Mechanochromic Luminescence and Liquid Crystallinity of Molecular Copper Clusters

Abstract

Molecular copper iodide clusters with the [Cu 4 I 4 ] cubane core have been functionalized by phosphine ligands carrying protomesogenic gallate-based derivatives bearing either long alkyl chains (C8, C12, and C16) or cyanobiphenyl (CBP) fragments. The mesomorphic properties of the functionalized clusters were studied by combining differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS) experiments. Whereas clusters functionalized solely with long alkyl chains present amorphous or crystalline states, the cluster carrying CBP fragments displays liquid crystal properties with the formation of a smectic A mesophase from room temperature up to 100 °C. Temperaturedependent photoluminescence measurements show that the CBP derivative displays an unusual luminescence thermochromism which is possibly due to a resonance energy transfer mechanism between the emissive [Cu 4 I 4 ] inorganic and CBP moieties. The emission properties of this original cluster are also sensitive to variation of local order of the molecular assembly. Moreover, the liquid crystalline properties imported on the inorganic core allow for a facile deformation of its local environment, leading to mechanochromic properties related to modulation of intramolecular interactions. Indeed, mechanical constraints on the molecularly self-assembled structure induce changes at the molecular level by modification of the [Cu 4 I 4 ] inorganic cluster core geometry and in particular of the strength of the cuprophilic interactions.

Ambipolar Discotic Liquid Crystals Built Around Platinum Diimine‐Dithiolene Cores

“…In the mesomorphic domains, the SAXS patterns display several sharp peaks in the small‐angle region that can tentatively be indexed as the (11), (02), (22), (31), and (24) reflections of a rectangular lattice corresponding to the plane group C2mm ( h + k =2 n ) ( a =42.8 Å, b =42.5 Å at 30 °C) (Figure ). Two broad peaks are also observed in the wide‐angle region at 5.6 and 4.4 and are attributed to the mean distance between the CBP fragments and the carbon chains, respectively . A last broad peak is also observed at 3.4 Å and is attributed to the stacking distance between the disk molecules.…”

“…The absorption spectrum of the bpyCBP ligand 3 b also displays an absorption band centered at 298 nm but the band is much sharper and the associated extinction coefficient is much higher compared with 3 a (Figure a and S1). This absorption band is assigned to π–π* transitions localized on the bipyridine core but also to π–π* transitions localized on the cyanobiphenyl fragments . Coordination of the PtCl 2 fragment leads to a new absorption band at 412 nm for 4 a and 429 nm for 4 b (Figure S2 and 2a) with weak extinction coefficients.…”

Functional Liquid Crystals towards the Next Generation of Materials

“…These order-order and order-disorder transitions are important for the functionalization of liquid crystals. [4,6,71] Forthermotropic liquid crystals, transitions are mainly triggered by increasing or decreasing the temperature.I na ddition to such thermal stimuli, photo [72][73][74][75][76] and mechanical stimulation, [6,[77][78][79][80][81][82][83][84][85][86] and electric fields [87][88][89][90][91][92][93][94][95][96] have been found to induce phase transitions.…”

“…[4,9,20,67,68] Mechano-stimuli can also trigger phase transitions of LC material. [77][78][79][80][81][82][83][84][85][86] Recently,m echanoinduced LC phase transitions have been shown to be useful for the development of new chromic materials ( Figure 3a). [77] Kato and Sagara reported mechanochromic nanostructured liquid crystals that change luminescent colors by as hearinduced micellar cubic-columnar phase transition.…”

“…[83] Application of mechanical stimuli converts the assembled structures of this amphiphilic dipolar molecule from metastable aggregates into more stable structures,w hich induces luminescent color changes.T hey also demonstrated that molecule 11 can be applied for invisible inkjet imaging.N ew metallomesogens composed of copper iodide clusters and phosphine ligands bearing three cyanobiphenyl mesogens show mechanochromic luminescence as ar esult of the modulation of intramolecular interactions. [84] Mechanochromic properties were also induced for LC polymers. [85,86] An LC block copolymer composed of polythiophene and polyphenylisocyanide derivatives exhibits vapor-induced liquid crystallinity and selfrecover mechanochromism under ambient conditions.…”