Back intersystem crossing

To conclude this section, many systems are complicated by the presence of two (or more) low lying states (as in Figure 6). If these states can intercommunicate, one must add to the kinetic scheme various intersystem crossing processes. Thus for the Crin case, one may need to consider the processes in equation (22), where and bisc are the rate constants for intersystem crossing and for back intersystem crossing, respectively. The kinetic analysis at this point can become rather complex.27-28... 

Figure I. State energies (a) and qualitative potential energy surfaces (b) for Cr(NH3) +. Alternative mechanistic proposals for (2E)Cr(III) de cay are illustrated in 1(b) a, back intersystem crossing b, direct reaction to yield electronically correlated products c, surface crossing to some ground state intermediate potential energy surface.

The pathway for the relaxation of the excited doublet state is described in Ref. 159. Three models for the relaxation of the lowest-energy exited states were employed (a) the quenching of the excited state by direct chemical reaction (b) the back-intersystem crossing to excited quartet state (c) would account for the... 

Chromium(III) photosubstitution can be discussed in terms of two limiting mechanisms [97] (1) intersystem crossing from quartet states formed by initial excitation to the doublet manifold followed by reaction from the relatively long-lived D0 or (2) reaction from the quartet state(s) via thermally promoted back-intersystem crossing from the doublet and/or promptly upon formation via initial excitation. Various elaborations on these themes include (3) competitive ligand labilization from both states and (4) formation of a GS intermediate followed by competitive reactions to give products or reform reactants. 

For other Cr(III) complexes, however, neither sensitization nor quenching can fully discriminate between the T2g and Eg excited states because these are very close in energy and therefore back intersystem crossing from Eg to can also occur. The data obtained from quenching experiments, however, revealed (20, 21, 22, 23, 24) that the photoreaction is less quenched than the phosphorescence emission. This indicates that at least part of the photoreaction originates directly from the quartet state. 

The issue of quartet versus doublet reactivity becomes considerably more complicated for most other Cr complexes such as [Cr(en)3] +, tranj-[Cr(NCS)4(NH3)2] and tran5-[Cr(NCS)2(en)2]. Both phosphorescence and photochemistry can be quenched in these systems, with the former process being affected to a greater degree. While the unquenchable portion of the photochemistry most likely involves prompt reaction from the lowest excited quartet state, the quenchable component could result from either (i) delayed reaction from the quartet following back-intersystem crossing from the doublet, or (ii) reaction from the doublet state itself. Despite a rather extensive series of investigations, the question as to which pathway (i or ii) dominates is still contentious. ... 

Often more complicated excited state decay behavior is observed. For example, if the Si—Ti energy gap is small (Figure 1), back intersystem crossing may occur and the fluorescence (Si —> So) will consist of a prompt decay and a much longer-lived decay that is associated with the repopulation of the Si state from the Ti state. An excellent discussion of such cases is given in books by Ferraudi and Demas. ... 

Intersystem crossing refers to the process whereby an electronic excited state may be converted to another excited state of similar or lower energy. Back intersystem crossing refers to the reverse process. 

The system in Figure 7.1 could be expanded to include formation of products from the initially populated state I or from other photoactive states produced from I or A. Furthermore, there could be back intersystem crossing from A to I. It is also possible to have the simpler case in which I is the only photoactive state. 

The rather contentious question of the relative importance of either doublet or quartet states in the photoaquation of [Cr(en)3] ion has been examined by three groups. " The evidence reported earlier in favor of a slow reaction from the doublet state has been reexamined by Kirk, who concludes that the involvement of a quartet state is more likely. Similarly, from a study of the photoaquation of A-[Cr(en)3] between 365 and 685 nm in aqueous acidic solutions, it is concluded that the lowest lying quartet state is photoreactive, with back intersystem crossing from the lowest doublet state to the quartet state responsible for the delayed reaction. " The wavelength-independent quantum yields of the three products, A-cis, A-cis-, and frans-[Cr(en)2(enH)(H02)] are 0.10, 0.03, and 0.24, respectively. Very recently, however, good evidence in favor of doublet state reactivity has been found. Irradiation at 669.2 nm populates the Eg state and enhances the reaction efficiency by 50%. At this wavelength there is little or no competitive absorption from the quartet state. ... 

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