[S2-->S0 radiative transition of 2,2'-bipyridine doped in silica gel glass]

Doping 2,2'-bipyridine molecules in silica gel glass leads to special spectroscopic properties compared to those dissolved in liquid solutions. For the sample without heat treatment, two broad bands centered at 400 nm and 454 nm respectively, induced by the radiative transition from the first excited level S1 to the ground level S0 and attributed to the emission from an excited dimeric 2,2'-bipyridine species respectively, were observed. After heat-treated at 200 degrees C, the excimer emission at 454 nm disappeared, and the sample exhibited another emission band centered at 325 nm due to the radiative transition from the second excited level S2 to the ground level. S2-->S0 transition resulted from restricting 2,2'-bipyridine molecules in silica networks. Only the structure emission of S2-->S0 transition was observed when heat-treated at 550 degrees C, which indicates that most of 2,2'-bipyridine molecules were entrapped in the cages of Si-O network. The above results can be used to characterize the microstructural evolution of hybrid organic-inorganic optical materials.