Paolo P. Mazzeo

Reversible Interconversion between Luminescent Isomeric Metal–Organic Frameworks of [Cu4I4(DABCO)2] (DABCO=1,4‐Diazabicyclo[2.2.2]octane)

The metal-organic frameworks (MOF) of cluster [Cu(4)I(4)(DABCO)(2)] (DABCO=1,4-diazabicyclo[2.2.2]octane) have been prepared and characterized as two different crystalline forms, I and II. Form I is obtained by reaction of DABCO and CuI in aqueous solution or by solvothermal reaction, while form II is obtained by reacting DABCO and CuI in acetonitrile. Their luminescence properties in the solid state have been analyzed at room temperature and at 77 K. MOF II has bright emission with a maximum at 556 nm that shifts bathochromically at low temperature in conjunction with a marked change in the colour of the emission. The emission of MOF I has a maximum at 580 nm and a less pronounced temperature dependence. The peculiar luminescence properties of the two isomers have been interpreted by utilising current knowledge on the excited states properties of Cu(I) cubane clusters. The two isomers exhibit a high degree of porosity and can release the disordered solvent molecules trapped in the channels, whilst preserving the crystal structure. Isomer I can be converted into II on exposure to acetonitrile or methanol vapour, whereas II reverts to I when heated in a closed pan at 250 degrees C.

Solid-state reactivity of copper(I) iodide: luminescent 2D-coordination polymers of CuI with saturated bidentate nitrogen bases

Solid-state reactions of copper(I) iodide with 1,4-diazabicyclo[2.2.2]octane (DABCO) and piperazine in different conditions (kneading, vapour digestion, thermal treatment) resulted in the formation of four new luminescent systems, which have been characterized viaX-ray diffraction and photo-luminescence spectroscopy. At room temperature all compounds exhibit a bright emission in the solid state that markedly shifts bathochromically on cooling to 77 K. The luminescence of the almost isostructural 2D-coordination polymers [Cu2I2(L)2]∞ (L = DABCO, piperazine) exhibits an unusually long lifetime (125 and 53 μs, respectively), which becomes even longer (609 and 420 μs, respectively) at 77 K.

Tuning the colour and efficiency in OLEDs by using amorphous or polycrystalline emitting layers

We investigated the nature of the emissive states in newly synthesized cyclometallated Pt complexes containing a chelating 2-pyridyl tetrazolate (2-PTZ) ligand, namely Pt(ppy)(2-PTZ), and Pt(F2ppy)(2-PTZ) as a solid-state phosphor, by examining their structural properties versus their phosphorescence (PH) and electroluminescence (EL) characteristics. It is found that the observed tuning of both PH and EL spectra, their red shift and shortening decay with increasing concentration in the complex blends are due to the competition between three emissive states: monomer, excimer and dimer. The pure dimer emission appeared in neat films, reaching a high PH quantum yield of about 75% and external EL efficiency approaching 10% for the OLED based on the neat Pt(F2ppy)(2-PTZ) complex film as an emitting layer (EML). X-ray diffraction proved the high structural order of the latter thin film. These findings have a direct impact on the design of a new OLED generation based on single phosphor multi-emission controlled by the structural order degree of the EML.

Phosphorescence quantum yield enhanced by intermolecular hydrogen bonds in Cu4I4 clusters in the solid state.

Organo-copper(i) halide complexes with a Cu4I4 cubane core and cyclic amines as ligands have been synthesized and their crystal structures have been defined. Their solid state photophysical properties have been measured and correlated with the crystal structure and packing. A unique and remarkably high luminescence quantum yield (76%) has been measured for one of the complexes having the cubane clusters arranged in a columnar structure and held together by N-HI hydrogen bonds. This high luminescence quantum yield is correlated with a slow radiationless deactivation rate of the excited state and suggests a rather strong enhancement of the cubane core rigidity bestowed by the hydrogen bond pattern. Some preliminary thin film deposition experiments show that these compounds could be considered to be good candidates for applications in electroluminescent devices because of their bright luminescence, low cost and relatively easy synthesis processes.

