From tectons to luminescent supramolecular ionic liquid crystals

International audienceNew phosphorescent and room-temperature liquid-crystalline materials were obtained by combining dicyanometallate anions with dicationic bisamidinium based tectons bearing four peripheral lipophilic pyrogallate moieties. Liquid crystals (LCs) constitute a class of soft materials, combining order with fluidity within various types of mobile and low-dimensional periodic structures. 1 Their dynamic nature, function-integration and stimuli-responsiveness abilities render them interesting for technological applications. 2 However, the control of self-assembling and self-organizing processes from functional nano-segregated structures to micrometric scale by molecular engineering still remains a challenge. Over the last two decades, the supramolecular approach, based on molecular recognition events, 3 has been considered 4 for generating novel LC assemblies integrating new functions. The majority of reported supramolecular LCs is mainly based on the use of non-ionic 5 or ionic 6 H-bonding. Functional mesophases displaying emissive properties are mostly achieved using either purely organic luminophores 7 or transition metal 8 and lanthanide 9 complexes. Hereafter, we report on new phosphorescent LC materials elaborated by molecular tectonics, an approach until now sparingly used in this area. Molecular tectonics, 10 a strategy allowing the assembly of molecular building blocks (tectons) into pre-designed 1-3D periodic architectures in crystalline phases or on surfaces, 11 seems particularly well-suited for the generation of LCs based on the formation of molecular networks composed of at least one mesogenic tecton. Indeed, using charge-assisted H-bonding, 12 it was shown that dicationic bisamidinium-based tectons combined with alkoxybenzoates lead to the formation of LC mesophases. 6c Moreover, association of such dicationic tectons with cyanometallate anions induces the formation of porous networks in the crystalline phase. 13 In particular, the combination of compound 1 2+ 14 (Scheme 1) with M(CN) 2 À (M = Ag or Au) leads to the formation of neutral 1D H-bonded networks (Fig. 1a). 15 The formation of the latter results from both electrostatic charge-charge interactions and directional H bonds between the acidic H atoms of 1 2+ and N atoms of CN ligands of M(CN) 2 À. Furthermore, analogues of 1 2+ bearing alkyl chains 2 2+ (propyl, 16 hexyl or dodecyl groups) were found to behave in a similar fashion, demonstrating that the presence of alkyl substituents at these two positions do not alter the recognition pattern observed between the dication and M(CN) 2 À (Fig. 1a). 17 In order to avoid crystallization and to induce the formation of mesophases, while maintaining the formation of 1D networks (Fig. 1b), the dicationic tecton 3 2+ bearing four peripheral lipophilic pyrogallate units (Scheme 1) was considered as an appropriate unit to be combined with M(CN) 2 À. As stated above, the design of 3 + is based on the tetra H-bond-donor-unit bisamidinium central core bearing four mesogenic 3,4,5-trisalkoxybenzyl groups offering 12 flexible chains disposed in a symmetrical fashion. The phenyl spacer connecting the two cyclic amidinium moieties was chosen because not only it increases considerably the stability of the tecton towards hydrolysis but in addition it imposes the required distances between the two acidic H atoms on each face of the tecton and thus a distance of ca. 4.0-4.2 Å between the two metal centers (Ag or Au) and furthermore enhances the axial anisotropy of the system. This rather short MM distance of ca. 4.0-4.2 Å leads to metallophilic interactions 18 between adjacent metallic centers (Ag or Au). Scheme 1 Fig. 1 Schematic representation of 1D H-bonded networks (ladder-like type) formed upon the combination of the dicationic organic tecton 1 2+ (a) or lipophilic 3 2+ (b) with dicyanometallates. w Electronic supplementary information (ESI) available: Complete synthesis of [3 2+ ][Cl À ] 2 and [3 2+ ][Ag(CN) 2 À ], DSC measurements and SA-XRD for [3 2+ ][Cl À ] 2. Se