Titania (TiO2) nanoparticles are active photocatalysts, and isoprene (C5H8) is a biogenic volatile organic compound that contributes crucially to global particulate matter generation. Herein, the direct photooxidation of isoprene by titanium oxide cluster anions with dimensions up to a nanosize by both ultraviolet (UV) and visible (Vis) light excitations has been successfully identified through mass spectrometric experiments combined with quantum chemistry calculations. The potential role of "dry" titania in atmospheric isoprene oxidation has been revealed, and a clear picture of the photooxidation mechanism on titanium oxide nanoparticles has been provided explicitly at the molecular level. The adsorption of isoprene on the atomic oxygen radicals (O center dot-) of titanium oxide clusters leads to the formation of the crucial interfacial state (IS) within the band gap of titanium oxides. This IS is demonstrated to be the significant factor in delivering the electron from the pi orbital of C5H8 to the Ti-3d orbital in the photooxidation process (C5H8 + Ti4+-O center dot--> C5H8O + Ti3+) and creating photoactivity in the Vis region. It is revealed that after the photogeneration of the O center dot- radicals by UV excitation on the TiO2 particle surface, the subsequent reactions can be induced by Vis excitation through the IS. This multicolor strategy in both the UV and Vis regions can enhance the efficiency of solar energy harvesting and improve the product yield of the photocatalysis on TiO2 nanoparticles. New insights have been provided into both the atmospheric chemistry of isoprene and the photochemistry of TiO2 nanoparticles.