Reaction of the excited states

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). 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

Chemical reaction of the excited state (secondary charge separation, isomerisation, disassociation etc). 

Bimolecular Electron Transfer Reactions of the Excited States of Transition Metal Complexes... 

But we are not doing a thermal reaction. If you look back at the orbital diagram above, you will see that it is the HOMO/HOMO and LUMO/LUMO interactions that now matter in the reactions of the excited state. The sizes of the coefficients in the LUMO of the alkene are the other way round to those in the HOMO. There is one electron in this pair of orbitals—in the LUMO of the enone in fact, as the enone has been excited by the light—so overlap between the two LUMOs (shown in the frame)... 

In equation (1) K y is referred to as the Stern-Volmer constant Equation (1) applies when a quencher inhibits either a photochemical reaction or a photophysical process by a single reaction. <1>° and M° are the quantum yield and emission intensity (radiant exitance), respectively, in the absence of the quencher Q, while <1> and M are the same quantities in the presence of the different concentrations of Q. In the case of dynamic quenching the constant K y is the product of the true quenching constant kq and the excited state lifetime, t°, in the absence of quencher, kq is the bimolecular reaction rate constant for the elementary reaction of the excited state with the particular quencher Q. Equation (1) can therefore be replaced by the expression (2)... 

The p,Y-acetyIenic ketone 6 undergoes photochemical C—C cleavage at a position which is propargylic to the triple bond (equation 11) , but this is more likely to be an a-cleavage reaction of the excited state of the ketone group rather than a reaction of the alkyne excited state. 

Photochemical reactions involving photo-excited states can also be catalyzed as well as the thermal reactions of ground states. However, the lifetimes of excited states are usually very short, particularly for the singlet excited states, and accordingly reactions of the excited state should be fast enough to compete with the decay of the excited state to the ground state (typically the lifetime is 10" -10 " s). Hence there seems to be little chance of catalysis to accelerate the reactions of excited states, which are already fast. There are many cases, however, such that photochemical reactions can be accelerated by some added substances which act as catalysts in the photochemical reactions [62-65]. Photoinduced electron transfer reactions can also be accelerated by the presence of an appropriate catalyst [52]. 

Further evidence for the long-lived excited state of acetylene is found in the chem-ionization experiments of Koyano et at 1216 A. The intensities of the ions C4H3 and C4H2 were found to be second order in acetylene pressure. The ions were attributed to the reaction of the excited state of acetylene with a ground state acetylene molecule to yield the ion, an electron and the appropriate hydrogen fragment. 

Although the reaction of the excited states caimot be ruled out for the shock reaction in question, Mimura [140] proposed that a tlrermochemical reaction of tlie ground states dominates the shock reaction within tlie experimental limits. This hypothesis is discussed below. 

Fully thermalised excited states may be treated as distinct chemical species with their own equilibrium thermodynamic properties, including redox potentials. We may therefore define standard redox potentials U°.. and f/%. for reactions of the excited states D and A ... 

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