This manuscript reports on an unusual self-assembly of small adenine-based molecules leading to complex, functional systems. Molecules feature an adenine nucleobase substituted at the N9 position with a triarylamine unit through a flexible spacer. Hydrogen bonding interactions prompt the formation of unprecedented adenine hexameric rosettes, which organize in dimers and then into helical columnar assemblies exhibiting hexagonal columnar liquid crystal phases, even with nonchiral molecules. Theoretical calculations including geometry optimization and prediction of vibrational modes have provided essential insight into the configuration of hydrogen bonds between adenine units to form stable hexads, and experimental and simulated X-ray diffraction (XRD) patterns are consistent with the unique helical self-assembly. Furthermore, molecular design including chirality in the flexible spacer and triarylamine electron-donor units steers these nanostructured materials toward functionalities related to the control of supramolecular chirality and semiconductivity. This is confirmed by thin film circular dichroism measurements for chirality and the space charge-limited current method for hole transport.

We present a liquid-crystal laser device based on the chiral ferroelectric nematic phase (NF*). The laser medium is obtained by mixing a ferroelectric nematic material with a chiral agent and a small proportion of a fluorescent dye. Notably, in the NF* phase very low electric fields perpendicular to the helical axis are able to reorient the molecules, giving rise to a periodic structure whose director profile is not single harmonic but contains the contribution of various Fourier components. This feature induces the appearance of several photonic bandgaps whose spectral ranges depend on the field, which can be exploited to build tunable laser devices. Here we report the characterization of home-made NF* lasers that can be tunable under low electric fields and present laser action in two of the photonic bands of the material. The obtained results open a promising route for the design of new and more versatile liquid-crystal based lasers.

This work reports new soft photothermal materials based on mesomorphic nickel bis(dithiolene) complexes bearing pentakis(dodecyloxy)triphenylene units, in which the triphenylene core and the metal complex are linked through alkyl connectors.

Organic semiconductors with well-defined architectures pose a suitable alternative to amorphous silicon-based inorganic semiconductors. Encouraged by the development of organic semiconductors based on columnar liquid crystals, herein, we report on a family of C-3-symmetric star-shaped mesogens based on triphenylamine (TPA), a functional unit with strong electron donor character. Highly stable columnar phases with high hole mobility values were obtained out of this nonplanar functional unit, and this was achieved by using flexible amide spacers to join the TPA units to a tris(triazolyl)triazine (T) star-shaped core, allowing the formation of intermolecular hydrogen bonds. The presence of hydrogen bonds results in a stabilization of the columnar architectures either in bulk or in the presence of solvents by reinforcing pi-stacking and van der Waals interactions, as deduced by Fourier-transform infrared (FTIR) and X-ray diffraction (XRD) studies. Furthermore, the introduction of a stereogenic center in the flexible spacer prompts the formation of chiral aggregates in the liquid crystal state and in the organogel formed in 1-octanol, as demonstrated by circular dichroism spectroscopy.

Antiaromatic expanded porphyrins designed to self-assemble into columnar liquid crystalline (LC) structures are synthesized and characterized by multiple techniques. The substituents were found to play a crucial role in modulating the LC behaviour.

The synthesis, supramolecular self-assembly and structural characterization of a new family of tetraethylene oxide (TEG)-based bent-core compounds and their 1/1 lithium-containing complexes are reported. TEG-based bent-core amphiphiles, even joining the TEG/Li+ tandem, are suitable building-blocks to achieve supramolecular nanostructures, in some cases showing chiral features from achiral molecules either in the mesophase or in solvents. The thermal and liquid crystal behavior of these materials studied by polarizing optical microscopy and X-ray diffraction confirmed that Li-based materials stabilized polar smectic C and helical nanofilament-type mesophases, in contrast to the non-liquid crystalline pure TEG-compounds. Alternatively, both the pure amphiphiles and the lithium-doped materials self-assemble into physical gels in non-polar solvents, displaying three-dimensional networks composed of long fibers with lamellar molecular organizations as shown by transmission electron microscopy and X-ray diffraction. Interestingly, biphenyl- and azobenzene-based bent core amphiphiles aggregate in solvent into chiral nanostructured morphologies with supramolecular trends comparable to their molecular arrangement in their liquid crystalline phases.

