Intersystem-crossing processes complexes

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Table II. Rate Constants for Intersystem-Crossing Processes in Co(II), Fe(II) and Fe(III) Complexes.

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... 

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. 

The spin state lifetimes in solution of the complexes II and III have been measured directly with the laser Raman temperature-jump technique189). Changes in the absorbance at 560 nm (CT band maximum) following the T-jump perturbation indicate that the relaxation back to equilibrium occurs by a first-order process. The spin-state lifetimes are r(LS) = 2.5 10 6 s and r(HS) =1.3 10 7 s. The enthalpy change is AH < 5 kcal mol-1, in good agreement with that derived from x(T) data in Ref. 188. The dynamics of intersystem crossing processes in solution for these hexadentate complexes and other six-coordinate ds, d6, and d7 spin-equilibrium complexes of iron(III), iron(II), and cobalt(II) has been discussed by Sutin and Wilson et al.u°). 

In the non-CT radiationless transition the change in electronic charge interacts with the nuclei in a similar maimer both before and after the transition. Two types of processes can be identified internal conversion processes in which the transition is between spin states of the same multiplicity and intersystem crossing process in which the transition is between states of different spin multiplicity. For non-CT internal conversion processes the full BO (Bom—Oppenheimer) adiabatic wave-functions for the supramolecular complex are used as the zero-order basis [42-44]. The perturbations that cause the transition are the vibronic coupling between the nuclear and electron motions. These are just the terms that are neglected in the BO approximation [45]. The terms are expanded (normally to first order) in the normal vibrational coordinates of the nuclei as is customarily done for optical vibronic transitions. Thus one obtains Eq. 61b for cases when only one normal mode couples the two states... 

In order to illustrate the complexity of excited states reactivity in transition metal complexes two selected examples are reported in the next section dedicated to the ab initio (CASSCF/MR-CI or MS-CASPT2) study of the photodissociation of M(R)(CO)3(H-DAB) (M=Mn, R=H M=Re, R=H, Ethyl) complexes. Despite the apparent complexity and richness of the electronic spectroscopy, invaluable information regarding the photodissociation dynamics can be obtained on the basis of wave packet propagations on selected 1-Dim or 2-Dim cuts in the PES, restricting the dimensionality to the bonds broken upon visible irradiation (Metal-CO or Metal-R). The importance of the intersystem crossing processes in the photoreactivity of this class of molecules will be illustrated by the theoretical study of the rhenium compound. 

The high spin-orbit coupling constant of the platinum nucleus (x = 4,481 cm ) should facilitate both the S T intersystem crossing process and the T S radiative decay. However, the extent to which it does so in a complex will depend upon the contribution of metal atomic orbitals to the excited state. In many simple Pt(II) complexes with relatively small hgands, the metal s involvement is such that triplet state formation is very fast, of the order of 10 s [7]. Since this greatly exceeds typical singlet radiative rate constants of aromatic ligands, emissimi is then... 

Triplet state peculiar features of inclusion complexes (spectral changes, lifetimes) proved to be useful for investigating both thermodynamic and kinetic aspects of binding and gave information on complex conformational dynamics. The rate for the intersystem crossing process is mainly controlled by intramolecular spin-orbit electronic interactions, so that it is not much influenced by the CD environment. However, the triplet quantum yield and decay kinetics of included guests are often strongly modified and, sometimes, a room temperature phosphorescence appears. 

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