White luminescence achieved by a multiple thermochromic emission in a hybrid organic–inorganic compound based on 3-picolylamine and copper(I) iodide

Three copper(i) complexes have been obtained by the reaction of CuI with 3-picolylamine in acetonitrile solution and characterized by X-ray powder diffraction, both from synchrotron and laboratory radiation. Photophysical investigations in the solid state revealed highly efficient thermally-activated delayed fluorescence (TADF) with photoluminescence quantum yields (PLQYs) up to 18%. Notably, the complex [Cu2I2(3pica)]∞ displays a strong luminescence thermochromism due to the presence of both 1,3(X + M)LCT excited states and a lower-lying cluster-centered (3CC) one, leading to multiple emission at room temperature; as a result, a white luminescence is achieved with a PLQY of 4.5%.

SCSC metal–organic framework transmetallation: a successful case

Metal-Organic Frameworks (MOFs) are well known porous materials, extensively studied in the last decade and widely used in manifold applications such as gas absorption/storage, catalysis, controlled guest-delivery and many other [1]. A desired MOFs could be ex novo synthesized by direct reaction between metallic precursor and organic bridging ligand(s), usually via solvothermal reaction or, alternatively, it could be obtained via post-synthetic modification from an ad hoc starting framework [2]. Up to the present, the latter method is rarely described in the literature and only few successful examples of metallic node exchange have been reported. A single-crystal-to-single-crystal Zn-to-Co metathesis is here proposed in a post-synthetic metal-organic framework modification performed in mild and reproducible experimental conditions. The modified MOF shows a complete Zn-to-Co transmetallation and results to be isostructural with respect to the starting framework, which only acted as a mould. The new Co-MOF has been widely characterized via SR-SCXRD, SR-XRPD and ESEM analyses: results will be here described in details and compared with the native MOF. In addition, the transmetallated as well as the native MOFs were then exposed to different environmental conditions: they both results to be temperature sensitive going through a similar reversible phase transformation which has been characterized via SCXRD. A deep structure analysis will be also proposed with a description of the hypothesis of the active role of solvent/counter-ion in the transmetallation process. An ex novo approach was also followed to obtain the Co-MOF via direct metal-ligand reaction but none of the synthetic routes proposed has given the same structure obtained via PSM of the native Zn-MOF; on the other hand, a non-porous more stable 1D coordination polymer was obtained. A particular phase transformation of the 1D coordination polymer in which temperature and solvent play a synergistic role is also described. [1] H. Furukawa et al. (2013). Science 341, 6149, 974 [2] M. Lalonde et al. (2013) J. Mater. Chem. A, 1, 5453

Adding flavours to our MOFs

MOFs are highly versatile materials that are made by connecting metal ions with prefixed coordination geometry with rigid ligands acting as spacers, hence affording three-dimensional coordination polymers. The accurate design of the building units allows to design porous MOFs, obtaining cavities of considerable size, which usually accommodate loosely bound solvent molecules. The scope of this work is to find a systematic way to embed small molecular aggregates inside porous crystalline materials, with the multiple aims to explore the structural aspects of nanoconfinement, and of the stabilization of guest molecules inside the cavities of the structure. The feasibility of this approach stems from both the recently developed crystalline-sponge method that describes the structural determination of single molecules trapped inside a microporous framework [1]. The guest that we are here considering for inclusion in MOFs pores are some important compounds for the human health and nutrition which occur as liquids at room temperature, such as eugenol (a major component of clove oil), carvacrol (extracted from the oregano essential oil), thymol (present in the oil of thyme); these are part of a naturally occurring class of compounds known as biocides, with strong antimicrobial attributes. In order to embed these guest in crystalline materials we needed to carefully tailor the MOF cavities; thus we designed a small library of new ligands which could afford flexible MOF architectures. We were able to characterize a series of new materials which are able to release the guests in a controlled way, opening the way to engineer additives for active food packaging. In particular the structure and inclusion properties of PUM168 will be discussed. This new heteroleptic MOF shows triple interpenetration, and nevertheless can accommodate all the guests belonging to the oil family by exhibiting a remarkable flexibility; the selectivity towards mixtures will also be discussed. In the Figure the triple interpenetration of the MOF architecture is shown on the left, and an example of MOF...guest interaction with carvacrol via hydrogen bonds is shown on the right. [1] Inokuma Y., et al., (2013), Nature, 495, 461–466