We have developed a numerical method for calculating the second-harmonic generation (SHG) generated by an anisotropic material whose optical properties present an arbitrary modulation in one dimension. The method is based on the Berreman 4 × 4 matrix formalism, which is generalized to include nonlinear optical phenomena. It can be used under oblique incidences of the input beam, and is valid even when the SHG frequency is close to photonic bands, where the usual slowly-varying-amplitude approximation breaks down. As an example of application, we have studied the SHG performance of ferroelectric and helielectric fluids. The obtained results indicate that the present procedure may contribute to improving the structural design and enlarging the variety of nonlinear optical materials for application in optical devices.

Modulation of columnar segregation inmetal-organictriphenylene liquid crystals is accomplished by blending two structurallydissimilar metallomesogens able to self-associate through complementaryelectron donor-acceptor interactions. This work reports an uncommon modulation of columnarsegregationof metal-organic triphenylene liquid crystals by blending twostructurally dissimilar metallomesogens that can self-associate throughcomplementary electron donor-acceptor interactions. The constituentmolecules are cis-[PtCl2(CNR)(2)] (CNR = 2-(6-(4-isocyanophenoxy)hexyloxy)-3,6,7,10,11-pentakisdodecyloxytriphenylene)that displays an organic/inorganic segregated columnar mesophase and[PtCl2(Bipy)] (Bipy = didodecyl 2,2 & PRIME;-bipyridyl-4,4 & PRIME;-dicarboxylate)that shows a lamellar mesomorphism. The phase diagram of this systemwas constructed using polarized optical microscopy (POM), differentialscanning calorimetry (DSC), and X-ray scattering data. The phase diagramcorresponds to a typical binary system with an intermediate compound(in this case a supramolecular aggregate) of stoichiometry [PtCl2(CNR)(2)]/2[PtCl2(Bipy)], which is maintainedin solution. This species shows an unusual columnar mesophase formedby the stacking of alternating organic/inorganic fragments. Quantumchemical calculations show that the columnar structure is mainly supportedby complementary & pi; electron donor-acceptor interactionsbetween each triphenylene group of the isocyanide complex and a platinum-bipyridinemolecule. This induces the elimination of the organic/inorganic columnarsegregation of the isocyano parent component and constitutes an unconventionalexample of modulation of organic/inorganic segregation in columnarmesophases by the intercalation of metal complexes into hexaalkoxytriphenylenestacks.

This work reports the synthesis and characterization of a new family of star-shaped tricarboxamides with C3-symmetry that have flexible amide spacers linking a tris(triazolyl)triazine core with three trialkoxyphenyl groups. The presence of amide groups allows the formation of intermolecular hydrogen bonds that reinforce π-stacking and van der Waals interactions, promoting liquid crystalline behavior, and self-assembly in solvents leading to organogels. As determined by polarized optical microscopy, differential scanning calorimetry and X-ray diffraction on powder samples, all the three reported molecules present a hexagonal columnar (Colh) phase stable at room temperature. Interestingly, they show a transition to a cubic micellar mesophase (BCC) at high temperatures. A mechanism for this transition, which is consistent with the fragmentation of columns to form supramolecular spheres, was elucidated from X-ray studies on aligned samples. Moreover, on cooling from the BCC to the columnar phase a preferential orientation of columns occurs, according to which each cubic domain gives rise to four hexagonal domains. The ability of the synthesized structures to aggregate in solvent media was studied in a variety of organic solvents, and all of them were able to gel 1-octanol at low concentrations. X-ray studies of gels and xerogels were carried out and showed a molecular organization consistent with Colh order. FTIR studies were carried out to analyze the formation of hydrogen bonds and the influence of the length of the flexible spacer in the liquid crystalline state and in the organogels. Furthermore, the presence of a stereogenic center in the flexible amide spacer leads to macroscopic chirality in the liquid crystal state and the organogels in 1-octanol as demonstrated by circular dichroism spectroscopy.

