From Blue to Green: Fine-Tuning of Photoluminescence and Electrochemiluminescence in Bifunctional Organic Dyes.

We describe the synthesis, computational analysis, photophysics, electrochemistry and electrochemiluminescence (ECL) of a series of compounds formed of two triphenylamines linked by a fluorene or spirobifluorene bridge. The phenylamine moieties were modified at the para-position of the two external rings by electron-withdrawing or electron-donating substituents. These modifications allowed for fine-tuning of the photoluminescence (PL) and ECL emission from blue to green, with an overall wavelength span of 73 (PL) and 67 (ECL) nm, respectively. For all compounds, we observed a very high PL quantum yield (79-89%) and formation of stable radical ions. The ECL properties were investigated by direct annihilation of the electrogenerated radical anion and radical cation. The radical-ion annihilation process is very efficient and causes an intense greenish-blue ECL emission, easily observable even by naked eye, with quantum yield higher than the standard 9,10-diphenylanthracene. The ECL spectra show one single band that almost matches the PL band. Because the energy of the annihilation reaction is higher than that required to form the singlet excited state, the S-route is considered the favored pathway followed by the ECL process in these molecules. All these features point to this type of molecular system as promising for ECL applications.

Proton in a Confined Space: Structural Studies of H+ ⊂Crypt-111 Iodide and Some Halogen-Bonded Derivatives.

Experimental observations and modeling data are reported on the solid-state structural features of crypt- 111⋅HI (1) and the three-component co-crystals that 1 forms with α,ω-diiodoperfluoroalkanes 2 a-d. X-ray analyses indicate that, in all five systems and at low temperature, the caged proton is covalently bonded to a single nitrogen atom and is involved in a network of intramolecular hydrogen bonds. In contrast, room-temperature, solid-state 15 N NMR spectroscopy suggests magnetic equivalency of the two N atoms of crypt-111 in both 1 and co-crystals of 1 with diiodoperfluoroalkanes. Computational modelling confirms that the acidic hydrogen inside the cavity preferentially sits along the internitrogen axis and is covalently bonded to one nitrogen. The computed energy barriers suggest that the hopping of the encapsulated proton between the two N atoms of the cage can occur in the halogen-bonded co-crystals of 1⋅2, but it is hardly possible in the pure H+ ⊂crypt-111 iodide 1. These different pictures of the proton position and dynamics obtained by using different techniques and conditions confirm the unique characteristics of the confined space within the cavity of crypr-111 and the distinctive features of processes occurring therein.

The Role of Ligand Topology in the Decomplexation of Luminescent Lanthanide Complexes by Dipicolinic Acid.

In this study, we present the aqueous solution behavior of two luminescent lanthanide antenna complexes (Eu3+ ⊂1, Dy3+ ⊂9) with different ligand topologies in the presence of dipicolinic acid (DPA, pyridine-2,6-dicarboxylic acid). Macrocyclic (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, DO3A, 9) and acyclic (1,4,7-triazaheptane-1,1,7,7-tetraacetic acid, DTTA, 1) ligands have been selected to form a ratiometric pair in which Dy3+ ⊂9 acts as a reference and Eu3+ ⊂1 acts as a probe for the recognition of DPA. The pair of luminescent complexes in water reveals the capability to work as a DPA luminescent sensor. The change of emission intensity of Eu3+ indicates the occurrence of a new sensitization path for the lanthanide cation through excitation of DPA. NMR evidence implies the presence of free 1 and mass spectrometry shows the formation of emitting [EuDPA2 ]- as a result of a ligand exchange reaction.

Efficient luminescence from fluorene- and spirobifluorene-based lanthanide complexes upon near-visible irradiation.

We describe herein the synthesis and photophysical characterization of new lanthanide complexes that consist of a (9,9-dimethylfluoren-2-yl)-2-oxoethyl or a (9,9'-spirobifluoren-2-yl)-2-oxoethyl unit as the antenna, covalently linked to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) unit as the Ln(3+) (Gd(3+), Eu(3+), Sm(3+), Tb(3+), Dy(3+)) coordination site. We were able to translate the spectroscopic properties of the innovative bipartite ligands into the formation of highly luminescent europium complexes that exhibit efficient emission (ϕ(se)>0.1) upon sensitization in the near-visible region, that is, with an excitation wavelength above 350 nm. The luminescence of the Eu(3+) complexes is clearly detectable at concentrations as low as 10 pM. Furthermore, the structural organization of these bipartite ligands makes the complexes highly soluble in aqueous solutions and chemically stable over time.

