Exited-State Lifetimes

Below is the function Data Em iss ion. m to generate the data. Note that experimental emission decays are exponentials and that the lifetime x is used instead of the more customary rate constant in kinetics. Also, we use the notation C representing the concentration of the exited states, not the normal concentration. 

The first model implies the rupture of the coordination bonds. This is inconsistent with the inertness of the macrobicyclic complex because its decomposition causes the rupture not only of M-N bonds, but also of C-C and C-H bonds. Therefore, this model predicts a relatively long lifetime of the state for the [Cr(sep)]3+ cation. In fact, the E states for [Cr(en)s] and [Cr(sep)] + cations have very similar lifetimes (of the order of 10 ps) in DMF at 0°C and similar spectral characteristics (both of them show an intense band at 15 151 cm-i and possess a quantitatively similar low-intensity vibronic structure). The macrobicyclic ligand ensures that ligand dissociation will have a large activation barrier, even in metal-centred electronic exited states. Neither the first model nor the second one adequately accounts for the photolytic similarities of [Cr(sep)]3+, [Cr(en)3] + and [Cr(NH3)e] cations. The third model seems the most realistic alternative [159]. 

In the previous chapters experimental data on ablation of the designed polymers have been shown. Polyimide was applied as reference polymer to compare the ablation behavior of the designed polymers versus a commercial polymer which exhibits similar absorption properties. Polyimide is most probably also the most studied polymer in ablation, and numerous reference data about ablation, but also about the chemical properties, exist (e.g., thermal diffusivity, heat capacity, reflectivity, lifetime of exited states etc.). This is also the reason why many models are benchmarked against ablation data of polyimide. 

The fluorescent properties of 12-AS have been widely used in membrane research. The fluorescence of the 12-AS molecules adsorbed at the gold surface should be quenched because of the reduced lifetime of the exited state. In contrast, the molecule should fluoresce if desorbed forming aggregates in the electrolyte layer near the electrode surface. These properties indicate that, if surfactant... 

In WiUstatter s mechanism, this would have required that chlorophyll was constantly associated with carbon anhydride otherwise the short lifetime of exited state would have caused a large loss. Traces of formaldehyde were detected from illuminated leaves, but a relation with oxygen developed was not demonstrated, and at any rate the process occurring in chloroplast was not duplicated in solution, suggesting a role of something more complex than a single molecule of chlorophyll in chloroplasts function. 

M. Broyer, G. Delacretaz, N. Guoquan, J.P. Wolf, and L. Woste, Lifetimes and Relaxation Processes in Electronically Exited States of Nas , Chem. Phys. Lett. 145, 232 (1988). 

The efficiency of energy transfer is the same as the quantum yield of energy transfer. It is the number of times that molecules take the energy transfer pathway divided by the number of times that the donor molecules have been excited. This is the same as the ratio of the number of times the excited donor exits by transferring energy to the number of times the excited molecules exit by any process to return to the ground state. In terms of the lifetimes of the donor, this is ... 

Energy transfer occurs in a long-lived collision complex. An exited molecule is often very polarizable and may form a collision complex with the Q molecule in the ground state. The collision complex A Q has a longer lifetime than the corresponding AQ collision complex. The formation of an exciplex provides the energy transfer by a collision mechanism. 

Focusing on comparisons to measurable quantities, the relative probability of a reaction (exit) channel can be written as the ratio of the cross section for that channel, (r to the total reaction cross section, 07. The ratios are labeled as the relative decay widths, T, in a notation that is, unfortunately, easy to confuse with the number of states discussed above. The sum of the decay widths is the total width of the state and can be used to calculate the lifetime of the excited state. Thus,... 

It is worthwhile, however, pointing out that the existence of a long-lived intermediate state and the absence of a barrier in the exit channel do not necessarily imply statistical product state distributions. The fragment distributions in the dissociation of weakly bound van der Waals molecules are usually neither thermal nor statistical, despite the extremely long lifetime of the complex. We will come back to this in Chapter 12. 

This has very important consequences The alignment of v and j with respect to the space-fixed z-axis is diminished by overall rotation before the excited complex dissociates. A long lifetime destroys the alignment. The correlation of j with v, on the other hand, is not established until the bond breaks and the two fragments recoil such that rotation of the parent molecule prior to dissociation is irrelevant. It represents a new observable which can provide additional information about the bond rupture and the exit channel dynamics other than the final state distributions. 

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

In equation (1) K y is referred to as the Stern-Volmer constant Equation (1) applies when a quencher inhibits either a photochemical reaction or a photophysical process by a single reaction. <1>° and M° are the quantum yield and emission intensity (radiant exitance), respectively, in the absence of the quencher Q, while <1> and M are the same quantities in the presence of the different concentrations of Q. In the case of dynamic quenching the constant K y is the product of the true quenching constant kq and the excited state lifetime, t°, in the absence of quencher, kq is the bimolecular reaction rate constant for the elementary reaction of the excited state with the particular quencher Q. Equation (1) can therefore be replaced by the expression (2)... 

The rates of diffusion of solutes and surfactants in and out of micelles have been measured using photophysical techniques. The most commonly used method is to measure the deactivation of excited states of the probe by added quenchers, which are only soluble in the aqueous phase. The measurement of either the decrease in emission intensity or a shortening of the emission lifetime of the probe can be employed to determine exit and entrance rates out of and into micelles 7d). The ability of an added quencher to deactivate an excited state is determined by the relative locations and rates of diffusion of the quenchers and excited states. Incorporation of either the quencher or excited state into a surfactant allows one to determine the rates of diffusion of surfactants. Because of the large dynamic range available with fluorescent and phosphorescent probes (Fig. 3), rates as fast as... 

The lifetime of an excited molecule (in seconds) is the inverse of the sum of the kinetic rates (in s ) for all the pathways for exiting the excited state. Thus in the absence of an acceptor, the lifetime is... 

The CM angular distribution, T(0), is isotropic at the lowest collision energy investigated (see Figure 14.9). An isotropic distribution is backward-forward symmetric and, therefore, implies either a lifetime of the decomposing complex(es) longer than its rotational period or a symmetric exit transition state. Since the electronic structure calculations depict no symmetric exit transition state(s) [84], we conclude that the lifetime of the... 

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