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Photophysical steps

This is the process used in most photocopying machines. It is largely a mechanical process which uses one key photophysical step based on photo-induced electron transfer of a rather special kind. [Pg.192]

ENERGY TRANSFER STEP a photophysical step in which an excited state of the donor is deactivated to a lower-lying energy level while simultaneously, a different species, the acceptor, is raised to a higher energy level. [Pg.190]

Photophysical steps E => T2 back-transfer The photoactive states Direct reaction The photosolvation mechanism Cr(CO)e Co(CN)i-Rh(III) complexes Summary and conclusions References... [Pg.215]

The questions to be addressed in this context have historically been characterized as photophysical and photochemical, categories whose boundaries are sometimes ill-defined. The photophysical steps include intrastate vibrational relaxation, photon emission (fluorescence and phosphorescence), and interstate radiationless transitions (internal conversion between states of the same multiplicity and intersystem crossing between states of different multiplicity). The major aim in this area is to determine the rates of the individual steps and the relationship between molecular structure and these rates. Other goals are to identify the photoactive state and to detail the reaction mechanism. [Pg.216]

The same approach applies to photoprocesses, with one significant difference. In the ground state, when no chemical reaction obtains, the lifetime of any A species is governed only by the interconversion rate between the various solvates. In excited states, competition between the photophysical steps, e.g., Af => A(, and solvent motion must be considered. Two extreme limits can be treated (1) a rigid environment where there is no environmental movement on the time scale of any excited state process, and (2) a very fluid environment where the converse prevails. The luminescence of complexes that occupy multiple sites in crystals is an example of the first limit, while long-lived luminescence in low viscosity solvents conforms to the second limit. [Pg.227]

Photoreactions are treated as pseudo first order kinetics. As demonstrated in Section 2.1.3.3, besides the photochemical step, a variety of photophysical steps such as radiationless transitions are included in the mechanism. Excited states are intermediates. The considerations typical in thermal kinetics as, for example, Bodenstein s hypothesis, can also be applied to these steps. Thus an overall treatment of the total mechanism becomes possible. [Pg.4]

In contrast to thermal reactions, any photochemical reaction is accompanied by a number of photophysical processes which all have to be taken into account in the reaction scheme. Most of these processes are thermal reactions. The mechanism of the photoreaction and - as we will see in the examples - the photochemical quantum yield depends on these photophysical steps. One of the most simple photoreactions is a photo-isomerisation... [Pg.40]

Conclusion Most of the photophysical steps are thermal reactions, for which the Bodenstein hypothesis is valid. The photochemical quantum yield depends on these elementary steps. [Pg.43]

The reduction of the reaction scheme because of linear dependencies between partial steps works for photoreactions in the same way as for thermal reactions. In addition it has to be considered that intermediates are negligible and allow the application of the Bodenstein hypothesis. Therefore the number of steps reduces to one per photoreactive step. Between these steps, linear relationships can happen. Photoreactions and true thermal reactions (not the photophysical steps) can exhibit linear dependencies. The simplest case is a photoisomerisation superimposed on a thermal back-reaction. [Pg.51]

As demonstrated the Jacobi matrices are more complex in the case of photokinetics, since instead of the rate constant the quantum yield together with the photokinetic factor and the absorption coefficient have to be taken. Furthermore all the photochemical steps are accompanied by photophysical steps. Tliese problems are dealt with in the next chapter. [Pg.143]

Based on the fundamental considerations in Sections 1.3 and 1.4, the principle of quantum yield was introduced in Section 2.1.2 to allow a treatment of photochemical reactions in a way comparable to thermal reactions. The difference from thermal reactions has been demonstrated by taking account of the photophysical steps in Sections 2.1.3.3 and 2.1.4.3. These have to be considered in detail to find out whether the partial photochemical quantum yield depends on the intensity of the irradiation source or even on concentrations. Furthermore the definitions derived in Chapter 2.1 and summarised in Table 2.2 are used. In particular the definitions for the degrees of advancement for partial steps in general x, for photophysical steps in special x-, and for linearly independent steps x of the reaction procedure have to be remembered. [Pg.145]

In the following in addition to the examples in the sections mentioned above and the derivations in Appendix 6.2 many of the possible mechanisms are reviewed and classified in brief. To avoid repetition, in some cases details of the derivations are found in Appendix 6.6. However, since the formulae derived depend on the chosen numbering of the photophysical steps and the reaction scheme, in most cases both have to be set up to make the relationships obtained obvious. [Pg.145]

As in the case of photoisomerisations, the reaction can proceed either via the singlet or the triplet intermediate. The component B is part of the reaction in contrast to the sensitisation mechanism discussed above. In addition both pathways via the singlet or triplet state can be quenched. The photophysical steps are summarised in Table 3.2 and the reaction scheme is derived in Appendix 6.6.1.1. In the following examples this information is used to derive the time laws and the quantum yields for 4 types of photoaddition reactions according to either the normal addition reaction... [Pg.149]

Time-resolved measurements were initiated both by physicists, who were principally interested in photophysical processes that left the chemical structures of the molecules intact, and by chemists, who were mainly interested in the chemical alterations of the irradiated molecules, but also in the associated photophysical steps. The parallel development of these two lines of research is reflected in the terminology. For example, the term flash photolysis, as used by chemists, applies to time-resolved measurements of physical property changes caused by chemical processes induced by the absorption of a light flash (pulse). Flash photolysis serves to identify short-lived intermediates generated by bond breakage, such as free radicals and radical ions. Moreover, it allows the determination of rate constants of reactions of intermediates. Therefore, this method is appropriate for elucidating reaction mechanisms. [Pg.39]

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... [Pg.165]

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. [Pg.212]

E. Lippert, Photophysical primary steps In solutions of aromatic compounds , Arc. Chem. Res., 3, 1970, 74. [Pg.358]

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. [Pg.417]

Photomorphogenesis, 231 Phycocyanobilin, 237 Phytochrome, 230 molecular weight of, 232 native undegraded, 232 P(r (far-red absorbing form), 236 bilatrene chromophore structure of, 236 photophysical properties of, 236 Pfr — Pr transformation, 234 Pr — Pfr transformation of, 234 chemical nature of individual reaction steps of, 263... [Pg.384]

Photophysics and photochemistry are relatively young sciences, a real understanding of light-induced processes going back some 50 or 60 years. The development of quantum mechanics was an essential step, as classical physics cannot account for the properties of excited states of atoms and molecules. In the past 30 years the advent of new experimental techniques has given a major impetus to research in new areas of photochemistry, and these are the subject of this final chapter. It must of course be realized that these developments advance all the time, and that we talk here of a moving frontier, as it is in 1992. [Pg.256]


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