Electron transfer reactions Spin forbidden

Similar to other d -d systems, the drnuclear iridium(I) complex [Ir(/x-pz)(COD)]2 (23) showed spin-allowed and spin-forbidden (da — pa) absorption bands at 498 and 585 nm, respectively. Under ambient conditions, the complex displayed fluorescence at 564 nm and phosphorescence at 687 nm, which were assigned to singlet and triplet excited states of (da — pa) character. The triplet excited state of the complex was a powerful reductant with an excited-state reduction potential E° (Ir2+ ) of-1.81 V vs. SSCE. Facile electron transfer reactions occurred between the excited complex and methyl viologen and other pyridinium acceptors. The absence of an inverted effect for the forward electron transfer reactions, and the presence of such inverted behavior for the back-electron-transfer reactions were observed and explained. ... 

One area of research that has not been explored very much by the techniques covered in this chapter is biological chemistry. Many enzymes carry out electron-transfer reactions, and can undergo intersystem crossing. Many of the methodologies discussed in this chapter are applicable to biological systems exhibiting spin-forbidden transitions. 

This reaction as written is energy-deficient, i.e. the first excited singlet state of Ru(II) is inaccessible directly. The emission of the Ru(II) species, produced in this electron-transfer reaction, was therefore ascribed to a charge transfer, spin-forbidden luminescence [41],... 

Thus, the change in oxidation state that would alter the spin is either essential to cater for spin-forbidden transitions or should not occur, to let a spin-allowed reaction to occur rapidly. This principle of specificity was identified in a large array of metal-oxo cluster reactions with small molecules [53, 54] and should be transferable to real catalyst under the concept of local electronic structures at active sites. 

The only difference from the single-channel EM outlined above (Section V.A) is the substitution of k et by the sum of the spin-allowed and spin-forbidden transfer rates k et + k c, to the ground and triplet states, respectively. Like k-eh the intersystem crossing rate kKC does not depend on viscosity. Moreover, EM does not separate the two different steps of the forbidden transition spin conversion to the triplet RIP and subsequent allowed electron transfer into the triplet product [212-216]. However, as has been shown in Section XI.A, even in the case of a single channel but spin-forbidden reaction (I), one should discriminate between the spin conversion and subsequent recombination through electron transfer. The qualitative difference between the spin-allowed and... 

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