Pronounced Supramolecular Order in Discotic Donor–Acceptor Mixtures

An Appraisal for Providing Charge Transfer (CT) Through Synthetic Porous Frameworks for their Semiconductor Applications

In recent years, there has been considerable focus on the development of charge transfer (CT) complex formation as a means to modify the band gaps of organic materials. In particular, CT complexes alternate layers of aromatic molecules with donor (D) and acceptor (A) properties to provide inherent electrical conductivity. In particular, the synthetic porous frameworks as attractive D-A components have been extensively studied in recent years in comparison to existing D-A materials. Therefore, in this work, the synthetic porous frameworks are classified into conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) and compare high-quality materials for CT in semiconductors. This work updates the overview of the above porous frameworks for CT, starting with their early history regarding their semiconductor applications, and lists CT concepts and selected key developments in their CT complexes and CT composites. In addition, the network formation methods and their functionalization are discussed to provide access to a variety of potential applications. Furthermore, several theoretical investigations, efficiency improvement techniques, and a discussion of the electrical conductivity of the porous frameworks are also highlighted. Finally, a perspective of synthetic porous framework studies on CT performance is provided along with some comparisons.

Monolayer and Bilayer Formation of Molecular 2D Networks Assembled at the Liquid/Solid Interfaces by Solution-Based Drop-Cast Method

In recent years, extending self-assembled structures from two-dimensions (2D) to three-dimensions (3D) has been a paradigm in surface supramolecular chemistry and contemporary nanotechnology. Using organic molecules of p-terphenyl-3,5,3′,5′-tetracarboxylic acid (TPTC), and scanning tunneling microscopy (STM), we present a simple route, that is the control of the solute solubility in a sample solution, to achieve the vertical growth of supramolecular self-assemblies, which would otherwise form monolayers at the organic solvent/graphite interface. Presumably, the bilayer formations were based on π-conjugated overlapped molecular dimers that worked as nuclei to induce the yielding of the second layer. We also tested other molecules, including trimesic acid (TMA) and 1,3,5-tris(4-carboxyphenyl)-benzene (BTB), as well as the further application of our methodology, demonstrating the facile preparation of layered assemblies.

Donor-Acceptor Type Covalent Organic Frameworks

Intermolecular charge transfer (ICT) effect has been widely studied in both small molecules and linear polymers. Covalently-bonded donor-acceptor pairs with tunable bandgaps and photoelectric properties endow these materials with potential applications in optoelectronics, fluorescent bioimaging, and sensors, etc. However, owing to the lack of charge transfer pathway or effective separation of charge carriers, unfavorable charge recombination gives rise to inevitable energy loss. Covalent organic frameworks (COFs) can be mediated with various geometry- and property-tailored building blocks, where donor (D) and acceptor (A) segments are connected by covalent bonds and can be finely arranged to form highly ordered networks (namely D-A COFs). The unique structural features of D-A COFs render the formation of segregated D-A stacks, thus provides pathways and channels for effective charge carriers transport. This review highlights the significant progress on D-A COFs over the past decade with emphasis on design principles, growing structural diversities, and promising application potentials.

Influence of Vibronic Coupling on Ultrafast Singlet Fission in a Linear Terrylenediimide Dimer

Singlet fission (SF) is a photophysical process capable of boosting the efficiency of solar cells. Recent experimental investigations into the mechanism of SF provide evidence for coherent mixing between the singlet, triplet, and charge transfer basis states. Up until now, this interpretation has largely focused on electronic interactions; however, nuclear motions resulting in vibronic coupling have been suggested to support rapid and efficient SF in organic chromophore assemblies. Further information about the complex interactions between vibronic excited states is needed to understand the potential role of this coupling in SF. Here, we report mixed singlet and correlated triplet pair states giving rise to sub-50 fs SF in a terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the two TDI molecules are covalently linked by a direct N-N connection at one of their imide positions, leading to a linear dimer with perpendicular TDI π systems. We observe the transfer of low-frequency coherent wavepackets between the initial predominantly singlet states to the product triplet-dominated states. This implies a non-negligible dependence of SF on nonadiabatic coupling in this dimer. We interpret our experimental results in the framework of a modified Holstein Hamiltonian, which predicts that vibronic interactions between low-frequency singlet modes and high-frequency correlated triplet pair motions lead to mixing of the pure basis states. These results highlight how nonadiabatic mixing can shape the complex potential energy landscape underlying ultrafast SF.

