Fluorescent liquid crystals with rod-shaped π-conjugated hydrocarbon core

Calamitic fluorophores are presented possessing either two phenylethylene or phenylacetylene fragments which are connected via an aromatic entity. The new compounds exhibit smectic C and/or nematic liquid crystalline phases. The chromophores show fluorescence in the blue spectral region. Grafting acrylate functional groups to the termini of naphthalene centred members of the rod-shaped dyes provides new luminescent reactive mesogens. Uniaxial orientation of mixed systems of the acrylate fluorophores with non-emissive reactive benzoate nematogens was achieved by surface-assisted alignment on polyimide orientation layers. The photocrosslinked oriented films display linear polarised photoluminescence upon isotropic excitation.     

Insulated π-conjugated metallopolymers

Recently, there has been a progressive development of insulated π-conjugated metallopolymers with accumulated features of π-conjugated bridging units, transition metal complexes, and encapsulating moieties, as higher-order functionalized materials. A number of insulated conjugated metallopolymers have been successfully synthesized and their fascinating properties have been reported. In addition to the conventional features derived from π-conjugation and transition metals, their insulated structures can compensate for solubility, a disadvantage in conventional metallopolymers, and enhance their functionalities, such as sensing, luminescence, and conduction. In this review, we summarize the synthetic methodologies, structural characteristics, and functionalities of one-dimensional insulated π-conjugated metallopolymers, while focusing on the effect of transition metals and insulation on their properties.     

Dimensionally oriented redox-active π-conjugated systems

Porphyrins bearing the redox-active phenylenediamine pendant groups are synthesized to afford dimensionally oriented π-conjugated systems. The structural and electronic characteristics depend on the atropisomers. In the fluorescence emission spectra, the emission from the porphyrin moiety is almost completely quenched. Zinc complexation of the αααα isomers gives the corresponding zinc porphyrins bearing four phenylenediamine strands. Treatment with a bidentate ligand, DABCO, leads to the sandwich dimer complex, in which the porphyrin moieties are surrounded by π-conjugated pendant groups. p-t-Butylcalix[4]arenes bearing four redox-active phenylenediamine pendant groups on the lower rim are synthesized and characterized both spectroscopically and electrochemically. The interconversion of the oxidation states of the pendant groups is demonstrated both chemically and electrochemically.     

Emissive ‘zinc(II)-salphen’ core: building block for columnar liquid crystals

A series of half-discoid [N₂O₂]-donor tetradentate alkoxy substituted salicylaldimine ligands, [N,N′-di- (4-n-alkoxysalicylidene)-4-Cl-l,2-diamino-benzene] (L; n?=?12, 14, 16 and 18) have been prepared. The reaction of the ‘salphen’-type Schiff base ligand with Zn(OAc)₂.4H₂O afforded a series of mononuclear zinc(II) complexes. The ligands and the corresponding zinc(II) complexes were characterised by elemental analysis, Fourier transform infrared, proton nuclear magnetic resonance and ultraviolet-visible spectroscopy. Although the ligands are non-mesogenic, columnar mesomorphism is induced upon complexation with the metal. The p2gg symmetry of the rectangular columnar phase is confirmed by variable temperature powder X-ray diffraction study. Two ‘half-disc’-shaped molecules with four alkoxy legs are presumed to self-assemble via a dimeric interaction filling the space. In contrast to the ligands, the zinc(II) complexes all exhibited moderately intense green emission at room temperature both in solution and in the solid state. Density functional theory calculation, carried out using a DMol3 program, revealed a distorted square planar geometry for the complexes. The mesomorphic and photoluminescence property of the zinc complexes are collated as a function of spacer substituent, as well as alkoxy carbon chain length.     

