Luminescence of Intraporous Cerium(III) Complexes in Alkyl Sulfonic Mesoporous Silica

Mesoporous silica materials derivatized with propyl-sulfonic groups (SBA-15) with a monovalent cation-exchange capacity of ∼1 meq/g were exchanged with Ce(III) ions to ∼0.5 meq/g to form Ce(III)SBA material displaying high-intensity purple photoluminescence. XPS and NIR-DRS analysis suggests the Ce(III) ion is bound to two proximal propyl-sulfonate groups and capped with a hydroxyl in a complex [≡Si−(CH 2 ) 3 SO 3 ] 2 Ce(III)OH· x H 2 O ( x > 4), where the coordinated water can be almost completely removed by anoxic dehydration at 350 C to x < 0.2. The white hydrated ( x ≥ 4) CeSBA and the brown-colored dehydrated ( x < 0.2) CeSBA exhibit fluorescence with comparable integrated intensities at 368 and 394 nm, respectively. The Ce(III) 4f electron is located by XPS above the top of the Si 3p O 2p valence band, thus identifying the Ce(III) 4f orbital as the HOMO. Photoluminescence emission and excitation spectra of the 5d → 4f transition display Stokes shifts and structure sensitivity consistent with a direct transition process not involving, in these SBA materials, a multistep energy transfer previously invoked for oxide cerium-containing phosphors. A quantitative assessment of the relaxation was made on the basis of calculation of vibrational frequencies in DFT-optimized structures of a disulfonate−La−OH species with and without additional coordinated water.