Potential applications of metal-organic frameworks

The persistent radical effect: a principle for selective radical reactions and living radical polymerizations.

Nanographenes, or extended polycyclic aromatic hydrocarbons, have been attracting renewed and more widespread attention since the first experimental demonstration of graphene in 2004. However, the atomically precise fabrication of nanographenes has thus far been achieved only through synthetic organic chemistry. The precise synthesis of quasi-zero-dimensional nanographenes, i.e. graphene molecules, has witnessed rapid developments over the past few years, and these developments can be summarized in four categories: (1) non-conventional methods, (2) structures incorporating seven- or eight-membered rings, (3) selective heteroatom doping, and (4) direct edge functionalization. On the other hand, one-dimensional extension of the graphene molecules leads to the formation of graphene nanoribbons (GNRs) with high aspect ratios. The synthesis of structurally well-defined GNRs has been achieved by extending nanographene synthesis to longitudinally extended polymeric systems. Access to GNRs thus becomes possible through the solution-mediated or surface-assisted cyclodehydrogenation, or “graphitization,” of tailor-made polyphenylene precursors. In this review, we describe recent progress in the “bottom-up” chemical syntheses of structurally well-defined nanographenes, namely graphene molecules and GNRs.

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New advances in nanographene chemistry

The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.

Control of magnetic properties through external stimuli.

Nanographenes, namely polycyclic aromatic hydrocarbons (PAHs) with nanoscale dimensions (>1 nm), are atomically precise cutouts from graphene. They represent prime models to enhance the scope of chemical and physical properties of graphene through structural modulation and functionalization. Defined nitrogen doping in nanographenes is particularly attractive due to its potential for increasing the number of π-electrons, with the possibility of introducing localized antiaromatic ring elements. Herein we present azomethine ylide homocoupling as a strategy to afford internally nitrogen-doped, non-planar PAH in solution and planar nanographene on surfaces, with central pyrazine rings. Localized antiaromaticity of the central ring is indicated by optical absorption spectroscopy in conjunction with theoretical calculations. Our strategy opens up methods for chemically tailoring graphene and nanographenes, modified by antiaromatic dopants. Polyaromatic hydrocarbons can be precisely manipulated to yield ever more complex and discrete graphene analogs, such as nanographenes. Here, the authors use azomethine ylide homocoupling to insert an antiaromatic pyrazine ring into the core of a nanographene, and characterize the molecule’s unique electronic character.

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Exploration of pyrazine-embedded antiaromatic polycyclic hydrocarbons generated by solution and on-surface azomethine ylide homocoupling

A large first-order magnetic phase transition in an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl, is described. The transition occurs with a wide thermal hysteresis loop over the temperature range 230 to 305 kelvin. The high-temperature phase is paramagnetic, and its structure consists of a uniform one-dimensional stacking of the radical. The low-temperature phase is diamagnetic because of strong dimerization along the stacking direction. The results may have applications in thermal sensors, switching units, and information storage media based on organic radical crystals.

https://science.sciencemag.org/content/sci/286/5438/261.full.pdf

Room-Temperature Magnetic Bistability in Organic Radical Crystals

Magnetic properties of the layered materials Cu2(OH)3(n-CmH2m+1COO) (m = 0, 1, 7, 8, and 9) have been examined. X-ray powder diffraction patterns indicate that the interlayer distance increases in the order of m. The interlayer distances of the m = 0 and 1 materials are shorter than the molecular heights of the intercalated carboxylates, respectively, while the distances of the m = 7−9 materials are nearly double those of the molecular heights of the corresponding carboxylates, indicating a bilayer structure in them. The temperature dependence of the paramagnetic susceptibilities of the m = 0 and 1 materials suggests an intralayer ferromagnetic interaction and an interlayer antiferromagnetic interaction. The metamagnetic behavior is supported by S-shape dependence of their magnetization curves. The temperature dependence of the paramagnetic susceptibilities of the m = 7−9 materials indicates an intralayer antiferromagnetic interaction, in opposition to those in the former two. Further, they show a diverge...

Magnetic Properties of Cu2(OH)3(alkanecarboxylate) Compounds: Drastic Modification with Extension of the Alkyl Chain

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1–4 were investigated using steady-state UV–vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1–T1 energy gap (ΔES–T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV–vis–NIR absorption, ESR, and NMR spectroscopies for the chemically generated r...

Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

A phase control by photoirradiation is successfully achieved in a spin-Peierls system of the organic radical crystal, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which exhibits optical and magnetic bistability around room temperature with a large hysteresis loop. A nanosecond laser pulse is found to induce a transition from a diamagnetic low-temperature phase to a paramagnetic high-temperature phase both inside (296 K) and outside (11 K) the hysteresis loop. Comparison of the excitation energy dependence between transition efficiency and photoconductivity suggests that the photoinduced transition is driven by suppression of the spin-Peierls instability by the accumulation of photocarriers.

Wataru Fujita

Papers

Room-Temperature Magnetic Bistability in Organic Radical Crystals

Magnetic Properties of Cu2(OH)3(alkanecarboxylate) Compounds: Drastic Modification with Extension of the Alkyl Chain

Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

Photoinduced phase transition in an organic radical crystal with room-temperature optical and magnetic bistability.

Reversible Structural Transformation and Drastic Magnetic Change in a Copper Hydroxides Intercalation Compound