An imidazole-copper(I) metallacycle complex exhibiting thermochromic fluorescence

In this paper, we firstly report the synthesis and structural characterization of a discrete coordination metallacycle complex, [CuI (bipy)]2 (1). The x-ray diffraction structure, temperature-dependent electronic absorption, and photoluminescence spectra have been investigated. The solid-state fluorescence at variable temperatures shows that complex 1 exhibits an obvious thermochromic fluorescence. At low temperature, the dual fluorescence with peaks at 590 and 694 nm was observed. The emission color significantly changes from red at 77 K to yellow at 200 K and blue-green at 330 K. The thermochromic fluorescent molecular materials show great potential as temperature sensing.

Cubane Dimerization: Cu4 vs Cu8 Copper Iodide Clusters

Copper(I) halides are well-known for their structural diversity and rich photoluminescence properties, showing great potential for the development of solid-state lighting technology. A series of four molecular copper iodide clusters based on the [Cu4I4] cubane geometry is reported. Among them, [Cu8I8] octanuclear clusters of rare geometry resulting from dimerization of the tetranuclear counterparts were also synthesized. Two different phosphine ligands were studied, bearing either a styrene or an ethyl group. Therefore, the effect of the dimerization and of the ligand nature on the photophysical properties of the resulting clusters is investigated. The structural differences were analyzed by single-crystal X-ray diffraction (SCXRD), solid-state nuclear magnetic resonance (NMR), infrared, and Raman analyses. Compared to the ethyl group, the styrene function appears to greatly impact the photophysical properties of the clusters. The luminescence thermochromic properties of the ethyl derivatives and the intriguing photophysical properties of the clusters with styrene function were rationalized by density functional theory (DFT) calculations. Thus, the styrene group significantly lowers in energy the vacant orbitals and consequently affects the global energetic layout of the clusters. From this study, it was found that the nuclearity of copper iodide clusters eventually has less influence on the photophysical properties than the nature of the ligand. The design of proper ligands should therefore be considered when developing materials for specific lighting applications.

Chain Length Effect on the Structural and Emission Properties of the CuI/Bis((4-methoxyphenyl)thio)alkane Coordination Polymers

A systematic chain length variation of the ligand para-MeOC₆H₄S(CH₂)_mSC₆H₄OMe (1 ≤ m ≤ 8) was performed to study its effect on the structures and photophysical properties of the coordination polymers (CP) when reacted with CuI. Indeed, direct correlations are noted between these features and m. When m is an odd number, the secondary building unit is systematically the common closed-cubane Cu₄I₄ cluster, rendering the material strongly luminescent (i.e., emission quantum yield, Φ_e > 20%), and the CP is one-dimensional (1D). However, when m is 2, 4, and 6, the SBUs exhibit rare polymeric motifs of (Cu₂I₂)_n: staircase ribbon, fused poly(rhombic pseudo-dodecahedron), and accordion ribbon, respectively, and the emission intensities are either very weak (Φ_e < 0.001%) or of medium intensity (Φ_e ∼ 10% when m = 6). When m = 8 (i.e. the most flexible chain), the SBU is a closed-cubane Cu₄I₄ and the emission intensity is medium (Φ_e ∼ 10%). A special case was observed for m = 3, where a co-crystallization of the molecular cluster Cu₄I₄(NCCH₃)₄ is observed in the lattice, which turns out to be quite important for the stability of the network.

A Fused Poly(truncated rhombic dodecahedron)-Containing 3D Coordination Polymer: A Multifunctional Material with Exceptional Properties

