Photoreactive State

It should be emphasized that solvation of excited electronic states is fundamentally different from the solvation of closed-shell solutes in the electronic ground state. In the latter case, the solute is nonreactive, and solvation does not significantly perturb the electronic structure of the solute. Even in the case of deprotonation of the solute or zwitterion formation, the electronic structure remains closed shell. Electronically excited solutes, on the other hand, are open-shell systems and therefore highly perceptible to perturbation by the solvent environment. Empirical force field models of solute-solvent interactions, which are successfully employed to describe ground-state solvation, cannot reliably account for the effect of solvation on excited states. In the past, the proven concepts of ground-state solvation often have been transferred too uncritically to the description of solvation effects in the excited state. In addition, the spectroscopically detectable excited states are not necessarily the photochemically reactive states, either in the isolated chromophore or in solution. Solvation may bring additional dark and photoreactive states into play. This possibility has hardly been considered hitherto in the interpretation of the experimental data. 

The well-known photochromic transformations of anthraquinones are closely associated with the photoinduced migration of hydrogen, acyl, or aryl groups. Although photochromism of these compounds fits the reaction shown in Scheme 9, the processes of photochromic transformation exhibit some features related to the nature of the photoreactive state and details of the mechanism of the photochromic transformations. 

The nature of the photoreactive state, as opposed to the luminescing state, is still the source of some debate, as states other than the lowest triplet have been proposed to be important. Evidence that an excited singlet may be involved comes from time-resolved studies of the luminescence of some haloammine complexes, where a weak fluorescence, with a lifetime of about lOOps, was... 

Whatever the nature of the photoreactive state, photolysis of a Rh]I1 amine leads to increased electron density in the metal-centered, a eg orbitals. Ligand labilization (especially of strongly cr-donating ligands), and subsequent solvolysis, is the anticipated (and observed) reaction. Photoinduced substitutions have now been reported for a large number of Rhm amines, and some of the results are summarized in Table 49. 

By contrast, lifetimes of a nanosecond or longer can be inferred for the actual photoreactive state. First, since quantum yields are rarely greater than a few tenths, the dominant dissipative process is not one... 

The temperature dependence of was reported for a number of systems. With Cr(III) complexes, values of E range from near zero up to 10 or more kcal/mole 14, 15), Ecr values of 12 or more kcal/mole thus seem not uncommon. One may then make the following analysis. The rate constant for a first-order reaction should be about 10 exp-(—E /RT), neglecting activation entropy. For Ecr = E c l2 kcal/ mole, the rate constant kcr would be about 10 sec at room temperature. Even for an Ecr of 4 kcal/mole, kcr is about 10 sec which still corresponds to an excited-state lifetime of thousands of vibrational periods. In summary, the prevalence of activated photochemistry strongly suggests that the photoreactive state is much longer lasting than would be expected were it a Franck-Condon state. The situation is that expected for a thexi state. 

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