Highly excited solvent states

The chemistry of "hot" intermediates pre-thermalized charges, highly excited solvent states, energetic fragments (such as "hot" H atoms). What is the nature of these states What role do they play in radiolysis How do they relax and react What happens to the heat dissipated in radiolytic reactions What is the mechanism for vibrational deactivation of the products Could the heat be... 

Another form of radiationless relaxation is internal conversion, in which a molecule in the ground vibrational level of an excited electronic state passes directly into a high vibrational energy level of a lower energy electronic state of the same spin state. By a combination of internal conversions and vibrational relaxations, a molecule in an excited electronic state may return to the ground electronic state without emitting a photon. A related form of radiationless relaxation is external conversion in which excess energy is transferred to the solvent or another component in the sample matrix. 

Tanaka et al reported that photoreduction of methylene blue in different solvents is accomplished, not only through the interaction between the lowest triplet state of MB and the reducing agent, but that the excited triplet state also plays an important role owing to the exceedingly high laser intensity. 

Indeed, things are slightly more complicated, because the electrons of the solvent can respond on the timescale of the absorption. Thus, in discussing solvent effects, it is helpful to separate the bulk dielectric response of the solvent, which is a function of s, into a fast component, depending on where n is the solvent index of refraction, and a slow component, which is the remainder after the fast component is removed from the bulk. The initially formed excited state interacts with the fast component in an equilibrium fashion, but with the slow component frozen in its ground-state-equilibrium polarization. The fast component accounts for almost the entire bulk dielectric response in very non-polar solvents, like alkanes, and about one-half of the response in highly polar solvents. 

In chloroform solvent, the platinum dithiolene complexes Pt(S2C2R2)2 are photooxidized between 300 and 350 nm, providing the complexes used are those for which the R groups result in redox potentials in the 0.1 to 0.5 V (vs SCE) range. The results are consistent with the reaction shown in equation (534).1849 Further work on this system identifies the fact that several excited states are probably photoreactive in this process,1850 and no definitive answer on the excited state reactivities is yet available. Very recently, however, highly structured solid-state emissions have been observed for Pt S2C2(CN)2 P(OR)3 2, and this data may help resolve some of these questions.1851... 

Figure 3.51 Diagram of the crossing of two excited states of different dipole moments as a result of solvent stabilization. State (a), e.g. a CT state is stabilized below state (b) in highly polar solvents. f(D) is the Onsager polarity function of the solvents shown by arbitrary symbols...

---