Dongzhong Chen

Self-assembled hierarchical structure evolution of azobenzene-containing linear-dendritic liquid crystalline block copolymers

In this paper, the phase behaviors and self-assembled structures of a series of liquid crystalline linear-dendritic block copolymers (LDBCs) in the bulk state have been systematically investigated in combination with DSC, POM, SAXS, WAXS, and TEM. The LDBCs consist of a linear poly(ethylene glycol) (PEG) block (degree of polymerization, DP = 49) and dendritic polyamidoamine (PAMAM) segments of generation G0 to G3, functionalized with photoactive azobenzene mesogenic units bearing an octyloxy tail and a flexible ten-methylene spacer (AZO). An unprecedented hierarchical structure evolution with increasing dendron generation has been demonstrated for this series of azobenzene-containing LDBCs in bulk. Well-defined lamellar structures are formed for liquid crystalline LDBCs of G0, G1, and G2 with only a slight change in layer spacing between 12.2 and 13.0 nm. A simple lamellar structure is generated by mPEG-G0-(AZO)2, while tetragonal-within-lamellar and lamellar-within-lamellar hierarchical structures are created in mPEG-G1-(AZO)4 and mPEG-G2-(AZO)8, respectively, due to further suborganization between PAMAM and AZO segments within the dendritic blocks. For the highest generation copolymer investigated in this work, mPEG-G3-(AZO)16, the structure changes dramatically from lamellar for lower generations into rectangular columnar with lattice parameters a = 8.5 nm and b = 6.5 nm or higher temperature disordered columnar mesophases. The introduction of the alkyloxy tail in the azobenzene mesogenic unit is of crucial importance for constructing such hierarchical structures. Moreover, those complex lamellar mesophases transform into micellar or network cubic structures upon cooling to ambient solidification temperature with PEG crystallization showing a complete inverse phase change tendency in virtue of the curvature effect as a result of the specific linear-dendritic architecture. Very interestingly, the films of azobenzene-containing liquid crystalline LDBCs exhibit generation/morphology dependent photophysical characteristics, which may afford a convenient way for designing and fine-tuning novel optical storage materials.

Discotic columnar liquid-crystalline polymer semiconducting materials with high charge-carrier mobility via rational macromolecular engineering

Discotic liquid crystal (DLC) polymers in columnar phases are fascinating and promising organic semiconducting materials as they combine the advantages of DLCs with the flexibility and good processability of polymers. Synthetic challenges hinder progress in this area, particularly in the preparation of such polymers in a well-controlled way. A group of butoxy-substituted triphenylene (TP)-based side-chain DLC polymers have been prepared by reversible addition fragmentation chain transfer (RAFT) polymerization via rational macromolecular engineering with particular emphasis on the effects of spacer length and molecular weight. The DLC polymers with shorter alkyl spacers exhibit various ordered columnar LC or columnar plastic phases and easily realize macroscopic homeotropic or planar columnar alignments. Very high hole mobilities over 0.1 cm2 V−1 s−1 are achieved by the well-defined, high-molecular-weight side-chain DLC polymers with shorter spacers, as evaluated by time-of-flight measurements. These high hole mobilities are mainly attributed to positive coupling between the side-chain TP discogens and the polymer backbones. These DLC polymers and the applied macromolecular engineering principles may pave the way for cost-effective, solution-processable organic semiconducting materials for various electronic and optoelectronic device applications.

Broad hexagonal columnar mesophases formation in bioinspired transition-metal complexes of simple fatty acid meta-octaester derivatives of meso-tetraphenyl porphyrins.

A series of meta-substituted fatty acid octaester derivatives and their transition-metal complexes of meso- tetraphenyl porphyrins (TPP-8OOCR, with R = C(n-1)H(2n-1), n = 8, 12, or 16) have been prepared through very simple synthesis protocols. The thermotropic phase behavior and the liquid crystalline (LC) organization structures of the synthesized porphyrin derivatives were systematically investigated by a combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM), and variable-temperature small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) techniques. The shorter octanoic acid ester substituted porphyrin (C8-TPP) did not show liquid crystallinity and its metal porphyrins exhibited an uncommon columnar mesophase. The lauric acid octaester (C12-TPP) and the palmitic acid octaester (C16-TPP) series porphyrins generated hexagonal columnar mesophase Colh. Moreover, the metal porphyrins C12-TPPM and C16-TPPM with M = Zn, Cu, or Ni, exhibited well-organized Colh mesophases of broad LC temperature ranges increasing in the order of TPPNi<TPPCu≤TPPZn with their increased effective ionic radii in the square-planar coordination. The simplicity in synthesis, the well intercolumnar organization of Colh mesophase, the broadness of the discotic LC range, and the specific UV/Vis absorption and fluorescence emission behaviors make the symmetrically substituted fatty acid octaester porphyrins and their metal complexes very attractive for variant applications.

