Bimolecular Excited State Processes

Case C Q is not in large excess and mutual approach of M and Q is possible during the excited-state lifetime. The bimolecular excited-state process is then diffusion-controlled. This type of quenching is called dynamic quenching (see Section 4.2.2). At high concentrations of Q, static quenching may occur in addition to dynamic quenching (see Section 4.2.4). 

The uranyl ion luminesces in fluid solutions at room temperature2 201 202 thus providing a tool for the study of bimolecular excited state processes. In several cases, however, this study is complicated by the fact that the uranyl ion forms complexes with a variety of chemical species, so that it is often difficult to distinguish between intramolecular photochemical processes involving uranyl ion-ligand complexes and intermolecular photochemical processes involving reaction between an electronically excited UO + species and the substrate2,201,202 ... 

Bimolecular excited state electron transfer reactions have been investigated extensively during the last decade (1-3). Electron transfer is favored thermodynamically when the excitation energy E of an initially excited molecule A exceeds the potential difference of the redox couples involved in the electron transfer process. 

Table I. Bimolecular Excited State Quenching Processes...

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.

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 )... 

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. 

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... 

Radical substitution plays a part in the thermal chemistry of aromatic compounds, but not in the photochemistry, except in so far as many radicals that attack aromatic compounds are generated by photochemical methods from other addends. The reason for this is that reactive radicals exist only in low concentrations, and electronically excited states similarly are formed only in low concentrations the rate of bimolecular reaction between two such reactive species is generally much lower than the rates of alternative processes such as attack of the radical on ground-state aromatic compounds. 

The reaction between ground state oxygen atoms 0(3P) and the monoflu-orocarbene species CHF(X1A ) possesses all three features of the PES discussed above. The reaction proceeds at almost gas-kinetic rate at room temperature [128,129], and the reaction channel (12) to produce CO and HF products in their ground electronic states (in a spin-forbidden process) is one of the most exothermic bimolecular reactions known, and several other product channels, such as reactions (13) and (14) as well as the production of electronically excited states, can occur. Pulsed IR chemiluminescence was observed following IRMPD of 10-40 mTorr of CH2F2 in the presence of O atoms (5-25 mTorr, and measured by titration), and was passed through the SS interferometer and recorded by one of three detectors InSb (1840-... 

A third type of bimolecular reaction, summarized by equation (24), is that of excited state proton transfer where Q is now a proton acceptor. This type of process was proposed in the case of the OH" and CO32- quenching of emission from [RhCl(NH3)5]2+.48... 

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... 

This provides an illustration of the way in which the chemical outcome of a light-induced reaction can depend on the nature of the reactive excited state triplet states lead to long-lived primary products which can take part in further bimolecular processes, whereas singlet states lead to fast uni-... 

Concerted cycloadditions can exist in principle when the photochemical process originates from a singlet excited state. Such reactions are rather exceptional in bimolecular cycloadditions, simply because singlet excited states have short lifetimes (in the ns time-scale), so that encounter with a... 

Chemiluminescence can occur when a thermal (dark) reaction is so exothermic that its energy exceeds that of the electronically excited state of one of the product molecules. The major pathway for these reactions is the decomposition of cyclic peroxides, and this is at the basis of most bioluminescence processes. There are some other physico-chemical processes which can lead to the formation of excited states and thereby to the emission of light these are based on the bimolecular recombination of high-energy species such as free radicals and radical ions. 

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