Chromophore quenchers

Early work on the kinetics of photoinduced ET in transition metal complex systems focused exclusively on bimolecular reactions between transition metal chromophores and electron donors or acceptors. However, concomitant with the advances in rapid photochemical kinetic methods and chemical synthetic methodology, emphasis shifted to photoinduced ET in chromophore-quencher assemblies that comprise a metal complex chromophore covalently linked to an organic electron donor or acceptor [24]. These supramolecular compounds afford several... 

Fig. 15 Photoinduced electron transfer in Re-based dyads (chromophore-quencher complexes)...

Another type of chromophore-quencher complex, developed by Schanze [151— 154], contains an axial ligand L, which undergoes irreversible intraligand bond... 

In the chromophore-quencher system (1) a ferrocenyl unit is connected to a Ru(tpy)2 + unit via a p-phenylene-acetylene spacer [141]. The weak emission of the Ru(tpy)2 chromophore is completely quenched in the dyad [142]. For this system, an EnT mechanism could be inferred by analogy with the behavior of related bimolecular systems [140], but PET followed by fast back electron transfer cannot be ruled out. This mechanistic ambiguity is common to most studies on related chro-mophore-ferrocene dyads [143-146]. 

By far the most popular electron donor used in inorganic chromophore-quencher systems has been phenothiazine (PTZ). The pioneering studies of Meyer and coworkers [162, 163] on the chromophore-quencher complex (3) showed rapid (<10 ns) quenching of the MLCT excited state of the chromophore, with formation of a charge-separated state containing a reduced Re(I) complex (i.e., a Re(I)-bpy species) and the PTZ+ radical cation. Charge recombination takes place in several tens of nanoseconds, with rates depending on the X-substituents. More recent work... 

The most widely used electron acceptors in inorganic chromophore-quencher systems have been bipyridinium dications, often called viologens (quatemarized derivatives of 4,4 -bipyridine) or diquat (cyclic quatemarized derivatives of 2,2 -bipyridine). The classical studies of Elliott, Schmehl, and Mallouk have been concentrated on dyads of types (4) and (5). For dyads (4) [168, 169], oxidative PET takes place, with forward processes in the 80 to 1700-ps time scale and very fast (<30 ps) charge recombination. The main observations are that (i) electron transfer to the diquat quencher occurs from the directly linked bipyridine ligand (ii) fast equilibration between the MLCT excited states on the three bipyridine ligands precedes electron transfer (iii) the electron transfer rates are in the normal Marcus... 

A number of interesting inorganic-organic dyads have been recently investigated that, rather than chromophore-quencher, could be called chromophore-sensitizer ... 

Chromophore quencher systems where the organic molecular components are aryl hydrocarbons are under intense study [67, 190-205], In these systems an inorganic moiety (based on polypyridine complexes of Ru(II), Os(II), Re(I)) and an aryl hydrocarbon (naphthalene, anthracene, pyrene) are covalently linked by flexible aliphatic [187-201] or rigid conjugated bridges [49, 190, 191, 193). 

Photoinduced Charge Shift in Chromophore-Quencher Systems... 

The enol silyl ether (56) undergoes a photoinduced reaction in the presence of chloranil to give cyclohexenone and the adduct (57) and the currently available evidence suggests that the reaction proceeds by electron transfer to the photo-activated chloranil to give (56) A photophysical study has appeared of the chromophore-quencher compounds [Au(CCPh)(L )] (L = 10-[(diphenylphosphi-no)methyl]anthracene) and [Au(CCPh)(L )]BPh4 (L = l-[2-(diphenylphosphin-oxy)ethyl]pyridinium], 1 -[2-(diphenylphosphinoxy)ethyl]-4-methyl pyridinium, 1 -[2-(diphenylphosphinoxy)ethyl]-4-tert-butylpyridinium, and 1 -[2-(diphenylpho-sphinoxy)ethyl]triethylammonium in which the Au(CCPh) chromophore is linked by a flexible tether to the acceptors. 

Chromophore/quencher-iabeied singie-stranded PNA probe... 

Chromophore-quencher" complexes have been prepared in which the same processes occur but intramolecularly, e.g., eq. 3. The sequence of events that... 

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