Dynamical dissociation quenching

For to = S = 0, the initial wavepacket is considered to be prepared instantaneously at maximum intensity. In contrast, if to is set equal to T/4 (T = In/co), corresponding to 5 = tt/2, the initial state preparation occurs at the start of an optical cycle, i.e., at zero-field infensity. The two situations result into completely different dynamics, the former leading to dissociation quenching, while the latter is monitored by a barrier suppression mechanism. This distinction can best be understood by viewing the dynamics as taking place on the time-dependent adiabatic potential surfaces W R,t) which arise from diagonalizing the potential energy operator of Eq. (61). 

Global compartmental analysis can be used to recover association and dissociation rate constants in some specific cases when the lifetimes are much shorter than the lifetimes for the association and dissociation processes. An example is the study for the binding dynamics of 2-naphthol (34, Scheme 14) with / -CD.207 Such an analysis is possible only if the observed lifetimes change with CD concentration and at least one of the decay parameters is known independently, in this case the lifetime of the singlet excited state of 33 (5.3 ns). From the analysis the association and dissociation rate constants, as well as intrinsic decay rate constants and iodide quenching rate constants, were recovered. The association and dissociation rate constants were found to be 2.5 x 109M-1 s 1 and 520 s 1, respectively.207... 

M s in DMSO-water in the presence and absence of amylose. Thus quenching is reduced about 30-fold for iodide by amylose incorporation of the stilbene chromophore. While it is somewhat uncertain as to what precisely the nature of the quenching of stilbene by iodide is, it is reasonable to assume that the reduced quenching constants imply a more difficult approach of the iodide ion to the complexed stilbene than to the free. We are currently exploring many aspects of reactivity of amylose-incorporated chromophores. We find for example that amylose is able to extract totally insoluble hydrophobic stilbene molecules into water and we are presently trying to obtain crystal structural data on the complex molecule. The dynamics of complex formation and dissociation are currently under investigation. 

Over the past few years it has often been observed that the photochemical behaviour of adsorbed molecules is distinctly different to that of their gas phase counterparts. Even direct dissociations of molecules physisorbed on insulator substrates were found to have different dynamics to the analagous gas phase reaction, and exhibited a dependence on the coverage. This needs to be understood. For adsorbed molecules a new kind of "dissocation" is possible, namely desorption, Photolytic (non thermal) desorption has been reported from all kinds of substrate. On metal surfaces it is often found that the quantum yield for a direct photodissociation reaction is much lower than in the isolated molecule. This must be accounted for. Finally, the observation which has stimulated a great deal of research in surface photochemistry, photolysis is observable at energies where the gas phase molecules are transparent. It turns out that all of these interrelated effects can be interpreted by a delicate interplay of excitation mechanism and transient quenching. The fine details of course depend on particular adsorbate-substrate systems, which are described in section 4. 

The value of is related to the reactivity within the supramolecular system. This rate constant will depend on the mobility of the quencher with respect to the probe inside the supramolecular structure, as well as on the chemical reactivity for the quenching process. Several models have been described for the mobility of quenchers with respect to probes in micelles [58-65]. Thus, the value of has a dynamic component to it, but it is not related to the association or dissociation processes of the quencher with the supramolecular system, which is the focus of this review. The values of will only be discussed when relevant to the association/dissociation studies. 

Models with increasing sophistication for the analysis of dynamic processes in supramolecular systems, notably micelles, as well as for the determination of other parameters have been developed over the past two decades. The basic conceptual framework has been described early on [59,60,95,96] and has been classifred into different cases which take into account the extent of quencher mobility and the mechanism of quenching [95]. Two of those cases lead to information about mobility and will be discussed. It is important to emphasize that this analysis is only applicable to self-assembled system such as micelles and vesicles it cannot be applied to host-guest complexes. This model assumes that the probe is exclusively bound to the supramolecular system and that no probe migration occurs during its excited state lifetime. The distribution of probe and quencher has been modeled by different statistical distributions, but in most cases, data are consistent with a Poisson distribution. The Poisson distribution implies that the quencher association/dissociation rate constants to/from the supramolecular system does not depend on how many... 

The second case firom which dynamic information can be recovered is also the more general description of the model. The quencher is assumed to be mobile and the dissociation rate constant of the quencher from the supramolecu-lar system ( g ) competes with the quenching process The mechanistic scheme of Fig. 1 is valid taking into account the general assumptions mentioned above. The fluorescence decay can be described by a function with four parameters [60,97] ... 

---