Cyclic polymers with pendant triphenylene discogens: convenient synthesis and topological effect on thermotropic liquid crystal behavior and fluorescence enhancement

Novel discotic liquid crystalline cyclic polymers prepared through RAFT polymerization and intrachain azide–alkyne click cyclization, exhibit fully inhibited or significantly weakened bulk columnar ordering with increased molecular weight or shortened spacer length, while exhibiting remarkably enhanced fluorescence emission in solution as a result of the cyclic topological constraint effect.

Formation of persistent ordered lamellar mesophases in azobenzene-containing silver thiolates and their application in the controlled synthesis of silver nanomaterials

Metal thiolates have aroused intensive interest mainly due to their precursor-based preparation of nanostructured metal or metal chalcogenides. In this paper, a series of azobenzene-containing thiol ligands with different length alkoxy tails and their corresponding silver thiolates AgS–C10H20–Ph–NN–Ph–OCnH2n+1 with n = 1, 6, 8, 12, have been successfully synthesized, and their thermal properties and phase behavior have been systematically investigated by differential scanning calorimetry (DSC), variable-temperature SAXS/WAXS and temperature dependent FTIR. By introducing azobenzene mesogen for the first time into the silver thiolates, ordered lamellar liquid crystalline mesophases persisting throughout a higher temperature have been achieved derived from their specific orthorhombic crystalline structures, which are in sharp contrast to the micellar or hexagonal columnar mesophases reported for silver alkane thiolates with a longer aliphatic alkyl chain AgSCmH2m+1 (m ≥ 12) owing to the interplay of azobenzene mesogen π–π stacking and the inorganic skeleton binding of a Ag–S slab. Furthermore, the intermediate nanoparticles formation and silver nanodisks preparation through an in situ thermolytic reaction based on such mesogenic precursors have been demonstrated in principle. As a kind of functional metallomesogen precursor the silver mesogenic thiolates with persistent ordered lamellar mesophases provide an ideal two-dimensional (2D) confined environment for the investigation of a layered-precursor-to-lamellar-nanomaterial (LPLM) mechanism of solventless thermolysis and the fascinating controlled preparation of variant 2D shaped metal or metal sulfide nanomaterials.

Competition and compromise between discotic and calamitic mesogens in triphenylene and azobenzene based shape-amphiphilic liquid crystals

Clarifying the relationship and interplay between disc-like (discotic) and rod-like (calamitic) mesogens is highly appealing. In this article, a series of novel disc-rod shaped amphiphilic liquid crystalline dimers and side-chain polymers based on typical discotic triphenylene (TP) and calamitic azobenzene (AZO) mesogens have been well synthesized and systematically investigated. It was found that lamello-columnar mesophases were characteristic for disc-rod hybrids and bridge the lamellar and columnar phases typical for calamitic or discotic mesogens alone. For the TP6 based hybrid dimers with peripheral hexyloxy substituents, the phase structure evolves from a sequence of lamello-columnar mesophases Colr/L and slim ColSr/L for dimers TP6–AZO6 and TP6–AZO8 with shorter hexyloxy or octyloxy tails attached on AZO, to a single Colr/L mesophase persisting to lower temperature for TP6–AZO12 and TP6–AZO16 with longer dodecyloxy or hexadecyloxy tails. With the extension of TP peripheral substituents to decyloxy, although TP10–AZO12 exhibits only a stable lamello-columnar mesophase Colr/L, the TP10–AZO6 represents an ideal sample of majority discotic moieties and just the right calamitic fraction, which demonstrates a fascinating phase transformation from a simple lamellar phase to a disordered oblique columnar phase Colob-d through an intermediate lamello-columnar mesophase Colr/L. Although the methacrylate monomer derived from the hybrid TP6–AZO10 exhibits similar lamello-columnar mesophase Colr/L, the corresponding polymer displays only a simple lamellar phase due to the constraint of the polymethacrylate backbone. The elucidation of complex self-assembly behavior and in-depth understanding of the competition and compromise between different shape aromatic moieties in disc-rod shaped amphiphiles is of crucial importance for clarifying the aromatic interactions involved in complex biological systems and guiding molecular design of advanced organic materials for various optoelectronic applications.

