Jean-Claude Bunzli - Ln(III)-cored complexes based on boron dipyrromethene (Bodipy) ligands for NIR emission

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  Ryu,J.H., Eom,Y.K., Bunzli,J.C.G., Kim,H.K. New Journal of   Chemistry 2012 36 (3) 723-731

  Abstract: Two Bodipy-based chromophores have   been synthesized, which bear two methoxyphenyl substituents in   the 1,7 positions and either hydrogen (L1) or bromine (L2) atoms   in the 2,6 positions, with the aim of testing their ability to   sensitize the luminescence of near-infrared emitting Ln(III)   ions. Despite the presence of the two halogen substituents in L2,   the two chromophores display very similar absorption and emission   properties, the only difference being a drop in the quantum yield   from 31% for L1 to 21% for L2. In addition, no triplet state   emission could be evidenced for both chromophores as well as for   their tris complexes with Gd-III ions. This is traced back to   unfavorable conformation of the molecule, preventing efficient   intersystem crossing. Complexes with stoichiometry   [Ln(Li)(3)(tpy)] have been isolated for Ln = Gd, Er, Yb and fully   characterized. Ligand emission is partially quenched (20-40%) in   the Er-III and Yb-III complexes and typical metal-centred   luminescence is detected at 1530 (Er-III) and 978 (Yb-III) nm   upon excitation in the orange portion of the visible spectrum   (583 nm). In these complexes, the main energy transfer pathway   involves the singlet state(s) of the ligands: the lowest (1)pi pi   state emission overlaps the Er(F-4(9/ 2) <- I-4(15/2))   absorption band and the (1)pi pi-Yb(F-2(5/2)) gap amounts to only   5700 cm (1). This study demonstrates that the Bodipy framework is   adequate for sensitizing the luminescence of NIR-emitting   lanthanide ions, allowing excitation wavelengths extending into   the orange portion of the visible spectrum and yielding complexes   with large molar absorption coefficients (log epsilon approximate   to 5.0-5.2). Additionally it also points to the importance of the   conformation of the chromophores on the yield of intersystem   crossing

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