Photoluminescence from silsesquioxanes R8(SiO1.5)8

UV absorption, UV light induced emission (photoluminescence, PL) and PL-excitation spectra of H8(SiO1.5)8 and alkyl-substituted R8(SiO1.5)8 in solution, powder, and “gel” have been studied systematically. All investigated molecules show similar spectral structure, consisting of intensive absorption bands at about 6 eV and two intensive emission bands peaking at about 3.7 and 4.2 eV. The emission band at higher energy can be explained in terms of the calculated densities of states and potential energies of H8(SiO1.5)8, published to date. This transition is connected with a charge transfer from the Si–O cage to the ligand. Second emission band peaking at 3.7 eV is probably due to an intermolecular interaction which results in dimer or exciplex (“excited complex”) formation. For larger ligands, the density of states increases because of additional vibration, rotation and torsion modes. This results in an increase of absorption and emission intensity for larger ligands.