Excited states decay constants

Rate constants" for the excited state decay of 7-azaindole in H/D solvent mixtures... 

FIG. 12 Simulation of fluorescent decays for dye species located in the aqueous phase following laser pulses in TIR from the water-DCE interface according to Eq. (38). A fast rate constant of excited state decay (10 s ) was assumed in (a). The results showed no difference between infinitely fast or slow kinetics of quenching. On the other hand, a much slower rate of decay can be observed for other sensitizers like Eu and porphyrin species. Under these conditions, heterogeneous quenching associated with the species Q can be readily observed as depicted in (b). (Reprinted with permission from Ref 127. Copyright 1997 American Chemical Society.)... 

The concentration of the iron porphyrins was adjusted to be between 0.2 and 0.3 OD for 2 mm cell at 530 nm. All relaxation times were calculated from the first order kinetic curves of excited state decay or ground state reappearance. This procedure eliminates error in delay times between the excitation and different wavelength probe pulses ("chirp") since constant delay times are subtracted out of the kinetic curves. There may, however, be some error introduced in the shorter decay times because of the excitation pulse and the probe pulse may overlap at the earliest points of the kinetic curve calculations. 

Determination of decay constants or lifetimes of excited states. Decay... 

Note that in all of these situations, one can begin to unravel the excited state rate constants if measurements of i° and of i can be made. There are several ways in which this can be done. The most common one is from the decay of the emission from A, following pulse excitation. Alternatively, it has been possible in several cases to observe the UV-visible absorption spectrum of A, and the decay of this absorption again gives x° or t (see, for examples, refs. 22-26). In rare cases, it has been possible to observe the rate at which the primary photoproduct P grows in, again following a pulse excitation it should do this with the lifetime of A. 27... 

In contrast to the reference compounds 4a-c, the singlet excited state deactivates in the dyads and triads much faster than in the oFL references with rate constants typically on the order of 1012 s-1 (Fig. 8.13). These values match the quantitative quenching of the oFL fluorescence in 5 and 6. A comparison between the decay rates of the conjugates carrying one C60 or two C60 moieties, reveals a 2-fold acceleration of the o/igo(fluorene) deactivation in the latter. Conclusively, placing two Ceos instead of one onto the oFL backbone significantly accelerates the excited state decay. 

In contrast to the references, 23a,b, the singlet-excited state deactivation in the Fc-oFL -C60 conjugates, 24a,b, occurs with rate constants of around 1010 s-1 (Fig. 9.64). The values are in good agreement with the observed quantitative quenching of the oFL fluorescence (Fig. 9.62a). At the conclusion of the singlet-excited state decay, two important transient maxima resemble the successful formation of the radical ion pair state, namely a weak shoulder of the transient... 

Figure 3 Recombination of oppositely charged, statistically independent carriers (e, h) can lead to the creation of an emitting excited state through a Coulombically correlated charge pair (e—h). The charge pair formation time (diffusion motion time) and its capture time are indicated in the figure as im and tc, respectively. The excited states decay radiatively (hi/) with the rate constant k and non-radiatively with an overall rate constant kn. After Ref. 21a.

With the advent of convenient techniques in recent years, it has become popular to measure "lifetimes" of excited states. Decay characteristics are useful adjuncts to quantum yields as they provide further knowledge about the mechanism of reaction, particularly with regard to evaluation of rate constants for excited molecule reactions. Only when the decay is exponential can a unique lifetime be defined. 

The experimental ilc computed from Eq. (21) can be related to the mechanistic rate constants. In general, the relationship is very complex, but in some limiting cases the correspondance of the experimental and mechanistic rate constants can be made by inspection. For example, in the dissociative mechanism when CrL is formed but is negligible, k computed from Eq. (21) is ka- In an associative mechanism when CrLJ is not reformed by geminate recombination, = k while in a concerted mechanism Kx = rp + rp- However when geminate recombination reforms CrLJ, the excited state decay is nonexponential. At long times is reduced by the recombination and k is a more complicated function of the several rate constants that appear in the mechanistic description. 

PURELY ROTATIONAL COHERENCE AND SUB-DOPPLER SPECTROSCOPY. Guided by the theoretical decay simulations of Fig. 46, the first unambiguous observation of thermally averaged rotational coherence effects was made for excitation and detection of the S, - S00° band of jet-cooled t-stilbene.47 Observed fluorescence decays are shown in Fig. 47 theory and experiment match very well. The recurrences associated with rotational coherence effects in fluorescence have been observed for a number of other species as well. Among these species are t-stilbene-, 2, t-stilbene-argon complexes,48 and t-stilbene-he-lium complexes.71 The recurrences allow the determination of the excited-state rotational constants to a high degree of accuracy. [For example, for t-stilbene we find j(B + C) to be 0.00854 + 0.00004 cm-1.] The indications are that with currently available temporal resolution, rotational coherence effects should be observable in a multitude of species and should allow the accurate determination of such species excited-state rotational constants. 

The first case from which information about the dynamics of quencher mobility can be recovered occurs when the quencher is partially solubilized and where the quenching is much more efficient than the exit of the quencher from the self-assembly. The excited state decay is first order and the observed decay rate constant is given by... 

Although this appears to be a simple experiment for systems in which the excited state decays via both fluorescence and dissociation, it is in fact not a simple matter to establish that k can be attributed exclusively to dissociation. The excited state can also decay nonradiatively by internal conversion to the ground electronic state, by intersystem crossing to a triplet state followed by collisional deactivation, etc. That is, dissociation is only one of several nonradiative paths that may be important in the decay of an excited state. Thus, considerable care must be exercised in the interpretation of such data. However, these data can always be used to place an upper limit on the dissociation rate constant. 

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