Azobenzene mesogens mediated preparation of SnS nanocrystals encapsulated with in-situ N-doped carbon and their enhanced electrochemical performance for lithium ion batteries application*

In this work, azobenzene mesogen-containing tin thiolates have been synthesized, which possess ordered lamellar structures persistent to higher temperature and serve as liquid crystalline precursors. Based on the preorganized tin thiolate precursors, SnS nanocrystals encapsulated with in-situ N-doped carbon layer have been achieved through a simple solventless pyrolysis process with the azobenzene mesogenic thiolate precursor served as Sn, S, N, and C sources simultaneously. Thus prepared nanocomposite materials as anode of lithium ion batteries present a large specific capacity of 604.6 mAhg−1 at a current density of 100 mAg−1, keeping a high capacity retention up to 96% after 80 cycles, and display high rate capability due to the synergistic effect of well-dispersed SnS nanocrystals and N-doped carbon layer. Such encouraging results shed a light on the controlled preparation of advanced nanocomposites based on liquid crystalline metallomesogen precursors and may boost their novel intriguing applications.

Self-assembled helical columnar superstructures with selective homochirality

Electron donor–acceptor (EDA) complexes of triphenylene (TP) based side-chain discotic liquid crystalline polymers doped with chiral acceptors exhibit a remarkable supramolecular polymer effect upon reaching a critical doping amount, complying with the discrete columnar stack (DCS) based modular assembly mechanism. Furthermore, selective left- or right-handed homochirality determined by the enantiomeric nature of chiral dopants has been achieved, with a helical polymer backbone surrounded by chiral supramolecular columns through modular assembly between the chiral dopant and DCS subunits, revealing a strong circular dichroism (CD) activity reminiscent of the fascinating hierarchically assembled superstructure of tobacco mosaic virus (TMV). Such polymeric EDA stoichiometric complexation and generation of single-handedness provides inspiring insight into and guidance for constructing various chiral functional materials via non-covalent interactions in a facile way, and also enhances in-depth understanding of the origin of biological helical structures and chiral basis of life.

Side-Chain Liquid Crystalline Polymers: Controlled Synthesis and Hierarchical Structure Characterization

In this Chapter, we have reviewed the recent research progress in the side-chain liquid crystalline polymers (LCPs) of both rod-like and disc-like types mainly based on typical azobenzene (AZO) calamitic mesogens with photoresponsive characteristics and triphenylene (TP) derivatives discotic mesogens. Side-chain liquid crystalline homopolymers and block copolymers prepared by typical controlled radical polymerization (CRP) techniques such as atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain-transfer (RAFT) are the primary systems to be introduced, together with the linear–dendritic liquid crystalline block copolymers with side-chain type dendritic liquid crystalline segments synthesized through iterative reactions. The phase behaviors and hierarchical structure evolution characterized by a combination of various techniques such as DSC, POM, SAXS/WAXS, TEM and AFM have been summarized and concisely commented. The relationship between the phase structures and morphologies and their optoelectronic properties has also been briefly discussed.

Highly efficient luminescent side-chain polymers with short-spacer attached tetraphenylethylene AIEgens via RAFT polymerization capable of naked eye explosive detection

Tetraphenylethylene (TPE) is one of the well-known typical aggregation-induced emission (AIE) luminogens (AIEgens) and TPE-containing polymers possess excellent processing properties which facilitates the fabrication of various AIE systems and devices. For AIE active side-chain polymers, increasing the fluorescence quantum yield through suitable molecular design is very appealing. In this work, we present the well-controlled synthesis of a series of TPE polyacrylate side-chain polymers of Pm with short spacers with methylene number m = 0–5 via RAFT polymerization. All the obtained polymers exhibit typical and strong AIE characteristics in THF/H2O solution mixtures and in solid state films. In particular, greatly enhanced fluorescence emission is achieved with reduced spacer lengths, and further significant improvement achieved through annealing, which is ascribed to the restriction of intramolecular motions (RIM) and the blocking of the non-radiative channels due to overcrowding and confinement thanks to the strong positive coupling effect and the jacketing effect of side-chain polymers with very short spacers. The quantum yields of 34.4%, 32.0% and 31.4%, respectively, for the polymer films P0, P1 and P2 are several times higher than those of homologous polymers with longer spacers reported in the literature. Based on these TPE side-chain polymers of very short spacers, a simple, economic and visual method for detection of the explosive 2,4,6-trinitrotoluene (TNT) is demonstrated. Therefore, a facile approach and feasible strategy of simply adopting very short spacers for side-chain AIE active polymer preparation to significantly enhance the luminescence efficiency has been properly established and proposed, which may provide guidance and have a wealth of implications for the rational design and preparation of highly efficient luminescent AIE polymer materials.