Perfluorinated nitrosopyrazolone-based erbium chelates: a new efficient solution processable NIR emitterw Luca Beverina,* Maurizio Crippa, Mauro Sassi, Angelo Monguzzi, Francesco Meinardi, Riccardo Tubino and Giorgio A. Pagani* Received (in Cambridge, UK) 1st April 2009, Accepted 9th July 2009 First published as an Advance Article on the web 24th July 2009 DOI: 10.1039/b906494j We show the design and synthesis of new perfluorinated nitroso- pyrazolone-based ligands and the original method employed for their complexation of erbium ions in the presence of the co-ligand perfluorotriphenylphosphine oxide; the resulting chelate is non- hygroscopic, solution processable and possesses a NIR emission with lifetimes as long as 16 ls. Interest in the development of efficient NIR emitting organic derivatives is connected with the recent development of plastic optical fibers. 1 While in fact it is accepted that silica-based optical technologies will remain unchallenged for long distance informa- tion transmission, short distance telecommunications, such as LAN, are expected to profit from alternative, low cost, plastic- based solutions. However, the more the absorption (and emission) of an organic compound is shifted towards low energies, the less the compound becomes able to stand long term exposure to ambient atmosphere, especially under the device operating conditions (illumination, temperature, moisture...). This observation indicates how challenging the task of designing efficiently NIR operating organic materials may be. The development of lanthanide-based emitting chelates represents a valuable solution. These particular compounds profit from the favorable properties of a conjugated organic ligand (efficient absorption in the UV-Vis spectrum, stability, solubility, compatibility with plastic fibers and polymers in general) and from the peculiar emission capabilities of lanthanide ions, in this case the Er 31 4 I 13/2 - 4 I 15/2 transition at 1550 nm, matching well the POF (plastic optical fiber) transmission–loss dispersion minima. 2 Lanthanide ions cannot be efficiently pumped directly; their absorption transitions are in fact prohibited on symmetry grounds. The population of the rare-earth excited states is best achieved through energy transfer from suitable organic ligands. 3 This strategy has been so far successfully applied to several lanthanides, including europium, whose complexes are widely employed as emitting materials in LED technologies. 4 The case of erbium chelates is however more complex. Erbium emission is in fact particularly prone to non-radiative relaxation due to dipole coupling of the ion excited states with the high energy vibrational modes of the ligands and the coordinating solvent molecules. In this respect the presence of O–H, N–H and even C–H groups in close proximity to erbium is a well known and a very efficient relaxation pathway. 5 In the case of erbium tri(hydroxyquinolinate) (ErQ 3 ), the most studied erbium chelate, the excited state lifetime drops from the milliseconds commonly observed in erbium doped glass-based devices to a mere 0.2–2 ms depending on the environment. 6 Recently, the use of fluorinated ligands afforded a viable solution to the vibrational quenching issue. 7 The most notable drawback of this approach is the relatively high absorption energy of the ligands employed so far, whose absorption cut-off scarcely reaches the 400 nm wavelength. 8 In view of practical applications it would be highly profitable to employ ligands having absorption spectra compatible with broad- band, low cost light sources such as high-power LED now available in the near UV region, i.e. 407 nm GaN laser diodes. Moreover most of the known perfluorinated chelates have been processed by high temperature vacuum evaporation, a relatively high cost process especially when compared with spin or blade coating. We here propose a new class of perfluorinated ligands, pertaining to the class of nitrosopyrazolone derivatives, possessing efficient erbium chelating capabilities, relatively low band gaps and facile preparation procedures. Moreover, the compounds we obtained are highly soluble in common organic solvents, thus enabling the preparation of films by spin coating, and they are completely hydrophobic. The new ligand 2 was prepared according to the classic pyrazolone synthetic scheme. Pentafluorophenyl hydrazine was thus reacted with ethyl 4,4,4-trifluoroacetoacetate in refluxing acetic acid and under microwave irradiation to give the pyrazolone derivative 1, nitrosation of which with isoamylnitrite in THF afforded the pure ligand 2 in an overall yield of 30%. See Scheme 1. A slight modification of the same procedure afforded ligand 3, the hydrogenated analog of ligand 2 which was made for comparison (see the ESIw). Scheme 1 University of Milano-Bicocca, Department of Materials Science, 20215, Milan, Italy. E-mail: luca.beverina@mater.unimib.it w Electronic supplementary information (ESI) available: Synthetic procedures, IR-spectra and DSC traces for ligands and complexes. Details on the photophysical characterization. See DOI: 10.1039/b906494j This journal is  c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 5103–5105 | 5103 COMMUNICATION www.rsc.org/chemcomm | ChemComm