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Crossing ISC

Next ISC a b is considered. In this case, if the spin-orbit coupling Hso is not large, it can be used as a perturbation for ISC, i.e., [43,44] [Pg.195]

On the other hand, if the spin-orbit coupling is large and the electronic gap is small, the singlet state and triplet state will strongly mix these states should be diagonalized and the radiationless fraction can be treated in the IC manner. For the displaced surfaces, Eq. (72) becomes [Pg.195]

It should be noted that the expressions for IC and ISC cases Eqs. (71) and (72) are quite similar except for the electronic matrix elements and energy gaps. Although the Fourier integral involved in Wl fb given above can easily be carried out numerically, analytical expressions are often desired for this purpose, the method of steepest-descent [45-51] (saddle-point method) is commonly used. Take Eq. (73) as an example. Wl b will first be written as [Pg.196]

This equation indicates that it is a real number and that when Wbal i is large, only the high frequency accepting modes play an important role in radiationless transitions, for example, if one compares the term elt a for coj = 100 cm-1 and for coj = 3000 cm-1. To apply this approximation method, it is necessary to first solve Eq. (79) for it this can be accomplished by introducing an average frequency (usually the frequency of major accepting modes) it is then obtained [Pg.196]

This is usually referred to as the energy gap expression for radiationless transitions first derived by Engleman and Jortner [52]. [Pg.197]


The sensitizer in our experiments is benzophenone (BP) which reacts as shown in Scheme 2. UV light of 300 to 400 nm is absorbed and excites the aromatic ketone group to a singlet state which by intersystem crossing (ISC) reverts to a triplet state, abstracts a... [Pg.172]

High stereospecificity is observed when the rotation of the diradical intermediate is slow in comparison with cyclization to cycloadduct or reversion to reactants. With the presence of external heavy atoms, it could facilitate the intersystem crossing (ISC) of the first-formed singlet diradical to the longer-lived triplet counterpart. The triplet diradical will have a chance to undergo rotation before it reverts back to singlet and cyclizes or cleaves to reactants. This then accounts for the reduced stereospecificity. The alternative possibility of a zwitterionic intermediate is considered unlikely because there is no interception of zwitterions by water. [Pg.393]

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)... Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...
A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). [Pg.191]

The same experiment can be carried out quantitatively. By taking into account radiationless processes, namely, internal conversion fcIC, intersystem crossing isc, and bimolecular quenching kQ[Q] with a quencher Q, the time-dependent concentrations of the donor D and the acceptor A in the excited singlet state Si, [Ds,] and [As,] can be expressed as follows ... [Pg.52]

Intersystem crossing (ISC) is the intramolecular crossing from one state to another of different multiplicity without the emission of radiation. In Fig. 3.9 (ISC), shows the transfer from the first excited singlet state S, to the first excited triplet state T,. Since the process is horizontal, the total energy remains the same and the molecule initially is produced in upper vibrational and rotational levels of T, from which it is deactivated as shown by the vertical wavy line. Similarly, (ISC)2 shows the intersystem crossing from T, to upper vibrational and rotational states of the ground state S(l, from which vibrational deactivation to v" = 0 then occurs. [Pg.50]

Zafiriou (1983), for example, suggested that absorption of light by organics, followed by intersystem crossing (ISC) to the triplet state (T) as described in Chapter 3, could occur. The subsequent reaction of the organic in a triplet state with 02 could then give 02 ... [Pg.315]

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. [Pg.519]

In the liquid phase, singlet phenylnitrene is rapidly relaxed by collision with solvent and cannot surmount the barrier to form cyanocyclopentadiene at ambient temperature. Under these conditions PN isomerizes over a small barrier to form cyclic ketenimine K. Later, computational work of Karney and Borden would show this to be a two-step process involving benzazirine BZ, the species trapped by ethanethiol (Scheme 2). In the liquid phase, PN prefers rearrangement to intersystem crossing (ISC) to the lower-energy triplet state at ambient temperature. Intersystem crossing is not an activated process and its rate is not expected to vary with temperature. The rate of... [Pg.258]

Absorption by the chromophore ligand is commonly (6) followed by inter-system crossing (ISC) to the triplet state of the ligand. This state can decay either by emission (ligand phosphorescence) or by... [Pg.363]

Figure 14.11. (a) Orbital and (b) state correlation diagrams for the decomposition of dioxetane. The observed chemiluminescence may be due to intersystem crossing (ISC) to the triplet manifold in the region denoted by the shaded circle in (b). [Pg.208]


See other pages where Crossing ISC is mentioned: [Pg.434]    [Pg.462]    [Pg.462]    [Pg.315]    [Pg.48]    [Pg.148]    [Pg.254]    [Pg.162]    [Pg.229]    [Pg.623]    [Pg.724]    [Pg.172]    [Pg.175]    [Pg.80]    [Pg.85]    [Pg.61]    [Pg.42]    [Pg.129]    [Pg.34]    [Pg.132]    [Pg.291]    [Pg.482]    [Pg.300]    [Pg.31]    [Pg.2]    [Pg.196]    [Pg.211]    [Pg.239]    [Pg.127]    [Pg.128]    [Pg.128]    [Pg.162]   


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ISC

Intersystem crossing, ISC

IscS

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