Synthesis and properties of cyclic sandwich compounds.

Cyclic nanometre-scale sandwich complexes assembled from individual building blocks were synthesized. Sandwich complexes, in which a metal ion is π-coordinated by two planar aromatic organic rings belong to the foundations of organometallic chemistry. They have been successfully used in a wide variety of applications ranging from catalysis, synthesis and electrochemistry to nanotechnology, materials science and medicine1,2. Extending the sandwich structural motif leads to linear multidecker compounds, in which aromatic organic rings and metal atoms are arranged in an alternating fashion. However, the extension to a cyclic multidecker scaffold is unprecedented. Here we show the design, synthesis and characterization of an isomorphous series of circular sandwich compounds, for which the term 'cyclocenes' is suggested. These cyclocenes consist of 18 repeating units, forming almost ideally circular, closed rings in the solid state, that can be described by the general formula [cyclo-MII(µ-η8:η8-CotTIPS)]18 (M = Sr, Sm, Eu; CotTIPS = 1,4-(iPr3Si)2C8H62-). Quantum chemical calculations lead to the conclusion that a unique interplay between the ionic metal-to-ligand bonds, the bulkiness of the ligand system and the energy gain on ring closure, which is crucially influenced by dispersion interactions, facilitate the formation of these cyclic systems. Up to now, only linear one-dimensional multidecker sandwich compounds have been investigated for possible applications such as nanowires3-10. This textbook example of cyclic sandwich compounds is expected to open the door for further innovations towards new functional organometallic materials.

Modulating Aryl Azide Photolysis: Synthesis of a Room-Temperature Phosphorescent Carboline in Cucurbit[7]uril Host.

Cucurbit[7]uril (CB7), a supramolecular host, is employed to control the pathway of photolysis of an aryl azide in an aqueous medium. Normally, photolysis of aryl azides in bulk water culminates predominantly in the formation of azepine derivatives via intramolecular rearrangement. Remarkably, however, when this process unfolds within the protective confinement of the CB7 cavity, it results in a carboline derivative, as a consequence of a C─H amination reaction. The resulting carboline caged by CB7 reveals long-lived room temperature phosphorescence (RTP) in the solid state, with lifetimes extending up to 2.1 s. These findings underscore the potential of supramolecular hosts to modulate the photolysis of aryl azides and to facilitate novel phosphorescent materials.

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters.

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Phase-Dependent Long Persistent Phosphorescence in Coumarin-Phosphine-Based Coinage Metal Complexes.

A coumarin functionalized aminodiphosphine has been introduced as a bidentate ligand in coinage metal chemistry. Mono-, di-, and trimetallic copper and silver complexes were synthesized with this ligand. The hybrid character of the ligand led to compounds with rich luminescence properties. These include coumarin-based blue fluorescence, observed as a sole emission in solution at room temperature, and green phosphorescence, which is efficient at low temperatures and dominates the spectra of the metal complexes. In the rigid environment of frozen solutions, the green phosphorescence shows an unusually long (for metal complexes) decay on the seconds timescale in high quantum yield. In addition, a red phosphorescence, which may be assigned to the triplet state localized in the phosphine-M3 Cl2 (M=Cu, Ag), is observed for the trinuclear complexes at low temperature. Neither the second-long phosphorescence nor the red emission is observed for the coumarin ligand, thus they must be a result of the coordination to coinage metal clusters. The excited states in these compounds were also investigated by femtosecond transient absorption spectroscopy and quantum chemical calculations.

Non-Palindromic C∧C∧P Platinum and Palladium Pincer Complexes Showing Intense Phosphorescence via Direct Spin-Forbidden S0 → T1 Excitation.

The synthesis of C∧C∧P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of PtII and PdII via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of PtII as well as PdII emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S0 → T1) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S0 → T1 excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S0 → Sn → T1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.

Heterobimetallic Gold/Ruthenium Complexes Synthesized via Post-functionalization and Applied in Dual Photoredox Gold Catalysis.

