Jahn–Teller coupling and the influence of strain in Tg and Eg ground and excited states – A ligand field and DFT study on halide MIIIX6 model complexes [M=TiIII–CuIII; X=F−, Cl−]

Understanding Pressure Effects on Structural, Optical, and Magnetic Properties of CsMnF4 and Other 3dn Compounds

The pressure dependence of structural, optical, and magnetic properties of the layered compound CsMnF4 are explored through first-principles calculations. The structure at ambient pressure does not arise from a Jahn-Teller effect but from an orthorhombic instability on MnF63- units in the tetragonal parent phase, while there is a P4/n → P4 structural phase transition at P = 40 GPa discarding a spin crossover transition from S = 2 to S = 1. The present results reasonably explain the evolution of spin-allowed d-d transitions under pressure, showing that the first transition undergoes a red-shift under pressure following the orthorhombic distortion in the layer plane. The energy of such a transition at zero pressure is nearly twice that observed in Na3MnF6 due to the internal electric field and the orthorhombic distortion also involved in K2CuF4. The reasons for the lack of orthorhombic distortion in K2MF4 (M = Ni, Mn) or CsFeF4 are also discussed in detail. The present calculations confirm the ferromagnetic ordering of layers in CsMnF4 at zero pressure and predict a shift to an antiferromagnetic phase for pressures above 15 GPa consistent with the reduction of the orthorhombicity of the MnF63- units. This study underlines the usefulness of first-principles calculations for a right interpretation of experimental findings.

Synthesis of pentameric chlorotin carboxylate clusters for high resolution EUV photoresists under small doses

This work reports the synthesis and characterization of a novel pentameric tin chloro cluster, (vinylSn)3Sn2Cl5O2(OH)2(t-BuCO2)6 (1), and explores its application as an efficient negative-tone photoresist in a 1 : 2 weight ratio blend with [(n-BuSn)12O14(OH)6](BF4)2 (2). Through e-beam lithography, a small high-resolution pattern (HP = 20 nm) is achieved for the blend photoresist (3) at a dose of 2080 μC cm-2. Additionally, EUV lithography demonstrates the development of a high-resolution pattern (HP = 16 nm) at an EUV dose of 70 mJ cm-2. Mechanistic studies by reflective FTIR indicate a significant decomposition of Sn-carbon and SnO2(t-Bu) moieties starting at J = 35 mJ cm-2, which is accompanied by growth of the Sn-O absorption intensity. A collapse of the cluster frameworks of clusters (1) and (2) is observed at J > 70 mJ cm-2. High-resolution X-ray photoelectron spectroscopy (HRXPS) reveals that low EUV light predominantly decomposes Sn-butyl and Sn-Cl bonds. As EUV doses increase, primary photolytic reactions involve cleavage of Sn-butyl, Sn-O2CBut, and Sn-vinyl bonds. Notably, the photolytic decomposition of Sn-Cl bonds is distinctive, with only two out of five bonds being cleaved, even at high EUV doses, resulting in a break in film growth at J = 27-35 mJ cm-2 in the EUV contrast curve. Moreover, HRXPS analysis suggests that radical propagation on the vinyltin end of the blend is unlikely, providing concise mechanistic insights into the photochemical processes governing the behavior of this advanced photoresist.

Achieving Enhanced Visible-Near-Infrared Light Absorption in Stable Lead-Free Vanadium-Based Perovskite Nanocrystals via Structural Regulation

Lead-free halide perovskites are promising materials for solar energy applications. However, their efficiency is hindered by poor light absorption in the visible-near-infrared region. Herein, we introduce vanadium (V) with low-lying ground/excited-state energy levels to form two types of stable lead-free V-based perovskite (Cs2NaVCl6 and Cs3V2Cl9) colloidal nanocrystals (NCs) with strong light absorption covering the ultraviolet to near-infrared region. We find the absorption can be further enhanced by structural regulation, in which the zero-dimensional (0D) Cs3V2Cl9 NCs show stronger and red-shifted (up to 1400 nm) light absorption compared to the three-dimensional Cs2NaVCl6 NCs. In 0D Cs3V2Cl9 NCs, [V2Cl9]3- dimers play a vital role in governing strong visible-near-infrared light absorption. We demonstrated their application for photocatalytic CO2 reduction. Our work sheds light on the structure-property relationship governing the absorption behavior, providing a novel route for tuning the light absorption ability of lead-free halide perovskites.

Cyclometalated platinum(II) complexes with acyclic diaminocarbene ligands for OLED application

A novel series of cyclometalated platinum(II) complexes bearing acyclic diaminocarbene (ADC) ancillary ligands were designed and prepared. Their photophysical properties were systematically studied through experimental and theoretical investigations. All complexes exhibit green phosphorescence with a quantum efficiency of up to 45% in 2 wt% doped PMMA film at room temperature. The complexes are used as light-emitting dopants for organic light-emitting diode (OLED) fabrication. The devices displayed a green emission with a maximum current efficiency of 2.9 cd A^-1 and a luminance of 2700 cd m^-2. These results show that these cyclometalated platinum(II) complexes can be used as efficient green emitting components of OLED devices.

Tuning the luminescence of transition metal complexes with acyclic diaminocarbene ligands

Organometallics featuring acyclic diaminocarbene ligands have recently emerged as powerful emitters for use in electroluminescent technologies.

