Wei-Shi Li

Dendrimer porphyrins and phthalocyanines.

The development of dendrimer porphyrins has been inspired and prompted by many interesting functions of porphyrin derivatives in biological systems. Natural porphyrin derivatives, including hemes, chlorophylls, and bacteriochlorophylls, are integrated into protein scaffolds that are essential for their biological activities.1 For example, the protein matrices of hemoproteins provide active centers with a special hydrophobic pocket having a specific steric hindrance. In the case of hemoglobin and myoglobin, the embedded active iron centers are protected sterically and hydrophobically against irreversible oxidation, thereby enabling reversible O2-binding to take place.2 In the lightharvesting complexes of purple bacteria, wheel-like supramolecular assemblies of many bacteriochlorophyll units are also realized with the assistance of proteins; they play an important role in the efficient capturing of energy from sunlight and transferring it to the photosynthetic reaction center.3-6 Because of their morphological similarities to proteins, three-dimensional dendrimer architectures can act as attractive scaffolds for the site-specific positioning of porphyrin functionalities in nanoscale size regimes. Dendrimers are characterized as regularly branched, threedimensional architectures with various properties that can be tailored and controlled, such as the size and shape of the molecule and the position of functional groups either in the core of the molecule, in the branching units, and/or on the exterior surface.7,8 Dendrimer porphyrins were first reported in 1993 by one of the present authors: a porphyrin unit was integrated into the center of a poly(benzyl ether) dendrimer (1n ·2H) to form a synthetic model of a hemoprotein (Chart 1).9 Half of a year later, Diederich et al. reported the synthesis and redox properties of poly(ether-amide) dendrimer porphyrins 2n ·Zn (Chart 2).10 In 1996, Suslick et al. studied the action of manganese porphyrin-cored poly(aryl ester) dendrimers for oxidation.11 After the publication of such pioneering works, dendrimer porphyrins began to attract considerable attention. The main purpose of this review Article is to highlight some of the unique structural and functional aspects of dendrimer porphyrins in relation to the biological activities of hemoproteins and photosynthetic systems, as well as in the context of host-guest chemistry, self-assembly, materials sciences, and biomedical applications. Although this Article may feature some studies that have already been highlighted in other reviews and books,12-18 they are included again to discuss the structure-property-function relationships of dendrimer porphyrins, which could provide a guiding principle for the rational molecular design of functional molecules and materials on the nanometer scale. Because phthalocyanines have molecular structures and properties similar to those of porphyrins, dendrimer phthalocyanines are also included in this review.19-21

Amphiphilic Molecular Design as a Rational Strategy for Tailoring Bicontinuous Electron Donor and Acceptor Arrays: Photoconductive Liquid Crystalline Oligothiophene−C60 Dyads

For tailoring solution-processable optoelectronic thin films, a rational strategy with amphiphilic molecular design is proposed. A donor-acceptor dyad consisting of an oligothiophene and C60, when modified with a hydrophilic wedge on one side and a paraffinic wedge on the other (1Amphi), forms over a wide temperature range a photoconducting smectic A liquid crystal having bicontinuous arrays of densely packed donor and acceptor units. In contrast, when modified with only paraffinic wedges (1Lipo), the dyad forms a smectic A liquid crystalline mesophase, which however is poorly conductive. As indicated by an absorption spectral feature along with a synchrotron radiation small-angle X-ray scattering profile, 1Lipo in the lamellar structure does not adopt a uniform head/tail orientation. Such defective donor and acceptor arrays likely contain a large number of trapping sites, leading to short-lived charge carriers, as observed by a flash photolysis time-resolved microwave conductivity study.

Use of Side-Chain Incompatibility for Tailoring Long-Range p/n Heterojunctions: Photoconductive Nanofibers Formed by Self-Assembly of an Amphiphilic Donor–Acceptor Dyad Consisting of Oligothiophene and Perylenediimide

To tailor organic p/n heterojunctions with molecular-level precision, a rational design strategy using side-chain incompatibility of a covalently connected donor-acceptor (D-A) dyad has been successfully carried out. An oligothiophene-perylenediimide dyad, when modified with triethylene glycol side chains at one terminus and dodecyl side chains at the other (2(Amphi)), self-assembles into nanofibers with a long-range D/A heterojunction. In contrast, when the dyad is modified with dodecyl side chains at both termini (2(Lipo)), ill-defined microfibers result. In steady-state measurements using microgap electrodes, a cast film of the nanofiber of 2(Amphi) displays far better photoconducting properties than that of the microfiber of 2(Lipo). Flash-photolysis time-resolved microwave conductivity measurements, in conjunction with transient absorption spectroscopy, clearly indicate that the nanofiber of 2(Amphi) intrinsically allows for better carrier generation and transport properties than the microfibrous assembly of 2(Lipo).