The design of new and inexpensive metal-containing functional materials is of great interest. Herein is reported a unique thermochromic near-IR emitting coordination polymer, 3D-[Cu₈I₈(L1)₂]_n, CP2, which is formed when ArS(CH₂)₄SAr (L1, Ar = 4-C₆H₄OMe) reacts with 2 equiv of CuI in EtCN. In MeCN, CP1 ([Cu₄I₄(L1)(MeCN)₂]_n, consisting of an alternating [-Cu₄I₄-L1-Cu₄I₄-L1-]_n chain where the Cu₄I₄ cubane units bear two metal-bound MeCN molecules, is formed. Heat-driven elimination of these MeCN's in solid CP1 also leads to CP2 through a predisposed organization of the Cu₄I₄ units prone to fusion after MeCN eliminations (i.e., a rare case of template effect). The CP2 structure exhibits parallel 1D-(Cu₈I₈)_n chains, (z-axis; designated 1D-[CuI]_n) as secondary building units (SBU) held together by parallel thioether ligands (x,y-axes), forming a nonporous 3D network. The structure of this 1D-[CuI]_n SBU is unprecedented and consists of a series of fused and twisted open Cu₄I₄ cubanes forming a fused poly(truncated rhombic dodecahedron). Unexpectedly, the compact 3D CP2 exhibits a solid-to-solid phase transition at 100 °C and a hysteresis of ∼20 °C. CP1 emits intensively (298 K: λ_emi = 564 nm; Φ_e = 0.35), whereas CP2 presents a strongly red-shifted weaker emission (298 K: λ_emi ∼ 740 nm, Φ_e < 0.0001). Moreover, CP2, which is stable over long periods of time, exhibits thermochromism where the emission intensity of the near-IR band decreases significantly at the benefit of a ligand-centered phosphorescence at 415 nm. Altogether, these properties listed above make CP2 exceptional. The low-energy singlet and triplet excited states have been assigned to ligand/metal-to-ligand charge transfer based on DFT and TD-DFT computations.

Self-Assembly and Aggregation-Induced Emission in Aqueous Media of Responsive Luminescent Copper(I) Coordination Polymer Nanoparticles

Luminescent copper(I)-based compounds have recently attracted much attention since they can reach very high emission quantum yields. Interestingly, Cu(I) clusters can also be emissive, and the extension from small molecules to larger architecture could represent the first step towards novel materials that could be obtained by programming the units to undergo self-assembly. However, for Cu(I) compounds the formation of supramolecular systems is challenging due to the coordinative diversity of copper centers. This works shows that this diversity can be exploited in the construction of responsive systems. In detail, the changes in the emissive profile of different aggregates formed in water by phosphine-thioether copper(I) derivatives were followed. Our results demonstrate that the self-assembly and disassembly of Cu(I)-based coordination polymeric nanoparticles (CPNs) is sensitive to solvent composition. The solvent-induced changes are related to modifications in the coordination sphere of copper at the molecular level, which alters not only the emission profile but also the morphology of the aggregates. Our findings are expected to inspire the construction of smart supramolecular systems based on dynamic coordinative metal centers.

Thermally induced transformation of a Cu4I4-based cluster to a Cu2I2-based cluster under mild conditions

A reaction of 6,6'-bis((benzylthio)methyl)-2,2'-bipyridine (L) with CuI at room temperature led to one Cu4I4-based cluster, which could be thermally transformed to a Cu2I2-based one under mild conditions due to the formation of a Cu-S bond. Along with the structural transformation, remarkable changes in the color and luminescence have been triggered.

From Short-Bite Ligand Assembled Ribbons to Nanosized Networks in Cu(I) Coordination Polymers Built Upon Bis(benzylthio)alkanes (BzS(CH2)nSBz; n = 1-9)

