Quenching rate constants, free energy

In the equations, k (0) is the hypothetical quenching rate constant when AE ( AG) = 0 and AG is the free energy change on quenching. 

Eqs. 9 and 10 make clear predictions about the dependence of quenching rate constants on the free energy change in the quenching step. One way of testing the theory is to observe the quenching of the excited state by a series of related quenchers where the parameters kq(0), K, and k j) should remain sensibly constant and yet where the potentials of the quenchers as oxidants or re-ductants can be varied systematically. Such experiments have been carried out, most notably with the MLCT excited state, Ru(bpy)3 + (1). The experiments have utilized both a series of oxidative nitroaromatic and alkyl pyridinium quenchers, and a series of reductive quenchers based on aniline derivatives. From the data and known redox potentials for the quenchers, plots of RTlnk q vs. 

Fig. 2. Fluorescent quenching rate constants of excited /J-naphtholate anion correlated with the electron transfer free energy [96]...

Fig. 1. Plot of rate constant for fluorescence quenching kq vs free energy AG for E.T. The solid line is calculated by Rehm and Weller for an E.T, process...

Fig. 2. Plot of the fluorescence quenching rate constants k, vs the free energy changes AG for the electron transfer process (a) Marcus relationship, (b) Rehm-Weller relationship...

Table 3. Free energy change, AG°ct, and rate constants, of photoinduced electron transfer from group 14 organometallic electron donors to C, observed rate constants, obs, triplet quenching rate constants, kq, and limiting quantum yields, Oco, in the photoaddition of the donors to in benzonitrile at 298 K [212].

Table 5. Reduction potentials ( red), excited triplet (singlet for NTAB and CTAB) energies, free energy changes (AGet) and quenching rate constants (A q, measured for n-butyltriphenylborate anion) of the light absorbing molecules used as initiators of free-radical polymerization.

Regular dependence (Fig. 28) of quenching rate constants, feq, on the free-energy driving force AGq(AGq = — AGe) strongly suggests another reason for the observed ECL behavior. 

The quenching efficiencies of the free ligands fall off rapidly as the excitation energy of the donor (Ex) decreases from 70 to 60 kcal/mol. The quenching rate constants of the nickel(II) complexes show a slight decrease In this region but are then fairly constant (kq = 3 X 10 M s ) down to Ex =... 

The driving-force dependence of the quenching rate constant and the redox potential of the ES couple can be obtained from Eq. (d) or (0 if the free-energy dependences of AGj3 and AG are known. In terms of the classical formalism, AG23 is given by... 

Progress in the understanding of ES reactions now allows ESs to be so well characterized that quencher exchange rates and potentials are being determined from the free-energy dependence of [ RuL3] + quenching rate constants . Study of ES electron transfer permits a detailed examination of very exothermic reactions and provides a probe of the intimate details of electron transfer. 

Figure 1. Comparison of calculations of the quenching rate constant as a function of free energy for the quenching reaction [Eq. (d)] using the classical [solid line, Eq. (g)] and the empirical [dashed line. Eq. (h)] free-energy expressions k = 3X 10 M s kj, = k = lO s. Vj, = = 6 X 10" s, (ACp =...

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