the number of appended alkoxy substituents.…”

“…Compounds with the 1,3,4‐oxadiazole motif are generally chemically stable and have diverse luminescent properties, and are often emissive mesogens with good electron‐transport properties 9. Although the crystal structures of nonmesogenic 2,5‐disubstituted‐1,3,4‐oxadiazoles with different functionalities have been reported,10 structural information on mesogenic materials containing 2,5‐diaryl‐1,3,4‐oxadiazole is lacking, owing to difficulties in obtaining single crystals suitable for X‐ray diffraction studies 11a–d. To the best of our knowledge, related studies on extended 2,5‐diaryl‐1,3,4‐oxadiazoles are less‐explored 11e.…”

A new series of liquid‐crystalline bi‐1,3,4‐oxadiazole derivatives: synthesis and mesomorphic behaviour

“…1,3,4-oxadiazole derivatives, five-membered heterocyclic compounds constituting nitrogen and oxygen atoms, are one of the more widely investigated classes, both as additives and as mesogens (2). More recently, several discotic 1,3,4-oxadiazole mesomorphic compounds, showing columnar mesophases, in which the oxadiazole moiety was used as the discotic core have been reported (3)(4)(5)(6). For example, a star-shaped discotic compound containing 1,3,5-triethynylbezene core and oxadiazole-based rigid arms exhibited a discotic nematic phase (N Col ) (3).…”

“…For example, a star-shaped discotic compound containing 1,3,5-triethynylbezene core and oxadiazole-based rigid arms exhibited a discotic nematic phase (N Col ) (3). Lai et al reported that 2,5-bis(3,4,5-trialkoxy phenyl)-1,3,4-oxadiazoles exhibited hexagonal columnar phases (Col h ), while their complexes with metals, Col r and Col h phases (5,6). In our recent paper, we synthesised new types of polycartenar molecules by using bi-1,3,4-oxadiazole as the rigid core, namely 2,29-bis(3,4,5-trialkoxyphenyl)-bi-1,3,4-oxadiazole.…”