Excited states bimolecular reactions

There are significant differences between tliese two types of reactions as far as how they are treated experimentally and theoretically. Photodissociation typically involves excitation to an excited electronic state, whereas bimolecular reactions often occur on the ground-state potential energy surface for a reaction. In addition, the initial conditions are very different. In bimolecular collisions one has no control over the reactant orbital angular momentum (impact parameter), whereas m photodissociation one can start with cold molecules with total angular momentum 0. Nonetheless, many theoretical constructs and experimental methods can be applied to both types of reactions, and from the point of view of this chapter their similarities are more important than their differences. 

Research Opportunities. The presence of a long-lived fluorescing state following either 532 nm or 1064 nm excitation of PuF6(g) provides a valuable opportunity to study the extent to which electronic energy in a 5f electron state is available in photochemical and energy transfer reactions. Such gas phase bimolecular reactions would occur in a weak interaction limit governed by van der Waals forces. Seen from the perspective of potential photochemical separations in fluoride volatility... 

Figure 4. Energy diagram for 532 nm excitation of PuF g). The 5f electron states of PuF are shown at the left. The solid arrows indicate photon absorption or emission processes. The wavy arrows indicate nonradiative processes by which excited states of PuFg may be lost. The laser-fluence dependent fluorescence decay found at this excitation wavelength can be explained in terms of a bimolecular reaction between PuFg(g) in its 4550 cm l state and PuF (g) to form PuFj(g). It is assumed that PuF (g) is formed via dissociation of the initially populated PuF state.

Balzani, V., Bolletta, F., Gandolfi, M. T., and Maestri, M. Bimolecular Electron Transfer Reactions of the Excited States of Transition Metal Complexes. 75, 1-64 (1978). 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

The lowest excited state, Ru(bpy)j+, has a sufficiently long lifetime (-0.6 ps) to allow bimolecular reactions to compete with other... 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

In these bimolecular reactions the lifetime of the reacting excited state is a very important factor. In view of lifetime considerations, longer-lived triplets might be more likely candidates for the reacting excited states than singlets. Most nucleophilic aromatic photosubstitutions seem to proceed via (7r,7t )-triplet states, but cases of the intermediacy of singlet states are also known. 

Again, a hnear relationship of and [CNS] i shows a bimolecular reaction between the excited triplet state of 1 and the nucleophile to take place. The triplet lifetime of la is 4.7 X 10 s in water and 1.2xl0 8s in aqueous 10 2M solutions of potassium cyanide as determined from quenching studies The nitro group in 7 a is likewise replaced photochemically by methoxide and cyanate ions. 

These kinetic data suggest a pathway in which the nitrophenyl ester or ether, brought into an excited state by absorption of a light quantum, reacts in a bimolecular process with the nucleophile or returns to the ground state of the original molecule. At high nucleophile concentrations every excited molecule has one or more encounters with the nucleophilic reaction partner and the... 

The rate of intersystem crossing is just as important as its efficiency. Obviously, if the rate of intersystem crossing is faster than that of diffusion in solution (usually on the order of 1010 sec"1), bimolecular reactions of the excited singlet are precluded. Unfortunately, the intersystem crossing rates are available for only a few carbonyl compounds.11,12 It is known that the rate of intersystem crossing for aliphatic carbonyl compounds (e.g., acetone) is slow (4-20 x 107 sec-1)30 in comparison to that for aromatic carbonyl compounds. Thus, aliphatic (and perhaps some aromatic) carbonyl compounds have an opportunity to react in the excited singlet state. 

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