With the objective to establish a correlation between the spacer distance and halide dependence on the structural features of coordination polymers (CPs) assembled by the reaction between CuX salts (X = Cl, Br, I) and dithioether ligands BzS(CH₂)_nSBz (n = 1-9; Bz = benzyl), a series of 26 compounds have been prepared and structurally investigated. A particular attention has been devoted to the design of networks with extremely long and flexible methylene spacer units between the SBz donor sites. Under identical conditions, CuI and CuBr react with BzSCH₂Bz (L1) affording respectively the one-dimensional (1D) CPs {Cu(μ₂-I)₂Cu}(μ-L1)₂]_n (CP1) and {Cu(μ₂-Br)₂Cu}(μ-L1)₂] (CP2), which incorporate Cu(μ₂-X)₂Cu rhomboids as secondary building units (SBUs). The hitherto unknown architecture of two-dimensional (2D) layers obtained with CuCl (CP3) differs from that of CP1 and CP2, which bear inorganic -Cl-Cu-Cl-Cu-Cl- chains interconnected through bridging L1 ligands, thus forming a 2D architecture. The crystallographic characterization of a 1D CP obtained by reacting CuI with 1,3-bis(benzylthio)propane (L2) reveals that [{Cu(μ₂-I)₂Cu}(μ-L2)₂]_n (CP4) contains conventional Cu₂I₂ rhomboids as SBUs. In contrast, unusual isostructural CPs [{Cu(μ₂-X)}(μ₂-L2)]_n (CP5) and (CP6) are obtained with CuX when X = Br and Cl, respectively, in which the isolated Cu atoms are bridged by a single μ₂-Br or μ₂-Cl ion giving rise to infinite [Cu(μ₂-X)Cu]_n ribbons. The crystal structure of the strongly luminescent three-dimensional (3D) polymer [{Cu₄(μ₃-I)₃(μ₄-I)(μ-L3)_1.5]_n (CP7) issued from reacting 2 equiv of CuI with BzS(CH₂)₄SBz (L3) has been redetermined. CP7 features unusual [(Cu₄I₃)(μ₄-I)]_n arrays securing the 3D connectivity. In contrast, mixing CuI with an excess of L3 provides the nonemissive material [{Cu(μ₂-I)₂Cu}(μ-L3)₂]_n (CP8). Treatment of CuBr and CuCl with L3 leads to [{Cu(μ₂-Br)₂Cu}(μ-L3)₂]_n (CP9) and the 0D complex [{Cu(μ₂-Cl)₂Cu}(μ-L3)₂] (D1), respectively. The crystallographic particularity for CP9 is the coexistence of two topological isomers within the unit cell. The first one, CP9-1D, consists of simple 1D ribbons running along the a axis of the unit cell. The second topological isomer, CP9-2D, also consists of [Cu(μ₂-Br)₂Cu] SBUs, but these are interconnected in a 2D manner forming 2D sheets placed perpendicular to the 1D ribbons. Four 2D CPs, namely, [{Cu₄(μ₃-I)₄}(μ-L4)₂]_n (CP10), [{Cu(μ₂-I)₂Cu}(μ-L4)₂]_n (CP11), [{Cu(μ₂-Br)₂Cu}(μ-L4)₂]_n (CP12), and [{Cu(μ₂-Cl)₂Cu}(μ-L4)₂]_n (CP13), stem from the self-assembly process of CuX with BzS(CH₂)₆SBz (L4). A similar series of 2D materials comprising [{Cu₄(μ₃-I)₄}(μ-L5)₂]_n (CP14), [{Cu(μ₂-I)₂Cu}(μ-L5)₂]_n (CP15), [{Cu(μ₂-Br)₂Cu}(μ-L5)₂]_n (CP16), and [{Cu(μ₂-Cl)₂Cu}(μ-L5)₂]_n (CP17) result from the coordination of BzS(CH₂)₇SBz (L5) on CuX. Ligation of CuX with the long-chain ligand BzS(CH₂)₈SBz (L6) allows for the X-ray characterization of the luminescent 2D [{Cu₄(μ₃-I)₄}(μ-L6)₂]_n (CP18) and the isostructural 1D series [{Cu(μ₂-X)₂Cu}(μ-L6)₂]_n CP19 (X = I), CP20 (X = Br) and CP21(X = Cl). Noteworthy, BzS(CH₂)₉SBz (L7) bearing a very flexible nine-atom chain generated the crystalline materials 2D [{Cu₄(μ₃-I)₄}(μ-L7)₂]_n (CP22) and the isostructural 1D series [{Cu(μ₂-X)₂Cu}(μ-L6)₂]_n CP23 (X = I), CP24 (X = Br), and CP25 (X = Cl), featuring nanometric separations between the cubane- or rhomboid-SBUs. This comparative study reveals that the outcome of the reaction of CuX with the shorter ligands BzS(CH₂)_nSBz (n = 1-4) is not predictable. However, with more flexible spacer chains BzS(CH₂)_nSBz (n = 6-9), a clear structural pattern can be established. Using a 1:1 CuX-to-ligand ratio, [{Cu(μ₂-X)₂Cu}(μ-L4-7)₂] CPs are always formed, irrespectively of L4-L7. Employing a 2:1 CuX-to-ligand ratio, only CuI is able to form networks incorporating Cu₄(μ₃-I)₄ clusters as SBUs. All attempts to construct polynuclear cluster using CuBr and CuCl failed. The materials have been furthermore analyzed by powder X-ray diffraction, Raman spectroscopy, and thermogravimetric analysis, and the photophysical properties of the emissive materials have been studied.

