Chelates heavy atom effect

This enhancement of intersystem crossing by combining heavy atom and paramagnetic effects explains the relative insensitivity of the Gd phosphorescence lifetime (Table IV) to any additional heavy atom effect (as in the chelate with iodo-BTFA), or to deuteration of solvent or ligand which, by inhibiting nonradiative deactivation, usually increases the lifetime of organic phosphorescence. This insensitivity of the lifetime of the Gd chelate permits us to assign the value of ca. 3 X sec." as the intrinsic radiative rate for the triplet state for Gd BTFA chelates, and a similar value should apply for the Eu compounds. 

From the best fits, the kinetic rate constants, and kb, and the ratio of the radiative rate constants, A/B, of the resonantly coupled spin levels could be obtained. The lifetimes of the triplet sublevels in the various chelates [56] are collected in Table 2. Evidently, the sublevel lifetimes are on the millisecond time scale, and about three orders of magnitude shorter than the phosphorescence lifetimes of the free ligand molecules (bpy Tp = 0.8 s phen Tp=1.4 s [60, 61]). For [Rh(bpy)3] (0104)3 the radiative rate constants are in the ratio T Ty. Tz= 10 1 2, showing that the sublevel is the most active in the radiative as well as non-radiative processes [62]. The shortening of the triplet state sublevel lifetimes for the Rh(III)-chelates as compared to the triplet sublevel lifetimes of the free ligand molecules is reminiscent of the heavy atom effect as, for example, observed for halonaphthalenes [33-35]. In the latter, the mixing of states with UTT and Vtt excitations is enhanced by SOC within the heavy atom. In... 

The method chosen to achieve this stabilization is to have the olefinic group form part of a molecule containing a class b donor atom, usually P or As but occasionally 0 or S. It is found, as expected, that the strongest chelate effect occurs when the chelate ring has 5.5 or 6.5 members, that, is, when there are two or three carbon atoms between heavy atom and vinyl group. Typical ligands, with the abbreviations used here, are shown in Tables 1 and 2. This review excludes compounds such as the one in Fig. 1, where no true chelation, considered as the... 

The hydrolysis of polyphosphate chains is catalysed by heavy metal cations, the effect being most pronounced with cations of high charge and small radius. The effect is believed to be connected with chelation of the cations by the oxygen atoms, which probably increases the susceptibility of the P atom to nucleophilic attack. 

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