Photochemical reactions nonradiative decay

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

In the absence of photochemical reactions, the lowest excited states of [Re(L) (CO)3(N,N)]m complexes decay to the ground state both radiatively and nonradiatively. The lifetimes in fluid solutions range from tens of nanoseconds to microseconds, depending on L, N,N and the medium. Complexes where N,N = 1,4-diazabutadiene are mostly nonemissive in fluid solutions, having excited-state lifetimes of hundreds of picoseconds. Excited-state decay of Re complexes is a much studied phenomenon. It has been dealt with in several review articles [1, 91] and Chap. 2 of this book. Herein, we will only stress some crucial aspects ... 

The data in Table 1 refer to the nonradiative decay rate k , in DCS and DCM and are indicative of the reaction to the photochemical funnel through double-bond twisting. They reveal that k, is highly polarity dependent, slowest in strongly and fastest in weakly polar solvents (negative solvatokinetic effect). In view of the above, we recognize this as signifying that the funnel state P is of less polar nature than the precursor state E. ... 

As noted in the introduction photochemical reactions are just one example of a radiationless process. In this case there are rearrangements of chemical bonds and/or breaking of chemical bonds. It is expected that much of the theoretical developments which have provided an understanding of the simpler radiationless processes should also be useful in a proper description of photochemical reactions. The utility of the theories of these simpler processes does not arise only from the fact that photochemical processes often occur simultaneously with the simpler nonradiative decays. It is expected that experimental and theoretical studies of the product yield dependence of photochemical reac-... 

Apart from forming catalysts for photochemical reactions, some rare-earth ion complexes may also form efficient luminescent materials after incorporation into microporous crystals. Alvaro et al. loaded a europium complex into zeolite Y,111831 mordenite, and ZSM-5. Because of their confinement in the zeolite framework, the chance for the luminescent centers to decay nonradiationally is reduced, and as a result the lifetime is increased in comparison with that in solution. In the meantime, upon formation of the complex, the luminescence intensity of Eu3+ ion is distinctly increased. Therefore, it is possible to prepare valuable composite luminescent materials using microporous crystals as hosts and complexes as guests. 

Figure 16. Excitation energy dependence of nonradiative decay rates in vapor-phase pyridine, pyrazine, and pyrimidine. The dashed lines represent S, - T intersystem crossing, whereas the solid lines represent the second nonradiative process attributed to photochemical reaction. (From ref. [11] with permission.)...

A molecule in an excited state must either decay to the ground state or form a photochemical product. Therefore, the total number of molecules deactivated by radiative processes, nonradiative processes, and photochemical reactions must be equal to the number of excited species produced by absorption of light. We conclude that the sum of primary quantum yield , for all physical changes and photochemical reactions i must be equal to 1, regardless of the number of reactions involving the excited state. It follows that... 

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