An interesting ligand for the preparation of luminescent plastics: The picrate ion

Electrolytes formed with poly(oxyethylene), POE, and europium picrate, [Eu(pic)(2)(OH2)(6)]pic . 6H(2)O (where pic denotes the picrate anion) or simply Eu(pic)(3)xH(2)O, have been investigated by infrared and photoluminescence spectroscopies. The complexes have been represented by POEnEu(pic)(3)xH(2)O (where n represents the molar ratio of (OCH2CH2) units per Eu3+ ion). Materials with n ranging from 133 to 11 have been examined. A tentative attribution for the absorption bands of the mid-infrared spectra is presented. The spectral changes detected in the mid-infrared region and the modifications observed in the XRD patterns at increasing salt content show that Eu(pic)(3)xH(2)O exerts an effective plasticizing role which leads to the complete supression of crystallinity at n = 11. The emission spectra of the complexes and their signature in the nu C-O spectral region provide conclusive evidence that the Eu3+ ions are coordinated to the oxygens atoms of the polyether chains over the whole range of compositions studied. The photoluminescence spectra of the POEnEu(pic)(3)xH(2)O electrolytes indicate that the Eu3+/ether oxygen complexation occurs with the concomitant partial substitution of water molecules from the cation coordination shell. The luminescence data obtained confirm that the latter aspect is quite advantageous from the standpoint of optical properties. The incorporation of the picrate salt into POE results in a substantial increase in the calculated total radiative contribution for the depopulation of the D-5(0) level (from 0.58 to 1.94 ms(-1)) and in the quantum efficiency (from 5.6 to 41.9%). The analysis of the nu(a)NO(2), nu(s)NO(2) and nu(phen)CO picrate anion characteristic bands of the spectra of the POE-based complexes suggests that the coordinated picrate ligands of the picrate salt are unaffected by the presence of POE and never leave the lanthanide coordination shell. The absence of diffraction peaks of Eu(pic)(3)xH(2)O in the diffractograms of the electrolytes and the recognition that the crystalline regions are essentially composed of pure POE allow us to conclude that the complexation effect occurs exclusively in the amorphous phase. It is probable that the charge carrier in these electrolytes is the non-hydrogen-bonded and uncoordinated picrate ligand of the salt, while the complex cation [Eu(pic)(2)(OH2)(y)(OR2)(z)](+) (where R2O stands for POE, y<6 and z greater than or equal to 1) is expected to be immobile. (C) 2000 American Institute of Physics. [S0021-9606(00)70106-1].