A Single Crystal Hybrid Ligand Framework of Copper(II) with Stable Intrinsic Blue-Light Luminescence in Aqueous Solution

Single-crystal solid-liquid dual-phase hybrid organic-inorganic ligand frameworks with reversible sensing response facilitated by external stimuli have received significant attention in recent years. This report presents a significant leap in designing electronic structures that display reversible dual-phase photoluminescence properties from single-crystal hybrid ligand frameworks. Three-dimensional Cu(C₃N₂H₄)₄Cl₂ complex frameworks were formed through the intermolecular hydrogen bonding and π⋯π stacking supramolecular interactions. The absorption band peaks at 627 nm were assigned to d-d transition showing 10Dq = 15,949 cm^-1 and crystal field stabilization energy (CFSE) = 0.6 × 10Dq = 114.4 kJmol^-1, while the ligand-to-metal charge transfer (LMCT) of complexes was displayed at 292 nm. The intense luminescence band results from LMCT present at 397 nm. Considering its structure, air stability, framework forming and stable luminescence in aqueous solution, the Cu(C₃N₂H₄)₄Cl₂ complex shows potential for luminescence Cu-based sensors using emission intensity to detect heavy metal ion species.

Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex I: synthesis, spectroscopy and static quantum chemistry

In spite of intense, recent research efforts, luminescent transition metal complexes with Earth-abundant metals are still very rare owing to the small ligand field splitting of 3d transition metal complexes and the resulting non-emissive low-energy metal-centered states. Low-energy excited states decay efficiently non-radiatively, so that near-infrared emissive transition metal complexes with 3d transition metals are even more challenging. We report that the heteroleptic pseudo-octahedral d2-vanadium(iii) complex VCl3(ddpd) (ddpd = N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine) shows near-infrared singlet → triplet spin-flip phosphorescence maxima at 1102, 1219 and 1256 nm with a lifetime of 0.5 μs at room temperature. Band splitting, ligand deuteration, excitation energy and temperature effects on the excited state dynamics will be discussed on slow and fast timescales using Raman, static and time-resolved photoluminescence, step-scan FTIR and fs-UV pump-vis probe spectroscopy as well as photolysis experiments in combination with static quantum chemical calculations. These results inform future design strategies for molecular materials of Earth-abundant metal ions exhibiting spin-flip luminescence and photoinduced metal-ligand bond homolysis.

Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr(bpmp)₂]³⁺ (bpmp = 2,6-bis(2-pyridylmethyl)pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D₂O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr(bpmp)₂]³⁺ and its facile reduction to [Cr(bpmp)₂]²⁺ result in a high excited state redox potential. The ligand's methylene bridge acts as a Brønsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru(α-diimine)₃]²⁺ charge transfer complexes, which are commonly employed in optical sensing and photo(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Synthesis of a d1-titanium fluoride kagome lattice antiferromagnet

The kagome lattice, composed of a planar array of corner-sharing triangles, is one of the most geometrically frustrated lattices. The realization of a spin S = 1/2 kagome lattice antiferromagnet is of particular interest because it may host an exotic form of matter, a quantum spin liquid state, which shows long-range entanglement and no magnetic ordering down to 0 K. A few S = 1/2 kagome lattice antiferromagnets exist, typically based on Cu²⁺, d⁹ compounds, though they feature structural imperfections. Herein, we present the synthesis of (CH₃NH₃)₂NaTi₃F₁₂, which comprises an S = 1/2 kagome layer that exhibits only one crystallographically distinct Ti³⁺, d¹ site, and one type of bridging fluoride. A static positional disorder is proposed for the interlayer CH₃NH₃⁺. No structural phase transitions were observed from 1.8 K to 523 K. Despite its spin-freezing behaviour, other features-including its negative Curie-Weiss temperature and a lack of long-range ordering-imply that this compound is a highly frustrated magnet with unusual magnetic phase behaviours.

Electrically switchable magnetic exchange in the vibronic model of linear mixed valence triferrocenium complex

A vibronic model for the electric field control of antiferromagnetic exchange is developed for the linear mixed-valence triferrocenium complex Fe(iii)–Fe(ii)–Fe(iii), which is proposed as possible molecular candidate for the implementation of a quantum logic gate.

The electronic effects of ligands on metal-coordination geometry: a key role in the visual discrimination of dimethylaminopyridine and its application towards chemo-switch

Visual discrimination of p-DMAP has been demonstrated by alternation of the geometry of Cu(ii)–terpyridine complexes after selective ligand coordination, and their feasibility towards chemo-switch has also been realized.

Spin-Crossover Anticooperativity Induced by Weak Intermolecular Interactions

As a rule, rational design of cooperative spin-crossover (SCO) molecular switches is largely based on consideration of sizes and structures of individual building blocks, whereas a meticulous analysis of crystal packing, including the weakest intermolecular interactions, is often assumed to play a secondary role or is even fully neglected. By investigating cobalt(II) clathrochelates, which do not change the molecular volume upon SCO, we showed that even weak (1.2 kcal/mol) π···Cl intermolecular interactions can cause a pronounced anticooperativity of SCO, being more gradual in the solid state than in solution. Our results clearly demonstrate that the "chemical pressure" concept is not as general as it is thought to be, and the successful design of molecular switches requires in-depth analysis of intermolecular interactions, however weak they seem.