Intersystem crossing implications

Both l3A"-4b and 23A"-4b are predicted to be minima on the potential surface at the CASSCF level.61 This is in contrast with the situation for 4a, where one triplet state is calculated to be a transition state for pseudorotation of the other.55-57 A derivative of triplet 4b has been implicated in solution trapping experiments,49 but there have been no reports of the direct observation of 4b. Facile intersystem crossing from the triplet ground state (13A") to the open-shell singlet state ( A"), followed by vibrational relaxation to one of the enantiomers of 3b (Fig. 12), would be expected to rapidly depopulate the 13A" state of 4b. Thus, direct detection of triplet 4b may prove to be much more difficult than detection of 4a was.lla... 

This article examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes which are relevant to the dynamics studies are first reviewed. Then the techniques used to observe these rapid processes are described. Some aspects of solid-state dynamics are mentioned. Finally, some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins are described. 

Furan-2-carbaldehyde has been much studied. A thorough analysis of the first two major electronic transitions has been carried out. Practical work is hampered by the resinification of the compound and by the presence of a trace impurity which gives rise to a long-lived pressure-independent component in the phosphorescence spectrum.23 The absence of n ->n excited emission and other facts implicate a very efficient double intersystem crossing.14 24 Whether or not sensitized by mercury, photodecomposition of the aldehyde gives much carbon monoxide, propyne, and allene. Small amounts of furan, carbon dioxide, and acetylene are also formed. 

Yang, N. C., Kimura, M., Eisenhardt, W., Paterno Biichi Reactions of Aromatic Aldehydes with 2 Butenes and their Implication on the Rate of Intersystem Crossing of Aromatic Aldehydes, J. Am. Chem. Soc. 1973, 95, 5058 5060. 

The chemiluminescent reactions of e aq with dyes represent a new process which may have broader implications. The formation of excited dye in Reaction 8 should lead to triplet formation via intersystem crossing, although the yield would be low because of competition from Reaction 1. 

The most important implication of these data, however, is the statement they make about the nonradiative relaxation from the levels below 2500 A. If the chemical activity in the thermally equilibrated experiments is limited to the high levels reached by thermal activation, then nonradiative relaxation from the levels below about 2500 cm is a single process— intersystem crossing to the triplet state. Measurement of nonradiative decay rates from those levels could then provide an explicit picture of the dependence of intersystem crossing on the initial vibrational modes and energies. Unfortunately, proof of this proposition cannot be derived from the available data. 

They observed an initial fast decrease in conductivity (r, =lxl0 s) followed by a slower change (r, lOxlO" s). The major process (>67%) is the slower one and it has the same rate as the phosphorescent decay, which is from a doublet state. They suggested that the fast conductivity change is due to photoaquation from the quartet state and the slower and dominant decay is from the doublet state. The implication is that both doublet and quartet states can be photoactive, with their relative amounts depending on the efficiency of the intersystem crossing between these states, which in turn will depend on the ligands on the Cr(IIl). 

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