Tunable electronic interactions in small lanthanide(III) nanoclusters: The comparative effects of OH− and O2− supramolecular glues on europium(III)-to-dysprosium(III) energy transfer

A comparative study of Eu(III)–Ln(III) (Ln = Y, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) electronic interactions in two series of nanoclusters, [EuxLn4-x(μ4-O)(μ2-OH)2(μ2-L)6]X2 (2) (L = 2,6 diformyl-p-cresolate) and [EuxLn4-x(μ3-OH)4(μ2-L)4]X4 (3) (X=ClO4- or CF3SO3-), featuring tetrahedral disposition of Ln3+ ions was undertaken in order to investigate the effect of supramolecular glues on lanthanide metal–metal interactions. Specifically the influence of bridging, polarizable O2− on one hand and highly polar OH− on the other was investigated. For comparative purposes, simple pair-wise Eu(III)–Ln(III) electronic interactions were established using heterodinuclear complexes, [EuLn(μ2-L)2(L)4], featuring discrete molecules of Eu3+–Ln3+ hetero-couples. Electronic coupling interactions were found to increase with the number of OH− bridges; e.g. the order of Eu3+–Dy3+ electronic interaction strength is [Dy4−xEux(μ3-OH)4(μ2-L)4]4+ > [Dy4−xEux(μ4-O)(μ2-OH)2(μ2-L)6]2+ > [(Dy1−xEux)2(μ2-L)4L2]. Surprisingly, Eu(III)–Yb(III) interactions are also strong and consistent with this trend; those in [Yb4−xEux(μ4-O)(μ2-OH)2(μ2-L)6]2+ being stronger than those in [YbEu(μ2-L)4L2]. The preponderant evidence indicates that these unusual electronic Eu(III)–Yb(III) interactions involve simultaneous coupling of electronic Eu3+(5D0) and Yb3+(2F7/2) states to at least two vibrational states (including νCO). Since highly polar OH− ions are more efficient Eu3+–Ln3+ electronic couplers than the more polarizable O2−, μ3-OH− being more effective than μ2-OH−, we conclude that the dipole moment of a supramolecular glue is more important for lanthanide(III) metal–metal electronic coupling than its polarizability. This is consistent with the dominance of dipolar components of crystal electric fields, which through interaction with electric dipoles of f-electrons become major sources of intensity for the otherwise parity-forbidden Ln3+ f–f transitions.

Supramolecular glues have significant influence on the nature and strength of electronic coupling among proximal Eu3+ and Ln3+ ions. The potency of OH− as a transmitter of electronic effects in Ln(III) nanoclusters increases with the number of Ln–OH–Ln bridges it is engaged in. The energy transfer efficiency order is: μ5-OH− > μ3-OH− > μ2-OH−.Figure optionsDownload as PowerPoint slide