Co-assembly of functionalized donor-acceptor molecules within block copolymer microdomains via the supramolecular assembly approach with an improved charge carrier mobility

Here, we demonstrate the three-component self-assembly of functionalized small molecules (donor and acceptor) and a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer using the supramolecular approach. The introduction of functional groups on both the donor (1-pyrenebutyric acid, PBA) and acceptor (functionalized naphthalene diimide, FNDI) molecules can form stable charge-transfer (CT) complexes within the block copolymer domains and these supramolecules exhibited a charge carrier mobility of around 1.01 × 10^-4 cm² (V s)^-1. In this case, both the molecules can form H-bonding with P4VP chains, and as well as π-π stacking between the PBA and FNDI molecules is also possible within the block copolymer domains. These noncovalent interactions lead to the formation of stable hierarchical structures and CT complexes between PBA and FNDI, where bilayer donor-acceptor (D-A) stacks formed within the block copolymer microdomains. Overall, the organization of both functionalized donor and acceptor molecules within the block copolymer domain exhibits an enhanced charge carrier mobility, which is potentially useful in the fabrication of organic photovoltaic cells and organic light-emitting diodes.

Photoconductive Core-Shell Liquid-Crystals of a Perylene Bisimide J-Aggregate Donor-Acceptor Dyad

A novel core-shell structured columnar liquid crystal composed of a donor-acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self-assembles in solution into helical J-aggregates guided by π-π interactions and hydrogen bonds which organize into a liquid-crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs' (J-type) transition dipole moment parallel and the bithiophene side arms' (H-type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self-assembled core-shell structure enables the implementation of the dye in two-contact photoconductivity devices giving rise to a 20-fold increased photoresponse compared to a reference dye without bithiophene donor moieties.

Quintet-triplet mixing determines the fate of the multiexciton state produced by singlet fission in a terrylenediimide dimer at room temperature

Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T₁T₁) state whose spin dynamics influence the successful generation of uncorrelated triplets (T₁). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI₂), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T₁T₁) state. The spin dynamics of the (T₁T₁) state and the processes leading to uncoupled triplets (T₁) were studied at room temperature for TDI₂ aligned in 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T₁T₁) state has mixed ⁵(T₁T₁) and ³(T₁T₁) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the ³(T₁T₁) state opens a pathway for triplet-triplet annihilation that produces a single uncorrelated T₁ state. The presence of the ⁵(T₁T₁) state at room temperature and its relationship with the ¹(T₁T₁) and ³(T₁T₁) states emphasize that understanding the relationship among different (T₁T₁) spin states is critical for ensuring high-yield T₁ formation from singlet fission.

Comparative Study on the Supramolecular Assemblies Formed by Calixpyridinium and Two Alginates with Different Viscosities

In this work, a comparative study on the supramolecular assemblies formed by calixpyridinium and two alginates with different viscosities was performed. We found that sodium alginate (SA) with medium viscosity (SA-M) had a better capability to induce aggregation of calixpyridinium in comparison with SA with low viscosity (SA-L) because of the stronger electrostatic interactions between calixpyridinium and SA-M. Therefore, the morphology of calixpyridinium-SA-M supramolecular aggregates was a compact spherical structure, while that of calixpyridinium-SA-L supramolecular aggregates was an incompact lamellar structure. As a result, adding much more amount of 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt to calixpyridinium-SA-M solution was required to achieve the balance of the competitive binding, and in comparison with calixpyridinium-SA-L supramolecular aggregates, calixpyridinium-SA-M supramolecular aggregates were more sensitive to alkali. However, for the same reason, in comparison with calixpyridinium-SA-M supramolecular aggregates, calixpyridinium-SA-L supramolecular aggregates were much more stable in water not only at room temperature but also at a higher temperature, and even in salt solution. Therefore, in comparison with calixpyridinium-SA-L supramolecular aggregates, calixpyridinium-SA-M supramolecular aggregates exhibited a completely opposite response to acid because of the generation of salt. Because SA is an important biomaterial with excellent biocompatibility, it is anticipated that this comparative study is extremely important in constructing functional supramolecular biomaterials.

Encapsulating propeller-like columnar liquid crystals with an aromatic outer shell: influence of phenoxy-terminated side chains on the phase behaviour of triphenylbenzenes

Tailoring of phase transition temperatures of columnar liquid crystals by side chain variation is often associated with an undesired change in the mesophase type and/or geometry. To overcome this problem phenoxy-terminated side chains rather than alkyl side chains were grafted onto triphenylbenzenes, which resulted in reduced clearing points, while melting points were little affected. More importantly, helical columnar self-assembly was not compromised.

