Triplet decay

In the general case where triplet decay occurs by both first and second order processes we obtain... 

That charge-transfer effects are not involved follows from the fact that the rate of triplet decay in perfluorobenzene is larger than that in benzene. If the benzophenone triplet were to act as acceptor and the benzene derivative as donor in a charge-transfer complex, the substitution of perfluorobenzene for benzene should render this type of process much less probable due to the strongly electron-withdrawing character of the fluorine atoms. 

The triplet decay rates kd for (65) and (69) are so similar that it is not reasonable to suggest that decay is dominated by unsuccessful migration, but must be due to a radiationless transition. 

Since we have inhibited one pathway leading to triplet depopulation by deuteration, it is clear that it will take longer for the triplet to decay by the radiative pathway and the lifetime of the triplet is increased. If phosphorescence were the sole pathway leading to triplet decay, the measured triplet lifetime would correspond to the radiative lifetime and would be equal to... 

Thus we see that in molecules possessing ->- 77 excited states inter-combinational transitions (intersystem crossing, phosphorescence, and non-radiative triplet decay) should be efficient compared to the same processes in aromatic hydrocarbons. This conclusion is consistent with the high phosphorescence efficiencies and low fluorescence efficiencies exhibited by most carbonyl and heterocyclic compounds. 

Fig. 12.2. FRET calculator. (A) Table of input parameters arbitrary concentration and time units. The rate constants selected for singlet decay (kdd = kd, excluding FRET see Eq. (12.1)) and for FRET (kt = kt) are the same, corresponding to a FRET efficiency E = 0.5. The acceptor has been assigned quite different parameters, except for triplet decay (kT — ki). The data are plotted for the indicated time scales (10, 150). (B) specification of desired output signals (see Fig. 12.1 b) xl, sg = excited donor ground state...

The triplet decay will also be governed by four rates ... 

Reversible electron transfer within Mg and Zn-substituted hemoglobin hybrids is initiated by flash photoproduction of the long-lived triplet state ( MP). According to Scheme I, the triplet return to the ground state involves two pathways, intrinsic triplet decay (with rate constant kp) and electron transfer quenching (with rate constant k,). 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

Fig. 2. Normalized triplet decay curves for [MP, FeP] hybrids. For a given M, (Zn, Mg) the arrow is directed from the curve for the Fe state toward that for the Fe state...

The triplet decay curves of ZnCcP and of all [ ZnCcP, Fe Cc] complexes, where electron transfer has been blocked by prior reduction of the Cc heme, are... 

Our early reports of the I A reaction in [ZnCcP, Cc] complexes were interpreted within the context of the simple ET cycle. Scheme I [8b, c]. As noted above, there are two limiting cases for this scheme, i) The I A reaction is slow I(t) appears with the triplet decay rate constant kp and disappears with the thermal ET rate constant k. ii) It is rapid here I(t) appears with k and disappears with kp. We reported that ZnCcP complexes with heterologous Cc s [8d] and with aliphatic Phe-82 mutants of yeast Cc [8b] fell into the former class, while the intermediate for fungal Cc appears rapidly [8d]. 

Lower), as reported earlier, but also rises more rapidly than kp (Fig. 13 Upper). This result immediately requires a more complex kinetic scheme than that of Scheme I. Excellent self-consistent fits to the time evolution of [1] (t) are obtained with an expression that is the sum of three kinetic phases, all having a common rate constant for triplet decay, kp, but with differing values of the rate constants for the decay of 1(3000 s , 40s , 5s ). We have further seen that complexes with different Cc show similar behavior, but with the fractional contribution of these multiple phases varying with species. 

Siebrand. W., Williams, D. F. Isotope rule for radiationless transitions with an application to triplet decay in aromatic hydrocarbons. J. Chem. Phys. 46, 403 (1967). 

Whereas a straight Stem-Volmer plot could not be obtained in quenching studies with octafluomaphthalene, the orders of magnitude of the rate constants of triplet decay ka) and of reaction with olefin ( r) could still be estimated Ad = 6 X 10 s i and ki =2 X 10 1 mole is i >). In view of this rapid decay, a high concentration of olefin is evidently necessary for effective addition. 

This change in the triplet decay rate constant was verified by ESE measurements where spin lattice relaxation effects can be minimized from the decay rate constant measurements (14). Observed decay rate constants are given in Table I for the y triplet level... 

It is found that carotenoids with carbonyl functional groups increase the triplet decay rate constants of the porphyrins TPP or TPPS in frozen toluene/ethanol glasses possibly by triplet-triplet transfer. The triplet-triplet transfer could be facilitated by the presence of hydrogen bonding between carotenoids containing carbonyl groups and the prophyrins. Carotenoids with no polar functional groups like 3 Totene do not interact with porphyrin so as to affect the triplet life time of the latter. 

Equation (15) permits us to examine the circumstances under which back transfer will become important. If we assume that energy transfer to the acceptor is the only mode by which the sensitizer triplet decays to the ground state (i.e., kd 0), Eq. (15) reduces to ... 

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