Lanthanide-Containing 2,2′-Bipyridine Bridged Urea Cross-Linked Polysilsesquioxanes

Abstract

Urea-based bis-silylated 2,2 0 -Bipyridine (bpy) organic-inorganic hybrids incorporating different lanthanide (Ln 3þ ) ions (Eu 3þ , Gd 3þ , Tb 3þ or Eu 3þ =Tb 3þ ) were obtained by the sol-gel process. The structure and the emission characteristics of the hybrids were ascertained using X-ray diffraction, nuclear magnetic resonance, Fourier transform infrared spectroscopy, photoluminescence, and quantum yield measurements. The hybrids feature both the emission of the host and the Eu 3þ and=or Tb 3þ transitions allowing a fine-tuning of the color from the blue to the red, orange, or green spectral regions. Bpy-to-Ln 3þ and Tb 3þ -to-Eu 3þ energy transfer mechanisms are demonstrated and the hybrids present slightly distinct Ln 3þ coordination spheres due to the different bpy=Ln 3þ ratios.

Luminescent coatings from bipyridine-based bridged silsesquioxanes containing Eu3+ and Tb3+ salts

“…The emission features were quantified through the measurement of the absolute emission quantum yields of F4-Tb (0.06 AE 0.01) and F4-Eu (0.05 AE 0.01) (excited at 290 nm). The values found are lower than those of M4-Tb (0.12 AE 0.01) and M4-Eu (0.08 AE 0.01), 13 excited at 320 nm. The differences presumably arise from differences in the nanostructures and/or compositions of the coatings and corresponding bulk materials.…”

“…6) show a series of peaks, attributed to the Tb 3+ 5 D 4 / 7 F 6-0 and Eu 3 + 5 D 0 / 7 F 0-4 transitions, and a broad band (380-480 nm), whose energy and fwhm depend on the excitation wavelength. The broad band resembles that observed for the Ln 3+ -containing and pristine bulk hybrid materials, 13,16,28,29 despite the blue-shift from $400 to 550 nm for the former materials to $370 to 480 nm for the films. The band is attributed to the superposition of three distinct components: the bpy-related triplet state and electron-hole recombinations originating in the urea cross-linkages and in the siliceous nanoclusters.…”

“…5) featuring a broad absorption band within the near-UV range (250-330 nm), which is blue-shifted compared to that already reported for bulk M4-Eu. 13 This blue shift (also reported for analogous hybrid materials) 12,16 is induced by differences in the gelation times (hours for the bulk and seconds for the films) which modulates the degree of organisation of the materials. It is interesting to note that there is no evidence of the presence of intra-4f 8 and intra-4f 6 bands in the spectra of the films (which would arise from the direct excitation of the Tb 3+ or Eu 3+ ions, respectively), while the previously reported spectra of the bulk M4-Tb 13 and M4-Eu exhibit the 7 F 6 / 5 D 4 and 7 F 0 / 5 L 6 , 5 D 0 , 7 F 0,1 / 5 D 1 transitions, respectively.…”

“…In addition to the blue-shift discussed above, the energy and relative intensity of the 5 D 0 / 7 F 0-4 Eu 3+ transitions are significantly different. In particular, the non-degenerate 5 D 0 / 7 F 0 is shifted to lower energy by $9 cm À1 for F4-Eu (17261.2 AE 0.4 cm À1 compared to 17270.8 AE 0.4 cm À1 for M4-Eu, 13 with a comparable fwhm of $41 cm À1 ) and the Stark splitting of the 7 F 1,2 levels is much less evident for the thin film. Differences in the energy and relative intensity of the intra-4f 8 lines are also discerned for F4-Tb and M4-Tb (Fig.…”

“…Ln 3+ -containing bridged silsesquioxanes (BS) 3,4 obtained by hydrolysis-condensation of organo-bridged polytrialkoxysilanes (RO) 3 Si-X-Si(OR) 3 (X ¼ organic function, R ¼ Et, Me) in the presence of lanthanide salts have been the subject of several recent studies. 1,2,[5][6][7][8][9][10][11][12][13][14][15][16][17] These materials exhibit a high loading of the lanthanide complex (up to 11.2 wt%) 16 covalently bonded to the siloxane network, with excellent homogeneity. However, to date, few studies have been devoted to the processing of BS as thin films, 6,8,[16][17][18][19][20] with most investigations focusing on the synthesis of PMO-derived thin films from low molecular weight organosilane precursors.…”