The synthesis of heterobimetallic AuI /RuII complexes of the general formula syn- and anti-[{AuCl}(L1∩L2){Ru(bpy)2 }][PF6 ]2 is reported. The ditopic bridging ligand L1∩L2 refers to a P,N hybrid ligand composed of phosphine and bipyridine substructures, which was obtained via a post-functionalization strategy based on Diels-Alder reaction between a phosphole and a maleimide moiety. It was found that the stereochemistry at the phosphorus atom of the resulting 7-phosphanorbornene backbone can be controlled by executing the metal coordination and the cycloaddition reaction in a different order. All precursors, as well as the mono- and multimetallic complexes, were isolated and fully characterized by various spectroscopic methods such as NMR, IR, and UV-vis spectroscopy as well as cyclic voltammetry. Photophysical measurements show efficient phosphorescence for the investigated monometallic complex anti-[(L1∩L2){Ru(bpy)2 }][PF6 ]2 and the bimetallic analogue syn-[{AuCl}(L1∩L2){Ru(bpy)2 }][PF6 ]2 , thus indicating a small influence of the {AuCl} fragment on the photoluminescence properties. The heterobimetallic AuI /RuII complexes syn- and anti-[{AuCl}(L1∩L2){Ru(bpy)2 }][PF6 ]2 are both active catalysts in the P-arylation of aryldiazonium salts promoted by visible light with H-phosphonate affording arylphosphonates in yields of up to 91 %. Both dinuclear complexes outperform their monometallic counterparts.

Triggering near-infrared luminescence of vanadyl phthalocyanine by charging.

Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state-dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser-induced fluorescence (LIF) spectra of samples comprising both the mass-selected neutral molecule (VOPc⋅, a stable radical) and its cation produced upon electron ionization (EI) isolated in 5 K neon matrices. Ionization of the essentially non-emissive VOPc⋅ forms a high-spin diradical cation (VOPc+.. ) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR-emitting electro-optic devices.

18-Crown-6 Coordinated Metal Halides with Bright Luminescence and Nonlinear Optical Effects.

The crown-ether coordination compounds ZnX2(18-crown-6), EuX2(18-crown-6) (X: Cl, Br, I), MnI2(18-crown-6), Mn3Cl6(18-crown-6)2, Mn3I6(18-crown-6)2, and Mn2I4(18-crown-6) are obtained by ionic-liquid-based synthesis. Whereas MX2(18-crown-6) (M: Zn, Eu) show conventional structural motives, Mn3Cl6(18-crown-6)2, Mn3I6(18-crown-6)2, and Mn2I4(18-crown-6) exhibit unusual single MnX4 tetrahedra coordinated to the crown-ether complex. Surprisingly, some compounds show outstanding photoluminescence. Thus, rare Zn2+-based luminescence is observed and unexpectedly efficient for ZnI2(18-crown-6) with a quantum yield of 54%. Unprecedented quantum yields are also observed for Mn3I6(18-crown-6)2, EuBr2(18-crown-6), and EuI2(18-crown-6) with values of 98, 72, and 82%, respectively, which can be rationalized based on the specific structural features. Most remarkable, however, is Mn2I4(18-crown-6). Its specific structural features with finite sensitizer-activator couples result in an extremely strong emission with an outstanding quantum yield of 100%. Consistent with its structural features, moreover, anisotropic angle-dependent emission under polarized light and nonlinear optical (NLO) effects occur, including second-harmonic generation (SHG). The title compounds and their optical properties are characterized by single-crystal structure analysis, X-ray powder diffraction, chemical analysis, density functional theory (DFT) calculations, and advanced spectroscopic methods.

Enantiopure Calcium Iminophosphonamide Complexes: Synthesis, Photoluminescence, and Catalysis.

The synthesis of calcium complexes ligated by three different chiral iminophosphonamide ligands, L-H (L=[Ph2 P{N(R)CH(CH3 )Ph}2 ]), L'-H (L'=[Ph2 P{NDipp}{N(R)CH(CH3 )Ph}]), (Dipp=2,6-i Pr2 C6 H3 ), and L''-H (L''=[Ph2 P{N(R)CH(CH3 )naph}2 ]), (naph=naphthyl) is presented. The resulting structures [L2 Ca], [L'2 Ca], and [L''2 Ca] represent the first examples of enantiopure homoleptic calcium complexes based on this type of ligands. The calcium complexes show blue-green photoluminescence (PL) in the solid state, which is especially bright at low temperatures. Whereas the emission of [L''2 Ca] is assigned to the fluorescence of naphthyl groups, the PL of [L2 Ca] and [L'2 Ca] is contributed by long-lived phosphorescence and thermally activated delayed fluorescence (TADF), with a strong variation of the PL lifetimes over the temperature range of 5-295 K. Furthermore, an excellent catalytic activity was found for these complexes in hydroboration of ketones at room temperature, although no enantioselectivity was achieved.

