Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Multimetallic catalysis based on heterometallic complexes and clusters.

Solid-state batteries (SSBs) using a solid electrolyte show potential for providing improved safety as well as higher energy and power density compared with conventional Li-ion batteries. However, two critical bottlenecks remain: the development of solid electrolytes with ionic conductivities comparable to or higher than those of conventional liquid electrolytes and the creation of stable interfaces between SSB components, including the active material, solid electrolyte and conductive additives. Although the first goal has been achieved in several solid ionic conductors, the high impedance at various solid/solid interfaces remains a challenge. Recently, computational models based on ab initio calculations have successfully predicted the stability of solid electrolytes in various systems. In addition, a large amount of experimental data has been accumulated for different interfaces in SSBs. In this Review, we summarize the experimental findings for various classes of solid electrolytes and relate them to computational predictions, with the aim of providing a deeper understanding of the interfacial reactions and insight for the future design and engineering of interfaces in SSBs. We find that, in general, the electrochemical stability and interfacial reaction products can be captured with a small set of chemical and physical principles. The reliable operation of solid-state batteries requires stable or passivating interfaces between solid components. In this Review, we discuss models for interfacial reactions and relate the predictions to experimental findings, aiming to provide a deeper understanding of interface stability.

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Understanding interface stability in solid-state batteries

A critical current density on stripping is identified that results in dendrite formation on plating and cell failure. When the stripping current density removes Li from the interface faster than it can be replenished, voids form in the Li at the interface and accumulate on cycling, increasing the local current density at the interface and ultimately leading to dendrite formation on plating, short circuit and cell death. This occurs even when the overall current density is considerably below the threshold for dendrite formation on plating. For the Li/Li6PS5Cl/Li cell, this is 0.2 and 1.0 mA cm−2 at 3 and 7 MPa pressure, respectively, compared with a critical current for plating of 2.0 mA cm−2 at both 3 and 7 MPa. The pressure dependence on stripping indicates that creep rather than Li diffusion is the dominant mechanism transporting Li to the interface. The critical stripping current is a major factor limiting the power density of Li anode solid-state cells. Considerable pressure may be required to achieve even modest power densities in solid-state cells. A ceramic electrolyte with a lithium metal anode can offer advantages over liquid electrolytes for Li-ion battery performance. A critical current density on stripping in a solid-state cell is identified, resulting in dendrite formation on plating and failure.

Critical stripping current leads to dendrite formation on plating in lithium anode solid electrolyte cells.

The appearance of ferrocene in the middle of the 20th century has revolutionized organometallic chemistry and is now providing applications in areas as varied and sometimes initially unexpected as optical and redox devices, battery and other materials, sensing, catalysis, including asymmetric and enantioselective catalysis, and medicine. The author presents here a general, although personal, view of ferrocene's chemistry, properties, functions, and applications through a literature survey involving both historical and up-to-date trends and including examples of his group's research in a number of these areas. The review gathers together general features of ferrocene chemistry and representative examples of the salient aspects. Its focus is on ferrocene's basic properties, ferrocene-containing ligands, the ferrocene/ferricinium redox couple, ferrocene mixed-valence and average-valence systems, the ferricinium/ferrocene redox shuttle in catalysis, ligand-exchange reactions, ferrocene-containing polymers, ferrocene-containing structures for cathodic battery and other materials, ferrocenes in supramolecular ensembles, liquid crystals, and nonlinear optical materials, ferrocene-containing stars and their electrostatic effects, ferrocene-containing dendrons, dendrimers, and nanoparticles (NPs) and their application in redox sensing and catalysis, and ferrocenes in nanomedicine.

Why is Ferrocene so Exceptional

Electronic communication in heterobinuclear organometallic complexes through unsaturated hydrocarbon bridges

In this review, the properties of the most significant examples of multi(ferrocenyl) cations containing a number of ferrocenyl units from two to six are discussed and the results are compared with the outcomes of some of our recent studies on conjugated ferrocenyl complexes, in order to give an overview of how the nature of the bridging ligand, the distance between the redox-active centres, and the medium affect the electronic and electrostatic properties of the molecules.

Key multi(ferrocenyl) complexes in the interplay between electronic coupling and electrostatic interaction

The extent of metal-metal electronic coupling was quantified for a series of syn and anti stereoisomers of (FeCp)(2)-, (RhL(2))(2)- and (FeCp)(RhL(2))- (L(2)=1,5-cyclooctadiene (cod), L=CO) as-indacenediide mixed-valent ions by spectroelectrochemical and DFT studies. The effect of the syn/anti orientation of the metal units with respect to the planar aromatic ligand indicates that electron transfer occurs through the bridge rather than through space. The nature of the metal was found to be crucial: while homobimetallic diiron species are localised valence-trapped ions (Class II), the dirhodium analogues are almost delocalised mixed-valent ions (borderline and Class III). Finally, despite their redox asymmetry, even in the heterobimetallic iron-rhodium as-indacenediide complexes, strong metal-metal coupling is present. In fact, oxidation of the iron centre is accompanied by electron transfer from rhodium to iron and formation of a reactive 17-electron rhodium site. syn and anti Fe-Rh as-indacenediide complexes are rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species.

Metal-metal electronic coupling in syn and anti stereoisomers of mixed-valent (FeCp)2-, (RhL2)2-, and (FeCp)(RhL2)-as-indacenediide ions.

Heterobimetallic indenyl complexes. Synthesis and structure of cis-[Cr(CO)3(indenyl)RhL2 (L2 = norbornadiene, (CO)2)

A twisted intramolecular charge-transfer (TICT) process has been identified in (2-ferrocenyl)indene. This photochemical process explains the anti-Kasha’s rule fluorescence emission observed for this system. Experimental and model investigations on (2-ferrocenyl)tetramethylindene and (2-ferrocenyl)-hexamethylindene were also performed, in order to evaluate the effect of a steric hindrance on the TICT mechanism. The energy of the lowest main excited states was computed with a TD-DFT approach, as a function of the rotation of the dihedral angle between the indene and the cyclopentadienyl planes. To the best of our knowledge, this is the first example of TICT generated by metal-to-ligand charge transfer (MLCT) in a ferrocene-containing complex and, more generally, the first case of complexes in which a metal center is directly involved.

Saverio Santi

Papers

Electronic communication in heterobinuclear organometallic complexes through unsaturated hydrocarbon bridges

Key multi(ferrocenyl) complexes in the interplay between electronic coupling and electrostatic interaction

Metal-metal electronic coupling in syn and anti stereoisomers of mixed-valent (FeCp)2-, (RhL2)2-, and (FeCp)(RhL2)-as-indacenediide ions.

Heterobimetallic indenyl complexes. Synthesis and structure of cis-[Cr(CO)3(indenyl)RhL2 (L2 = norbornadiene, (CO)2)

Anti-Kasha’s Rule Fluorescence Emission in (2-Ferrocenyl)indene Generated by a Twisted Intramolecular Charge-Transfer (TICT) Process