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.

No abstract

The synthesis and electrochemical and photophysical properties of a series of heterodinuclear ruthenium-iridium complexes linked by a modular para-phenylene bridge [Ir-ph(n)-Ru]3+ (Ir=Ir(ppyFF)2bpy, Ru=Ru(bpy)3, ppyFF=2-(2,4-difluorophenyl)pyridine), bpy=2,2'-bipyridine, ph=phenylene, n=2, 3, 4, 5) are reported. The use of a high-energy iridium complex, which can act as an energy donor when coupled to the lower energy ruthenium-based component, allows the investigation of photoinduced energy transfer from the excited iridium-centre to the ruthenium fragment (energy acceptor). The rate constants of the energy-transfer processes are determined by time-resolved emission and sub-picosecond transient absorption spectroscopy. Interestingly, there is almost no decrease in transfer efficiency or rates as the length between the two chromophores (number of spacers) is increased. This "molecular wire" behavior indicates the dominance of the incoherent hopping mechanism, allowing a very fast energy transfer over long distances (with n = 5 the metal-to-metal distance is estimated to be 32.5 Angstrom). This is the first case in which such behavior is observed for metal complexes, and could lead to new development in molecular electronics.

No abstract

In the search for light-addressable nanosized compounds we have synthesized 10 dinuclear homometallic trisbipyridyl complexes of linear structure with the general formula [M(bpy) 3 -BL-M(bpy) 3 ] 4+ [M ) Ru(II) or Os(II); BL ) polyphenylenes (2, 3, 4, or 5 units) or indenofluorene; bpy ) 2,2′-bipyridine]. By using a "chemistry on the complex" approach, different sizes of rodlike systems have been obtained with a length of 19.8 and 32.5 Å for the shortest and longest complex, respectively. For one of the ruthenium precursors, [Rubpy-ph 2 -Si(CH 3 ) 3 ][PF 6 ] 2 , single crystals were obtained by recrystallization from methanol. Their photophysical and electrochemical properties are reported. All the compounds are luminescent both at room and low temperature with long excited-state lifetimes due to an extended delocalization. Nanosecond transient absorption showed that the lowest excited state involves the chelating unit attached to the bridging ligand. Electrochemical data indicated that the first reduction is at a slightly more positive potential than for the reference complexes [M(bpy) 3 ] 2+ (M ) Ru, Os). This result confirms that the best acceptor is the bipyridine moiety connected to the conjugated spacers. The role of the tilt angle between the phenylene units, in the two series of complexes, for the ground and excited states is discussed.