L. De Cola

Electroluminescent device with reversible switching between red and green emission

Research on new materials for organic electroluminescence has recently focused strongly on phosphorescent emitters, with the aim of increasing the emission efficiency and stability. Here we report the fabrication of a simple electroluminescent device, based on a semiconducting polymer combined with a phosphorescent complex, that shows fully reversible voltage-dependent switching between green and red light emission. The active material is made of a polyphenylenevinylene (PPV) derivative molecularly doped with a homogeneously dispersed dinuclear ruthenium complex, which fulfils the dual roles of triplet emitter and electron transfer mediator. At forward bias (+4 V), the excited state of the ruthenium compound is populated, and the characteristic red emission of the complex is observed. On reversing the bias (-4 V), the lowest excited singlet state of the polymer host is populated, with subsequent emission of green light. The mechanism for the formation of the excited state of the PPV derivative involves the ruthenium dinuclear complex in a stepwise electron transfer process that finally leads to efficient charge recombination reaction on the polymer.

Cationic Heteroleptic Cyclometalated Iridium(III) Complexes Containing Phenyl-Triazole and Triazole-Pyridine Clicked Ligands

Novel heteroleptic iridium complexes containing the 1-substituted-4-phenyl-1H-1,2,3-triazole (phtl) cyclometalating ligand have been synthesized. The 3+2 Huisgen dipolar cycloaddition method (‘click’ chemistry) was utilized to prepare a class of bidentate ligands (phtl) bearing different substituents on the triazole moiety. By using various ligands (phtl-R1 and pytl-R2) (R1=adamantane, methyl and R2=adamantane, methyl, β-cyclodextrin, ursodeoxycholic acid), we prepared a small library of new luminescent ionic iridium complexes [Ir(phtr-R1)2(pytl-R2)]Cl and report on their photophysical properties. The flexibility of the clicking approach allows a straightforward control on the chemical-physical properties of the complexes by varying the nature of the substituent on the ligand.

Photochemistry of supramolecular and species

As molecules are made up by atoms, supermolecules are made up by suitably arranged molecular components. Investigations on the great variety of supermolecules that are now available because of the progress made by synthetic methods offer the dual opportunity to broaden our knowledge of the fundamental principles of photochemistry and to discover new functions and applications of photoinduced processes. Photochemical studies on some selected supramolecular species, including covalently linked components, ion pairs, host- guest systems, and cage-type complexes, are briefly reviewed.

Extending excited-state lifetimes by interchromophoric triplet-state equilibration in a pyrene-Ru(II)diimine dyad system

The synthesis and the spectroscopic properties of a bichromophoric ruthenium trisbipyridyl-1,4-diethynylenebenzene-pyrene system (Ru-b-Py) and the corresponding pyrene ligand (b-Py) are reported. The ruthenium model systems Ru-b-OH, Ru-b-Ph are also presented. UV–Vis absorption and emission at room and low temperature and time-resolved spectroscopy are discussed. For the Ru-b-Py dyad, a mixing of the 3MLCT state of the ruthenium-based component and the triplet state of pyrene, 3Py, is observed. Time-resolved transient absorption studies performed on the Ru-b-Py and on the b-Py ligand show that the lowest energy absorption is due to the population of the triplet state localized on the pyrene-component. Time-resolved studies also evidenced a relatively slow forward triplet equilibration rate, in the order of 2×105 s-1 (5 μs), and an even slower back energy transfer rate, 3.3×104 s-1, still faster than the intrinsic decay time of the pyrene (200 μs).

Cyclodextrin-modified zeolites: host-guest surface chemistry for the construction of multifunctional nanocontainers.

