Jishan Wu

Poly(2,7-carbazole) and perylene tetracarboxydiimide: a promising donor/acceptor pair for polymer solar cells

A highly soluble polycarbazole (PCz) has been synthesized, and used as a donor material with perylene tetracarboxydiimide (PDI) as an acceptor and light harvesting material in bulk-heterojunction solar cells. This donor/acceptor (D/A) pair shows a broad absorption fit within the solar spectrum, and balanced potential levels for charge separation at the D/A interface. The best photovoltaic device exhibits a high external quantum efficiency (EQE) of 16% at 490 nm and a power efficiency of 0.6% under illumination with solar light. The morphology of PCz/PDI films studied by SEM showed the formation of a favorable micro-phase separation, which is important in obtaining high efficiency. Incorporation of poly(3-hexyl)thiophene (P3HT) instead of PCz as donor produced a much lower Voc and thus a lower efficiency in solar cells.

Nanostructured MnO2/graphene composites for supercapacitor electrodes: the effect of morphology, crystallinity and composition

Nanostructured MnO2 with different morphologies, i.e. amorphous, lamellar and needle-like, is incorporated with tetrabutylammonium hydroxide stabilized graphene (GTR) with different mass ratios. A systematical approach has been used to investigate the morphology, structure and electrochemical performances of these materials for supercapacitor electrodes. It is found that the morphology, crystallinity and composition all play important roles in the capacitor performance. Needle-like MnO2 (N-Mn)/GTR composites with high surface area and good crystallinity show better performance compared with the other two systems. A new morphology emerges in N-Mn/GTR13; meanwhile high specific capacitances of 280 F g−1 for the N-Mn/GTR13 composite and 631 F g−1 for MnO2 are achieved. The inclusion of graphene significantly improves the cycling stability.

Graphene oxide/ferric hydroxide composites for efficient arsenate removal from drinking water.

A series of novel composites based on graphene oxide (GO) cross-linked with ferric hydroxide was developed for effective removal of arsenate from contaminated drinking water. GO, which was used as a supporting matrix here, was firstly treated with ferrous sulfate. Then, the ferrous compound cross-linked with GO was in situ oxidized to ferric compound by hydrogen peroxide, followed by treating with ammonium hydroxide. The morphology and composition of the composites were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The ferric hydroxide was found to be homogenously impregnated onto GO sheets in amorphous form. These composites were evaluated as absorbents for arsenate removal from contaminated drinking water. For the water with arsenate concentration at 51.14 ppm, more than 95% of arsenate was absorbed by composite GO-Fe-5 with an absorption capacity of 23.78 mg arsenate/g of composite. Effective arsenate removal occurred in a wide range of pH from 4 to 9. However, the efficiency of arsenate removal was decreased when pH was increased to higher than 8.

Surfactant-intercalated, chemically reduced graphene oxide for high performance supercapacitor electrodes

A series of surfactant-stabilized graphene materials were prepared by intercalation of graphene oxide (GO) with different surfactants, tetrabutylammonium hydroxide (TBAOH), cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzene sulfonate (SDBS), followed by reduction using hydrazine. The materials were fully characterized, and the surfactants were found to be successfully intercalated in both GO and the reduced graphene oxide. As well as stabilizing the morphology of single layer or few-layer structure of graphene sheets during reduction, the presence of surfactants in graphene materials can also enhance the wettability of the graphene surface and thus improve its performance as a supercapacitor electrode. When the graphene materials were used as an electrode for a supercapacitor, the highest specific capacitance of 194 F g−1 was obtained from the TBAOH stabilized graphene at a specific current density of 1 A g−1 in 2 M H2SO4 electrolyte.

Surfactant-stabilized graphene/polyaniline nanofiber composites for high performance supercapacitor electrode

Polyaniline nanofibers were prepared by in situ polymerization of aniline in the presence of surfactants such as tetrabutylammonium hydroxide and sodium dodecyl benzenesulfonate stabilized graphene under acidic condition. A homogeneous dispersion of individual graphene sheets within the polymer matrix was achieved due to the good dispersibility of surfactant-stabilized graphene in aqueous phase. The morphology and electrochemical properties of both components were well preserved due to the mild reaction conditions. The composite materials were used for supercapacitor electrode and a high specific capacitance of 526 F g−1 was obtained at a current density of 0.2 A g−1 with good cycling stability.

