This work discusses quantitatively the energy transfer mechanism that occurs in the white-light emission of sol−gel derived amine- and amide-functionalized hybrids. The white-light photoluminescence (PL) results from a convolution of the emission originated in the NH/CO groups of the organic/inorganic cross-links with electron-hole recombinations occurring in the siloxane nanoclusters, both emissions typical of donor−acceptor pairs. Two model compounds that reproduce separately the two hybrid's emissions were synthesized and characterized by X-ray diffraction, 29Si/H/13C magic-angle spinning NMR, diffuse reflectance, Fourier transform−IR, and photoluminescence spectroscopy to support their use as organic and inorganic structural models for the two counterparts of the hybrids. The comparison between the lifetimes of the two emissions of the inorganic and organic model compounds with those of the hybrids, the Arrhenius dependence with temperature of the siliceous-related lifetime in the hybrids, and the nonexponential behavior of the decay curve of the siliceous-related emission under lower excitation wavelengths are experimental evidence supporting the occurrence of energy transfer in the hybrids. This energy transfer rate is quantitatively estimated for d-U(600) (the diureasil host with smaller number of polymer repeat units) generalizing the ideas proposed recently for the intramolecular energy transfer between singlet and triplet ligand levels and ligand-to-metal charge transfer states in lanthanide coordination compounds. The dipole−dipole energy transfer rate between the two emitting centers is 1.3 × 109 s-1, larger than the value estimated for the transfer rate mediated by the exchange mechanism, 3.7 × 108 s-1. The predicted room-temperature emission quantum yield of that diureasil hybrid is comparable to the corresponding experimental value (7 ± 1 %), pointing out a strong dependence of the radiative component values of the two emissions with temperature, induced by the glass−rubber phase transition of the hybrid's polymer chains.

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This work discusses quantitative aspects of energy transfer occurring in sol-gel derived organic-inorganic di-ureasil hybrids incorporating either [Eu(btfa) 3 (4,4′-bpy)(EtOH)] (btfa ) benzoyltrifluoroacetonate, 4,4′bpy ) 4,4′-bipyridine) or Eu(CF 3 SO 3 ) 3 . Host-to-Eu 3+ energy transfer occurs either via ligand singlet and triplet (T) excited states or directly from the hybrid emitting centers through the dipole-dipole, dipole-2 λ pole (λ ) 2, 4, and 6) and exchange mechanisms. This latter process is dominant for all discussed energy transfer pathways. The ligand-to-Eu 3+ energy transfer rate is typically 1 order of magnitude larger than the value estimated for direct hybrid-to-Eu 3+ transfer (3.75 × 10 10 and 3.26 × 10 9 s -1 , respectively, to the 5 D 1 level). The most efficient luminescence channel is (S 0 ) Hybrid f (T) Hybrid f (T) Ligand f ( 5 D 1 , 5 D 0 ) f 7 F 0-6 . The predicted emission quantum yield of the di-ureasil incorporating [Eu(btfa) 3 (4,4′-bpy)(EtOH)] is in excellent agreement with the corresponding experimental value (53 and 50 ( 5%, respectively), pointing out that the optimization of the ground state geometry by the Sparkle/AM1 model can, under certain conditions, be implemented in Eu 3+ -based organic-inorganic hybrids. For di-ureasils incorporating Eu(CF 3 SO 3 ) 3 , the energy transfer rates could not be quantitatively predicted because of the higher computational effort necessary for calculating the singlet and triplet excited states in complex structures, such as these di-ureasils. Instead, the classic Fo ¨rster and Dexter approaches were applied. Although less efficient, as compared with the di-ureasil incorporating [Eu(btfa) 3 (4,4′-bpy)(EtOH)], the hybrid-to-Eu 3+ energy transfer is also dominated by the exchange (Dexter) interaction.

The bridged silsesquioxane precursor (EtO)3Si(CH2)3NH(CO)NH-(CH2)12-NH(CO)NH(CH2)3 Si(OEt)3, combining polymerizable silylated groups, urea functionalities, and alkyl chains, undergoes fluoride (F−)-catalyzed sol−gel reactions in the presence or absence of EuCl3·6H2O. Supramolecular self-assembly of the growing structure relies primarily on the establishment of strong and ordered hydrogen bonding interactions. In the case of the Eu3+-containing hybrids the lanthanide ions play a totally unparallel dual-role acting simultaneously as structure directing agents and structural probes to sense locally morphological alterations. In the early stages of the synthesis, while a fraction of the Eu3+ ions promotes the formation of a unidirectional urea−urea hydrogen-bonded array, the remaining ions coordinate to silanol (Si−OH) groups inhibiting the growth of the siloxane network and yielding small anisotropic flakes (∼200 nm long). These are subsequently assembled on the micrometer scale in a brick-like tile-to-tile arrangement that ends up with the formation of fibers or twisted bundles (3.0−4.0 μm long and 0.5−1.0 μm wide). At higher Eu3+ concentrations, Eu3+-based ionic interfiber cross-links hinder the solvent flow and force adjacent fibers to adopt a bow-tie form (3.0−4.0 μm long and 3.0 μm wide at the tips). The hybrids are room temperature multiwavelength emitters because of the convolution of the hybrids’ intrinsic emission and the Eu3+ intra-4f6 transitions. The photoluminescence features (5D0→7F0 energy, 5D0 quantum efficiency, number of coordinated water molecules, and experimental intensity parameters) as a function of the Eu3+ content and acidic- and F−-catalyzed conditions used in the synthesis are compared to address the effect of the morphology in the photoluminescence features of the hybrid materials.

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In this article a detailed study of the optical properties of lanthanide doped lamellar nanohybrids synthesized by the "benzyl alcohol route" is presented. The synthetic approach results in the formation of a highly ordered lamellar nanocomposite consisting of yttrium or gadolinium oxide crystalline layers with a confined thickness of about 0.6 nm, separated from each other by organic layers of intercalated benzoate molecules. When the inorganic layers are doped with optically-active lanthanide ions they show outstanding emission properties in the green (Tb(3+)), red (Eu(3+)) and near infrared (Nd(3+)). The local environment of the emitting ions and the energy transfer processes involving the phenyl ring of the benzoate complexes and the lanthanide ions are presented, as well as radiance and lifetime measurements. The radiance values are comparable and in some cases even larger than those of standard phosphors, proving that these nanohybrids can compete, from an emission efficiency point of view, with commercial phosphors. Furthermore, in these nanohybrids it is possible by simply changing the excitation wavelength, to tune the emission colour chromaticity without loosing the radiance.

We report a metal‐free bpy‐bridged silsesquioxane displaying an efficient and tuneable blue‐to‐green emission (maximum quantum yield and luminance values of 0.22 ± 0.02 cd m–2 and 63 ± 3 cd m–2, respectively) when excited by commercial InGaN light emitting diodes (LEDs) (385–405 nm). This is the highest quantum yield value reported so far for metal‐free phosphors easily excited by commercial InGaN long‐UV/blue LEDs bringing organic/inorganic hybrids on the way to applications as metal‐free phosphors for smart LED‐based lighting and LED‐addressed displays. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)