Singlet Fission in Covalent Terrylenediimide Dimers: Probing the Nature of the Multiexciton State Using Femtosecond Mid-Infrared Spectroscopy

Singlet fission (SF) is a spin-allowed process that involves absorption of a photon by two electronically interacting chromophores to produce a singlet exciton state, ¹(S₁S₀), followed by rapid formation of two triplet excitons if the singlet exciton energy is about twice that of the triplet exciton. The initial formation of the multiexciton correlated triplet pair state, ¹(T₁T₁), is thought to involve the agency of charge transfer (CT) states. The dynamics of these electronic states were studied in a covalent slip-stacked terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the conformation of two TDI molecules is determined by a xanthene spacer (XanTDI₂). Femtosecond mid-infrared (fsIR) spectroscopy shows that the multiexciton ¹(T₁T₁) state has absorptions characteristic of the T₁ state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the C═C stretch region. The simultaneous presence of CT and triplet state features in both high dielectric constant CH₂Cl₂ and low dielectric constant 1,4-dioxane throughout the multiexciton state lifetime suggests that this state has both CT and triplet character.

Molecular aggregation of naphthalimide organic semiconductors assisted by amphiphilic and lipophilic interactions: a joint theoretical and experimental study

Amphiphilic and lipophilic donor-acceptor naphthalimide-oligothiophene assemblies exhibiting almost identical intramolecular properties, but differing in their intermolecular interactions, have been synthesized. Here we analyze the effect of replacing the normally used lipophilic alkyl chains with hydrophilic ones in directing molecular aggregation from an antiparallel to a parallel stacking. This different molecular packing of the amphiphilic, NIP-3TAmphi, and lipophilic, NIP-3TLipo, systems is assessed by electronic spectroscopies, scanning electronic microscopy and DFT quantum-chemical calculations. Theoretical calculations indicate that the presence of amphiphilic interactions promotes a face-to-face parallel arrangement of neighbor molecules, which induces improved electronic coupling and therefore enhances the charge transport ability and photoconducting properties of this type of materials. Time of flight and photoconducting measurements are used to determine the impact of the amphiphilic and lipophilic interactions on their possible performance in optoelectronic devices.

Tunable Supramolecular Hexagonal Columnar Structures of Hydrogen-Bonded Copolymers Containing Two Different Sized Dendritic Side Chains

Polymer structures with tunable symmetry and sizes are desired for applications such as lithography, filtration membranes, and separation. Here we report the self-assembled supramolecular hexagonal columnar (Φ_H) structures with tunable lattice size varying from 5 to 7 nm by constructing hydrogen-bonded copolymers bearing poly(4-vinylpyridine) (P4VP) and two dendritic molecular additives, 1-[4'-(3″,4″,5″-tridecyloxybenzoyloxy)phenyleneoxycarbonyl]-3-[(4″-hydroxyphenyl)oxycarbonyl]benzene (12CBP) and 4-hydroxyphenyl (3,4,5-tridecyloxy)benzoate (12CTB). Despite the distinct molecular size difference between 12CBP and 12CTB, the resulting ternary supramolecular copolymers, P4VP(12CBP)_x(12CTB)_y, possess a homogeneous Φ_H phase at x ≥ 0.1 and y ≥ 0.2. Each column is constructed with P4VP as the backbone tethered with mixed side chains. The column diameter is between the size of the corresponding P4VP(12CBP)_x+y and P4VP(12CTB)_x+y and could be easily tuned by varying x and y. The enhancement of Φ_H in supramolecular copolymers is attributed to the entropy effect of the mixed side chain and enthalpy effect from hydrogen bonding interaction of the P4VP backbone and two molecules (12CBP and 12CTB).

Relationship between Side-Chain Polarity and the Self-Assembly Characteristics of Perylene Diimide Derivatives in Aqueous Solution

Perylene-3,4,9,10-tetracarboxylic acid diimides (PDIs) have recently gained considerable interest for water-based biosensing applications. PDIs have been studied intensively in the bulk state, but their physical properties in aqueous solution in interplay with side-chain polarity are, however, poorly understood. Therefore, three perylene diimide based derivatives were synthesized to study the relationship between side-chain polarity and their self-assembly characteristics in water. The polarity of the side chains was found to dictate the size and morphology of the formed aggregates. Side-chain polarity rendered the self-assembly and photophysical properties of the PDIs-both important for imminent water-based applications-and these were revealed to be especially responsive to changes in solvent composition.

