Ligand Ionisation

Eu(III) excitation spectroscopy is extremely useful for monitoring ionisation of Eu(III) complexes as a function of pH. An example of this is shown in Fig. 8.8 where a blue shifted excitation peak grows in with increasing pH for Eu(CNPHC)(OH2) [60], The pATa of this group is 7.5 and correlates to that measured by pH potentiometric titrations. It is not certain whether this new species arises from deprotonation of an alcohol group or of the water ligand. 

Ionisation of Eu(III) in water to give hydroxide complexes generally gives comphcated solution chemistry with too many excitation peaks to be resolved. For example, the Fq excitation spectrum of Eu(III) salts in aqueous solution as a function of pH attests to the complicated solution chemistry of Eu(III) under these conditions (Fig. 8.9). 

A further example of complicated solution speciation for Eu(lll) complexes as a function of increasing solution basicity as probed by direct excitation Eu(lll) spectroscopy is shown for Eu(C104)3 and Eu(CF3S03)3 in DMSO/water mixtures [62]. Addition of Mc4NOH to these solutions leads to a very broad asymmetric Eu(III) excitation peak consistent with the formation of multiple species (Fig. 8.11). 

Curiously, the excitation spectra differ for C104 compared to CF3S03 salts, possibly due to the coordination of perchlorate anion to the Eu(III) species. Water counting studies were consistent with dehydration of the Eu(lII) ion at more basic pH. However, luminescence lifetime decays were also reflective of the presence of multiple species with different hfetimes (Fig. 8.12) [62]. 

This study shows the difficulty of smdying complex speciation by lifetime decays alone because it is difficult to obtain reliable data from multi-exponential fits. LRET studies with the Eu(in)Arb(ni) couple in DMSO were consistent with multinuclear species as shown by energy transfer. The presence of dinuclear hydroxo complexes was also supported by pH potentiometric titration data [62]. 

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The final system of the metallocene series, Ni(Cp)2, and its dimethyl derivative, sup-ly only a small amount of information from their photoelectron spectra, since only a single peak due to a d-electron ionisation is observed in each case. This band is obviously due to ionisation of a 7r d-electron from the 32 (o2 tt2 54) ground level to yield a single ion state, 2Il(a2 tt 54), and its intensity relative to the ligand ionisation region rules out the possibility of other d-electron ionisations being coincident with it. 

From an inspection of the predicted intensities in Table 24 it is evident that absorptions due to triplet levels are likely to predominate within the other three d-ionisation bands, and first order ligand field theory gives the energies of these, relative to the 12+(a2 54)level, as 3IT(a2 tt 53) = 34>(a27r 53) = At + A2 - 9 B - 3 C, and 3Ila7r 54) =... 

In all the cases above, the ligand is water and we are looking at the effect of co-ordination upon the first and second ionisation processes (Fig. 2-15). 

Figure 2-15. Co-ordinated water ligands show deprotonation processes corresponding to the first and second ionisation steps.

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