Emission quenching

Fig. 17 Rate constants in [26] for intramolecular emission quenching in various solvents. (Reproduced with permission from Winnik, 1977)...

Differential quenching in emission - Quenching by EefCN) - su ests the presence of 2 species on DNA + 51,52... 

Emission quenching is also observed with mononucleotides. In that case the quenching efficiency decreases from GMP (guanosine 5 monophosphate) to AMP (adenosine 5 monophosphate) i.e. it also follows the redox potentials of the bases, as G is more easily oxidisable than A, although the oxidation potential valura reported in the literature are rather different from one author to the other [101-104], Moreover the quenching rate constant by GMP in a Kries of different TAP and HAT complexes plotted versus the reduction potential of the excited state (Fig. 12) [95] is consistent with an electron transfer process. Indeed, as will be demonstrated in Sect. 4.3.1, these quenchings (by the mono-and polynucleotides) originate from such processes. 

Fig. 6 Emission quenching of polymer 25 by different transition metal ions. The polymer concentrations are held fixed at 3.08x10" M corresponding to receptor unit. Transition metal ions are 1.54x10 M. (Reprinted with permission from Ref. [26]. Copyright 2002 American Chemical Society)...

Emission quenching, steady state quantum yield measurements, and flash photolysis studies established that the initiation process could be represented by eqs. 13 or 14, and 15 ... 

The simplest excitation-emission-quenching scheme (which does not allow for radiationless decay processes) is given in Equations 13.15-13.17. 

If the total excitation-emission quenching scheme is more complicated than shown in Equation 13.15-13.17 in that it also includes radiationless decay processes for D, then, instead of Equation 13.19, one obtains Equation 13.21, where ku is the sum of all the unimolecular decay processes of D. ... 

The inclusion of 5 wt% PCBM [l-(3-methoxycarbonyl)propyl-l-phe-nyl-[6,6]C6i] in the spin-coating solutions resulted in efficient polymer emission quenching for all the polythiophenes studied. The transient absorption spectra of the amorphous poly(541)/PCBM blend film. At 10 ps exhibited an absorption peak around 700 nm, similar to that observed for the poly(541) pristine film. The shape of the transient spectrum varied with time, with the absorption peak shifting from 700 nm at 10 is to 900 nm for time delays >100 (is, demonstrating the formation of two distinct transient species in the blend film. The monoexponential lifetime was t = 8 (is under Ar atmosphere and significantly shortened under 02 atmosphere. Monoexponential phase is therefore assigned to the decay of poly(541) triplet excitons. 

Zinc(II) cyclene was linked to phenothiazene group which served as electron-donor (Scheme 22). To the complexed zinc(II), a riboflavin tetraacetate molecule coordinated. Upon irradiation, the flavin became a strong oxidant and the transfer of electrons could be easily observed by emission quenching. 

A possible explanation for the lack of electron-transfer characteristics in the trimer 9d is derived when extrapolating the linear relationship in Fig. 9.8 to the distance of the trimer. As a matter of fact, the charge-separation would not be able to compete with the intrinsic singlet lifetime of C6o (i.e. dashed line). This, in turn, explains the lack of fullerene emission quenching in 9d. Nevertheless, the photophysical assays clearly established that oPPE bridges effectively mediate electron-transfer processes over distances up to 20 A. These findings were further corroborated by quantum mechanical calculations. 

Next, a strong solvent dependence emerges. When varying the solvent polarity from, for example, toluene to benzonitrile, the Cemission quenching increases (Fig. 9.44). In summary, such observations imply charge-transfer interactions between the Ceo electron acceptor and exTTF electron donor. Obviously, the charge transfer pathway passes the transiently formed C,l0 singlet excited state. 

Although Barton and Turro did not provide direct proof that ET was the mechanism of Ru(II) emission quenching in this tethered Ru/Rh-DNA system, recent work involving bimolecular ET quenching [97] leaves little doubt that closely related racemic-Ru(DMP)2(dppz)2+, where DMP = 4,7-dimethylphen-anthroline, and A-Rh(phi)2(bpy)3+ intercalate into duplex DNA, exhibit static Ru(II) emission quenching (t < 10 ns), and form a Ru(III) photoinduced ET... 

It is tempting to use static Ru(II) -emission quenching in bimolecular studies involving donors and acceptors intercalated into DNA to learn about the distance dependence of the ET quenching reaction. However, these studies are always open to two interpretations. One interpretation is that the donors and... 

In light of the above bimolecular studies of static and dynamic Ru(II) -emission quenching by donors and acceptors intercalated in DNA, it is even more important to focus on the results of the tethered Ru/Rh-DNA duplex described above. The basic result is that Ru(II) -emission quenching is interpreted as... 

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