Chromophore quencher complexes

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... 

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... 

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

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... 

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). 

In complex (81), the electron-donating phenothiazine moiety is separated from the Ru(bpy)2 " unit by a triazole bridge that carries a formal negative charge. An investigation of this system shows that such anionic bridges can mediate electron transfer between chromophore and quencher. ... 

When [Rh(phi)2(phen)]3+ is titrated into a solution containing [Ru(phen)2(dppz)]2+ and B-form DNA, the photoinduced luminescence of the ruthenium(II) complex is quenched dramatically (53). In these experiments, luminescence is monitored by laser flash as quencher is added. Data are then plotted in Stern-Volmer format, where the ratio of initial intensity/intensity (I0/I) is given as a function of quencher concentration [Q. The degree of lifetime quenching can also be described by plotting the inverse of the lifetime (r0/r) versus [Q. Normally, when chromophore and quencher interact bimolecularly, Stern-Volmer graphs are linear with [Q] and the slope for r0/r is the same as that for I0/I. 

Custom modifications have previously been developed whereby a non-fluorescent chromophore can be attached to the DNA sequence to provide a strong SE(R)RS signature which is indicative of the DNA sequence present. This has been done previously using DABCYL, phthalocyanines and black hole quenchers (BHQs) as well as specifically designed simple azo dyes which contain moieties to aid in their binding ability to metal surfaces such as the benzotriazole motif which has been shown to be very effective at complexing onto silver nanoparticles [12, 13, 40, 41]. 

Viologens have been used as covalently linked quenchers for Cu(I) bipyridine chromophores, leading to fast (<10 ns) charge separation and remarkably slow (30 ns to 2 ps, depending on solvent) charge recombination [174]. More complex systems, similar to (5) but with two viologens on the same bypyridine of the Ru(II) chromophore, have been designed to mimic the presence of two ET in the reaction center of natural photosynthesis [175]. 

Typical examples of the stabilizers generally used to prevent the above chain reaction are (a) UV absorbers — 2(2-hydroxy-3-tert-butyl-5-methylpheny l)-5-c hi or ob en zo tr ia zo le and 2-h yd ro xy-4-octoxybenzophenone (b) Antioxidants — 3,5-di-tert-butyl-4-hydroxy-toluene and octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propa-noate (c) Peroxide decomposers — dilauryl thiodipropionate. In addition quenchers such as the organic nickel complex, Ni(II) bis-(diisopropyl dithiocarbamate) are used for the deactivation of the excited states of the chromophoric groups responsible for light initiation. 

Quenchers [9-12] deactivate chromophores in excited polymer molecules through an energy-transfer mechanism before these excited states can undergo reactions that would result in the polymer s degradation [3], This process is shown in Figure 4.6. Here the superscript 0 represents the ground state and shows an excited state. Likewise, an excited state complex may be formed and this complex could then... 

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