Stefano Stagni

Essential role of the ancillary ligand in the color tuning of iridium tetrazolate complexes.

We report on the synthesis and physical chemical characterization of a class of heteroleptic mononuclear cyclometalated bis(phenylpyridine)iridium(III) complexes with tetrazolate chelate ligands, such as the deprotonated form of 2-(1 H-tetrazol-5-yl)pyridine ( PyTzH), 2-(1 H-tetrazol-5-yl)pyrazine ( PzTzH), and 5-bromo-2-(1 H-tetrazol-5-yl)pyridine ( BrPyTzH). The electrochemical and photophysical investigations of the resulting iridium(III) complexes revealed a rather wide span of redox and emission properties as a consequence of the nature of the ancillary tetrazolate ligand. In particular, within a series of the three neutral species, the emission observed changes from the blue-green of the pyridyltetrazolate complex to the red of that containing the pyrazinyltetrazolate ligand. The bromo-containing species, despite it displaying poor photophysical performances, is a synthetically attractive building block for the construction of polymetallic architectures. Moreover, the investigation of the reactivity toward electrophiles of one of the neutral mononuclear complexes, by methylation of the coordinated tetrazolate ligand, has also allowed further tuning of the electronic properties. In the latter case, the emission color tuning is also associated with a simple method for the conversion of a neutral species, a potentially triplet emitter for organic light-emitting devices, into the corresponding methylated cation, which might be used as a dopant for light-emitting electrochemical cell type devices or as a marker for biological labeling.

Ultrasound-promoted hydrogelation of terpyridine derivatives

Terpyridine derivatives proved to be versatile hydrogelators when subjected to ultrasound irradiation in their bis-protonated form. The chelating ability of the terpyridine ligands allows for the capture of metal cations resulting in stable gels with tunable emissive properties.

Photophysical and Photochemical Trends in Tricarbonyl Rhenium(I) N-Heterocyclic Carbene Complexes

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed.

Modulation of the organelle specificity in Re(I) tetrazolato complexes leads to labeling of lipid droplets

The biological behaviour in terms of cellular incubation and organelle specificity for two complexes of the type fac-[Re(CO)3(phen)L], where phen is 1,10-phenanthroline and L is either 3-pyridyltetrazolate or 4-cyanophenyltetrazolate, are herein investigated. The emission signal detected from the live insect Drosophila and human cell lines, generated by exploiting two-photon excitation at 830 nm to reduce cellular damage and autofluorescence, suggests photophysical properties that are analogous to those measured from dilute solutions, meaning that the complexes remain intact within the cellular environment. Moreover, the rhenium complex linked to 4-cyanophenyltetrazolate shows high specificity for the lipid droplets, whereas the complex bound to 3-pyridyltetrazolate tends to localise within the lysosomes. This differential localisation implies that in these complexes, organelle specificity can be achieved and manipulated by simple functional group transformations thus avoiding more complex bioconjugation strategies. More importantly, these results highlight the first example of phosphorescent labeling of the lipid droplets, whose important cellular functions have been recently highlighted along with the fact that their role in the metabolism of healthy and diseased cells has not been fully elucidated.

Self-Assembly of [Pt3n(CO)6n]2− (n = 4−8) Carbonyl Clusters: from Molecules to Conducting Molecular Metal Wires

A comprehensive study discussing the different parameters that influence the self-assembly of [Pt(3n)(CO)(6n)](2-) (n = 4-8) clusters with miscellaneous mono- and dications into 0-D, 1-D, 2-D, and 3-D materials is herein reported. As an unexpected bonus, the use of Ru(II) dications allowed the first structural characterization of the previously unknown [Pt(21)(CO)(42)](2-) dianion. 0-D structures, which contain isolated ions, are electrical insulators in solid form. Conversely, as soon as infinite chains of clusters are formed, the electrical resistivity, measured in pressed pellets, decreases to 10(5)-10(6), 10(4), and 10(2) ohms cm for discontinuous, semicontinuous, and continuous chains, respectively. Therefore, the resemblance of these materials to molecular metal wires is not only morphological but also functional. Preliminary results of possible self-assembly phenomena in a solution of [Pt(15)(CO)(30)](2-) and [Pt(18)(CO)(36)](2-) according to dynamic light scattering experiments are also reported.

