Diimines excited states

The cationic monomeric complexes Re(CO)4(diimine) formed in the reaction of Eq. (6,39) when L=CO show excited state redox reactions. Reductive quenching experiments of the excited state with substituted methoxybenzenes and with chloride ion leads to an sq)proximate value of 1.5 V being found for the Re(CO)4(di-imine) /Re(CO)4(diimine) excited state potential. Photosubstitution is also observed with these complexes, although a definitive assignment of the photoactive states that lead to these different reactions has not yet been achieved. 

The requirement of hydrogen peroxide or oxygen in diazaquinone chemiluminescence appears to be another argument against the hypothesis of Albrecht 128> or of Kautsky and coworkers 132> that the diazaquinone is simply hydrolyzed to yield aminophthalate and diimine which in turn reduces other diazaquinone molecules to luminol in an excited state so that it then emits. 

There is a small solvent dependence for [Ru(L)3]2+complexes which depends on the structure of L. The emission spectra of the Ru(bpy)32+ is mildly sensitive to media, the Ru(phen)32+ much less so, and the Ru(Ph2phen)32+ is virtually media independent. This decreasing sensitivity to solvent perturbation is a consequence of the excitation being localized in the metal a-diimine portion of the complex (-N=C-C=N).(27) The more extended the complex, the greater the shielding of the excited portion and the smaller the solvent perturbations of the emission spectrum. In particular, the bulky phenyl groups are extremely effective at shielding the excited state from environmental perturbations. 

Ru(CN)jNO reactions with OH , SH and SOj" resemble those of the nitroprusside ion, with attack at the coordinated nitrosyl to give analogous transients and similar second-order rate constants. Ruthenium(II) complexes of the general type Ru(N2), Nj = biden-tate hgands, are important reactants. The relative inertness of Ru(NH3) + and Ru(diimine)f+ towards substitution makes these complexes definite, although weak, outer-sphere reductants (Tables 5.4, 5.5, 5.6 and 5.1). Ruthenium(ll) complexes of the general type Ru(diimine)f +, and particularly the complex Ru(bpy)j+, have unique excited state properties. They can be used as photosensitizers in the photochemical conversion of solar energy. Scheme 8.1 ... 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

The intriguing photochemistry of these complexes in relationship to their excited state properties certainly deserves more attention.Subject to further study are also the stability and electron distribution of the radicals M (CO)3(a-diimine) and the identification and chemical properties of the highly reducing species M (CO)3(a-diimine)(PR3). 

Since an initial report by Eisenberg in 1989 [38], a large number of plat-inum(II) diimine dithiolate complexes are now known which are luminescent in fluid solution. Although their emission is typically weak under ambient conditions, and some earlier examples had been noted to be emissive only at 77 K [39], this class of complex is of considerable interest owing to the wide range over which the excited state energies and redox poten-... 

Pioneering works by Harriman and Sauvage reported the zinc(II)-gold(III) bis(porphyrin)-type complexes with diimine linkers, where the photoinduced electron transfer process from the zinc(II) porphyrin excited state to the... 

Keywords TDDFT Excitation energies Excited states Transition metal complexes Electronic spectra Metallotetrapyrroles Metallocarbonyls a-diimine complexes Porphyrins Porphyrazines Phthalocyanines Werner complexes Sandwich complexes... 

---