Water-mediated structural tunability of an alkyl/siloxane hybrid: from amorphous material to lamellar structure or bilamellar superstructure

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Mono-amide cross-linked alkyl/siloxane hybrids (classed as mono-amidosils and represented by the notation m-A(x), where m and A stand for mono and amide, respectively, and x is the number of methylene repeat units) with water-mediated tunable structures have been prepared by means of sol–gel chemistry and self-direct assembly routes from the organosilanes CH3(CH2)x–C(O)NH–(CH2)3–Si–(OCH2CH3)3. The amorphous sample m-A(8) has been produced under the stoichiometric conditions (molar ratio Si : ethanol : water = 1 : 4 : 2) used previously to obtain the lamellar bilayer highly organized m-A(14) hybrid material, showing that the length of the pendant alkyl chains affects the degree of order of the materials. Three structurally ordered samples identified as AC-m-A(8) (1 : y) (where AC represents acid catalysis and y is the number of moles of water per mole of Si) have been obtained using the acid catalyzed hydrolytic route and different water contents (y = 600, 300 and 100). Water plays a unique role in the organized mono-amidosils: it not only reverses the natural tendency of the precursor molecule with x = 8 to yield a disordered material, but it also allows the induction of order, leading to the formation of a lamellar structure exclusively in AC-m-A(8) (1 : 100). The presence of a higher water content promotes extra ordering in AC-m-A(8) (1 : 300) and AC-m-A(8) (1 : 600), where the lamellar structure coexists with a bilamellar superstructure. Upon heating the AC-m-A(8) (1 : 600) sample to 120 °C and then cooling it to room temperature, the lamellar structure remained unaffected, while the superstructure was destroyed. The occurrence of the superstructure, although basically associated with the preferential entrapment of water molecules every two lamellae at the siliceous nanodomains, is also correlated with the amide–amide hydrogen bonded array. The m-A(8) and AC-m-A(8) (1 : 600) hybrids are multi-wavelength emitters under UV/VIS excitation. The emission spectra exhibit a broad band (300–650 nm) resulting from two contributions: a “blue” band, due to electron–hole recombinations in the NH/CO groups of the amide cross-links, and a “purplish-blue” band, due to oxygen-related defects •O–O–Si≡(CO2) in the siliceous nanoclusters. The absolute emission quantum yields determined for m-A(8) and AC-m-A(8) (1 : 600) in the 360–380 nm excitation wavelength range were 0.10 ± 0.01 and 0.15 ± 0.01, respectively. While the lifetime values of the high and low wavelength components of the siliceous-related emission are practically the same for both samples (∼3.5 ms), the lifetime value of the NH/CO component for m-A(8) is higher than that of AC-m-A(8) (1 : 600) (∼292 and ∼156 ms, respectively), suggesting a lower non-radiative transition probability for the former hybrid.