The functionalization of nanoporous zeolite L crystals with β-cyclodextrin (CD) has been demonstrated. The zeolite surface was first modified with amino groups by using two different aminoalkoxysilanes. Then, 1,4-phenylene diisothiocyanate was reacted with the amino monolayer and used to bind CD heptamine by using its remaining isothiocyanate groups. The use of the different aminoalkoxysilanes, 3-aminopropyl dimethylethoxysilane (APDMES) and 3-aminopropyl triethoxysilane (APTES), led to drastic differences in uptake and release properties. Thionine was found to be absorbed and released from amino- and CD-functionalized zeolites when APDMES was used, whereas functionalization by APTES led to complete blockage of the zeolite channels. Fluorescence microscopy showed that the CD groups covalently attached to the zeolite crystals could bind adamantyl-modified dyes in a specific and reversible manner. This strategy allowed the specific immobilization of His-tagged proteins by using combined host-guest and His-tag-Ni-nitrilotriacetic acid (NTA) coordination chemistry. Such multifunctional systems have the potential for encapsulation of drug molecules inside the zeolite pores and non-covalent attachment of other (for example, targeting) ligand molecules on its surface.

Electronic Energy Transfer in Dinuclear Metal Complexes Containing Meta Substituted Phenylene Units

The synthesis and photophysical properties of heterometallic dinuclear complexes based on ruthenium and osmium trisbipyridine units, Ru-mPh3-Os and Ru-mPh5-Os, in which the metal complexes are linked via an oligophenylene bridge centrally connected in the meta position, are described. Electronic energy transfer from the excited ruthenium-based component (donor) to the osmium moiety (acceptor) has been investigated using steady-state and time-resolved spectroscopy. The results obtained for the meta-substituted compounds are compared with the analogous systems in which the phenylene spacers are substituted in the para position. The mechanism of energy transfer is discussed.

Strong fluorescence enhancement of 2-bromo-3-(1H-indol-3-yl)maleimide upon coordination to a Lewis-acidic metal complex

The emission intensity of an indolyl maleimide derivative increases approximately 80-fold by reversible coordination to (1,4,7,11-tetraazacyclododecane)zinc(II), which makes the system a promising new signalling motif for molecular sensors.

Electronic Properties and Supramolecular Organization of Terminal Bis(alkylethynyl)-Substituted Benzodithiophenes

Benzodithiophene (BDT) was symmetrically bisubstituted in the terminal positions with five different alkynes C≡C-(C(n)H(2n+1)) with n = 4, 6, 8, 10, 12. The materials were characterized as potential materials for field-effect transistor applications. Electrochemical measurements in solution and photophysical measurements in solution and in the solid state, together with UV photoelectron spectroscopy in air and quantum-chemical calculations, elucidate the nature of the frontier orbitals and of the excited states as well as their deactivation pathways. Structural information on the molecular assembly in the solid state, both at room temperature and at elevated temperatures, is obtained by a combination of DSC, polarized optical microscopy, and 2D-WAXS, which point to the crystallinity of the compounds in all phases and reveal π-stacking arrangements independently of the length of the alkyl side chains.

Electrochemical and Photophysical Properties of Ruthenium(II) Bipyridyl Complexes with Pendant Alkanethiol Chains in Solution and Anchored to Metal Surfaces

Luminescent ruthenium trisbipyridine complexes containing one or two mercapto-alkyl chain(s) on one of the bipyridyl units have been synthesized through a new strategy. The electrochemical and photophysical properties, determined in solution and in the solid state were compared. Deposition on electrode surfaces (gold, platinum and indium tin oxide) was realized by self-assembly and the re- sulting adsorbed layers were characterized by electrochemistry and fluorescence confocal microscopy. Voltammetric measurements of the films, in aqueous and in acetonitrile solution, allowed the determination of the surface coverages and the oxidation potentials of the complexes. The effect of the number of chains and the chain length in the complexes is highlighted. Emission of the adsorbed complexes was strongly quenched by the metallic surfaces, while confocal microscopy images showed aggregate formation on am length scale. The latter results provide considerable insight into the nature of the adsorbed layers and support deductions from the voltammetric data.

New luminescent ruthenium complexes with extended π systems

New Ru(II) complexes are presented as potential luminescent sensors, fluorescent labels and DNA photoprobes.