Bimolecular process, energy transfer

CFIDF end group, no selective reaction would occur on time scales above 10 s. Figure B2.5.18. In contrast to IVR processes, which can be very fast, the miennolecular energy transfer processes, which may reduce intennolecular selectivity, are generally much slower, since they proceed via bimolecular energy exchange, which is limited by the collision frequency (see chapter A3.13). 

As seen from (1) and (2), intermolecular processes may reduce essentially the lifetime and the fluorescence quantum yield. Hence, controlling the changes of these characteristics, we can monitor their occurrence and determine some characteristics of intermolecular reactions. Such processes can involve other particles, when they interact directly with the fluorophore (bimolecular reactions) or participate (as energy acceptors) in deactivation of S) state, owing to nonradiative or radiative energy transfer. Table 1 gives the main known intermolecular reactions and interactions, which can be divided into four groups ... 

Besides the excited molecule can interact physically with a second molecule, i.e. undergo bimolecular processes. These are either energy transfer (1.7) or exciplex formation (1.8) depending on the relative excitation energies of the molecule to be studied and its partner. 

Collisions which place energy into, or remove energy from, internal modes in one molecule without producing any chemical change are very important in some processes. The transfer of this energy into reactant A is represented by the bimolecular process... 

Guldi DM, Hungerbuhler H, Asmus KD (1999) Inhibition of cluster phenomena in truly water soluble fullerene derivatives bimolecular electron and energy transfer processes. Journal of Physical Chemistry B 103 1444-1453. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

There are also numerous examples of bimolecular energy transfer processes involving CTTL excited states. The Ru - bipy CT state in [Ru(bipy)3]2+, for example, photosensitizes the reactions of a number of organic and inorganic substrates by this pathway.103... 

A wide range of condensed matter properties including viscosity, ionic conductivity and mass transport belong to the class of thermally activated processes and are treated in terms of diffusion. Its theory seems to be quite well developed now [1-5] and was applied successfully to the study of radiation defects [6-8], dilute alloys and processes in highly defective solids [9-11]. Mobile particles or defects in solids inavoidably interact and thus participate in a series of diffusion-controlled reactions [12-18]. Three basic bimolecular reactions in solids and liquids are dissimilar particle (defect) recombination (annihilation), A + B —> 0 energy transfer from donors A to unsaturable sinks B, A + B —> B and exciton annihilation, A + A —> 0. 

In addition to unimolecular reactions, the excited state may participate in several bimolecular processes. At high concentrations, dimer formation, excimer formation, exciplex formation, solute-solvent complexation, energy transfer, and collosional deactivation may occur. The high-concentration conditions are often experienced when the guest molecules are loaded onto the layered materials with high coverages and specific examples will be provided shortly. 

Bimolecular quenching of the excited states of metal complexes generally involves electron transfer or energy transfer processes ( 1). Recently, however, Pt2(pop)4 " has been found to undergo a photochemical reaction involving atom abstraction as a primary photoprocess (.26). The reaction involves the catalytic conversion of isopropanol to acetone ... 

All these studies on bimolecular processes indirectly indicate the involvement of acz -nitromethane, but more detailed studies will be required to estimate the energy requirements for its formation. Our own preliminary analysis for the bimolecular transfer of hydrogen between carbon and oxygen shows this process to be spontaneous for the transfer from acz-nitromethane to the nitromethide anion, whereas that from nitromethane is 10 kcal/mol endothermic (Scheme VI). Thus, under gas phase conditions H-transfer from acz -nitromethane to the nitromethide anion should occur exclusively, but the reverse process may well occur in liquid and solid phases as the endothermicity is only modest. This is further supported by the 12 kcal/mol stabilization that results on the spontaneous carbon to oxygen transfer of hydrogen from protonated nitromethane to nitromethane [28]. 

Excited-state relaxation can proceed spontaneously in monomolecular processes or can be stimulated by a molecular entity (quencher) that deactivates (quenches) an excited state of another molecular entity, by energy transfer, electron transfer, or a chemical mechanism [lj.The quenching is mostly a bimolecular radiationless process (the exception is a quencher built into the reactant molecule), which either regenerates the reactant molecule dissipating an energy excess or generates a photochemical reaction product (Figure 4.1). 

Secondly, the rate coefficients of unimolecular bond fissions and of bimolecular combinations depends, not only on the temperature, but also on the concentrations of the species which are not chemically transformed by the elementary process under consideration, but which play a role in energy transfer processes. Various theoretical treatments of this effect have been suggested (see, for example, refs. 1—15). 

At high temperatures, both simplifications and complications of the above mechanism occur. Bimolecular initiation processes (involving at least one unsaturated molecule) cannot be excluded (see, for example, ref. 15). Transfer processes must be included since chains are no longer long. H abstraction from alkenes generates not only allylic type radicals, but also vinylic type radicals. As the temperature increases, allylic type radicals become thermally unstable. As the activation energy of unimolecular fissions of radicals is much higher than that of bimolecular processes such as metatheses, when the temperature increases the relative concentration of the p- radicals, compared with that of the thermally stable / and Y- radicals, decreases. Therefore, termination processes involve mainly / radicals (except for H- radicals, because they are very reactive and recombine in a third-order process) and Y-radicals. Finally, the addition of the most concentrated / and Y- radicals to unsaturated molecules can play a role, because this process is followed by a very fast unimolecular fission. For reasons of size limitation, the addition of radicals (e.g. H- and CH3-) will mainly be considered. Of course, the above a priori hypotheses about relative radical concentrations or reaction rates must be checked a posteriori, when numerical calculations have been carried out. 

As overlap of orbitals is necessary for energy transfer by an exchange mechanism, the maximum efficiency one would expect for this process corresponds to a diffusion controlled reaction. The rate constant for any bimolecular reaction, which depends only on the rate of diffusion of the reactants together, may be estimated from the following formula derived by Debye21... 

These complexes also usually exhibit substantial photostability under visible light irradiation and, due to their relatively long-lived triplet excited-state characteristics, the emission lifetimes are easily quenched by bimolecular electron- and/or energy-transfer processes in solution [6, 76], The electronic structures of MLCT excited molecules of diimine rhenium(I) tricarbonyl complexes can be viewed as a charge-separated species, [LRen(CO)3(diimine ")], with an essentially oxidized... 

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