Lamellar mono-amidosil hybrids incorporating monomethinecyanine dyes

Abstract

Mono-amide cross-linked alkyl–siloxane hybrids synthesized through the sol–gel process and the self-directed assembly have been doped with variable concentrations of monomethinecyanines SSCH₂(C₆H₅)I, SSCH₃Cl and SSC₂H₅ClO₄. The host matrix m-A(14) is a lamellar structured hybrid composed of short methyl-capped alkyl chains grafted to the siliceous framework through amide groups. The doped mono-amidosil hybrid materials were obtained as solid powders. The bulky SSCH₂(C₆H₅)I dye inhibited the condensation of the silica network. In the SSCH₃Cl- and SSC₂H₅ClO₄-doped m-A(14)-based materials the original lamellar structure coexists with a new lamellar ordered phase with a lower interlamellar distance in which the alkyl chains adopt essentially all-trans conformations. The efficient encapsulation of these two dyes by m-A(14), which ensures their complete dissolution, is due to the coordination of the S atoms of SSCH₃Cl and SSC₂H₅ClO₄ to the carbonyl oxygen atoms of the amide cross-links. This process leads to a major breakdown of the disordered and ordered hydrogen-bonded aggregates of m-A(14), but does not affect globally the strength of the hydrogen-bonded array. The most dilute SSC₂H₅ClO₄-doped mono-amidosil studied displays room temperature emission in the visible spectral region (380–680 nm) due to the convolution of the intrinsic emission (blue spectral region) of m-A(14) with that of the SSC₂H₅ClO₄ dye (green and red spectral regions) due to the formation of fluorescent J-dimers. The excitation spectra (monitored along the emission spectra) reveal the occurrence of host-to-monomethinecyanine energy transfer.