Double level selection in a constitutional dynamic library of coordination driven supramolecular polygons.

A constitutional dynamic library (CDL) of Cu(II) metallo-supramolecular polygons has been studied as a bench test to examine an interesting selection case based on molecular recognition. Sorting of the CDL polygons is achieved through a proper guest that is hosted into the triangular metallo-macrocycle constituent. Two selection mechanisms are observed, a guest induced path and a guest templated self-assembly (virtual library approach). Remarkably, the triangular host can accommodate several guests with a degree of selectivity ranging from ∼1 to ∼10(4) for all possible guest pairs. A double level selection operates: guests drive the CDL toward the triangular polygon, and, at the same time, this is able to pick a specific guest from a set of competitive molecules, according to a selectivity-affinity correlation. Association constants of the host-guest systems have been determined. Guest competition and exchange studies have been analyzed through variable temperature UV-Vis absorption spectroscopy and single crystal X-ray diffraction studies. Molecular structures and electronic properties of the triangular polygon and of the host-guest systems also have been studied by means of all electrons density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations including dispersive contributions. DFT outcomes ultimately indicate the dispersive nature of the host-guest interactions, while TDDFT results allow a thorough assignment of the host and host-guests spectral features.

Tuning the nature of the fluorescent state: a substituted polycondensed dye as a case study.

An extensive spectroscopic analysis is presented of an elongated polycondensed dye with a donor-acceptor substitution. The charge-transfer (CT) state, polarized along the long molecular axis, is close in energy to a local excitation (LE) of the polycondensed system, roughly polarized along the short molecular axis, which makes this system particularly suitable to investigate the subtle LE/CT interplay. An essential-state model is presented that quantitatively reproduces absorption and fluorescence spectra, as well as fluorescence emission and excitation anisotropy spectra collected in solvents of different polarity and viscosity, which sets a sound basis for the understanding of how solvent polarity and solvent relaxation affect the nature of low-lying excitations. The markedly different fluorescence emission and excitation anisotropy spectra measured in glassy and liquid polar solvents unambiguously demonstrate the major role played by solvent relaxation in the definition of fluorescence properties of the dye.

Efficient greenish blue electrochemiluminescence from fluorene and spirobifluorene derivatives.

The spectroscopic and electrochemical behavior as well as electrogenerated chemiluminescence (ECL) of a series of donor-π-donor derivatives bearing triphenylamine groups as donor connected to a fluorene, 2,7-bis-(4-(N,N-diphenylamino)phen-1-yl)-9,9'-dimethylfluorene (1), or spirobifluorene core, 2,7-bis-(4-(N,N-diphenylamino)phen-1-yl)-9,9'-spirobifluorene (2) and 2,2',7,7'-tetrakis(4-(N,N-diphenylamino)phen-1-yl)-9,9'-spirobifluorene (3), were investigated. Besides a high photoluminescence (PL) quantum yield in solution (between 81 and 87%), an efficient radical ions annihilation process induces intense greenish blue ECL emission that could be seen with the naked eye. Only the tetrasubstituted spirobifluorene derivative (compound 3) shows weak ECL obtained by a direct annihilation mechanism. Because the energy of the annihilation reaction is higher than the energy required to form the singlet excited state, the S-route could be considered the pathway followed by the ECL process in these molecules. The ECL emissions recorded by direct ion-ion annihilation show two bands compared to the single structureless PL band. The ECL spectra obtained by a coreactant approach using benzoylperoxide as a coreagent show no differences relative to that produced by annihilation, except for an increasing of ECL intensity for all compounds.

Dinuclear Cu(II) complexes of isomeric bis-(3-acetylacetonate)benzene ligands: synthesis, structure, and magnetic properties.

Highly versatile coordinating ligands are designed and synthesized with two ß-diketonate groups linked at the carbon 3 through a phenyl ring. The rigid aromatic spacer is introduced in the molecules to orient the two acetylacetone units along different angles and coordination vectors. The resulting para, meta, and ortho bis-(3-acetylacetonate)benzene ligands show efficient chelating properties toward Cu(II) ions. In the presence of 2,2'-bipyridine, they promptly react and yield three dimers, 1, 2, and 3, with the bis-acetylacetonate unit in bridging position between two metal centers. X-ray single crystal diffraction shows that the compounds form supramolecular chains in the solid state because of intermolecular interactions. Each of the dinuclear complexes shows a magnetic behavior which is determined by the combination of structural parameters and spin polarization effects. Notably, the para derivative (1) displays a moderate antiferromagnetic coupling (J = -3.3 cm(-1)) along a remarkably long Cu···Cu distance (12.30 Å).

