Excitons and excess electrons in nanometer size molecular polyoxotitanate clusters: electronic spectra, exciton dynamics, and surface states.

The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti17(μ4-O)4(μ3-O)16(μ2-O)4(OPri)20 (in short Ti17) was studied using femtosecond pump–probe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17•–, are blue-shifted in comparison with bulk TiO2 and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17* and Ti17•– indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the ∼1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to pl...