Luminescence liquid-crystalline phase

In summary, metallomesogens of crown ethers are interesting compounds that combine a variety of properties (1) appropriate metal centers can show luminescence, (2) all properties can be tuned by the addition of alkaline metal salts to the crown ether, and (3) ordered, liquid crystalline phases are possible. With these hybrid materials, interesting applications can be foreseen in the near future. 

Interestingly enough there is another sharp drop in luminescence intensity at the transition to the liquid-crystalline phase 215). This is shown in Fig. 45. In the latter phase there is an orientation of the phthalocyanine molecules that is more favorable for migration, so that more quenching centers are reached. The transition shows hysteris (Fig. 45) and coincides with thermodynamic measurements. Therefore the luminescence is used to probe the crystalline to liquid-crystalline transition. A further analysis yielded an estimation of the number of phthalocyanine molecules in the stack. 

Columnar phases could also be obtained by reacting disk-shaped molecules such as crown ethers with lanthanide salts. BiinzU and coworkers reported, for example, the preparation of thermotropic hexagonal columnar phases from nonmesogenic crown ethers and several different ions of the lanthanide family [76], An additional property of these compounds is a metal-centered emission, in the case of the liquid-crystalline phases containing Eu and Tb, making them attractive for the design of luminescent liquid-crystalline materials [76],... 

S. Abraham, V.A. Mallia, K.V. Ratheesh, N. Tamaoki, S. Das, Reversible thermal and photochemical switching of liquid crystalline phases and luminescence in diphenylbutadiene-based mesogenic dimers. J. Am. Chem. Soc. 128, 7692-7698 (2006)... 

Linearly polarized fluorescence (LPF) and circularly polarized fluorescence (CPF) provide complementary techniques to LCLD and LCICD for the assignment of solute electronic transition moments and for investigating solute orientational ordering and liquid crystalline properties [1,2, 304, 326-328]. Additionally, these techniques can provide information on the electronic structure of excited state complexes (excim-ers and exciplexes) [329, 330]. Time-re-solved luminescence depolarization experiments have been used by various workers to study the ordering and mobility of molecules in liquid crystalline phases [317-319, 331, 332] the information obtainable from these studies in analogous to that obtained by NMR relaxation experiments. Since luminescence depolarization is the main result of probe molecular motions and is consequently very rapid, it leads to complica-... 

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. 

Kozhevnikov et al. observed that the luminescence colour of vitrified mesophase of liquid-crystalline N,C,N-coordinated platinum(II) complexes (Figure 2.32) is different from that observed for a film of the same compound obtained by fast cooling from the isotropic phase. The samples that were fast cooled from the liquid crystal phase displayed monomer emission, whereas the samples fast cooled from the isotropic state showed excimer-like emission. Spin-coated thin films exhibited excimer-like emission, whereas heat treatment of the sample to 110 °C followed by cooling to room temperature resulted in a drastic change of the luminescence colour from the red of the excimer to the yellow of a mixture of the monomer and the excimer. However, rubbing of the heat treated film resulted in a return of the red excimer emission. 

Phase transitions in crystalline materials have also been followed by luminescence measurements. Examples are the transition crystalline —> liquid crystalline in octa-n-dodecoxy substituted phihalocyanine [15] where the luminescence disappears, and the para- to ferroelectric transition in Cr -doped Li2Ge70is [16], where the emission lines are split by the phase transition. 

---