Synthesis and reactivity of a new Fe(II) 5-(4-pyridyl)-tetrazolate complex and X-ray structure of its doubly protonated derivative.

Abstract The synthesis of the new Fe(II) complex [CpFe(CO)2(N4C–C5H4N)] (2) is described. 1H- and 13C-NMR spectroscopy data of (2) indicate the presence of interannular conjugation in the pyridyl-tetrazolate ligand, implying coplanarity between the two rings. Addition of electrophiles to 2 resulted in the formation of cationic complexes such as [CpFe(CO)2(4-MeN4C–C5H4N)][O3SCF3] (3), [CpFe(CO)2(4-HN4C–C5H4N)][O3SCF3] (4) and of the doubly protonated complex [CpFe(CO)2(4-HN4C–C5H4N–H)][O3SCF3]2 (5). In all cases, the out-of-plane rotation of the pyridyl ring occurred as a consequence of the quaternization of the N-4 of tetrazole ring. X-ray structure of complex 5 indicates a torsion angle of 20.9(2)° between the aromatic rings. Protonation reactions were found to be reversible and complexes 4–5 were easily converted into the starting complex 2 by addition of a base.

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.

A new tetraarylcyclopentadienone based low molecular weight gelator: synthesis, self-assembly properties and anion recognition

A new class of tetraarylcyclopentadienones bearing 3-hydroxy-1-propynyl substituents has been synthesized. One of them, 3,4-bis (4-(3-hydroxy-3-methylbut-1-ynyl) phenyl)-2,5-diphenylcyclopenta-2,4-dienone, exhibits pronounced aggregation properties in various organic solvents responding to thermal and ultrasound stimuli and represents the first example of a tetraarylcyclopentadienone based low molecular weight organogelator. The hydroxydimethyl group on the ethynyl substituent proved to be essential to perform the gelation process. The 1H NMR analysis and FT-IR spectroscopy suggested that the intermolecular π–π and hydrogen bonding interactions of the gelator with the solvent are the main driving forces for the supramolecular assembly. The SEM images of xerogels show the characteristic gelation morphologies of 3D fibrous network structures. Fluorescence and UV/Vis absorption studies provided more information to define the molecular packing model in the gelation state. In addition the obtained gels show selective response to the fluoride anion.

Enhanced deep-blue emission from Pt(II) complexes bound to 2-pyridyltetrazolate and an ortho-xylene-linked bis(NHC)cyclophane

The coordination of 2-pyridyltetrazolate and ortho-xylene-linked bis(NHC)cyclophane to Pt(II) yielded a novel complex characterised by enhanced pure deep-blue emission, whose intensity can be modulated via methylation of the tetrazole ring.

Investigating Intracellular Localisation and Cytotoxicity Trends for Neutral and Cationic Iridium Tetrazolato Complexes in Live Cells

A family of five neutral cyclometalated iridium(III) tetrazolato complexes and their methylated cationic analogues have been synthesised and characterised. The complexes are distinguished by variations of the substituents or degree of π conjugation on either the phenylpyridine or tetrazolato ligands. The photophysical properties of these species have been evaluated in organic and aqueous media, revealing predominantly a solvatochromic emission originating from mixed metal-to-ligand and ligand-to-ligand charge transfer excited states of triplet multiplicity. These emissions are characterised by typically long excited-state lifetimes (∼hundreds of ns), and quantum yields around 5-10 % in aqueous media. Methylation of the complexes caused a systematic red-shift of the emission profiles. The behaviour and the effects of the different complexes were then examined in cells. The neutral species localised mostly in the endoplasmic reticulum and lipid droplets, whereas the majority of the cationic complexes localised in the mitochondria. The amount of complexes found within cells does not depend on lipophilicity, which potentially suggests diverse uptake mechanisms. Methylated analogues were found to be more cytotoxic compared to the neutral species, a behaviour that might to be linked to a combination of uptake and intracellular localisation.