Alkali Metal Complexes of a Bis(diphenylphosphino)methane Functionalized Amidinate Ligand: Synthesis and Luminescence.

A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM-C(N-Dipp)2 H) (Dipp=2,6-diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM-C(N-Dipp)2 }(Ln )] (M=Li, Na, K, Rb, Cs; L=thf, Et2 O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal-π-interactions. In addition, a caesium derivative [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six-membered ring structure in the solid state. All complexes were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 metallocycle exhibits a blue emission in the solid state, which is significantly red-shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.

Novel Cofacial Porphyrin-Based Homo- and Heterotrimetallic Complexes of Transition Metals.

We present a straightforward and generally applicable synthesis route for cofacially linked homo- and heterotrimetallic trisporphyin complexes. The protocol encompasses synthesising the first aryl-based, trans-o-phenylene trisporphyrin starting from pyrrole and benzaldehyde with an overall yield of 3.6 %. It also allows investigating the respective cis-isomer as the first conformationally restricted planar-chiral trisporphyrin. The free-base ligand was used in subsequent metalation reactions to afford the corresponding homotrimetallic Mn(III)-, Fe(III)-, Ni(II)-, Cu(II)-, Zn(II)- and Pd(II) complexes - additionally, a small adaptation of the protocol resulted in the defined Ni(II)Fe(III)Ni(II) complex in a total yield of 2.3 %. By monitoring Ni(II) insertion into the empty trimeric ligands, we affirmed that the outer porphyrin rings are filled before the internal ring. The molecular species were characterised by 1 H NMR, UV-Vis, photoluminescence, IR, MS, CID, and high-resolution IMS measurements.

Thermally Activated Delayed Fluorescence and Phosphorescence Quenching in Iminophosphonamide Copper and Zinc Complexes.

The synthesis of copper and zinc complexes of four variably substituted iminophosphonamide ligands is presented. While the copper complexes form ligand-bridged dimers, the zinc compounds are monomeric. Due to different steric demand of the ligand the arrangement of the ligands within the dimeric complexes varies. Similar to the structurally related iminophosphonamide complexes of alkali metals and calcium, the steady-state and time-resolved photoluminescence (PL) of four of the seven compounds studied here as solids in a temperature range of 5-295 K can be described within the scheme of thermally activated delayed fluorescence (TADF). Accordingly, they exhibit bright blue-green phosphorescence at low temperatures (<100 K), which turns into delayed fluorescence by increasing the temperature. However, unusually, the fluorescence is practically absent in two copper complexes which otherwise still conform to the TADF scheme. In these cases, the excited singlet states decay essentially non-radiatively and their thermal population from the corresponding low-lying triplet states efficiently quenches PL (phosphorescence). Three other copper and zinc complexes only exhibit prompt fluorescence, evidencing a wide variation of photophysical properties in this class of compounds. The excited states of the copper complex with especially pronounced phosphorescence quenching were also investigated by low-temperature time-resolved infrared spectroscopy and quantum chemical calculations.

Near-Infrared Luminescence in Trinuclear Mixed-Metal Chalcogenolate Complexes of the Types [M2Ti(EPh)6(PPh3)2] (M = Cu, Ag; E = S, Se) and [Na(thf)3]2[Ti(SPh)6].

The optical properties of four new trinuclear chalcogenolato bridged metal complexes [Ag2Ti(SPh)6(PPh3)2], [Na(thf)3]2[Ti(SPh)6], [Cu2Ti(SePh)6(PPh3)2], and [Ag2Ti(SePh)6(PPh3)2] have been investigated by absorption and photoluminescence spectroscopy as well as time-dependent density functional theory (TDDFT) calculations and compared to the results published recently for [Cu2Ti(SPh)6(PPh3)2]. All of these compounds are distinguished by efficient near-infrared luminescence at ∼880-1200 nm in the solid state at low temperatures, which remains quite intense for the copper-titanium complexes at ambient temperature with PL quantum yields of 9.5 and 4.8% at λPL = 1090 and 1240 nm for [Cu2Ti(EPh)6(PPh3)2], E = S, Se, respectively. According to the calculations, a peculiar feature of the lowest-energy electronic transitions in these complexes is their high localization on the metal and chalcogen atoms, with negligible contributions of the "external" ligand groups. Correspondingly, the type of atoms in the M2TiE6 (M = Cu, Ag, Na) core structure determines optical properties such as the absorption and emission wavelengths and PL lifetime.