Controlled density patterning of tolylterpyridine-tagged oligonucleotides.

An efficient surface anchoring strategy of tolylterpyridine-tagged DNA single strands (ssDNA-ttpy) synthesized on gold electrodes is reported. The method is based on exchange reactions between Fe(II)bis-terpyridine complexed SAMs and ssDNA-ttpy, and allows efficient hybrydization of the cDNA strands. Moreover, by using low-current focused ion beam lithography, micropatterned arrays are obtained, homogeneously covered with ssDNA-ttpy. The surface adsorption kinetics of ssDNA-ttpy, as well as its hybridization efficiency, was monitored by in situ quartz crystal microbalance with dissipation monitoring (QCM-D) technique. The effective confinement of the ssDNA-ttpy at the micrometer level has been monitored by time of flight secondary ion mass spectrometry (ToF-SIMS) and ellipsometric surface imaging experiments, providing laterally resolved chemical and topographic mapping.

Luminescent Ir(III) complex exclusively made of polypyridine ligands capable of intercalating into calf-thymus DNA.

Efficient intercalation of a luminescent Ir(III) complex exclusively made of polypyridine ligands in natural and synthetic biopolymers is reported for the first time. The emission of the complex is largely enhanced in the presence of [poly(dA-dT)(2)] and strongly quenched in the presence of [poly(dG-dC)(2)]. By comparing the emission decays in DNA and in synthetic polynucleotides, it is proposed that the emission quenching of the title compound by guanine residues in DNA is no longer effective over a distance of four dA-dT base pairs.

Highly conductive approximately 40-nm-long molecular wires assembled by stepwise incorporation of metal centres.

One of the main goals of molecular electronics is to achieve electronic functions from devices consisting of tailored organic molecules connecting two metal electrodes. The fabrication of nanometre-scale spaced electrodes still results in expensive, and often scarcely reproducible, devices. On the other hand, the 'conductance' of long organic molecules--generally dominated by the tunnelling mechanism--is very poor. Here, we show that by incorporating a large number of metal centres into rigid molecular backbones we can obtain very long (up to 40 nm) and highly 'conductive' molecular wires. The metal-centre molecular wires are assembled in situ on metal surfaces via a sequential stepwise coordination of metal ions by terpyridine-based ligands. They form highly ordered molecular films of elevated mechanical robustness. The electrical properties, characterized by a junction based on Hg electrodes, indicate that the 'conductance' of these metal-centre molecular wires does not decrease significantly even for very long molecular wires, and depends on the nature of the incorporated redox centre. The outstanding electrical and mechanical characteristics of these easy-to-assemble molecular systems open the door to a new generation of molecular wires, able to bridge large-gap electrodes, and to form robust films for organic electronics.

Functionalization of oxide surfaces by terpyridine phosphonate ligands: surface reactions and anchoring geometry.

A strategy for creating a general-purposes surface functionalization platform is reported, based on direct attachment of phosphate groups onto hydroxylated surfaces and subsequent formation of a terpyridine-based monolayer. Such a platform is suitable for the construction, onto technologically relevant oxide surfaces, of single- and multilayer structures of interest in technological applications. In particular, the paper describes the successful attachment of 4-(2,2':6',2''-terpyridine-4-yl)benzenephosphonic acid (1, PPTP) onto a SiO(2) surface previously functionalized by means of Zr-phosphate groups. Two alternative anchoring strategies of the PPTP were explored: (i) a direct one-step way, implying no protection of terpyridinic functionality, and (ii) a three-step way, implying protection and successive deprotection of this group. It was found that, in the first case, the PPTP ligand anchoring to the Zr-containing phosphate layer takes place by means of terpyridinic group. At variance of this, in the second case, due to the protection of the terpyridinic functionality, the anchoring process takes place through the phosphonic group, making the terpyridinic moiety available for further reactions, i.e., multilayer constructs. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to study the functionalized surfaces, providing information on coverage, chemical structure, and stoichiometry of the various functionalized layers and, among the others, clear evidence of the PPTP linkage and orientation.

Synthesis, characterization, absorption spectra, and luminescence properties of multinuclear species made of Ru(II) and Ir(III) chromophores.

