Pathways of Radiative and Nonradiative Transitions of Tb3+in DMSO

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Abstract
Recent luminescence studies of Eu3+ in organic solvents and in aqueous solutions have revealed that the fluorescence efficiency of the rare-earth ion depends upon both the primary and the secondary solvation spheres of the ion. Primary and secondary fluorescence quenching rate constants of Eu3+ have been determined for various compounds such as CH3COOH, CF3COOH, CH3COCl(1) as well as for such specific groups as the ←C-H and -OH groups of CH3COOH(2). In these studies the primary solvation sphere of Eu3+ consisted mainly of CH3COOH i.e. [CH3COOH]/[EuCl3] = 17, whereas the second solvent or any excess of CH3COOH were located in the secondary solvation sphere. In cases in which the rare-earth salt was soluble in both solvents, such as in DMSO-H2O, D2O-ETOH, D2O-CH3COOD, DMSO-D2O or D2O-H2O, a gradually varying primary solvation sphere exhibited a paramount effect upon both the fluorescence efficiency and the transition mechanisms of Eu3+. A drastic effect due to traces of DMSO upon the 7Fo ←5D2 and 5Do ← 7F2 transitions of Eu3+ in D2O-DMSO or in H2O - DMSO, compared to an almost negligible effect upon the 7Fo ← 5L6 transition, led us to conclude that the former transitions were mostly “electric-quadrupole” in nature(3) rather than “forced electric-dipole”(4), a model which can account very well for the 7Fo ← 5L6 transition. Our conclusion was verified by the fact that the aforementioned effect was almost exclusively associated with the primary rather than the secondary solvation sphere.(3)