Spatially resolved mapping of phase transitions in liquid-crystalline materials by X-ray birefringence imaging.

The X-ray Birefringence Imaging (XBI) technique, first reported in 2014, is a sensitive method for spatially resolved mapping of the local orientational properties of anisotropic materials. We report the first application of the XBI technique to characterize molecular orientational ordering in a liquid crystalline material, demonstrating significant potential for exploiting XBI measurements to advance structural understanding of liquid crystal phases.

Self-Assembling and Luminescent Properties of Chiral Bisoxadiazole Derivatives in Solution and Liquid-Crystalline Phases.

Herein, we report the synthesis, self-assembly, and electroluminescence characteristics of a new green-emitting, pseudodiscoid chiral molecule, OXDC, containing an electron-donating stilbene core and an electron-accepting oxadiazole substituent. The helical organization and specific interaction of the chiral pseudodiscoid molecule resulted in the formation of self-assembled nanofibers with a columnar superstructure. Macroscopic chirality was observed in both the liquid-crystalline phases and the self-assembled nanofibers of OXDC, a feature which was absent in the analogous achiral oxadiazole derivative reported earlier [ Sivadas , A. P. ; Supergelation via Purely Aromatic π-π Driven Self-Assembly of Pseudodiscotic Oxadiazole Mesogens . J. Am. Chem. Soc. 2014 , 136 , 5416 - 5423 ]. A high-performance organic light-emitting device was demonstrated using OXDC as the emitting material, with a luminous intensity of 10 115 cd m-2 at 5 V and chromaticity coordinates of (0.32, 0.51).

Mesomorphism and Photophysics of Some Metallomesogens Based on Hexasubstituted 2,2':6', 2''-Terpyridines.

The luminescent and mesomorphic properties of a series of metal complexes based on hexacatenar 2,2':6',2''-terpyridines are investigated using experimental methods and density functional theory (DFT). Two types of ligand are examined, namely 5,5''-di(3,4,5-trialkoxyphenyl)terpyridine with or without a fused cyclopentene ring on each pyridine and their complexes were prepared with the following transition metals: Zn(II) , Co(III) , Rh(III) , Ir(III) , Eu(III) and Dy(III) . The exact geometry of some of these complexes was determined by single X-ray diffraction. All complexes with long alkyl chains were found to be liquid crystalline, which property was induced on complexation. The liquid-crystalline behaviour of the complexes was studied by polarising optical microscopy and small-angle X-ray diffraction. Some of the transition metal complexes (for example, those with Zn(II) and Ir(III) ) are luminescent in solution, the solid state and the mesophase; their photophysical properties were studied both experimentally and using DFT methods (M06-2X and B3LYP).

Supergelation via purely aromatic π-π driven self-assembly of pseudodiscotic oxadiazole mesogens.

A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12) exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported supergelators, is driven purely by π-stacking interactions of the constituent molecules. The d-spacing ratios estimated from XRD analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the mesogens and the self-assembled nanofibers formed in the gelation process.

Trigonal 1,3,4-oxadiazole-based blue emitting liquid crystals and gels.

Star-shaped molecules consisting of a 1,3,4-oxadiazole core derivatized with alkoxy-substituted phenyl ethynylenes, FD12 (dodecyl) and FD16 (hexadecyl) were synthesized. These molecules exhibited enantiotropic columnar mesophases over a wide temperature range, with the liquid crystalline phases exhibiting strong blue fluorescence. On cooling, FD12 transformed into a transparent glass at room temperature wherein the liquid crystalline texture was retained. The glassy film remained stable over a period of one year and exhibited blue luminescence with an absolute quantum yield of 26%. The oxadiazole derivatives formed stable luminescent gels in decane and study of their morphology by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated formation of interlocked network of self-assembled fibers. X-ray diffraction (XRD) analysis of the xerogel of these derivatives indicated oblique columnar ordering of the molecules within the fibers. The length of the alkyl substituent was observed to have a significant effect on the absorption and fluorescence properties of the gels, which was attributable to the role of the alkyl substituents in controlling the nature of the molecular packing within the self -assembled fibers of the gels.

