CT transitions

Figure 4. The calculated spectrum of the complex after a Lorentzian band convolution. Region I is dominated by bridging-sulfur-to-iron CT transitions, while region II is mostly due to organic-sulfur-to-iron electron transitions. Regions I and II are explained in a MO diagram. The vertical lines correspond to the experimental bands observed in the absorption spectrum of the [Fe2 (J. - S2) P o - CqH4S) ) ] complex, from Reference 1.

These two-step features, which will be further proved by the FTIR spectra of adsorbed CO, can be summarized as follows. The adsorption of CO, being accompanied by the increase of the coordination munber due to the formation of mono- and dicarbonyl species, causes a shift of the d - d transitions toward the values more typical of the octahedral coordination. Furthermore, in the presence of CO (electron donor molecule) more energy is required to transfer electrons from O to Cr as a consequence, the O Cr(II) CT transition shifts at higher frequencies (from 28000-30000 to 33 700cm ). At increasing CO pressure the CO Cr(II) CT transition also becomes visible (band at 33400 cm ). Analogous features have been reported in the past for NO adsorption on the reduced Cr/Si02 system [48,82]. 

In agreement with the ionization potential involved, it has been observed that the MMCT transition between ions M(d") and M(d°) shifts to lower energy if the valency of the former (the donor) ion decreases [17,19]. This transition is for M(IV) donor ions hardly distinguishable from the CT transition in the M(d°)0 complex, because they overlap in the spectra. For M(III) donor ions (like Cr(III), see above) the MMCT is observed below the CT transition and the sample concerned is strongly colored. For M(II) donor ions (like Fe(II), see above) the MMCT is even at still lower energies. 

This type of centre has also been observed in chlorides [35], Whereas Cs2ZrCl6 and CsjHfClg show blue luminescence due to a CT transition in the ZrClg and HfClg octahedra, the introduction of Pb(II) yields a red luminescence due to MMCT between Pb(II) and Zr(IV)/Hf(IV). 

A CT transition which is very similar to the -> MMCT transition has been observed by Vogler et al. [55] for complexes [M(2,2 -bipyridyl)X3] with X = Cl, Br, I and M = Sb, Bi. These authors report MLCT transitions involving the promotion of an electron from the lone pair to the n orbital of the bipyridyl ligand. For example, for M = Sb and X = Br they observe an orange color for the complex due to an absorption band with a maximum at 435 nm. In the complexes considered by us the transition is to an antibonding n orbital (with pronounced d character) on the filled-shell transition-metal complex ion. 

A description as a MMCT transition is not very obvious for this case. However, there is no essential difference between the physical origin of the colors of Pb(N02)2 and, for example, CU2WO4. Unfortunately the literature shows sometimes discussions on the nature of their excited states in terms of either MMCT or metal-ion-induced CT transitions. To us, such a discussion does not seem to be very fruitful. In the classification it is a matter of taste which nomenclature is used, in the (more difficult) characterization it is essential to determine the coefficients which indicate the amount of configuration interaction. The latter describe the nature of the excited state. 

A similar situation obtains in a series of para-substituted tribenzylbor-anes (48) bearing X = H, F, Me, and MeO groups47 the UV spectra of 48 exhibit a CT band of medium intensity in the region of 240-285 nm. Such an assignment is supported by the existence of a linear correlation of the CT transition energies of the boranes with the ionization potentials of Ph—X (Eq. 16). 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

The large molar extinction coefficients of the spin-allowed CT transitions make them much easier to pump optically. The intense 454-nm visible band of Ru(bpy)32+is an MLCT transition. 

The resonance Raman enhancement profiles In Figures 7 and 8 show that the maximum Intensity of the Fe-O-Fe symmetric stretch falls to correspond to a distinct absorption maximum In the electronic spectrum. This Implies that the 0x0 Fe CT transitions responsible for resonance enhancement are obscured underneath other, more Intense bands. Although strong absorption bands In the 300-400 nm region (e > 6,000 M" cm"l) are a ubiquitous feature of Fe-O-Fe clusters, the Raman results make It unlikely that they are due to 0x0 -> Fe CT. An alternative possibility Is that they represent simultaneous pair excitations of LF transitions In both of the... 

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