Quenching by Chemical Reaction

The quenching of an excited state of a transition metal complex by chemical reaction can occur, in principle, by means of any of the intermolecular reactions which transition metal complexes are able to undergo. It should be noted, however, that intermolecular excited state reactions can only occur if they are fast enough to compete with the intramolecular deactivation modes of the excited state and with the other quenching processes (Fig. 2). 

The most important classes of bimolecular reactions of transition metal complexes are ligand substitutions, reactions of the coordinated ligands and inner and outer sphere oxidation-reduction reactions28.  

Some intermolecular reactions involving the coordinated ligands (e.g., hydrogen or proton transfer) may be fast enough to compete with the excited state decay. However, except for a few cases [e.g., Ru(bpy)2(CN)2 34 and Ru(bpy)2 (bpy-4,4 -(COOH)2 )2 + 35)] reactions of this kind have not yet been well documented for transition metal complexes, although they are very common for organic molecules19, 36,37.  

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

In dealing with neutral organic molecules in a fluid solution, it has been found that the formation of the electron transfer products in an excited state quenching process may be the result of a very complicated series of events. In particular, the encounter complex may give rise to an excited state complex (exciplex), ie., a situation in which the excited state and the quencher are linked by some kind of interaction4, 38-44) (Fig. 4). Depending on its own properties and on the ex- 

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Chemical reaction is an important quenching mechanism of electronically excited states. Because of the short lifetime (generally less than 1 /xsec) of excited states in fiuid solution at room temperature, quenching by chemical reaction must be very fast if it is to occur. We shall consider here only outer-sphere electron transfer reactions of excited states since these reactions are certainly fast enough to compete with the other deactivation modes. 

For a flame to be quenched the flame arrester passageways must be small enough to extract heat from the flame faster than it can be generated by chemical reactions. The surface to volume ratio of flame arresters is impor-... 

Primary zone size is important with regard to efficiency and limits also. Within practical limits, a larger primary zone cross-sectional area will provide the best performance 138). Possible reasons arc lower velocities, less wall impingement by fuel, larger zone of low velocity, and less wall quenching of chemical reactions. The best axial distribution of open area of a combustor will depend on required operating conditions, the pressure loss characteristics, and the shape of the air entry ports. It will also depend on fuel-injection and fuel-volatility characteristics, as these factors will affect the amount of vapor fuel present at any location. If proper burning environment is to be obtained, these factors must be matched, and compromises in performance must be expected. 

In the past, combustion modeling was directed towards ffuid mechanics that included global heat release by chemical reaction. The latter was often described simply with the help of thermodynamics, assuming that the chemical reactions are much faster than the other processes like diffusion, heat conduction, and flow. However, in most cases chemistry occurs on time scales which are comparable with those of flow and molecular transport. As a consequence, detailed information about the individual elementary reactions is required if transient processes like ignition and flame quenching or pollutant formation shall be successfully modeled. The fundamental concept of using elementary reactions to describe a macroscopic... 

Fluorescence initiated by chemical reactions is called chemiluminescence. It is a common phenomenon in flames in which free radicals are oxidized by molecular oxygen but occurs rarely in solution because many of the reactive species necessary for the generation of chemiluminescence have very short lifetimes in solution. Phosphorescence as a form of chemiluminescence does not exist in solution because of quenching of the triplet state in the liquid phase. 

Similarly, enzymatic reactions can be performed directly on a MALDI plate, quenched, and the reaction products analyzed [43]. Various laser desorption/ionization (LDI)-based techniques facilitate such off-line measurements [44,45]. When the operations of initiating and quenching the chemical reactions are carried out manually, the temporal resolution of the LDI-MS-based methods is typically in the order of a few minutes. However, by implementing flow mixing and quenching methodology, one can perform observations of sub-second phenomena with this kind of off-line MS detection (see, e.g., [46-48]). 

ABSTRACT. We describe an apparatus by which the detonation products of an explosive can be identified and whose relative concentrations can be determined quantitatively. These measurements can be made on products that have been formed in less than one microsecond after the passage of the detonation wave. The technique is based on the rapid quenching of chemical reactions by virtue of the free expansion of the products into vacuum. Of course, products that have been formed over a longer period of time and under different pressure/temperature conditions can also be studied. Time resolved molecular-beam mass spectrometry is used, so that whether detonation occurred or not in forming the products can be determined. We describe optical techniques, principally Schlieren photographs, that also confirm detonation. We report measurements made on six standard explosives, PETN, RDX, HMX, HNS, TNT and TATB, and one research explosive, nitric oxide. For none of the standard explosives do we measure product distributions that agree with model predictions based on equilibrium assumptions. A computer model of the free expansion is described briefly and its importance to the interpretation of the data is emphasized. 

Figure 2 shows a schematic Jablonski diagram for a typical molecule where deactivation by chemical reaction and quenching processes are not represented. Light absorption by the ground state molecule, A, leads mostly to the spin-allowed excited states A(aj), A(a2), A(a3), etc. The spin-forbidden excited states, A(fj), A(f3>, etc., can be populated by... 

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