Time-dependent triplet decay

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

The transient T-T absorption in the gas phase has been measured recently for aromatic molecules such as naphthalene (119,211) and anthracene (80,81) using flash kinetic spectroscopy and tandem laser pulse absorption techniques. Particularly, the later technique (211) provides time-dependent absorption spectra of the "isolated" unrelaxed triplet molecules because of its capability for rapid monochromatic excitation and detection. It will certainly provide a wealth of Important kinetic and spectroscopic information about the evolution and decay of triplet states. Direct observation of the formation of transient hot ground-state (Sq) molecules through an internal conversion process has also been achieved with laser excitation and laser... 

We can see from Eq. (11-70) that the diagonal matrix elements are time independent in the absence of longitudinal relaxation. Eq. (11-69) illustrates the difference between coherent mixing (Q) and relcixation (VV2). The former causes an oscillatory behaviour of the off-diagonal elemebts, while the latter causes an irreducible decay of the elements. In order to illustrate how this time dependence causes transitions between initial triplet and singlet spin... 

Non-exponential phosphorescence decay is frequently observed for various aromatic chromophores molecularly dispersed in polymer matrices. Various possible mechanisms for non-exponential decay are reviewed, and a dynamic quenching mechanism by polymer matrices including the effect of a time-dependent transient term in the rate coefficient is discussed in some detail. The biphotonic triplet-triplet annihilation mechanism is also introduced for the non-exponential decay under high-intensity and/or repeated laser irradiation. 

Recently, Richert and Baessler(37) regarded the non-exponential decay of benzophenone as a dispersive triplet transport phenomenon to trap sites, and approximated it by a stretched exponential fit (In l(t) + t/T = -C(t/t ) ) with a time dependent dispersion parameter a. 

Triplet Decay and Exciton Migration. The rate of triplet state decay following a photoexcitation pulse is conveniently followed by monitoring the time dependence of the delayed fluorescence. A limitation of this approach is that absolute triplet concentrations cannot usually be evaluated and so rate constants for processes having a kinetic order greater than unity cannot be determined. 

Ring, et al., (1998 and 1999) have used a time-dependent magnetic field and the combination of a static magnetic field in a direction perpendicular to that of a time-dependent field to create and manipulate novel coherences and to monitor the quantum beats associated with specifiable details of the time evolution of these coherences. The frequencies and decay rates of different classes of coherence (AMj = 2 and 1 polarization beats, AMj = 0 singlet triplet population beats) may be sampled and modified selectively. 

An apparatus for measuring the time dependence in the xs range of lightscattering intensity has been used to investigate the degradation of poly(phenyl vinyl ketone) (PVK) in solution.205-250 The PVK was irradiated with 25 ns pulses at 347.1 nm, and butyrophenone was also investigated in the same way. In both cases a transient absorption, identified by sensitization measurements as the PVK and butyrophenone triplet state, was shown to have a first-order decay rate-constant of 1.0 0.2 x 107 s-1. The quantum yield of triplet state formation in PVK was estimated to be between 0.1 and 0.3, whereas the quantum yield of main-chain scission was found to be 0.4—0.6, insensitive to the presence of Oa. 

From magnetic resonance spectroscopy [49] it is well-known that IB effects are adequately circumvented by the tricks of a spin echo experiment. For instance, in a two-pulse echo experiment, IB effects are averaged out and one probes spin dephasing determined by time-dependent fluctuations characteristic of HB only (and not IB). More specifically, a nll-r-n microwave pulse sequence is applied, where the first nil pulse creates a coherent superposition state for which a la = 1 and the n pulse, applied at time r after the first pulse, generates a spin coherence (the echo) at time 2r after the initial pulse. The echo amplitude is traced with r. The echo amplitude decay time is characteristic of the pure dephasing dynamics. For phosphorescent triplet states it is possible to make the echo optically detectable by means of a final nil probe pulse applied at time f after the second pulse [44]. In Fig. 3b, the optically detected echo amplitude decay for the zero-field transition at 2320 MHz of... 

The time dependence of the decay of the delayed fluorescence, luF(fl. is thus found according to Eq. (6.33b) from the time dependence of the square of the triplet exciton concentration [Ti]. The latter can be obtained from Eq. (6.30), which is readily solved for two limiting cases ... 

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