RÉGIMEN PATRIMONIAL

The development of organic-inorganic hybrid materials was greatly advanced through the use of soft chemical synthetic methods such as sol-gel processing.62 This one pot synthesis allows for mild processing conditions ensuring the survival of low weight organic components.52, 62 This technique is based on the polymerisation of metal alkoxide species M(OR)n.

52

The rich chemistry of silicone has been exploited to synthesise stable, low cost and efficient M(OR)n.-based white

light photoluminescent organic-inorganic hybrids lacking metal activator ions.63 Siloxane based materials also possess attractive optical features such as high laser efficiency and photostability,64 electroluminescence64, 65 and nonlinear optical properties.66 Of particular interest in this thesis are a class of siloxane-based organic-inorganic hybrids called di-ureasils. Di-ureasils are composed of poly(ethylene oxide) (PEO)/poly(propylene oxide) (PPO) block copolymer chains chemically grafted to a siliceous network through two urea linkages,67-69 giving rise to the di-ureasil nomenclature. It is possible to prepare a variety of di-ureasil species through alteration of the organic linker species with syntheses containing block copolymer with molecular weights of 600,

17

900, 2000, 4000 g mol-1 previously reported.68, 70, 71 Di-ureasils are prepared through hydrolysis and subsequent condensation of a precursor solution designated d-UPTES, the structure of which is shown in Fig. 1.12 for the 600 g mol-1 block copolymer.

Figure 1.12. Structure of the d-UPTES precursor solution. Red circles highlight the two urea linkages that give rise to the di-ureasil name. The expanded structure on the far right represents the siliceous network formed following hydrolysis and condensation of the Si moieties at either end of the d-UPTES structure.

Undoped di-ureasils are transparent materials with an elastomeric nature imparted by the flexibility of the PEO/PPO chains. Di-ureasils are also inherently photoluminescent, exhibiting an emission band in the purple-blue spectral region, the maximum of which is dependent on excitation wavelength.68, 72 Due to their desirable optical properties and ease of processability, di-ureasils have been frequently employed as host materials for a range of dopant species. Incorporation of lithium perchlorate (LiClO4) produces materials with high conductivity that is attributed to the

flexibility of the host allowing faster movement of the Li+ ions.73 Increasing the flexibility by inclusion of further PPO chains within the organic linker causes the conductivity to almost double.74 These results suggest the utility of di-ureasils as solid polymer electrolytes, which are currently under investigation in the area of Li-ion batteries in attempts to reduce leakage, eliminate the use of volatile organic liquids and increase mechanical flexibility and thermal stability.73, 74 Furthermore, addition of methacrylic acid modified zirconium clusters within a di-ureasil host is found to modulate the material’s refractive index,75 while simultaneously inducing a red-shift of the excitation wavelength which moves the hybrid hosts intrinsic emission intensity from the UV (365 nm) to the blue (420 nm). An enhancement in the emission quantum yield is also observed, reaching 9.0% for the zirconium doped di-ureasil.76

There are also frequent reports of di-ureasils doped with lanthanide ions with the intention of creating materials with interesting optical properties.77-80 The incorporation of an Eu-β-diketonate complex within a di-ureasil hybrid has been attempted in an effort to enhance its light emission and photostability properties.78, 81 It was found that on incorporation of this complex into a di-ureasil host, the carbonyl groups of the urea linkages replaced the ethanol species in the coordination sphere of the Eu3+ ion observed in solution causing a dramatic enhancement in the quantum efficiency of the complex in the solid state. This is attributed to the removal the OH oscillator from the coordination sphere of the Eu3+ ion. Efficient energy transfer from the di-ureasil host to the

18

Eu3+ metal centre was also observed along with increased photostability of the incorporated complex. A similar rearrangement in the lanthanide coordination sphere of a Tb(III)- (acetylacetonate)3 was also observed on inclusion in a di-ureasil host, with the carbonyl of the host

replacing the coordinated water molecules within the complex.79 The temperature dependence of both an Eu-β-diketonate and Tb-β-diketonate complex has recently been developed for use as a self-referencing thermometer with high temperature sensitivity and spatial resolution.82 The intensity of the green Tb3+ luminescence was found to strongly decrease as the temperature increases, while the intensity of the red Eu3+ lines begin to increase at precisely the same temperature at which the Tb3+ emission decreases. Thus, temperature may be determined by the relative intensities of the Eu3+ and Tb3+ emission intensities. The inclusion of these complexes within a di-ureasil host shifts the range of operation of the thermometer to room temperature by increasing the activation energy between the triplet state of the host and the excited state of the lanthanide. This makes these materials sufficiently sensitive to be used in the physiological temperature range. Addition of siloxane-based hybrid magnetic nanoclusters allows for the same temperature dependence of the lanthanide complexes (Fig. 1.13a) while increasing the sensitivity to 300-350 K (Fig. 1.13b).83

Figure 1.13. (a) Integrated intensity of the 5D0 →7F2 (605-635 nm, red circles) and 5D4→7F5 (535-560 nm,

green circles) transitions as a function of temperature for the magnetic nanoclusters co-doped with Eu-β- diketonate and Tb-β-diketonate containing complex discussed in the text. Dashed lines are solely to guide the eye. (b) Relative sensitivity computed from experimental data for the pure Eu-β-diketonate and Tb-β- diketonate containing nanoparticles (NPS-1.3) and the di-ureasil coating incorporating the magnetic nanoclusters co-doped with Eu-β-diketonate and Tb-β-diketonate complexes (NPU-1.3) The shadowed areas correspond to the temperatures where the relative sensitivity is greater than 0.5%. Adapted from ref 83.