π-conjugated columnar polyacetylenes prepared with Rh complex catalysts

Highly stereospecific polymerization of monosubstituted acetylenes was carried out using the Rh complex, [Rh(norbornadiene)Cl]₂ catalysts. The resulting polyacetylenes were characterized in detail by ¹HNMR, ESR, laser Raman, diffuse reflective UV, and wide angle X-ray diffraction methods. The data showed that the Rh complex were the preferred catalyst to selectively yield the corresponding cis-transoid polymers even at room temperature when alcohol, triethylamine or water was used as the polymerization solvent. Additionally, the resulting cis polyacetylenes were found to have a helical form whose polymer is amorphous or composed of pseudohexagonal structures called π-conjugated columnar as self-assembly or super structure. Further compression of the amorphous cis polymers resulted in cis to trans isomerization at room temperature under vacuum, breaking rotationally the cis C=C bonds giving π-radicals called solitons as the origin of a polymer magnet. On the other hand, the π-conjugated columnar was also found to show an extremely longer wavelength absorption compared with that of the amorphous one, although the absorption maximum was shifted to a shorter wavelength when the columnar was destroyed by the compression. Therefore, the formation of the π-conjugated columnar can be considered as a new and quite useful control method concerning color of such conjugated polymers, i.e., a new concept concerning the color of conjugated polymers.     

Spectroscopic probes with changeable π-conjugated systems

Spectroscopic probes have been extensively investigated and used widely in many fields because of their powerful ability to improve analytical sensitivity, and to offer greater temporal and spatial resolution (in some cases a molecule event may be visualized by the naked eye). So far, different photophysical mechanisms, such as charge transfer, photo-induced electron transfer and fluorescent resonance energy transfer, have been employed to develop various spectroscopic probes with superior properties. However, these photophysical mechanisms depend on the energy levels of molecular orbitals, which are usually difficult to accurately determine. This would lead to the poor prediction of analytical performance of the designed probe. Instead, the change of π-conjugated systems induced by chemical reactions is often accompanied by a distinct alteration in spectroscopic signal, which is more predictable and is of high signal/background ratio. This mechanism can serve as an effective measure for developing excellent spectroscopic probes, but to our knowledge, has not been systematically summarized. In this feature article, we review the development of spectroscopic probes with changeable π-conjugated systems, which is catalogued according to the fluorochromes: fluorescein, rhodamine, spiropyran, squaraine, coumarin, cyanine, etc. Two main strategies for constructing these spectroscopic probes, including ring-closing reaction and nucleophilic addition reaction, are summarized, and the merits and limitations of the probes are discussed.     

Second-order dispersion interactions in π-conjugated polymers

We calculate the ground state and excited state second-order dispersion interactions between parallel π-conjugated polymers. The unperturbed eigenstates and energies are calculated from the Pariser-Parr-Pople model using CI-singles theory. Based on large-scale calculations using the molecular structure of trans-polyacetylene as a model system and by exploiting dimensional analysis, we find that: (1) For inter-chain separations, R, greater than a few lattice spacings, the ground-state dispersion interaction, ΔE(GS), satisfies, ΔE(GS)～L(2)/R(6) for L ? R and ΔE(GS)～L/R(5) for R ? L, where L is the chain length. The former is the London fluctuating dipole-dipole interaction while the latter is a fluctuating line dipole-line dipole interaction. (2) The excited state screening interaction exhibits a crossover from fluctuating monopole-line dipole interactions to either fluctuating dipole-dipole or fluctuating line dipole-line dipole interactions when R exceeds a threshold R(c), where R(c) is related to the root-mean-square separation of the electron-hole excitation. Specifically, the excited state screening interaction, ΔE(n), satisfies, ΔE(n) ～ L∕R(6) for R(c) < L ? R and ΔE(n) ～ L(0)∕R(5) for R(c) < R ? L. For R < R(c) < L, ΔE(n) ～ R(-ν), where ν ? 3. We also investigate the relative screening of the primary excited states in conjugated polymers, namely the n = 1, 2, and 3 excitons. We find that a larger value of n corresponds to a larger value of ΔE(n). For example, for poly(para-phenylene), ΔE(n = 1) ? 0.1 eV, ΔE(n = 2) ? 0.6 eV, and ΔE(n = 3) ? 1.2 eV (where n = 1 is the 1(1)B(1) state, n = 2 is the m(1)A state, and n = 3 is the n(1)B(1) state). Finally, we find that the strong dependence of ΔE(n) on inter-chain separation implies a strong dependency of ΔE(n) on density fluctuations. In particular, a 10% density fluctuation implies a fluctuation of 13 meV, 66 meV, and 120 meV for the 1(1)B(1), m(1)A state, and n(1)B(1) states of poly(para-phenylene), respectively. Our results for the ground-state dispersion are applicable to all types of conjugated polymers. However, our excited state results are only applicable to conjugated polymers, such as the phenyl-based class of light emitting polymers, in which the primary excitations are particle-hole (or ionic) states.     

