Luminescence liquid crystals

Self-assembly of Pt(II) complexes yielding luminescent liquid crystals (210,211,274) and AIE (275-279) have been already described, but metal complexes forming soft structures with such intense phosphorescence are very rare. 

By this approach, it was therefore possible to retain the intrinsic emission properties of PAMAM and PPI dendrimers and manufacturing liquid crystals luminescent in the visible region, without introducing conventional fluorophores [112], Incorporation of rigid fluorophores in a mesophase quite often alters the phase transition temperatures of liquid-crystal materials to higher values, so these results are interesting because they provide the possibility of preparing luminescent liquid crystals close to room temperature. 

An alternative approach to obtain luminescent liquid crystals is to use the host-guest concept a luminescent lanthanide complex is dissolved in a hquid-crystalline matrix. This approach allows one to optimize the luminescence and mesomorphic properties of the liquid-crystal mixture independently. Boyaval et al. investigated the luminescence of lanthanide complexes in a cholesteric hquid crystal mixture (Boyaval et al., 1999, 2001). Birmemans and Moors (2002) showed that a nematic liquid crystal can be an interesting host matrix to study the spectroscopic properties of luminescent lanthanide complexes. These authors doped the /3-diketonate complex [Eu(tta)3(phen)] into the nematic liquid crystals N-(4-methoxybenzyhdene)-4-butylaniline (MBBA) and 4-n-pentyl-4 -cyanobiphet5rl (5CB) and observed narrow-band red photoluminescence with a well-resolved crystal field sphttmg. Later on, these authors extended their studies to lanthanide j8-diketonate complexes emitting in the near-infrared (Ln = Nd, Er, Yb) (Van Deun et al., 2003). 

Besides this, the remarkable properties of gold(I) compounds, which often give rise to aurophilic interactions and/or to luminescence, are of interest when these properties are transported into the liquid crystal field. Although there is much still to be studied, it is already clear that luminescence can survive in the condensed but mobile state of a mesophase, and even in the isotropic liquid state of a molten gold compound. It also seems that aurophilicity can contribute in some cases to the formation of mesophases. 

Bayon, R., Coco, S. and Espinet, P. (2005) Gold Liquid Crystals Displaying Luminescence in the Mesophase and Short F... F Interactions in the Solid State. Chemistry - A European Journal, 11, 1079-1085. 

Binary molecular co-crystals of 2,5-bis(3-pyridyl)-l,3,4-oxadiazole and 2,5-bis-(4-pyridyl)-l,3,4-oxadiazole with benzene-1,3,5-tricarboxylic and benzene-1,2,4,5-tetracarboxylic acids were studied by X-ray and thermogravimetric analysis of mass loss <2005MI1247>. Dipole moments were used to study the flexoelectric effect in guest-host mixtures of 2,5-(4-pentylbenzene)-l,3,4-oxadiazole with commercial liquid crystal hosts <2005CM6354>. The luminescence properties of many other copolymers were also investigated (see Section 5.06.12.3). 

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

M O Neill and SM Kelly, Liquid crystals for charge transport, luminescence, and photonics, Adv. Mater., 15 1135-1146,2003. 

Cyclic trinuclear gold(I) complexes provide a novel and productive strategy for achieving supramolecular structures. While molecules of this type have been known for more than twenty years, some of their remarkable properties have only been recognized recently. Some can form liquid crystals at room temperature [41], while others lead to luminescent materials with surprising properties. We will now summarize some selected examples to illustrate the behavior of these trinuclear systems. 

The corresponding pyrazole mononuclear derivatives [AuCl(pzH)] have been prepared with the same ligands (Figure 7.41) [38]. Only the monosubstituted non-mesomorphic pyrazole leads to a liquid crystal, not the disubstituted mesomorphic ligand. The complex displays a SmA mesophase on cooling to 69.2 °C, although this is mixed with some crystallization. Moreover, this derivative is luminescent at 77 in the solid state. In addition, the corresponding trinuclear and mononuclear derivatives, namely [ Au(pz) 3] and [Au(Hpz)2]+, which could be expected to be liquid crystals are non-mesomorphic. 

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

---