Massimo La Deda

Electrofluorochromism in π-conjugated ionic liquid crystals

Materials in which photoluminescence is modulated by redox processes are known as electrofluorochromic. Intrinsically switchable fluorophores, incorporating both redox and fluorescent moieties, could be ideal electrofluorochromic materials if they possess high fluorescence quantum yields in at least one of their redox states. Fluorescent liquid crystals with redox active centres could combine the above requirements with the advantage to work in bulk anisotropic phases. However, electrofluorochromic liquid crystals have not been reported yet because their synthesis is challenging due to aggregation-caused fluorescent quenching. Here we show the first examples of electrofluorochromic π-conjugated ionic liquid crystals based on thienoviologens. These ordered materials, combining ionic and electronic functions, are highly fluorescence in the bulk state (quantum yield>60%). Their direct electrochemical reduction leads to fast and reversible bulk electrofluorochromic response in both columnar and smectic phases allowing for fluorescence intensity modulation and colour tuning.

Synthesis and characterization of a homologous series of mononuclear palladium complexes containing different cyclometalated ligands

Abstract New mononuclear palladium(II) complexes formed by with 2-hydroxy-4-( n -hexyloxybenzylidene)-4′- n -hexyaniline ( HL ) and 2-phenylpyridine ( I ), benzo[h]quinoline ( II ), azobenzene ( III ), 2-benzoylpyridine ( IV ) or phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone ( V ) have been synthesized and characterized by analytical and spectroscopic methods and the single-crystal structures of [(IVa)Pd(L)] and [(Va)Pd(L)] have been established. The spectroscopic and diffractrometric data account for molecular structures wherein the palladium(II) center is part of two chelate rings involving HL and one of the I – V ligands which form a five-member N,C{[(Ia)PdL], [(IIa)Pd(L)] and [(IIIa)Pd(L)]}, a six-member N,C{[(IVa)Pd(L)]}, or a six-member N , N ′{[(Va)Pd(L)]} metallacycle. The electronic spectra of [(IIIa)Pd(L)] and [(Va)Pd(L)] show an absorption maxima, at 495 ( e ∼3×10 3 M −1 cm −1 ) and 531 nm ( e ∼2×10 4 M −1 cm −1 ), respectively, which results are significantly affected by the polarity of the solvent. These chromophores should therefore be of interest for the preparation of nonlinear optical materials.

Synthesis and spectroscopic characterization of organometallic chromophores for photoluminescent materials: cyclopalladated complexes

Abstract A homologous series of isostructural and isoelectronic palladium acetylacetonato complexes containing different cyclometalated ligands HL (HL=2-phenylpyridine H(PhPy), benzo[h]quinoline H(BhQ), 9-diethylamino-benzo[a]phenoxazin-5-one H(NR), azobenzene H(Azo), 5-[(4′-methyl)-phenylazo]-8-methoxyquinoline H(QAzo), 2-benzoylpyridine H(BKPy), phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone, H(BHPy)) has been prepared. The photophysical characterization of the newly synthesized products shows that many of them are luminescent in solution at room temperature. The best performances are exhibited by the H(NR) derivative (Φem=0.23, λem=660, in dichloromethane solution), which improves the emission features displayed by the unmetalated ligand (Φem=0.13, λem=602, in dichloromethane solution). H(NR) is a well known laser dye commonly named nile red and to the best of our knowledge [(NR)Pd(acac)] is the first example reported up to now of its incorporation in a transition metal complex. Interestingly, the results of the present investigation prove that “[(NR)Pd]” can be an useful building block for the preparation of luminescent organometallic complexes.

Dispersed and encapsulated gain medium in plasmonic nanoparticles: a multipronged approach to mitigate optical losses.

The performance of all metamaterial-based applications is significantly limited by the inherent and strong energy dissipation present in metals, especially in the visible range. In fact, these materials suffer from rather strong damping of the plasmon fields which can become obstructive for most optical and photonic applications. Therefore, eliminating losses in optical metamaterials is critical for enabling their numerous potential applications. We experimentally demonstrate that the incorporation of gain material (fluorophores) in the high-local-field areas of a metamaterial subunits (gold core/silica shell nanoparticles) makes it possible to induce resonant energy transfer processes from gain units to plasmonic nanoparticles. A comparison between gain-assisted (nanoparticle-dye dispersion) and gain-functionalized (dye encapsulated into the shell) systems is reported. Fluorescence quenching and time-resolved spectroscopy along with modification of Rayleigh scattering and transmission of a probe beam as a function of impinging energy are key evidence of the strong coupling occurring between NPs and gain medium. The multipronged approach used to compensate losses in these metal-based subunits permits one to obtain important advances in materials science and paves the way toward further promising scientific research aimed to enable the wide range of electromagnetic properties of optical metamaterials.