ABSTRACT We present a generalisation of the Maier-Saupe (MS) theory that incorporates the description of the ferroelectric nematic phase (NF) within its scope of application. The extension of the theory is carried out in a natural way, by adding to the nematic potential of MS a term proportional to cosθ and to the polar order parameter , where θ is the angle between the long molecular axis and the spontaneous polarisation. The NF phase can be reached by cooling from the isotropic phase with or without the formation of an intermediate normal nematic phase, depending on the relative intensity of the polar and non-polar terms of the nematic potential. For both phase sequences all the transitions are first order. The temperature dependence of the polar and non-polar order parameters is calculated, and some results derived from the theory are compared with the experimental observations. It is argued that the theory is a valuable first approximation to describe some of the properties of ferroelectric nematics. GRAPHICAL ABSTRACT

We have carried out a spectroscopic study on a prototype ferroelectric nematic material (RM734) doped with a non-polar chiral compound. The mixture presents two chiral nematic phases, one conventional (N*) and the other polar (N-F*), whose behaviours under electric fields E are totally different. On the one hand, the N* phase shows a single reflection band if E is not very high, while fields perpendicular to the helix of a few V/mm are already sufficient to produce multiple bands in the N-F* phase. On the other hand, the pitch of the N-F* phase grows notably on increasing fields, however remaining practically constant in the N* phase. Both effects have been explained in terms of the different type of interaction with E in each phase (ferroelectric in N-F* and dielectric in N*). We argue that the N-F* phase behaves as a photonic material with multiple gaps tunable by small fields, which presents important potentials for applications. A study of the bandgaps has been carried out through the analysis of the dispersion relation of the optical eigenmodes in the N-F* phase under field. Finally, an example of the potentials of the N-F* phase within the field of nonlinear optics is briefly presented.

We present an experimental study of an electrically tunable laser based on liquid-crystal elements and analyse theoretically its main working principles. The laser structure includes two polymer-stabilised cholesteric-liquid-crystal slabs that act as polarisation-sensitive reflectors, and a dye-doped nematic layer sandwiched between them. The wavelength of lasing can be changed by modifying the optical retardation of the nematic layer by means of a low-amplitude square wave. For all wavelengths the laser light is circularly polarised with opposite handedness to that of the cholesteric mirrors. Good laser performance has been achieved, with low thresholds and quasi-continuous tuning ranges. The laser threshold varies with the wavelength, and was found to be highly dependent on the spatial location of the indium-tin-oxide (ITO) electrodes, inside or outside the laser resonator. A theoretical account for this effect is given using simulations based on the Berreman 4 x 4 matrix method.

Materials that exhibit high nonlinear optical (NLO) susceptibilities are considered as promising candidates for a wide range of photonic and electronic applications. Here, we argue that the ferroelectric nematic (N-F) materials have sufficient potentialities to become materials for the next-generation of NLO devices. We have carried out a study of the efficiency of optical second-harmonic generation in a prototype N-F material, finding a nonlinear susceptibility of 5.6 pm.V-1 in the transparent regime, one of the highest ever reported in ferroelectric liquid crystals. Given the fact that the studied molecule was not specifically designed for NLO applications, we conclude there is still margin to obtain N-F materials with enhanced properties that should allow their practical use.

Chiral gold(i) isocyanide complexes [Au(p-C6F4OR1)(p-CNC6H4(O2CC6H4OR2)] with R1, R2 or both being (R)-2-octyl, display an interesting and unprecedented response to mechanical or thermal stress.

The spot shapes of cholesteric liquid crystal lasers have been studied experimental and theoretically. A broad variety of profiles has been found depending on the oscillator architecture, thickness, and lasing conditions. Simple cells, lasing at the short-wavelength edge of the gap, give rise to simple spots, whereas if the laser wavelength is at the long-wavelength edge the spots are surrounded by rings. On the other hand, complex cells, which incorporate passive cholesteric mirrors connected with glass substrates, produce spots with many sharp concentric rings. In all cases we have demonstrated that the different intensity profiles can be explained as due to anomalous light scattering of the main laser propagating along the helical axis towards oblique directions. For simple cells, the beam divergence is essentially determined also by scattering, showing a decreasing tendency with the sample thickness. Complex cells have a much smaller beam divergence, and its value is limited by diffraction.

