Chelates intersystem crossing

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. 

The intensity of the ligand-localized phosphorescence again suggests that the intersystem crossing efficiency for the tris-chelated complexes is near unity, but the lower intensity for the bis-chelated complexes suggests that population of the metal-centered state is less efficient. Emission from... 

Ohnesorge [23] have postulated that this is the result of a crossover to a ta/CgV spin state during the lifetime of the initial excited state of the iron(II) chelates. Such a spin state is paramagnetic and undergoes rapid intersystem crossing and rapid internal conversion to the ground state before emission can occur. 

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