About Supramolecular Assemblies of π-Conjugated Systems

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Covalent Organic Frameworks: Design, Synthesis, and Functions.

Functional π-Gelators and Their Applications

Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given (234 references).

Discotic liquid crystals: a new generation of organic semiconductors

A new series of C3 -symmetric, π-conjugated molecules was designed, synthesized and characterized. The materials were derived from electron-accepting s-triazine, appended covalently to electron-donating styrylbenzene arms, and were readily prepared in excellent yield with high purity by means of three-fold condensation of triphosphonate with n-alkoxybenzaldehydes under Horner-Wadsworth-Emmons reaction conditions. Examination of the phase transitional properties by several complementary techniques evidenced self-assembly into a hexagonal columnar phase, occurring over wide and reasonable thermal ranges. The photophysical properties were studied both in solution and in the fluid/frozen columnar states by UV/Vis absorption and photoluminescence spectroscopy. The emission spectra obtained as a function of the temperature rule out the breaking-up of larger columns and a non-radiative, thermally activated process. A study carried out on thin films of the glassy columnar state, which accounts for conserved fluorescence, defect-free orientation, and freezing ionic species, with the help of atomic force microscopy (AFM) images, suggested a homogeneous granular morphology comprising fibrillar structures. Dissimilarities in the surface morphology and birefringence of thin films of the solid and frozen columnar states were clearly shown by Raman spectroscopy. An electrochemical investigation revealed a LUMO energy of -4.0 eV. Thus, the discotic motifs presented herein meet certain criteria of organic materials, which are essential for developing electronic devices.

s‐Triazine‐Based Functional Discotic Liquid Crystals: Synthesis, Mesomorphism and Photoluminescence

We report the results of high-pressure studies on the photoinduced nematic-isotropic phase transition. It is observed that pressure has a significant influence on the shift in the transition temperature caused by uv radiation. We propose a possible mechanism to explain the observed results.

Effect of pressure on the photoinduced nematic-isotropic phase transition.

The article discusses the observation of a fluid phase having three-dimensional orientational order and its transformation to a similar phase but with an additional one-dimensional positional order in a low molar system. X-ray and optical observations establish the phases to be biaxial nematic and biaxial smectic A respectively. The organic system used is a unique combination of a bent core molecule linked to a rod-like unit through a flexible aliphatic spacer. We argue that this special combination is responsible for the observation of especially the biaxial nematic phase and propose possible molecular arrangements in the two phases.

Biaxial Nematic and Smectic A Phases in a “Peelable Banana-Shaped” Molecule

Based on the chemical incompatibility between the aromatic mesogenic cores and the flexible end aliphatic (methylene units) chains, which can lead to the formation of polarization modulated layered undulated phases, four-ring bent-core compounds have been designed and synthesized. The designed compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding, highly fluorescent and exhibit 2D-polarization modulated layer undulated (PMLU) smectic phases. The phase transition temperatures have been confirmed by differential scanning calorimetry and the phases are confirmed by polarizing optical microscopy and X-ray diffraction studies. For compounds 1-10-11 and 1-15-11 the peaks are indexed to tilted columnar lattice (Colob), while for 2-12-11 the peaks are indexed to the rectangular lattice belonging to the PMLU family The influence of the direction of ester linking group played an important role in the mesophase as well as the evaluated parameters of the dipole moment, the bending angle and the asymmetry parameter by spin-restricted DFT calculations using the B3LYP exchange-correlation functional and the 6-311G(d,p) basis set.

D. S. Shankar Rao

Papers

s‐Triazine‐Based Functional Discotic Liquid Crystals: Synthesis, Mesomorphism and Photoluminescence

Effect of pressure on the photoinduced nematic-isotropic phase transition.

Biaxial Nematic and Smectic A Phases in a “Peelable Banana-Shaped” Molecule

Fluorescent unsymmetrical four-ring bent-core mesogens: 2D modulated phases

Dielectric studies under high pressure on strongly polar liquid crystals exhibiting monolayer smectic A phase