Thermo-mechanically responsive crystalline organic cantilever

Dynamic molecular crystals lift weights up to ∼100× heavier than themselves powered by a thermally induced single-crystal to single-crystal phase transition.

Induced stabilization of columnar phases in binary mixtures of discotic liquid crystals

Three discotic liquid-crystalline binary mixtures, characterized by their extent of bidispersity in molecular thickness, were investigated with molecular dynamics simulations. Each equimolar mixture contained A-type (thin) and B-type (thick) discogens. The temperature-dependence of the orientational order parameter reveals that A-type liquid samples produce ordered phases more readily, with the (hexagonal) columnar phase being the most structured variant. Moderately and strongly bidisperse mixtures produce globally-segregated samples for temperatures corresponding to ordered phases; the weakly bidisperse mixture displays microheterogeneities. Ordered phases in the B-type liquid are induced partially by the presence of the A-type fluid. In the moderately bidisperse mixture, order is induced through orientational frustration: a mixed prenematic-like phase precedes global segregation to yield nematic and columnar mesophases upon further cooling. In the strongly bidisperse mixture, order is induced less efficiently through a paranematic-like mechanism: a highly-ordered A-type fluid imparts order to B-type discogens found at the interface of a fully-segregated sample. This ordering effect permeates into the disordered B-type domain until nematic and columnar phases emerge upon further cooling. At sufficiently low temperatures, all samples investigated exhibit the (hexagonal) columnar mesophase.

Interfacial Donor-Acceptor Engineering of Nanofiber Materials To Achieve Photoconductivity and Applications

Self-assembly of π-conjugate molecules often leads to formation of well-defined nanofibril structures dominated by the columnar π-π stacking between the molecular planes. These nanofibril materials have drawn increasing interest in the research frontiers of nanomaterials and nanotechnology, as the nanofibers demonstrate one-dimensionally enhanced exciton and charge diffusion along the long axis, and present great potential for varying optoelectronic applications, such as sensors, optics, photovoltaics, and photocatalysis. However, poor electrical conductivity remains a technical drawback for these nanomaterials. To address this problem, we have developed a series of nanofiber structures modified with different donor-acceptor (D-A) interfaces that are tunable for maximizing the photoinduced charge separation, thus leading to increase in the electrical conductivity. The D-A interface can be constructed with covalent linker or noncovalent interaction (e.g., hydrophobic interdigitation between alkyl chains). The noncovalent method is generally more flexible for molecular design and solution processing, making it more adaptable to be applied to other fibril nanomaterials such as carbon nanotubes. In this Account, we will discuss our recent discoveries in these research fields, aiming to provide deep insight into the enabling photoconductivity of nanofibril materials, and the dependence on interface structure. The photoconductivity generated with the nanofibril material is proportional to the charge carriers density, which in turn is determined by the kinetics balance of the three competitive charge transfer processes: (1) the photoinduced electron transfer from D to A (also referred to as exciton dissociation), generating majority charge carrier located in the nanofiber; (2) the back electron transfer; and (3) the charge delocalization along the nanofiber mediated by the π-π stacking interaction. The relative rates of these charge transfer processes can be tuned by the molecular structure and nanoscale interface engineering. As a result, maximal photoconductivity can be achieved for different D-A nanofibril composites. The photoconductive nanomaterials thus obtained demonstrate unique features and functions when employed in photochemiresistor sensors, photovoltaics and photocatalysis, all taking advantages of the large, open interface of nanofibril structure. Upon deposition onto a substrate, the intertwined nanofibers form networks with porosity in nanometer scale. The porous structure enables three-dimensional diffusion of molecules (analytes in sensor or reactants in catalysis), facilitating the interfacial chemical interactions. For carbon nanotubes, the completely exposed π-conjugation facilitates the surface modification through π-π stacking in conjunction with D-A interaction. Depending on the electronic energy levels of D and A parts, appropriate band alignment can be achieved, thus producing an electric field across the interface. Presence of such an electric field enhances the charge separation, which may lead to design of new type of photovoltaic system using carbon nanotube composite.

