The known electron acceptor systems whereby the redox centers are linked by reversible noncovalent interactions are in most cases restricted to organic solVents. A kinetically labile coordinative bond has been designed for self-assembly of an electron donor (phenothiazine) and a photoinducible electron acceptor (riboflavin) in water at neutral pH. The pH dependent formation of the donor-acceptor complex in water was investigated by potentiometric titrations showing a binding constant of log K ) 5.9. The strong binding constant supports the observed large fluorescence deactivation of the riboflavin emission by the phenothiazine zinc complex. The riboflavin fluorescence lifetime was found to be constant (τ ) 4.7 ns) whatever the quencher concentration, clear evidence for a static quenching mechanism. A strong thermodynamical driving force and the observation of the riboflavin radical anion and phenothiazine radical cation by transient spectroscopy provide evidence for intramolecular electron transfer as the likely mechanism for the fluorescence quenching.
The synthesis and the photophysical properties of a series of noncovalently assembled donor-acceptor systems, dyads, is reported. The presented approach uses an "innocent" coordination compound, a scandium(III) acetyl acetonate derivative, as core and promotor of the dyad formation. Intercomponent photoinduced energy transfer or electron transfer within the dynamic assembly, which yields to a statistical library of donor-acceptor systems, is reported. The assemblies for energy-transfer processes are constituted by an energy donor, Ru(bpy)(3)(2+)-based component (bpy = 2,2'-bipyridine), and by an energy-acceptor moiety, anthracene-based unit, both substituted with a chelating ligand, acetyl acetone, that via coordination with a scandium ion will ensure the formation of the dyad. If N,N,N'N'-tetramethyl-2,5-diaminobenzyl-substituted acetyl acetonate ligands are used in the place of 9-acyl-anthracene, intramolecular photoinduced electron transfer from the amino derivative (electron donor) to the Ru(bpy)(3)(2+)-unit was detected upon self-assembly, mediated by the scandium complex. The photophysical processes can be studied on the lifetime of the kinetically labile complexes.
(Ferrocenylviny1)arenes 3, 5, and 7 are obtained from vinylferrocene (1) and substituted aromatic and heteroaromatic halides by palladium-catalyzed Heck-type reactions. Up to three ferrocene units are introduced in one step by the multifold reaction of 1,2-dibromo-(4) or 1,3,5-tribromobenzene (6) with 1. The first crystal structure of a bis(ferrocenylviny1)benzene chromophore (5) is reported.There is considerable interest in the synthesis of new materials with large second-order optical non-linearities because of their potential use in optical devices for information processing [']. It is now well established that molecular structures that possess both differences between ground-state and excited-state dipole moments and large transition dipole moments will have large second-order nonlinearities[*]. Molecules with R donor-acceptor interactions are promising candidates to fulfil these requirements. But in comparison with the great efforts focussed on the synthesis of organic materials such properties ['], organometallic compounds have received little attention until recently. The vinylferrocene moiety has now been used as a 71 electron donor in several with high second harmonic generationL51 (SHG) efficiencies.However, most synthetic routes to (ferroceny1vinyl)arenes are based on the Wittig reaction of ferrocenecarboxaldehyde with ylide~[6',~~] or (ferrocenylmethy1)triphenylphosphonium iodide with aldehydes [hh]. Over the last decade palladium-catalyzed coupling reactions of vinyl and aryl halides with alkenes (Heck reactions [']) have become a useful synthetic method with numerous applications. In the course of our studies of an application of this powerful methodology to the synthesis of defined polynuclear metal complexes, we have observed the facile coupling of vinylferrocene with various aryl halides.Iodobenzene reacts with vinylferrocene under the phase-transfer conditions described by Jefferyr'] to yield (E)-styrylferrocene[9] as the only product. The coupling reaction can be extended to electron-deficient or electron-rich aromatic rings with nitro or methoxy substituents. Heterocyclic examples['0] of the procedure are the reactions of 1 with 2-bromo-6-methylpyridine (Zd) and 2-bromo-p~-rimidine (Ze).
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