Quenching bimolecular processes

A nonlinear plot of loge[ecd vs. T indicates that bimolecular processes such as triplet-triplet annihilation or triplet quenching are contributing to triplet state deactivation. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

Quenching is a bimolecular process that is, it involves the collision of both Si and Q molecules. Thus, in the presence of the quencher, where the rate constant is kQ and the rate of deactivation by quenching is QJ ... 

Since any quenching action is a bimolecular process, it is essential that the molecules M and Q should be in relatively close contact, but not necessarily in hard sphere (van der Waals) contact. Theoretical models lead to the distance dependence of the quenching rate constants as exponentials or sixth powers of r, the centre-to-centre distance of M and Q. Since these distance dependences are very steep, it is convenient to define a critical interaction distance r at which the quenching efficiency is, this being the distance at which 50% of the molecules M decay with emission of light (or undergo a chemical reaction) and 50% are quenched by some nearby molecule Q. 

Note that all the rate constants are unimolecular or pseudo-unimolecular (in the case of bimolecular processes like quenching or chemical reaction M + N-+P). 

The results clearly indicate that quenching of benzophenone triplets in polar solvents is a bimolecular process. This means that the ions exist mostly as separated ions and that the electron transfer process occurs mainly as an inter-ion-pair reaction [66, 67]. Analysis of the products after irradiation of A-(p-benzoylbenzyl)-A,A,N-tri-n-butylammonium triphenyl-n-butylborate (BTAB, Figure 14) indicates formation of p,p -(benzoyl)dibenzyl, p-pentylbenzophenone and tributylamine. This indicates that electron transfer from the borate anion to the acceptor excited state leads to homolytic C-N bond scission and formation of the tertiary amine. 

Alternatively, although Process II cannot be the rate determining process it may still be the emission process provided that the quenching of emission determines the rate of decay. If the bimolecular processes (Processes III and IV) are the important quenching processes, Equation 3 remains applicable. Also, as delayed fluorescence has been shown to be unimportant, then either k4 or the factor f is small and Equation 2 reduces to ... 

Case 4 Only the quencher migrates. This is the most common situation with excited singlet state probes. The quenching efficiency will be determined by the rate of access of the quencher to the supramolecular strucmre ( g+[H]) and the efficiency of quenching within the supramolecular system The association to the supramolecular complex is a bimolecular process, whereas the quenching efficiency within the supramolecular structure is viewed as a uni-molecular process. This picture is concepmally analogous to the formation of an encounter complex in solution before reaction, where the volume of the encounter complex is defined by the supramolecular structure. Thus, an overall effective quenching rate constant [ q(eff)] can be defined which takes into account the association process and the intrinsic reactivity ... 

Besides the earlier addressed monomolecular processes of the Sj state, bimolecular processes are also important. Interaction of an excited molecule M with another molecule mostly leads to the reduction of its fluorescence (quenching). Different quenching processes are possible, for instance ... 

Ge(CeH5)4 forms a labile donor-acceptor complex with O2 in CeHi4 at 77 K. The reversible reaction involves molecular oxygen coordination to the phenyl ring. This leads to a reduction in the intensity of the emission spectrum, the process of phosphorescence quenching being predominant among other deactivation processes of triplet states. The bimolecular process obeys the Stern-Volmer equation + Kq [O2] (I and I02 are intensities of phospho-... 

We ve seen that bimolecular processes involving excited states can take many forms. Collision can facilitate relaxation to the ground state (quenching) or formation of an excited state complex (exciplex or excimer). Alternatively, bimolecular association can occur prior to excitation, leading to an absorption complex. In this and the next two sections we consider a new outcome for the interactions of an excited state, D, with another molecule, A (Eq. 16.8). Now the result is energy transfer from one molecule to another, producing electronically excited A (A ). Different mechanisms are possible, and these energy transfer processes are very important in photochemistry and other fields. 

If the biradical cleaves to produce starting material, we have a bimolecular process that returns an excited state to its ground state. This is an example of quenching, which we discussed in Section 16.2.2. Indeed, olefins and especially polyenes are efficient quenchers of carbonyl excited states. This illustrates the statement we made in Section 16.2.2 that quenching is often closely related to a photochemical process. 

We now consider the relative rates of these processes. In the simplest type of system there is one fluorophore, one excited state, and one quencher and external quenching is an irreversible bimolecular process in which the molecules encounter each other by diffusion, and all the resulting encounter-complexes lead on to quenching. The reaction scheme obtained by combining the above processes is then summarised in Figure 6.3. 

The effect of excimer kinetics on fluorescence decays of monomers and excimers upon excitation with a short pulse was studied first by Birks et al. [119]. They took into account all the relevant processes that proceed after the excitation of a low fraction of monomers by an ultrashort pulse and derived the rate equations describing the monomer and excimer decays. Most processes involved in the Birks scheme are monomolecular and depend only on the concentration of the excited species and on the first-order rate constant one of them is a bimolecular process and depends on the concentrations of both the excited and ground-state molecules. They include (1) monomer fluorescence, (rate constant fM), (2) internal monomer quenching, M —>M, ( iM). (3) excimer formation, M - -M D (bimolecular reaction, i.e., the rate depends on the product of the rate constant and concentration of the ground-state... 

The reaction was also performed with a reduced nicotinamide molecule having a propyl side chain. The reverse reaction, however, does not take place. Free radical quenching agents had no effect on the rate of the reaction, suggesting an ionic rather than a radical mechanism. However, isotope effects with deuterated NADH show that under nonenzymatic conditions the reaction with FAD is not a simple bimolecular process. 

Important bimolecular processes in the excited state include complex formation and energy transfer processes. When these processes produce non-fluorescent species, fluorescence or phosphore>scence quenching aJone is observed. Quenching is the general word used to describe any... 

The electrochemical and photophysical properties of a variety of mixed-metal supramolecular complexes incorporating Ru(II)/Os(II)-polyazine LAs to reactive Rh(III) systems have been investigated. The coupling of Rh(III) to ruthenium and osmium chromophores has been explored in some detail due to the known energy and electron transfer quenching of MLCT states of ruthenium and osmium by Rh(III) complexes in bimolecular processes. The systems studied to date most frequently included tris(bidentate) or bis(tridentate) coordination on Rh(III). While these studies provide considerable insight into the intramolecular excited state dynamics, these coordination environments typically prevent reactivity at the rhodium site. 

As in experiments on fluorescence quenching, a unimolecular and a bimolecular process are in competition. In the simplest analysis, one makes the assumption of strong collisions, that is, whatever the initial excitation, the first collision is assumed to lower the internal energy of the excited molecule below and hence render it unreactive. If this were true, a simple two-state treatment of the kinetics would suffice, and... 

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