Fluorinated graphene: facile solution preparation and tailorable properties by fluorine-content tuning

Fluorinated graphene is one of the most important two-dimensional carbon nanomaterials derived from graphene, and possesses specific and outstanding properties. However, it lacks a cost-effective and large-scale preparation method. Here, we describe a novel and facile solution approach using graphene oxide (GO) and liquid diethylaminosulfur trifluoride as starting materials under mild conditions. The chemical composition and the structure of so-prepared fluorinated graphene were characterized in detail by elemental analysis, solid state F-19 NMR, XPS, FT-IR, Raman, SEM, TEM, and AFM. These studies reveal that some oxygen-containing moieties in GO are converted into C-F bonds, while some are eliminated during the reaction. More interestingly, the fluorine-loading amount can be well tuned by simply altering the reaction medium, and has a significant impact on the optical, electronic, and conductive properties of the product. Preliminary experiments on its application as an electrode material for solid-state supercapacitors were finally presented.

Influence of moiety sequence on the performance of small molecular photovoltaic materials

The purpose of this work is to study the impact of moiety sequence in the chemical structure of small molecular photovoltaic materials on their basic properties and photovoltaic performance. For this aim, two isomeric compounds, namely BDT(ThBTTh)2 and BDT(BTTh2)2, have been designed and synthesized by exchanging benzothiadiazole and thiophene positions with a structural variation. As compared with BDT(BTTh2)2, BDT(ThBTTh)2 possesses a lower melting point, a blue-shifted absorption spectrum in solution, and slightly lower-lying highest occupied and lowest unoccupied molecular orbitals. More interestingly, the hole mobility of the BDT(ThBTTh)2 neat film is 0.1 cm2 V−1 s−1, which is three-orders of magnitude larger than that of BDT(BTTh2)2. Furthermore, these two compounds display significantly different photovoltaic performance, 4.53% for BDT(ThBTTh)2versus 1.58% for BDT(BTTh2)2 in terms of their power conversion efficiency.

Control of Molecular Structures and Photophysical Properties of Zinc(II) Porphyrin Dendrimers Using Bidentate Guests: Utilization of Flexible Dendrimer Structures as a Controllable Mold

We have prepared supramolecular assemblies of hexaaryl-anchored polyester zinc(II) porphyrin dendrimers (6P(Zn)W, 12P(Zn)W, and 24P(Zn)W) with various bipyridyl guests (C(n)Py2; n = 1, 2, 4, 6, and 8) through self-assembled coordination to control the structures and photophysical properties. We comparatively investigated the photophysical properties of porphyrin dendrimers with and without guest binding by using ensemble and single-molecule spectroscopy. The spectrophotometric titration data of dendrimers with guest molecules provide a strong indication of the selective intercalation of bipyridyl guests into porphyrin dendrimers. The representative dendrimer assembly 12P(Zn)W [symbol: see text] C6Py2 exhibits increased fluorescence quantum yield and lifetime in ensemble measurements, as well as higher initial photon count rates with stepwise photobleaching behavior in the single-molecule fluorescence intensity trajectories (FITs) compared to 12P(Zn)W. At the single-molecule level, the higher photostability of 12P(Zn)W [symbol: see text] C6Py2 can be deduced from the long durations of the first emissive levels in the FITs. We attribute the change in photophysical properties of the dendrimer assemblies to their structural changes upon intercalation of guest molecules between porphyrin units. These results provide new insight into the control of porphyrin dendritic structures using appropriate bidentate guests in poor environmental conditions.