Recent advances with π-conjugated salen systems

Schiff base ligands have long been successfully employed as ligands in combination with various metals to give catalysts capable of realizing a variety of synthetic transformations. One of the most widely used Schiff base ligands, the "salen" ligand, has been extensively researched. Recently, there has been increased interest in π-conjugated salen systems, known as "salphen" ligands, as a result of the differences in reactivity of the complexes in catalytic applications compared with the salen analogues. Complexes of salphen ligands display interesting photophysical and supramolecular properties which are not always observed with salen systems as a result of their π-conjugation. This tutorial review therefore describes the most significant advances recently made with salphen and related π-conjugated ligand systems.     

Diboron-containing fluorophores with extended ladder-type π-conjugated skeletons

A series of ladder type π-conjugated diboron complexes 1-4 have been designed and synthesized by a very simple synthetic procedure. Single crystals of complex 3 were grown and the molecular structure determined by X-ray diffraction analysis demonstrated that this type of diboron ladder has a rather planar skeleton. All complexes possess very high melting points (275-383 °C) and decomposition temperatures (T(d5): 343-400 °C), indicative of their high thermal stabilities. The electrochemical and photophysical properties as well as theoretical calculations were investigated, suggesting the possibility of these boron complexes as efficient emitters in optoelectronics.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

End-functionalized π-conjugated oligomeric materials for photoelectrical applications

Soluble poly(diphenylacetylene)s (PDA) capped with PhCO- groups (PDA-C) and Ph₂C= groups (PDA-P) were prepared via McMurry reductive coupling of benzil and carbonyl-olefin exchange reaction of tetraphenylethene and benzil, respectively. Fluoren-9-ylidene groups have been introduced into PDA by the McMurry coupling of PDA-C with fluoren-9-one and via copolymerization of benzil and fluoren-9-one. The oligomers prepared are stable in air, soluble in a variety of solvents. They can be processed by casting to form good-quality thin films suitable for measurements of electrical and optoelectrical properties.     

Highly stereoselective synthesis of N-substituted π-conjugated phthalimides

A new regio- and stereoselective synthesis of (E)-N-(2-arylvinyl)phthalimides as well as phthalimide-containing (E,E)-buta-1,3-dienes and (E)-but-1-en-3-ynes has been developed. The one-pot ruthenium-catalyzed silylative coupling/iododesilylation sequence provides (E)-N-(2-iodovinyl)phthalimide 1, which undergoes palladium-catalyzed Suzuki–Miyaura or Sonogashira cross-coupling to afford stereodefined highly π-conjugated phthalimides and functionalized dienimides containing phthalimide groups.     

Boron-bridged π-conjugated ladders as efficient electron-transporting emitters

Four diboron-bridged ladder molecules 1-4 have been designed and synthesized. X-ray diffraction analysis revealed that the bulky phenyl substituents on boron centers efficiently prevented π stacking of the luminescent ladder unit. Characterizations of these complexes demonstrated that the construction of diboron-containing ladder-type skeletons endowed these materials with good thermal stability, high fluorescence quantum yields, and strong electron affinity. The highly efficient nondoped organic light-emitting diodes using complexes 1 and 2 as electron-transporting emitters exhibited maximum luminance values of 16,930 and 18,060 cd/m(2) with turn-on voltages of 3.5 and 2.5 V as well as maximum luminous efficiencies of 6.4 and 5.4 cd/A, respectively.     

