External species

Another class of photochemically relevant polyphosphazenes is formed by macromolecules having chromophores able to absorb light in a selective way and to transfer it to external species, thus inducing different reactions by energy transfer processes. In some cases electron transfer processes are also involved. These situations are described by Formula below and the corresponding polymers and external reagents are reported in Table 26. 

The photophysical properties of lanthanide ions are influenced by their local environment, the nature of the quenching pathways available to the excited states of sensitizing chromophores, and the presence of any available quenchers (as we have seen when discussing bioassay). All of these factors can be exploited for the sensing of external species. 

The external factors comprise the nature of the membrane, the substrate concentration in the aqueous phase and any other external species that may participate in the process. They may strongly influence the transport rates via the phase distribution equilibria and diffusion rates. When a neutral ligand is employed to carry an ion pair by complexing either the cation or the anion, the coextracted uncomplexed counterion will affect the rate by modifying the phase distribution of the substrate. The case of a cationic complex and a counteranion is illustrated schematically in Figure 10 (centre). 

Figure 6.32. Molecular structures of dendrimers modified with long-chain aliphatic cores. Unlike traditional dendritic structures with smaller cores, as the generation size increases, there is an available channel for external species to enter the dendrimer interior. Reproduced with permission from Watkins, D. M. Sayed-Sweet, Y Klimash, J. W Turro, N. J. Tomalia, D. A. Langmuir 1997,13, 3136. Copyright 1997 American Chemical Society.

FIG. 7.13 Schematic representation of a "command" surface. The response of tiie sur ce to an external species is controlled by the state of the interface. 

If initially there are a variety of chemicals that form a complex network of mutual catalyzation, this system may be robust against the invasion of parasitic molecules. Such an idea resembles the stability of an ecosystem, where a complex network of several species may resist to invasion of external species. Hence we need to study if replication of complex reaction network can be sustained. In this case, from the beginning, there are many molecular species that mutually catalyze, allowing for the existence of many parasitic molecules. Here, complete replication of the system is probably difficult. Then the question we have to address is whether such a complex network can maintain molecules that catalyze the synthesis of the network species. This question was addressed by Dyson [9], as a possibility of a loose reproduction system. 

Doped materials, where a structural component of the material becomes partially substituted by a dopant species or when external species ingress in the original material as an interstitial ion. The term doping is thus applied to, for instance, yttria-doped zirconias used for potentiometric determination of O2 but also to describe the incorporation of Li in polymers and nanostructured carbons. 

In the sections that follow, the discussions progress through simple diastereoselection, control from indigenous chirality present in the substrate, control from a removable chiral auxiliary, ultimately to absolute stereodirection from an external species not formally bound in a covalent fashion to the substrate. There have been several previous reviews of the ene reaction, although none has focused on both relative and absolute stereochemical aspects 6-14 20. 

Fluorescence Quenching Measurements Addition of an external species, known as a quencher (Q), which is capable of deactivating the excited state through collision can provide information concerning the extent/degree of exposure of a fluorescent species. The process is outlined below... 

Exploitation of photons by supramolecular systems for information purposes can be performed by two different routes (Scheme I). The first one ("photon-writes"), involves the occurrence of a photorcaction in a supramolecular species that causes ("writes") some changes in the properties of the species, reflected in a monitorable signal. The second route ("photon-reads") is based on a some kind of interaction between components of a supramolecular system and an external species which affects the photon response of the system. Some excited state manifestation (most usually, luminescence) can therefore be used "to read" the interaction. 

Introduction. A representation of "photon-reads" devices is shown in Scheme III. Such devices are made of a photoactive component P bound to another component, M, which can undergo interaction with some external species X. The interaction with X perturbs the properties of M. This modifies the excited-state behavior of P. Thus, the photochemical and photophysical behavior of P can be used to reveal ("read") the interaction taking place between M and X. The "photon-reads" devices can be classified according to the type of interaction which modulates the photochemical and photophysical properties of the supramolecular system. 

This network is particularly interesting for vanishingly small concentrations of the external species B such that with the usual product ansatz for the fluxes along Ri... 

The total reaction rate on the surface for each species (including storage reactions) is equal to the local external species mass transfer to/from the exhaust gas ... 

Here species i moves only (1) due to concentration gradient of species i (since C,Vx( may be written as VC,), (2) the external species i-spedfic forces and (3) the bulk motion (if any are considered via v ). If there is a concentration gradient of any other species or some external forces specific to that other species, they are not supposed to influence AT, as written above except through their contribution to the bulk velocity. ... 

The kinetics of nanocomposite formation is much less understood. In clay-polymer mixtures, it is still unclear how the surfactant or polymer get into clay gallery and form the intercalated or exfoliated stmcture since the gallery is initially separated by less than 1 nm and thus hinders the infusion of external species. Thus, some important issues must be addressed (i) How do the galleries open for the accommodation of intercalating surfactant or polymer (ii) By what mechanisms do the chains enter the galleries (iii) What factors may affect the penetration of polymers into the galleries (iv) How does the diffiisivity of the intercalated chains compare to that of bulk chains Some theoretical efforts have recently been made toward such kinetics issues of nanocomposite formation. 

In the models of formal reaction kinetics, a species is called an internal species if its concentration change is important for the simulation of the reactimi system. These species are denoted by letters from the end of the Latin alphabet (X, Y, Z). The concentrations of the external species are either constant or change slowly in time (A and Ma) (pool chemical) or have no effect on the concentrations of the other species (P). 

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