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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, [Eu x Ln 4- x (μ 4 -O)(μ 2 -OH) 2 (μ 2 - L ) 6 ]X 2 ( 2 ) ( L = 2,6 diformyl-p-cresolate) and [Eu x Ln 4- x (μ 3 -OH) 4 (μ 2 - L ) 4 ]X 4 ( 3 ) ( X = ClO 4 - or CF 3 SO 3 - ), featuring tetrahedral disposition of Ln 3+ ions was undertaken in order to investigate the effect of supramolecular glues on lanthanide metal–metal interactions. Specifically the influence of bridging, polarizable O 2− 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(μ 2 - L ) 2 ( L ) 4 ], featuring discrete molecules of Eu 3+ –Ln 3+ hetero-couples. Electronic coupling interactions were found to increase with the number of OH − bridges; e.g. the order of Eu 3+ –Dy 3+ electronic interaction strength is [Dy 4− x Eu x (μ 3 -OH) 4 (μ 2 - L ) 4 ] 4+ > [Dy 4− x Eu x (μ 4 -O)(μ 2 -OH) 2 (μ 2 - L ) 6 ] 2+ > [(Dy 1− x Eu x ) 2 (μ 2 - L ) 4 L 2 ]. Surprisingly, Eu(III)–Yb(III) interactions are also strong and consistent with this trend; those in [Yb 4− x Eu x (μ 4 -O)(μ 2 -OH) 2 (μ 2 - L ) 6 ] 2+ being stronger than those in [YbEu(μ 2 - L ) 4 L 2 ]. The preponderant evidence indicates that these unusual electronic Eu(III)–Yb(III) interactions involve simultaneous coupling of electronic Eu 3+ ( 5 D 0 ) and Yb 3+ ( 2 F 7/2 ) states to at least two vibrational states (including ν C O ). Since highly polar OH − ions are more efficient Eu 3+ –Ln 3+ electronic couplers than the more polarizable O 2− , μ 3 -OH − being more effective than μ 2 -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 Ln 3+ f–f transitions. |
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