Donor-acceptor cocrystal based on hexakis(alkoxy)triphenylene and perylenediimide derivatives with an ambipolar transporting property

An organic donor-acceptor cocrystal with an ambipolar transporting property was constructed based on N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) and 2,3,6,7,10,11-hexakis-(hexyloxy)-triphenylene (H6TP). The cocrystal with an alternating stacking of H6TP and EP-PDI molecules was formed through both drop-casting and spin-coating processes, especially at the optimized ratios of H6TP/EP-PDI (2/1, 1/1). The formation of the cocrystal was driven by the strong π-π interaction and the weaker steric hindrance, resulting from the smaller side groups, between the donor and acceptor molecules. Field effect transistors (FETs) based on the H6TP/EP-PDI cocrystal exhibited relatively balanced hole/electron transport, with a hole mobility of 1.14 × 10(-3) cm(2) V(-1) s(-1) and an electron mobility of 1.40 × 10(-3) cm(2) V(-1) s(-1).

Liquid crystalline perylene diimide outperforming nonliquid crystalline counterpart: higher power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) cells and higher electron mobility in space charge limited current (SCLC) devices

In this work, we propose the application of liquid crystalline acceptors as a potential means to improve the performances of bulk heterojunction (BHJ) organic solar cells. LC-1, a structurally-simple perylene diimide (PDI), has been adopted as a model for thorough investigation. It exhibits a broad temperature range of liquid crystalline (LC) phase from 41 °C to 158 °C, and its LC properties have been characterized by differental scanning calorimetry (DSC), polarization optical microscopy (POM), and X-ray diffraction (XRD). The BHJ devices, using P3HT:LC-1 (1:2) as an organic photovoltaic active layer undergoing thermal annealing at 120 °C, shows an optimized efficiency of 0.94 %. By contrast, the devices based on PDI-1, a nonliquid crystalline PDI counterpart, only obtain a much lower efficiency of 0.22%. Atomic force microscopy (AFM) images confirm that the active layers composed of P3HT:LC-1 have smooth and ordered morphology. In space charge limited current (SCLC) devices fabricated via a spin-coating technique, LC-1 shows the intrinsic electron mobility of 2.85 × 10(-4) cm(2)/(V s) (at 0.3 MV/cm) which is almost 5 times that of PDI-1 (5.83 × 10(-5) cm(2)/(V s)) under the same conditions for thermal annealing at 120 °C.

Coexistence of helical morphologies in columnar stacks of star-shaped discotic hydrazones

Discotic hydrazone molecules are of particular interest as they form discotic phases where the discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic behavior and solid-state organizations of three discotic hydrazone derivatives with dendritic groups attached to their outer peripheries, containing six, eight, and ten carbons of linear alkoxy chains. On the basis of two-dimensional wide angle X-ray scattering (2DWAXS), the elevated temperature liquid crystalline (LC) phases were assigned to a hexagonal columnar (Colh) organization with nontilted hydrazone discs for all three compounds. With WAXS, advanced solid-state nuclear magnetic resonance (SSNMR) techniques, and ab initio computations, the compounds with six and ten carbons of achiral alkoxy side chains were further subjected to studies at 25 °C, revealing complex crystalline phases with rigid columns and flexible side chains. This combined approach led to models of coexisting helical columnar stacking morphologies for both systems with two different tilt/pitch angles between successive hydrazone molecules. The differences in tilt/pitch angles between the two compounds illustrate that the columns with short alkoxy chains (six carbons) are more influenced by the presence of other stacks in their vicinity, while those with long side chains are less tilted due to a larger alkoxy (ten carbons) buffer zone. The formation of different packing morphologies in the crystalline phase of a columnar LC has rarely been reported so far, which suggests the possibility of complex stacking structures of similar organic LC systems, utilizing small molecules as potential materials for applications in organic electronics.

pH-responsive supramolecular vesicles based on water-soluble pillar[6]arene and ferrocene derivative for drug delivery

The drug delivery system based on supramolecular vesicles that were self-assembled by a novel host-guest inclusion complex between a water-soluble pillar[6]arene (WP6) and hydrophobic ferrocene derivative in water has been developed. The inclusion complexation between WP6 and ferrocene derivative in water was studied by (1)H NMR, UV-vis, and fluorescence spectroscopy, which showed a high binding constant of (1.27 ± 0.42) × 10(5) M(-1) with 1:1 binding stoichiometry. This resulting inclusion complex could self-assemble into supramolecular vesicles that displayed a significant pH-responsive behavior in aqueous solution, which were investigated by fluorescent probe technique, dynamic laser scattering, and transmission electron microscopy. Furthermore, the drug loading and in vitro drug release studies demonstrated that these supramolecular vesicles were able to encapsulate mitoxantrone (MTZ) to achieve MTZ-loaded vesicles, which particularly showed rapid MTZ release at low-pH environment. More importantly, the cellular uptake of these pH-responsive MTZ-loaded vesicles by cancer cells was observed by living cell imaging techniques, and their cytotoxicity assay indicated that unloaded vesicles had low toxicity to normal cells, which could dramatically reduce the toxicity of MTZ upon loading of MTZ. Meanwhile, MTZ-loaded vesicles exhibited comparable anticancer activity in vitro as free MTZ to cancer cells under examined conditions. This study suggests that such supramolecular vesicles have great potential as controlled drug delivery systems.