Bright Luminescence in Three Phases-A Combined Synthetic, Spectroscopic and Theoretical Approach.

Combining phase-dependent photoluminescence (PL) measurements and quantum chemical calculations is a powerful approach to help understand the influence of the molecular surroundings on the PL properties. Herein, a phosphine functionalized amidinate was used to synthesize a recently presented bimetallic gold complex, featuring an unusual charge separation. The latter was subsequently used as metalloligand to yield heterotetrametallic complexes with an Au-M-M-Au "molecular wire" arrangement (M=Cu, Ag, Au) featuring metallophilic interactions. All compounds show bright phosphorescence in the solid state, also at ambient temperature. The effect of the molecular environment on the PL was studied in detail for these tetrametallic complexes by comparative measurements in solution, in the solid state and in the gas phase and contrasted to time-dependent density functional theory computations.

Efficient Blue Phosphorescence in Gold(I)-Acetylide Functionalized Coinage Metal Bis(amidinate) Complexes.

The synthesis of linear symmetric ethynyl- and acetylide-amidinates of the coinage metals is presented. Starting with the desilylation of the complexes [{Me3 SiC≡CC(NDipp)2 }2 M2 ] (Dipp=2,6-diisopropylphenyl) (M=Cu, Au) it is demonstrated that this compound class is suitable to serve as a versatile metalloligand. Deprotonation with n-butyllithium and subsequent salt metathesis reactions yield symmetric tetranuclear gold(I) acetylide complexes of the form [{(PPh3 )AuC≡CC(NDipp)2 }2 M2 ] (M=Cu, Au). The corresponding Ag complex [{(PPh3 )AuC≡CC(NDipp)2 }2 Ag2 ] was obtained by a different route via metal rearrangement. All compounds show bright blue or blue-green microsecond long phosphorescence in the solid state, hence their photophysical properties were thoroughly investigated in a temperature range of 20-295 K. Emission quantum yields of up to 41 % at room temperature were determined. Furthermore, similar emissions with quantum yields of 15 % were observed for the two most brightly luminescent complexes in thf solution.

Pressure-Modulated Broadband Emission in 2D Layered Hybrid Perovskite-Like Bromoplumbate.

Two-dimensional (2D) layered hybrid bromoplumbate perovskites are promising candidates for solution-processed light-emitting materials. Here, we report the synthesis and characterization of two novel layered bromoplumbates: (4BrPhMA)2PbBr4 (1) and (4BrPhA)6Pb3Br12 (2), where 4BrPhMA is (4-bromophenyl)methylammonium and 4BrPhA is (4-bromophenyl) ammonium. Despite similar optical absorption, these materials show remarkably different photoluminescence properties: 1 emits a narrow exciton band at ca. 395 nm with a very small bandwidth (particularly at low temperatures of 15-50 K) and Stokes shift, while 2 exhibits a broad emission at ca. 560 nm with a large Stokes shift, both at low and ambient temperatures. However, under several kbar of hydrostatic pressure, the broad emission diminishes and a new band reversibly develops at ca. 395 nm, similar to that in 1. Our results emphasize organic layer flexibility as an important design factor for this class of perovskite-like materials featuring broadband emission.

Nanoscale patterning at the Si/SiO2/graphene interface by focused He+ beam.