A series of new mixed-metal Ru(II)-Ir(III) trinuclear complexes have been prepared and characterized, together with their mononuclear parents and a series of closely related dinuclear and trinuclear homometallic Ru(II) and Ir(III) species, and their absorption spectra and luminescence properties (both at 77 K in rigid matrix and at room temperature in fluid solution) have been studied. The absorption spectra and luminescence properties of the Ru(II) species and subunits are dominated by metal-to-ligand charge-transfer (MLCT) transitions and excited states, whereas ligand centered (LC) transitions and excited states govern the spectroscopic and photophysical properties of most of the Ir(III) species here studied, with MLCT states playing an important role when cyclometalated Ir(III) subunits are present. Each metal-based subunit retains in the multinuclear arrays its own spectroscopic properties, but in the case of the mixed Ru-Ir species an efficient, additional decay channel is opened for the excited states involving the Ir-centered subunits, that is, photoinduced energy transfer to the lower-lying MLCT state(s) involving the Ru centers.

Enhancing the efficiency of two-photon absorption by metal coordination.

The intensity of the two-photon absorption (TPA) spectrum of a terpyridine ligand acting as a D-pi-A chromophore (D = donor and A = acceptor) is enhanced by a factor of about 2 upon coordination to ZnCl(2). Based on an analysis of linear absorption and fluorescence spectra of both the ligand and its Zn(II) complex, we have defined essential-state models for the two species. Linear and TPA spectra of the ligand are well reproduced in terms of a two-state model accounting for the D-pi-A <--> D(+)-pi-A(-) charge resonance. However, the enhancement of the TPA response of its Zn(II) complex can only be understood by extending the model to account for the active role of the "ZnCl(2)" moiety acting as a virtual A(v) acceptor group of a D-pi-AA(v) structure. The virtual D(+)AA(v)(-) state of the relevant three-state model has negligible weight in the ground state but contributes to the first excited state. The resulting increase of the excited-state dipole moment is responsible for the enhancement of the TPA cross section, and also explains the increase of the second order nonlinear optical response upon coordination.

A Cu(ii) metallocycle for the reversible self-assembly of coordination-driven polyrotaxane-like architectures.

We report the design and synthesis of a Cu(ii) metallocycle (1) and use the possibility to expand the Cu(ii) coordination sphere to self-assemble mechanically interlocked species via interpenetration. Metallocycle 1 can be used as a platform to reversibly assemble a 1D + 1D → 1D coordination-driven polyrotaxane (3), where 1 acts as the hosting ring as well as the stopper, and 4,4'-bipyridine is the guest-axle. A coordinating solvent can substitute the 4,4'-bipyridine axle to disassemble the polyrotaxane (3 → 2) that is easily restored by further adding 4,4'-bipyridine (2 → 3). Other polyrotaxanes can be isolated by reacting 1 with pyridine (4) and phenylpyridine (5). Interconversion among the presented species is demonstrated and ensured by the open position of each copper center in platform 1.

A highly fluorescent water soluble spirobifluorene dye with a large Stokes shift: synthesis, characterization and bio-applications.

The first water-soluble spirobifluorene derivative has been synthesized, which exhibits high fluorescence quantum yield and a large Stokes shift (>100 nm). Proteins induce changes in the emission color, allowing to reach the nanomolar detection limit. Cellular uptake and cytotoxicity studies in living cells revealed its biocompatibility, indicating potential application for live cell imaging.

Photophysical properties and tunable colour changes of silica single layers doped with lanthanide(III) complexes.

Highly stable Eu(III) and Tb(III) complexes, emitting in the red and green visible regions, respectively, have been anchored onto a single SiO(2) transparent layer, yielding ca. 40 nm thick films; this allows high loading of tailored proportions of the red and green emitters within the films and results in highly uniform and easily colour-tunable luminescent layers.

The unexpected similar second-order NLO response for nearly planar and largely twisted push-pull stilbazole chromophores: EFISH and theoretical TD-DFT evidence.

The molecular second-order nonlinear optical properties of planar (E)-4-[2-(4-(N-methyl-N-hexadecylaminophenyl)ethenyl]pyridine (L1) and [cis-Ir(CO)2ClL1] and of the significantly twisted new chromophores (E)-4-[(5,6,7,8-tetrahydro-5-isoquinolylidene)methyl]-N-methyl-N-hexadecylaniline (L2) and [cis-Ir(CO)2ClL2] are reported, evidencing for the first time that planarity of a conventional (donor)(pi-bridge)(acceptor) structure is not compulsory to reach a large NLO response.

Two-color luminescence from a tetranuclear Ir(III)/Ru(II) complex.

A new tetranuclear compound containing Ru(II) and Ir(III) polypyridine subunits exhibits two independent emissions at room temperature, as a consequence of weak interchromophoric coupling; in contrast, at 77 K energy transfer from Ir-based chromophores to the Ru-based ones is quantitative.