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Reductive Quenching of Photoexcited Cu(dipp)2+ and Cu(tptap)2+ by Ferrocenes (dipp = 2,9-Diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-Tetraphenyl-1,4,5,8-tetraazaphenanthrene)

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journal contribution
posted on 21.07.1998, 00:00 by Kurstan L. Cunningham, David R. McMillin
The metal-to-ligand charge-transfer excited states of Cu(NN)2+ systems tend to be good reducing agents but poor oxidants for kinetic and thermodynamic reasons. However, this report demonstrates that reductive electron-transfer quenching is an important pathway for ferrocenes that react with the photoexcited states of Cu(dipp)2+ and Cu(tptap)2+ in methylene chloride (dipp = 2,9-diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-tetraphenyl-1,4,5,8-tetraazaphenanthrene). In the case of the dipp complex the bulky isopropyl substituents inhibit structural relaxation within the excited state, and the self-exchange rate for reductive quenching is quite favorable, k** ≈ 2 × 108 M-1 s-1. Even in the absence of a significant kinetic barrier to reaction, however, for energetic reasons only extensively methylated ferrocene derivatives with relatively negative reduction potentials are capable of transferring an electron to the excited state. In contrast, every ferrocene derivative investigated, except diacetyl ferrocene, reacts with the charge-transfer excited state of the tptap complex by an electron-transfer mechanism. This is mainly due to a difference in driving force which is about 0.5 V greater for the tptap complex. This system also has a favorable self-exchange rate, k** ≈ 5 × 107 M-1 s-1, evidently because the juxtapositioned phenyl substituents inhibit low-symmetry distortions within the ground state as well as the excited state. Although energy transfer to ferrocene is also possible, this is a less competitive process with the tptap complex because the zero−zero energy of the reactive excited state is rather low (3E00 ≈ 1.7 V).