Metals, photoinduced electron emission from

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

Additional work by the Forster group, making use of transient emission spectroscopy, probed the rate of photoinduced electron transfer between metal centers within a novel trimeric complex [Os(II)(bpy)2(bpe)2 ] [Os(II) (bpy)2Cl]2 4+, where bpy is 2,2/-bipyridyl and bpe is fra s-l,2-bis-(4-pyridyl) ethylene. Transient emission experiments on the trimer, and on [Os(bpy)2(bpe)2]2+ in which the [Os(bpy)2Cl]+ quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 0.1 x 108s-1. The electron transfer is believed to be from the peripheral Os(II)Cl metal centers to the [Os(bpy)2(bpe)2]2+ chro-mophore. Comparison to rate constants for reduction of monolayers at a Pt electrode reveals that the photoinduced process is significantly faster [39]. 

Another example of modulation of the electronic interaction between two metal units is schematically represented by compound 41 in Scheme 2(d), for which upon complexation of the vacant 2,2 -bipyridine a perturbation of the luminescence properties of the ruthenium moiety was reported [100]. Methylation caused a quenching of the emission, most likely due to photoinduced electron transfer from the terminal chromophores to the central viologen-type unit. 

Dopants also influence the emission processes from PPVs. Improved red dopants have been based on pyran dyes" while Qo doping appears to be variable" "". Doping with electron transport materials such as oxadiazoles give polymers with balanced properties for hole transport". The avoidance of low molecular weight material in the synthesis of cyano based PPVs is important" as are head to head and tail to tail chain sequences in thiophene based polymers. Head to tail tetramer sequences were the most fluorescent. Metal ion doped PPV s are claimed to be good chemosensors and broad emission is observed from titania doped PPV . Electron rich dopants enhance the emission in the red region while electro and photoinduced infrared bands from PPV are similar . ... 

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