Self-localized domain walls at π-conjugated branching junctions

Self-localized domain walls are found trapped at the potential wells created by π-conjugated branching junctions due to the intrinsic electron-phonon couplings. The potential well depths are 0.14 eV for soliton, 0.28 eV for polaron, and 0.32 eV for exciton using the adapted Su-Schrieffer-Heeger model Hamiltonian, as compared to 0.23 eV for soliton, 0.25 eV for positively charged polaron, 0.33 eV for negatively charged polaron, and 0.21 eV for exciton using the ab initio Hartree-Fock method. Once the junction trapping wells are filled, however, branching junctions turn repulsive to additional self-localized domain walls. Torsions around the branching junction center have significant effects on the junction band gap and electron localizations.     

C-H/π-interaction-guided self-assembly in π-conjugated oligomers

We report CH/π hydrogen-bond-driven self-assembly in π-conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single-crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π-stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single-crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π-ring (H-bond acceptor) and alkyl C-H (H-bond donor). The four important crystallographic parameters, d(c-x)=3.79 ?, θ=21.49°, φ=150.25° and d(Hp-x)=0.73 ?, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close-packing of mesogens in x-y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid-crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11%. The orientation and translational ordering of mesogens in the liquid-crystalline (LC) phases induced H- and J-type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H- and J-type molecular arrangements were found to emit a blue or yellowish/green colour. Time-resolved fluorescence decay measurements confirmed longer lifetimes for H-type smectic OPVs relative to that of loosely packed one-dimensional nematic hydrocarbon-tailed OPVs.     

Synthesis of extended, π-conjugated isoindolin-1-ones

The synthesis and characterization of extended, conjugated molecules containing isoindolinone units was explored. Nucleophilic cyclizations between an amide and an alkyne were found to be an efficient method of producing the desired isoindolin-1-ones in high yields. A variety of derivatives were synthesized, demonstrating that a number of structural alterations could be made while maintaining good regio- and stereospecificity in the cyclized product.     

Self-sorting self-complementary assemblies of π-conjugated acyclic anion receptors

Pyrrole-based π-conjugated anion-responsive molecules bearing an anionic moiety form self-complementary dimers, which exhibit self-sorting behaviours depending on the substituted positions of anionic sites.     

Solvent-dependent supramolecular assemblies of π-conjugated anion-responsive acyclic oligopyrroles

Amide-attached pyrrole-based π-conjugated anion receptors showed solvent-dependent assembled modes such as H-aggregates that were soluble in octane and highly organized structures that provided supramolecular gels in CH(2)Cl(2) and 1,4-dioxane in the absence of π-π stacking.     

Isolable chiral aggregates of achiral π-conjugated carboxylic acids

The induced aggregation of achiral building blocks by a chiral species to form chiral aggregates with memorized chirality has been observed for a number of systems. However, chiral memory in isolated aggregates of achiral building blocks remains rare. One possible reason for this discrepancy could be that not much is understood in terms of designing these chiral aggregates. Herein, we report a strategy for creating such isolable chiral aggregates from achiral building blocks that retain chiral memory after the facile physical removal of the chiral templates. This strategy was used for the isolation of chiral homoaggregates of neutral achiral π-conjugated carboxylic acids in pure aqueous solution. Under what we have termed an "interaction-substitution" mechanism, we generated chiral homoaggregates of a variety of π-conjugated carboxylic acids by using carboxymethyl cellulose (CMC) as a mediator in acidic aqueous solutions. These aggregates were subsequently isolated from the CMC templates whilst retaining their memorized supramolecular chirality. Circular dichroism (CD) spectra of the aggregates formed in the acidic CMC solution exhibited bisignated exciton-coupled signals of various signs and intensities that were maintained in the isolated pure homoaggregates of the achiral π-conjugated carboxylic acids. The memory of the supramolecular chirality in the isolated aggregates was ascribed to the substitution of COOH/COOH hydrogen-bonding interaction between the carboxylic acid groups within the aggregates for the hydrogen-bonding interactions between the COOH groups of the building blocks and the chiral templates. We expect that this "interaction-substitution" procedure will open up a new route to isolable pure chiral aggregates from achiral species.