We have studied the capability of He+ focused ion beam (He+-FIB) patterning to fabricate defect arrays on the Si/SiO2/Graphene interface using a combination of atomic force microscopy (AFM) and Raman imaging to probe damage zones. In general, an amorphized 'blister' region of cylindrical symmetry results upon exposing the surface to the stationary focused He+ beam. The topography of the amorphized region depends strongly on the ion dose, DS , (ranging from 103 to 107ions/spot) with craters and holes observed at higher doses. Furthermore, the surface morphology depends on the distance between adjacent irradiated spots, LS . Increasing the dose leads to (enhanced) subsurface amorphization and a local height increase relative to the unexposed regions. At the highest areal ion dose, the average height of a patterned area also increases as ∼1/LS . Correspondingly, in optical micrographs, the µm2-sized patterned surface regions change appearance. These phenomena can be explained by implantation of the He+ ions into the subsurface layers, formation of helium nanobubbles, expansion and modification of the dielectric constant of the patterned material. The corresponding modifications of the terminating graphene monolayer have been monitored by micro Raman imaging. At low ion doses, DS , the graphene becomes modified by carbon atom defects which perturb the 2D lattice (as indicated by increasing D/G Raman mode ratio). Additional x-ray photoionization spectroscopy (XPS) measurements allow us to infer that for moderate ion doses, scattering of He+ ions by the subsurface results in the oxidation of the graphene network. For largest doses and smallest LS values, the He+ beam activates extensive Si/SiO2/C bond rearrangement and a multicomponent material possibly comprising SiC and silicon oxycarbides, SiOC, is observed. We also infer parameter ranges for He+-FIB patterning defect arrays of potential use for pinning transition metal nanoparticles in model studies of heterogeneous catalysis.

Size Matters: From Two-Dimensional AuI -TlI Metallopolymers to Molecular Complexes by Simple Variation of the Steric Demand.

Bis(acetylido) aurates(I) and thallium(I) trifluoromethylsulfonates were used to synthesize AuI -TlI metallopolymers, displaying novel and unusual structural motifs of the metal-metal backbones in the solid state: a discrete molecular cluster, 1D chains of interconnected dimers, tetramers, or dodecamers of Au-Tl units, and a 2D-plane network, consisting of alternating edge-linked (AuTl)6 and (AuTl)4 cycles. The formation of the different architectures was primarily controlled by the steric demand of the acetylide-substituent groups. Thus, the bulkiest 2,6-diisopropylphenyl derivative yielded a molecular cluster [Tl2 Au3 ]. Most compounds showed bright visible photoluminescence with quantum yields of up to 25 % at ambient temperature. The color of the emitted light significantly differs with the network structure. Furthermore, theoretical studies of singlet excitations in the molecular cluster, as well as NMR and mass-spectrometric investigations of the fragmentation of the metallopolymers in solution are described in detail.

Tuning the Metal/Chalcogen Composition in Copper(I)-Chalcogenide Clusters with Cyclic (Alkyl)(amino)carbene Ligands.

A series of phosphorescent homo- and heterometallic copper(I)-chalcogenide clusters stabilized by cyclic (alkyl)(amino)carbene ligands [Cu4M4(µ3-E)4(CAACCy)4] (M = Cu, Ag, Au; E = S, Se) has been synthesized by the reaction of the new copper(I) trimethylsilylchalcogenolate compounds [(CAACCy)CuESiMe3] with ligand-supported group 11 acetates. The clusters are emissive at 77 K in solution and the solid state, with emission colors that depend on the metal/chalcogen composition. Electronic structure calculations point to a common 3[(M++E2-)LCT] emissive state for the series.

Bimetallic d10 -Metal Complexes of a Bipyridine Substituted N-Heterocyclic Carbene.

The hybrid ligand 3-(2,2'-bipyridine-6-ylmethyl)-1-mesityl-1H-imidazolylidene (NHCBipy ) featuring both carbene and N-donor sites, was selectively complexed with various d10 metal cations in order to examine its coordination behavior with regard to homo and heterometallic structures. Respective silver complexes can be obtained by the silver oxide route and are suitable transmetallation reagents for the synthesis of gold(I) compounds. Starting from the mononuclear complexes [(NHCBipy )AuCl], [(NHCBipy )Au(C6 F5 )] and [(NHCBipy )2 Au][ClO4 ], open-chain as well as cyclic heterobimetallic complexes containing Cu+ , Ag+ , Zn2+ , Cd2+ , and Hg2+ were synthesized. Furthermore, the homobimetallic species [(NHCBipy )2 M2 ][ClO4 ]2 (M=Cu, Ag) were obtained. All bimetallic compounds were fully characterized including single-crystal X-ray analysis. Their photoluminescence (PL) properties were investigated in the solid state at temperatures between 15 and 295 K and compared with those of the mononuclear species. There is a clear difference in PL properties between the open chain and the cyclic heterobimetallic complexes. The latter species show different PL properties, depending on the metals involved. In addition, collision-induced dissociation (CID) experiments were performed on electrosprayed cations of the cyclic heterobimetallic compounds, to compare the metal binding at the carbene and N-donor sites.