Steady-state quenching

We determine the SCP as it is terminated at the substrate in the following manner. First, we calculate I = dl/dt for the linear portion of the intensity decay. A°linear least squares fit to the data in the time interval 170 - 190 sec produces a value of dl/dt = 1150 counts/sec. This represents the steady state quenching rate which is intimately related to the steady state SPR. Secondly, we calculate I. = dl/dt at one-second intervals as the steady state endsrwith the arrival of the SCP at the substrate. For example, for the data in Figure 2,... 

In steady-state quenching experiments [57,58], usually the ratio of quantum yields of products or fluorescence photons, respectively, measured in the absence (

linear dependence on the quencher concentration c. From... 

An instance of photoreduction of ketones by complexation with alkylbenzenes177,178 (as electron donors) is shown in Scheme 41. Products shown in Scheme 41 have been formed by radical coupling reactions. The investigations (using a combination of flash kinetics, steady-state quenching and quantum yield measurements) of the substituents and isotope (H/D) effect indicate that ketones react predominantly through CT complexes. 

Figure 3 Steady-state quenching of A-[Ru(phen)2dppz]2+ luminescence by metal complexes in the presence of CT-DNA. (Data compiled from Ref. 27.) ( ) A-[Rh(phi)2phen]3+ and ( ) [Ru(NH3)6]3+ [Ru(phen)2dppz]2 t = 10 p.M [bp] = 500 p.M buffer is 5 mM Tris-HCy50 mM NaCl (pH 7.2).

Figure 6 Steady-state quenching of A-[Ru(phen)2dppz]2+ luminescence by viologens in the presence of poly(dA-dT)poly(dA-dT). ( ) 75 bp/Ru (5 p,M Ru + 375 p.M bp), (o) 25 bp/Ru (5 jjlM Ru + 125 jxM bp), (A) 10 bp/Ru (5 Ru + 50 bp), and ( ) 25 bp/Ru (20 p.M Ru + 500 p.M bp). Upper panel MV2+ and lower panel Me2DAP2+. [Ru(phen)2dppz]2+ = 5 jjlM except for [Ru(phen)2dppz]2+ = 20 jjlM buffer is 5 mM sodium phosphate (pH 7). The insets are re-scaled to the same X-scale as Fig. 2 to show how much less efficiently these viologens quench [Ru(phen)2dppz]2+ compared to EB+ emission.

Identification of the photoactive state is a prerequisite to the calculation of the photochemical rates by comparison of the photochemical yields and luminescence lifetimes. Although measurements of steady state quenching and the dynamics of intermediate formation have been used to infer the reactive states in several complexes, there is still much to be done in this area. The effect of thermally accessible higher energy states can be minimized by reducing the temperature. 

The steady state quenching can be obtained by integrating the time dependent emission (see eqn. (16)). Using Gjj (t)e we obtain ... 

The Steady state quenching curve can be evaluated as a function of... 

The temperature dependence on the steady-state quenching is typified by DPA (Figure 8). There was a dramatic increase in the degree of quenching at a given concentration of oxygen with a decrease in temperature when oxygen gas-phase concentrations were used as the independent variable. 

The monomer and exclmer fluorescence decay profiles obtained from P2NMA and 2NMA-co-6.2% BDHM dissolved In benzene at room temperature are compared In Figures 4 (i) and (ii) respectively. The steady-state quenching Is accompanied by a significant decrease in both the monomer and excimer fluorescence decay times as well as a reduction of the risetime of the excimer fluorescence to one which is response-limited. The decay of the excimer fluorescence from the copolymer Is nonexponential but is fitted successfully by the Forster quenching function [5,13] given In equation (6). 

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