Photo-sensitive liquid crystals for optically controlled diffraction gratings

We report on the fabrication and characterization of diffraction gratings based on a polymeric microstructure including a light responsive material, which enables an all-optical control of the diffraction of light. The enhanced performances of a mesogenic azo dye are combined with the outstanding properties of reconfigurability and fluidity of nematic liquid crystals; the mesogenic affinity between the two substances enables realization of a photosensitive liquid crystal whose optical properties can be controlled by means of low power light. Long range order and high mechanical stability of the photosensitive liquid crystal composition are obtained by using the universal organizing capability of a channeled polymeric template, without the need for any kind of surface treatment. The structure exhibits strong contrast ratio and a high diffraction efficiency, both optically controlled.

Organometallic red-emitting chromophores: a computational and experimental study on cyclometallated Nile Red complexes of palladium(II) and platinum(II) acetylacetonates and hexafluoroacetylacetonates

A comparative photophysical, electrochemical and computational study of a series of Nile Red cyclopalladated and newly synthesised cycloplatinated complexes is reported. All complexes are luminescent at room temperature and the emission properties are governed by the nature of the cyclometallated dye. However, the presence of platinum(II) as metal centre decreases severely the emission quantum yield, while the electronegativity of the ancillary hexafluoroacetylacetonato ligand enhanced the emission properties of the Nile Red complexes.

Liaisons between photoconductivity and molecular frame in organometallic Pd(II) and Pt(II) complexes

The synthesis and a comprehensive experimental study of molecular structure, electrochemical properties, photoconductivity, absorption and emission properties of two homologous series of square planar cyclometallated Pd(II) and Pt(II) complexes are described. In these complexes, a five member (C,N) metallacycle is formed by either azobenzene, 2-phenylpyridine or benzo[h]quinoline, while the ancillary (O,N) ligand required to complete the metal coordination sphere is the Schiff base resulting from the condensation of 4-n-hexylaniline with 2-hydroxy-4-n-hexyloxybenzaldehyde. The study is complemented by Density Functional Theory calculations of molecular structure in the ground state, frontier orbitals distribution and energies. Absorption energies, intensities for the transitions and composition of the excited states are characterized by Time Dependent Density Functional Theory computations. Results show that for these complexes the HOMO is located on the Schiff base and the LUMO on the cyclometalated ligand. As also indicated by spectroscopic and photoconductivity results, the photogeneration of charge carriers might be associated with the spatial separation of HOMO and LUMO, while an additional contribution could derive from a conformational variation in the excited state. The separation of the frontier orbitals on different ligands provides simple synthetic routes towards band-gap tuning and energy matching with other molecular species and electrodes, both important features for several applications of molecular semiconductors. The results of the present investigation suggest that these square planar cyclometalated Pd(II) and Pt(II) complexes actually form a new class of photoconductors.

Plasmon mediated super-absorber flexible nanocomposites for metamaterials

A flexible host has been selected to achieve, for the first time, functional nanocomposites based on CdSe@ZnS core-shell type quantum dots (QDs) and Au nanoparticles (NPs), simultaneously dispersed in a polymer matrix. Coherent interactions between QDs and plasmonic Au NPs embedded in PDMS films have been demonstrated to lead to a relevant enhancement of the absorption cross-section of the QDs, remarkably modifying the optical response of the entire system. Optical and time resolved spectroscopy studies revealed an active gain-plasmon feedback behind the super-absorbing overall effect.

Rheological and photophysical investigations of chromonic-like supramolecular mesophases formed by luminescent iridium(III) ionic complexes in water

ABSTRACT Luminescent complexes with general formula [(C^N)2Ir(N^N)]OCOCH3, where C^N = 2-phenylpyridine (ppy) and N^N = 2,2ʹ-bipyridine (bpy), self-assemble into ordered ‘soft’ viscous mesophases of chromonic type. By changing the ancillary ligand with 1,10-phenanthroline (phen), a new mesomorphic complex was obtained. The self-assembly into ordered liquid crystalline phases of chromonic type in water was investigated by comprehensive rheological and photophysical studies. By changing the ancillary ligand bpy with phen, assembly into smaller, more symetric aggregates is favoured, resulting in lower viscosity and more dynamic mesophase. Graphical Abstract

Synthesis and photophysical characterisation of luminescent zinc complexes with 5-substituted-8-hydroxyquinolines

New 5-substituted-8-hydroxyquinoline ligands HLn (1–3), in which the quinoline and the p-N,N-dimethylphenyl fragments are connected through the –NN– (1), –CHN– (2) or –CH2–NH– (3) group, were designed and synthesised. Their reaction with a Zn(II) salt gave the pertinent Zn(Ln)2 complexes (4–6). The compounds 1–4 and 6 are stable in solution and their photophysical properties were explored. Zinc complexes 4 and 6 are both luminescent (4, λem: 620 nm, Φ < 1 × 10−4; 6, λem: 475 nm, Φ = 7 × 10−3), although the states responsible for the radiative deactivation processes were different: CT in nature for 4 and from a π–π* transition localised on the quinoline fragment for 6. The roles exerted by both the metal and bridge on the photophysical properties are discussed.