Discotic liquid crystals of transition metal complexes 47: synthesis of phthalocyanine-fullerene dyads showing spontaneous homeotropic alignment

In previous work, we successfully synthesized the first columnar liquid crystalline 1:1 phthalocyanine(Pc)-fullerene( C60 ) dyad, C12(OMalC60)PcCu (4), showing spontaneous homeotropic alignment, by using Bingel reaction. However, it gave undesirable 2:1 and 3:1 Pc- C60 by-products at the same time, so that the 1:1 Pc- C60 dyad (4) could be obtained in a very low yield (20%). On the other hand, in this work we have prepared novel Pc- C60 dyads, C12(OFbaC60)PcM (8: M = Co (a), Ni (b) and Cu (c)), by using Prato reaction. Very interestingly, it gave almost only the target 1:1 Pc- C60 dyads 8a–8c in very high yields (81–96%) with negligible amount of Pc- C60 by-products. These dyads 8a–8c and the precursor Pc derivatives were characterized by polarizing microscope, differential scanning calorimeter and temperature-dependent X-ray diffractometer. Thus, it was revealed that each of the dyads and precursors shows spontaneous homeotropic alignment in their Coltet mesophase.

Discotic liquid crystals of transition metal complexes 44: synthesis of hexaphenoxy-substituted phthalocyanine derivatives showing spontaneous perfect homeotropic alignment

We have synthesized novel hexaphenoxy-substituted phthalocyanine derivatives, 2-(12-hydroxydodecyloxy)-3-methoxy-9,10,16,17,23,24-hexakis(3,4-di-n-alkoxyphenoxy)phthalocyaninato copper(II) (abbreviated as Cn(OC12OH)PcCu : n = 10, 12, 14), and investigated their columnar mesomorphism and homeotropic alignment property. These hexaphenoxy-substituted Pc derivatives could be successfully isolated and purified from the mixture products by polarity difference. It was revealed by using polarizing optical microscopic observations, differential scanning calorimetry and temperature-dependent X-ray diffraction studies that each of the hexaphenoxy-substituted Pc derivatives has plural mesophases, and that the tetragonal columnar (Coltet) mesophase shows spontaneous perfect homeotropic alignment between two non-surface-treated glass plates without any defects and polydomain boundaries.

Hydrogen-bonding induced alternate stacking of donor (D) and acceptor (A) chromophores and their supramolecular switching to segregated states

This paper reports comprehensive studies on the mixed assembly of bis-(trialkoxybenzamide)-functionalized dialkoxynaphthalene (DAN) donors and naphthalene-diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge-transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure-dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self-assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self-assembly propensity, the mode of co-assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen-bonding interaction instead of a weak CT interaction. Consequently, in a highly non-polar solvent like methylcyclohexane (MCH), although kinetically controlled CT-gelation was initially noticed, within a few hours the system sacrificed the CT-interaction and switched over to the more stable self-sorted gel to maximize the gain in enthalpy from the hydrogen-bonding interaction. In contrast, in a relatively less non-polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT-state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D-A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co-assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co-assembly, morphology, and the unprecedented supramolecular-switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.

Self-assembling porphyrins and phthalocyanines for photoinduced charge separation and charge transport

Large π-conjugated compounds are promising building blocks for organic thin-film electronics such as organic light-emitting diodes, organic field-effect transistors, and organic photovoltaics. Utilization of porphyrins and phthalocyanines for this purpose is highly fascinating because of their excellent electric, photophysical, and electrochemical properties as well as intense self-assembling abilities arising from π-π stacking interactions. This paper focuses on fundamental aspects of self-assembled structures that have been obtained from porphyrin and phthalocyanine building blocks and more complex composites for photoinduced charge separation and charge transport toward the potential applications to organic thin-film electronics.