Dendronized graphenes: remarkable dendrimer size effect on solvent dispersity and bulk electrical conductivity

Graphene as a newly emerged carbon material has attracted considerable attention due to its outstanding properties and a wide range of fascinating applications. However, its real use is limited due to the lack of a method for mass production. The reduction from graphene oxide has been considered as one of the potential ways for mass-scalable preparation. However, it suffers from re-stacking of the final graphene sheets after reduction due to the strong intersheet interactions. To address this, we report here a strategy using three-dimensional and bulky dendritic structure to functionalize graphene sheets. We found that the treatment of the acylchlorinated graphene oxide with dendritic anilines can easily load dendritic wedges to graphene oxide sheets and simultaneously reduce graphene oxide to graphene. The afforded dendronized graphene products possess excellent dispersibility in a variety of solvents. The dispersity shows a great dependence on the size of the dendritic structure, in which the larger dendritic substituents afford a better dispersity. Surprisingly, dendronization with an appropriate size of dendritic structure does not hamper but can even greatly enhance the bulk electric conducting capability.

Construction of a long range p/n heterojunction with a pair of nanometre-wide continuous D/A phases

A p/n heterojunction is the basic setup for light-electric conversion. It has been widely accepted that the ideal configuration for organic photovoltaics is formed by the joint of a pair of long-range continuous but nanometre-wide phases consisting of electron-donating (D) and -accepting (A) components, respectively. Such a p/n heterojunction can provide not only a large D/A interface essential to efficient photoinduced charge separation, but also the transportation pathways for both electrons and holes. This review article summarizes the present approaches including D-A double cables, diblock copolymers, and small molecular D-A dyads and multiads, to construct such an ideal p/n heterojunction. Each approach is introduced by a few selected representative works, with highlights on their molecular design strategies and the relationship of chemical structure-packing order-property. Such information would be useful for the next research in the field.

Long-term thermally stable organic solar cells based on cross-linkable donor–acceptor conjugated polymers

The real-life application of polymer solar cells (PSCs) requires both a high power conversion efficiency (PCE) and a long enough lifetime. In order to avoid microstructure evolution and enhance device thermal stability, various different amounts of terminal vinyl moieties have been integrated into the side chains of poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione), a previously reported high performance donor–acceptor photovoltaic polymer, to produce a series of crosslinkable polymers named PBDTTPD-Vx (where x is defined as the molar content of vinyl units). It has been found that the larger the vinyl content the polymer contains, the larger the amount of polymer remaining on the substrate after thermal crosslinking and solvent washing. However, the optimized PSC device based on such a polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) displayed a decreased efficiency. These studies have discovered that a vinyl content as small as 2.5% is enough for this family of crosslinkable polymers to achieve effective crosslinking, while at the same time maintaining their high photovoltaic performance. The optimized PBDTTPD-V0.025/PC71BM device showed a power conversion efficiency (PCE) of 6.06% after thermal crosslinking, which represents the highest recorded efficiency among PSC devices with crosslinked active layers. Furthermore, this crosslinked device successfully retained 91% of its initial PCE after thermal treatment at 150 °C for 40 h, which was much better than the non-crosslinkable PBDTTPD-V0/PC71BM cell.

High-performance flexible transparent conductive films achieved by cooperation between 1D copper nanowires and 2D graphene materials

Flexible transparent conductive films (TCFs) fabricated from indium tin oxide (ITO)-alternative materials are highly desirable for a variety of present and future (opto-)electronics. In this contribution, we report that the hybridization of a kind of two-dimensionally electro-conductive material and a kind of one-dimensionally electro-conductive material, i.e. reduced graphene oxide (rGO) and copper nanowires (CuNWs), is a good choice to meet such desire. Different combination ratios between these two kinds of materials by either adding CuNWs into rGO bulk or vice versa were tested. It was found that a significant synergistic effect in improving TCF performance takes place between two-dimensional (2D) rGO nanosheets and one-dimensional (1D) copper nanowires. That is, 1D metallic CuNWs are superior to 2D rGO nanosheets as a conducting additive to improve the performance of TCFs mainly based on the rGO material, while 2D rGO nanosheets rather than 1D CuNWs are very good additives for CuNW-based TCFs to decrease sheet resistance with a small sacrifice in film transparency. Moreover, the hybridization of CuNWs with rGO can not only significantly reduce datum fluctuation in sheet resistance, but also improve the anti-oxidation and anti-foldability properties of TCFs mainly based on CuNWs. Finally, flexible TCFs with a transmittance at 550 nm larger than 80% and a sheet resistance down to 50 Ω sq−1 have been achieved on a polyethylene terephthalate (PET) substrate.