Unusual triplet-triplet annihilation in a 3D copper(i) chloride coordination polymer

A new coordination polymer (CP) defined as [Cu₂Cl₂(EtS(CH₂)₄SEt)₄]_n (CP2) was prepared by reacting EtS(CH₂)₄SEt with CuCl in acetonitrile in a 1 : 2 stoichiometric ratio. The X-ray structure reveals formation of non-porous 3D material composed of parallel 2D-[Cu₂Cl₂S₂]_n layers of Cl-bridged Cu₂(μ-Cl)₂ rhomboids assembled by EtS(CH₂)₄SEt ligands. A weak triplet emission (Φ_e < 0.0001) is observed in the 400-500 nm range with τ_e of 0.93 (298 K) and 3.5 ns (77 K) as major components. CP2 is the only 2nd example of emissive thioether/CuCl-containing material and combined DFT/TDDFT computations suggest the presence of lowest energy M/XLCT excited states. Upon increasing the photon flux (i.e. laser power), a triplet-triplet annihilation (TTA) is induced with quenching time constants of 72 ps (k_Q = 1.3 × 10¹⁰ s^-1) and 1.0 ns (k_Q = 7.1 × 10⁸ s^-1) at 298 and 77 K, respectively, proceeding through an excitation energy migration operating via a Dexter process. Two distinct (I_o)^1/2 (I_o = laser power) dependences of the emission intensity are depicted, indicating saturation as the observed emission increases with the excitation flux. These findings differ from that previously reported isomorphous CP [Cu₂Br₂(μ-EtS(CH₂)₄SEt)₄]_n (CP1), which exhibits no TTA behaviour at 77 K, and only one (laser power)² dependence at 298 K. The ∼18-fold increase in k_Q upon warming CP2 from 77 to 298 K indicates a temperature-aided TTA process. The significant difference between the presence (slower, CP2) and absence (CP1) of TTA at 77 K is explained by the larger unit cell contraction of the former upon cooling. This is noticeable by the larger change in inter-rhomboid CuCu separation for CP2.

1,3-Dithianes as Assembling Ligands for the Construction of Copper(I) Coordination Polymers. Investigation of the Impact of the RC(H)S2C3H6 Substituent and Reaction Conditions on the Architecture of the 0D-3D Networks

