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, [Eu(x)Ln(4-x)(mu(4)-O)(mu(2)-OH)(2)(mu(2)-L)(6)]X-2 (2) (L = 2.6 diformyl-p-cresolate) and [Eu(x)Ln(4-x)(mu(3)-OH)(4)(mu(2)-L-4)]X-4 (3) (X = ClO4- or CF3SO3-), 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 O2- on one hand and highly polar OH- the other was investigated. For comparative purposes, simple pair-wise Eu(III)-Ln(III) electronic interactions were established using heterodinuclear complexes, [EuLn(mu(2)-L)(2)(L)(4)], featuring discrete molecules of Eu3+-Ln(3+) 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(mu(3)-OH)(4)(mu(2)-L)(4)](4+) > [Dy4-xEux(mu(4)-O)(mu(2)-OH)(2)(mu(2)-L)(6)](2+) > [(Dy1-xEux)(2)(mu(2)-L)(4)L-2]. Surprisingly, Eu(III)-Yb(III) interactions are also strong and consistent with this trend; those in [Yb4-xEux(mu(4)-O)(mu(2)-OH)(2)(mu(2)-L)(6)](2+) being stronger than those in [YbEu(mu(2)-L)(4)L-2]. The preponderant evidence indicates that these unusual electronic Eu(III)-Yb(III) interactions involve simultaneous coupling of electronic Eu3+(D-5(0)) and Yb3+(F-2(7/2)) states to at least two vibrational states (including nu(C=O)). Since highly polar OH- ions are more efficient Eu3+-Ln(3+) electronic couplers than the more polarizable O2-. mu(3)-OH- being more effective than mu(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. (C) 2009 Elsevier Ltd. All rights reserved.