Luminescence quenching by electron transfer

Fig. 30. Schematic representation of luminescence quenching by electron transfer.

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

Fig. 4.13. Luminescence quenching by electron transfer. 1 hc ground state a consists of two species, A + B. In the excited states b and c the A ion is excited A +B. State d is the electron-transfer state A Luminescence from the level c is quenched by electron transfer as indicated by the arrow...

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Figure 3.40 Example of the Perrin plot of static quenching. Luminescence of a metal complex [Ru (bpy)32+] in rigid glycerol in the presence of increasing concentrations of methylviologen (quenching by electron transfer)...

In case of co-facial quinone-capped porphyrins (P and Q are linked by four tetraamidophenoxy bridges and are located at a distance of 10 A from each other), the quantum yield of charge separation is much bigger and reaches 30% for short distances between P and Q [53, 54]. Luminescence quenching via electron transfer from P to Q is observed for both singlet- and triplet-excited states of the porphyrin fragment of P-Q. The appearance of the additional channel for luminescence decay via electron transfer manifests itself in the biphase character of P-Q luminescence decay kinetics. 

The fundamental concepts concerning such proton-sensitive devices are illustrated in Fig. 12. A fluorescent component (e.g. an aromatic hydrocarbon) is connected by a spacer to a proton receptor (e.g., an amine group). When the proton receptor is not protonated, the luminescent excited state of the fluorophore is quenched by electron transfer from the HOMO of the receptor (i.e., the non-bonding electron pair of the amine). When the receptor is protonated, this orbital is strongly stabilized, so that electron transfer is no longer thermodynamically allowed. The luminescence of the fluorophore, therefore, is not quenched. 

The simplest aliphatic thiyl radicals luminesce in the gas phase but not in the condensed phase [48-51]. The one-electron oxidation potential of the RS /RS couple is estimated to be about +0.92 V in aqueous solution [76, 77] while the emitting excited states are at least 2.5 eV above the ground state. Thus photoex-cited thiyl radicals are expected to be powerful oxidants with oxidative potential exceeding +3.4 V, which must be effectively quenched by electron transfer in any matrix except perhaps fluorinated hydrocarbons and noble gases. Oxidation of inorganic anions and water by photoexcited HS and CH3S has been observed experimentally [78]. 

Fig. 30. Schematic representation of a photo-electroswitch where the emission properties of a photosensitive centre are modulated by the electrochemical interconversion of a redox centre inducing luminescence quenching by energy or electron transfer.

A new probe of solvent accessibility of bound sensitizers has been described and tested for the particular case of a series of Ru" and Os photosensitizers bound to sodium lauryl sulphate micelles. The method depends upon the large solvent deuterium effect on excited-state lifetimes, and a correlation has been established between accessibility of bound complexes and hydrophobicity of the ligands. Luminescence properties of amphiphilic annelide-type complexes of ruthenium in micellar phases have been described. In the case of [4,4 -bis(nonadecyl)-2,2 -bipyridyl]bis-[4,4 -di-(10,13,16-trioxaundecyl)-2,2 -bipyridyl]ruthenium dichloride, intramicellar self-quenching effects have an influence on the excited-state lifetime, and the mechanism of self-quenching has been determined. Deactivation of [Ru(bipy)3] by [Co(EDTA)] has been studied in a micellar environment and found to occur by electron transfer at diffusion-controlled rates a stereoselective effect has been observed. ... 

QUENCHING OF LUMINESCENCE FROM METAL COMPLEXES BY ELECTRON TRANSFER... 

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