The parent compound 1,3-dithiane (L1) was reacted with CuI providing the 1D coordination polymer [{Cu(μ₂-I)₂Cu}(μ₂-L1)₂] _n (CP1), an isostructural compound [{Cu(μ₂-Br)₂Cu}(μ₂-L1)₂] _n (CP2) was isolated upon treatment of CuBr with L1. In contrast, treatment of L1 with CuCl results in the formation of 2D polymeric [{Cu(μ₂-Cl)₂Cu}(μ₂-L1)] _n (CP3), in which each sulfur atom acts as a 4-electron donor. The 1D compounds [{Cu(μ₂-X)₂Cu}(μ₂-L2)₂] _n (CP7, X = Br, and CP8, X = Cl) resulting from treatment of 2-methyl-1,3 dithiane (L2) with CuBr and CuCl are isostructural with their CuI homologue [{Cu(μ₂-I)₂Cu}(μ₂-L2)₂] _n (CP5), reported previously. Using CuCN, a 2D CP of composition [{Cu(μ₂-CN)₂Cu}(μ₂-L2)₂] _n (CP9) has been isolated. Complexation of 2-isobutyl-1,3-dithiane (L3) on CuI generates a 2D material [{Cu₃(μ₃-I)(μ₂-I)₂(μ₂-L3)₂}] _n (CP10), incorporating the usual trinuclear μ₃-I-capped Cu clusters as SBUs, whereas 2D-polymeric compounds [{Cu(μ₂-Br)₂Cu}(μ₂-L3)₂] _n (CP11) and [{Cu(μ₂-Cl)₂Cu}(μ₂-L3)₂] _n (CP12) were obtained with CuBr and CuCl. Treatment of 2-Me₃Si-1,3-dithiane (L4) with CuX yields the series [{Cu₂(μ₄-X)(μ₂-X)}(μ₂-L4)] _n (CP13-CP15). With 2-phenyl-1,3-dithiane (L5), the outcome of the reaction with CuI depends on the reaction conditions. Reaction with CuI in MeCN provides a 1D ribbon [{Cu(μ₂-I)₂Cu}(MeCN)₂(μ₂-L5)₂] _n (CP16), whereas treatment of CuI with L5 in hot EtCN yields 2D-polymeric[{Cu₃(μ₃-I)(μ₂-I)₂(μ₂-L5)₂}] _n (CP17). A reversible phase transition from triclinic P1̅ to monoclinic P2₁/ m is observed when recording the structure of CP16 at five different temperatures in the 100-300 K range. Ligand L6 containing a ferrocenyl function at the 2-position was also probed as organometallic dithioether ligand. Reaction of L6 with 1 equiv of CuI produces the 0D dinuclear complex [{Cu(μ₂-I)₂Cu}(η¹-L6)₂(MeCN)₂] (D1), whereas treatment with 2 equiv of CuI affords the novel 1D CP [{Cu(μ₃-I)₂Cu}(μ-L6)] _n (CP18), in which both S atoms of one L6 molecule span two copper centers of the infinite (CuI) _n ribbon. Some selected results of thermal analyses and luminescence measurements are also presented.

The trans-Bis(p-thioetherphenylacetynyl)bis(phosphine)platinum(II) Ligands: A Step towards Predictability and Crystal Design

Two organometallic ligands L1 (trans-[p-MeSC₆H₄C≡C-Pt(PR₃)₂-C≡CC₆H₄SMe; R = Me]) and L2 (R = Et) react with CuX salts (X = Cl, Br, I) in MeCN to form one-dimensional (1D) or two-dimensional (2D) coordination polymers (CPs). The clusters formed with copper halide can either be step cubane Cu₄I₄, rhomboids Cu₂X₂, or simply CuI. The formed CPs with L1, which is less sterically demanding than L2, exhibit a crystallization solvent molecule (MeCN), whereas those formed with L2 do not incorporate MeCN molecules in the lattice. These CPs were characterized by X-ray crystallography, thermogravimetric analysis, IR, Raman, absorption, and emission spectra as well as photophysical measurements in the presence and absence of crystallization MeCN molecules for those CPs with the solvent in the lattice (i.e., [(Cu₄I₄)L1·MeCN] _n (CP1), [(Cu₂Br₂)L1·2MeCN] _n (CP3), and [(Cu₂Cl₂)L1·MeCN] _n (CP5)). The crystallization molecules were removed under vacuum to evaluate the porosity of the materials by Brunauer-Emmett-Teller (N₂ at 77 K). The 2D CP shows a reversible type 1 adsorption isotherm for both CO₂ and N₂, indicative of microporosity, whereas the 1D CPs do not capture more solvent molecules or CO₂.

Isolation of an unusual [Cu6] nanocluster through sequential addition of copper(i) to a polynucleating ligand

Addition of 2 equiv. of CuI to a [Cu]₄ multimetallic complex results in cluster formation leading to the isolation of a rare bicapped tetrahedral [Cu₆I₂] cluster that is stabilised by two conformationally constrained polynucleating ligands.