Quenching viologen

The DPA moiety is less active in forming the CT complex with viologens than the pyrene moiety e.g., for PMAvDPA the KCT values with MV2+ and SPV are 1.3 x 103 M 1 and almost zero, respectively, at pH 8-9 [60, 77], whereas for PMAvPY they are 7.8 xlO4 and 6.3 x 102 M, respectively, at pH 11 [77]. Therefore, the polymer-bound pyrene system undergoes much more static quenching than the polymer-bound DPA system. As will be discussed in Chapter 6, it is very important for charge separation whether the fluorescence quenching is static or dynamic. 

Methyl viologen (l,T-dimethyl-4,4 -bipyridylium dichloride, MV " ) promotes photoanodic dissolution in aerated CdS solution Figure 8 shows how the rate of dissolution depends on the concentration. The colloid has a weak fluorescence at 620 nm which is quenched by. The curves for fluorescence and dissolution in Fig. 8 are symmetric, which indicates that the two processes have a common intermediate that reacts with M. These effects are explained by the following mechanism ... 

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

They found almost complete quenching of the emission from the ruthenium complex and in addition the covalently linked compound considerably enhanced electron transfer to relay systems of aligned viologen units on micelles and polymers. 

In the presence of earlier generations, the luminescence quenching of aqueous solutions of Ru(L)32+ by methyl viologen (Figure 13.11) was found to follow Stern-Volmer bimolecular kinetics with a quenching constant /cq 5 x 109 M-1s-1, typical for the bimolecular quenching in homogeneous solutions. This... 

Frechet and co-workers [32] studied the ability of the dendrimer shell to provide site isolation of the core porphyrin moiety, using benzene-terminated dendrimers Zn[G-n]4P (i.e. 6). From the cyclic voltammograms in CH2C12, the interfacial electron transfer rate between the porphyrin core and the electrode surface decreased with increasing dendrimer generation. However, small molecules like benzyl viologen could still penetrate the shell of 6 to access the porphyrin core as observed from the quenching of porphyrin fluorescence. Their results also revealed that the dendritic shell did not interfere electrochemically or photochemically with the porphyrin core moiety. 

The kinetics of the quenching of Ru(bpy)3+ by the methyl viologen species, MV2+, is monitored by measuring the absorbance of the resulting strongly-absorbing blue MV+ at 600 nm, rather than the... 

DiCesare N, Pinto MR, Schanze KS, Lakowicz JR (2002) Saccharide detection based on the amplified fluorescence quenching of a water-soluble poly(phenylene ethynylene) by a boronic acid functionalized benzyl viologen derivative. Langmuir 18 7785-7787... 

The apparent binding constant K y obtained by Stern-Volmer quenching studies is the product of the number of receptors visited by the exciton and the binding constant of to the cyclophane receptor. For this reason polymer 3 and its monoreceptor model 2 were designed so that the binding constant for methyl viologen to the receptor was known for both systems. This allowed the calculation of the true amplification factor of 67. 

The emission from Ru(bpy)2+ of these compounds was extremely weak because of the rapid quenching by the adjacent viologen unit. The systems of mixed micells composed... 

The ionic domain of polyelectrolytes also affects the photoinduced charge separation between the coexisting low molecular charged compounds38). The quenching of Ru(bpy)2 + by a zwitterion compound, dibenzylsulphonates viologen (18, BSV), was enhanced about 4 times by poly(vinylsulphate) (PVS)39), and the back reaction... 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

Some other covalently bound porphyrin-acceptor complexes such as por-phyrin-viologen [68-73] and pyromellitimide-bridged porphyrins [74, 75] have been synthesized and studied. As in the case of P-Q complexes, strong fluorescence quenching and ion radical pair formation were observed in these systems under irradiation of complexes in porphyrin absorption bands. 

There is no evidence for ground state charge-transfer complex formation between stilbenes and neutral amines. Amine cations and dications are powerful electron acceptors and can form ground state complexes in which t-1 serves as the electron donor. Complex formation between t-1 and the organic dication methyl viologen is responsible for quenching of the fluorescence of surfactant stilbenes in organized assemblies (112). 

It has been known for some time116 that various organic molecules will quench [Ru(bipy)3]2+, but of all the compounds examined, methyl viologen (MV2+) has been the most extensively studied especially in systems for photochemical generation of hydrogen. This is no accident and this depth of study owes much to several important factors. 

Photochemical hydrogen production via oxidative quenching of [Ru(bipy)3 2+ by chemically modified viologens... 

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