On the Formation of the Thermotropic Cubic Phase:  Insights from Monoacetylide Complexes of Pt(II)

Abstract

In the liquid crystal phase behavior of surfactants, cubic phases are common and can, in principle, be found between any pair of adjacent phases. 1 In a binary water/surfactant phase diagram, the formation of phases as a function of water content depends on interactions between the polar headgroups and the water solvent, and also on the size of the polar headgroup versus the volume occupied by the hydrophobic chains. In an organized assembly of these surfactant molecules, one can imagine that a surface between the two incompatible parts, that is the hydrophilic and hydrophobic parts, is generated and that this interface possesses a variable curvature, with the degree of curvature depending on the above parameters. Thus, the formation of a particular phase depends strongly on the idea of interfacial curvature, that is the curvature at the hydrophilic/hydrophobic interface.In thermotropic liquid crystals, there exists a family of mesogens where interfacial curvature is also crucial in determining the phase behavior. 2 These are the polycatenar mesogens 3 which are composed of an extended, rigid core and three or more terminal chains and here the interface in question is that between the aromatic core and the terminal alkyl chains. Of these mesogens, perhaps the most interesting are those with four terminal chains with two found at each end in the 3,4-positions of phenyls rings. Here, at short chain lengths, nematic and smectic C phases are formed, while at longer chains lengths, columnar phases are observed. The lamellar (S C ) phase forms because in order to accommodate the greater cross-section of the chains relative to the core, the core tilts. However, as the chains grow longer, they occupy a much greater volume compared to the core which induces curvature in the lamellar phase, causing the lamellae to break down and forming a columnar phase composed of aggregates of molecules. 4 In certain cases, cubic phases are observed at intermediate chain length 5 and hence at intermediate curvature; the building block here is also clearly related to aggregate formation.The mesomorphism of calamitic (rodlike) mesogens is typically characterized by the formation of nematic and a whole family of smectic phases and interfacial curvature does not normally play a role in explanations of the observed behavior as the interface is planar. For example, smectic phase formation can be accounted for by the microphase separation between the central, more rigid part of the molecules and the flexible terminal chains. Cubic phases are seen here, too, and the first examples were reported by Gray in the late 1950s. 6 Although much more common than they were, cubic phases still remain rather rare and the driving force for their formation is not well understood in the case of calamitic mesogens. 7 Some time ago, we reported on the observation of cubic phases in some formally ionic, calamitic complexes of silver(I) ( Figure 1). 8 The phase behavior of these materials is noteworthy because they show nematic, 9 S A and S C phases in addition t...

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“…18) which required the use of three equivalents of acetylene and rather short reaction times (sometimes only ten minutes). 32 The mesomorphism of these new complexes was established by optical microscopy and, perhaps unsurprisingly, we found that the bis(acetylide) complexes showed a nematic phase as we had found with the alkanoate complexes, although curiously, we were also able to observe a SmC phase, too. However, we were most keen to find out about the behaviour of the monoacetylide complexes, for now we had direct analogues of the silver complexes except that these platinum complexes have a single chain but no ionic component to the metal-ligand interaction.…”

“…Thus, while we cannot attribute cubic phase formation solely to intermolecular electrostatic interactions, we can identify these interactions as crucial. 32 Now, is this explanation specific to these complexes, or does it find resonance in the structures of other calamitic mesogens which form cubic phases? The answer, we believe, is that the explanation is not strictly specific, rather that it hints at another factor.…”

Molecular Engineering of Mesomorphic Fluorene-Bridged Triphenylene Triads: Thermotropic Nematic/Columnar Mesophases, and p-Type Semiconducting Behavior

“…After the pioneering work on hexakis­(phenyl­ethynyl)­aryl derivatives with either a benzene, naphthalene, or triphenylene central ring, − more recent works include some hexakis­(4-ethylene oxide benzoate)­triphenylenes mixed with alkali salts, phenylene ethynylene macrocycles, , phenyl­ethynyl­benzene derivatives, , as well as truxenes, thiatruxenes, and oxatruxenes . LC systems with structures deviating slightly from a conventional disc, such as triphenylene twins, − nonuniformly alkyl-chains substituted phthalo­cyanines and di­ben­zo­tetra­aza­[14]­annulenes, cyclo­[6]­aramides, and shape-persistent aromatic oligoamides, soft spacer-connected triphenylene-pentaalkynylbenzene dyads, as well as some polycatenar mesogens, with unsymmetrical substitution or lateral chains − have also been reported to exhibit a nematic mesomorphism. Very recently, new materials for molecular electronics have shown high performances in conjunction with the appearance of nematic properties.…”

Control over the Self‐Assembly Modes of PtII Complexes by Alkyl Chain Variation: From Slipped to Parallel π‐Stacks

“…31 Among them, the self-assembly features of hydrophobic Pt(II) complexes with acetylide, [32][33][34][35][36] bidentate, [37][38][39] tridentate N-donor, [40][41][42] and cyclometalating ligands [43][44][45] are relatively well understood. 31 In contrast, their hydrophobic counterparts featuring non-chelating ligands and, in particular, bis(pyridyl) Pt(II) complexes have been limited to liquid crystalline materials [46][47][48] and hydrogen-bonded metallogelators. 49,50 Thus, there is a need for a deeper understanding of this class of Pt(II) complexes, in which not only the nature of the metal ion but also a wide range of weak unconventional forces stemming from the ligands attached to the metal fragment 51,52 anticipate the discovery of new supramolecular synthons.…”