Effect of Cross-Linking and Organic Groups on the Visible Photoluminescence Characteristics of n-Octadecylsiloxanes

The photoluminescence (PL) properties of properly annealed n-octadecylsiloxanes, which are newly recognized highly emissive hybrid materials reported recently by us (Nishimura, A.; Sagawa, N.; Uchino, T. J. Phys. Chem. C 2009, 113, 4260), with different cross-linking structures are investigated using steady-state and time-resolved PL spectroscopy. We find that the observed PL intensity and the PL decay dynamics are strongly dependent on the degree of cross-linking of the organic−inorganic hybrid materials. We attribute these changes to the initial concentration of the residual silanol groups, which can be converted into emission and electron-trapping centers through the condensation reaction during annealing. These PL features are then compared with those of purely inorganic silica gels to evaluate the effect of organic groups on the PL characteristics of the n-octadecylsiloxanes. It has been found that organic groups contribute to the PL decay component in the time scale of ∼70 to ∼100 ns. Also, organic groups will kinetically stabilize the condensation-derived emission centers through steric hindrance and will provide a good decoupling between the electronic states of the emission centers and the lattice vibrations, probably explaining the efficient PL emission from the hybrid materials.