Zethrenes, Extended p -Quinodimethanes, and Periacenes with a Singlet Biradical Ground State

Researchers have studied polycyclic aromatic hydrocarbons (PAHs) for more than 100 years, and most PAHs in the neutral state reported so far have a closed-shell electronic configuration in the ground state. However, recent studies have revealed that specific types of polycyclic hydrocarbons (PHs) could have a singlet biradical ground state and exhibit unique electronic, optical, and magnetic activities. With the appropriate stabilization, these new compounds could prove useful as molecular materials for organic electronics, nonlinear optics, organic spintronics, organic photovoltaics, and energy storage devices. However, before researchers can use these materials to design new devices, they need better methods to synthesize these molecules and a better understanding of the fundamental relationship between the structure and biradical character of these compounds and their physical properties. Their biradical character makes these compounds difficult to synthesize. These compounds are also challenging to physically characterize and require the use of various experimental techniques and theoretic methods to comprehensively describe their unique properties. In this Account, we will discuss the chemistry and physics of three types of PHs with a significant singlet biradical character, primarily developed in our group. These structures are zethrenes, Z-shaped quinoidal hydrocarbons; hydrocarbons that include a proaromatic extended p-quinodimethane unit; and periacenes, acenes fused in a peri-arrangement. We used a variety of synthetic methods to prepare these compounds and stabilized them using both thermodynamic and kinetic approaches. We probed their ground-state structures by electronic absorption, NMR, ESR, SQUID, Raman spectroscopy, and X-ray crystallography and also performed density functional theory calculations. We investigated the physical properties of these PHs using various experimental methods such as one-photon absorption, two-photon absorption, transient absorption spectroscopy, electrochemistry, and spectroelectrochemistry. These systematic studies revealed that aromaticity played a very important role in determining their singlet biradical character, which is critically related to both their physical properties and their chemical reactivity. In particular, we found that Clars aromatic sextet rule, which is useful for the closed-shell PAHs, can also predict the relative biradical character of benzenoid PH-based singlet biradicaloids. Other factors, such as structural flexibility of the biradical and quinoid resonance forms and the participation of the substitution in the π-conjugation, also influence the biradical character. These molecular materials demonstrate a number of unique properties such as near-infrared absorption, redox amphotericity, large two-photon absorption cross section, short excited state lifetime, stimuli-responsive magnetic activity, and singlet fission, which suggests promise for future applications.

Kinetically Blocked Stable Heptazethrene and Octazethrene: Closed-Shell or Open-Shell in the Ground State?

Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.

Soluble and Stable Heptazethrenebis(dicarboximide) with a Singlet Open-Shell Ground State

A soluble and stable heptazethrene derivative was synthesized and characterized for the first time. This molecule exhibits a singlet biradical character in the ground state, which is the first case among zethrene homologue series. Exceptional stability of this heptazethrenebis(dicarboximide) raises the likelihood of its practical applications in materials science.

Stable Tetrabenzo-Chichibabin’s Hydrocarbons: Tunable Ground State and Unusual Transition between Their Closed-Shell and Open-Shell Resonance Forms

Stable open-shell polycyclic aromatic hydrocarbons (PAHs) are of fundamental interest due to their unique electronic, optical, and magnetic properties and promising applications in materials sciences. Chichibabins hydrocarbon as a classical open-shell PAH has been investigated for a long time. However, most of the studies are complicated by their inherent high reactivity. In this work, two new stable benzannulated Chichibabins hydrocarbons 1-CS and 2-OS were prepared, and their electronic structure and geometry in the ground state were studied by various experiments (steady-state and transient absorption spectra, NMR, electron spin resonance (ESR), superconducting quantum interference device (SQUID), FT Raman, X-ray crystallographic etc.) and density function theory (DFT) calculations. 1-CS and 2-OS exhibited tunable ground states, with a closed-shell quinoidal structure for 1-CS and an open-shell biradical form for 2-OS. Their corresponding excited-state forms 1-OS and 2-CS were also chemically approached and showed different decay processes. The biradical 1-OS displayed an unusually slow decay to the ground state (1-CS) due to a large energy barrier (95 ± 2.5 kJ/mol) arising from severe steric hindrance during the transition from an orthogonal biradical form to a butterfly-like quinoidal form. The quick transition from the quinoidal 2-CS (excited state) to the orthogonal biradicaloid 2-OS (ground state) happened during the attempted synthesis of 2-CS. Compounds 1-CS and 2-OS can be oxidized into stable dications by FeCl(3) and/or concentrated H(2)SO(4). The open-shell 2-OS also exhibited a large two-photon absorption (TPA) cross section (760 GM at 1200 nm).

Pushing extended p-quinodimethanes to the limit: Stable tetracyano-oligo(N-annulated perylene)quinodimethanes with tunable ground states

p-Quinodimethane (p-QDM) is a fundamental building block for the design of π-conjugated systems with low band gap and open-shell biradical character. However, synthesis of extended p-QDMs has usually suffered from their intrinsic high reactivity and poor solubility. In this work, benzannulation together with terminal cyano-substitution was demonstrated to be an efficient approach for the synthesis of a series of soluble and stable tetracyano-oligo(N-annulated perylene)quinodimethanes nPer-CN (n = 1-6), with the longest molecule having 12 para-linked benzenoid rings! The geometry and electronic structures of these oligomers were investigated by steady-state and transient absorption spectroscopy, nuclear magnetic resonance, electron spin resonance, superconducting quantum interference device, and FT Raman spectroscopy assisted by density functional theory calculations. They showed tunable ground states, varying from a closed-shell quinoidal structure for monomer, to a singlet biradical for dimer, trimer, and tetramer, and to a triplet biradical for pentamer and hexamer. Large two-photon absorption cross-section values were observed in the near-infrared range, which also exhibited a clear chain-length dependence.