Fluorescent reporters of monoamine transporter distribution and function.

Serotonin is a monoamine serving as a chemical messenger in diverse brain regions, as well as in blood and various other organs. We synthesized six ethylamine functionalized fluorophores as fluorescent probes for serotonin. The one with best spectral properties and aqueous solubility, 6-amino-2-(2-aminoethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione, was studied in detail both in vivo and in vitro. It was shown to act as a ligand for serotonin transporter (SERT) without acute cerebral or cardiovascular toxicity or adverse effects. Fluorescent serotonin analogs can be used for direct visualization of SERT distribution and activity in live tissue.

Highly chromic, proton-responsive phenyl pyrimidones.

Aryl pyrimidones are pharmacologically relevant compounds whose optical properties have only been partially explored. We report the synthesis and optical characterization of a series of aryl- and diaryl-2(1H)-pyrimidones. The electronic transitions of these chromophores are modulated by the extent of conjugation between the pendant phenyl ring and the pyrimidone core as well as the presence of electron-donating auxochromes. Monoprotonation of the pyrimidone ring results in large hyperchromic and bathochromic shifts as well as switching of fluorescence making these phenyl pyrimidones of interest as sensory materials.

Luminescent charge-transfer complexes: tuning emission in binary fluorophore mixtures.

Charge-transfer (CT) complexes composed of a π-electron-poor naphthalene diimide (NDI) derivative combined with a series of π-electron-rich donors were investigated. Solutions of the CT complexes are nonemissive; however, solid-state complexes and aqueous suspensions display emission that is dependent on the energy of the HOMO of the electron donor. Crystallographic analysis of a pyrene-NDI complex reveals columnar packing and a high degree of frontier molecular orbital (FMO) overlap that likely contributes to the observed optical properties. The fluorescent CT particles are utilized as imaging agents; additional luminescent CT complexes may be realized by considering FMO energies and topologies.

Evidence of preferential pi-stacking: a study of intermolecular and intramolecular charge transfer complexes.

Nine combinations of pi-electron donors and acceptors were examined by UV-vis, fluorescence and (1)H-NMR spectroscopy to identify pi-stacked charge transfer complexes in macromolecular and supramolecular constructs. The high association constant of pyrene and naphthalene diimide suggests a preferentially pi-stacking pair rationalized by frontier orbital congruence.

Photophysical investigation of 3-substituted 4-alkyl and/or 7-acetoxy coumarin derivatives--a study of the effect of substituents on fluorescence.

In the present work an array of novel substituted 2H-chromen-2-one (coumarin) derivatives has been subjected to photophysical analysis. Though the influence of the electron-donating groups such as amino, substituted amino, hydroxyl, alkoxy groups, etc. at position 7 of the coumarin ring system has been extensively studied, the luminescent properties of the coumarin moieties with an acetoxy substituent have not been explored. Herein it is attempted to study the variation of fluorescence behavior of substituted coumarin derivatives with change of nature and position of the substituents on the 2H-chromen-2-one skeleton. Effect of a methyl substituent at position 4 which imposes abnormal photophysical behavior to the chromenone unit has also been briefly described.

Butadiene-based photoresponsive soft materials.

The creation of stimuli-responsive materials offers considerable challenges in the area of material science. The use of light as an external stimulus has particular advantages because it can bring about rapid transformations in remote regions in a very precise manner. Naturally occurring photoresponsive systems provide the motivation for developing corresponding artificial systems using molecular self-assembly to address issues such as quantum efficiency, selectivity, and amplification. A practical strategy for developing photoresponsive materials is to utilize molecules that can undergo considerable change in shape on photoisomerization. Although the photoisomerization of polyenes between their linear all-trans isomer and bent cis isomers has been extensively investigated in solution and in organized media because of its relevance to naturally occurring photoresponsive systems, its use in developing artificial photoresponsive systems has not been well explored. This feature article provides an overview of photoresponsive soft materials such as liquid crystals and gels with special emphasis on our recent studies related to the use of the butadiene chromophore for the design of such materials. The role of molecular self-assembly in controlling the photochemical and photophysical properties of these molecules is also discussed.