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Principles Of Fluorescence Spectroscopy

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 search for hydrogen storage materials capable of efficiently storing hydrogen in a compact and lightweight package is one of the most difficult challenges for the upcoming hydrogen economy. Liquid chemical hydrides with high gravimetric and volumetric hydrogen densities have the potential to overcome the challenges associated with hydrogen storage. Moreover, the liquid-phase nature of these hydrogen storage systems provides significant advantages of easy recharging, and the availability of the current liquid fuel infrastructure for recharging. In this review, we briefly survey the research progress in the development of diverse liquid-phase chemical hydrogen storage materials, including organic and inorganic chemical hydrides, with emphases on the syntheses of active catalysts for catalytic hydrogen generation and storage. Moreover, the advantages and drawbacks of each storage system are discussed.

Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage

The classical division of polymeric materials into thermoplastics and thermosets based on covalent network structure often implies that these categories are distinct and irreconcilable. Yet, the past two decades have seen extensive development of materials that bridge this gap through incorporation of dynamic crosslinks, enabling them to behave as both robust networks and moldable plastics. Although their potential utility is significant, the growth of covalent adaptable networks (CANs) has obscured the line between “thermoplastic” and “thermoset” and erected a conceptual barrier to the growing number of new researchers entering this discipline. This Perspective aims to both outline the fundamental theory of CANs and provide a critical assessment of their current status. We emphasize throughout that the unique properties of CANs emerge from the network chemistry, and particularly highlight the role that the crosslink exchange mechanism (i.e., dissociative exchange or associative exchange) plays in the res...

Adaptable Crosslinks in Polymeric Materials: Resolving the Intersection of Thermoplastics and Thermosets.

Dynamic covalent polymer networks (DCPN) have historically attracted attention for their unique roles in chemical recycling and self-healing, which are both relevant for sustainable societal development. Efforts in these directions have intensified in the past decade with notable progress in newly discovered dynamic covalent chemistry, fundamental material concepts, and extension toward emerging applications including energy and electronic devices. Beyond that, the values of DCPN in discovering/designing functional properties not offered by classical thermoplastic and thermoset polymers have recently gained traction. In particular, the dynamic bond exchangeability of DCPN has shown unparalleled design versatility in various areas including shape-shifting materials/devices, artificial muscles, and microfabrication. Going beyond this basic bond exchangeability, various molecular mechanisms to manipulate network topologies (topological transformation) have led to opportunities to program polymers, with notable concepts such as living networks and topological isomerization. In this review, we provide an overview of the above progress with particular focuses on molecular design strategies for the exploitation of functional material properties. Based on this, we point out the remaining issues and offer perspectives on how this class of materials can shape the future in ways that are complementary with classical thermoplastic and thermoset polymers.

Dynamic Covalent Polymer Networks: A Molecular Platform for Designing Functions beyond Chemical Recycling and Self-Healing.

Current vitrimer technology uses only a handful of distinct reactions for cross-linking. New dynamic reactions can diversify vitrimer functionality and properties. In this paper, reversible cross-links formed by conjugate addition–elimination of thiols with a Meldrum’s acid derivative enable compression–remolding of silicone elastomers. After 10 remolding cycles, there is no discernible deterioration of mechanical properties (Young’s modulus, Tg, rubbery plateau E’), nor is there a change in stress relaxation activation energy. This robust new cross-linker could be implemented in any number of systems that currently use permanent thiol–ene cross-linking, expanding the scope of recyclable materials.

Vitrimeric Silicone Elastomers Enabled by Dynamic Meldrum’s Acid-Derived Cross-Links

The synthesis of two parental BN anthracenes, 1 and 2, was developed, and their electronic structure and reactivity behavior were characterized in direct comparison with all-carbon anthracene. Gas-phase UV-photoelecton spectroscopy studies revealed the following HOMO energy trend: anthracene, −7.4 eV; BN anthracene 1, −7.7 eV; bis-BN anthracene 2, −8.0 eV. The λmax of the lower energy band in the UV–vis absorption spectrum is as follows: anthracene, 356 nm; BN anthracene 1, 359 nm; bis-BN anthracene 2, 357 nm. Thus, although the HOMO is stabilized with increasing BN incorporation, the HOMO–LUMO band gap remains unchanged across the anthracene series. The emission λmax values for the three investigated anthracene compounds are at 403 nm. The pKa values of the N-H proton for BN anthracene 1 and bis-BN anthracene 2 were determined to be approximately 26. BN anthracenes 1 and 2 do not undergo heat- or light-induced cycloaddition reactions or Friedel–Crafts acylations. Electrophilic bromination of BN anthracen...

Two BN isosteres of anthracene: synthesis and characterization.

The least stable isomer of the parental BN naphthalene series has been synthesized in a simple four-step sequence. Its experimental electronic structure characterization via UV-PES, cyclic voltammetry, and UV–vis spectroscopy in direct comparison with three other known BN naphthalene isomers has established two guiding principles for predicting the electronic structures of BN acene compounds: (1) Orientational BN isomers have similar HOMO–LUMO gaps. (2) For each pair of orientational BN isomers, the more thermodynamically stable compound has the lower HOMO energy. Furthermore, we demonstrate that BN/CC isosterism in the context of BN-9,1-Naph can impact crystal packing to favor a cofacial π-stack motif.

The Least Stable Isomer of BN Naphthalene: Toward Predictive Trends for the Optoelectronic Properties of BN Acenes

Homogeneous metal catalysis for conversion between aromatic and saturated compounds

https://par.nsf.gov/servlets/purl/10143575

Jacob S. A. Ishibashi

Papers

Vitrimeric Silicone Elastomers Enabled by Dynamic Meldrum’s Acid-Derived Cross-Links

Two BN isosteres of anthracene: synthesis and characterization.

The Least Stable Isomer of BN Naphthalene: Toward Predictive Trends for the Optoelectronic Properties of BN Acenes

Homogeneous metal catalysis for conversion between aromatic and saturated compounds

Cross-linker control of vitrimer flow