This paper reports the synthesis, liquid-crystal behavior, and charge-transport properties in the mesophase of triphenylene Schiff bases and their copper(II), nickel(II), and oxovanadium(IV) complexes. The thermal and electronic properties of the Schiff bases are modulated by coordination to the corresponding metal moieties, which have the ability to self-assemble into linear structures and help the alignment of the triphenylene columns. This produces two kinds of electronically nonconnected columnar regions, one purely organic and one more inorganic. The most remarkable effect is a striking charge mobility enhancement in the metal-containing mesophases, due to the contribution of the more inorganic columns: in comparison to values of hole mobility along the columnar stacking for the purely organic columnar mesophases, on the order of 10-7 cm2 V-1 s-1, these values jump to 1-10 cm2 V-1 s-1 in these hybrid inorganic/organic columnar materials.

We have carried out polarisation and angle-resolved measurements of the light scattered from photonic cholesteric liquid crystals. Both in samples doped with laser dyes and in inactive (non-doped) samples we have observed pronounced directional dependences of the scattered light, finding angular ranges where the scattering is greatly enhanced and regions where the effect is almost suppressed. Moreover, the total amount of scattered light has also been found to depend strongly on the polarisation and direction of the incident beam. All the results have been interpreted successfully in terms of a simple expression proposed for the scattering cross section, in which the density of states of the ingoing and outgoing beams plays a major role. The expression would be applicable not only to cholesteric liquid crystals but to any one-dimensional photonic material.

We study the optical properties of a cholesteric liquid crystal doped with a fluorescent dye in the regime of highly distorted helix without full helix unwinding. The distortion was achieved by applying a pulsed AC electric field, perpendicular to the helix axis. If the pulse is in the millisecond range, the helix is deformed but keeps its original pitch even for electric fields higher than the theoretical critical field for helix unwinding. In this field regime, very pronounced high-order photonic band gaps are observed, in agreement with our calculations. We theoretically explore the possibility of obtaining viable laser emission at the second-order photonic band gap, and experimentally find that lasing is not only possible but has a figure of merit similar to that of the usual laser at the main-gap region. Therefore, electric-field-induced high-order photonic band gaps are potentially useful for multiline laser applications. [GRAPHICS]

The topic of cholesteric-liquid-crystal lasers is a rapidly expanding research area in the field of soft-matter photonics. The increasing interest in this field is due to the high versatility that these lasers may possibly present and the prospects of giving rise to new miniaturized devices. However, further improvements in their operation capabilities are still required for potential applications. In this paper, we critically analyze the main strategies proposed up to now to optimize their performance. We show theoretically and experimentally that possible innovations in the device structure cannot produce lasers with threshold energies below a certain limit. This limit is determined by the light scattering and absorption losses inside the liquid crystal. Even assuming the case of samples free of defects and perfectly non-absorbing, an intrinsic light scattering, typical of mesogens, still remains. Numerical estimates of the thresholds indicate that these lasers could hardly be driven by compact light sources such as current electroluminescent or light-emitting diodes. Since the improvement possibilities regarding cell architecture seem to be exhausted, the advance must come from the use of new dye molecules. These molecules should show enhanced emission cross-sections and be efficiently integrable within the mesogenic solvent. In addition, the fluorescent systems must present very small quantum yields to triplet states if continuous-wave lasing is sought. In this respect, quantum dots are an alternative to explore for further investigations.

The out-of-normal emission in cholesteric liquid-crystal lasers is studied experimental and theoretically. Apart from the well-known dominant laser in the direction of the helix axis, three other types of radiation, with cone-shaped spatial patterns, are identified and characterized. The physical mechanisms responsible for the different emissions are clarified. One of the radiations is a weak colorful lasing emission whose wavelength changes continuously depending on the propagation direction. The wavelengths of the other two radiations take place at the long wavelength and short-wavelength edges of the photonic bandgap. These emissions are attributed to an anomalous scattering phenomenon that gives rise to energy transfer from the main laser beam to some specific directions where the amount of final photonic states is high. An expression for the scattering cross section, reminiscent of Fermi's golden rule for spontaneous emission in photonic structures, is proposed. Some other phenomena independent of the lasing occurrence but driven by the anomalous scattering are briefly presented.